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The effect of thermal annealing on photovoltaic devices comprising poly(3-hexylthiophene):[6,6]-phenyl C61 butyric acid methyl ester (P3HT:PCBM) with thicknesses up to 1200 nm was investigated. Without thermal annealing, the efficiency of the as-prepared devices decreased with increasing active layer thickness, reflecting largely a reduction in the short-circuit current density and an inverse photocurrent spectral response. Thermal annealing of the full devices was found to substantially recover thick-film device efficiencies while reducing the thin-film device efficiencies. The profound variations in photovoltaic characteristics were interpreted in terms of vertical phase separation in the P3HT:PCBM blend film and Li+ diffusion from the LiF/Al contact. © 2010 American Institute of Physics.

Thesis (Ph. D.)--University of Rochester. Dept. of Chemical Engineering, 2010., Through light absorption and emission as well as charge carrier generation, transport and recombination, π-conjugated molecules are central to electronic devices including organic field-effect transistors, organic light-emitting diodes, and organic solar cells. This thesis reports on materials development via molecular design, material synthesis and processing, device fabrication and characterization. Major accomplishments are summarized as follows. A series of oligo(fluorene-co-bithiophene)s, OF2Ts, have been synthesized and characterized for an investigation of the effects of oligomer length and pendant aliphatic structure on thermotropic properties, light absorption and emission, and anisotropic field-effect mobilities. Solvent-vapor annealing at room temperature was shown to be capable of orienting OF2Ts into monodomain glassy-nematic films with an orientational order parameter emulating that achieved with conventional thermal annealing on a rubbed polyimide alignment layer. Comprising hole- and electron-transporting moieties with flexible linkages, non-conjugated bipolar compounds have been developed for use as hosts for electrophosphorescence. These materials are characterized by an elevated glass transition temperature, morphological stability against crystallization, LUMO and HOMO levels unaffected by chemical bonding, and triplet energy unconstrained by the electrochemical energy gap. Phosphorescent OLEDs containing solution-processed emitting layers were fabricated with TRZ-3Cz(MP)2, TRZ-1Cz(MP)2 and Cz(MP)2 hosting Ir(mppy)3 for an illustration of how chemical composition and hence charge transport properties affect device performance. Bulk heterojunction organic solar cells comprising an active layer of P3HT:PCBM blend at a 1:1 mass ratio with thickness from 130 to 1200 nm have been fabricated and characterized before and after thermal annealing. Before thermal annealing, both short circuit current density and power conversion efficiency decrease with an