Cyclopentadithiophene cored A-π-D-π-A non-fullerene electron acceptor in ternary polymer solar cells to extend the light absorption up to 900 nm

Conjugated small molecular non-fullerene electron acceptors (NFA) are considered as one of the critical materials for achieving high performance and low cost of polymer solar cells, and received much attention in the last few years. However, most of the NFAs are based on large fused π-aromatic core, which requires complicate synthesis efforts. In addition, the relatively weak light absorption limited to 800 nm of most the NAFs limits the energy harvesting capability of the solar cells. In this paper, we report an A-π-D-π-A type molecule cored with a cyclopentadithiophene unit, which can be easily synthesized in two steps from commercially available starting materials. This compound shows a broad absorption up to 900 nm in thin solid film, which is ascribed to the relatively high highest occupied molecular orbital (HOMO) energy level as confirmed by cyclic voltammery and theoratical calculation. Application of the compound in polymer solar cells was also investigated both in binary and in ternary systems. The optimized power conversion efficiency (PCE) in binary solar cell with PTB7-Th as donor is 5.76% with an open circuit voltage (V_OC) of 0.838 V, a short circuit current (J_SC) of 14.81 mA/cm² and a fill factor (FF) of 46.4%. In the ternary solar cells which includes a second acceptor, PC₇₁BM, the highest PCE achieved is 9.39% with a V_OC of 0.803 V, a J_SC of 19.01 mA/cm², a FF of 61.6%, which is over 20% enhancement compared to the PTB7-Th:PC₇₁BM system (PCE of 7.58%). This work develops a simple small molecule non-fullerene acceptor which can largely enhance the photo response in near infrared region to improve the performance of fullerene based organic solar cell.