Theoretical Investigations on Donor-Acceptor Conjugated Copolymers Based on Naphtho[1,2-c:5,6-c]bis[1,2,5]thiadiazole for Organic Solar Cell Applications

Conjugated polymers with donor-acceptor architectures have been successfully applied in bulk heterojunction solar cell devices. Tuning the electron-withdrawing capability in donor-acceptor (D-A) conjugated polymers allows for design of new polymers with enhanced electrical and optical properties. In this paper, a series of D-A copolymers, PBDFDTBT (P1a), PBDTDTBT (P2a), PNDTDTBT (P3a), and PQDTDTBT (P4a), were selected and theoretically investigated using PBE0/6-311G** and TD-PBE0/6-311G**//PBE0/6-311G** methods. The calculated results agree well with the available experimental data of HOMO energy levels and band gaps. We further designed and studied four novel copolymers, P1b, P2b, P3b, and P4b, by substituting the 2,1,3-benzothiadiazole (BT) unit in P1a-P4a with a stronger unit of naphtho[1,2-c:5,6-c]bis[1,2,5]thiadiazole (NT), respectively. Compared with P1a-P4a, the newly designed polymers of P1b-P4b show better performance with the smaller band gaps and lower HOMO energy levels. The PCEs of similar to 5%, similar to 7%, similar to 7% and similar to 7% for P1b-P4b, predicted by Scharber diagrams, are much higher than those of P1a-P4a when used in combination with PCBM. These results clearly reveal that tuning the electron-withdrawing capability in D-A conjugated polymers is an effective way to improve the electrical and optical properties and the efficiency of the photovoltaic device.