Dhandapani Venkataraman

Organic semiconductors, particularly conjugated polymers, are critical for advancing flexible electronics. Their electrical conductivity is enhanced by increasing carrier concentration through doping by blending small atoms or molecules, which can introduce Coulombic traps and disrupt packing. Here, we elucidate a molecular design principle that leverages dopant energy depth quantified by Δ E , which is the difference between appropriate frontier orbital energies of the doped polymer and its counterion, to mitigate disorder and simultaneously improve conductivity and the Seebeck coefficient. Using a modified Gaussian disorder model that incorporates carrier screening, we simulate the density of states and validate our findings experimentally with iodine‐ and Magic Blue‐doped poly(3‐hexylthiophene) (P3HT). Our results reveal that deeper dopant frontier energy levels significantly reduce energetic disorder and enhance transport properties when the carrier‐counterion separation is small. We identify a saddle‐point behavior in conductivity as a function of Δ E and show that Δ E can significantly boost conductivity when the dopant is located close to the backbone. This work introduces Δ E as a powerful molecular lever for designing high‐performance organic semiconductors, offering a new design rule to overcome the long‐standing trade‐off between conductivity and Seebeck coefficient, thereby increasing thermoelectric efficiency in doped conjugated polymers.

The interface between the hole transport layer (HTL) and perovskite in p‐i‐n perovskite solar cells (PSCs) plays a vital role in the device performance and stability. However, the impact of this interface on the vertical phase segregation of mixed halide perovskite remains insufficiently understood. This work systematically investigates the impact of chemical and electronic properties of HTL on vertical halide segregation of mixed‐halide perovskites. This work shows that incorporating a poly[bis(4‐phenyl) (2,4,6‐trimethylphenyl) amine] (PTAA)/CuI_xBr_1‐x bilayer as the HTL significantly suppresses light‐induced vertical phase segregation in MAPb(I_0.7Br_0.3)₃. This work uses grazing‐incidence X‐ray diffraction (GIXRD) to capture the depth‐resolved composition change of MAPb(I_0.7Br_0.3)₃ at the interface and within the bulk under illumination. By changing the illumination direction and the electronic properties of HTL, this work elucidates the roles of charge carrier extraction and interfacial defects on vertical phase segregation. The PTAA/CuI_xBr_1‐x bilayer, with its synergistic passivation and efficient hole extraction ability, stabilizes the interface and bulk of the mixed halide perovskite layer and prevents phase segregation. This work underscores that synergetic passivation and efficient hole extraction pack a more powerful punch for arresting the vertical phase segregation in mixed‐halide perovskite.

Copper iodide (CuI) is a promising material for use as hole‐transport layers in electronic devices due to their solution processability and efficient hole conductivity. CuI has a rich chemistry with halide salts and solvents to which it may be exposed during device fabrication. Thus, care must be taken during device fabrication when CuI is used. We present a study using CuI as a hole transport layer in a p‐i‐n perovskite photovoltaic architecture. We studied how each of the components present in the perovskite precursor solution impacts the integrity of CuI films using power x‐ray diffraction, UV‐vis spectroscopy and impedance spectroscopy. Based on these studies, we show that DMSO, mixtures of γ‐butyrolactone:DMSO (v/v 7 : 3) and DMF:DMSO (v/v 8 : 2), and iodide ions can dissolve the CuI layer. We also how that by coating a layer of copper(II) acetate and utilizing the known Cu(II)/Cu(I) redox chemistry with iodide ions, we can preserve CuI in the presence of halide salts and solvents.

Conjugated polymers need to be doped to increase charge carrier density and reach the electrical conductivity necessary for electronic and energy applications. While doping increases carrier density, Coulomb interactions between the dopant molecules and the localized carriers are poorly screened, causing broadening and a heavy tail in the electronic density‐of‐states (DOS). The authors examine the effects of dopant‐induced disorder on two complimentary charge transport properties of semiconducting polymers, the Seebeck coefficient and electrical conductivity, and demonstrate a way to mitigate them. Their simulations, based on a modified Gaussian disorder model with Miller‐Abrahams hopping rates, show that dopant‐induced broadening of the DOS negatively impacts the Seebeck coefficient versus electrical conductivity trade‐off curve. Increasing the dielectric permittivity of the polymer mitigates dopant‐carrier Coulomb interactions and improves charge transport, evidenced by simultaneous increases in conductivity and the Seebeck coefficient. They verified this increase experimentally in iodine‐doped P3HT and P3HT blended with barium titanate (BaTiO₃) nanoparticles. The addition of 2% w/w BaTiO₃ nanoparticles increased conductivity and Seebeck across a broad range of doping, resulting in a fourfold increase in power factor. Thus, these results show a promising path forward to reduce the dopant‐charge carrier Coulomb interactions and mitigate their adverse impact on charge transport.