Optoelectronic properties of cyclopentadithiophene-based donor–acceptor copolymers as donors in bulk heterojunction organic solar cells: A theoretical study

Abstract

Cyclopentadithiophene and benzothiadiazole derivatives have promising characteristics for use in organic solar cells. In this study, we theoretically investigated two donor–acceptor copolymers comprising 2-(benzo[c][1,2,5]thiadiazol-4-yl)-4H-cyclopenta[2,1-b:3,4-b']dithiophen-4-one and 2-(2-(benzo[c][1,2,5]thiadiazol-4-yl)-4H-cyclopenta[2,1-b:3,4-b']dithiophen-4-ylidene)malononitrile with alternating donor–acceptor building units. Different copolymer chain lengths (n = 1–5) were extended to investigate their structural, electronic, and photo-physical properties based on density functional theory calculations. The copolymers exhibited high planarity, reduced band gap energies, and broad absorption in the visible range, and thus they may enhance the performance of organic solar cells. The transition density matrix plots were simulated for the copolymers and used to evaluate hole–electron localization interactions, donor–acceptor interactions, and the electronic excitation processes in their excited states. The charge transfer properties, fill factors, and open-circuit voltages were estimated. Based on our simulations, we found that the trimer was the most suitable conformation among the copolymers for designing high performance bulk heterojunction (BHJ) solar cells. The power conversion efficiency of the BHJ-based blended copolymers-[70]PCBM was estimated as 9.5%. Thus, the appropriate design of conjugated materials could considerably enhance the donor properties to further improve the performance of BHJ solar cells.