Improvement in the Efficiency of Alkylsilyl Functionalized Copolymer for Polymer Solar Cells: Face‐On Orientation Enhanced by Random Copolymerization

Abstract

Herein, an alkylsilyl functionalized alternative (D‐A1) copolymer with high crystallization property as the polymer matrix and planar [1,2‐c:4,5‐c]dithiophene‐4,8‐dione (BDD) block as the second acceptor unit (A2) are selected to construct two D‐A1‐D‐A2 type random copolymers PBDT‐TZ‐BDD‐1/19 and PBDT‐TZ‐BDD‐1/9. It is found that incorporation of a small amount of BDD block into the alkylsilyl functionalized copolymer by random copolymerization can effectively manipulate the energy levels, light absorption, molecular packing and the photovoltaic properties when blended with ITIC (indacenodithieno[3,2‐b]thiophene (IT) as the central donor unit and 2‐ (3‐oxo‐2,3‐dihydroinden‐1‐ylidene)malononitrile (IC) as end groups). More importantly, random copolymerization provides a beneficial trade‐off that the slightly reduced periodic sequence promotes the compatibility with the acceptor, whereas introduction of planar BDD units allows a preferred face‐on orientation with enhanced π–π stacking of the random copolymer to facilitate the charge transfer. As a result, the random copolymer PBDT‐TZ‐BDD‐1/19 delivers a significantly higher power conversion efficiency (11.02%) than the alternative binary copolymer counterpart together with the remarkably improved short circuit current and fill factor. These results demonstrate that random polymerization of a small amount of planar units into the highly crystalline polymer matrix is a promising strategy to develop high‐performance polymer solar cells.