Thin-film photovoltaic devices incorporating low-bandgap push-pull molecules dispersed in passive polymeric matrices

Abstract

Active layers in many thin-film organic photovoltaic devices (OPVs) contain light-absorbing polymers that serve as electron donors, mixed with appropriate electron acceptors. In principle, the polymers can be replaced by small molecules with suitable bandgaps, which offer multiple advantages, including well-defined structures and methods of synthesis and purification that provide uniform samples. However, such materials often undergo separation of phases and crystallization, so making long-lived films that remain smooth, homogeneous, flexible, and transparent is not easy. We have found that effective OPVs can be made by dispersing mixtures of low-bandgap push–pull small molecules as electron donors and [6,6]-phenyl-C71-butyric acid methyl ester (PC71BM) as electron acceptor in matrices of optoelectronically passive conventional polymers, including polystyrene, poly(methyl methacrylate), poly(vinyl chloride), poly(ethylene glycol), and poly(dimethylsiloxane). By varying the identity of the matrix, its molecular weight, the loading of active components, and the conditions of annealing, we have produced efficient OPVs from components that would otherwise have undergone phase separation and crystallization, leading to poor performance. Layers with up to 35% matrix were found to be effective and could be fabricated at room temperature by simple processes. To probe the role of the polymers as dispersants, morphologies of composite films were examined by atomic force microscopy and electron microscopy. © 2017 Wiley Periodicals, Inc. J. Polym. Sci., Part B: Polym. Phys. 2017