Theoretical Investigation of Donor–Acceptor Copolymers Based on C-, Si-, and Ge-Bridged Thieno[3,2-b]dithiophene for Organic Solar Cell Applications

Abstract

The aim of this work is to modify the electron-donating block in donor–acceptor (D–A) copolymers to improve their electronic and photophysical properties for organic solar cell (OSC) applications. Based on the reported polymer PCPDTTTTz (Pa1), which includes electron-rich cyclopenta[2,1-b:3,4-b′]dithiophene (CPDT), electron-withdrawing tetrazine, and bridge thiophene, we substituted CPDT with electron-rich dithienocyclopentadithiophene, dithienosiloledithiophene, and dithienogermolodithiophene to design three D–A copolymers (Pa2 to Pa4). The calculation results indicate that Pa3 and Pa4 show lower highest occupied molecular orbital (HOMO)/lowest unoccupied molecular orbital (LUMO) energy levels and larger open-circuit voltage (V oc) than Pa1. Polymers Pa2 to Pa4 exhibit better performance with stronger and wider optical absorption and good hole transport properties in comparison with Pa1. The predicted power conversion efficiencies for the designed polymers Pa2 to Pa4 in OSC applications are ∼5.7%, ∼5.9%, and 6.0%, respectively. These results clearly indicate that modifying the electron-donating block in D–A copolymers can effectively improve their electronic and photophysical properties and OSC performance. The designed polymers Pa2 to Pa4 may be promising donor candidates for OSC applications.