Isoindigo-based copolymers for polymer solar cells with efficiency over 7%
A series of isoindigo-based low-band-gap copolymers (PnTI) containing an extended thiophene unit in the donor segment of the polymer were synthesized. The results show that the extended thiophene unit with centrosymmetric conformation simultaneously broadens the polymer absorption and enhances the crystallinity and, thus, hole mobility. Consequently, with additional improved solubility, the polymer P6TI exhibits the highest PCE of 7.25% (and a high Jsc of 16.24 mA cm−2) among isoindigo-based low-band-gap copolymers. This work demonstrates that by simply adjusting the donor segment and with relatively simple synthetic schemes, a material for high-performance and scalable PSCs will become available.