Sabyasachi Mukhopadhyay

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-x)Clx 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 CH3NH3PbI(3-Clx ones, where LD values are comparable for both charge types, do not.

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 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.

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)n-phenyl, with n = 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.

High-Resolution Photocurrent Imaging of Bulk Heterojunction Solar Cells.

Images obtained from photocurrent scanning of organic bulk heterojunction solar cell devices provide a direct measure of correlation of the morphology to the performance parameters. The peripheral photocurrent induced from light coupled to probe tips in the near-field regime of bulk heterojunction layers permits in situ scanning of active solar cells with asymmetric electrodes. We present a methodology involving a combination of atomic force microscopy, near-field optical microscopy, and near-field photocurrent microscopy to decipher the carrier generation and transport regions in the bulk heterojunction layer. The angular Fourier transformation technique is implemented on these images to rationalize the optimum blend concentration in crystalline and amorphous donor systems and provide insights into the role of the bulk heterojunction morphology.

Protein bioelectronics: a review of what we do and do not know

We review the status of protein-based molecular electronics. First, we define and discuss fundamental concepts of electron transfer and transport in and across proteins and proposed mechanisms for these processes. We then describe the immobilization of proteins to solid-state surfaces in both nanoscale and macroscopic approaches, and highlight how different methodologies can alter protein electronic properties. Because immobilizing proteins while retaining biological activity is crucial to the successful development of bioelectronic devices, we discuss this process at length. We briefly discuss computational predictions and their connection to experimental results. We then summarize how the biological activity of immobilized proteins is beneficial for bioelectronic devices, and how conductance measurements can shed light on protein properties. Finally, we consider how the research to date could influence the development of future bioelectronic devices.

Rationalization of donor-acceptor ratio in bulk heterojunction solar cells using lateral photocurrent studies

A robust bicontinuous network for charge transport is a central requirement for efficient bulk heterojunction polymer solar cells. Factors affecting the network morphology include crystallinity and the relative concentration of the constituent materials. These factors are closely followed using a scanning approach which involves monitoring the photocurrent decay from the cathode-periphery in asymmetric device structures. The decay length correlates with the efficiency and the network connectivity of the ensuing microstructure.

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 bacterioruberins ends proximal to the two electrodes that contact the protein; thus, coupling to these electrodes may facilitate the activation-less current across the contacts. Oxidation of bacterioruberin eliminates its conjugation, causing the ETp to become temperature dependent (>180 K). Remarkably, even elimination of the retinal-protein covalent bond, with the fully conjugated bacterioruberin still present, leads to temperature-dependent ETp (>180 K). These results suggest that ETp via phR is cooperatively affected by both retinal and bacterioruberin cofactors.

Characteristic noise features in light transmission across membrane protein undergoing photocycle

We demonstrate a technique based on noise measurements which can be utilized to study dynamical processes in protein assembly. Direct visualization of dynamics in membrane protein system such as bacteriorhodopsin (bR) upon photostimulation are quite challenging. bR represents a model system where the stimulus-triggered structural dynamics and biological functions are directly correlated. Our method utilizes a pump-probe near field microscopy method in the transmission mode and involves analyzing the transmittance fluctuations from a finite size of molecular assembly. Probability density distributions indicating the effects of finite size and statistical correlations appear as a characteristic frequency distribution in the noise spectra of bR whose origin can be traced to photocycle kinetics. Valuable insight into the molecular processes were obtained from the noise studies of bR and its mutant D96N as a function of external parameters such as temperature, humidity or presence of an additional pump source.

Lateral Photocurrent Scanning of Donor and Acceptor Polymers on Graphene Coated Substrates

Graphene provides a two-dimensional surface which can be utilized to interface with a variety of molecular species to modify opto-electronic processes. We use a scanning photocurrent technique to study the effect of graphene-coated anode substrates in semiconducting polymer device structures. The approach involves the measurement of the spatially varying photocurrent generated by an active semiconducting-polymer film sandwiched between a patterned substrate and a top electrode, where the narrow-incident light beam scans regions beyond the overlapping electrodes. We observe substantial difference in the spatial decay profile of the photocurrent upon introduction of graphene layers in the structure. Using simple circuit model and spreading impedance analysis we discuss the modification in carrier transport and recombination processes by underlying graphene layer in solar cell devices.

Monitoring intermediate states of bacteriorhodopsin monolayers using near-field optical microscopy.

We demonstrate single-molecule-level features using near-field optical microscopy on bacteriorhodopsin (bR), a membrane protein that functions as a light-driven proton pump. The photophysical properties of bR are utilized in this imaging technique, using a combination of photoexcitation sources, to accurately identify the active regions and quantify the optical parameters. The studies of bR monolayers are carried out on inert quartz substrates as well as active conducting polymer (polyaniline) substrates. The substrate also plays an important role in the photocycle quantum efficiencies. We speculate on mechanisms governing the higher near-field absorption strength of bR molecules.