Conjugated Cofactor Enables Efficient Temperature-Independent Electronic Transport Across ∼6 nm Long Halorhodopsin

We observe temperature-independent electron transport, characteristic of tunneling across a ∼6 nm thick Halorhodopsin (phR) monolayer. phR contains both retinal and a carotenoid, bacterioruberin, as cofactors, in a trimeric protein-chromophore complex. This finding is unusual because for conjugated oligo-imine molecular wires a transition from temperature-independent to -dependent electron transport, ETp, was reported at ∼4 nm wire length. In the ∼6 nm long phR, the ∼4 nm 50-carbon conjugated bacterioruberin is bound parallel to the α-helices of the peptide backbone. This places 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

Nanoscale Electron Transport and Photodynamics Enhancement in Lipid-Depleted Bacteriorhodopsin Monomers

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

Why Lead Methylammonium Tri-Iodide Perovskite-Based Solar Cells Require 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 CH₃NH₃PbI_3–<i>x</i>Cl_<i>x</i> ones. A spatial map of charge separation efficiency in working cells shows p-i-n structures for both thin film cells. Effective diffusion lengths, <i>L</i>_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 CH₃NH₃PbI_3–<i>x</i>Cl_<i>x</i> ones, where <i>L</i>_D values are comparable for both charge types, do not

Odd–Even Effect in Molecular Electronic Transport via an Aromatic Ring

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

Interface-Dependent Ion Migration/Accumulation Controls 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 <i>within the perovskite</i>. 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 <i>different</i> 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 <i>V</i>_OC as a function of temperature shows that the main recombination mechanism is trap-assisted (Shockley-Read Hall, SRH) in the space charge region, similar to what is the case for other thin film inorganic solar cells