Rational Design of Nanostructured Hybrid Materials for Photovoltaics
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Rational Design of Nanostructured Hybrid Materials for Photovoltaics Ioan Botiz and Seth B. Darling Center for Nanoscale Materials, Argonne National Laboratory, 9700 S. Cass Ave., Argonne, IL 60439, U.S.A. ABSTRACT To develop efficient organic and/or hybrid organic-inorganic solar energy devices, it is necessary to use, among other components, an active donor–acceptor layer with highly ordered nanoscale morphology. In an idealized morphology, the effectiveness of internal processes is optimized leading to an efficient conversion of photons to electricity. Using a poly(3hexylthiophene)-block-poly(L-lactide) rod-coil block copolymer as a structure-directing agent, we have rationally designed and developed an ordered nanoscale morphology consisting of selfassembled poly(3-hexylthiophene) donor domains of molecular dimension, each of them separated by fullerene C60 hydroxide acceptor domains. Using this morphological control, one can begin to probe structure-property relationships with unprecedented detail with the ultimate goal of maximizing the performance of future organic/hybrid photovoltaic devices. INTRODUCTION Traditional photovoltaic materials are well-suited to some markets, these technologies are currently too expensive to implement on a global scale, so scientists are directing fresh attention toward new generations of organic and/or hybrid low-cost photovoltaic (PV) devices.1,2 Demonstrated efficiencies of such PV energy devices are far short of the thermodynamic limit. Efficient conversion of photons to electricity in organic and hybrid materials depends on optimization of factors including light absorption,3 exciton separation,4 and charge carrier migration.5 Various PV device concepts have been developed, including polymer-based solar cells. The most efficient such devices are made by blending a semiconducting polymer either with fullerenes,2,6 other polymers,7 or nanocrystals.8 One conclusion that appears when dealing with polymer-based PV devices is that donor–acceptor bulk heterojunction (BHJ) devices address key internal processes, but disorder on the nanoscale results in inefficiencies due to exciton recombination and poor mobility. In order to develop high-performance organic and/or hybrid organic-inorganic solar energy devices, it is necessary to control the active layer morphology on the nanoscale.9 One promising approach is to design block copolymers possessing one or more optoelectronic active blocks.10 These copolymers can undergo a microphase separation process that leads to ordered morphologies on surfaces11 characterized by nanostructured domains of characteristic distance tuned by the polymer block lengths. The goal of the work presented here is to experimentally implement the idea of morphological control using common PV active component materials: poly(3-hexylthiophene) and C60. While the standard approach is to mix these two compounds and manipulate processing conditions to try to optimize nanoscale phase separation in the BHJ layer, we have used a novel fabrication process
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