Publications

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.

The inorganic lead halide perovskite CsPbBr3 promises similar solar cell efficiency to its hybrid organic-inorganic counterpart CH3NH3PbBr3 but shows greater stability. Here, we exploit this stability for the study of band alignment between perovskites and carrier selective interlayers. Using ultraviolet, X-ray, and inverse photoemission spectroscopies, we measure the ionization energy and electron affinities of CsPbBr3 and the hole transport polymer polytriarylamine (PTAA). We find that undoped PTAA introduces a barrier to hole extraction of 0.2-0.5 eV, due to band bending in the PTAA and/or a dipole at the interface. p-doping the PTAA eliminates this barrier, raising PTAA's highest occupied molecular orbital to 0.2 eV above the CsPbBr3 valence band maximum and improving hole transport. However, IPES reveals the presence of states below the PTAA lowest unoccupied molecular level. If present at the CsPbBr3/PTAA interface, these states may limit the polymer's efficacy at blocking electrons in solar cells with wide band gap materials like CsPbBr3 and CH3NH3PbBr3. Published by AIP Publishing.

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.

We present a measurement of the energies and capture cross-sections of defect states in methylammonium lead bromide (MAPbBr(3)) 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. Published by AIP Publishing.

The realization of high-quality optoelectronic properties in halide perovskite semiconductors through low-temperature, low energy processing is unprecedented. Understanding the unique aspects of the formation chemistry of these semiconductors is a critical step toward understanding the genesis of high quality material via simple preparation procedures. The toolbox of preparation procedures for halide perovskites grows rapidly. The prototypical reaction is that between lead iodide (PbI2) and methylammonium iodide (CH3NH3I, abbr. MAI) to form the perovskite CH3NH3PbI3 (MAPbI(3)), which we discuss in this work. We investigate the conversion of small, single-crystalline PbI2 crystallites to MAPbI(3) by two commonly used synthesis processes: reaction with MAI in solution or as a vapor. The single crystal nature of the PbI2 precursor allows definitive conclusions to be made about the relationship between the precursors and the final product, illuminating previously unobserved aspects of the reaction process. From in situ photoluminescence microscopy, we find that the reaction in solution begins via isolated nucleation events followed by growth from the nuclei. We observe via X-ray diffraction and morphological characterization that there is a strong orientational and structural relationship between the final stage of the solution-reacted MAPbI(3) product and the initial PbI2 crystallite. In all these measurements, we find that the reaction does not proceed below a certain MAI threshold concentration, which allows the first experimental determination of a free energy of formation for a widely used synthetic procedure of similar to 0.1 eV. From these conclusions, we present a more detailed hypothesis about the reaction pathway than has yet been proposed: Our results suggest that the reaction in solution begins with a topotactic nucleation event followed by grain growth by dissolution-reconstruction. By similar techniques, we find the reaction via vapor phase produces

Photovoltaic solar cells operate under steady-state conditions that are established during the charge carrier excitation and recombination. However, to date no model of the steady-state recombination scenario in halide perovskites has been proposed. In this Letter we present such a model that is based on a single type of recombination center, which is deduced from our measurements of the illumination intensity dependence of the photoconductivity and the ambipolar diffusion length in those materials. The relation between the present results and those from time-resolved measurements, such as photoluminescence that are commonly reported in the literature, is discussed.

Advances in Perovskite Solar Cells

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.

CONSPECTUS: Hybrid 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

To experimentally (dis)prove ferroelectric effects on the properties of lead-halide perovskites and of solar cells, based on them, we used second-harmonic-generation spectroscopy and the periodic temperature change (Chynoweth) technique to detect the polar nature of methylammonium lead bromide (MAPbBr 3). We find that MAPbBr 3 is probably centrosymmetric and definitely non-polar; thus, it cannot be ferroelectric. Whenever pyroelectric-like signals were detected, they could be shown to be due to trapped charges, likely at the interface between the metal electrode and the MAPbBr 3 semiconductor. These results indicate that the ferroelectric effects do not affect steady-state performance of MAPbBr 3 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 (MAPbI(3), MAPbBr(3), 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.

We observe temperature-independent electron transport, characteristic of tunneling across a approximately 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 approximately 4 nm wire length. In the approximately 6 nm long phR, the approximately 4 nm 50-carbon conjugated bacterioruberin is bound parallel to the alpha-helices of the peptide backbone. This places bacterioruberin's 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.

2015 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.Electron transport (ETp) across met-myoglobin (m-Mb), as measured in a solid-state-like configuration between two electronic contacts, increases by up to 20 fold if Mb is covalently bound to one of the contacts, a Si electrode, in an oriented manner by its hemin (ferric) group, rather than in a non-oriented manner. Oriented binding of Mb is achieved by covalently binding hemin molecules to form a monolayer on the Si electrode, followed by reconstitution with apo-Mb. We found that the ETp temperature dependence (>120?K) of non-oriented m-Mb virtually disappears when bound in an oriented manner by the hemin group. Our results highlight that combining direct chemical coupling of the protein to one of the electrodes with uniform protein orientation strongly improves the efficiency of ET across the protein. We hypothesize that the behavior of reconstituted m-Mb is due to both strong protein-substrate electronic coupling (which is likely greater than in non-oriented m-Mb) and direct access to a highly efficient transport path provided by the hemin group in this configuration.

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. 2015 Elsevier B.V.

A small molecule based on N,N'-dialkyl perylenediimide (PDI) as core derivatized with thiophene moieties (Th-PDI) was synthesized. Its HOMO (highest occupied molecular orbital) level was measured to be between 5.7 and 6.3 eV vs. local vacuum level depending on doping and measurement method. Th-PDI was successfully applied as hole-transporting material (HTM) in CH3NH3PbBr3 hybrid perovskite solar cells. Three different cell architectures, each with a different mode of operation, were tested: (1) using a mesoporous (mp) TiO2 substrate; (2) mp-Al2O3 substrate; (3) planar dense TiO2 substrate. The first gave the best overall efficiency of 5.6% while the mp-Al2O3 gave higher open-circuit photovoltage (VOC) but lower efficiency (2.2%). The cells exhibited good reproducibility with very little J-V hysteresis (the mp-Al2O3 showed a more appreciable hysteresis of individual photovoltaic parameters but little dependence of efficiency on scan direction). Storage of unencapsulated cells in 25-30% relative humidity demonstrated fairly good stability with

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

Electron transfer (ET) proteins are biomolecules with specific functions, selected by evolution. As such they are attractive candidates for use in potential bioelectronic devices. The blue copper protein azurin (Az) is one of the most-studied ET proteins. Traditional spectroscopic, electrochemical, and kinetic methods employed for studying ET to/from the protein's Cu ion have been complemented more recently by studies of electrical conduction through a monolayer of Az in the solid-state, sandwiched between electrodes. As the latter type of measurement does not require involvement of a redox process, it also allows monitoring electronic transport (ETp) via redox-inactive Az-derivatives. Here, results of macroscopic ETp via redox-active and -inactive Az derivatives, i.e., Cu(II) and Cu(I)-Az, apo-Az, Co(II)-Az, Ni(II)-Az, and Zn(II)Az are reported and compared. It is found that earlier reported temperature independence of ETp via Cu(II)-Az (from 20 K until denaturation) is unique, as ETp via all other derivatives is thermally activated at temperatures >approximate to 200 K. Conduction via Cu(I)-Az shows unexpected temperature dependence >approximate to 200 K, with currents decreasing at positive and increasing at negative bias. Taking all the data together we find a clear compensation effect of Az conduction around the Az denaturation temperature. This compensation can be understood by viewing the Az binding site as an electron trap, unless occupied by Cu(II), as in the native protein, with conduction of the native protein setting the upper transport efficiency limit.

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.

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 CH3NH3PbI3-xClx 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 diffusion length. In this work we investigate the crystallization parameters relevant to fabrication of perovskite materials based on CH(3)NH(3)Pbl(3) and CH3NH3PbBr3. We find that the addition of PbCl2 to the solutions used in the perovskite synthesis has a remarkable effect 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 perovsldte, 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.

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.

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.

Reproducible molecular junctions can be integrated within standard CMOS technology. Metal-molecule-semiconductor junctions are fabricated by direct Si-C binding of hexadecane or methyl-styrene onto oxide-free H-Si(111) surfaces, with the lateral size of the junctions defined by an etched SiO2 well and with evaporated Pb as the top contact. The current density, J, is highly reproducible with a standard deviation in log(J) of 0.2 over a junction diameter change from 3 to 100 mu m. Reproducibility over such a large range indicates that transport is truly across the molecules and does not result from artifacts like edge effects or defects in the molecular monolayer. Device fabrication is tested for two n-Si doping levels. With highly doped Si, transport is dominated by tunneling and reveals sharp conductance onsets at room temperature. Using the temperature dependence of current across medium-doped n-Si, the molecular tunneling barrier can be separated from the Si-Schottky one, which is a 0.47 eV, in agreement with the molecular-modified surface dipole and quite different from the bare Si-H junction. This indicates that Pb evaporation does not cause significant chemical changes to the molecules. The ability to manufacture reliable devices constitutes important progress toward possible future hybrid Si-based molecular electronics.

A distinct odd even effect on the electrical properties, induced by monolayers of alkyl-phenyl molecules directly bound to Si(111), is reported. Monomers of H2C=CH- Even (CH2) phenyl, with n = 2-5, were adsorbed onto Si I-I and formed high-quality monolayers with a binding density of 50-60% Si(111) surface atoms. Molecular dynamics simulations suggest that the binding proximity is close enough to allow efficient /r,r interactions and therefore distinctly different packing and ring orientations for monomers with odd or even numbers of rnethylenes in their alkyl spacers. The odd even alternation in molecular tilt was experimentally confirmed by con. tact ellipsometry, and XPS with a close quantitative matc,t to the simulation results. The orientation. an even s of bot h ring ng p ane and the long axis f the alkyl spacer are more perpendicular pi-pi the with substrate lane for molecules number of niethylenes than for those with an odd number ofi7rriethilYlenesd. with Interestingly! those th an even number conduct better than the effectively thinner mono.laYers Of the Plialeatrin_s le with respect odd number of inethylenes. We attribute this to a change in the orientation of the electron density,on the aromatic to the shortest tunneling path, which increases the harrier for electron transport through the Odd monolaye,rsof,The hig h sensitivity of molecular charge transport o orientation t the oentation of an aromatic moiety might be relevant to better control over th electronic properties of interfaces n organic electronics.

In recent years, conducting and semiconducting polymers such as PEDOT:PSS and P3HT have become commercially available, and as a result, a new type of polymer/Si heterostructure solar cell is emerging. With a conducting polymer (a degenerate semiconductor) as emitter, such an organic/inorganic hybrid heterojunction is likely to achieve high conversion efficiencies only if the inorganic semiconductor is pushed into strong inversion to reduce dramatically the space-charge recombination and to mitigate the poor lateral conductance of the polymeric layer. We explain this notion through a review of the types of solar cells based on an inversion layer, induced in the semiconductor absorber by a metal, by a dielectricmaterial with fixed charges, or by another semiconductor. In these types, which include the metal-insulator-semiconductor (MIS), semiconductor-insulator-semiconductor, and MIS inversion layer solar cells, interfaces play a crucial role, even more so than in other forms of solid-state photovoltaics. We also point out the strategy by which atomic-layer-deposited Al2O3 can be used to form an inversion layer solar cell on an n-Si emitter.

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 similar to 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.

To understand the title topic a model system of single crystal SiC, modified with an interfacial molecular monolayer of alkyl siloxane molecules, with polycrystalline pentacene deposited on it, was fabricated. In this way a change in the length of the alkyl chain could change the structural order of the pentacene film by changing the surface's hydrophobicity, while no significant variation was found in the surface potential, and, thus, in the surface dipole. The pentacene film grown on top of the monolayers showed, with increasing alkyl chain length, increased lateral order and decreased band gap state density, as observed by X-ray diffraction and surface photovoltage spectroscopy. The V-oc, J(sc) and fill factor of solar cells, made with these material combinations, improved with increasing alkyl chain length. We explain this as a result of increased 2D film growth with increasing alkyl chain length of the monolayer, as the surface becomes more hydrophobic, which increases ordering of the pentacene film. Thus, this model system illustrates the role of ordering in charge separation and recombination.

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.

Charge separation at organic-organic (O-O) interfaces is crucial to how many organic-based optoelectronic devices function. However, the mechanism of formation of spatially separated charge carriers and the role of geminate recombination remain topics of discussion and research. We review critically the contributions of the various factors, including electric fields, long-range order, and excess energy (beyond the minimum needed for photoexcitation), to the probability that photogenerated charge carriers will be separated. Understanding the processes occurring at the O/O interface and their relative importance for effective charge separation is crucial to design efficient solar cells and photodetectors. We stress that electron and hole delocalization after photoinduced charge transfer at the interface is important for efficient free carrier generation. Fewer defects at the interface and long-range order in the materials also improve overall current efficiency in solar cells. In efficient organic cells, external electric fields play only a small role for charge separation.

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 SN reaction, whereas with UV irradiation RCR dominates, with SN as a secondary path. We show that the site-sensitive SN 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 SN 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.

Transition voltage spectroscopy (TVS) has become an accepted quantification tool for molecular transport characteristics, due to its simplicity and reproducibility. Alternatively, the Taylor expansion view, TyEx, of transport by tunneling suggests that conductance-voltage curves have approximately a generic parabolic shape, regardless of whether the tunneling model is derived from an average medium view (e.g., WKB) or from a scattering view (e.g., Landauer). Comparing TVS and TyEx approaches reveals that TVS is closely related to a bias-scaling factor, V-0, which is directly derived from the third coefficient of TyEx, namely, the second derivative of the conductance with respect to bias at 0 V. This interpretation of TVS leads to simple expressions that can be compared easily across primarily different tunneling models. Because the basic curve shape is mostly generic, the quality of model fitting is not informative on the actual tunneling model. However internal correlation between the conductance near 0 V and V-0 (TVS) provides genuine indication on fundamental tunneling features. Furthermore, we show that the prevailing concept that V-0 is proportional to the barrier height holds only in the case of resonant tunneling, while for off-resonant or deep tunneling, V-0 is proportional to the ratio of barrier height to barrier width. Finally, considering TVS as a measure of conductance nonlinearity, rather than as an indicator for energy level spectroscopy, explains the very low TVS values observed with a semiconducting (instead of metal) electrode, where transport is highly nonlinear due to the relatively small, bias-dependent density of states of the semiconducting electrode.

Rapid changes in energy availability lead to the question of whether the sustainable availability of energy implies the sustainable availability of materials and vice versa. In particular, many researchers assume that materials can be produced from any resource type, irrespective of scarcity, by providing enough energy. We revisit this issue here for two reasons: (1) To avoid significant disruptions in daily life, no more than a few percent of total energy production and materials usage can be diverted to support a transition to new energy sources. (2) Such a transition could also be problematic if it requires large quantities of materials that are byproducts of other large-scale production cycles, as any increase in the production of a byproduct typically requires an almost proportional increase in the production of the primary product. In turn, increased production of the primary product could require materials and energy expenditures that are too large to be practical. Both limitations have to be taken into account in future energy planning.

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.

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, beta, while for "short" AP ML junctions beta 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.

Electrical transport studies across nm-thick dielectric films can be complicated, and datasets compromised, by local electrical breakdown enhanced by nm-sized features. To avoid this problem we need to know the minimal voltage that causes the enhanced electrical breakdown, a task that usually requires numerous measurements and simulation of which is not trivial. Here we describe and use a model system, using a "floating'' gold pad to contact Au nanoparticles, NPs, to simultaneously measure numerous junctions with high aspect ratio NP contacts, with a dielectric film, thus revealing the lowest electrical breakdown voltage of a specific dielectric-nanocontact combination. For a 48 +/- 1.5 angstrom SiO2 layer and a similar to 7 angstrom monolayer of organic molecules (to link the Au NPs) we show how the breakdown voltage decreases from 4.5 +/- 0.4 V for a flat contact, to 2.4 +/- 0.4 V if 5 nm Au NPs are introduced on the surface. The fact that larger Au NPs on the surface do not necessarily result in significantly higher breakdown voltages illustrates the need for combining experiments with model calculations. This combination shows two opposite effects of increasing the particle size, i.e., increase in defect density in the insulator and decrease in electric field strength. Understanding the process then explains why these systems are vulnerable to electrical breakdown as a result of spikes in regular electrical grids. Finally we use XPS-based chemically resolved electrical measurements to confirm that breakdown occurs indeed right below the nm-sized features.

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, Lip 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 Lip 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.

Since the first report of Si-C bound organic monolayers on oxide-free Si almost two decades ago, a substantial amount of research has focused on studying the fundamental mechanical and electronic properties of these Si/molecule surfaces and interfaces. This feature article covers three closely related topics, including recent advances in achieving high-density organic monolayers (i.e., atomic coverage >55%) on oxide-free Si(111) substrates, an overview of progress in the fundamental understanding of the energetics and electronic properties of hybrid Si/molecule systems, and a brief summary of recent examples of subsequent functionalization on these high-density monolayers, which can significantly expand the range of applicability. Taken together, these topics provide an overview of the present status of this active area of research.

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 strong QD-electrode interaction. That interaction is the origin for the highly efficient QD to electrode charge transfer, and it also bears on the possibility of hot carrier injection in these types of cells.

Despite many recent research efforts, the influence of grain boundaries (GBs) on device properties of CuIn1-xGaxSe2 solar cells is still not fully understood Here, we present a microscopic approach to characterizing GBs in polycrystalline CuIn1-xGaxSe2 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-Sigma 3 GBs, while Sigma 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. (C) 2011 Elsevier B.V. All rights reserved.

Basic scientific interest in using a semiconducting electrode in molecule- based electronics arises from the rich electrostatic landscape presented by semiconductor interfaces. Technological interest rests on the promise that combining existing semiconductor (primarily Si) electronics with (mostly organic) molecules will result in a whole that is larger than the sum of its parts. Such a hybrid approach appears presently particularly relevant for sensors and photovoltaics. Semiconductors, especially Si, present an important experimental test-bed for assessing electronic transport behavior of molecules, because they allow varying the critical interface energetics without out, to a first approximation, altering the interfacial chemistry. To investigate semiconductor-molecule electronics we need reproducible, high-yield preparations of samples that allow reliable and reproducible data collection. Only in that way can we explore how the molecule/electrode interfaces affect or even dictate charge transport, which may then provide a basis for models with predictive power. To consider these issues and questions we will, in this Progress Report, review junctions based on direct bonding of molecules to oxide-free Si. describe the possible charge transport mechanisms across such interfaces and evaluate in how far they can be quantified. investigate to what extent imperfections in the monolayer are important for transport across the monolayer. revisit the concept of energy levels in such hybrid systems.

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 tran

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

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.

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

Temperature-depen dent 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.

Electronic current transport through alkoxy and alkyl monolayer-based junctions is presented. Monolayers are prepared on n-Si(100) with sufficiently high quality to reliably investigate the actual molecular effect of each monolayer on their current-voltage characteristics. The results show that extending the Si-binding chemistry from alkene to alcohol is feasible which should significantly facilitate preparation of monolayers with modified molecules.

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.

Using a dense organic monolayer, self-assembled and directly bount to n-Si, as high quality insulator with a thickness that can be varied from 1.5-2.5 nm, we construct a Metal-organic Insulator-Semiconductor (MOIS) structure, which, if fabricated with semi-transparent top electrode, performs as ahybrid organic-inorganic photovoltaic device. The feasibility of the concept and the electrical properties of the insulating layer were first shown with a Hg top electrode, allowing use of prior know-how from electron transport through molecular monolayers, but with photon collection only from around the electrode. We then used another bottom-up fabrication technique, in addition to molecular self-deposition yields an electrically continuous, porous and semi-transparent top electrode, improving photon harvesting. Aside form being a nearly ideal insulator, the monolayer acts to passivate and protect the interfacial Si layer from defects and to decrease the surface state density. In addition the cell, like any MIS solar cell, benefits from that the light needs only to cross a few thin transparent layers (anti-reflective coating, organic insulator) to reach the photovoltaically active cell part. This helps to generate carriers close to the junction area, even by short wavelength photons, and, thus, to increase light collection, compared to p-n junction solar cells. While probably the most important use of MOIS cells will be to allow systematic exploration of directions for general MIS solar cell optimization, low temperature cell fabrication without high vaccum steps, may make this approach also interesting for low cost solar cells.

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 14 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

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 An 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.

A series of p- and n- GaAs- S- C(n)H(2)n(+1) vertical bar vertical bar Hg junctions are prepared, and the electronic transport through them is measured. From current-voltage measurements, we find that, for n- GaAs, transport occurs by both thermionic emission and tunneling, with the former dominating at low forward bias and the latter dominating at higher forward bias. For p- GaAs, tunneling dominates at all bias voltages. By combining the analysis of the transport data with results from direct and inverse photoemission spectroscopy, we deduce an energy band diagram of the system, including the tunnel barrier and, with this barrier and within the Simmons tunneling model, extract an effective mass value of 1.5 - 1.6me for the electronic carriers that cross the junctions. We find that transport is well- described by lowest unoccupied and highest occupied states at 1.3 - 1.4 eV above and 2.0 - 2.2 eV below the Fermi level. At the same time, the photoemission data indicate that there are continua of states from the conduction band minimum and the valence band maximum, the density of which varies with energy. On the basis of our results, it appears likely that, for both types of junctions, electrons are the main carrier type, although holes may contribute significantly to the transport in the p- GaAs system.

Studying electron transport (ET) through proteins is hampered by achieving reproducible experimental configurations, particularly electronic contacts to the proteins. The transmembrane protein bacteriorhoclopsin (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.

We study how partial monolayers of molecular dipoles at semiconductor/metal interfaces can affect electrical transport across these interfaces, using a series of molecules with systematically varying dipole moment, adsorbed on n-GaAs, prior to Au or Pd metal contact deposition, by indirect evaporation or as "ready-made" pads. From analyses of the molecularly modified surfaces, we find that molecular coverage is poorer on low-than on high-doped n-GaAs. Electrical charge transport across the resulting interfaces was studied by current-voltage-temperature, internal photoemission, and capacitance-voltage measurements. The data were analyzed and compared with numerical simulations of interfaces that present inhomogeneous barriers for electron transport across them. For high-doped GaAs, we confirm that only the former, molecular dipole-dependent barrier is found. Although no clear molecular effects appear to exist with low-doped n-GaAs, those data are well explained by two coexisting barriers for electron transport, one with clear systematic dependence on molecular dipole (molecule-controlled regions) and a constant one (molecule-free regions, pinholes). This explains why directly observable molecular control over the barrier height is found with high-doped GaAs: there, the monolayer pinholes are small enough for their electronic effect not to be felt (they are "pinched off"). We conclude that molecules can control and tailor electronic devices need not form high-quality monolayers, bind chemically to both electrodes, or form multilayers to achieve complete surface coverage. Furthermore, the problem of stability during electron transport is significantly alleviated with molecular control via partial molecule coverage, as most current flows now between, rather than via, the molecules.

We study the effect of monolayer quality on the electrical transport through n-Si/CnH2n+1/Hg junctions ( n = 12, 14, and 18) and find that truly high quality layers and only they, yield the type of data, reported by us in Phys. ReV. Lett. 2005, 95, 266807, data that are consistent with the theoretically predicted behavior of a Schottky barrier coupled to a tunnel barrier. By using that agreement as our starting point, we can assess the effects of changing the quality of the alkyl monolayers, as judged from ellipsometer, contact angle, XPS, and ATR-FTIR measurements, on the electrical transport. Although low monolayer quality layers are easily identified by one or more of those characterization tools, as well as from the current-voltage measurements, even a combination of characterization techniques may not suffice to distinguish between monolayers with minor differences in quality, which, nevertheless, are evident in the transport measurement. The thermionic emission mechanism, which in these systems dominates at low forward bias, is the one that is most sensitive to monolayer quality. It serves thus as the best quality control. This is important because, even where tunneling characteristics appear rather insensitive to slightly diminished quality, their correct analysis will be affected, especially if layers of different lengths are also of different quality.

We show that the electrode/molecule chemical bond does not change the tunneling barrier for charge transport through alkyl chain monolayers sandwiched between Si and Hg electrodes. This observation can be understood if the interfacial bond mainly governs the monolayer's structure, while the electronic states due to molecule-electrode bonding do not contribute significantly to tunneling. Yet, the nature of the bond affects the Schottky barrier inside the semiconductor due to changes in the interface dipole.

Two-dimensional arrangements of molecules can show remarkable cooperative electronic 1771 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.

Molecular modification of dye-sensitized, mesoporous TiO2 electrodes changes their electronic properties. We show that the open-circuit voltage (V-oc) of dye-sensitized solar cells varies linearly with the dipole moment of coadsorbed phosphonic, benzoic, and dicarboxylic acid derivatives. A similar dependence is observed for the short-circuit current density (I-sc). Photovoltage spectroscopy measurements show a shift of the signal onset as a function of dipole moment. We explain the dipole dependence of the V-oc in terms of a TiO2 conduction band shift with respect to the redox potential of the electrolyte, which is partially followed by the energy level of the dye. The I-sc shift is explained by a dipole-dependent driving force for the electron current and a dipole-dependent recombination current.

Indirect e-beam evaporation of metal on a cooled substrate that allows making reproducible and gentle electrical contact to molecular films of organic molecules yields strikingly different results with Pd and Au. This is attributed to different growth modes of the metals, which lead to different molecule/metal interactions and to Au penetration in between the molecules. These differences can radically change the effect of the molecules on the resulting junctions. (C) 2005 American Institute of Physics.

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-oc) 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.

Molecular control over charge transport across a metal/semiconductor interface persists even if there is only a partial monolayer of polar molecules at the interface. This is because the long-range electrostatic effect of the dipole layer also affects the semiconductor regions under the film's pinholes. Thus, all types of polar molecules that show average order at the interface can be used.

Electronic mapping of cross sections of a polycrystalline device, the n-CdS/p-CdTe solar cell, show that the photovoltaic and metallurgical junctions coincide to within experimental resolution (50 nm), which rules out both type conversion of CdS and buried homojunctions. Compositional analysis of the CdS supports this. Mapping was done using scanning capacitance, complemented by scanning Kelvin probe microscopy. Our results explain why a high-resistance transparent conducting oxide layer is needed as contact to the CdS for successful device operation. They define limits on inputs for modeling performance of these devices. (C) 2003 American Institute of Physics.

CdTe/CdS solar cells were subjected to heat stress at 200 degreesC in the dark under different environments (in N-2 and in air), and under illumination (in N-2). We postulate that two independent mechanisms can explain degradation phenomena in these cells: i) Excessive Cu doping of CdS: Accumulation of Cu in the CdS with stress, in the presence of Cl, will increase the photoconductivity of CdS. With limited amounts of Cu in CdS, this does NOT affect the photovoltaic behavior, but explains the crossover of light/dark current-voltage (J-V) curves. Overdoping of CdS with Cu can be detrimental to cell performance by creating deep acceptor states, acting as recombination centers, and compensating donor states. Under illumination, the barrier to Cu cations at the cell junction is reduced, and, therefore, Cu accumulation in the CdS is enhanced. Recovery of light-stress induced degradation in CdTe/CdS cells in the dark is explained by dissociation of the acceptor defects. ii) Back contact barrier: Oxidation of the CdTe back surface in O-2/H2O-containing environment to form an insulating oxide results in a back-contact barrier. This barrier is expressed by a rollover in the J-V curve. Humidity is an important factor in air-induced degradation, as it accelerates the oxide formation. Heat treatment in the dark in inert atmosphere can stabilize the cells against certain causes of degradation, by completing the back contact anneal.

Differences between junctions of metals on ionic or covalent semiconductors persist for junctions, prepared by wet solution methods with a molecular layer at the junctions' interface. A series of molecules that controls the junction of Au with n-GaAs, does so even stronger with ZnO (300 instead of similar to100 mV barrier height change). With ZnO the interface behavior index is found to be 0.55, five times that with GaAs. This agrees remarkably well with results for junctions of these materials with different metals, prepared in ultrahigh vacuum. Thus, the free semiconductor surface, e.g., surface state density, rather than direct metal-semiconductor interactions, appears to dominate junction behavior. (C) 2003 American Institute of Physics.

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/alkylthiolsip-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 cur-rent 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 (less than or equal toC(12)), resulting in an additional route to charge transport by tunneling through space. In contrast, long chains (greater than or equal toC(14)) 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.

Grafting organic molecules onto solid surfaces can transfer molecular properties to the solid. We describe how modifications of semiconductor or metal surfaces by molecules with systematically varying properties can lead to corresponding trends in the (electronic) properties of the resulting hybrid (molecule + solid) materials and devices made with them. Examples include molecule-controlled diodes and sensors, where the electrons need not to go through the molecules (action at a distance), suggesting a new approach to molecule-based electronics.

This article examines a somewhat counter-intuitive approach to molecular-based electronic devices. Control over the electronic energy levels at the surfaces of conventional semiconductors and metals is achieved by assembling on the solid surfaces, poorly organized, partial monolayers (MLs) of molecules instead of the more commonly used ideal ones. Once those surfaces become interfaces, these layers exert electrostatic rather than electrodynamic control over the resulting devices, based on both electrical monopole and dipole effects of the molecules. Thus electronic transport devices, incorporating molecules, can be constructed without current flow through the molecules. This is illustrated for a gallium arsenide (GaAs) sensor as well as for gold-silicon (Au-Si) and Au-GaAs diodes. Incorporating molecules into solid interfaces becomes possible, using a 'soft' electrical contacting procedure, so as not to damage the molecules. Because there are only a few molecular restrictions, this approach opens up possibilities for the use of more complex (including biologically active) molecules as it circumvents requirements for ideal MLs and for molecules that can tolerate actual electron transport through them.

The electron affinity of semiconductors and the work function of metals can be systematically varied by the adsorption of a series of organic molecules with varying dipole moment. This effect can be used to induce molecular control over the electrical characteristics of some metal/semiconductor diodes. The variety of molecules that can be bonded to metals, together with those that are available for bonding to semiconductors, makes the method quite flexible and may enable the build up of more sophisticated interfacial structures.

We suggest an alternative technique for electroluminescent device fabrication, based on our earlier findings of electric field (E-field)-induced bipolar transistor creation in Si, doped with Li. An external electric field served to induce Irm sized electroluminescent device structures in Si, that had been doped prior to E-field application, with Li, and Er via thermal in-diffusion. Such devices exhibit low temperature, near infrared (IR) electroluminescence at similar to 1.16 and 1.55 mum, corresponding to transitions associated with Li and Er levels, respectively, in the forbidden gap. While Li also creates radiative recombination centers in Si, the Er-based IR radiation is the most desirable one. At the same time Li-doping is what makes E-field-induced p-n junction fabrication possible. (C) 2001 Elsevier Science B.V. All rights reserved.

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 ECB 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 ECB 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.

Nitric oxide (NO) acts as a signal molecule in the nervous system, as a defense against infections, as a regulator of blood pressure, and as a gate keeper of blood flow to different organs. In vivo, it is thought to have a lifetime of a few seconds. Therefore, its direct detection at low concentrations is difficult. Mie report on a new type of hybrid, organic-semiconductor, electronic sen-sor that makes detection of nitric oxide in physiological solution possible. The mode of action of the device is described to explain how its electrical resistivity changes as a result of NO binding to a layer of native hemin molecules. These molecules are self-assembled on a GaAs surface to which they are attached through a carboxylate binding group. The new sensor provides a fast and simple method for directly detecting NO at concentrations down to 1 muM in physiological aqueous (pH=7.4) solution at room temperature.

We explain the cause for the photocurrent and photovoltage in nanocrystalline, mesoporous dye-sensitized solar cells, in terms of the separation, recombination, and transport of electronic charge as well as in terms of electron energetics. On the basis of available experimental data, we confirm that the basic cause for the photovoltage is the change in the electron concentration in the nanocrystalline electron conductor that results from photoinduced charge injection from the dye. The maximum photovoltage is given by the difference in electron energies between the redox level and the bottom of the electron conductor's conduction band, rather than by any difference in electrical potential in the cell, in the dark. Charge separation occurs because of the energetic and entropic driving forces that exist at the dye/electron conductor interface, with charge transport aided by such driving forces at the electron conductor/contact interface. The mesoporosity and nanocrystallinity of the semiconductor are important not only because of the large amount of dye that can be adsorbed on the system's very. large surface, but also for two additional reasons: (1) it allows the semiconductor small particles to become almost totally depleted upon immersion in the electrolyte (allowing for large photovoltages), and (2) the proximity of the electrolyte to all particles makes screening of injected electrons, and thus their transport, possible.

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

Stability aspects of the Mo/Cu(In,Ga)Se-2/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)Se-2 (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.

CuInSe2 is a semiconductor used in solar cells that has appreciable Cu ion conductivity. These authors have used 0.25 Angstrom synchrotron X-radiation to analyze its structure. It is shown that the application of a strong electric field leads to a decrease in electron density on the Cu sites, which sheds light on the role of Cu electromigration in transistor formation (see Figure) and in the photovoltaic activity of CuInSe2.

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.

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-1n and the defect pair (2V(Cu)-In-Cu) occur in the investigated specimens in considerable concentrations. (C) 2000 Elsevier Science S.A. All rights reserved.

We show how phase separation, in the form of a redistribution of impurities (dopants in a semiconductor), can occur at impurity concentrations that are more than one order of magnitude lower than hitherto observed. This phenomenon results from the balance between long-range electrostatic repulsion and the elastic attraction of the dopants, which deforms the anisotropic host lattice. We observed such a phase separation for Ag in (Cd, Hg)Te at Ag concentrations <0.02 at. %. This also leads to the formation of a thermodynamically (as opposed to kinetically) stable p-n junction in the a-phase region. Searching for phase separation at such low concentrations requires highly sensitive analyses, here made possible because of the difference in conductivity type between the phases.

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 M Omega cm), following annealing in the temperature range of 350-1000 degrees C. Resulting crystals showed n-type conductivity with a free carrier concentration in the range: of 10(15)-10(18) cm(-3) and carrier mobility of 100-750 cm(2)/(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 degrees C, the In and the free electron concentrations are roughly the same. (C) 1999 Elsevier Science B.V. All rights reserved.

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) <0.8. The 10(7) times higher diffusivity at x(Cd) <0.8 can be understood by realizing that the percolation threshold for an ideal FCC lattice is at 0.19. While normally Ag is a donor, it can be an acceptor by stabilizing the Hg(I) state. This is possible by interaction with 2 Hg neighbors, a process that will be favorable above the Hg percolation limit. The fast Ag diffusion also holds the clue for the occurrence of ultra-low concentration phase separation in this system, the result of a balance between elastic attraction and Coulombic repulsion between the charged dopants. Prima facie evidence for this phase separation comes from coulometric Ag titration in and out of MCT. (C) 1999 Elsevier Science B.V. All rights reserved.

We present "design rules" for the selection of molecules to achieve electronic control over semiconductor surfaces, using a simple molecular orbital model. The performance of most electronic devices depends critically on their surface electronic properties, i.e., surface band-bending and surface recombination velocity. For semiconductors, these properties depend on the density and energy distribution of surface states. The model is based on a surface state-molecule, HOMO-LUMO-Like interaction between molecule and semiconductor. We test it by using a combination of contact potential difference, surface photovoltage spectroscopy, and time- and intensity-resolved photoluminescence measurements. With these, we characterize the interaction of two types of bifunctional dicarboxylic acids, the frontier orbital energy levels of which can be changed systematically, with air-exposed CdTe, CdSe, InP, and GaAs surfaces. The molecules are chemisorbed as monolayers onto the semiconductors. This model explains the widely varying electronic consequences of such interaction and shows them to be determined by the surface state energy position and the strength of the molecule-surface state coupling. The present findings can thus be used as guidelines for molecule-aided surface engineering of semiconductors.

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 alpha-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 alpha-phase, towards In- and Ga-rich compositions. We also investigate the interplay between phase segregation and junction formation in polycrystalline Cu(In, Ga)Sen 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 beta-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)Se-2 solar cells, and one due to Cu-electromigration.

Post-deposition air-annealing effects of Cu(In,Ga)Se-2 based thin films and heterojunction solar cell devices are studied by photoelectron spectroscopy and admittance spectroscopy. Ultraviolet photoelectron spectroscopy reveals type inversion at the surface of the as-prepared films, which is eliminated after exposure of several minutes to air due to the passivation of surface Se deficiencies. X-ray photoelectron spectroscopy demonstrates that air annealing at 200 degrees C leads to a decreased Cu concentration at the film surface. Admittance spectroscopy of complete ZnO/CdS/Cu(In,Ga)Se-2 heterojunction solar cells shows that the Cu(In,Ga)Se-2 surface type inversion is restored by the chemical bath used for CdS deposition. Air annealing of the finished devices at 200 degrees C reduces the type inversion again due to defect passivation. Our results also show that oxygenation leads to a charge redistribution and to a significant compensation of the effective acceptor density in the bulk of the absorber. This is consistent with the release of Cu from the absorber surface and its redistribution in the bulk. (C) 1999 American Institute of Physics. [S0021-8979(99)02813-3].

Systematic errors are likely to affect the results of indirect methods used for measuring dopant diffusion in semiconductors, which, for this purpose should be considered as mixed electronic-ionic conductors. The highest contribution to these errors is introduced by the presence of an internal electric field, i.e., by space charge effects. The electric field can be the result either of a dopant concentration gradient or of external bias, applied during the measurement. We consider here three methods in detail, viz. measurement of p-n junction motion, of current or potential decay, and of the time dependence of capacitance (transient ion drift). We show that space charge effects can lead to overestimating diffusion coefficients by a few orders of magnitude. We use the results of our analyses to review and compare the experimental data obtained by different direct and indirect methods, for Cu diffusion in CuInSe2, an issue of considerable current interest for solar cells. (C) 1998 American Institute of Physics.

By growing CuInSe2 with the traveling heater method, from an In melt (at a temperature below that of the sphalerite-chalcopyrite transition), twinning in the resulting single crystals was suppressed. The resulting crystals were n-type and could be converted to p-type by Se anneal. Both types were characterized structurally by X-ray diffraction, for composition by microprobe analyses, morphologically by AFM and SEM, and electrically. They were found to be mostly homogeneous, with mirror-like cleavage and without twins.

A doped semiconductor can be viewed as a mixed electronic-ionic conductor, with the dopants as mobile ions. Normally the temperature range where this becomes true is not even close to that where the (opto)electronic properties of the material are of interest. However notable exceptions exist and some examples of these are reviewed here. We limit ourselves to those cases where semiconductivity is preserved when the (mobile) dopant concentration changes and ambipolar behaviour can be obtained by dopant mobility. Dopant diffusion and drift are of interest not only in materials such as Si:Li, known from its use in radiation detectors, but also in ternary semiconductors, such as (HE,Cd)Te and CuInSe2. Understanding the phenomena is important not only for low-temperature doping, but also because of the implications that it has for device miniaturization, as dopant diffusion and drift impose chemical limits on device stability.

In this study we present the determination of the defect chemistry and the electrical properties of CuInSe2 after controlled annealing steps. The samples were investigated with the positron annihilation method and the admittance spectroscopy.

Control over semiconductor surface energetics can be achieved using different chemisorbed organic molecules with diverse electronic properties. We find evidence of such control over CdTe upon adsorption of dicarboxylic acid derivatives with different substituted phenyl rings. FT-IR measurements show that the dicarboxylic acid derivatives bind as carboxylates to form approximately one monolayer. Such chemisorption modifies both the band bending and the electron affinity (up to 500 and 700 mV, respectively), as measured by contact potential difference (CPD). Changes in band bending result from a coupling between molecular orbitals and surface states close to the valence band and depend on the withdrawing character of the phenyl substituent. A model is presented to interpret and explain the data. (C) 1998 Elsevier Science B.V.

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.

Ion potential diagrams can facilitate the description of systems in which ionic species are mobile. They depict qualitatively the spatial dependence of the potential energy for mobile ions, somewhat akin to band diagrams for electrons. We construct ion potential diagrams for the mixed conducting (oxide), optically active electrodes of five-layer electrochromic devices, based on reversible Li+ intercalation. These serve to analyze stability problems that arise in these systems. We then use them as building blocks to arrive at ion diagrams for complete devices. This allows analyses of (dis)coloration kinetics.

The interactions between adsorbed organic molecules and the electronic charge carriers in specially made GaAs structures are studied by time- and wavelength-dependent measurements of the photocurrent. The adsorption of the molecules modifies the photocurrent decay time by orders of magnitude. The effects are molecularly specific, as they depend on the electronic properties and absorption spectrum of the molecules. These observations are rationalized by assuming that new surface states are created upon adsorption of the molecules and that the character of these states is controlled by the relative electronegativity of the substrates and the adsorbed molecules. The relevance for surface passivation and for construction of semiconductor-based sensors is indicated. (C) 1998 Elsevier Science B.V.

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.

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.

Control of the work function of GaAs single crystals, under ambient conditions, was achieved by chemisorption of a series of benzoic acid derivatives with varying dipole moments. Quantitative Fourier transform infrared spectroscopy shows that the benzoic acid derivatives bind as carboxylates, via coordination to oxidized Ga or As atoms, with a surface coverage of about one layer and a binding constant of 2.1 10(4) M(-1) for benzoic acid. Contact potential difference measurements reveal that molecules affect the work function by changing the electron affinity while band bending is not affected significantly. The direction of the electron affinity changes depends on the direction of the dipole moments, and the extent of the change increases linearly with the dipole's magnitude. Investigation of the surface composition by X-ray photoelectron spectroscopy shows that the etched surface, onto which the molecules adsorb, is covered by an oxide layer. This may prevent the molecules from affecting band bending.

Homogeneously mixed molecular assemblies of defined stoichiometry were created by adsorption of asymmetric, trifunctional ligands on gold and CuInSe2 (CISe). The ligands rely on cyclic disulfide groups for binding to the substrate and can in addition-possess two different substituents, one polar substituent (p-cyanobenzoyl or anisoyl) and one long-chain, aliphatic residue (palmitoyl). Because the substituents are covalently connected, no phase segregation will occur upon surface binding. Adsorption of these ligands on conducting surfaces changed both the surface potential (because of the polar substituent) and hydrophobicity (because of the aliphatic residue). Larger changes of surface potential were obtained by adsorption of the symmetric, dipolar ligands than by adsorption of the asymmetric ligands, and larger changes occurred on gold than on CuInSe2 (up to 1.2 V between extreme modifications on Au and 0.3 V on CISe). The magnitude and direction of the observed contact potential difference changes were found to depend on the extent of coverage (as derived from electrochemical and contact angle measurements) and on the orientation of the ligands (estimated from ellipsometry and FTIR data) and could also be reconstructed using a simple, electrostatic model. These findings demonstrate that the present methodology enables simultaneous grafting of two desired properties onto solid surfaces and illustrate the predictive power of a simple, electrostatic model for molecule-controlled surface engineering.

The effects of chemisorbed molecules on the electronic transport through an ungated high electron mobility transistor and through an ungated field effect transistor are examined, Current versus voltage measurements in the dark reveal that the adsorbed molecules and their chemical nature have a pronounced effect on the structures' performance, as they reduce the current by up to one order of magnitude. The molecular specificity of the devices is expressed in the wavelength dependence of photo current decay. The decay time, which increases by several orders of magnitude upon adsorption of the molecules, changes drastically when the excitation wavelength matches the absorption of the adsorbed molecules. Effect of Cu ions caught by adsorbed organic molecules on the photo current decay is clearly demonstrated. The observations open new possibilities in constructing semiconductor based light and chemical sensors.

The diffusion and electromigration of Ag in crystals of CdxHg1-x is studied, as a function of original doping level and of the concentration of mercury. In materials with x = 0.55-0.8, Ag dopes p-type, when diffusing in at <125 degrees C. This should be contrasted to what is found in n-CdTe; where in-diffusion of Ag at 200 degrees C increases the net donor density, leaving the material n-type. Our results show that the higher is the mercury content or the hole concentration in CdxHg1-xTe (x = 0.55-0.8), the faster Ag will; diffuse in these materials; We explain our results, building on earlier suggestions made for Hg-rich materials, by assuming that silver diffuses by way of a substitutional-interstitial mechanism; i.e., it is present as two species with opposite charge, one of which : dominates nd is practically immobile, while the minority, species diffuses rapidly These forms equilibrate, at room temperature, within a few seconds, something that can be understood by postulating silver-mercury complex formation. If both forms of silver are bound to mercury, then this hypothesis explains the strong influence of mercury content on the diffusion behavior.

Assembling quinolinium-based chromophores on silicon surfaces provides a new route to electronic control over such semiconducting surfaces. The two-step process by which the molecules are grafted on to the surface involves first coupling the organic functionality to silicon, followed by chromophore anchoring. These synthetic steps are monitored by XPS, UV-Vis and FTIR spectroscopies. Using contact potential difference measurements we found that the electron affinity of the modified silicon is a function of the molecule's dipole moment. The same technique shows a pronounced effect of the sub-nanometer siloxane-based, coupling-agent, layer by itself on the band bending and band-bending modification as function of chromophore adsorption. (C) 1997 Published by Elsevier Science B.V.

We show that the transient ion drift (TID) method, which is based on recording junction capacitance under constant reverse bias [A. Zamouche, T. Heiser, and A. Mesh, Appl. Phys. Lett. 66, 631 (1995)], can be used not only for measurements of the diffusion coefficient of mobile impurities, but also to estimate the concentration of mobile species as part of the total dopant density. This is illustrated for CdTe, contaminated by Cu, and intentionally doped by Li or Ag and for CuInSe2. We show also that, with some restrictions, the TID method can be used if the mobile ions are major dopants. This is demonstrated using Schottky barriers on CdTe, and p-n junction devices in (Hg,Cd)Te, and CuInSe2. The values that we obtain for the diffusion coefficients (for Li, Ag, and Cu in CdTe and for Cu in CuInSe2) agree well with measured or extrapolated values, obtained by other methods, as reported in the literature. Furthermore, we could distinguish between diffusion and chemical reactions of dopants, as demonstrated for the case of Cu in CdTe and Ag-doped (Hg,Cd)Te. In the former case this allows us to separate copper-free from contaminated CdTe samples. (C) 1997 American Institute of Physics.

When p-n junctions are formed by doping with a dopant that diffuses via a dissociative diffusion mechanism, dopant diffusion is suppressed and dopants can pile up near the junction, well above their original concentration. Calculations confirm this, if no local neutrality is assumed. The results agree well with published and our own experimental data for Zn diffusion in InP. The increased built-in electric field due to this pileup is expelled nearly completely to the side of the junction without the pileup. This effect has important consequences for devices containing thin and/or small regions doped with such dopants because such regions may be completely depleted. (C) 1997 American Institute of Physics.

We have measured the widths of the transition regions at p-n-p junctions that are formed on CuxAg1-xInSe2 by high electric field at room temperature, by using a scanning tunneling microscope with capacity for measuring current-voltage curves on physical contact. We find marked transition regions of low conductivity with widths in the range of 1.3-5 mu m. This shows that the junction created by the electromigration mechanism in this material, while diffuse by the standards of those produced by high-temperature or high-energy methods, are remarkably sharp by solid-state ionic measures. (C) 1996 American Institute of Physics.

Micrometer-sized homojunction structures can be formed by applying strong electric pulses, at ambient temperatures, to Li-doped, floating zone n-Si. Two such junctions, arranged back to back, act as a transistor, as evidenced by electron-beam-induced current and current-voltage measurements, The structures are created during a time ranging from similar to 100 ms to a few seconds, depending on the size of the structure. The phenomenon is similar to what was observed earlier in CuInSe2 and was explained there by thermally assisted electromigration of Cu. In the case of Si doped with Li we can use secondary-ion-mass spectrometry to detect the redistribution of Li after electric-field application. Such a redistribution is indeed found and corresponds to an n(+)-p-n structure with the p region extending at least similar to 20 mu m into the bulk of Si. Structures created in Si doped with Li in this way are stable for at least 13 months after their creation. We ascribe this to the large difference between Li diffusivity at the local temperature that is reached during structure formation (similar to 400 degrees C; 10(-8) cm(2)/s) and at room temperature (similar to 10(-15) cm(2)/s). (C) 1996 American Institute of Physics.

A number of configurations can be used to form p-n junctions in semiconductors, by applying an electrical potential difference to them under conditions where dopants, native or foreign, show ionic conductivity, In all these cases the electrical potential difference guides the dopants to create compositional inhomogeneities. This can lead to actual type conversion and thus to p-n junction formation, Junction formation can be localized on a scale that is small compared to the sample size, by working under conditions far from equilibrium and by limiting thermal effects.

In view of our recent experimental finding of self-restoration of p/n junctions in Ag-doped (Cd,Hg)Te after their electrical or thermal perturbation, we ask the question if, and if so, when can, a mixed electronic semiconductor/ionic conductor support a built-in electric field. The question is of interest because common p/n junctions are merely kinetically stabilized systems. We study the problem by deriving the thermodynamically stable states of mixed conductors. This shows that (1) as long as all components of a multicomponent system behave ideally, no stable concentration gradient and built-in field may exist; (2) a thermodynamically stable concentration gradient and thus a built-in field can exist in a multicomponent system, if at least one of its components behaves nonideally (and thus, from the Gibbs-Duhem relation, at least one additional component must behave nonideally, too); and (3) the likelihood of finding a thermodynamically stable concentration gradient increases with the number of components of the system. While the first of these results is intuitively obvious, the rigorous proof given here is necessary to deduce that actual observation of self-restoration of p/n junctions implies nonideal behavior of at least two of the mobile species in the system. We show that our results can be used to derive the built-in electric field for a given variation of activity coefficients of one or more of the mobile species and vice versa.

Controlled surface modification of CdTe single crystals and CdTe and CuInSe2 solar cell quality thin films was achieved by chemisorption of a series of organic ligands with varying dipole moments. Contact potential difference measurements in air showed that adsorption of benzoic or hydroxamic acid derivatives on the thin films or crystals changes the semiconductors' electron affinity without significantly affecting band bending. The magnitude and direction of surface potential changes, which reach 670 mV between extreme modifications, correlate with the ligands' dipole moments. Ligand dipole moments were controlled by varying the substituents of the ligand. Quantitative Fourier transform infrared (FTIR) spectroscopy showed that benzoic acid surface coverage is about one monolayer. Finally, FTIR spectral analysis showed that the benzoic acid derivatives adsorb via coordination to Cd on CdTe and that hydroxamic acids bind to Cd on CdTe and to In on CuInSe2. These phenomena occur in several systems (two semiconductor compounds, two types of binding groups, and two types of surface morphologies were examined) and may prove useful in band edge engineering.

The interaction between a chalcogenide semiconductor surface and a specific chelating ligand was studied by a number of complementary spectroscopic methods (UV/VIS, FTIR, XPS). FTIR suggests that the chelating ligand (diphenyl hydroxamic acid) complexes an In3+ ion in CuInSe2, and a Cd2+ ion in CdTe, accompanied by the loss of a proton. Contact potential measurements showed that the adsorbed ligand changes the semiconductor electron affinity without significantly influencing the band bending. On the basis of these and other results, we suggest that the molecular dipole of the chelating ligand is responsible for the change in surface potential. Because this dipole can be modified without changing the ligand's binding group, this finding opens the way to control surface potential by varying the dipole moment of the adsorbed ligand without changing the binding functional group.

We report here on the optimization of ohmic contacts to p-CuInSe2 (CISe) single crystals. A low resistance ohmic contact is required to minimize current losses due to series resistance; e.g. in Schottky diodes. Both In-Ga (eutectic)/CISe and gold (evaporated)/CISe contacts have been fabricated on crystals with different orientations and bulk properties. Gold contacts were found to have a lower resistance and to be more stable than In-Ga ones, from the slope of the linear current-voltage plot of the junctions. The resistance of the Au/CISe ohmic contact was decreased by etching the CISe crystal surface chemically in a 0.5% solution of Br2 in methanol for 30 sec at room temperature, prior to gold deposition, while that of the In-Ga contact increased by this etch. Wetting experiments and contact angle measurements showed evidence for changes in the polarity of the surface due to chemical etches.

YBa2Cu3O7 (1237) was reduced and reoxidized at room temperature (RT), by using wet electrochemistry to extract, or solid state electrochemistry to re-insert oxygen. With thin films this led to homogeneous products, as judged by X ray diffraction and temperature dependence of electrical resistivity. Because reduction leads to loss of superconductivity and re-oxidation restores it, this method allows room temperature control over superconducting properties of (parts of) films and, thus, can be used for patterning of (1237) thin films. We showed this by preparing SNS-like structures by this method. The method affords control not only over bulk properties, but also over the quality of the intergranular contact, possibly via control of oxygen concentration and ordering in the outermost layers of grains (in films and pellets). The results imply rapid, field-assisted, oxygen motion and this was confirmed by independent measurements.

Solar cells based on CuInSe2 polycrystalline thin films (see Figure) are one of the few realistic alternatives to polycrystalline Si for practical conversion of solar energy into electricity. A model explaining the important role of the secondary copper chalcogenide phases in the deposition of the CuInSe2 films by physical vacuum evaporation is presented.

Quantitative data are presented that show partial ionic conductivity of Cu or Ag in ternary and quaternary electronic semiconductors, with idealized stoichiometry CuxAg1-xInSe2. A trend of increasing facility of ionic motion with increasing Ag content was observed. Ionic transference numbers up to 0.13 and 0.55 were measured for CuInSe2 and AgInSe2, respectively. This trend can be correlated with the degree of compactness of the structure. It is supported by results from measurements of effective values of chemical diffusion coefficients, obtained by a potentiostatic current decay technique. Those results show a generally higher diffusivity in AgInSe2 than in CuInSe2. A clear trend of increasing diffusion coefficient (up to 10(-7) cm2/s) with decreasing concentration of IB metal was observed. No obvious general correlation is seen between net electronic carrier concentration (or resistivity) and diffusion coefficients, except that overall the highest diffusion coefficients are found for Cu-poor (or Ag-poor) samples which are also the most resistive. The effect of temperature (between 20 and 100-degrees-C) on the diffusivity is small. On the basis of our observations we conclude that diffusion occurs predominantly via a vacancy mechanism and suggest that this possibility of coexistence of significant ionic mobility with true semiconductivity can be useful for electronic doping by native defects.

We present a summary of our defect chemical microscopic model for the effect that annealing in air or oxygen has on CuInSe2 and CdTe-based polycrystalline thin film solar cells and explain its generalization to other chalcogenide-based semiconductor devices. The summary includes a hypothesis for specific O2 (molecule) and surface interaction From the point of view of device performance, the model provides a specific chemical explanation for the conclusions obtained from device analyses that the performance of the present generation of polycrystalline cells of this type is mainly limited by recombination at grain surfaces and boundaries.