Relation between carrier mobility and cell performance in bulk heterojunction solar cells consisting of soluble polythiophene and fullerene derivatives

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Abstract
The effect of carrier mobility on the cell performance was examined in the bulk heterojunction solar cells consisting of soluble polythiophene (PHTh) and fullerene derivatives (PCBM). The hole mobility decreased from 6.3×10−3cm2V−1s−1 in the pure PHTh by blending PCBM, while the electron mobility decreased from 2.4×10−2cm2V−1s−1 in the pure PCBM by blending the PHTh. When blending ratio R=PHTh∕(PHTh+PCBM) by weight was 0.5–0.85, ambipolar carrier conduction was possible, showing the best-balanced ambipolar carrier mobility of ca. 10−4cm2V−1s−1 at R=0.7. The power conversion efficiency (η) of the PHTh:PCBM bulk heterojunction solar cells under AM1.5, 85mWcm−2 illumination significantly depended on the blending ratio R and the maximum η of 2.6% was observed at R=0.7, where both electrons and holes are conducting well. Thus, it was concluded that the ambipolar carrier conduction limits the performance of PHTh:PCBM bulk heterojunction solar cells.