Conjugated D-π-A photovoltaic polymers containing thieno[3,2-b]thiophene π-bridge

The continuously emerging new alternating donor-pi-acceptor (D-pi-A) copolymers over the past decades have recently boosted the power conversion efficiency (PCE) of organic solar cells (OSCs) >19%, which mainly relies on a deep understanding of the relationship between D or A units and photovoltaic properties. Bridge units between D and A were somewhat neglected although they significantly affect the molecular packing, electronic structure, and film morphology of polymers. Compared to the commonly used bridges such as thiophene (T) and 2,2 '-bithiophene (2T), the polymer with thieno[3,2-b]thiophene (TT) as pi-bridge usually exhibit linear backbones because of the directly opposite linkage and thus usually display (i) strong conjugation degree, (ii) wide absorption spectrum, (iii) large absorption coefficient, (iv) high-lying energy level, and (v) good crystallinity. In this review, we focus on the correlations between pi-bridges and polymer properties including rigidity, thermodynamic stability, conformation, phase transformation, crystallinity, and device photovoltaic performance. The commonly used T and 2T bridges are discussed for comparison. This review suggests a new perspective for D-pi-A polymer design to tune the polymer characteristics, understand the intrinsic limitations of PCE, evaluate the application of TT bridges in high-efficient polymers, and draw much attention to the application of TT-based copolymers in ternary blend OSCs in the near future.