Nanomorphology and Charge Generation in Bulk Heterojunctions Based on Low‐Bandgap Dithiophene Polymers with Different Bridging Atoms

Carbon bridged (C‐PCPDTBT) and silicon‐bridged (Si‐PCPDTBT) dithiophene donor–acceptor copolymers belong to a promising class of low bandgap materials. Their higher field‐effect mobility, as high as 10−2 cm2 V−1 s−1 in pristine films, and their more balanced charge transport in blends with fullerenes make silicon‐bridged materials better candidates for use in photovoltaic devices. Striking morphological changes are observed in polymer:fullerene bulk heterojunctions upon the substitution of the bridging atom. XRD investigation indicates increased π–π stacking in Si‐PCPDTBT compared to the carbon‐bridged analogue. The fluorescence of this polymer and that of its counterpart C‐PCPDTBT indicates that the higher photogeneration achieved in Si‐PCPDTBT:fullerene films (with either [C60]PCBM or [C70]PCBM) can be correlated to the inactivation of a charge‐transfer complex and to a favorable length of the donor–acceptor phase separation. TEM studies of Si‐PCPDTBT:fullerene blended films suggest the formation of an interpenetrating network whose phase distribution is comparable to the one achieved in C‐PCPDTBT:fullerene using 1,8‐octanedithiol as an additive. In order to achieve a balanced hole and electron transport, Si‐PCPDTBT requires a lower fullerene content (between 50 to 60 wt%) than C‐PCPDTBT (more than 70 wt%). The Si‐PCPDTBT:[C70]PCBM OBHJ solar cells deliver power conversion efficiencies of over 5%.