Publications

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

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.

Halide perovskite (HaP) semiconductors are revolutionizing photovoltaic (PV) solar energy conversion by showing remarkable performance of solar cells made with HaPs, especially tetragonal methylammonium lead triiodide (MAPbI(3)). In particular, the low voltage loss of these cells implies a remarkably low recombination rate of photogenerated carriers. It was suggested that low recombination can be due to the spatial separation of electrons and holes, a possibility if MAPbI(3) is a semiconducting ferroelectric, which, however, requires clear experimental evidence. As a first step, we show that, in operando, MAPbI(3) (unlike MAPbBr(3)) is pyroelectric, which implies it can be ferroelectric. The next step, proving it is (not) ferroelectric, is challenging, because of the material's relatively high electrical conductance (a consequence of an optical band gap suitable for PV conversion) and low stability under high applied bias voltage. This excludes normal measurements of a ferroelectric hysteresis loop, to prove ferroelectricity's hallmark switchable polarization. By adopting an approach suitable for electrically leaky materials as MAPbI(3), we show here ferroelectric hysteresis from well-characterized single crystals at low temperature (still within the tetragonal phase, which is stable at room temperature). By chemical etching, we also can image the structural fingerprint for ferroelectricity, polar domains, periodically stacked along the polar axis of the crystal, which, as predicted by theory, scale with the overall crystal size. We also succeeded in detecting clear second harmonic generation, direct evidence for the material's noncentrosymmetry. We note that the material's ferroelectric nature, can, but need not be important in a PV cell at room temperature.

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

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.

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.

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

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

The soft character of organic materials leads to strong coupling between molecular, nuclear and electronic dynamics. This coupling opens the way to influence charge transport in organic electronic devices by exciting molecular vibrational motions. However, despite encouraging theoretical predictions, experimental realization of such approach has remained elusive. Here we demonstrate experimentally that photoconductivity in a model organic optoelectronic device can be modulated by the selective excitation of molecular vibrations. Using an ultrafast infrared laser source to create a coherent superposition of vibrational motions in a pentacene/C-60 photoresistor, we observe that excitation of certain modes in the 1,500-1,700 cm(-1) region leads to photocurrent enhancement. Excited vibrations affect predominantly trapped carriers. The effect depends on the nature of the vibration and its mode-specific character can be well described by the vibrational modulation of intermolecular electronic couplings. This presents a new tool for studying electron-phonon coupling and charge dynamics in (bio) molecular materials.

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.

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

The remarkable advances over the past few years in performance of photovoltaic cells, including the advent of new absorber materials, call for an update to the previous assessment of prospects for future progress. The same simple criteria with some refinements, based on cell and module performance data, serve to evaluate and compare most types of solar cells. Apart from Si and InP, for all types the best cells have improved in conversion performances (and crystalline Si modules have made major strides in cost reduction). New cell types, such as perovskite, sustainable chalcogenide, and quantum dot cells, are included. CdTe results bring those cells in line with other well-developed ones, lending some credence to the idea that the criteria provide the reader with knowledge, useful for gauging possible future technological developments. Additionally, the developments of the past few years show that, while the advent of more new cell types cannot be predicted, it can be aided and stimulated by innovative, daring, and creative new materials research.

Conducting Probe AFM. CP-AFM, was used to follow how chemical etching, oxidation, and sulfurization affect the surface nanoscale electrical characteristics of polycrystalline Cu(In,Ga)Se-2 (CIGS) thin films. Band bending at grain boundaries (GBs) on the surface was studied and analyzed by CP-AFM - measured photocurrents. We find that both oxidation and sulfurization can passivate the GBs of the CIGS films; oxidation increases n-type band bending, which impedes the transport of photogenerated electrons, while sulfurization increases p-type band bending at GBs, which helps this transport. Differences in effects between surface terminations by sulfide, selenide and oxide were analyzed. The effects of these treatments on the electrical activity of the GBs of the films, as well as the importance of the use of chemical bath deposition of the CdS buffer, are explained within a defect surface chemistry model. (C) 2013 Elsevier B.V. All rights reserved.

CH3NH3PbI3-based solar cells were characterized with electron beam-induced current (EBIC) and compared to CH3NH3PbI3-xClx ones. A spatial map of charge separation efficiency in working cells shows p-i-n structures for both thin film cells. Effective diffusion lengths, L-D, (from EBIC profile) show that holes are extracted significantly more efficiently than electrons in CH3NH3PbI3, explaining why CH3NH3PbI3-based cells require mesoporous electron conductors, while CH3NH3PbI3-xClx ones, where L-D values are comparable for both charge types, do not.

The field of organo-lead perovskite absorbers for solar cells is developing rapidly, with open-circuit voltage of reported devices already approaching the maximal theoretical voltage. Obtaining such high voltages on spun-cast or evaporated thin films is intriguing and calls for detailed investigation of the source of photovoltage in those devices. We present here a study of the roles of the selective contacts to methylammonium lead iodide chloride (MAPbI(3-x)Cl(x)) using surface photovoltage spectroscopy. By depositing and characterizing each layer at a time, we show that the electron-extracting interface is more than twice as effective as the hole-extracting interface in generating photovoltage, for several combinations of electrode materials. We further observe the existence of an electron-injection related spectral feature at 1.1 eV, which might bear significance for the cell's operation. Our results illustrate the usefulness of SPV spectroscopy in highlighting gaps in cells efficiency and for deepening the understanding of charge injection processes in perovskite-based photovoltaics.

Organometallic lead-halide perovskite-based solar cells now approach 18% efficiency. Introducing a mixture of bromide and iodide in the halide composition allows tuning of the optical bandgap. We prepare mixed bromide-iodide lead perovskite films CH3NH3Pb(I1-xBrx)(3) (0

Integrating proteins in molecular electronic devices requires control over their solid-state electronic transport behavior. Unlike `` traditional'' electron transfer (ET) measurements of proteins that involve liquid environments and a redox cycle, no redox cofactor is needed for solid-state electron transport (ETp) across the protein. Here we show the fundamental difference between these two approaches by macroscopic area measurements, which allow measuring ETp temperature dependence down to cryogenic temperatures, via cytochrome C (Cyt C), an ET protein with a heme (Fe-porphyrin) prosthetic group as a redox centre. We compare the ETp to electrochemical ET measurements, and do so also for the protein without the Fe (with metal-free porphyrin) and without porphyrin. As removing the porphyrin irreversibly alters the protein's conformation, we repeat these measurements with human serum albumin (HSA), 'doped' (by non-covalent binding) with a single hemin equivalent, i.e., these natural and artificial proteins share a common prosthetic group. ETp via Cyt C and HSA-hemin are very similar in terms of current magnitude and temperature dependence, which suggests similar ETp mechanisms via these two systems, thermally activated hopping (with similar to 0.1 eV activation energy) > 190 K and tunneling by superexchange o190 K. Also, ET rates to and from the Fe redox centres (Fe2+reversible arrow Fe3+ + e(-)), measured by electrochemistry of HSA-hemin are only 4 times lower than those for Cyt C. However, while removing the Fe redox centre from the porphyrin ring markedly affects the ET rate, it hardly changes the ETp currents through these proteins, while removing the macrocycle (from HSA, which retains its conformation) significantly reduces ETp efficiency. These results show that solid-state ETp across proteins does not require the presence of a redox cofactor, and that while for ET the Fe ion is the main electron mediator, for ETp the porphyrin ring has this function.

Organic monolayers derived from omega-fluoro-1-alkynes of varying carbon chain lengths (C-10-C-18) were prepared on Si(111) surfaces, resulting in changes of the physical and electronic properties of the surface. Analysis of the monolayers using XPS, Infrared Reflection Absorption Spectroscopy, ellipsometry and static water contact angle measurements provided information regarding the monolayer thickness, the tilt angle, and the surface coverage. Additionally, PCFF molecular mechanics studies were used to obtain information on the optimal packing density and the layer thickness, which were compared to the experimentally found data. From the results, it can be concluded that the monolayers derived from longer chain lengths are more ordered, possess a lower tilt angle, and have a higher surface coverage than monolayers derived from shorter chains. We also demonstrate that by substitution of an H by F atom in the terminal group, it is possible to controllably modify the surface potential and energy barrier for charge transport in a full metal/monolayer-semiconductor (MOMS) junction.

Measuring solid-state electron transport (ETp) across proteins allows studying electron transfer (ET) mechanism(s), while minimizing solvation effects on the process. ETp is, however, sensitive to any static (conformational) or dynamic (vibrational) changes in the protein. Our macroscopic measurements allow extending ETp studies to low temperatures, with the concomitant resolution of lower current densities, because of the larger electrode contact areas. Thus, earlier we reported temperature-independent ETp via the copper protein azurin (Az), from 80 K until denaturation, whereas for apo-Az ETp was temperature dependent above 180 K. Deuteration (H/D substitution) may provide mechanistic information on the question of whether the ETp involves H-bonds in the solid state. Here we report results of kinetic deuterium isotope effect (KIE) measurements on ETp through holo-Az as a function of temperature (30-340 K). Strikingly, deuteration changed ETp from temperature independent to temperature dependent above 180 K. This H/D effect is expressed in KIE values between 1.8 (340 K) and 9.1 (

Contact potential difference (CPD) measurements of the relative work functions of a range of organic semiconductor thin films show that oxygen causes effective p-type doping (with work functions increasing 0.1-0.3 eV). This doping effect is found to be reversible by exposure to high vacuum or heating in inert atmosphere. The mechanism of doping is explained by a model, based on a reversible formation of an O-substrate charge transfer state. Conductivity measurements of p-phthalocyanine films at variable temperatures support this doping model. The oxygen doping effect is consistent with filling of tail states in the gap, as shown by the increase of activation energy of hole transport with decreased O-doping, and by the good fit between experimental data and simulations of the in-gap density of states. A model hybrid solar cell configuration also shows the effect of doping by O-2 and corroborates the fact that O-doping fills the tail states in the system. (C) 2013 Elsevier B.V. All rights reserved.

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.

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.

Compositional uniformity of Cu(In,Ga)Se-2 (CIGS) solar cells was studied, using thin cross sections of complete cells prepared by focused ion beam (FIB) and examined in the transmission electron microscope (TEM). This methodology revealed the compositional variations at the nm-scale. The Ga and In compositions vary not only between neighboring grains, but also inside individual single crystal grains along their growth direction, which explains the electrical non-uniformity seen in electron beam-induced current (EBIC) measurements. The improved compositional uniformity with increase in sample preparation temperature correlates with higher solar cell efficiency. (C) 2011 Elsevier B.V. All rights reserved.

The photovoltaic performance of solar cells, based on a Cu(In1-x Ga-x)Se-2 (CIGS) absorber layer, is directly correlated with Ga composition. We have used scanning capacitance microscopy and conducting probe atomic force microscopy (CP-AFM) to provide microscopic electrical characterization of CIGS films with different Ga content. We found p- to n-type inversion at grain boundaries of the polycrystalline CIGS film, especially for Ga-poor compositions. The fraction of grain boundaries undergoing inversion dramatically decreased for Ga compositions above x = 0.32, the composition corresponding to a sharp efficiency drop of the complete cells. CP-AFM measurements showed a marked current drop at grain boundaries as the Ga composition rose above x = 0.32.

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.

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

We demonstrate a solar cell that uses fixed negative charges formed at the interface of n-Si with Al2O3 to generate strong inversion at the surface of n-Si by electrostatic repulsion. Built-in voltages of up to 755mV are found at this interface. In order to harness this large built-in voltage, we present a photovoltaic device where the photocurrent generated in this inversion layer is extracted via an inversion layer induced by a high work function transparent organic top contact, deposited on top of a passivating and dipole-inducing molecular monolayer. Results of the effect of the molecular monolayer on device performance yield open-circuit voltages of up to 550mV for moderately doped Si, demonstrating the effectiveness of this contact structure in removing the Fermi level pinning that has hindered past efforts in developing this type of solar cell with n-type Si. (C) 2012 American Institute of Physics. [http://dx.doi.org/10.1063/1.4769041]

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.

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

The electronic structure of the prototypical self-assembled monolayer (SAM) system, i.e. alkanethiol molecules on Au, is investigated via ultraviolet and inverse photoemission spectroscopy measurements. The determination of the density of filled and empty states of the system reveals that the metal Fermi level is significantly closer to the lowest unoccupied molecular orbital (LUMO) of the molecules than to their highest occupied molecular orbital (HOMO). The results suggest that charge carrier tunneling is controlled by the LUMO, rather than by the HOMO, in contrast to what is commonly assumed. (C) 2011 Elsevier B.V. All rights reserved.

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.

Molecular electronics is a flourishing area of nano-science and -technology, with a promise for cheap electronics of novel functionality. Here we outline the major challenges for molecular electronics becoming an established scientific discipline, including models with predictive power.

Metal-organic molecule-semiconductor junctions are controlled not only by the molecular properties, as in metal-organic molecule-metal junctions, but also by effects of the molecular dipole, the dipolar molecule-semiconductor link, and molecule-semiconductor charge transfer, and by the effects of all these on the semiconductor depletion layer (i.e., on the internal semiconductor barrier to charge transport). Here, we report on and compare the electrical properties (current-voltage, capacitance-voltage, and work function) of large area Hg/organic monolayer-Si junctions with alkyl and alkenyl monolayers on moderately and highly doped n-Si, and combine the experimental data with simulations of charge transport and electronic structure calculations. We show that, for moderately doped Si, the internal semiconductor barrier completely controls transport and the attached molecules influence the transport of such junctions only in that they drive the Si into inversion. The resulting minority carrier-controlled junction is not sensitive to molecular changes in the organic monolayer at reverse and low forward bias and is controlled by series resistance at higher forward bias. However, in the case of highly doped Si, the internal barrier is smaller, and as a result, the charge transport properties of the junction are affected by changing from an alkyl to an alkenyl monolayer. We propose that the double bond near the surface primarily increases the coupling between the organic monolayer and the Si, which increases the current density at a given bias by increasing the contact conductance.

Studying solid-state electronic conductance of biological molecules requires interfacing the biomolecules with electronic conductors without altering the molecules. To this end, we developed and present here a simple, solution-based approach of conjugating Bacteriorhodopsin (bR)-containing membranes with metallic clusters. Our approach is based on selective electroless deposition of Pt nanoparticles on suspended membrane fragments through chemical interaction of the Pt precursor with the protein's residues. Optical absorption measurements show that the membranes retain their photoactivity after this procedure. The result of the Pt deposition is best shown by conductive probe atomic force microscopy mapping of electronic current transport across such soft biological layers, which allows reproducible microscopic electrical characterization of the electronic conductance of the resulting junctions. The maps show that chemical contact between the protein and the deposited electrode yields better electronic coupling than a physical contact, demonstrating that also with biomolecules, the type and method of deposition of the electrical contact are critical to the behavior of the resulting junctions.

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

We report on electronic transport measurements through dense monolayers of CH(3)(CH(2))(n)PO(3)H(2) molecules of varying chain lengths, with a strong and stable bond through the phosphonic acid end group to a <100 > GaAs surface and a Hg top contact. The monolayers maintain their high quality during and after the electrical measurements. Analyses of the electronic transport measurements of junctions, and of UV and inverse photoemission spectroscopy data on band alignments of free surfaces, yield insight about the electrical transport mechanism. Transport characteristics for n-GaAs junctions at low forward bias are identical for different chain lengths, a strong indication of high-quality monolayers. Tunneling barrier and carrier effective mass values for n- and p-GaAs samples were deduced from the transport data. In this way we find a tunneling barrier for n-GaAs of 1.3 eV, while UPS data for the lowest unoccupied system orbital (LUSO) point to a 2.4 eV barrier. This discrepancy can be understood by invoking states, closer to the Fermi level than the LUSO state, that contribute to charge transport. Such states lead to a manifold of transitions, each having a different probability, both because of differences in the tunnel barrier and because of differences in density of these interface-induced states; i.e., the single barrier, deduced from J-V measurements, is an effective value only.

Electronic transport across n-Si-alkyl monolayer/Hg junctions is, at reverse and low forward bias, independent of alkyl chain length from 18 down to 1 or 2 carbons! This and further recent results indicate that electron transport is minority, rather than majority carrier dominated, occurs via generation and recombination, rather than (the earlier assumed) thermionic emission, and, as such, is rather insensitive to interface properties. The (m)ethyl results show that binding organic molecules directly to semiconductors provides semiconductor/metal interface control options, not accessible otherwise.

After some definitions to establish common ground and illustrate the issues in terms of orders of magnitude, we note that meeting the Energy challenge will require suitable materials. Luckily, we can count on the availability of natural resources for most materials. We briefly illustrate the connection between materials and energy and review the past and the present situations, to focus on the future. We wrap up by arguing that more than bare economics is required to use the fruits of science and technology towards a world order, built an sustainable energy band materials resources.

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.

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.

Controlling the orientation of bacteriorhodopsin (bR) monolayers is an important step in studying and utilizing such membranes in a solid-state configuration in., for example, photoelectric applications. Macroscopic monolayers of bR have been fabricated in a variety of ways, but characterization of the distribution of the two possible orientations in which the membrane fragments can adsorb has not yet been addressed experimentally. Here, an approach is presented that labels only one of the membrane surfaces by electroless growth of metal nanoparticles on top of the solid-supported membranes. In, this way, it is possible to observe which surface of the membranes is actually adsorbed to the substrate. How this technique serves to interface the membranes with a top metal contact for further electrical measurements is also demonstrated.

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

Electron irradiation can alter electronic charge transport through Si-CH2(CH2)(12)-CH3//Hg molecular junctions. Applying UPS, XPS, Auger, NEXAFS, and electrical transport measurements, we show that irradiation induces defects, most likely CC bonds and C-C cross-links, which introduce new electronic states into the HOMO-LUMO gap of the alkyl chains, and, hence, effectively dope these layers. We demonstrate a 1-2 order of magnitude enhancement in current, clearly distinguishable from that of defects in as-prepared layers.

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.

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

We elucidate the electronic structure of both filled and empty states of ordered alkyl chains bound to the Si(111) surface by combining direct and inverse photoemission spectroscopy with first principles calculations based on density functional theory. We identify both filled and empty interface-induced gap states, distinguish between those and states extending throughout the monolayer, and discuss the importance of these findings for interpreting transport experiments through such monolayers.

Monolayers of alkyl chains, attached through direct Si-C bonds to Si(111), via phosphonates to GaAs(100) surfaces, or deposited as alkyl-silane monolayers on SiO(2), are investigated by ultraviolet and inverse photoemission spectroscopy and X-ray absorption spectroscopy. Exposure to ultraviolet radiation from a He discharge lamp, or to a beam of energetic electrons, leads to significant damage, presumably associated with radiation-or electron-induced H-abstraction leading to carbon-carbon double-bond formation in the alkyl monolayer. The damage results in an overall distortion of the valence spectrum, in the appearance of (occupied) states above the highest occupied molecular orbital of the alkyl molecule, and in a characteristic (unoccupied state) pi* resonance at the edge of the carbon absorption peak. These distortions present a serious challenge for the interpretation of the electronic structure of the monolayer system. We show that extrapolation to zero damage at short exposure times eliminates extrinsic features and allows a meaningful extraction of the density of state of the pristine monolayer from spectroscopy measurements.

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.

A dipole-layer approach is adapted to describe the electrostatic potential and electronic transport through metal/semiconductor junctions with a discontinuous monolayer of polar molecules at the metal/semiconductor interface. The effective barrier height of those junctions, which have small pinholes, embedded in a molecular layer, which introduces a negative {positive} dipole (i.e., a dipole whose negative {positive} pole is the one that is closest to the semiconductor surface) on an n-type {p-type} semiconductor, is often "tunable" by the magnitude and density of the dipoles. If the lateral dimensions of a molecule-free pinhole at the interface exceed the semiconductor depletion width, carrier transport is not influenced by the molecular layer and the "effective" barrier height is the nominal metal/semiconductor barrier height. If the molecular layer introduces a positive {negative) dipole on an n-type {p-type) semiconductor, enhanced field emission at edges of small pinholes might lead to a leakage- and/or an edge-current component resulting in an effective barrier height lower than the nominal one. We support these conclusions by direct measurements of the nm-scale electronic behaviour of a Au/n-GaAs diode with a discontinuous monolayer of dicarboxylic acids at the interface, using Ballistic Electron Emission Microscopy (BEEM). (c) 2006 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.

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.

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

A contact-free method to measure the photovoltage that can be generated by an absorber, upon illumination, is presented. The measurement is based on Kelvin's well-known capacitor method which measures the contact potential difference that builds up between two sufficiently conducting materials of different work function that are electrically connected. We show that the photovoltage of an absorber, which is introduced into the Kelvin capacitor, can be measured accurately, even though it is not in electrical contact to any of the capacitor plates. Comparative measurements of the surface photovoltage of an n-type Si semiconductor surface in grounded and nongrounded mode as well as the interface photovoltage of mesoporous TiO2, deposited onto a conducting substrate, are presented to demonstrate the feasibility of the concept. This approach enables to measure the photovoltage of complete solar cells and also its single components (absorber, absorber + buffer layer, absorber + buffer layer + electron and/or hole conductor), a matter that is of particular importance for a better understanding of photovoltaic devices such as extreme thin absorber cells, dye sensitized solar cells, or organic (so-called plastic) solar cells. (C) 2004 American Institute of Physics.

Reproducible electrical contacts to organic molecules are created non-destructively by indirect electron beam evaporation of Pd onto molecular films on cooled substrates. In contrast, directly evaporated contacts damage the molecules seriously. Our conclusions are based on correlating trends in properties of a series of molecules with systematically varying, exposed functional groups, with trends in the electrical behaviour of Pd/molecule/GaAs junctions, where these same molecules are part of the junctions.

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

We use scanning probe microscopy-based methods for direct characterization of a single grain boundary and a single grain surface in solar cell-quality CdTe, deposited by closed-space vapor transport. We find that scanning capacitance microscopy can serve to study polycrystalline electronic materials, notwithstanding the strong topographical variations. In this way, we find a barrier for hole transport across grain boundaries, a conclusion supported by the much more topography-sensitive scanning kelvin probe microscopy, with some variation in barrier height between different boundaries. (C) 2003 American Institute of Physics.

Epitaxial films of CuInSe2 on Si(1 1 1) were modified by the application of an electric field through a movable tip. The electric field induces stable junction regions which are identified by separation and collection of electron beam-induced charge carriers. The movable tip allows scribing of these junction regions and one-sided connection to a contact pad. The junction formation is mainly due to electric field application and, in contrast to what was found to be the case for bulk samples, is not accompanied by a significant temperature rise. The junctions can be explained by symmetrical p/p(+)/n/p(+)/p regions formed within the CuInSe2 epilayers. The reported method presents a new way for junction patterning in two dimensions. (C) 2003 Elsevier Science B.V. All rights reserved.

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

An information transfer mechanism through a molecular bilayer, which does not involve charge/mass transfer or conformational changes of membrane-spanning molecular structures, is proposed. We tested this proposal by measuring changes in the electric potential at a Si/SiOx surface, onto which an artificial bilayer had been constructed, in response to exposure of the adsorbed bilayer to different ambient. The bilayer was comprised of an OctadecylTrichloroSilane (OTS) monolayer, adsorbed onto the Si/SiOx surface and a second layer of stearic acid, deposited on the OTS monolayer by the Langmuir-Blodgett technique. Changes of the band bending (1313) at the Si/SiOx surface in response to exposing the bilayer to different solutions were measured by Kelvin Probe. These changes indicate that external stimuli at the bilayer's exterior induce a change in the electric potential at the bilayer's interior, a change that is sensed at the surface of the Si/SiOx. The mechanism proposed to explain the results is based on electrostatic interactions at the bilayer's exterior with dipole- or monopole-carrying molecules. (C) 2002 Elsevier Science B.V. All rights reserved.

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.

Progress reports are a new type of article in Advanced Materials, dealing with the hottest current topics, and providing readers with a critically selected overview of important progress in these fields. It is not intended that the articles be comprehensive, but rather insightful, selective, critical, opinionated, and even visionary. We have approached scientists we believe are at the very forefront of these fields to contribute the articles, which will appear on an annual basis. The article below describes the latest advances in molecular electronics.

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.

High efficiency CdTe/CdS thin-film solar cells require low resistance contacts to p-CdTe, which is frequently achieved by addition of Cu. Decreases in cell efficiency over time. however. have been associated with Cu from the contact. The question that is considered here is if Cu is really detrimental to cell performance? By performing a series of thermal stress tests the authors reach a far more optimistic conclusion than what has hitherto been assumed. The Figure shows the proposed model for action of Cu (and Cl) in the CdTe/CdS cell.

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.

A semiconductor that contains dopants can be considered as a mixed electronic-ionic conductor, with the dopants as mobile ions. The temperature range in which this normally becomes true is far from where the opto-electronic properties of the material are of interest. However, exceptions exist. In this chapter we consider several important cases. Dopant diffusion and drift are relevant not only for materials such as Si:Li, used in radiation detectors, but also for other semiconductors, ranging from II-VIs and related compounds, such as CdTe, (Hg,Cd)Te and CuInSe2, to III-Vs, including GaN, and potential high temperature semiconductors, such as SiC. Better understanding of the phenomena is important also because of the implications that it has for device miniaturization. as dopant diffusion and drift put chemical limits to device stability. Such understanding can also make dopant electromigration useful for loci-temperature doping.

The recent literature regarding the stability of CdTe/CdS photovoltaic cells las distinguished from modules) is reviewed. Particular emphasis is given to the role of Cu as a major factor that can limit the stability of these devices. Cu is often added to improve the ohmic contact to p-CdTe and the overall cell photovoltaic performance. This may be due to the formation of a Cu2Te/CdTe back contact. Excess Cu also enhances the instability of devices when under stress. The Cu, as Cu+, from either Cu2Te or other sources, diffuses via grain boundaries to the CdTe/CdS active junction. Recent experimental data indicate that Cu, Cl and other diffusing species reach (and accumulate at) the CdS layer, which may not be expected on the basis of bulk diffusion. These observations may be factors in cell behavior and degradation, for which new mechanisms are suggested and areas for future study are highlighted. Other possible Cu-related degradation mechanisms, as well as some non-Cu-related issues for cell stability are discussed. (C) 2000 Elsevier Science B.V. All rights reserved.

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.

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.

The chemical effects of oxygenation of Cu(In,Ga)Se-2 (CIGS) interfaces are analyzed and are shown to involve passivation of Se deficiencies and Cu removal. The former effect is beneficial at grain boundaries, but detrimental at the CdS/CIGS interface. The latter effect is purely detrimental. Na and chemical bath deposition (CBD) treatments are shown to isolate the 'good' oxygenation effect from the 'bad' ones. Na is shown to promote oxygenation already before the deposition of the buffer and window layers, which allows a maximization of the benefits of Se deficiency passivation and a minimization of Cu removal. Next, the CBD of the CdS buffer layer restores the interface charge, due to creation of Cd-Cu interface donors and possibly a removal of O-Se interface accepters. This highlights the crucial role that interface redox engineering plays in optimizing the performance of CIGS-based solar cells. (C) 2000 Elsevier Science S.A. All rights reserved.

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

Single crystals of CuInSe2 were prepared under conditions to suppress the occurrence of twinning. This was accomplished by growth from an In melt by the traveling heater method to keep the growth temperature below that of the sphalerite-chalcopyrite phase transition. The resulting crystals were n-type and could be converted to p-type by a Se anneal. Both n and p-type crystals were characterized structurally by X-ray diffraction, compositionally by microprobe analyses, morphologically by atomic force and scanning electron microscopy, and electrically. They were found to be mostly homogeneous with mirror-like cleavage and without twins. (C) 1999 Elsevier Science B.V. All rights reserved.

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

A reinvestigation of the phase diagram of the Cu-In-Se system along the quasi-binary cut In2Se3-Cu2Se reveals an existence range of the chalcopyrite 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.

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

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. Primafacie evidence for this phase separation comes from coulometric Ag titration in and out of MCT. (C) 1999 Elsevier Science B.V. All rights reserved.

Hemispherical p/n/p transistor structures ranging from 100 mu m down to 0.05 mu m in diameter are fabricated in CuInSe(2), by application of a high electric field between a conducting diamond tip of an atomic force microscope and a CuInSe(2) crystal. This leads to electromigration of Cu ions in the bulk of the material. The results of this thermally assisted process are the transistor structures. These are characterized by scanning spreading resistance microscopy. For large devices these results are compared and found to agree with those of "conventional" electron-beam-induced current ones. Removing several tens of atomic layers from the top surface of a structure does not affect the spreading resistance image of the device. This indicates the three-dimensional hemispherical nature of the structures. [S0163-1829(99)13515-X].

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.

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.

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.

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

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.

By growing CuInSe(2) 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.

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.

We show how sub-mu m sized transistor structures (down to 50 nm cross section) can be fabricated by thermally assisted electromigration of mobile dopants inside the semiconductor CuInSe2. Small device structures are fabricated by application of an electric field to the sample via the contact, defined by a conducting atomic force microscope tip. The structures are characterized by nm scale scanning spreading resistance and scanning capacitance measurements to reveal the inhomogeneous doping profiles created by the electric field. (C) 1998 American Institute of Physics. [S0003-6951(98)03539-6].

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.

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

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

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.

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.

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.

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

Room temperature formation of ohmic contacts by electroplating gold on chemically treated surfaces of p-CuInSe2 and p-CdTe single crystals is reported. The effect of Br-2/methanol and KOH+KCN+H2O treatments prior to plating was analyzed in the case of CuInSe2. It is shown that the former treatment yields better ohmic contacts, with lower contact resistance, than the latter. While annealing these contacts made them highly non-ohmic, the method gives reasonably ohmic contacts on surfaces, that were purposely oxidized prior to contact preparation. In the case of p-CdTe stable, low resistance ohmic contacts were obtained at room temperature by electrochemical diffusion of Hg from solution, prior to gold plating,The treatment forms a highly degenerated p(+)-HgCdTe layer. The contacts, which have a very low contact to bulk resistivity ratio, were further improved by vacuum annealing.

Cu diffusion in chalcopyrite CuInSe2 was studied directly, using Cu-64 as a radioactive tracer. For diffusion from a thin surface layer, the Cu diffusion coefficients at 380 and 430 degrees C, were found to vary from 10(-8) to 10(-9) cm(2)/s. In case of diffusion from a volume source at 400 degrees C, a value of 10(-10) cm(2)/s was calculated from diffusion profiles. Electromigration of Cu was demonstrated, by applying a strong electric field to a sample and following the redistribution of Cu-64, that had been thermally diffused into the sample, prior to electric field application. (C) 1997 American Institute of Physics. [S0021-8979(97)03321-5].

Polymer electrochemical cells with ion blocking electrodes were reported to emit light under applied voltage. This work analyzes the current-voltage relations, internal electric fields, and point defect distribution in the polymer. The polymer is regarded as a mixed-ionic-electronic conductor. Two relevant defect models are investigated, A goad fit is obtained between experimental data and theory which also takes into consideration electrode over potentials. (C) 1997 American Institute of Physics.

Electron transfer (ET) in organic-inorganic hybrid structures is of interest for applications in optoelectronics. The study of these hybrid structures is difficult because contact formation is often destructive. We have experimented with the lift-off/float-on technique to create metal contacts in a gentle manner. We present results of these experiments and initial data on silicon/octadecyl trichlorosilane (OTS)/silver junctions where the metal contact was prepared by evaporation. The presence of the OTS greatly affects the current-voltage characteristics of such a junction.

A p-n junction is a stable situation for electronic carriers, but not for those dopants that are the cause of the junction. Thus the junction is stabilized only kinetically against equilibration of the dopant concentrations due to the electrical and chemical potential gradients that reign in the depletion regions that are associated with the junction. The situation changes, though, when a dopant can be present in two forms, with opposite effective charges, with the majority form being responsible for doping and the minority one for drifting. Then the electrical potential gradient can actually stabilize the junction. Ag, which acts as an acceptor in (Hg,Cd)Te, is an example of such a dopant. Silver creates a p-n junction in Cd-rich n-(Hg,Cd)Te. We show that this junction is capable of restoring itself after being smeared out by small electrical or thermal perturbations. This behaviour suggests that we have a thermodynamically, rather than a kinetically stabilized junction. It indicates that Ag dissolution in MCT strongly deviates from ideal behaviour. Reasons for this non-ideality are suggested.

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.

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.

Contact potential difference measurements in the dark and under illumination are used to derive the conduction band offset (Delta E(c)) in a solar cell quality junction formed by chemical bath deposition of CdS on a polycrystalline thin film of Cu(In,Ga)Se-2. Our experimental measurements and the estimates made for dipole contributions show that the junction is of type II, i.e., without a spike in the conduction band (Delta E(c) = 80 meV +/- 100 meV). This is consistent with the high performance of the actual solar cell. However, it differs from most previous results on junctions based on single crystals and/or vacuum deposited CdS, which indicated the existence of a conduction band spike. (C) 1995 American Institute of Physics.

Communication: Light emitting diodes where the efficiency of the junction electroluminescence has been improved over devices fabricated using thermal in-diffusion methods, is demonstrated for structures such as that shown in the figure, which is an electric-field-induced device. As E-field produced transistors are already available, combined monolithic LED/transistor structures produced using E-field-induced doping of semiconductors could be possible.

Surface photovoltage spectroscopy has been used as a clear, simple method to identify the two polar faces [(111) and (111BAR)] of undoped CdTe. The difference in electronic structure of the two faces is clearly observed in air on polished untreated samples. The (111BAR)Te surface is characterized by an acceptor level, at an energy 1.17 eV below the conduction-band minimum, which is assumed to result from TeO2-CdTe interaction. The (111)Cd surface is distinguished by two additional donor levels, at 1.11 and 1.33 eV above the valence-band maximum, which seem to be due to Cd surface atom displacements, accompanying the Te oxidation. These assignments are supported by the results obtained after various chemical treatments.

Semionic behaviour is demonstrated in certain ternary and multinary semiconductors, by following the effects of application of external electric fields at ambient temperature. We find that this can lead to stable, local changes in their microchemical composition and, via resulting changes in carrier concentration, in their electronic properties. Thus, actual device structures are embedded in the material. We summarize these results and our understanding of the mechanisms involved.

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.

Semionic behaviour is demonstrated in certain ternary and multinary semiconductors, by following the effects of application of external electric fields at ambient temperature. We find that this can lead to stable, local changes in their microchemical composition and, via resulting changes in carrier concentration, in their electronic properties. Thus, actual device structures are embedded in the material. We summarize these results and our understanding of the mechanisms involved.

Impedance measurements were used to evaluate the relative band edge positions of single crystal p-CuInSe2 electrodes in various aqueous electrolytes, by measuring the extrapolated flatband potentials, V(fb). We find that V(fb) can be shifted, depending on the extent of the potential scan and on the pH of the electrolyte used, over a range of up to 1.7 V (between pH 0-pH 14). In the pH range 0-6, V(fb) can be fixed at intermediate values, which, in their turn, are determined by the pH of the electrolyte.

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.

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.

Diffusion of oxygen at room temperature in polycrystalline pellets and thin films of YBa2Cu3O7-x (123) is measured from the current decay at constant potential in an electrochemical cell with a liquid electrolyte, using 123 as the cathode. The effective chemical diffusion coefficient is found to be 10(-11)-10(-12)cm2/s, thus explaining the relatively facile movement of oxygen in such samples.