Correlating domain purity with charge carrier mobility in bulk heterojunction polymer solar cells

We report on the impact of morphology onto charge carrier transport and photovoltaic properties in bulk heterojunction solar cells. The study is based on ternary blends of amorphous and semi-crystalline anthracene-containing poly(p-phenylene- ethynylene)-alt-poly(p-phenylene-vinylene) (PPE-PPV) copolymers, with [6,6]-phenyl-C61-butyric acid methyl ester (PCBM). Both copolymers exhibit the identical backbone, but bear different side-chain substitutions, which in turn yield changes in the conformation from semi-crystalline (AnE-PVab) to amorphous (AnE-PVba). Strongly phase separated domains are observed for binary AnE-PVab:PCBM blends, presumably dueto strong stacking tendency of AnE-PVab. On the other hand and due to good miscibility a fine-scaled homogeneously intermixed amorphous phase is obtained for binary AnE-PVba:PCBM blends. Upon ternary blending the phase separation between the polymers can readily be continuously tuned between coarse grained and fine-scaled. For each photoactive layer composition the dominant domain spacing was evaluated by resonant soft x-ray scattering (R-SoXS) and related to photovoltaic properties and charge carrier mobility. A with the AnE-PVba steadily decaying domain size and a strong correlation between phase separation and charge transport is demonstrated.