Donor–Acceptor Random versus Alternating Copolymers for Efficient Polymer Solar Cells: Importance of Optimal Composition in Random Copolymers

The backbone composition of conjugated copolymers is of great importance in determining the conjugated structure and intermolecular assembly and in manipulating their optical, electrochemical, and electronic properties. However, limited attention has been directed at controlling the backbone composition of donor–acceptor (D–A) type low bandgap polymers. Herein, we developed a series of D–A random copolymers (P­(BDTT-r-DPP)) composed of different compositions of electron-rich (D) thienyl-substituted benzo­[1,2-b:4,5-b′]­dithiophene (BDTT) and electron-deficient (A) pyrrolo­[3,4-c]­pyrrole-1,4-dione (DPP). The optical and electrical properties of D–A random copolymers could be controlled by tuning the ratios of BDTT to DPP (4:1, 2:1, 1:1, 1:2, and 1:4) in the polymer backbone; an increase in BDTT resulted in increased absorption in the range of 400–600 nm and a lower-lying highest occupied molecular orbital energy level, while a higher proportion of DPP induced stronger absorption in the range of 700–900 nm. The P­(BDTT-r-DPP) copolymer with a D:A ratio of 2:1 produced the highest power conversion efficiency (PCE) of 5.63% in the polymer solar cells (PSCs), which outperformed the D–A alternating copolymer, P­(BDTT-alt-DPP) (1:1)-based PSCs (PCE = 5.03%), because of the improved light absorption and open-circuit voltage. Thus, we highlight the importance of developing random copolymers with controlled D:A compositions for optimizing their optoelectronic properties and performances of PSCs. Also, we compared the polymer packing structure and the electrical properties between the P­(BDTT-r-DPP) and P­(BDTT-alt-DPP) copolymers and developed a quantitative understanding of the effect of the D:A monomer sequence on the structural, electrical, and photovoltaic properties of the D–A copolymers.