Publications

The fundamental question of \u201cwhat is the transport path of electrons through proteins?\u201d initially introduced while studying long-range electron transfer between localized redox centers in proteins in vivo is also highly relevant to the transport properties of solid-state, dry metal\u2013protein\u2013metal junctions. Here, we report conductance measurements of such junctions, Au-(Azurin monolayer ensemble)-Bismuth (Bi) ones, with well-defined nanopore geometry and ~10³ proteins/pore. Our results can be understood as follows. (1) Transport is via two interacting conducting channels, characterized by different spatial and time scales. The slow and spatially localized channel is associated with the Cu center of Azurin and the fast delocalized one with the protein\u2019s polypeptide matrix. Transport via the slow channel is by a sequential (noncoherent) process and in the second one by direct, off-resonant tunneling. (2) The two channels are capacitively coupled. Thus, with a change in charge occupation of the weakly coupled (metal center) channel, the broad energy level manifold, responsible for off-resonance tunneling, shifts, relative to the electrodes\u2019 Fermi levels. In this process, the off-resonance (fast) channel dominates transport, and the slow (redox) channel, while contributing only negligibly directly, significantly affects transport by intramolecular gating.

The term defect tolerance (DT) is used often to rationalize the exceptional optoelectronic properties of halide perovskites (HaPs) and their devices. Even though DT lacked direct experimental evidence, it became a "fact" in the field. DT in semiconductors implies that structural defects do not translate to electrical and optical effects (e.g., due to charge trapping), associated with such defects. We present pioneering direct experimental evidence for DT in Pb-HaPs by comparing the structural quality of 2-dimensional (2D), 2D-3D, and 3D Pb-iodide HaP crystals with their optoelectronic characteristics using high-sensitivity methods. Importantly, we get information from the materials' bulk because we sample at least a few hundred nanometers, up to several micrometers, from the sample's surface, which allows for assessing intrinsic bulk (and not only surface-) properties of HaPs. The results point to DT in 3D, 2D-3D, and 2D Pb-HaPs. Overall, our data provide an experimental basis to rationalize DT in Pb-HaPs. These experiments and findings will help the search for and design of materials with real DT.

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.

DOI: 10.1002/adfm.202204283 The following funding information is added to the Acknowledgments: This project received funding from the European Union's Horizon 2020 research and innovation program under the Marie Sklodowska-Curie grant agreement No. 893194. These errors do not affect the conclusions of the report. The authors apologize for any inconvenience caused.

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.

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\u2019 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.

The future of halide perovskites (HaPs) is beclouded by limited understanding of their long-term stability. While HaPs can be altered by radiation that induces multiple processes, they can also return to their original state by “self-healing.” Here two-photon (2P) absorption is used to effect light-induced modifications within MAPbI₃ single crystals. Then the changes in the photodamaged region are followed by measuring the photoluminescence, from 2P absorption with 2.5 orders of magnitude lower intensity than that used for photodamaging the MAPbI₃. After photodamage, two brightening and one darkening process are found, all of which recover but on different timescales. The first two are attributed to trap-filling (the fastest) and to proton-amine-related chemistry (the slowest), while photodamage is attributed to the lead-iodide sublattice. Surprisingly, while after 2P-irradiation of crystals that are stored in dry, inert ambient, photobrightening (or “light-soaking”) occurs, mostly photodarkening is seen after photodamage in humid ambient, showing an important connection between the self-healing of a HaP and the presence of H₂O, for long-term steady-state illumination, practically no difference remains between samples kept in dry or humid environments. This result suggests that photobrightening requires a chemical-reservoir that is sensitive to the presence of H₂O, or possibly other proton-related, particularly amine, chemistry.

Attempts to dope halide perovskites (HaPs) extrinsically have been mostly unsuccessful. Still, oxygen (O2) is an efficient p\u2010dopant for polycrystalline HaP films. To an extent, this doping is reversible, i.e., the films can be de\u2010doped 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\u2010surface interactions suffice to dope the bulk of the films. Such behavior fits what is known for other polycrystalline semiconductors, where surface charge transfer\u2010adducts 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.

Combinatorial material science crucially depends on robust, high-throughput characterization methods. While X-ray photoelectron spectroscopy (XPS) may provide detailed information about chemical and electronic properties, it is a time-consuming technique and, therefore, is not viewed as a high-throughput method. Here we present preliminary XPS data of 169 measurement spots on a combinatorial 72 × 72 cm2 CuxNi1−xOy compositional library to explore how characterization and evaluation routines can be optimized to improve throughput in XPS for combinatorial studies. In particular, two quantification approaches are compared. We find that a simple integration (of XPS peak regions) approach is suited for fast evaluation of, in the example system, the [Cu]/([Cu] + [Ni]) ratio. Complementary to that, the time-consuming (XPS peak-) fit approach provides additional insights into chemical speciation and oxidation state changes, without a large deviation of the [Cu]/([Cu] + [Ni]) ratio. This insight suggests exploiting the fast integration approach for ‘real time’ analysis during XPS data collection, paving the way for an ‘on-the-fly’ selection of points of interest (i.e., areas on the sample where sudden composition changes have been identified) for detailed XPS characterization. Together with the envisioned improvements when going from laboratory to synchrotron-based excitation sources, this will shorten the analysis time sufficiently for XPS to become a realistic characterization option for combinatorial material science.

Ultra-thin hydrophobic capping layers of two-dimensional (2D) onto three-dimensional (3D) metal halide perovskites (HaPs) are an attractive strategy for preventing ambient-induced degradation and minimizing interfacial non-idealities of 3D HaPs. However, it is not obvious in how far the unusual 3D HaP lattice dynamics affect 2D-on-3D HaP composites' stability, especially at their interface, an issue important for devices made with such composites. Using low electron-fluence, four-dimensional scanning transmission electron microscopy and nanobeam electron diffraction, we show formation of the 2D (n = 1) phase on top of 3D perovskite, using focused-ion beam-prepared cross-sections, under conditions that minimize radiation damage. The 2D-on-3D HaP composites were prepared by controlled gas-phase surface cation exchange of 3D MAPbI3 films to form A2PbI4, where A = (fluoro- )phenyl-ethyl-ammonium, (F)PEA. We provide direct evidence for 2D phase formation also inside the 3D matrix, likely via A cation grain boundary diffusion, and, over time, of quasi-2D phases near the surface. These results show that the 2D/3D heterointerface is dynamic; they imply that not only small, but also large A cations, (F)PEA+, migrate. Structural rearrangements, leading to quasi-2D phase formation can be followed with the electron beam, which provides hitherto unknown atomistic insights into such interfaces, needed to assess their (in)stability. Apart from directly illustrating effects of HaP lattice dynamics, our results help understanding extensive (semi)empirical data on engineering 2D-on-3D composites and provide guidance for enhancing stability of such systems. Critically, our direct observation of electron beam-induced loss of long-range periodicity defines conditions for damage-free atomicresolution studies of HaP samples, also in device-relevant configurations.

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.

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.

Direct detection of intrinsic defects in halide perovskites (HaPs) by standard methods utilizing optical excitation is quite challenging, due to the low density of defects in most samples of this family of materials (≤1015 cm-3 in polycrystalline thin films and ≤1011 cm-3 in single crystals, except melt-grown ones). While several electrical methods can detect defect densities 2 eV) HaPs. By measuring HaP layers on both hole- and electron-contact layers, as well as single crystals without contacts, we conclude that the observed deep defects are intrinsic to the Br-based HaP, and we propose a passivation route via the incorporation of a 2Dforming ligand into the precursor solution.

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\u2013hydrogen 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.

Perovskite photovoltaics has witnessed an unprecedented increase in power conversion efficiency over the last decade. The choice of transport layers, through which photo-generated electrons and holes are transported to electrodes, is a crucial factor for further improving both the device performance and stability. In this perspective, we critically examine the application of optical spectroscopy to characterize the quality of the transport layer-perovskite interface. We highlight the power of complementary studies that use both continuous wave and time-resolved photoluminescence to understand non-radiative losses and additional transient spectroscopies for characterizing the potential for loss-less carrier extraction at the solar cell interfaces. Based on this discussion, we make recommendations on how to extrapolate results from optical measurements to assess the quality of a transport layer and its impact on solar cell efficiency.

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.

Quantum tunneling is the basis of molecular electronics, but often its electron transport range is too short to overcome technical defects caused by downscaling of electronic devices, which limits the development of molecular-/nano-electronics. Marrying electronics with plasmonics may well present a revolutionary way to meet this challenge as it can manipulate electron flow with plasmonics at the nanoscale. Here we report on unusually efficient temperature-independent electron transport, with some photoconductivity, across a new type of junction with active plasmonics. The junction is made by assembly of SiO2 shell-insulated Au nanoparticles (Au@SiO2 NPs) into dense nanomembranes of a few Au@SiO2 layers thick and transport is measured across these membranes. We propose that the mechanism is plasmon-enabled transport, possibly tunneling (as it is temperature-independent). Unprecedentedly ultra-long-range transport across one, up to even three layers of Au@SiO2 in the junction, with a cumulative insulating (silica) gap up to 29 nm/NP layer was achieved, well beyond the measurable limit for normal quantum mechanical tunneling across insulators (similar to 2.5 nm at 0.5-1V). This finding opens up a new interdisciplinary field of exploration in nanoelectronics with wide potential impact on such areas as electronic information transfer.

Halide perovskites are promising optoelectronic materials. Despite impressive device performance, especially in photovoltaics, the femtosecond dynamics of elementary optical excitations and their interactions are still debated. Here we combine ultrafast two-dimensional electronic spectroscopy (2DES) and semiconductor Bloch equations (SBEs) to probe the room-temperature dynamics of nonequilibrium excitations in CsPbBr3 crystals. Experimentally, we distinguish between excitonic and free-carrier transitions, extracting a similar to 30 meV exciton binding energy, in agreement with our SBE calculations and with recent experimental studies. The 2DES dynamics indicate remarkably short,

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.

The author list of this publication should have included Janardan Dagar, from Young Investigator Group ‘Hybrid Materials Formation and Scaling’, Helmholtz-Zentrum Berlin für Materialien und Energie GmbH, Kekule-Strasse 5, 12489 Berlin, Germany. ́The authors express their sincere apologies to Dr. Dagar for this omission. Dr. Janardan Dagar prepared the iodide-based (mixed MAPI) solar cells used in this work, as shown in the main text in Figure 2b and described in the Supporting Information.Dr. Dagar made thus a significant contribution to this work.

The remarkable development in photovoltaic (PV) technologies over the past 5 years calls for a renewed assessment of their performance and potential for future progress. Here, we analyse the progress in cells and modules based on single-crystalline GaAs, Si, GaInP and InP, multicrystalline Si as well as thin films of polycrystalline CdTe and Culn(x)Ga(1-x),Se-2. In addition, we analyse the PV developments of the more recently emerged lead halide perovskites together with notable improvements in sustainable chalcogenides, organic PVs and quantum dots technologies. In addition to power conversion efficiencies, we consider many of the factors that affect power output for each cell type and note improvements in control over the optoelectronic quality of PV-relevant materials and interfaces and the discovery of new material properties. By comparing PV cell parameters across technologies, we appraise how far each technology may progress in the near future. Although accurate or revolutionary developments cannot be predicted, cross-fertilization between technologies often occurs, making achievements in one cell type an indicator of evolutionary developments in others. This knowledge transfer is timely, as the development of metal halide perovskites is helping to unite previously disparate, technology-focused strands of PV research.

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.

Lead-based halide perovskites (APbX(3)) are fascinating optoelectronic materials. Because of toxicity issues of Pb, Sn-based halide perovskites are studied, although less so, as an alternative. Adding SnF2 often improves the properties of Sn halide perovskite-based devices. This effect is usually ascribed to suppression of Sn2+ -> Sn4+ oxidation and/or decreased Sn vacancy concentration. These effects will change the doping, sometimes in opposite directions. Here we review the effect of addition of SnF2 during the formation of ASnX(3) layers as observed by different groups, both to the properties of the layers themselves and to photovoltaic cells made from these layers. SnF2 can affect many different properties of the ASnX(3) perovskites, including film morphology, doping, control over formation of unwanted crystal phases, material stability to various factors, and energy level positions. It also improves (in general) the performance of photovoltaic cells made with these layers. Besides focusing on all these issues, we also describe possible doping scenarios for the perovskites, including some that do not appear to have been considered before and conclude that the doping mechanism depends strongly on whether the oxidation of Sn2+ to Sn2+ occurs during the materials preparation or after the film is formed, and if oxygen is involved.

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.

We present a measurement of the energies and capture cross-sections of defect states in methylammonium lead bromide (MAPbBr₃) single crystals. Using Laplace current deep level transient spectroscopy (I-DLTS), two prominent defects were observed with energies 0.17 eV and 0.20 eV from the band edges, and further I-DLTS measurements confirmed that these two defects are bulk defects. These results show qualitative agreement with theoretical predictions, whereby all of the observed defects behave as traps rather than as generation-recombination centers. These results provide one explanation for the high efficiencies and open-circuit voltages obtained from devices made with lead halide perovskites.

Negative capacitance in photovoltaic devices has been observed and reported in several cases, but its origin, at low or intermediate frequencies, is under debate. Here we unambiguously demonstrate a direct correlation between the observation of this capacitance and a corresponding decrease in performance of a halide perovskite (HaP; CsPbBr3)-based device, expressed as reduction of open-circuit voltage and fill factor. We have prepared highly stable CsPbBr3 HaPs that do not exhibit any degradation over the duration of the impedance spectroscopy measurements, ruling out degradation as the origin of the observed phenomena. Reconstruction of current voltage curves from the impedance spectroscopy provided further evidence of the deleterious role of negative capacitance on photoconversion performance.

Materials are central to our way of life and future. Energy and materials as resources are connected, and the obvious connections between them are the energy cost of materials and the materials cost of energy. For both of these, resilience of the materials is critical; thus, a major goal of future chemistry should be to find materials for energy that can last longer, that is, design principles for self-repair in these.

Inserting molecular monolayers within metal/semiconductor interfaces provides one of the most powerful expressions of how minute chemical modifications can affect electronic devices. This topic also has direct importance for technology as it can help improve the efficiency of a variety of electronic devices such as solar cells, LEDs, sensors, and possible future bioelectronic ones. The review covers the main aspects of using chemistry to control the various aspects of interface electrostatics, such as passivation of interface states and alignment of energy levels by intrinsic molecular polarization, as well as charge rearrangement with the adjacent metal and semiconducting contacts. One of the greatest merits of molecular monolayers is their capability to form excellent thin dielectrics, yielding rich and unique current-voltage characteristics for transport across metal/molecular monolayer/semiconductor interfaces. We explain the interplay between the monolayer as tunneling barrier on the one hand, and the electrostatic barrier within the semiconductor, due to its space-charge region, on the other hand, as well as how different monolayer chemistries control each of these barriers. Practical tools to experimentally identify these two barriers and distinguish between them are given, followed by a short look to the future. This review is accompanied by another one, concerning the formation of large-area molecular junctions and charge transport that is dominated solely by molecules.

Using several metals with different work functions as solar cell back contact we identify majority carrier type inversion in methylammonium lead bromide (MAPbBr3, without intentional doping) as the basis for the formation of a p-n junction. MAPbBr₃ films deposited on TiO₂ are slightly n-type, whereas in a full device they are strongly p-type. The charge transfer between the metal electrode and the halide perovskite (HaP) film is shown to determine the dominant charge carrier type of the HaP and, thus, also of the final cells. Usage of Pt, Au and Pb as metal electrodes shows the effects of metal work function on minority carrier diffusion length and majority carrier concentration in the HaP, as well as on built-in voltage, band bending, and open circuit voltage (V_OC) within a solar cell. V_OC > 1.5 V is demonstrated. The higher the metal WF, the higher the carrier concentration induced in the HaP, as indicated by a narrower space charge region and a smaller minority carrier diffusion length. From the analysis of bias-dependent electron beam-induced currents, the HaP carrier concentrations are estimated to be ∼ 1 × 10¹⁷ cm^-3 with Au and 2-3 × 10¹⁸ cm^-3 with Pt. A model in which type-inversion stretches across the entire film width implies formation of the p-n junction away from the interface, near the back-contact metal electrode. This work highlights the importance of the contact metal on device performance in that contact engineering can also serve to control the carrier concentration in HaP.

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.

Direct comparison between perovskite-structured hybrid organic-inorganic methylammonium lead bromide (MAPbBr₃) and all-inorganic cesium lead bromide (CsPbBr₃), allows identifying possible fundamental differences in their structural, thermal and electronic characteristics. Both materials possess a similar direct optical band gap, but CsPbBr₃ demonstrates a higher thermal stability than MAPbBr₃. In order to compare device properties, we fabricated solar cells, with similarly synthesized MAPbBr₃ or CsPbBr₃, over mesoporous titania scaffolds. Both cell types demonstrated comparable photovoltaic performances under AM1.5 illumination, reaching power conversion efficiencies of ∼6% with a poly aryl amine-based derivative as hole transport material. Further analysis shows that Cs-based devices are as efficient as, and more stable than methylammonium-based ones, after aging (storing the cells for 2 weeks in a dry (relative humidity 15-20%) air atmosphere in the dark) for 2 weeks, under constant illumination (at maximum power), and under electron beam irradiation.

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.

We observe temperature-independent electron transport, characteristic of tunneling across a ∼6 nm thick Halorhodopsin (phR) monolayer. phR contains both retinal and a carotenoid, bacterioruberin, as cofactors, in a trimeric protein-chromophore complex. This finding is unusual because for conjugated oligo-imine molecular wires a transition from temperature-independent to -dependent electron transport, ETp, was reported at ∼4 nm wire length. In the ∼6 nm long phR, the ∼4 nm 50-carbon conjugated bacterioruberin is bound parallel to the α-helices of the peptide backbone. This places bacterioruberins ends proximal to the two electrodes that contact the protein; thus, coupling to these electrodes may facilitate the activation-less current across the contacts. Oxidation of bacterioruberin eliminates its conjugation, causing the ETp to become temperature dependent (>180 K). Remarkably, even elimination of the retinal-protein covalent bond, with the fully conjugated bacterioruberin still present, leads to temperature-dependent ETp (>180 K). These results suggest that ETp via phR is cooperatively affected by both retinal and bacterioruberin cofactors.

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 central vision in molecular electronics is the creation of devices with functional molecular components that may provide unique properties. Proteins are attractive candidates for this purpose, as they have specific physical (optical, electrical) and chemical (selective binding, self-assembly) functions and offer a myriad of possibilities for (bio-)chemical modification. This Progress Report focuses on proteins as potential building components for future bioelectronic devices as they are quite efficient electronic conductors, compared with saturated organic molecules. The report addresses several questions: how general is this behavior; how does protein conduction compare with that of saturated and conjugated molecules; and what mechanisms enable efficient conduction across these large molecules? To answer these questions results of nanometer-scale and macroscopic electronic transport measurements across a range of organic molecules and proteins are compiled and analyzed, from single/few molecules to large molecular ensembles, and the influence of measurement methods on the results is considered. Generalizing, it is found that proteins conduct better than saturated molecules, and somewhat poorer than conjugated molecules. Significantly, the presence of cofactors (redox-active or conjugated) in the protein enhances their conduction, but without an obvious advantage for natural electron transfer proteins. Most likely, the conduction mechanisms are hopping (at higher temperatures) and tunneling (below ca. 150-200 K).

Hybrid organic/lead halide perovskites are promising materials for solar cell fabrication, resulting in efficiencies up to 18%. The most commonly studied perovskites are CH₃NH₃PbI₃ and CH₃NH₃PbI_3-x Cl_x where x is small. Importantly, in the latter system, the presence of chloride ion source in the starting solutions used for the perovskite deposition results in a strong increase in the overall charge diff usion length. In this work we investigate the crystallization parameters relevant to fabrication of perovskite materials based on CH₃NH₃PbI₃ and CH₃NH₃PbBr₃. We find that the addition of PbCl₂ to the solutions used in the perovskite synthesis has a remarkable eff ect on the end product, because PbCl₂ nanocrystals are present during the fabrication process, acting as heterogeneous nucleation sites for the formation of perovskite crystals in solution. We base this conclusion on SEM studies, synthesis of perovskite single crystals, and on cryo-TEM imaging of the frozen mother liquid. Our studies also included the effect of different substrates and substrate temperatures on the perovskite nucleation efficiency. In view of our findings, we optimized the procedures for solar cells based on lead bromide perovskite, resulting in 5.4% efficiency and V_oc of 1.24 V, improving the performance in this class of devices. Insights gained from understanding the hybrid perovskite crystallization process can aid in rational design of the polycrystalline absorber films, leading to their enhanced performance.

Organometallic lead-halide perovskite-based solar cells now approach 18% efficiency. Introducing a mixture of bromide and iodide in the halide composition allows tuning of the optical bandgap. We prepare mixed bromide-iodide lead perovskite films CH₃NH₃Pb(I _1-xBr_x)₃ (0 ≤ x ≤ 1) by spin-coating from solution and obtain films with monotonically varying bandgaps across the full composition range. Photothermal deflection spectroscopy, photoluminescence, and X-ray diffraction show that following suitable fabrication protocols these mixed lead-halide perovskite films form a single phase. The optical absorption edge of the pure triiodide and tribromide perovskites is sharp with Urbach energies of 15 and 23 meV, respectively, and reaches a maximum of 90 meV for CH ₃NH₃PbI_1.2Br_1.8. We demonstrate a bromide-iodide lead perovskite film (CH₃NH₃PbI _1.2Br_1.8) with an optical bandgap of 1.94 eV, which is optimal for tandem cells of these materials with crystalline silicon devices.

The built-in voltage of poly(3,4-ethylenedioxythiophene): poly(styrenesulfonate)-nSi hybrid solar cells is demonstrated to indicate strong inversion over most substrate donor concentrations. This implies a p-n homojunction, induced in the Si surface by the high work function of the top contact. This induced homojunction is then used to form the source and drain electrodes in a field-effect transistor.

Electronic coupling to electrodes, G, as well as that across the examined molecules, H, is critical for solid-state electron transport (ETp) across proteins. Assessing the importance of each of these couplings helps to understand the mechanism of electron flow across molecules. We provide here experimental evidence for the importance of both couplings for solid-state ETp across the electron- mediating protein cytochrome c (CytC), measured in a monolayer configuration. Currents via CytC are temperature-independent between 30 and -130 K, consistent with tunneling by superexchange, and thermally activated at higher temperatures, ascribed to steady-state hopping. Covalent protein-electrode binding significantly increases G, as currents across CytC mutants, bound covalently to the electrode via a cysteine thiolate, are higher than those through electrostatically adsorbed CytC. Covalent binding also reduces the thermal activation energy, Ea, of the ETp by more than a factor of two. The importance of H was examined by using a series of seven CytC mutants with cysteine residues at different surface positions, yielding distinct electrode-protein(-heme) orientations and separation distances. We find that, in general, mutants with electrode-proximal heme have lower Ea values (from high-temperature data) and higher conductance at low temperatures (in the temperatureindependent regime) than those with a distal heme. We conclude that ETp across these mutants depends on the distance between the heme group and the top or bottom electrode, rather than on the total separation distance between electrodes (protein width).

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.

We report on the passivation properties of molecularly modified, oxide-free Si(111) surfaces. The reaction of 1-alcohol with the H-passivated Si(111) surface can follow two possible paths, nucleophilic substitution (S_N) and radical chain reaction (RCR), depending on adsorption conditions. Moderate heating leads to the S_N reaction, whereas with UV irradiation RCR dominates, with S_N as a secondary path. We show that the site-sensitive S_N reaction leads to better electrical passivation, as indicated by smaller surface band bending and a longer lifetime of minority carriers. However, the surface-insensitive RCR reaction leads to more dense monolayers and, therefore, to much better chemical stability, with lasting protection of the Si surface against oxidation. Thus, our study reveals an inherent dissonance between electrical and chemical passivation. Alkoxy monolayers, formed under UV irradiation, benefit, though, from both chemical and electronic passivation because under these conditions both S_N and RCR occur. This is reflected in longer minority carrier lifetimes, lower reverse currents in the dark, and improved photovoltaic performance, over what is obtained if only one of the mechanisms operates. These results show how chemical kinetics and reaction paths impact electronic properties at the device level. It further suggests an approach for effective passivation of other semiconductors.

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

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.

Electron transport (ETp) across bacteriorhodopsin (bR), a natural proton pump protein, in the solid state (dry) monolayer configuration, was studied as a function of temperature. Transport changes from thermally activated at T > 200 K to temperature independent at

We report near-perfect transfer of the electrical properties of oxide-free Si surface, modified by a molecular monolayer, to the interface of a junction made with that modified Si surface. Such behavior is highly unusual for a covalent, narrow bandgap semiconductor, such as Si. Short, ambient atmosphere, room temperature treatment of oxide-free Si(100) in hydroquinone (HQ)/alkyl alcohol solutions, fully passivates the Si surface, while allowing controlled change of the resulting surface potential. The junctions formed, upon contacting such surfaces with Hg, a metal that does not chemically interact with Si, follow the Schottky-Mott model for metal-semiconductor junctions closer than ever for Si-based junctions. Two examples of such ideal behavior are demonstrated: a) Tuning the molecular surface dipole over 400 mV, with only negligible band bending, by changing the alkyl chain length. Because of the excellent passivation this yields junctions with Hg with barrier heights that follow the change in the Si effective electron affinity nearly ideally. b) HQ/methanol passivation of Si is accompanied by a large surface dipole, which suffices, as interface dipole, to drive the Si into strong inversion as shown experimentally via its photovoltaic effect. With only ∼0.3 nm molecular interlayer between the metal and the Si, our results proves that it is passivation and prevention of metal-semiconductor interactions that allow ideal metal-semiconductor junction behavior, rather than an insulating transport barrier.

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.

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.

Despite many recent research efforts, the influence of grain boundaries (GBs) on device properties of CuIn_1-xGa_xSe₂ solar cells is still not fully understood Here, we present a microscopic approach to characterizing GBs in polycrystalline CuIn_1-xGa _xSe₂ films with x = 0.33. On samples from the same deposition process we applied methods giving complementary information, i.e., electron backscatter diffraction (EBSD), electron-beam induced current measurements (EBIC), conductive atomic force microscopy (c-AFM), variable-temperature Kelvin probe force microscopy (KPFM), and scanning capacitance microscopy (SCM). By combining EBIC with EBSD, we find a decrease in charge-carrier collection for non-σ3 GBs, while σ 3 GBs exhibit no variation with respect to grain interiors. In contrast, a higher conductance of GBs compared to grain interiors was found by c-AFM at low bias and under illumination. By KPFM, we directly measured the band bending at GBs, finding a variation from - 80 up to + 115 mV. Depletion and even inversion at GBs was confirmed by SCM. We comparatively discuss the apparent differences between the results obtained by various microscopic techniques.

Understanding how quantum dot (QD)-sensitized solar cells operate requires accurate determination of the offset between the lowest-unoccupied molecular orbital (LUMO) of the sensitizer quantum dot and the conduction band of the metal oxide electrode. We present detailed optical spectroscopy, low-energy photoelectron spectroscopy, and two-photon photoemission studies of the energetics of size-selected CdSe colloidal QDs deposited on TiO₂ electrodes. Our experimental findings show that in contrast to the prediction of simplified models based on bulk band offsets and effective mass considerations, band alignment in this system is strongly modified by the interaction between the QDs and the electrode. In particular, we find relatively small conduction band- LUMO offsets, and near "pinning" of the QD LUMO relative to the conduction band of the TiO₂ electrode, which is explained by the strongQD-electrode interaction. That interaction is the origin for the highly efficientQDto electrode charge transfer, and it also bears on the possibility of hot carrier injection in these types of cells.

The temperature dependence of current-voltage values of electron transport through proteins integrated into a solid-state junction has been investigated. These measurements were performed from 80 up to 400 K [above the denaturation temperature of azurin (Az)] using Si/Az/Au junctions that we have described previously. The current across the similar to 3.5 nm thick Az junction was temperature-independent over the complete range. In marked contrast, for both Zn-substituted and apo-Az (i.e., Cu-depleted Az), thermally activated behavior was observed. These striking temperature-dependence differences are ascribed to the pivotal function of the Cu ion as a redox center in the solid-state electron transport process. Thus, while Cu enabled temperature-independent electron transport, upon its removal the polypeptide was capable only of supporting thermally activated transport.

One of the major problems in molecular electronics is how to make electronically conducting contact to the "soft" organic and biomolecules without altering the molecules. As a result, only a small number of metals can be applied, mostly by special deposition methods with severe limitations. Transferring a predefined thin metal leaf onto a molecular layer provides a nondestructive, noninvasive contacting method that is, in principle, applicable to many types of metal and a variety of metal/molecules combinations. Here we report a modification of our earlier lift-off, float-on (LOFO) method, using as a basis its offspring, the polymer-assisted lift-off (PALO) method, where a backing polymer enables simultaneous deposition of multiple contacts as well as reduces wrinkles in the thin metal leaf. The modified PALO (MoPALO) method, reported here, adds lithography steps to obviate the need to punch through the polymer, as is done to complete PALO contacts. Morphological characterization of the electrodes indicates highly uniform, wrinkle-free contacts of negligible roughness. The good electrical performance of the MoPALO contacts was proven with metal/organic-monolayer/semiconductor (MOMS) junctions, which are known to be very sensitive to molecular degradation and metal penetration. We also show how MoPALO contacts enabled us to compare the effect of varying the metal work function and contact area on the current-voltage characteristics of MOMS devices.

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.

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.

The electronic band structure of different alkyl/Si(111) self-assembled monolayers (SAMs) was investigated using photoelectron spectroscopy (PES) with variable photon energy. We observe a significant dispersion in the valence-band spectra and a large density-of-states (DOS) effect. The dispersion can be described by quantum well states, which depend only on the local properties of the alkanes with a dispersion relation similar to polyethylene and without any significant influence of the Si/molecule interface. Furthermore, the DOS effect is due to averaging over molecules with different tilt angles and thus can be considered as indicator for the degree of orientational order within the SAM. Finally we present a structural model for a description of the PES data, which takes both aspects into account.

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.

A low-cost dichroic mirror can be used successfully for solar spectrum splitting to enhance solar to electrical energy conversion. The mirror is optimized for use with a polycrystalline silicon photovoltaic cell (pc-Si). With the dichroic mirror simultaneous excitation of a medium-efficient (11.1%) commercial pc-Si and a custom-made high band gap GaInP cell (12.3%), yields 16.8% efficiency, with both cells operating at maximum power. Our results clearly show that what is missing for this simple low-cost enhancement of Si solar cell efficiency are low-cost high band gap cells. (C) 2009 American Institute of Physics. [DOI: 10.1063/1.3081510]

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.

The interfacing of functional proteins with solid supports and the study of related protein-adsorption behavior are promising and important for potential device applications. In this study, we describe the preparation of bacteriorhodopsin (bR) monolayers on Br-terminated solid supports through covalent attachment. The bonding, by chemical reaction of the exposed free amine groups of bR with the pendant Br group of the chemically modified solid surface, was confirmed both by negative AFM results obtained when acetylated bR (instead of native bR) was used as a control and by weak bands observed at around 1610 cm^-1 in the FTIR spectrum. The coverage of the resultant bR monolayer was significantly increased by changing the pH of the purple-membrane suspension from 9.2 to 6.8. Although bR, which is an exceptionally stable protein, showed a pronounced loss of its photoactivity in these bR monolayers, it retained full photoactivity after covalent binding to Br-terminated alkyls in solution. Several characterization methods, including atomic force microscopy (AFM), contact potential difference (CPD) measurements, and UV/Vis and Fourier transform infrared (FTIR) spectroscopy, verified that these bR monolayers behaved significantly different from native bR. Current-voltage (I-V) measurements (and optical absorption spectroscopy) suggest that the retinal chromophore is probably still present in the protein, whereas the UV/Vis spectrum suggests that it lacks the characteristic covalent protonated Schiff base linkage. This finding sheds light on the unique interactions of biomolecules with solid surfaces and may be significant for the design of protein-containing device structures.

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.

The single-crystal outperformance of thin film polycrystalline (PX) CdTe/CdS solar cells was analyzed. The defect density of grains was reduced by gettering of defects at the grain boundaries (GB). Scanning capacitance microscopy (SCM) alongwith scanning Kelvin probe microscopy (SKPM) characterize the grain surface electrically. The results show that the separation and collection of photogenerated charge carriers was enhanced by CdTe GB.

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.

Using Hg/alkyl-chain-monolayer/p-Si devices we find that the type of contact between the chains and the electrodes (chemical bonding or not) is of critical importance for electronic transport across the junctions. As the semiconductor is p-type, the transport is that of holes. In agreement with theory we find that holes tunnel more efficiently through alkyl chains than do electrons.

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.

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.

The chemical effects of oxygenation of Cu(In,Ga)Se₂ (CIGS) interfaces are analyzed and are shown to involve passivation of Se deficiencies and Cu removal. The former effect is beneficial at grain boundaries, but detrimental at the CdS/CIGS interface. The latter effect is purely detrimental. Na and chemical bath deposition (CBD) treatments are shown to isolate the `good' oxygenation effect from the `bad' ones. Na is shown to promote oxygenation already before the deposition of the buffer and window layers, which allows a maximization of the benefits of Se deficiency passivation and a minimization of Cu removal. Next, the CBD of the CdS buffer layer restores the interface charge, due to creation of Cd_Cu interface donors and possibly a removal of O_Se interface acceptors. This highlights the crucial role that interface redox engineering plays in optimizing the performance of CIGS-based solar cells.

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.

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.