Time-dependent density functional theory study on cyclopentadithiophene- benzothiadiazole-based push-pull-type copolymers for new design of donor materials in bulk heterojunction organic solar cells

Push-pull-type copolymers - low-band-gap copolymers of electron-rich fused-ring units (such as cyclo-pentadithiophene; CPDT) and electron-deficient units (such as benzothiadiazole; BT) - are promising donor materials for organic solar cells. Following a design principles proposed in our previous study, we investigate the electronic structure of a series of new CPDTBT derivatives with various electron-withdrawing groups using the time-dependent density functional theory and predict their power conversion efficiency from a newly-developed protocol using the Scharber diagram. Significantly improved efficiencies are expected for derivatives with carbonyl [C=O], carbonothioyl [C=S], dicyano [C(CN) ₂] and dicyanomethylene [C=C(CN) ₂] groups, but these polymers with no long alkyl side chain attached to them are likely to be insoluble in most organic solvents and inapplicable to low-cost solution processes. We thus devise several approaches to attach alkyl side chains to these polymers while keeping their high efficiencies.