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Doped organic semiconductors are critical to emerging device applications, including thermoelectrics, bioelectronics, and neuromorphic computing devices. It is commonly assumed that low conductivities in these materials result primarily from charge trapping by the Coulomb potentials of the dopant counterions. Here, we present a combined experimental and theoretical study rebutting this belief. Using a newly developed doping technique based on ion exchange, we prepare highly doped films with several counterions of varying size and shape and characterize their carrier density, electrical conductivity, and paracrystalline disorder. In this uniquely large data set composed of several classes of high-mobility conjugated polymers, each doped with at least five different ions, we find electrical conductivity to be strongly correlated with paracrystalline disorder but poorly correlated with ionic size, suggesting that Coulomb traps do not limit transport. A general model for interacting electrons in highly doped polymers is proposed and carefully parametrized against atomistic calculations, enabling the calculation of electrical conductivity within the framework of transient localization theory. Theoretical calculations are in excellent agreement with experimental data, providing insights into the disorder-limited nature of charge transport and suggesting new strategies to further improve conductivities.

The self-assembly process and the correlation with device performance are studied through ZnO nanorods. It is seen that uniform ZnO nanorod films with defined nanorod alignment can be solution-deposited over large areas by controlling the surface energy of the nanorods through the choice of suitable ligands and by the fluid flow direction during growth.

Organic semiconductors consisting of conjugated oligomers or polymers are suitable in photodiodes, light emitting device and other related applications because of their optoelectronic properties. An innovative approach in designing and synthesizing organic semiconductors may also enhance its thin film transistor applications. It can be achieved through the use of thiophene derivatives.

Substituted side chains are fundamental units in solution processable organic semiconductors in order to achieve a balance of close intermolecular stacking, high crystallinity, and good compatibility with different wet techniques. Based on four air-stable solution-processed naphthalene diimides fused with 2-(1,3-dithiol-2-ylidene)malononitrile groups (NDI-DTYM2) that bear branched alkyl chains with varied side-chain length and different branching position, we have carried out systematic studies on the relationship between film microstructure and charge transport in their organic thin-film transistors (OTFTs). In particular synchrotron measurements (grazing incidence X-ray diffraction and near-edge X-ray absorption fine structure) are combined with device optimization studies to probe the interplay between molecular structure, molecular packing, and OTFT mobility. It is found that the side-chain length has a moderate influence on thin-film microstructure but leads to only limited changes in OTFT performance. In contrast, the position of branching point results in subtle, yet critical changes in molecular packing and leads to dramatic differences in electron mobility ranging from 0.001 to >3.0 cm2 V-1 s-1. Incorporating a NDI-DTYM2 core with three-branched N-alkyl substituents of C11,6 results in a dense in-plane molecular packing with an unit cell area of 127 Å2, larger domain sizes of up to 1000 x 3000 nm2, and an electron mobility of up to 3.50 cm2 V-1 s-1, which is an unprecedented value for ambient stable n-channel solution-processed OTFTs reported to date. These results demonstrate that variation of the alkyl chain branching point is a powerful strategy for tuning of molecular packing to enable high charge transport mobilities. [ABSTRACT FROM AUTHOR]

We report binary nanoparticle superlattices obtained by self-assembly of two different semiconductor quantum dots. Such a system is a means to include two discretized, quantum-confined, and complimentary semiconductor units in close proximity, for purposes of band gap matching and/or energy transfer. From a range of possible structures predicted, we observe an exclusive preference for the formation of Cuboctahedral AB13 and AB5 (isostructural with CaCu5) obtained in the system of 8.1 nm CdTe and 4.4 nm CdSe nanoparticles. For this system, a possible ionic origin for the formation of structures with lower packing densities was ruled out on the basis of electrophoretic mobility measurements. To understand further the principles of superlattice formation, we constructed space-filling curves for binary component hard spheres over the full range of radius ratio. In addition, the pair interaction energies due to core—core and ligand—ligand van der Waals (VDW) forces are estimated. The real structures are believed to form under a combined influence of entropic driving forces (following hard-sphere space filling principles) and the surface (due to ligand—ligand VDW). [ABSTRACT FROM AUTHOR]

A fused donor, thienobenzo[ b ]indacenodithiophene ( TBIDT ), was designed and synthesized using a novel acid-promoted cascade ring closure strategy, and then copolymerized with a benzothiadiazole ( BT ) monomer. The backbone of TBIDT is an expansion of the well-known indacenodithiophene ( IDT ) unit and was expected to enhance the charge carrier mobility by improving backbone planarity and facilitating short contacts between polymer chains. However, the optimized field-effect transistors demonstrated an average saturation hole mobility of 0.9 cm 2 V -1 s -1 , lower than the performance of IDT-BT (∼1.5 cm 2 V -1 s -1 ). Mobilities extracted from time-resolved microwave conductivity measurements were consistent with the trend in hole mobilities in organic field-effect transistor devices. Scanning tunneling microscopy measurements and computational modeling illustrated that TBIDT-BT exhibits a less ordered microstructure in comparison to IDT-BT . This reveals that a regular side-chain packing density, independent of conformational isomers, is critical to avoid local free volume due to irregular packing, which can host trapping impurities. DFT calculations indicated that TBIDT-BT , despite containing a larger, planar unit, showed less stabilization of planar backbone geometries in comparison to IDT-BT . This is due to the reduced electrostatic stabilizing interactions between the peripheral thiophene of the fused core and the BT unit, resulting in a reduction of the barrier to rotation around the single bond. These insights provide a greater understanding of the general structure-property relationships required for semiconducting polymer repeat units to ensure optimal backbone planarization, as illustrated with IDT -type units, guiding the design of novel semiconducting polymers with extended fused backbones for high-performance field-effect transistors.

We report the synthesis and polymerization of a novel thieno[3,2-b]thiophene-diketopyrrolopyrrole-based monomer. Copolymerization with thiophene afforded a polymer with a maximum hole mobility of 1.95 cm(2) V(-1) s(-1), which is the highest mobility from a polymer-based OFET reported to date. Bulk-heterojunction solar cells comprising this polymer and PC(71)BM gave a power conversion efficiency of 5.4%.

We report binary nanoparticle superlattices obtained by self-assembly of two different semiconductor quantum dots. Such a system is a means to include two discretized, quantum-confined, and complimentary semiconductor units in close proximity, for purposes of band gap matching and/or energy transfer. From a range of possible structures predicted, we observe an exclusive preference for the formation of Cuboctahedral AB13 and AB5 (isostructural with CaCu5) obtained in the system of 8.1 nm CdTe and 4.4 nm CdSe nanoparticles. For this system, a possible ionic origin for the formation of structures with lower packing densities was ruled out on the basis of electrophoretic mobility measurements. To understand further the principles of superlattice formation, we constructed space-filling curves for binary component hard spheres over the full range of radius ratio. In addition, the pair interaction energies due to core-core and ligand-ligand van der Waals (VDW) forces are estimated. The real structures are believed to form under a combined influence of entropic driving forces (following hard-sphere space filling principles) and the surface (due to ligand-ligand VDW).