Recent progress in high efficiency polymer solar cells by rational design and energy level tuning of low bandgap copolymers with various electron-withdrawing units

This review collects recent five-year publications on low bandgap semiconducting polymers, which are composed of electron donor (D) and electron acceptor (A) units, exhibiting the power conversion efficiency (PCE) higher than 6%. When the photovoltaic performances of different types of D-A semiconducting copolymers are compared after the copolymers are classified into several categories according to the type of A-units, it is realized that diketopyrrolopyrrole (DPP)-based copolymers exhibit high J_SCs owing to low bandgaps and low V_OCs due to high-lying HOMO levels, while thienopyrroledione (TPD)-based copolymers exhibit high V_OCs due to their deep HOMO levels and low J_SCs because of wide bandgaps. Benzothiadiazole- and thienothiophene-based copolymers show intermediate values of V_OC and J_SC between DPP- and TPD-based ones. For further enhancement of photovoltaic performance, DPP-based copolymers may be designed to have deeper HOMO level with the minimum widening of bandgap while TPD-based polymers may be designed to have lower bandgap with the minimum rise of HOMO level. Hence, the energy level tuning must be considered so as to minimize the adverse effect.