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David Cahen

Publications and source records attributed to David Cahen.

35 records · Page 2Linked to original sources
arXiv2016

Large-area, ensemble molecular electronics: Motivation and challenges

We review charge transport across molecular monolayers, which is central to molecular electronics (MoE) using large-area junctions (NmJ). We strive to provide a wide conceptual overview of three main sub-topics. First, a broad introduction places NmJ in perspective to related fields of research, and to single molecule junctions (1mJ), in addition to a brief historical account. As charge transport presents an ultra sensitive 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 MoE, 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 analyse the net electrical performance of molecular junctions, from a functional device perspective. Finally, we shortly point to creation of a built-in electric field as a key to achieve functionality, including non-linear current-voltage characteristics that originate in the molecules or their contacts to the electrodes.

cond-mat.mtrl-sci↗
arXiv2016

Molecular Electronics by Chemical Modification of Semiconductor Surfaces

Inserting molecular monolayers within metal / semiconductor interfaces provides one of the most powerful expressions of how minute chemical modifications can affect electronic devices. This topic also has direct importance for technology as it can help improve the efficiency of a variety of electronic devices such as solar cells, LEDs, sensors and possible future bioelectronic devices, which are based mostly on non-classical semiconducting materials (section 1). The review covers the main aspects of using chemistry to - control alignment of energy levels at interfaces (section 2): - passivate interface states (section 3), - insert molecular dipoles at interfaces (section 4), - induce charge rearrangement at and around interfaces (section 5). After setting the stage, we consider the unique current-voltage characteristics that result from transport across metal / molecular monolayer / semiconductor interfaces. Here we focus on the interplay between the monolayer as tunneling barrier on the one hand, and the electrostatic barrier within the semiconductor, due to its space-charge region (section 6), on the other hand, as well as how different monolayer chemistries control each of the these barriers. Section 7 provides practical tools to experimentally identify these two barriers, and distinguish between them, after which section 8 concludes the story with a summary and a view to the future. While this review is concerned with hybrid semiconductor / molecular effects (see Refs. 1,2 for earlier reviews on this topic), issues related to formation of monolayers and contacts, as well as charge transport that is solely dominated by molecules, have been reviewed elsewhere[3-6], including by us recently[7].

cond-mat.mtrl-sci↗
arXiv2016

Temperature-dependent Hysteresis in MAPbI3 Solar Cells

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

cond-mat.mtrl-sci↗
arXiv2015

Cesium Enhances Long-Term Stability of Lead Bromide Perovskite-Based Solar Cells

Direct comparison between perovskite-structured hybrid organic-inorganic - methyl ammonium 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 methyl ammonium-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.

cond-mat.mtrl-sci↗
arXiv2015

Are Mobilities in Hybrid Organic-Inorganic Halide Perovskites Actually 'High'?

We present an experimental and theoretical viewpoint on the electronic carrier mobilities of typical hybrid organic-inorganic perovskites (HOIPs). While these mobilities are often quoted as high, a review of them shows that although otherwise the semiconducting properties of HOIPs are impressively good, mobilities of HOIPs used in most solar cells are actually not that high. This is especially apparent if they are compared to those of inorganic semiconductors used in other high efficiency solar cells. We critically examine possible causes and focus on electron-lattice coupling mechanisms that are active at room temperature, and can lead to carrier scattering. From this, we propose scattering due to acoustic phonons or polarons as possible causes, but also point out the difficulties with each of these in view of additional experimental and theoretical findings in the literature. Further research in this direction will contribute to making HOIP solar cells even more efficient than they already are.

cond-mat.mtrl-sci↗
arXiv2015

Mechanical Properties of APbX3 (A=Cs or CH3NH3; X=I or Br) Perovskite Single Crystals

The remarkable optoelectronic, and especially photovoltaic performance of hybrid-organic-inorganic perovskite (HOIP) materials drives efforts to connect materials properties to this performance. From nano-indentation experiments on solution-grown single crystals we obtain elastic modulus and nano-hardness values of APbX3 (A = Cs, CH3NH3 and X = I, Br). The Youngs moduli are about 14, 19.5 and 16 GPa, for CH3NH3PbI3, CH3NH3PbBr3 and CsPbBr3, respectively, lending credence to theoretically calculated values. We discuss possible relevance of our results to suggested self-healing, ion diffusion and ease of manufacturing. Using our results, together with literature data on elastic moduli, we classified HOIPs amongst relevant materials groups, based on their elasto-mechanical properties.

cond-mat.mtrl-sci↗
arXiv2015

Mode-selective vibrational control of charge transport in $π$-conjugated molecular materials

The soft character of organic materials leads to strong coupling between molecular nuclear and electronic dynamics. This coupling opens the way to control charge transport in organic electronic devices by inducing molecular vibrational motions. However, despite encouraging theoretical predictions, experimental realization of such control has remained elusive. Here we demonstrate experimentally that photoconductivity in a model organic optoelectronic device can be controlled by the selective excitation of molecular vibrations. Using an ultrafast infrared laser source to create a coherent superposition of vibrational motions in a pentacene/C60 photoresistor, we observe that excitation of certain modes in the 1500-1700 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. Vibrational control thus presents a new tool for studying electron-phonon coupling and charge dynamics in (bio)molecular materials.

cond-mat.mtrl-sci↗
arXiv2014

Why Lead Methylammonium tri-IODIDE perovskite-based solar cells requires a mesoporous electron transporting scaffold (but not necessarily a hole conductor)

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, LD, (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 LD values are comparable for both charge types, do not.

physics.chem-ph↗
arXiv2012

A permanent, stable, and simple top-contact for molecular electronics on Si: Pb evaporated on organic monolayers

We show that thermally evaporated lead (Pb) preserves the electronic properties of organic monolayers on Si and the surface passivation of the Si surface itself. The obtained current-voltage characteristics are in accordance with results from the well-established hanging mercury drop method and preserve both the molecule-induced dipolar effect and length-attenuation of current. We rationalize our findings by the lack of interaction between the Pb and the Si substrate. Our method is fast, scalable, compatible to standard semiconductor processing, and can help to spur the large-scale utilization of silicon-organic hybrid electronics.

cond-mat.mtrl-sci↗
arXiv2012

Temperature and force dependence of nanoscale electron transport via the Cu protein Azurin

The mechanisms of solid-state electron transport (ETp) via a monolayer of immobilized Azurin (Az) was examined by conducting probe atomic force microscopy (CP-AFM), both as function of temperature (248 - 373K) and of applied tip force (6-12 nN). By varying both temperature and force in CP-AFM, we find that the ETp mechanism can alter with a change in the force applied via the tip to the proteins. As the applied force increases, ETp via Az changes from temperature-independent to thermally activated at high temperatures. This is in contrast to the Cu-depleted form of Az (apo-Az), where increasing the applied force causes only small quantitative effects, that fit with a decrease in electrode spacing. At low force ETp via holo-Az is temperature-independent and thermally activated via apo-Az. This observation agrees with macroscopic-scale measurements, thus confirming that the difference in ETp dependence on temperature between holo- and apo-Az is an inherent one that may reflect a difference in rigidity between the two forms. An important implication of these results, which depend on CP-AFM measurements over a significant temperature range, is that for ETp measurements on floppy systems, such as proteins, the stress applied to the sample should be kept constant or, at least controlled during measurement.

physics.bio-ph↗
arXiv2012

Doping Human Serum Albumin with Retinoate Markedly Enhances Electron Transport Across the Protein

Electrons can migrate via proteins over distances that are considered long for non-conjugated systems. Proteins' nano-scale dimensions and the enormous flexibility of their structures and chemistry makes them fascinating subjects for investigating the mechanism of their electron transport (ETp) capacity. One particular attractive research direction is that of tuning their ETp efficiency by doping them with external small molecules. Here we report that solid-state ETp across human serum albumin (HSA) increases by more than two orders of magnitude upon retinoate (RA) binding to HSA. RA was chosen because optical spectroscopy has provided evidence for the non-covalent binding of at least three RA molecules to HSA and indications for their relative structural positions. The temperature dependence of ETp shows that both the activation energy and the distance-decay constant decrease with increasing RA binding to HSA. Furthermore, the observed transition from temperature-activated ETp above 190K to temperature-independent ETp below this temperature suggests a change in the ETp mechanism with temperature.

physics.bio-ph↗
arXiv2012

Marked changes in electron transport through the blue copper protein azurin in the solid state upon deuteration

Measuring electron transport (ETp) across proteins in the solid-state offers a way to study electron transfer (ET) mechanism(s) that minimizes solvation effects on the process. Solid state ETp is sensitive to any static (conformational) or dynamic (vibrational) changes in the protein. Our macroscopic measurement technique extends the use of ETp meas-urements down to low temperatures and the concomitant lower current densities, because the larger area still yields measurable currents. Thus, we reported previously a surprising lack of temperature-dependence for ETp via the blue copper protein azurin (Az), from 80K till denaturation, while ETp via apo-(Cu-free) Az was found to be temperature de-pendent \geq 200K. H/D substitution (deuteration) can provide a potentially powerful means to unravel factors that affect the ETp mechanism at a molecular level. Therefore, we measured and report here the kinetic deuterium isotope effect (KIE) on ETp through holo-Az as a function of temperature (30-340K). We find that deuteration has a striking effect in that it changes ETp from temperature independent to temperature dependent above 180K. This change is expressed in KIE values between 1.8 at 340K and 9.1 at \leq 180K. These values are particularly remarkable in light of the previously reported inverse KIE on the ET in Az in solution. The high values that we obtain for the KIE on the ETp process across the protein monolayer are consistent with a transport mechanism that involves through-(H-containing)-bonds of the β-sheet structure of Az, likely those of am-ide groups.

physics.bio-ph↗
arXiv2011

Charge transport across metal/molecular (alkyl) monolayer-Si junctions is dominated by the LUMO level

We compare the charge transport characteristics of heavy doped p- and n-Si-alkyl chain/Hg junctions. Photoelectron spectroscopy (UPS, IPES and XPS) results for the molecule-Si band alignment at equilibrium show the Fermi level to LUMO energy difference to be much smaller than the corresponding Fermi level to HOMO one. This result supports the conclusion we reach, based on negative differential resistance in an analogous semiconductor-inorganic insulator/metal junction, that for both p- and n-type junctions the energy difference between the Fermi level and LUMO, i.e., electron tunneling, controls charge transport. The Fermi level-LUMO energy difference, experimentally determined by IPES, agrees with the non-resonant tunneling barrier height deduced from the exponential length-attenuation of the current.

cond-mat.mtrl-sci↗
arXiv2009

Proteins as Bioelectronic Materials: Electron Transport Through Solid-State, Protein Monolayer Junctions

Electron transfer (ET) through proteins, a fundamental element of many biochemical reactions, has been studied intensively in solution. We report the results of electron transport (ETp) measurements across proteins, sandwiched between two solid electrodes with a long-range goal of understanding in how far protein properties are expressed (and can be utilized) in such a configuration. While most such studies to date were conducted with one or just a few molecules in the junction, we present the high yield, reproducible preparation of large area monolayer junctions of proteins from three different families: Azurin (Az), a blue-copper ET protein, Bacteriorhodopsin (bR), a membrane protein-chromophore complex with a proton pumping function, and Bovine Serum Albumin (BSA). Surprisingly, the current-voltage (I-V) measurements on such junctions, which are highly reproducible, show relatively minor differences between Az and bR, even though the latter lacks a known ET function. ETp across both Az and bR is much more efficient than across BSA, but also for the latter the currents are still high, and the decay coefficients too low to be consistent with coherent tunneling. Rather, inelastic hopping is proposed to dominate ETp in these junctions. Other features such as asymmetrical I-V curves and distinct behavior of different proteins can be viewed as molecular signatures in the solid-state conductance.

cond-mat.mes-hall↗
arXiv2009

Molecular electronics at Metal / Semiconductor Junctions Si inversion by Sub-nm Molecular Films

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.

cond-mat.mes-hall↗
arXiv2005

How do electronic carriers cross Si-bound alkyl monolayers?

Electron transport through Si-C bound alkyl chains, sandwiched between n-Si and Hg, is characterized by two distinct types of barriers, each dominating in a different voltage range. At low voltage, current depends strongly on temperature but not on molecular length, suggesting transport by thermionic emission over a barrier in the Si. At higher voltage, the current decreases exponentially with molecular length, suggesting tunneling through the molecules. The tunnel barrier is estimated, from transport and photoemission data, to be ~1.5 eV with a 0.25me effective mass.

cond-mat.mtrl-sci↗