Publications

Self-healing (SH) of (opto)electronic material damage can have a huge impact on resource sustainability. The rising interest in halide perovskite (HaP) compounds over the past decade is due to their excellent semiconducting properties for crystals and films, even if made by low-temperature solution-based processing. Direct proof of self-healing in Pb-based HaPs is demonstrated through photoluminescence recovery from photodamage, fracture healing and their use as high-energy radiation and particle detectors. Here, the question of how to find additional semiconducting materials exhibiting SH, in particular lead-free ones is addressed. Applying a data-mining approach to identify semiconductors with favorable mechanical and thermal properties, for which Pb HaPs are clear outliers, it is found that the Cs₂Au^IAu^IIIX₆, (X = I, Br, Cl) family, which is synthesized and tested for SH. This is the first demonstration of self-healing of Pb-free inorganic HaP thin films, by photoluminescence recovery.

The class of materials termed halide perovskites has experienced a meteoric rise in popularity due to their potential for photovoltaic and related applications, rivaling the well-established silicon devices within a few short years of development. These materials are characterized by several intriguing properties, among them their mechanical behavior. The study of their response to stress is essential for proper device development, while being of fundamental scientific interest in its own right. In this perspective, we highlight the key concerns surrounding this topic, critically analyzing the measurement techniques and considering the challenges in the current level of understanding.

In terms of sustainable use, halide perovskite (HaP) semiconductors have a strong advantage over most other classes of materials for (opto)electronics, as they can self-heal (SH) from photodamage. While there is considerable literature on SH in devices, where it may not be clear exactly where damage and SH occur, there is much less on the HaP material itself. Here we perform “fluorescence recovery after photobleaching” (FRAP) measurements to study SH on polycrystalline thin films for which encapsulation is critical to achieving complete and fast self-healing. We compare SH in three photoactive APbI₃ perovskite films by varying the A-site cation ranging from (relatively) small inorganic Cs through medium-sized MA to large FA (the last two are organic cations). While the A cation is often considered electronically relatively inactive, it significantly affects both SH kinetics and the threshold for photodamage. The SH kinetics are markedly faster for γ-CsPbI₃ and α-FAPbI₃ than for MAPbI₃. Furthermore, γ-CsPbI₃ exhibits an intricate interplay between photoinduced darkening and brightening. We suggest possible explanations for the observed differences in SH behavior. This study’s results are essential for identifying absorber materials that can regain intrinsic, insolation-induced photodamage-linked efficiency loss during its rest cycles, thus enabling applications such as autonomously sustainable electronics.

Two-dimensional (2D) halide perovskites, HaPs, can provide chemical stability to three-dimensional (3D) HaP surfaces, protecting them from exposure to ambient species and from reacting with contacting layers. Both actions occur with 2D HaPs, with the general stoichiometry R2PbI4 (R: long or bulky organic amine) covering the 3D ones. Adding such covering films can also boost power conversion efficiencies of photovoltaic cells by passivating surface/interface trap states. For maximum benefit, we need conformal ultrathin and phase-pure (n = 1) 2D layers to enable efficient tunneling of photogenerated charge carriers through the 2D film barrier. Conformal coverage of ultrathin (

We demonstrate that the direction of current rectification via one of nature’s most efficient light-harvesting systems, the photosystem 1 complex (PS1), can be controlled by its orientation on Au substrates. Molecular self-assembly of the PS1 complex using four different linkers with distinct functional head groups that interact by electrostatic and hydrogen bonds with different surface parts of the entire protein PS1 complex was used to tailor the PS1 orientation. We observe an orientation-dependent rectification in the current–voltage characteristics for linker/PS1 molecule junctions. Results of an earlier study using a surface two-site PS1 mutant complex having its orientation set by covalent binding to the Au substrate supports our conclusion. Current–voltage–temperature measurements on the linker/PS1 complex indicate off-resonant tunneling as the main electron transport mechanism. Our ultraviolet photoemission spectroscopy results highlight the importance of the protein orientation for the energy level alignment and provide insight into the charge transport mechanism via the PS1 transport chain.

Efficient and stable electrocatalysts are critically needed for the development of practical overall seawater splitting. The nanocomposite of RuCoBO has been rationally engineered to be an electrocatalyst that fits these criteria. The study has shown that a calcinated RuCoBO-based nanocomposite (Ru2Co1BO-350) exhibits an extremely high catalytic activity for H2 and O2 production in alkaline seawater (overpotentials of 14 mV for H2 evolution and 219 mV for O2 evolution) as well as a record low cell voltage (1.466 V@10 mA cm-2) and long-term stability (230 h @50 mA cm-2 and @100 mA cm-2) for seawater splitting. The results show that surface reconstruction of Ru2Co1BO-350 occurs during hydrogen evolution reaction and oxygen evolution reaction, which leads to the high activity and stability of the catalyst. The reconstructed surface is highly resistant to Cl- corrosion. The investigation suggests that a new strategy exists for the design of high-performance Ru-based electrocatalysts that resist anodic corrosion during seawater splitting.

Halide perovskites (HaPs) are functional semiconductors that can be prepared in a simple, near-room-temperature process. With thin polycrystalline HaP films, excellent solar cells, light-emitting diodes (LEDs), and (also as single crystals) highenergy radiation detectors have been demonstrated. The very low single-crystal defect densities make HaP thin single crystals (TSCs), instead of polycrystalline HaP films an attractive option, to boost device performances and for fundamental research. However, growing TSCs is challenging primarily because of random multiple nucleations, which, in the often-used space-confined geometry, is favored at the substrate boundaries, where loss of organo-amines and solvents occurs. We show that fewer and better quality thin crystals nucleate and grow reproducibly away from the substrate edges in the substrate center, if we localize the heating (needed for inverse-temperature crystallization, the preferred crystal growth method) there. Using a further finding of ours that lowers the crystallization temperature, TSCs of methylammonium lead bromide (MAPbBr3), the HaP we focus on here, grow also directly on flexible substrates. 1H NMR measurements show how the observed lower crystallization temperature results from slow humidity-mediated chemical changes in the HaP precursor solution during its storage.

Humidity is often reported to compromise the stability of lead halide perovskites or of devices based on them. Here we measure the humidity dependence of the elastic modulus and hardness for two series of lead halide perovskite single crystals, varying either by cation or by anion type. The results reveal a dependence on bond length between, hydrogen bonding with, and polarizability/polarization of these ions. The results show an intriguing inverse relation between modulus and hardness, in contrast to their positive correlation for most other materials. This anomaly persists and is strengthened by the effect of humidity. This, and our overall findings are ascribed to the materials’ unique atomic-scale structure and properties, viz nano-polar domains and strong dynamic disorder, yet high-quality average order. Our conclusions are based on comparing results obtained from several different nano-indentation techniques, which separate surface from bulk elastic modulus, and probe different manifestations of the hardness.

The interfaces between inorganic selective contacts and halide perovskites (HaPs) are possibly the greatest challenge for making stable and reproducible solar cells with these materials. NiO x , an attractive hole-transport layer as it fits the electronic structure of HaPs, is highly stable and can be produced at a low cost. Furthermore, NiO x can be fabricated via scalable and controlled physical deposition methods such as RF sputtering to facilitate the quest for scalable, solvent-free, vacuum-deposited HaP-based solar cells (PSCs). However, the interface between NiO x and HaPs is still not well-controlled, which leads at times to a lack of stability and V oc losses. Here, we use RF sputtering to fabricate NiO x and then cover it with a Ni y N layer without breaking vacuum. The Ni y N layer protects NiO x doubly during PSC production. Firstly, the Ni y N layer protects NiO x from Ni3+ species being reduced to Ni2+ by Ar plasma, thus maintaining NiO x conductivity. Secondly, it passivates the interface between NiO x and the HaPs, retaining PSC stability over time. This double effect improves PSC efficiency from an average of 16.5% with a 17.4% record cell to a 19% average with a 19.8% record cell and increases the device stability.

Perovskite solar cells (PSCs) are being studied and developed because of the outstanding properties of halide perovskites as photovoltaic materials and high conversion efficiencies achieved with the best PSCs. However, leaching out of lead (Pb) ions into the environment presents potential public health risks. We show that thiol-functionalized nanoparticles provide an economic way of minimizing Pb leaching in the case of PSC module damage and subsequent water exposure (at most, ∼2.5% of today’s crystal silicon solar panel production cost per square meter). Using commercial materials and methods, we retain ∼90% of Pb without degrading the photovoltaic performance of the cells, compared with nonencapsulated devices, yielding a worst-case scenario of top-soil pollution below natural Pb levels and well below the U.S. Environmental Protection Agency limits.

Attempts to dope halide perovskites (HaPs) extrinsically have been mostly unsuccessful. Still, oxygen (O2) is an efficient p‐dopant for polycrystalline HaP films. To an extent, this doping is reversible, i.e., the films can be de‐doped by decreasing the O2 partial pressure. Here results are reported, aimed at understanding the mechanism of such reversible doping, as it has been argued that doping involves interaction of oxygen with defects inside bulk HaP. These experimental results clearly point out that O2‐surface interactions suffice to dope the bulk of the films. Such behavior fits what is known for other polycrystalline semiconductors, where surface charge transfer‐adducts can form and be removed. Thus, controlling the O2 partial pressure to which the HaP film is exposed, can, after proper encapsulation, achieve the desired bulk doping of the film.

Adding a 2D character to halide perovskite (HaP) active layers in ambient-protected cells can improve their stability drastically, which is not obvious from the hydrophobicity of the large cations that force the HaP into a 2D structure. Results of two-photon confocal microscopy are reported to study inherent photo-stability of 2D Pb iodide HaPs in the interior of single crystals. Compared to 3D HaP crystals, 2D ones have higher photo-stability and, under a few sun-equivalent conditions, self-heal efficiently after photo-damage. Using both photoluminescence (PL) intensities (as function of time after photo-damage) and spectra, self-healing dynamics of 2D HaP (C4H9NH3)(2)PbI4, 2D/3D (C4H9NH3)(2)(CH3NH3)(2)Pb3I10 and 3D MAPbI(3) are compared. Differences in response to photo-damage and self-healing ability from different degrees of photo-damage are found between these HaPs. Based on the findings, a possible chemical mechanism for photo-damage and self-healing of the 2D HaPs is suggested: the layered lattice arrangement limits out-diffusion of degradation products, facilitating damage reversal, leading to better 2D HaP photo-stability and self-healing uniformity than for 2D/3D HaPs. One implication of the layered structures' resilience to photo-damage is transfer of their increased stability to devices made with them, such as photovoltaic solar cells and light-emitting diodes.

Considering the toxicity of lead ions, substituting Pb with nontoxic elements in halide perovskites, HaPs, has become one of the most significant challenges associated with these materials. Here, we report on replacing Pb with Sn and Ge, focusing on an all-inorganic HaP, CsSn x Ge1–x Br3, and using a multihead spray deposition setup for thin-film formation to overcome the low solubility of the precursors and improve film coverage. We find that, in this way, we can form CsSn x Ge1–x Br3 films up to high x values as homogeneous solid solutions; i.e., we obtain a range of compositions with one crystal structure (rather than clusters of two phases). The cubic structure of pure CsSnBr3 is maintained up to 77 atom % Ge, with the lattice spacing decreasing with increasing Ge concentration. The optical band gap is tunable between 1.8 and 2.5 eV, from pure Sn to pure Ge HaP. Most importantly, the perovskite structural stability increases with increasing concentration of Ge, with less oxidation of both Ge and Sn to the +4 state, which can be ascribed to less octahedral tilting and stronger bonding. Electrical and electronic transport measurements show the potential of these materials as Pb-free absorbers for solar cells, particularly, given their band gap range as the top cell of a tandem photovoltaic device.

Among many riddles posed by halide perovskites, the surprising apparent near-absence of harmful defects stands out. This is commonly explained by invoking defect tolerance (DT), but the term is used loosely, sometimes interchangeably with self-healing (SH). Also, the relation between underlying physical and chemical mechanisms and device behavior is often murky. Here, we offer our views as to what DT and SH constitute, the evidence for and against them, and what research challenges remain.

A central issue in protein electronics is how far the structural stability of the protein is preserved under the very high electrical field that it will experience once a bias voltage is applied. This question is studied on the redox protein Azurin in the solid-state Au/protein/Au junction by monitoring protein vibrations during current transport under applied bias, up to ≈1 GV m
⁻¹, by electrical detection of inelastic electron transport effects. Characteristic vibrational modes, such as C-H stretching, amide (N-H) bending, and A-S (of the bonds that connect the protein to an Au electrode), are not found to change noticeably up to 1.0 V. At >1.0 V, the N-H bending and C-H stretching inelastic features have disappeared, while the Au-S features persist till ≈2 V, i.e., the proteins remain Au bound. Three possible causes for the disappearance of the N-H and C-H inelastic features at high bias, namely, i) resonance transport, ii) metallic filament formation, and iii) bond rupture leading to structural changes in the protein are proposed and tested. The results support the last option and indicate that spectrally resolved inelastic features can serve to monitor in operando structural stability of biological macromolecules while they serve as electronic current conduit.

We find significant differences between degradation and healing at the surface or in the bulk for each of the different APbBr3 single crystals (A = CH3NH3+, methylammonium (MA); HC(NH2)2+, formamidinium (FA); and cesium, Cs+). Using 1- and 2-photon microscopy and photobleaching we conclude that kinetics dominate the surface and thermodynamics the bulk stability. Fluorescence-lifetime imaging microscopy, as well as results from several other methods, relate the (damaged) state of the halide perovskite (HaP) after photobleaching to its modified optical and electronic properties. The A cation type strongly influences both the kinetics and the thermodynamics of recovery and degradation: FA heals best the bulk material with faster self-healing; Cs+ protects the surface best, being the least volatile of the A cations and possibly through O-passivation; MA passivates defects via methylamine from photo-dissociation, which binds to Pb2+. DFT simulations provide insight into the passivating role of MA, and also indicate the importance of the Br3− defect as well as predicts its stability. The occurrence and rate of self-healing are suggested to explain the low effective defect density in the HaPs and through this, their excellent performance. These results rationalize the use of mixed A-cation materials for optimizing both solar cell stability and overall performance of HaP-based devices, and provide a basis for designing new HaP variants.

Ion diffusion affects the optoelectronic properties of halide-perovskites (HaPs). Until now, the fastest diffusion has been attributed to the movement of the halides, largely neglecting the contribution of protons, on the basis of computed density estimates. Here, the process of proton diffusion inside HaPs, following deuterium–hydrogen exchange and migration in MAPbI₃, MAPbBr₃, and FAPbBr₃ single crystals, is proven through D/H NMR quantification, Raman spectroscopy, and elastic recoil detection analysis, challenging the original assumption of halide-dominated diffusion. The results are confirmed by impedance spectroscopy, where MAPbBr₃- and CsPbBr₃-based solar cells respond at very different frequencies. Water plays a key role in allowing the migration of protons as deuteration is not detected in its absence. The water contribution is modeled to explain and forecast its effect as a function of its concentration in the perovskite structure. These findings are of great importance as they evidence how unexpected, water-dependent proton diffusion can be at the basis of the ≈7 orders of magnitude spread of diffusion (attributed to I⁻ and Br⁻) coefficient values, reported in the literature. The reported enhancement of the optoelectronic properties of HaP when exposed to small amounts of water may be related to the finding.

We describe the growth of single crystals of the halide perovskites APbBr3, with varying ratios of Cs to methylammonium (MA) on the A site, by antisolvent vapor-assisted crystallization (AVC) and characterize the structural and compositional homogeneity of the grown crystals. We find improved thermostability for the Cs-rich mixed crystals and suggest that this is caused by a combination of locking-in of the relatively large MA in the smaller lattice of the Cs-rich material as well as by stronger hydrogen bonding between the nitrogen of MA and Br due to reduced lattice size and/or octahedral tilting with increased Cs. We also show that it is possible to grow either compositionally homogeneous crystals or core-shell, compositionally graded crystals by changing the ratio of antisolvent to solvent in the AVC method.

We show a simple modification of spin-coating to obtainpinhole-free, well-coveringanduniformfilms of an all-inorganic Pb-halide perovskite (Pb-HaP),on a flat substrate(i.e., not mesoporous)at standard temperature and pressure. In Pb-HaP-based devices the active film is generally fabricated by spin-coating. This is a challenging task for purely inorganic Pb-HaPs, as, except if nano-crystallites can be used, mostly rough and inhomogeneous thin films with poor optoelectronic quality are obtained. We describe agas flow-assistedmethod to fabricate thinconformalfilms of phase-pure CsPbBr3. First cells made with the resulting CsPbBr3-films, yield solar conversion efficiencies up to 2.5%. Electron beam-induced current measurements of device cross-sections show uniform charge generation profiles, implying >= similar to 0.2 mu m diffusion lengths. Non-encapsulated devices generate stable photocurrent for > 1 hr under continuous illumination at maximum power in ambient.

We study halide exchange in the prototypical halide perovskite, methylammonium lead trihalide, MAPbX(3) (X = halide), to test and possibly experimentally use halide diffusion in these materials. We use macroscopic single crystals to study the fundamental exchange process(es) so as to minimize possible grain boundary and surface diffusion effects. Initially, halide exchange creates normal concentration gradients of the outgoing and incoming halides, on a scale of a few microns to a few hundred microns. The depth (from the surface) of the substituted volume depends on the halide pair and on which one is exchanged and which is exchanging. The concentration gradient of the incoming halides decreases from the crystal surface toward its inner core and vice versa for the out-going halides; the profiles roughly fit diffusion in a semi-infinite specimen. This concentration gradient changes slowly with time, with the crystal becoming more homogeneous with storage time. Using the Boltzmann-Matano method and diffusion profiles from electron-dispersive spectroscopy, we evaluate the halide diffusion coefficients; these are not constant and depend on the halide couple. Although these gradients cause a lattice parameter change and may cause a symmetry change, X-ray diffraction shows that if the exchanging halides are of similar size (e.g., Br- and Cl-, Br- and I-, but not Cl- and I-), the resulting material remains single crystalline, prima facie evidence for bulk halide diffusion. These findings are valid, irrespective of which is the exchanged halide. These results suggest that for the similar-sized halide pairs the solid-state chemical exchange is topotactic such that the resulting crystal orientation is determined by that of the initial crystal. I-Cl exchange leads to loss of single crystallinity, suggesting lack of miscibility, a finding that might bear on the difficulty in finding Cl in MAPbI(3) samples grown from Cl-containing solutions.

We report on the chemical and electronic structure of cesium tin bromide (CsSiiBr(3)) and how it is impacted by the addition of 20 mol % tin fluoride (SnF2) to the precursor solution, using both surface-sensitive lab-based soft X-ray photoelectron spectroscopy (XPS) and near-surface bulk-sensitive synchrotron-based hard XPS (HAXPES). To determine the reproducibility and reliability of conclusions, several (nominally identically prepared) sample sets were investigated. The effects of deposition reproducibility, handling, and transport are found to cause significant changes in the measured properties of the films. Variations in the HAXPES-derived compositions between individual sample sets were observed, but in general, they confirm that the addition of 20 mol % SnF2 improves coverage of the titanium dioxide substrate by CsSnBr3 and decreases the oxidation of Sn-II to Sn-IV while also suppressing formation of secondary Br and Cs species. Furthermore, the (surface) composition is found to be Cs-deficient and Sn-rich compared to the nominal stoichiometry. The valence band (VB) shows a SnF2-induced redistribution of Sn 5s-derived density of states, reflecting the changing Sn-II/Sn-IV ratio. Notwithstanding some variability in the data, we conclude that SnF2 addition decreases the energy difference between the VB maximum of CsSnBr3 and the Fermi level, which we explain by defect chemistry considerations.

Extended x-ray absorption fine structure spectroscopy of the light-harvesting formamidinium lead bromide (FAPbBr(3)) perovskite, a system with attractive optoelectronic performance, shows anomalously large variance in Pb-Br bond length, some 50% larger than in its inorganic CsPbBr3 counterpart. Using first-principles molecular dynamics simulations, we find a significant contribution to this variance coming from the FA cation, and show that the FA does not just tumble in its cuboctahedral Br-12 cage, but instead stochastically sticks to, and detaches from one of the 12 nearest Br atoms after another, leading to the large variance in Pb-Br bond length. Our results demonstrate dynamic coupling between the FA-Br moiety and perovskite cage vibrations, and that tunability in dynamics can be achieved by changing the cation type and perovskite lattice parameter. Thus, our results provide information that needs to be considered in any of the intensely debated models of electron-phonon coupling in lead halide perovskites.

Solid-state electronic transport (ETp) via the electron-transfer copper protein azurin (Az) was measured in Au/Az/Au junction configurations down to 4 K, the lowest temperature for solid-state protein-based junctions. Not only does lowering the temperature help when observing fine features of electronic transport, but it also limits possible electron transport mechanisms. Practically, wire-bonded devices-on-chip, carrying Az-based microscopic junctions, were measured in liquid He, minimizing temperature gradients across the samples. Much smaller junctions, in conducting-probe atomic force microscopy measurements, served, between room temperature and the protein's denaturation temperature (similar to 323 K), to check that conductance behavior is independent of device configuration or contact nature and thus is a property of the protein itself. Temperature-independent currents were observed from similar to 320 to 4 K. The experimental results were fitted to a single-level Landauer model to extract effective energy barrier and electrode-molecule coupling strength values and to compare data sets. Our results strongly support that quantum tunneling, rather than hopping, dominates ETp via Az.

Electrocatalytical conversion of CO2 into various chemicals like hydrocarbons and CO is regarded as a promising approach to mitigate carbon emission and, meanwhile, to provide sustainable energy and value-added chemicals. Two different reactors are used in this work. One is based upon the two-electrode configuration powered by a DC power supply or Si solar cell, which is suitable for practical applications. Another is three-electrode one powered by a potentiostat, which is feasible to study the electrode performance. Polycrystalline Cu electrode is used as the cathode, and hematite is the anode. Performance of CO2 reduction using the two- and three-electrode configurations is studied by measuring electrode potential, cell voltage, current density, Faradaic efficiency, and reduction selectivity of CO2. Cu cathode used here exhibits a low overpotential for CO2 reduction, specifically for the cell with two-electrode configuration. No obvious difference can be observed between the two types of configurations at a low bias like -0.3 and -0.4 V; while the reactor with two-electrode configuration exhibits better performance at a high bias like -0.8 V than the one with three-electrode configuration. Thus, the reactors with two-electrode configuration are desirable for practical applications, specifically considering solar cells can be used as the power source to provide green and sustainable energy.

The Shockley-Queisser model is a landmark in photovoltaic device analysis by defining an ideal situation as reference for actual solar cells. However, the model and its implications are easily misunderstood. Thus, we present a guide to help understand and to avoid misinterpreting it.

A sample-type protein monolayer, that can be a stepping stone to practical devices, can behave as an electrically driven switch. This feat is achieved using a redox protein, cytochrome C (CytC), with its heme shielded from direct contact with the solid-state electrodes. Ab initio DFT calculations, carried out on the CytC-Au structure, show that the coupling of the heme, the origin of the protein frontier orbitals, to the electrodes is sufficiently weak to prevent Fermi level pinning. Thus, external bias can bring these orbitals in and out of resonance with the electrode. Using a cytochrome C mutant for direct S-Au bonding, approximately 80 % of the Au-CytC-Au junctions show at greater than 0.5 V bias a clear conductance peak, consistent with resonant tunneling. The on-off change persists up to room temperature, demonstrating reversible, bias-controlled switching of a protein ensemble, which, with its built-in redundancy, provides a realistic path to protein-based bioelectronics.

Lead bromide-based halide perovskites are of interest for wide-band-gap (>1.75 eV) absorbers for low-cost solar spectrum splitting to boost solar-to-electrical energy conversion efficiency/area by adding them to c-Si or Cu(In,Ga)Se-2 PV cells and for photoelectrochemical solar fuel synthesis. Deep in-gap electronic states in PV absorbers serve as recombination centers and are detrimental for the cell's photovoltaic performance, especially for the open-circuit voltage (V-oc). We find four different deep defect states in polycrystalline layers of mixed-cation lead tribromide from high-sensitivity modulated surface photovoltage (SPV) spectroscopy. Measurements were performed with different contact configurations, on complete solar cells and on samples before and after aging or stressing at 85 degrees C under illumination. Three of the four states, with energies of similar to 0.63, 0.73, and 1.35 eV below the conduction band edge, are assigned to intrinsic defects, whereas defect states in the middle of the band gap could be associated with (uncontrolled) impurities.

Design and modification of interfaces, always a critical issue for semiconductor devices, has become a primary tool to harness the full potential of halide perovskite (HaP)-based optoelectronics, including photovoltaics and light-emitting diodes. In particular, the outstanding improvements in HaP solar cell performance and stability can be primarily ascribed to a careful choice of the interfacial layout in the layer stack. In this review, we describe the unique challenges and opportunities of these approaches (section 1). For this purpose, we first elucidate the basic physical and chemical properties of the exposed HaP thin film and crystal surfaces, including topics such as surface termination, surface reactivity, and electronic structure (section 2). This is followed by discussing experimental results on the energetic alignment processes at the interfaces between the HaP and transport and buffer layers. This section includes understandings reached as well as commonly proposed and applied models, especially the often-questionable validity of vacuum level alignment, the importance of interface dipoles, and band bending as the result of interface formation (section 3). We follow this by elaborating on the impact of the interface formation on device performance, considering effects such as chemical reactions and surface passivation on interface energetics and stability. On the basis of these concepts, we propose a roadmap for the next steps in interfacial design for HaP semiconductors (section 4), emphasizing the importance of achieving control over the interface energetics and chemistry (i.e., reactivity) to allow predictive power for tailored interface optimization.

The incorporation of proteins as functional components in electronic junctions has received much interest recently due to their diverse bio-chemical and physical properties. However, information regarding the energies of the frontier orbitals involved in their electron transport (ETp) has remained elusive. Here we employ a new method to quantitatively determine the energy position of the molecular orbital, nearest to the Fermi level (E-F) of the electrode, in the electron transfer protein Azurin. The importance of the Cu(ii) redox center of Azurin is demonstrated by measuring gate-controlled conductance switching which is absent if Azurin's copper ions are removed. Comparing different electrode materials, a higher conductance and a lower gate-induced current onset is observed for the material with smaller work function, indicating that ETp via Azurin is LUMO-mediated. We use the difference in work function to calibrate the difference in gate-induced current onset for the two electrode materials, to a specific energy level shift and find that ETp via Azurin is near resonance. Our results provide a basis for mapping and studying the role of energy level positions in (bio)molecular junctions.

Junctions built from metallic nanoparticles (NPs) can circumvent the diffraction limit and combine molecular/nanoelectronics with plasmonics. However, experimental advances in plasmon-assisted electron transport at the nanoscale have been limited. We construct junctions of a robust, molecule-free, suspended film, built solely from AuNPs, capped by SiO2 shells (Au@SiO2), which give insulating tunneling gaps up to 3.6 nm between the NPs. Current measured across monolayers of such AuNPs shows ultra-long-range, plasmon-enabled electron transport (P-transport), beyond the range of normal electron tunneling across insulators. This finding challenges the present understanding of electron transport in such systems and opens possibilities for future combinations of plasmonics and nanoelectronics.

Multi-heme cytochrome c (Cytc) proteins are key for transferring electrons out of cells, to enable intracellular oxidation to proceed in the absence of O-2. In these proteins most of the hemes are arranged in a linear array suggesting a facile path for electronic conduction. To test this, we studied solvent-free electron transport across two multi-heme Cytc-type proteins: MtrF (deca-heme Cytc) and STC (tetra-heme Cytc). Transport is measured across monolayers of these proteins in a solid state configuration between Au electrodes. Both proteins showed 1000x higher conductance than single heme, or heme-free proteins, but similar conductance to monolayers of conjugated organics. Conductance is found to be temperature-independent (320-80 K), suggesting tunneling as the transport mechanism. This mechanism is consistent with I-V curves modelling, results of which could be interpreted by having protein-electrode coupling as rate limiting, rather than transport within the proteins.

Time-resolved, pulsed excitation methods are widely used to deduce optoelectronic properties of semiconductors, including now also Halide Perovskites (HaPs), especially transport properties. However, as yet, no evaluation of their amenability and justification for the use of the results for the above-noted purposes has been reported. To check if we can learn from pulsed measurement results about steady-state phototransport properties, we show here that, although pulsed measurements can be useful to extract information on the recombination kinetics of HaPs, great care should be taken. One issue is that no changes in the material are induced during or as a result of the excitation, and another one concerns in how far pulsed excitation-derived data can be used to find relevant steady-state parameters. To answer the latter question, we revisited pulsed excitation and propose a novel way to compare between pulsed and steady state measurements at different excitation intensities. We performed steady-state photoconductivity and ambipolar diffusion length measurements, as well as pulsed time-resolved microwave conductivity and time-resolved photoluminescence measurements as a function of excitation intensity on the same samples of different MAPbI(3) thin films, and found good quasi-quantitative agreement between the results, explaining them with a generalized single level recombination model that describes the basic physics of phototransport of HaP absorbers. Moreover, we find the first experimental manifestation of the boundaries between several effective recombination regimes that exist in HaPs, by analyzing their phototransport behavior as a function of excitation intensity. Published by AlP Publishing.

Electrochemical CO2 reduction on Cu electrode has attracted the attention of many researchers in the last decades, because of its potential to generate significant amounts of hydrocarbons at high reaction rates over sustained periods of time. As a result, substantial effort has been devoted to determining the unique catalytic performance of Cu and to elucidate the mechanism through which hydrocarbons are formed. Here we report new insights into CO2 reduction on Cu by electrochemical impedance spectroscopy (EIS) in terms of adsorption/desorption of the reduction intermediates. The potential dependence of charge transfer kinetics is discussed on the basis of EIS results. We revisit the mechanism of the formation of hydrocarbons, taking into account the pH adjacent to the electrode surface, adsorption of HCO3- and CO32-, and the role of active hydrogen. In addition to the enol-like intermediate, proposed previously, we proposed that *COOH center dot radicals, originating from the active involvement of HCO3- and/or CO32- upon reduction are key intermediates for the formation of a variety of C2 and C3 products. Thus, our results provide an additional crucial guideline for the design of future catalysts that can efficiently and selectively reduce CO2 into value-added chemicals.

Inorganic and organic lead halide perovskite materials attract great interest in the scientific community because of their potential for low-cost, high efficiency solar cells. In this report we add a new property of these materials, namely their photochemical activity in the visible light range. Both inorganic (CsPbBr3) and organic (CH3NH3PbBr3-MAPbBr(3)) perovskite thin films were demonstrated to promote photo-dissociation of adsorbed ethyl chloride (EC), employing 532 nm pulsed laser irradiation under ultra-high vacuum (UHV) conditions. From the post-irradiation temperature programmed desorption (TPD) analysis, the yield of photoproduct formation was found to be up to two orders of magnitude higher than for UV light-excited EC molecules on metallic and oxide surfaces. Photo-reactivity on top of the CsPbBr3 surface is almost an order of magnitude more efficient than on the CH3NH3PbBr3 surface, apparently due to the lower density of defect and surface states. A direct correlation was found between electron-induced luminescence and photoluminescence intensities and the photoreactivity cross-sections. We conclude that both the intense luminescence and the well-known photovoltaic properties associated with these halide perovskite materials are consistent with the efficiency of photo-reactivity in the visible range, reported here for the first time.

Metalloproteins, proteins containing a transition metal ion cofactor, are electron transfer agents that perform key functions in cells. Inspired by this fact, electron transport across these proteins has been widely studied in solid-state settings, triggering the interest in examining potential use of proteins as building blocks in bioelectronic devices. Here, we report results of low-temperature (10 K) electron transport measurements via monolayer junctions based on the blue copper protein azurin (Az), which strongly suggest quantum tunneling of electrons as the dominant charge transport mechanism. Specifically, we show that, weakening the protein-electrode coupling by introducing a spacer, one can switch the electron transport from off-resonant to resonant tunneling. This is a consequence of reducing the electrode's perturbation of the Cu(II)localized electronic state, a pattern that has not been observed before in protein-based junctions. Moreover, we identify vibronic features of the Cu(II) coordination sphere in transport characteristics that show directly the active role of the metal ion in resonance tunneling. Our results illustrate how quantum mechanical effects may dominate electron transport via protein-based junctions.

Peptide-based molecular electronic devices are promising due to the large diversity and unique electronic properties of biomolecules. These electronic properties can change considerably with peptide structure, allowing diverse design possibilities. In this work, we explore the effect of the side-chain of the peptide on its electronic properties, by using both experimental and computational tools to detect the electronic energy levels of two model peptides. The peptides include 2Ala and 2Trp as well as their 3-mercaptopropionic acid linker which is used to form monolayers on an Au surface. Specifically, we compare experimental ultraviolet photoemission spectroscopy measurements with density functional theory based computational results. By analyzing differences in frontier energy levels and molecular orbitals between peptides in gas-phase and in a monolayer on gold, we find that the electronic properties of the peptide side-chain are maintained during binding of the peptide to the gold substrate. This indicates that the energy barrier for the peptide electron transport can be tuned by the amino acid compositions, which suggests a route for structural design of peptide-based electronic devices.

Molecular monolayers at metal/semiconductor heterointerfaces affect electronic energy level alignment at the interface by modifying the interface's electrical dipole. On a free surface, the molecular dipole is usually manipulated by means of substitution at its external end. However, at an interface such outer substituents are in close proximity to the top contact, making the distinction between molecular and interfacial effects difficult. To examine how the interface dipole would be influenced by a single atom, internal to the molecule, we used a series of three molecules of identical binding and tail groups, differing only in the inner atom: aryl vinyl ether (PhO), aryl vinyl sulfide (PhS), and the corresponding molecule with a CH2 group allyl benzene (PhC). Molecular monolayers based on all three molecules have been adsorbed on a flat, oxide-free Si surface. Extensive surface characterization, supported by density functional theory calculations, revealed high-quality, well-aligned monolayers exhibiting excellent chemical and electrical passivation of the silicon substrate, in all three cases. Current voltage and capacitance voltage analysis of Hg/PhX (X = C, 0, S)/Si interfaces established that the type of internal atom has a significant effect on the Schottky barrier height at the interface, i.e., on the energy level alignment. Surprisingly, despite the formal chemical separation of the internal atom and the metallic electrode, Schottky barrier heights were not correlated to changes in the semiconductor's effective work function, deduced from Kelvin probe and ultraviolet photoemission spectroscopy on the monolayer-adsorbed Si surface. Rather, these changes correlated well with the ionization potential of the surface-adsorbed molecules. This is interpreted in terms of additional polarization at the molecule/metal interface, driven by potential equilibration considerations even in the absence of a formal chemical bond to the top Hg contact.

Halide perovskite film-based devices (e.g., solar cells and LEDs) have shown unique device performance. These films are commonly prepared from toxic solutions of metal salts (e.g., Pb²⁺ in DMF or DMSO). We describe a method to form halide perovskite films by simply reacting metal (Pb or Sn) films with alcoholic solutions of monovalent alkali metal or alkyl ammonium halides, which avoids the use of toxic Pb²⁺ solutions in the manufacturing step. We show how the morphology of the films can be controlled by variation in reaction parameters and also how mixed halide perovskite films can be prepared. A mechanism for the metal-to-perovskite conversion is suggested. We further show how electrochemically assisted conversion can allow control over the oxidation state of the metal and increase the reaction rate greatly.

Halide perovskite (HaP) semiconductors are revolutionizing photovoltaic (PV) solar energy conversion by showing remarkable performance of solar cells made with HaPs, especially tetragonal methylammonium lead triiodide (MAPbI₃). In particular, the low voltage loss of these cells implies a remarkably low recombination rate of photogenerated carriers. It was suggested that low recombination can be due to the spatial separation of electrons and holes, a possibility if MAPbI₃ is a semiconducting ferroelectric, which, however, requires clear experimental evidence. As a first step, we show that, in operando, MAPbI₃ (unlike MAPbBr₃) is pyroelectric, which implies it can be ferroelectric. The next step, proving it is (not) ferroelectric, is challenging, because of the material's relatively high electrical conductance (a consequence of an optical band gap suitable for PV conversion) and low stability under high applied bias voltage. This excludes normal measurements of a ferroelectric hysteresis loop, to prove ferroelectricity's hallmark switchable polarization. By adopting an approach suitable for electrically leaky materials as MAPbI₃, we show here ferroelectric hysteresis from well-characterized single crystals at low temperature (still within the tetragonal phase, which is stable at room temperature). By chemical etching, we also can image the structural fingerprint for ferroelectricity, polar domains, periodically stacked along the polar axis of the crystal, which, as predicted by theory, scale with the overall crystal size. We also succeeded in detecting clear second harmonic generation, direct evidence for the material's noncentrosymmetry. We note that thematerial's ferroelectric nature, can, but need not be important in a PV cell at room temperature.

We review charge transport across molecular monolayers, which is central to molecular electronics (MolEl), using large-area junctions (NmJ). We strive to provide a wide conceptual overview of three main subtopics. First, a broad introduction places NmJ in perspective to related fields of research and to single-molecule junctions (1mJ) in addition to a brief historical account. As charge transport presents an ultrasensitive probe for the electronic perfection of interfaces, in the second part ways to form both the monolayer and the contacts are described to construct reliable, defect-free interfaces. The last part is dedicated to understanding and analyses of current-voltage (I-V) traces across molecular junctions. Notwithstanding the original motivation of MolEl, I-V traces are often not very sensitive to molecular details and then provide a poor probe for chemical information. Instead, we focus on how to analyze the net electrical performance of molecular junctions, from a functional device perspective. Finally, we point to creation of a built-in electric field as a key to achieve functionality, including nonlinear current-voltage characteristics that originate in the molecules or their contacts to the electrodes. This review is complemented by a another review that covers metal-molecule-semiconductor junctions and their unique hybrid effects.

Recent studies showed that positron annihilation methods can provide key insights into the nanostructure and electronic structure of thin film solar cells. In this study, positron annihilation lifetime spectroscopy (PALS) is applied to investigate CdSe quantum dot (QD) light absorbing layers, providing evidence of positron trapping at the surfaces of the QDs. This enables one to monitor their surface composition and electronic structure. Further, 2D-Angular Correlation of Annihilation Radiation (2D-ACAR) is used to investigate the nanostructure of divacancies in photovoltaic-high-quality a-Si:H films. The collected momentum distributions were converted by Fourier transformation to the direct space representation of the electron-positron autocorrelation function. The evolution of the size of the divacancies as a function of hydrogen dilution during deposition of a-Si:H thin films was examined. Finally, we present a first positron Doppler Broadening of Annihilation Radiation (DBAR) study of the emerging class of highly efficient thin film solar cells based on perovskites.

Simple organic salts are used as a cheap alternative for hole-conducting materials in methylammonium lead bromide perovskite solar cells and obtaining power conversion efficiency of 4.4%. The findings suggest that the polar organic salts interact with the perovskite surface, leading to formation of a surface dipole or change of an existing one on the perovskites that changes its effective work function.

Solar cells based on "halide perovskites" (HaPs) have demonstrated unprecedented high power conversion efficiencies in recent years. However, the well-known toxicity of lead (Pb), which is used in the most studied cells, may affect its widespread use. We explored an all-inorganic lead-free perovskite option, cesium tin bromide (CsSnBr₃), for optoelectronic applications. CsSnBr₃-based solar cells exhibited photoconversion efficiencies (PCEs) of 2.1%, with a short-circuit current (J_SC) of ∼9 mA cm^-2, an open circuit potential (V_OC) of 0.41 V, and a fill factor (FF) of 58% under 1 sun (100 mW cm^-2) illumination, which, even though meager compared to the Pb analogue-based cells, are among the best reported until now. As reported earlier, addition of tin fluoride (SnF₂) was found to be beneficial for obtaining good device performance, possibly due to reduction of the background carrier density by neutralizing traps, possibly via filling of cation vacancies. The roles of SnF₂ on the properties of the CsSnBr₃ were investigated using ultraviolet photoemission spectroscopy (UPS) and X-ray photoelectron spectroscopy (XPS) analysis.

Charge migration for electron transfer via the polypeptide matrix of proteins is a key process in biological energy conversion and signaling systems. It is sensitive to the sequence of amino acids composing the protein and, therefore, offers a tool for chemical control of charge transport across biomaterial-based devices. We designed a series of linear oligoalanine peptides with a single tryptophan substitution that acts as a "dopant," introducing an energy level closer to the electrodes' Fermi level than that of the alanine homopeptide. We investigated the solid-state electron transport (ETp) across a selfassembled monolayer of these peptides between gold contacts. The single tryptophan "doping" markedly increased the conductance of the peptide chain, especially when its location in the sequence is close to the electrodes. Combining inelastic tunneling spectroscopy, UV photoelectron spectroscopy, electronic structure calculations by advanced density-functional theory, and dc current-voltage analysis, the role of tryptophan in ETp is rationalized by charge tunneling across a heterogeneous energy barrier, via electronic states of alanine and tryptophan, and by relatively efficient direct coupling of tryptophan to a Au electrode. These results reveal a controlled way of modulating the electrical properties of molecular junctions by tailormade "building block" peptides.

Hysteresis in the current-voltage characteristics of hybrid organic-inorganic perovskite-based solar cells is one of the fundamental aspects of these cells that we do not understand well. One possible cause, suggested for the hysteresis, is polarization of the perovskite layer under applied voltage and illumination bias, due to ion migration within the perovskite. To study this problem systemically, current-voltage characteristics of both regular (light incident through the electron conducting contact) and so-called inverted (light incident through the hole conducting contact) perovskite cells were studied at different temperatures and scan rates. We explain our results by assuming that the effects of scan rate and temperature on hysteresis are strongly correlated to ion migration within the device, with the rate-determining step being ion migration at/across the interfaces of the perovskite layer with the contact materials. By correlating between the scan rate with the measurement temperature, we show that the inverted and regular cells operate in different hysteresis regimes, with different activation energies of 0.28 ± 0.04 eV and 0.59 ± 0.09 eV, respectively. We suggest that the differences observed between the two architectures are due to different rates of ion migration close to the interfaces, and conclude that the diffusion coefficient of migrating ions in the inverted cells is 3 orders of magnitude higher than in the regular cells, leading to different accumulation rates of ions near the interfaces. Analysis of V_OC as a function of temperature shows that the main recombination mechanism is trap-assisted (Shockley-Read Hall, SRH) in the space charge region, similar to what is the case for other thin film inorganic solar cells.

We report valence and conduction band densities of states measured via ultraviolet and inverse photoemission spectroscopies on three metal halide perovskites, specifically methylammonium lead iodide and bromide and cesium lead bromide (MAPbI₃, MAPbBr₃, CsPbBr₃), grown at two different institutions on different substrates. These are compared with theoretical densities of states (DOS) calculated via density functional theory. The qualitative agreement achieved between experiment and theory leads to the identification of valence and conduction band spectral features, and allows a precise determination of the position of the band edges, ionization energy and electron affinity of the materials. The comparison reveals an unusually low DOS at the valence band maximum (VBM) of these compounds, which confirms and generalizes previous predictions of strong band dispersion and low DOS at the MAPbI₃ VBM. This low DOS calls for special attention when using electron spectroscopy to determine the frontier electronic states of lead halide perovskites.

ConspectusHybrid alkylammonium lead halide perovskite solar cells have, in a very few years of research, exceeded a light-to-electricity conversion efficiency of 20%, not far behind crystalline silicon cells. These perovskites do not contain any rare element, the amount of toxic lead used is very small, and the cells can be made with a low energy input. They therefore already conform to two of the three requirements for viable, commercial solar cells-efficient and cheap. The potential deal-breaker is their long-term stability. While reasonable short-term (hours) and even medium term (months) stability has been demonstrated, there is concern whether they will be stable for the two decades or more expected from commercial cells in view of the intrinsically unstable nature of these materials. In particular, they have a tendency to be sensitive to various types of irradiation, including sunlight, under certain conditions.This Account focuses on the effect of irradiation on the hybrid (and to a small degree, all-inorganic) lead halide perovskites and their solar cells. It is split up into two main sections. First, we look at the effect of electron beams on the materials. This is important, since such beams are used for characterization of both the perovskites themselves and cells made from them (electron microscopy for morphological and compositional characterization; electron beam-induced current to study cell operation mechanism; cathodoluminescence for charge carrier recombination studies). Since the perovskites are sensitive to electron beam irradiation, it is important to minimize beam damage to draw valid conclusions from such measurements. The second section treats the effect of visible and solar UV irradiation on the perovskites and their cells. As we show, there are many such effects. However, those affecting the perovskite directly need not necessarily always be detrimental to the cells, while those affecting the solar cells, which are composed of several other phases as well as the perovskite light absorber, are not always due to the perovskite itself. While we cannot yet say whether perovskite solar cells will or will not be stable over the long-term, the information in this Account should be a useful source to help achieve this goal.

Solution-processed hybrid organic-inorganic perovskites (HOIPs) exhibit long electronic carrier diffusion lengths, high optical absorption coefficients and impressive photovoltaic device performance. Recent results allow us to compare and contrast HOIP charge-transport characteristics to those of III-V semiconductors - benchmarks of photovoltaic (and light-emitting and laser diode) performance. In this Review, we summarize what is known and unknown about charge transport in HOIPs, with particular emphasis on their advantages as photovoltaic materials. Experimental and theoretical findings are integrated into one narrative, in which we highlight the fundamental questions that need to be addressed regarding the charge-transport properties of these materials and suggest future research directions.

Halide perovskite-based solar cells still have limited reproducibility, stability, and incomplete understanding of how they work. We track electronic processes in [CH₃NH₃]PbI₃(Cl) ("perovskite") films in vacuo, and in N₂, air, and O₂, using impedance spectroscopy (IS), contact potential difference, and surface photovoltage measurements, providing direct evidence for perovskite sensitivity to the ambient environment. Two major characteristics of the perovskite IS response change with ambient environment, viz. -1- appearance of negative capacitance in vacuo or post-vacuo N₂ exposure, indicating for the first time an electrochemical process in the perovskite, and -2- orders of magnitude decrease in the film resistance upon transferring the film from O₂-rich ambient atmosphere to vacuum. The same change in ambient conditions also results in a 0.5 V decrease in the material work function. We suggest that facile adsorption of oxygen onto the film dedopes it from n-type toward intrinsic. These effects influence any material characterization, i.e., results may be ambient-dependent due to changes in the material's electrical properties and electrochemical reactivity, which can also affect material stability.

The remarkable optoelectronic and especially photovoltaic performance of hybrid organic-inorganic perovskite (HOIP) materials drives efforts to connect materials properties to this performance. From nano-indentation experiments on solution-grown single crystals we obtain elastic modulus and nano-hardness values of APbX(3) (A = Cs, CH3NH3; X = I, Br). The Young's moduli are similar to 14, 19.5, and 16 GPa, for CH3NH3PbI3, CH3NH3PbBr3, and CsPbBr3, respectively, lending credence to theoretically calculated values. We discuss the possible relevance of our results to suggested self-healing, ion diffusion, and ease of manufacturing. Using our results, together with literature data on elastic moduli, we classified HOIPs amongst the relevant material groups, based on their elastomechanical properties.

We report a combined ultraviolet photoelectron spectroscopy (UPS) and density functional theory (DFT) study of the electronic structure of aromatic self-assembled monolayers covalently bound to Si, using several different aromatic groups (phenyl, biphenyl, and fluorene) and binding groups (O, NH, and CH₂). We obtain excellent agreement between theory and experiment, which allows for a detailed interpretation of the experimental results. Our analysis reveals a significant effect of the binding group on state hybridization at the organic/inorganic interface. Specifically, it highlights that lone-pair electrons in the binding atom facilitate hybridization between the aromatic system and the Si substrate, resulting in a significant induced density of interface states (IDIS). These interface states are manifested as a broadened HOMO peak in the experimental UPS data and are clearly observed in a theoretical spatially-resolved density of states map. This provides means to control the degree of coupling between substrate and molecule, which may prove useful in the design of transport across organic/inorganic interfaces.

Surprisingly efficient solid-state electron transport has recently been demonstrated through "dry" proteins (with only structural, tightly bound H₂O left), suggesting proteins as promising candidates for molecular (bio)electronics. Using inelastic electron tunneling spectroscopy (IETS), we explored electron-phonon interaction in metal/protein/metal junctions, to help understand solid-state electronic transport across the redox protein azurin. To that end an oriented azurin monolayer on Au is contacted by soft Au electrodes. Characteristic vibrational modes of amide and amino acid side groups as well as of the azurin-electrode contact were observed, revealing the azurin native conformation in the junction and the critical role of side groups in the charge transport. The lack of abrupt changes in the conductance and the line shape of IETS point to far off-resonance tunneling as the dominant transport mechanism across azurin, in line with previously reported (and herein confirmed) azurin junctions. The inelastic current and hence electron-phonon interaction appear to be rather weak and comparable in magnitude with the inelastic fraction of tunneling current via alkyl chains, which may reflect the known structural rigidity of azurin.

The soft character of organic materials leads to strong coupling between molecular, nuclear and electronic dynamics. This coupling opens the way to influence charge transport in organic electronic devices by exciting molecular vibrational motions. However, despite encouraging theoretical predictions, experimental realization of such approach has remained elusive. Here we demonstrate experimentally that photoconductivity in a model organic optoelectronic device can be modulated by the selective excitation of molecular vibrations. Using an ultrafast infrared laser source to create a coherent superposition of vibrational motions in a pentacene/C₆₀ photoresistor, we observe that excitation of certain modes in the 1,500-1,700 cm^-1 region leads to photocurrent enhancement. Excited vibrations affect predominantly trapped carriers. The effect depends on the nature of the vibration and its mode-specific character can be well described by the vibrational modulation of intermolecular electronic couplings. This presents a new tool for studying electron-phonon coupling and charge dynamics in (bio)molecular materials.

Hybrid organic-inorganic lead halide perovskite photovoltaic cells have already surpassed 20% conversion efficiency in the few years that they have been seriously studied. However, many fundamental questions still remain unanswered as to why they are so good. One of these is "Is the organic cation really necessary to obtain high quality cells?" In this study, we show that an all-inorganic version of the lead bromide perovskite material works equally well as the organic one, in particular generating the high open circuit voltages that are an important feature of these cells.

The great promise of hybrid organic-inorganic lead halide perovskite (HOIP)-based solar cells is being challenged by its Pb content and its sensitivity to water. Here, the impact of rain on methylammonium lead iodide perovskite films was investigated by exposing such films to water of varying pH values, simulating exposure of the films to rain. The amount of Pb loss was determined using both gravimetric and inductively coupled plasma mass spectrometry measurements. Using our results, the extent of Pb loss to the environment, in the case of catastrophic module failure, was evaluated. Although very dependent on module siting, even total destruction of a large solar electrical power generating plant, based on HOIPs, while obviously highly undesirable, is estimated to be far from catastrophic for the environment.

A distinct odd-even effect on the electrical properties, induced by monolayers of alkyl-phenyl molecules directly bound to Si(111), is reported. Monomers of H₂C=CH-(CH₂)_n-phenyl, with n = 2-5, were adsorbed onto Si-H and formed high-quality monolayers with a binding density of 50-60% Si(111) surface atoms. Molecular dynamics simulations suggest that the binding proximity is close enough to allow efficient π-π interactions and therefore distinctly different packing and ring orientations for monomers with odd or even numbers of methylenes in their alkyl spacers. The odd-even alternation in molecular tilt was experimentally confirmed by contact angle, ellipsometry, FT-IR, and XPS with a close quantitative match to the simulation results. The orientations of both the ring plane and the long axis of the alkyl spacer are more perpendicular to the substrate plane for molecules with an even number of methylenes than for those with an odd number of methylenes. Interestingly, those with an even number conduct better than the effectively thinner monolayers of the molecules with the odd number of methylenes. We attribute this to a change in the orientation of the electron density on the aromatic rings with respect to the shortest tunneling path, which increases the barrier for electron transport through the odd monolayers. The high sensitivity of molecular charge transport to the orientation of an aromatic moiety might be relevant to better control over the electronic properties of interfaces in organic electronics.

Potential future use of bacteriorhodopsin (bR) as a solid-state electron transport (ETp) material requires the highest possible active protein concentration. To that end we prepared stable monolayers of protein-enriched bR on a conducting HOPG substrate by lipid depletion of the native bR. The ETp properties of this construct were then investigated using conducting probe atomic force microscopy at low bias, both in the ground dark state and in the M-like intermediate configuration, formed upon excitation by green light. Photoconductance modulation was observed upon green and blue light excitation, demonstrating the potential of these monolayers as optoelectronic building blocks. To correlate protein structural changes with the observed behavior, measurements were made as a function of pressure under the AFM tip, as well as humidity. The junction conductance is reversible under pressure changes up to ∼300 MPa, but above this pressure the conductance drops irreversibly. ETp efficiency is enhanced significantly at >60% relative humidity, without changing the relative photoactivity significantly. These observations are ascribed to changes in protein conformation and flexibility and suggest that improved electron transport pathways can be generated through formation of a hydrogen-bonding network.

The field of organo-lead perovskite absorbers for solar cells is developing rapidly, with open-circuit voltage of reported devices already approaching the maximal theoretical voltage. Obtaining such high voltages on spun-cast or evaporated thin films is intriguing and calls for detailed investigation of the source of photovoltage in those devices. We present here a study of the roles of the selective contacts to methylammonium lead iodide chloride (MAPbI _3-xCl_x) using surface photovoltage spectroscopy. By depositing and characterizing each layer at a time, we show that the electron-extracting interface is more than twice as effective as the hole-extracting interface in generating photovoltage, for several combinations of electrode materials. We further observe the existence of an electron-injection related spectral feature at 1.1 eV, which might bear significance for the cell's operation. Our results illustrate the usefulness of SPV spectroscopy in highlighting gaps in cells efficiency and for deepening the understanding of charge injection processes in perovskite-based photovoltaics.

The work function (WF) of ZnO is modified by two types of dipole-bearing phenylphosphonate layers, yielding a maximum WF span of 1.2 eV. H ₃CO-phenyl phosphonate, with a positive dipole (positive pole pointing outwards from the surface), lowers the WF by ∼350 meV. NC-phenyl phosphonate, with a negative dipole, increases the WF by ∼750 meV. The WF shift is found to be independent of the type of ZnO surface. XPS data show strong molecular dipoles between the phenyl and the functionalizing (CN and OMe) tail groups, while an opposite dipole evolves in each molecular layer between the surface and the phenyl rings. The molecular modification is found to be invariant to supra-bandgap illumination, which indicates that the substrate's space charge-induced built-in potential is unlikely to be the reason for the WF difference. ZnO, grown by several different methods, with different degrees of crystalline perfection and various morphologies and crystallite dimensions, could all be modified to the same extent. Furthermore, a mixture of opposite dipoles allows gradual and continuous tuning of the WF, varying linearly with the partial concentration of the CN-terminated phosphonate in the solution. Exposure to the phosphonic acids during the molecular layer deposition process erodes a few atomic layers of the ZnO. The general validity of the treatment and the fine-tuning of the WF of treated interfaces are of interest for solar cells and LED applications.

Developments in organic-inorganic lead halide-based perovskite solar cells have been meteoric over the last 2 years, with small-area efficiencies surpassing 15%. We address the fundamental issue of how these cells work by applying a scanning electron microscopy-based technique to cell cross-sections. By mapping the variation in efficiency of charge separation and collection in the cross-sections, we show the presence of two prime high efficiency locations, one at/near the absorber/hole-blocking-layer, and the second at/near the absorber/electron-blocking-layer interfaces, with the former more pronounced. This 'twin-peaks' profile is characteristic of a p-i-n solar cell, with a layer of low-doped, high electronic quality semiconductor, between a p- and an n-layer. If the electron blocker is replaced by a gold contact, only a heterojunction at the absorber/hole-blocking interface remains.

The alignment between the energy levels of the constituents of an organic solar cell plays a central role in determining the open-circuit voltage. However, tuning the energy levels of electrodes and/or active components via molecular modifiers placed at interfaces is not straightforward. The morphology of organic materials is commonly controlled by the substrate onto which they are deposited, and differences in morphology often lead to differences in energetics. Such a change in morphology may reduce the effect of surface modifications, as the modified surface is part of an interface with the organic material. Here we show, in an experimental model system, that by using binary molecular monolayers, in which dipolar molecules are buried in a protective nonpolar matrix, we can transform changes in the electrode surface dipole into interface dipole changes without significantly affecting the growth of pentacene onto the molecular layer, thus enabling the use of the full range of dipolar-induced open-circuit-voltage tuning.

Low-cost solar cells with high V_OC, relatively small (E _G - qV_OC), and high qV_OC/E_G ratio, where E_G is the absorber band gap, are long sought after, especially for use in tandem cells or other systems with spectral splitting. We report a significant improvement in CH₃NH₃PbBr₃-based cells, using CH₃NH₃PbBr_3-xCl_x, with E_G = 2.3 eV, as the absorber in a mesoporous p-i-n device configuration. By p-doping an organic hole transport material with a deep HOMO level and wide band gap to reduce recombination, the cell's V_OC increased to 1.5 V, a 0.2 V increase from our earlier results with the pristine Br analogue with an identical band gap. At the same time, in the most efficient devices, the current density increased from ∼1 to ∼4 mA/cm².

Integrating proteins in molecular electronic devices requires control over their solid-state electronic transport behavior. Unlike "traditional" electron transfer (ET) measurements of proteins that involve liquid environments and a redox cycle, no redox cofactor is needed for solid-state electron transport (ETp) across the protein. Here we show the fundamental difference between these two approaches by macroscopic area measurements, which allow measuring ETp temperature dependence down to cryogenic temperatures, via cytochrome C (Cyt C), an ET protein with a heme (Fe-porphyrin) prosthetic group as a redox centre. We compare the ETp to electrochemical ET measurements, and do so also for the protein without the Fe (with metal-free porphyrin) and without porphyrin. As removing the porphyrin irreversibly alters the protein's conformation, we repeat these measurements with human serum albumin (HSA), 'doped' (by non-covalent binding) with a single hemin equivalent, i.e., these natural and artificial proteins share a common prosthetic group. ETp via Cyt C and HSA-hemin are very similar in terms of current magnitude and temperature dependence, which suggests similar ETp mechanisms via these two systems, thermally activated hopping (with ∼0.1 eV activation energy) >190 K and tunneling by superexchange

The interface level alignment of alkyl and alkenyl monolayers, covalently bound to oxide-free Si substrates of various doping levels, is studied using X-ray photoelectron spectroscopy. Using shifts in the C 1s and Si 2p photoelectron peaks as a sensitive probe, we find that charge distribution around the covalent Si-C bond dipole changes according to the initial position of the Fermi level within the Si substrate. This shows that the interface dipole is not fixed but rather changes with the doping level. These results set limits to the applicability of simple models to describe level alignment at interfaces and show that the interface bond and dipole may change according to the electrostatic potential at the interface.

Monolayers of the redox protein Cytochrome C (CytC) can be electrostatically formed on an H-terminated Si substrate, if the protein- and Si-surface are prepared so as to carry opposite charges. With such monolayers we study electron transport (ETp) via CytC, using a solid-state approach with macroscopic electrodes. We have revealed that currents via holo-CytC are almost 3 orders of magnitude higher than via the heme-depleted protein (→ apo-CytC). This large difference in currents is attributed to loss of the proteins' secondary structure upon heme removal. While removal of only the Fe ion (→ porphyrin-CytC) does not significantly change the currents via this protein at room temperature, the 30-335 K temperature dependence suggests opening of a new ETp pathway, which dominates at high temperatures (>285 K). These results suggest that the cofactor plays a major role in determining the ETp pathway(s) within CytC.

Mesoscopic solar cells, based on solution-processed organic-inorganic perovskite absorbers, are a promising avenue for converting solar to electrical energy. We used solution-processed organic-inorganic lead halide perovskite absorbers, in conjunction with organic hole conductors, to form high voltage solar cells. There is a dire need for low-cost cells of this type, to drive electrochemical reactions or as the high photon energy cell in a system with spectral splitting. These perovskite materials, although spin-coated from solution, form highly crystalline materials. Their simple synthesis, along with high chemical versatility, allows tuning their electronic and optical properties. By judicious selection of the perovskite lead halide-based absorber, matching organic hole conductor, and contacts, a cell with a ∼ 1.3 V open circuit voltage was made. While further study is needed, this achievement provides a general guideline for additional improvement of cell performance.

An eight-orders of magnitude enhancement in current across Hg/X-styrene-Si junctions is caused by merely altering a substituent, X. Interface states are passivated and, depending on X, the Si Schottky junction encompasses the full range from Ohmic to strongly rectifying. This powerful electrostatic molecular effect has immediate implications for interface band alignment and sensing.

Measuring solid-state electron transport (ETp) across proteins allows studying electron transfer (ET) mechanism(s), while minimizing solvation effects on the process. ETp is, however, sensitive to any static (conformational) or dynamic (vibrational) changes in the protein. Our macroscopic measurements allow extending ETp studies to low temperatures, with the concomitant resolution of lower current densities, because of the larger electrode contact areas. Thus, earlier we reported temperature-independent ETp via the copper protein azurin (Az), from 80 K until denaturation, whereas for apo-Az ETp was temperature dependent above 180 K. Deuteration (H/D substitution) may provide mechanistic information on the question of whether the ETp involves H-bonds in the solid state. Here we report results of kinetic deuterium isotope effect (KIE) measurements on ETp through holo-Az as a function of temperature (30-340 K). Strikingly, deuteration changed ETp from temperature independent to temperature dependent above 180 K. This H/D effect is expressed in KIE values between 1.8 (340 K) and 9.1 (≤180 K). These values are remarkable in light of the previously reported inverse KIE on ET in Az in solution. We ascribe the difference between our KIE results and those observed in solution to the dominance of solvent effects in the latter (larger thermal expansion in H ₂O than in D₂O), whereas in our case the KIE is primarily due to intramolecular changes, mainly in the low-frequency structural modes of the protein caused by H/D exchange. The observed high KIE values are consistent with a transport mechanism that involves through-H-bonds of the β-sheet structure of Az, likely also those in the Cu coordination sphere.

We report a method for preparing electrode-molecule-electrode junctions that incorporate nonsymmetrical azobenzene dithiols. Our approach is based on sequential deprotection of thiol moieties originally carrying two different protecting groups. The azobenzene derivatives retained their switching properties within monolayers and permitted the photocontrol of electrical conductance.

Keywords: Physics, Applied; Physics, Condensed Matter

Solid-state electron transport (ETp) via a monolayer of immobilized azurin (Az) was examined by conducting probe atomic force microscopy (CP-AFM), as a function of both temperature (248-373K) and applied tip force (6-15 nN). At low forces, ETp via holo-Az (with Cu²⁺) is temperature-independent, but thermally activated via the Cu-depleted form of Az, apo-Az. While this observation agrees with those of macroscopic-scale measurements, we find that for holo-Az the mechanism of ETp at high temperatures changes upon an increase in the force applied by the tip to the proteins; namely, above 310 K and forces >6 nN ETp becomes thermally activated. This is in contrast to apo-Az, where increasing applied force causes only small monotonic increases in currents due to decreased electrode separation. The distinct ETp temperature dependence of holo- and apo-Az is assigned to a difference in structural response to pressure between the two protein forms. An important implication of these CP-AFM results (of measurements over a significant temperature range) is that for reliable ETp measurements on flexible macromolecules, such as proteins, the pressure applied during the measurements should be controlled or at least monitored.

Good passivation of Si, both electrically and chemically, is achieved by monolayers of 1,9-decadiene, directly bound to an oxide-free Si surface. The terminal C - C bond of the decadiene serves for further in situ reaction, without harming the surface passivation, to -OH- or -Br-terminated monolayers that have different dipole moments. Such a two-step procedure meets the conflicting requirements of binding mutually repelling dipolar groups to a surface, while chemically blocking all surface reactive sites. We demonstrate a change of 0.15 eV in the Si surface potential, which translates into a 0.4 eV variation in the Schottky barrier height of a Hg junction to those molecularly modified n-Si surfaces. Charge transport across such junctions is controlled both by tunneling across the molecular monolayer and by the Si space charge. For reliable insight into transport details, we resorted to detailed numerical simulations, which reveal that the Si space charge and the molecular tunneling barriers are coupled. As a result, attenuation due to the molecular tunneling is much weaker than in metal/molecule/metal molecular junctions. Simulation shows also that some interface states are present but that they have a negligible effect on Fermi level pinning. These states are efficiently decoupled from the metal (Hg) and interact mostly with the Si.

Despite the rapid increase in solar cell manufacturing capacity (∼50 GW _p in 2011), maintaining this continued expansion will require resolving some major fabrication issues. Crystalline Si, the most common type of cell, requires a large energy input in the manufacturing process, which results in an energy payback time of years. CdTe/CdS thin film cells, which have captured around 10% of the global market, may not be sustainable for very large-scale use because of limited Te availability. Thus, research in this field is emphasizing cells that are energy efficient and inexpensive and use readily available materials. The extremely thin absorber (ETA) cell, the subject of this Account, is one of these new generation cells. Since the active light absorber in an ETA cell is no more than tens of nanometers thick, the direct recombination of photogenerated electrons and holes in the absorber should not compete as much with charge removal in the form of photocurrent as in thicker absorber materials. As a result, researchers expect that poorer quality semiconductors can be used in an ETA cell, which would expand the choice of semiconductors over those currently in use.We first describe the ETA cell, comparing and contrasting it to the dye-sensitized cell (DSC) from which it developed and describing its potential advantages and disadvantages. We then explain the mechanism(s) of operation of the ETA cell, which remain controversial: different ETA cells most likely operate by different mechanisms, particularly in their photovoltage generation. We then present a general description of how we prepare ETA cells in our laboratory, emphasizing solution methods to form the various layers and solution treatments of these layers to minimize manufacturing costs. This is followed by a more specific discussion of the various layers and treatments used to make and complete a cell with emphasis on solution treatments that are important in optimizing cell performance and explaining the possible modes of action of each of these treatments. Finally, we show how ETA cells have improved over the years, their present efficiencies, our expectations for the future, and the challenges that we foresee to fulfill these expectations.

Compositional uniformity of Cu(In,Ga)Se ₂ (CIGS) solar cells was studied, using thin cross sections of complete cells prepared by focused ion beam (FIB) and examined in the transmission electron microscope (TEM). This methodology revealed the compositional variations at the nm-scale. The Ga and In compositions vary not only between neighboring grains, but also inside individual single crystal grains along their growth direction, which explains the electrical non-uniformity seen in electron beam-induced current (EBIC) measurements. The improved compositional uniformity with increase in sample preparation temperature correlates with higher solar cell efficiency.

Solar cells based on crystalline semiconductors such as Si and GaAs provide nowadays the highest performance, but photovoltaic (PV) cells based on less pure materials, such as poly- or nano-crystalline or amorphous inorganic or organic materials, or a combination of these, should relax production requirements and lower the cost towards reliable, sustainable and economic electrical power from sunlight. So as to be able to compare the operation of different classes of solar cells we first summarize general photovoltaic principles and then consider implications of using less than ideal materials. In general, lower material purity means more disorder, which introduces a broad distribution of energy states of the electronic carriers that affects all the aspects of PV performance, from light absorption to the generation of voltage and current. Specifically, disorder penalizes energy output by enhanced recombination, with respect to the radiative limit, and also imposes a lowering of quasi-Fermi levels into the gap, which decreases their separation, i.e., reduces the photovoltage. In solar cells based on organic absorbers, such as dye-sensitized or bulk heterojunction solar cells, vibronic effects cause relaxation of carriers in the absorber, which implies an energy price in terms of obtainable output.

Freezing out of molecular motion and increased molecular tilt enhance the efficiency of electron transport through alkyl chain monolayers that are directly chemically bound to oxide-free Si. As a result, the current across such monolayers increases as the temperature decreases from room temperature to ∼80 K, i.e., opposite to thermally activated transport such as hopping or semiconductor transport. The 30-fold change for transport through an 18-carbon long alkyl monolayer is several times the resistance change for actual metals over this range. FTIR vibrational spectroscopic measurements indicate that cooling increases the packing density and reduces the motional freedom of the alkyl chains by first stretching the chains and then gradually tilting the adsorbed molecules away from the surface normal. Ultraviolet photoelectron spectroscopy shows drastic sharpening of the valence band structure as the temperature decreases, which we ascribe to decreased electron-phonon coupling. Although conformational changes are typical in soft molecular systems, in molecular electronics they are rarely observed experimentally or considered theoretically. Our findings, though, indicate that the molecular conformational changes are a prominent feature, which imply behavior that differs qualitatively from that described by models of electronic transport through inorganic mesoscopic solids.

A combined electronic transport-structure characterization of self-assembled monolayers (MLs) of alkyl-phosphonate (AP) chains on Al-AlOx substrates indicates a strong molecular structural effect on charge transport. On the basis of X-ray reflectivity, XPS, and FTIR data, we conclude that "long" APs (C14 and C16) form much denser MLs than do "short" APs (C8, C10, C12). While current through all junctions showed a tunneling-like exponential length-attenuation, junctions with sparsely packed "short" AP MLs attenuate the current relatively more efficiently than those with densely packed, "long" ones. Furthermore, "long" AP ML junctions showed strong bias variation of the length decay coefficient, β, while for "short" AP ML junctions β is nearly independent of bias. Therefore, even for these simple molecular systems made up of what are considered to be inert molecules, the tunneling distance cannot be varied independently of other electrical properties, as is commonly assumed.

The electronic structure of the prototypical self-assembled monolayer (SAM) system, i.e. alkanethiol molecules on Au, is investigated via ultraviolet and inverse photoemission spectroscopy measurements. The determination of the density of filled and empty states of the system reveals that the metal Fermi level is significantly closer to the lowest unoccupied molecular orbital (LUMO) of the molecules than to their highest occupied molecular orbital (HOMO). The results suggest that charge carrier tunneling is controlled by the LUMO, rather than by the HOMO, in contrast to what is commonly assumed.

What are the solar cell efficiencies that we can strive towards? We show here that several simple criteria, based on cell and module performance data, serve to evaluate and compare all types of today's solar cells. Analyzing these data allows to gauge in how far significant progress can be expected for the various cell types and, most importantly from both the science and technology points of view, if basic bounds, beyond those known today, may exist, that can limit such progress. This is important, because half a century after Shockley and Queisser (SQ) presented limits, based on detailed balance calculations for single absorber solar cells, those are still held to be the only ones, we need to consider; most efforts to go beyond SQ are directed towards attempts to circumvent them, primarily via smart optics, or optoelectronics. After formulating the criteria and analyzing known loss mechanisms, use of such criteria suggests-additional limits for newer types of cells, Organic and Dye-Sensitized ones, and their siblings,-prospects for progress and-further characterization needs, all of which should help focusing research and predictions for the future. Several simple criteria, based on cell and module performance data, are used to evaluate and compare all types of today's solar cells. Analyzing these data allows to gauge in how far significant progress can be expected for the various cell types and if basic bounds, beyond those known today, may exist, that can limit such progress.

Protein structures can facilitate long-range electron transfer in solution. But a fundamental question remains: can these structures also serve as solid-state electronic conductors? Answering this question requires methods for studying conductivity of the "dry" protein (which only contains tightly bound structured water molecules) sandwiched between two electronic conductors in a solid-state type configuration. If successful, such systems could serve as the basis for future, bioinspired electronic device technology. In this Account, we survey, analyze, and compare macroscopic and nanoscopic (scanning probe) solid-state conductivities of proteins, noting the inherent constraints of each of these, and provide the first status report on this research area. This analysis shows convincing evidence that "dry" proteins pass orders of magnitude higher currents than saturated molecules with comparable thickness and that proteins with known electrical activity show electronic conductivity, nearly comparable to that of conjugated molecules ("wires"). These findings suggest that the structural features of proteins must have elements that facilitate electronic conductivity, even if they do not have a known electron transfer function. As a result, proteins could serve not only as sensing, polar,or photoactive elements in devices (such as field-effect transistor configurations) but also as electronic conductors. Current knowledge of peptide synthesis and protein modification paves the way toward a greater understanding of how changes in a protein's structure affect its conductivity. Such an approach could minimize the need for biochemical cascades in systems such as enzyme-based circuits, which transduce the protein's response to electronic current. In addition, as precision and sensitivity of solid-state measurements increase, and as knowledge of the structure and function of "dry" proteins grows, electronic conductivity may become an additional approach to study electron transfer in proteins and solvent effects without the introduction of donor or acceptor moieties. We are particularly interested in whether evolution might have prompted the electronic carrier transport capabilities of proteins for which no electrically active function is known in their native biological environment and anticipate that further research may help address this fascinating question.

Using a semiconductor as the substrate to a molecular organic layer, penetration of metal contacts can be clearly identified by the study of electronic charge transport through the layer. A series of monolayers of saturated hydrocarbon molecules with varying lengths is assembled on Si or GaAs and the junctions resulting after further electronic contact is made by liquid Hg, indirect metal evaporation, and a "ready-made" metal pad are measured. In contrast to tunneling characteristics, which are ambiguous regarding contact penetration, the semiconductor surface barrier is very sensitive to any direct contact with a metal. With the organic monolayer intact, a metal-insulator-semiconductor (MIS) structure results. If metal penetrated the monolayer, the junction behaves as a metal-semiconductor (MS) structure. By comparing a molecule-free interface (MS junction) with a molecularly modified one (presumably MIS), possible metal penetration is identified. The major indicators are the semiconductor electronic transport barrier height, extracted from the junction transport characteristics, and the photovoltage. The approach does not require a series of different monolayers and data analysis is quite straightforward, helping to identify non-invasive ways to make electronic contact to soft matter.

Electron transfer (ET) through proteins, a fundamental element of many biochemical reactions, is studied Intensively in aqueous solutions. Over the past decade, attempts were made to Integrate proteins into solid-state Junctions in order to study their electronic conductance properties. Most such studies to date were conducted with one or very few molecules In the Junction, using scanning probe techniques. Here we present the high-yield, reproducible preparation of large-area monolayer Junctions, assembled on a Si platform, of proteins of three different families: azurln (Az), a blue-copper ET protein, bacterlorhodopsln (bR), a membrane proteln-chromophore complex with a proton pumping function, and bovine serum albumin (BSA). We achieve highly reproducible electrical current measurements with these three types of monolayers using appropriate top electrodes. Notably, the current-voltage (i-V) measurements on such Junctions show relatively minor differences between Az and bR, even though the latter lacks any known ET function. Electron Transport (ETp) across both Az and bR is much more efficient than across BSA, but even for the latter the measured currents are higher than those through a monolayer of organic, C18 alkyl chains that is about half as wide, therefore suggesting transport mechanlsm(s) different from the often considered coherent mechanism. Our results show that the employed proteins maintain their conformation under these conditions. The relatively efficient ETp through these proteins opens up possibilities for using such blomolecules as current-carrying elements in solid-state electronic devices.

Bromine-terminated alkyl-chain monolayers, bound to oxide-free Si substrates, were prepared by self-assembly. Infrared spectroscopy and atomic force microscopy imply that monolayer packing density improves after hydrolysis, despite an increase in the presence of oxide. The probable reason is that OH-mediated intermolecular H-bonding along the monolayer emerges after hydrolysis and rearranges the molecular components of the insulating layer. Current-voltage and differential capacitance measurements show that also the interfacial electronic properties of these junctions are changed by hydrolysis of the Br groups. This is expressed in an increased effective Schottky barrier height and a decreased junction ideality factor. We correlate the proposed structural changes of the monolayer with the change in the interfacial electronic properties, with the help of the inhomogeneous Schottky barrier height model. The role of oxide in the charge transport through the monolayer is discussed, as well.

We report on electronic transport measurements through dense monolayers of CH ₃(CH ₂) _nPO ₃H ₂ molecules of varying chain lengths, with a strong and stable bond through the phosphonic acid end group to a GaAs surface and a Hg top contact. The monolayers maintain their high quality during and after the electrical measurements. Analyses of the electronic transport measurements of junctions, and of UV and inverse photoemission spectroscopy data on band alignments of free surfaces, yield insight about the electrical transport mechanism. Transport characteristics for n-GaAs junctions at low forward bias are identical for different chain lengths, a strong indication of high-quality monolayers. Tunneling barrier and carrier effective mass values for n- and p-GaAs samples were deduced from the transport data. In this way we find a tunneling barrier for n-GaAs of 1.3 eV, while UPS data for the lowest unoccupied system orbital (LUSO) point to a 2.4 eV barrier. This discrepancy can be understood by invoking states, closer to the Fermi level than the LUSO state, that contribute to charge transport. Such states lead to a manifold of transitions, each having a different probability, both because of differences in the tunnel barrier and because of differences in density of these interface-induced states; i.e., the single barrier, deduced from J-V measurements, is an effective value only.

Alkyl chain molecules on n-Si were used to test the concept of hybrid metal-organic insulator-semiconductor (MOIS) solar cells. Test structures were made by binding alkyl chain molecules via Si-O-C bonds to oxide-free n-Si surfaces, using self-assembly. With thiol groups at the terminals away from the Si, binding of Au nanoparticles, followed by electroless Au plating yields semitransparent top contacts. First cells give, under 25 mW/ cm² white light illumination, open-circuit voltage V_oc =0.48 V and fill factor FF=0.58. Because with sulfur termination the molecules have a dipole that limits inversion of the Si, we also used methyl-terminated monolayers. Even though then we can work, at this point, only with a Hg top contact, without chemical bond to the molecules, we get, using only radiation (∼AM 1.5) collected around the contact, the expected higher V_oc =0.54 V, and respectable 0.8 FF, justifying further MOIS cell development.

Photon up-conversion (UC) and photon-induced multiple-exciton generation (MEG) are proposed directions that are of increasing interest for improving photovoltaic (PV) conversion efficiencies via "photon (or light) management". Straightforward analysis of these approaches for non-concentrated single-junction cells in the detailed balance limit yields a theoretical PV conversion limit of 49%, instead of 31% without UC and MEG. With what we estimate to be optimistic, maximal realistic efficiencies (25% for UC; 70% for MEG) this limit becomes

Interfacing functional proteins with solid supports for device applications is a promising route to possible applications in bio-electronics, -sensors, and -optics. Various possible applications of bacteriorhodopsin (bR) have been explored and reviewed since the discovery of bR. This tutorial review discusses bR as a medium for biomolecular optoelectronics, emphasizing ways in which it can be interfaced, especially as a thin film, solid-state current-carrying electronic element.

n-Si/C_nH_2n+1/Hg junctions (n = 12, 14, 16 and 18) can be prepared with sufficient quality to assure that the transport characteristics are not anymore dominated by defects in the molecular monolayers. With such organic monolayers we can, using electron, UV and X-ray irradiation, alter the charge transport through the molecular junctions on n- as well as on p-type Si. Remarkably, the quality of the self-assembled molecular monolayers following irradiation remains sufficiently high to provide the same very good protection of Si from oxidation in ambient atmosphere as provided by the pristine films. Combining spectroscopic (UV photoemission spectroscopy (UPS), X-ray photoelectron spectroscopy (XPS), Auger, near edge-X-ray absorption fine structure (NEXAFS)) and electrical transport measurements, we show that irradiation induces defects in the alkyl films, most likely C=C bonds and C-C crosslinks, and that the density of defects can be controlled by irradiation dose. These altered intra- and intermolecular bonds introduce new electronic states in the highest occupied molecular orbital (HOMO)-lowest unoccupied molecular orbital (LUMO) gap of the alkyl chains and, in the process, dope the organic film. We demonstrate an enhancement of 1-2 orders of magnitude in current. This change is clearly distinguishable from the previous observed difference between transport through high quality and defective monolayers. A detailed analysis of the electrical transport at different temperatures shows that the dopants modify the transport mechanism from tunnelling to hopping. This study suggests a way to extend significantly the use of monolayers in molecular electronics.

Introducing organic molecules in electronics, in general, and as active electronic transport components, in particular, is to no small degree limited by the ability to connect them electrically to the outside world. Making useful electrical contacts to them requires achieving this either without altering the molecules, or if they are affected, then in a controlled fashion. This is not a trivial task because most known methods to make such contacts are likely to damage the molecules. In this progress report we review many of the various ways that have been devised to make electrical contacts to molecules with minimal or no damage. These approaches include depositing the electronic conducting contact material directly on the molecules, relying on physical interactions, requiring chemical bond formation between molecule and electrode materials, "ready-made" contacts (i.e., contact structures that are prepared in advance), and contacts that are prepared in situ. Advantages and disadvantages of each approach, as well as the possibilities that they can be used practically, are discussed in terms of molecular reactivity, surface and interfacial science.

Temperature-dependent transport measurements through alkyl chain monolayers that are directly chemically bound to Si, show that the currents decrease as the temperature increases. We relate this temperature dependence primarily to a gradual un-tilting of the adsorbed molecules, which leads to increasing of the film thickness, resulting in a wider tunnel barrier. Following that, we conclude that a significant part of transport through these alkyl chain monolayers occurs "through space". The experimental finding and its interpretation result from the high reproducibility and accuracy of the transport results for the semiconductor/alkyl chain/metal junctions that we study.

(Figure Presented) Can we put organic molecules to use as electronic components? The answer to this question is to no small degree limited by the ability to contact them electrically without damaging the molecules. In this Account, we present some of the methods for contacting molecules that do not or minimally damage them and that allow formation of electronic junctions that can become compatible with electronics from the submicrometer to the macroscale. In "Linnaean" fashion, we have grouped contacting methods according to the following main criteria: (a) is a chemical bond is required between contact and molecule, and (b) is the contact "ready-made", that is, preformed, or prepared in situ? Contacting methods that, so far, seem to require a chemical bond include spin-coating a conductive polymer and transfer printing. In the latter, a metallic pattern on an elastomeric polymer is mechanically transferred to molecules with an exposed terminal group that can react chemically with the metal. These methods allow one to define structures from several tens of nanometers size upwards and to fabricate devices on flexible substrates, which is very difficult by conventional techniques. However, the requirement for bifunctionality severely restricts the type of molecules that can be used and can complicate their self-assembly into monolayers. Methods that rely on prior formation of the contact pad are represented by two approaches: (a) use of a liquid metal as electrode (e.g., Hg, Ga, various alloys), where molecules can be adsorbed on the liquid metal and the molecularly modified drop is brought into contact with the second electrode, the molecules can be adsorbed on the second electrode and then the liquid metal brought into contact with them, or bilayers are used, with a layer on both the metal and the second electrode and (b) use of preformed metal pads from a solid substrate and subsequent pad deposition on the molecules with the help of a liquid. These methods allow formation of contacts easily and rapidly and allow many types of monolayers and metals to be analyzed. However, in their present forms such approaches are not technologically practical. Direct in situ vacuum evaporation of metals has been used successfully only with bifunctional molecules because it is too invasive and damaging, in general. A more general approach is indirect vacuum evaporation, where the metal atoms and clusters, emitted from the source, reach the sample surface in an indirect line of sight, while cooled by multiple collisions with an inert gas. This method has clear technological possibilities, but more research is needed to increase deposition efficiency and find ways to characterize the molecules at the interface and to prevent metal penetration between molecules or through pinholes, also if molecules lack reactive termination groups. This Account stresses the advantages, weak points, and possible routes for the development of contacting methods. This way it shows that there is at present no one ideal soft contacting method, whether it is because of limitations and problems inherent in each of the methods or because of insufficient understanding of the interfacial chemistry and physics. Hopefully, this Account will present the latter issue as a research challenge to its readers.

A reliable and reproducible method for preparing bacteriorhodopsin (bR)-containing metal-biomolecule-monolayer-metal planar junctions via vesicle fusion tactics and soft deposition of Au top electrodes is reported. Optimum monolayer and junction preparations, including contact effects, are discussed. The electron-transport characteristics of bR-containing membranes are studied systematically by incorporating native bR or artificial bR pigments derived from synthetic retinal analogues, into single solid-supported lipid bilayers. Current-voltage (I-V) measurements at ambient conditions show that a single layer of such bR-containing artificial lipid bilayers pass current in solid electrode/bilayer/solid electrode structures. The current is passed only if retinal or its analogue is present in the protein. Furthermore, the preparations show photoconductivity as long as the retinal can isomerize following light absorption. Optical characterization suggests that the junction photocurrents might be associated with a photochemically induced M-like intermediate of bR. I-V measurements along with theoretical estimates reveal that electron transfer through the protein is over four orders of magnitude more efficient than what would be estimated for direct tunneling through 5 nm of water-free peptides. Our results furthermore suggest that the light-driven proton-pumping activity of the sandwiched solid-state bR monolayer contributes negligibly to the steady-state light currents that are observed, and that the orientation of bR does not significantly affect the observed I-V characteristics.

We show that electron beam evaporation of metal onto a monolayer of organic molecules can yield reproducible electrical contacts, if evaporation is indirect and the sample is on a cooled substrate. The metal contact forms without damaging even the molecules' outermost groups. In contrast, direct evaporation seriously damages the molecules. By comparing molecular effects on metal/molecular layer/GaAs junctions, prepared by indirect evaporation and by other soft contacting methods, we confirm experimentally that Au is not an optimal choice as an evaporated contact metal. We ascribe this to the ease by which Au can diffuse between molecules, something that can, apart from direct contact-substrate connections, lead to undesired and uncontrollable interfacial interactions. Such phenomena are largely absent with Pd as evaporated contact.

Local electrical transport measurements with scanning probe microscopy on polycrystalline (PX) p-CuInSe2 and p-Cu(In,Ga)Se-2 films show that the photovoltaic and dark currents for bias voltages smaller than 1 V flow mainly through grain boundaries (GBs), indicating inversion at the GBs. Photocurrent for higher bias flows mainly via the grains. Based on these results and our finding of similar to 100 meV GB band bending we deduce the potential landscape around the GBs. We suggest that high grain material quality, leading to large carrier mobilities, and electron-hole separation at the GBs, by chemical and electrical potential gradients, result in the high performance of these PX solar cells. (c) 2006 Elsevier B.V. All rights reserved.

Monolayers of alkyl chains, attached-through direct Si-C bonds to Si(111), via phosphonates to GaAs(100) surfaces, or deposited as alkyl-silane monolayers on SiO₂, are investigated by ultraviolet and inverse photoemission spectroscopy and X-ray absorption spectroscopy. Exposure to ultraviolet radiation from a He discharge lamp, or to a beam of energetic electrons, leads to significant damage, presumably associated with radiation- or electron-induced H-abstraction leading to carbon-carbon double-bond formation in the alkyl monolayer. The damage results in an overall distortion of the valence spectrum, in the appearance of (occupied) states above the highest occupied molecular orbital of the alkyl molecule, and in a characteristic (unoccupied state) π* resonance at the edge of the carbon absorption peak. These distortions present a serious challenge for the interpretation of the electronic structure of the monolayer system. We show that extrapolation to zero damage at short exposure times eliminates extrinsic features and allows a meaningful extraction of the density of state of the pristine monolayer from spectroscopy measurements.

Coupling of noble-metal nanoparticles (NPs) into electronics might lead to interesting new avenues in nanoelectronics. It was found that currents through metal//organic (or inorganic) insulator/Au NP monolayer//metal planar junctions (2) are orders of magnitude higher than what is obtained without the NP monolayer (1). (Graph Presented).

CdSe is homogeneously deposited into nanoporous TiO₂ films and used in liquid junction photoelectrochemical solar cells. The effect of the deposition parameters on the cell are studied, in particular differences between ion-by-ion and cluster deposition mechanisms. CdSe deposition on a Cd-rich CdS film that was deposited first into the TiO₂ film, or selenization of the Cd-rich CdS layer with selenosulphate solution improves the cell parameters. Photocurrent spectral response measurements indicate photocurrent losses due to poor collection efficiencies, as shown by the strong spectral dependence on illumination intensity. Cell efficiencies up to 2.8% under solar conditions have been obtained.

Studying electron transport (ET) through proteins is hampered by achieving reproducible experimental configurations, particularly electronic contacts to the proteins. The transmembrane protein bacteriorhodopsin (bR), a natural light-activated proton pump in purple membranes of Halobacterium salinarum, is well studied for biomolecular electronics because of its sturdiness over a wide range of conditions. To date, related studies of dry bR systems focused on photovoltage generation and photoconduction with multilayers, rather than on the ET ability of bR, which is understandable because ET across 5-nm-thick, apparently insulating membranes is not obvious. Here we show that electronic current passes through bR-containing artificial lipid bilayers in solid "electrode-bilayer-electrode" structures and that the current through the protein is more than four orders of magnitude higher than would be estimated for direct tunneling through 5-nm, water-free peptides. We find that ET occurs only if retinal or a close analogue is present in the protein. As long as the retinal can isomerize after light absorption, there is a photo-ET effect. The contribution of light-driven proton pumping to the steady-state photocurrents is negligible. Possible implications in view of the suggested early evolutionary origin of halobacteria are noted.

The superior performance of certain polycrystalline (PX) solar cells compared to that of corresponding single-crystal ones has been an enigma until recently. Conventional knowledge predicted that grain boundaries serve as traps and recombination centers for the photogenerated carriers, which should decrease cell performance. To understand if cell performance is limited by grain bulk, grain surface, and/or grain boundaries (GBs), we performed high-resolution mapping of electronic properties of single GBs and grain surfaces in PX p-CdTe/n-CdS solar cells. Combining results from scanning electron and scanning probe microscopies, viz., capacitance, Kelvin probe, and conductive probe atomic force microscopies, and comparing images taken under varying conditions, allowed elimination of topography-related artifacts and verification of the measured properties. Our experimental results led to several interesting conclusions: 1) current is depleted near GBs, while photocurrents are enhanced along the GB cores; 2) GB cores are inverted, which explains GB core conduction. Conclusions (1) and (2) imply that the regions around the GBs function as an extension of the carrier-collection volume, i.e., they participate actively in the photovoltaic conversion process, while conclusion (2) implies minimal recombination at the GB cores; 3) the surface potential is diminished near the GBs; and 4) the photovoltaic and metallurgical junction in the n-CdS/p-CdTe devices coincide. These conclusions, taken together with gettering of defects and impurities from the bulk into the GBs, explain the good photovoltaic performance of these PX cells (at the expense of some voltage loss, as is indeed observed). We show that these CdTe GB features are induced by the CdCl ₂ heat treatment used to optimize these cells in the production process.

Two-dimensional arrangements of molecules can show remarkable cooperative electronic effects. Such effects can serve to achieve direct electronic sensing of chemical and physical processes via electrostatic effects, i.e., without transfer of charge or matter between the locus of sensing and that of detection.

Diodes made by (indirectly) evaporating Au on a monolayer of molecules that are adsorbed chemically onto GaAs, via either disulfide or dicarboxylate groups, show roughly linear but opposite dependence of their effective barrier height on the dipole moment of the molecules. We explain this by Au-molecule (electrical) interactions not only with the exposed end groups of the molecule but also with its binding groups. We arrive at this conclusion by characterizing the interface by in situ UPS-XPS, ex situ XPS, TOF-SIMS, and Kelvin probe measurements, by scanning microscopy of the surfaces, and by current-voltage measurements of the devices. While there is a very limited interaction of Au with the dicarboxylic binding groups, there is a much stronger interaction with the disulfide groups. We suggest that these very different interactions lead to different (growth) morphologies of the evaporated gold layer, resulting in opposite effects of the molecular dipole on the junction barrier height.

The effect of surface treatments on p-CdTe/n-CdS solar cell performance was examined. Adsorption of organic molecules with various magnitudes and directions of the dipole moment on p-CdTe resulted in controlled changes in electron affinity and surface band bending. Similar adsorption on CdTe in state-of-the-art p-CdTe/n-CdS solar cells changes the cell performance, and we explain this by a combination of increased series resistance and changes in light absorption and in cell photovoltage. While at this stage no improvement in performance has been found with these cell structures, which are the result of years of empirical optimization, the molecular effect on the photovoltage shows that it is possible in this way to control the photovoltaic effect at this junction. Separate optimization may well lead to improvement by inserting a dipole layer near the photovoltaic interface. Our results also show that this is even possible when dipole adsorption is performed on the complete polycrystalline thin-film cell.

The electric potential distribution in dye-sensitized solar cells plays a major role in the operation of such cells. Models based on a built-in electric field which sets the upper limit for the open circuit voltage (V _∝) and/or the possibility of a Schottky barrier at the interface between the mesoporous wide band gap semiconductor and the transparent conducting substrate have been presented. We show that I-V characteristics in the dark and upon illumination are very well explained by electron tunneling, rather than transport over a Schottky barrier, at this interface. Our calculations, based on tunnel currents, show that a discontinuity of the conduction band at the TiO₂/FTO interface, rather than a built-in electric field, suffices for efficient electron transfer through this interface, and, thus, for efficient operation of this type of solar cell. Clearly, this will hold only if the photoinduced electrostatic potential barrier between the transparent conducting substrate and the mesoporous wide band gap semiconductor drops over a region that is sufficiently narrow to allow efficient tunneling through it.

We show reproducible, stable negative differential resistance (NDR) at room temperature in molecule-controlled, solvent-free devices, based on reversible changes in molecule-electrode interface properties. The active component is the cyclic disulfide end of a series of molecules adsorbed onto mercury. As this active component is reduced, the Hg-molecule contact is broken, and an insulating barrier at the molecule-electrode interface is formed. Therefore, the alignment of the molecular energy levels, relative to the Fermi levels of the electrodes, is changed. This effect results in a decrease in the current with voltage increase as the reduction process progresses, leading to the so-called NDR behavior. The effect is reproducible and repeatable over more than 50 scans without any reduction in the current. The stability of the system, which is in the "solid state" except for the Hg, is due to the molecular design where long alkyl chains keep the molecules aligned with respect to the Hg electrode, even when they are not bound to it any longer.

We review the status of the understanding of dye-sensitized solar cells (DSSC), emphasizing clear physical models with predictive power, and discuss them in terms of the chemical and electrical potential distributions in the device. Before doing so, we place the DSSC in the overall picture of photovoltaic energy converters, reiterating the fundamental common basis of all photovoltaic systems as well as their most important differences.

We compile, compare, and discuss experimental results on low-bias, room-temperature currents through organic molecules obtained in different electrode-molecule-electrode test-beds. Currents are normalized to single-molecule values for comparison and are quoted at 0.2 and 0.5 V function bias. Emphasis is on currents through saturated alkane chains where many comparable measurements have been reported, but comparison to conjugated molecules is also made. We discuss factors that affect the magnitude of the measured current, such as tunneling attenuation factor, molecular energy gap and conformation, molecule/electrode contacts, and electrode material.

Epitaxial films of CuInSe₂ on Si(1 1 1) were modified by the application of an electric field through a movable tip. The electric field induces stable junction regions which are identified by separation and collection of electron beam-induced charge carriers. The movable tip allows scribing of these junction regions and one-sided connection to a contact pad. The junction formation is mainly due to electric field application and, in contrast to what was found to be the case for bulk samples, is not accompanied by a significant temperature rise. The junctions can be explained by symmetrical p/p⁺/n/p⁺/p regions formed within the CuInSe₂ epilayers. The reported method presents a new way for junction patterning in two dimensions.

We present a concise, although admittedly non-exchaustive, but hopefully didactic review and discussion of some of the central and basic concepts related to the energetics of surfaces and interfaces of solids. This is of particular importance for surfaces and interfaces that involve organic molecules and molecular films. It attempts to pull together different views and terminologies used in the solid state, electrochemistry, and electronic device communities, regarding key concepts of local and absolute vacuum level, surface dipole, work function, electron affinity, and ionization energy. Finally, it describes how standard techniques like photoemission spectroscopy can be used to measure such quantities.

The effect of the presence or absence of chemical bonds between alkyl chain monolayers and the contacts in metal/molecule/semiconductor junctions on the current-voltage characteristics was studied. Three types of junctions were used: Hg/alkylthiols/SiO₂/p-Si, Hg/alkylthiols/p-Si-H, and Hg/alkylsilanes/SiO₂/p-Si. While in the first two junctions current is attenuated exponentially as a function of the length of the alkyl chain, a characteristic behavior of tunneling, the current through the third junction does not reveal such behavior, suggesting that current transport is different in this case. We postulate that this is because in the first two junctions the monolayers are covalently bound to the Hg, while in the third junction, the alkysilanes are anchored to the Si surface only at a few points and are best viewed as not bonded to either side of the junction. The mechanism of current flow through the first two junctions is thought to be through-bond tunneling, and our results indicate that a chemical bond to at least one of the electrode surfaces is essential for this mechanism to operate. Electrostriction causes changes in the current-voltage characteristics of the first two junctions. Evidence is presented suggesting that electrostriction tilts short chains (≤C₁₂), resulting in an additional route to charge transport by tunneling through space. In contrast, long chains (≥C₁₄) do not tilt under pressure; instead, gauche defects are formed in their initial all-trans configuration decreasing the efficiency of electronic coupling through them. The use of p-type Si in this study ensures that at low bias voltages holes are the dominant charge carriers. Holes are found to tunnel more efficiently than electrons in agreement with theoretical predictions.

The mechanism by which dicarboxylic acid molecules adsorb onto ambient-exposed GaAs (1 0 0) surfaces is studied by combining infrared spectroscopy and measurements that are sensitive to changes of the electric potential on the surface. For the latter we used the recently developed molecular controlled semiconductor resistor. By comparing the time dependences of the two measurements we conclude that adsorption proceeds sequentially, with virtually all of the rearrangement of electrical charge in the adsorbates taking place when the first carboxylic group, in each molecule, binds to the surface. This can be understood if charge rearrangement is necessary for forming a close-packed adsorbed layer. Since creation of such a layer, that is made up of molecules with significant molecular dipoles, requires some degree of depolarization of the molecules.

Junction function: Molecule-controlled devices based on metal-semiconductor junctions can exhibit negative differential resistance (NDR) at room temperature. The active component is a self-assembled monolayer of molecular dipoles (see schematic representation; X = CF₃, CN, H, OMe). A systematic change in the dipole moment of these molecules results in a corresponding systematic change in the NDR effect, thus establishing for the first time molecularly tunable NDR.

An information transfer mechanism through a molecular bilayer, which does not involve charge/mass transfer or conformational changes of membrane-spanning molecular structures, is proposed. We tested this proposal by measuring changes in the electric potential at a Si/SiO_x surface, onto which an artificial bilayer had been constructed, in response to exposure of the adsorbed bilayer to different ambient. The bilayer was comprised of an OctadecylTrichloroSilane (OTS) monolayer, adsorbed onto the Si/SiO_x surface and a second layer of stearic acid, deposited on the OTS monolayer by the Langmuir-Blodgett technique. Changes of the band bending (BB) at the Si/SiO_x surface in response to exposing the bilayer to different solutions were measured by Kelvin Probe. These changes indicate that external stimuli at the bilayer's exterior induce a change in the electric potential at the bilayer's interior, a change that is sensed at the surface of the Si/SiO_x. The mechanism proposed to explain the results is based on electrostatic interactions at the bilayer's exterior with dipole- or monopole-carrying molecules.

Epitaxial films of CuInSe₂ on Si(111) were modified by the application of an electric field through a movable tip. The electric field induces stable junction regions which are identified by efficient separation and collection of electron beam-induced charge carriers. The movable tip allows for scribing of these junction regions. The junctions can be explained by symmetrical p/p⁺ /n/p⁺ /p regions formed within the CuInSe₂ epilayers. The reported method presents an alternate way for junction patterning in two dimensions.

The onset wavelengths of the surface photovoltage (SPV) in dye-sensitized solar cells (DSSCs) with different mesoporous, wide-band gap electron conductor anode materials, viz., TiO₂ (anatase), Nb₂O₅ (amorphous and crystalline), and SrTiO₃, using the same Ru bis-bipyridyl dye for all experiments, are different. We find a clear dependence of these onset wavelengths on the conduction band edge energies (E_CB) of these oxides. This is manifested in a blue-shift for cells with Nb₂O₅ and SrTiO₃ compared to those with TiO₂. The E_CB levels of Nb₂O₅ and SrTiO₃ are known to be some 200-250 meV closer to the Vacuum level than that of our anatase films, while there is no significant difference between the optical absorption spectra of the dye on the various films. We, therefore, suggest that the blue shift is due to electron injection from excited-state dye levels above the LUMO into Nb₂O₅ and SrTiO₃. Such injection comes about because, in contrast to what is the case for anatase, the LUMO of the adsorbed dye in the solution is below the E_CB of these semiconductors, necessitating the involvement of higher vibrational and/or electronic levels of the dye, with the former being more likely than the latter. While for Nb₂O₅ hot electron injection has been proposed earlier, on the basis of flash photolysis experiments, this is the first evidence for such ballistic electron-transfer involving SrTiO₃, a material very similar to anatase but with a significantly smaller electron affinity. Additional features in the SPV spectra of SrTiO₃ and amorphous Nb₂O₅ (but not in those of crystalline Nb₂O₅) can be understood in terms of hole injection from the dye into the oxide via intraband gap surface states.

Fine tunning of Au/SiO₂/Si diode was realized by varying interfacial dipoles using molecular monolayers. Barrier heights of the Schottky junctions were modified by molecular monolayers dipoles. Current-voltage curves for a series of molecularly modified junctions showed that the presence of molecules with the positive dipole decreased the current compared to the molecules with the negative dipole.The presence of molecular layer resulted in the decrease of current relative to that of the unmodified junctions.

We explore chemical and physical limits to semiconductor device miniaturization. Minimal sizes for space charge-based devices can be estimated from Debye screening lengths of the materials used. Because a doped semiconductor can be viewed as a mixed electronic-ionic conductor, with the dopants as mobile ions, dopant intermixing across a p/n junction presents a chemical limit. Given a desired lifetime, simple relations can be derived between size and dopant intermixing for reverse- or forward-biased devices. Mostly, conditions for significant dopant mobility are far from those where the material is used. Thus, it is generally held that elemental and III-V-based p-n junctions are immune to this problem and persist because of kinetic stability. Indeed, we find this to be so for Si in the foreseeable future, but not for III-V- and II-VI-based ones. The limitation is more severe in structures with very thin undoped layers sandwiched between doped ones or vice versa, where even 1% intermixing can be critical. This decreases lifetime nearly 100 times. For example, for structures containing a 10 nm critical dimension, none of the components can have an average diffusion coefficient higher than 10^-24 cm²/s for a 3 year lifetime. Ways to overcome or mitigate this limitation are indicated.

Stability aspects of the Mo/Cu(In,Ga)Se₂/CdS/ZnO solar cell are reviewed and assessed. These include (i) the chemical stability of the various interfaces present in the device, (ii) the long-term behavior of metastable defects found in the Cu(In,Ga)Se₂ (CIGS) compound, and (iii) the impact of Cu migration on device performance and lifetime. We find that (i) all interfaces within the structure are chemically stable, (ii) metastable defects have a beneficial effect on performance, and (iii) Cu migration effects are reversible and their possible detrimental effects are eclipsed by the beneficial effect of the metastable states. Moreover, Cu out-diffusion from the CIGS layer is absent in photovoltaic-quality CIGS. Finally, we propose a model that explains the exceptional radiation hardness and impurity tolerance of CIGS-based devices, based on the synergetic effect of copper migration and point defect reactions.

The use of molecules to control electron transport is an interesting possibility, not least because of the anticipated role of molecules in future electronic devices(1). But physical implementations using discrete molecules are neither conceptually(2,3) simple nor technically straightforward (difficulties arise in connecting the molecules to the macroscopic environment). But the use of molecules in electronic devices is not limited to single molecules, molecular wires or bulk material. Here we demonstrate that molecules can control the electrical characteristics of conventional metal-semiconductor junctions, apparently without the need for electrons to be transferred onto and through the molecules. We modify diodes by adsorbing small molecules onto single crystals of n-type GaAs semiconductor. Gold contacts were deposited onto the modified surface, using a 'soft' method to avoid damaging the molecules(4). By using a series of multifunctional molecules whose dipole is varied systematically, we produce diodes with an effective barrier height that is tuned by the molecule's dipole moment. These barrier heights correlate well with the change in work function of the GaAs surface after molecular modification. This behaviour is consistent with that of unmodified metal-semiconductor diodes, in which the barrier height can depend on the metal's work function.

Numerical simulations of the defect distribution of CuInSe₂ were carried out as a function of the stoichiometry. The simulations are based on a new calculation of the intrinsic defects in this material. The results of the calculations were compared with earlier electrical and positron lifetime measurements. This leads to the assumption, that the single defects V_Se, V_Cu, Cu_In and the defect pair (2V_Cu-In_Cu) occur in the investigated specimens in considerable concentrations.

Sequential self-assembly of a two-component system on a solid support is described with respect to structure and function. Two ligands, which bind to the semiconductor surface through one end and axially ligate a heme analogue at the other end, are described. Monolayer assemblies of complexes formed by these ligands and iron-porphyrin perform reversible binding of molecular oxygen. In the monolayer, a metalloporphyrin (the sensing unit) is held by the intervening ligand that serves as a 'hinge', away from the solid surface. Sensing events based on porphyrin chemistry are communicated via the ligand to the solid support. The transduction manifests itself as a change in the solid's surface electronic properties. Synthesis of the ligands and analysis of its complex formation with Fen(III)-porphyrin are described. The anisotropic orientation of the porphyrin ring within the ligand cavity, due to restricted rotation around the Fe(III)-N imidazole bonds, was probed by ¹H NMR measurements in solution. We show that the porphyrin substituents stand as barriers for the free rotation even at room temperature. Molecular modeling supports the NMR evidence and reveals the stable conformations for the porphyrin's orientation relative to the solid support. The complexes were assembled as films on the (0001) surface of etched n-CdSe single crystals, and the films were characterized using transmission Fourier transform infrared (FTIR) and X-ray photoelectron (XPS) spectroscopies. Contact potential difference (CPD) and steady-state photoluminescence (PL) measurements of the derivatized CdSe show that the intervening ligands yield better conjugation and stronger binding of the sensing unit to the semiconductor surface, relative to direct adsorption of metalloporphyrins. Furthermore, the PL changes in the CdSe can be used to follow the interaction of the surface-bound Fe(III)-porphyrin-ligand complexes with molecular oxygen. A model is proposed to explain the electronic changes resulting from binding of O₂ to the monolayer.

A new generic transducer has been developed, based on a Molecular Controlled Semiconductor Resistor (MOCSER). It is based on a GaAs/(AI, Ga)As structure, to the surface of which the specially designed bifunctional organic molecules are covalently bound The electrical current through the device is very sensitive to the surface polential. Therefore, it changes when metal ions bind to the receptor site of the organic molecule. The new sensor has high sensitivity over a wide dynamic range, high selectivity, short measurement time and it is inexpensive to produce.

Nanocrystalline SrTiO₃ is synthesized by hydrothermal treatment of nanocrystalline titanium dioxide in the presence of strontium hydroxide. Working photoelectrochemical solar cells are produced using these nanometer-sized semiconductor particles as photoelectrode materials. At AM 1.5, measured open circuit voltages were roughly 100 mV higher than in solar cells produced using nanocrystalline titanium dioxide (anatase), in agreement with a simple relation between semiconductor conduction band edge and open circuit voltage for these cells. Photocurrents measured in the SrTiO₃ cells were roughly 1/3 those measured with TiO₂ (anatase) -based cells. On the basis of flash laser photolysis and absorptance studies, we suggest that low dye loading and possibly suboptimal dye-oxide interactions can be the cause for the relatively low photocurrents in the SrTiO₃ system.

Communication: Is "self-healing" the source of the stability of Cu(In, Ga)Se-2-based solar modules? The proven remarkable stability and radiation hardness of Cu(In,Ga)Se-2 (CIGS) solar cells stand in apparent contradiction to the fact that CIGS shows both short-range (metastable defect centers) and long-range (significant Cu migration) instabilities. The authors suggest that these instabilities may in fact be a prerequisite for CIGS's stability as they allow a degree of flexibility or "smartness" in accommodating externally imposed changes. Two self-healing cycles are proposed, in which copper species play a particularly important role.

A reinvestigation of the phase diagram of the Cu-In-Se system along the quasi-binary cut In₂Se₃-Cu₂Se reveals an existence range of the chalcopyrite α-phase that is much narrower than commonly accepted. The presence of 0.1% of Na or replacement of In by Ga at the at.% level widens the existence range of the α-phase, towards In- and Ga-rich cmnpositions. We also investigate the interplay between phase segregation and junction formation in polycrystalline Cu(In, Ga)Se₂ films. Here, we attribute the band bending observed at bare surfaces of the films to a positively charged surface acting as a driving force for the formation of a Cu-poor surface defect layer via Cu-electromigration. The electrical properties of this defect layer are different from those found for the bulk β-phase. We suggest that Cu-depletion is self-limited at the observed In/(In+Cu) surface composition of 0.75 because further Cu-depletion would require a structural transformation. Capacitance measurements reveal two types of junction metastabilities: one resulting from local defect relaxation, invoked to explain a light-induced increase of the open-circuit voltage of Cu(In, Ga)Se₂ solar cells, and one due to Cu-electromigration.

We show that percolation can control not only diffusion in solids, but in the case of semiconductors also their electrical activity, via the doping action of the diffusing species. This occurs in (Hg1-xCdx)Te (MCT) when x(Cd)

We have developed a new, efficient method to dope bulk single crystals of CdTe by In, via gas phase diffusion, using In₄Te₃ as the source. Doping was carried out on crystals of very high resistivity (>5 MΩ cm), following annealing in the temperature range of 350-1000°C. Resulting crystals showed n-type conductivity with a free carrier concentration in the range of 10¹⁵-10¹⁸ cm^-3 and carrier mobility of 100-750 cm²/(V s), depending on the annealing temperature and time, and on the cooling conditions. Incorporation of In was found to be a function of annealing time and temperature only. Up to 650°C, the In and the free electron concentrations are roughly the same.

We show that the chemisorption of dicarboxylic acids on GaAs (100) is described well by a two-site mechanism, in contrast to benzoic acid adsorption which fits to a one-site mechanism. We do so by using a novel electrical method for direct measurement of adsorption kinetics. In the method we measure the current through a GaAs/(Al, Ga)As-based device, where the bare surface between two contacts is used as the adsorption domain. The results, which are in agreement with FTIR absorption equilibrium data, are obtained in ambient notwithstanding the notorious instability of GaAs surfaces under such conditions. We conclude that these acids chemisorb on the GaAs surface and that binding is significantly stronger for the di-than for the monocarboxylic acids.

Control over the surface chemistry and physics of electronic and optical materials is essential for constructing devices and fine-tuning their performance. In the past few years we have started to explore the use of organic molecules for systematic modification of semiconductor surface electronic properties. In this paper, manipulation of silicon surfaces by self-assembly of various quinolinium-based chromophores is reported. The progress of the assembly process is monitored by XPS, UV-Vis, and FTIR spectroscopies as well as with surface wettability. The effect of the monolayer's dipole moment on the Si surface potential and the interaction with surface states is monitored by CPD measurements. A pronounced effect of a sub-nanometer coupling-agent layer alone on the electron affinity and band-bending of Si was observed. We also show a way to modulate the Si work-function by tuning the dipole strength of the chromophore-containing organic, self-assembled monolayer and of its orientation with respect to the silicon surface.

The structure of CuInSe2 was redetermined, using synchrotron X-radiation with 0.15 Angstrom wavelength, thus eliminating problems of uncertainties introduced by absorption corrections. This allowed us to look at the effect of subjecting crystals to strong electric fields, a process known to be able to type convert the material under relatively mild conditions. Proper refinement became possible after correcting for twinning. The main results are relatively high Cu temperature factors and significant electron density in octahedral interstitial sites. The main results of electric field application are a decrease in structure quality (increased R factor) and a slight increase in electron density on Cu sites. These preliminary results point to the need for further work with twin-free crystals.

We fabricate sub-micron sized diode and transistor structures (down to 100 nm in diameter) inside CuInSe2 crystals by inducing thermally assisted electromigration of mobile dopants. This is achieved by applying an electric field via a small area contact to the crystals, using a conducting Atomic Force Microscope tip. The structures are characterized by nm scale scanning spreading resistance and scanning capacitance measurements to reveal the inhomogeneous doping profiles, that result from the electric field action. Calculations suggest that the smallest of the structures that we made are very close to the lower size limit of possible p/n/p devices.

We show how sub-μm sized transistor structures (down to 50 nm cross section) can be fabricated by thermally assisted electromigration of mobile dopants inside the semiconductor CuInSe₂. Small device structures are fabricated by application of an electric field to the sample via the contact, defined by a conducting atomic force microscope tip. The structures are characterized by nm scale scanning spreading resistance and scanning capacitance measurements to reveal the inhomogeneous doping profiles created by the electric field.

Thermal diffusion, parallel to the photovoltaic junction and perpendicular to the direction of illumination, can be used to separate the contributions of injected and photogenerated carriers to the generation of heat in a photothermal experiment, and to demonstrate the influence of electronic carrier diffusion on the signal, already at relatively low modulation frequencies. We show this by using an experimental arrangement in which the distance is varied between the illuminated area of the photovoltaic cell and that over which the thermal signal is detected, and the two areas do not overlap. Our results agree with theoretical calculations used to simulate the photothermal responses of solar cells.

Cu diffusion in chalcopyrite CuInSe₂ was studied directly, using ⁶⁴Cu as a radioactive tracer. For diffusion from a thin surface layer, the Cu diffusion coefficients at 380 and 430 °C, were found to vary from 10^-8 to 10^-9 cm²/s. In case of diffusion from a volume source at 400 °C, a value of 10^-10 cm²/s was calculated from diffusion profiles. Electromigration of Cu was demonstrated, by applying a strong electric field to a sample and following the redistribution of ⁶⁴Cu, that had been thermally diffused into the sample, prior to electric field application.

The purpose of construction of ion potential diagrams is to facilitate the description of systems in which ionic species are mobile. These diagrams depict qualitatively the spatial dependence of the potential energy for mobile ions in a way similar to band diagrams for electrons. We specify and explain what types of experimental data are needed to construct these diagrams. We construct such diagrams for five layer electrochromic devices in which both optically active electrodes are oxides, capable of reversible lithium ion intercalation. We consider the systems at open circuit and under bias. We compare the behaviour of several electrochromic oxides with respect to intercalation and deintercalation reactions. On the basis of the diagrams we discuss electrode stability and switching time in electrochromic devices. Possible novel electrode materials, in terms of their electrical behaviour, are discussed.

Polymer electrochemical cells with ion blocking electrodes were reported to emit light under applied voltage. This work analyzes the current-voltage relations, internal electric fields, and point detect distribution in the polymer. The polymer is regarded as a mixed-ionic-electronic conductor. Two relevant defect models are investigated. A good fit is obtained between experimental data and theory which also takes into consideration electrode overpotentials.

Wavelength-dependent two-photon photoemission (WD-TPPE) spectroscopy was used to investigate the surface state properties of CdTe crystals before and after the adsorption of specially designed organic molecules. One photon was used to modify the population of the surface states and a second photon to eject electrons from the substrate. We measured the dependence of the photoemission signal on the energy of the first photon and on the delay between the two light pulses. The energy of surface states, relative to the bands, was found to correlate with the relaxation time of the semiconductor surface after being photoexcited. This is explained in terms of a simple kinetic model for electron transfer. These findings demonstrate that the properties of surface states of semiconductors can be manipulated by adsorbing suitable organic molecules on the semiconductor surface and that the WD-TPPE method is a useful tool for optoelectronic characterization of semiconductor surfaces, with sensitivity exceeding that of most commonly used techniques.

Control of the work function of GaAs single crystals, under ambient conditions, was achieved by chemisorption of a series of benzoic acid derivatives with varying dipole moments. Quantitative Fourier transform infrared spectroscopy shows that the benzoic acid derivatives bind as carboxylates, via coordination to oxidized Ga or As atoms, with a surface coverage of about one layer and a binding constant of 2.1 10⁴ M^-1 for benzoic acid. Contact potential difference measurements reveal that molecules affect the work function by changing the electron affinity while band bending is not affected significantly. The direction of the electron affinity changes depends on the direction of the dipole moments, and the extent of the change increases linearly with the dipole's magnitude. Investigation of the surface composition by X-ray photoelectron spectroscopy shows that the etched surface, onto which the molecules adsorb, is covered by an oxide layer. This may prevent the molecules from affecting band bending.

When p-n junctions are formed by doping with a dopant that diffuses via a dissociative diffusion mechanism, dopant diffusion is suppressed and dopants can pile up near the junction, well above their original concentration. Calculations confirm this, if no local neutrality is assumed. The results agree well with published and our own experimental data for Zn diffusion in InP. The increased built-in electric field due to this pileup is expelled nearly completely to the side of the junction without the pileup. This effect has important consequences for devices containing thin and/or small regions doped with such dopants because such regions may be completely depleted.