Novel Nanoscale Organic Materials for Optimal Photovoltaic Functions
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Novel Nanoscale Organic Materials for Optimal Photovoltaic Functions Lin X Chen1, Dmitrii Polshakov1, Shengqiang Xiao2, Yongye Liang2, and Luping Yu2 1 Chemistry, Argonne National Laboratory, 9700 South Cass Ave., Argonne, IL, 60439 2 Chemistry, The University of Chicago, 5735 South Ellis Ave., Chicago, IL, 60637
ABSTRACT Covalently linked electron donor (D) and electron acceptor (A) with conjugated organic building blocks are novel materials for potential solar cell applications, because these molecular p-n junctions can minimize the exciton diffusion and transform the charge separation from interdomain to intramolecular processes. Hence, the bottleneck of the exciton diffusion in many bulk heterojuction materials can be eliminated. Meanwhile, these planar conjugated assemblies, such as supermolecules, multiblock oligomers and polymers, have strong tendency to π-π stacking to form continuous channels for charge carriers to hop/diffuse to respective electrodes. A quartet D-A assembly has been synthesized with bis-oligothiophene (BOTH) and bisperylenediimide (BPDI) derivatives attached to a benzo template. The electronic structures and dynamics of photoinduced charge separation and recombination of this quartet molecule and reference compounds in solutions and films were studied at isolated the molecular level in solutions as well as at the molecular assembly level in films with stacked structures. Two different dynamics of charge separation and recombination associated with two types of donor/acceptor pair conformations in solution were observed. This molecular system exhibits a more efficient charge separation than charge recombination processes in both polar and nonpolar organic solvents, as well as films. More efficient charge separation and slower charge recombination due to the covalent linkage indicating that the material is a potential candidate for photovoltaic studies in solid-state. INTRODUCTION Most of the organic photovoltaic (OPV) materials studied so far, such as CuPc/C60 (Pc. Phthalocyanine),[1-8], and diblock copolymers/C60 composites,[9-12] are non-covalent microscopic mixtures of multiple organic compounds of electron donors (D) and electron acceptors (A). When excitons are generated in these materials by light illumination, they must be able to diffuse to the boundaries between the microscopic D/A domains in order to dissociate into opposite charge carriers, which then need to be collected by respective electrodes to generate electricity. The exiton diffusion lengths in organic semiconductor materials are on the order of multiple nanometer scales,[5, 7, 13, 14] often shorter than the D/A domain sizes in some current organic or hybrid solar cell materials. Consequently, the quantum yield of charge separation in these materials is low, which has been considered the bottle neck of the overall efficiency of organic photovoltaic solar cells.[15] Approaches have been proposed and implemented in growing interpenetrating microscopic donor and acceptor columns to enhance the yield of dissoci
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