Time-Dependent Density Functional Theory Study on Benzothiadiazole-Based Low-Band-Gap Fused-Ring Copolymers for Organic Solar Cell Applications

Organic solar cells need low-band gap polymers for efficient harvesting of sunlight. Alternating electron-rich and electron-deficient units in a copolymer can lower the band gap, and high efficiency would be reached with the optimum combination of those units. As the first step toward this goal, we constructed four systematically altered copolymers 1–4, where a benzothiadiazole electron-deficient unit is connected to each of four fused-ring electron-rich units (fluorene, carbazole, cyclopentadithiophene, and dithienopyrrole). The density functional theory (DFT) and the time-dependent DFT calculations at the B3LYP/6-311G(d,p) level on the dimer models of 1–4 reproduced very well the geometries, the HOMO/LUMO (highest occupied and lowest unoccupied molecular orbital) energy levels, and the band gaps – important parameters determining the solar-cell efficiency – of these copolymers. The calculation shows that the dithieno-type copolymers, 3 and 4, exhibit lower band gaps and stronger absorption of visible light than the dibenzo-type copolymers, 1 and 2. The calculation also indicates that 3, which has deeper HOMO/LUMO levels than 4, exhibits the highest solar cell efficiency (∼3%) among the four systems. On the basis of these results, we propose several derivatives of 3 with electron-withdrawing groups (6–8), which have even deeper HOMO/LUMO levels than 3, as promising donor copolymers for high-efficiency (∼6%) organic bulk heterojunction solar cells.