(521i) Domain Compositions and Fullerene Aggregation Govern Charge Photogeneration in Polymer/Fullerene Solar Cells

The relationship between active layer morphology and photovoltaic device performance of organic semiconductor mixtures continues to be obscured by its complex microstructure. For example, in the case of polymer/fullerene solar cells the ubiquitous presence of mixed phases in the active layer creates multiple morphologically distinct interfaces which are capable of either charge recombination or separation. Here, we demonstrate that domain compositions and fullerene aggregation can strongly modulate charge photogeneration at ultrafast timescales through studies of a model system, mixtures of a low band-gap polymer, poly[(4,4'-bis(2-ethylhexyl)dithieno[3,2-b:2',3'-d]germole)-2,6-diyl-alt-(2,1,3-benzothia-diazole)-4,7-diyl], and [6,6]-phenyl-C₇₁-butyric acid methyl ester.  We characterized the structure using energy-filtered transmission electron microscopy (EFTEM) and resonant soft X-ray scattering which show similar microstructures albeit the composition maps generated from EFTEM demonstrate that compositions of mixed domains in the microstructure vary significantly with overall film composition. Furthermore, the amount of polymer in the mixed domains is inversely correlated with device performance, which is corroborated by photo-induced absorption studies using ultrafast infrared spectroscopy that demonstrate that polaron concentrations are highest when mixed domains contain the least amount of polymer. Fullerene coherence lengths obtained from grazing-incidence X-ray scattering show correlation to polaron yields. Thus, for efficient charge photogeneration the purity of the mixed domains is critical as it modulates fullerene aggregation and electron delocalization.