A₁-A₂ Type Wide Bandgap Polymers for High-Performance Polymer Solar Cells: Energy Loss and Morphology

Introducing electron-withdrawing groups onto donor-acceptor (D-A) type conjugated materials is a commonly used method for lowering their highest occupied molecular orbital (HOMO) energy level to achieve higher open circuit voltage (V_oc) in polymer solar cells (PSCs). However, this method is rather costly due to the tedious synthesis and low yield involved in preparing the target monomers. Here, a novel design concept of using two different acceptor units to construct acceptor₁-acceptor₂ (A₁-A₂) type polymers with a deep HOMO level is proposed. Two A₁-A₂ type wide bandgap (WBG) polymers, PB24-3TDC and PB68-3TDC, were designed for PSCs. The developed polymers possess proper energy levels and complementary absorption with an efficient electron acceptor ITIC-Th. More importantly, by slightly regulating the alkyl side-chains, molecular stacking and photoluminescence (PL) emission energy loss of polymers can be alternated significantly. As a result, tuned V_oc from 0.9 to 1.0 V and short-circuit current (J_sc) from 9.4 to 17.0 mA cm^-2 can be achieved. The device based on PB24-3TDC:ITIC-Th exhibits a higher power conversion efficiency (PCE) of 10.3% compared to PB68-3TDC:ITIC-Th based device with a PCE of 7.88%. These results show that the design concept of A₁-A₂ type polymer donors have great potential for blending with non-fullerene acceptors for achieving high performance PSCs.