Distributed Bilayer Photovoltaics Based on Nematic Liquid Crystal Polymer Networks

We discuss a liquid crystal composite approach to provide a distributed interface to vertically separate electron-donating and electron-accepting films in an organic photovoltaic device. Two different methods are used to prepare a nematic liquid crystal polymer network with a porous surface and electron-donating properties. This is infilled with an electron-accepting organic semiconductor to form a bilayer device. The interface is diffuse rather than localized so that more photogenerated excitons can reach it to generate charge before they recombine. Photoinduced absorption of a blend of the donor and acceptor materials confirms that excitons dissociate at the heterointerface. The spatial features of the diffuse interface are examined by Fourier analysis of topographic images. We find a correlation between the in-plane spatial frequencies of the interface and photovoltaic device performance. The device performance is investigated as a function of input irradiance. Any charge combination is monomolecular rather than bimolecular, and the monochromatic power conversion efficiency varies between 0.8% and 0.3% with input irradiance. Equivalent circuit analysis shows that this is limited by a high series resistance, a blocking contact, and nonoptimized spatial features of the porous interface.