Ladder-type dithienocyclopentadibenzothiophene-cored wide-bandgap polymers for efficient non-fullerene solar cells with large open-circuit voltages†
Wide-bandgap (WBG) polymers exhibit strong absorption bands in the short wavelength region, and they can offer complementary absorption when blended with their low-bandgap (LBG) counterparts in multi-junction, ternary-blend, or non-fullerene polymer solar cells (PSCs) thereby further enhancing power conversion efficiencies (PCEs) of organic solar cells. In this work, two WBG polymers (PSS2 and PSS3) based on ladder-type dithienocyclopentadibenzothiophene (DTCD) are designed and synthesized successfully. PSS2 and PSS3 possess relatively deep-lying highest occupied molecular orbital (HOMO) energy levels of −5.36 and −5.46 eV with optical bandgaps of 2.01 and 1.92 eV, respectively. In combination with a non-fullerene acceptor (EH-IDTBR), PSS2 exhibited a power conversion efficiency (PCE) of 9.46% with a short-circuit current (JSC) of 15.70 mA cm−2, a fill factor (FF) of 58.3% and a VOC of 1.033 V. The 9.46% PCE is the highest among all copolymers based on ladder-type multi-ring-fused aromatics. With EH-IDTBR, PSS3 showed an inferior PCE of 4.74% with an enlarged VOC of 1.138 V. Furthermore, the best-performing PSS2-based device showed excellent stability with a PCE of 9.03% after storage for 120 days, which is 95% of its original value. Our results indicate that the ladder-type DTCD is a promising building block for designing WBG semiconductors for photovoltaic applications.