Design and Synthesis of Carbazole‐Cyclopentathiophene Alternating Copolymers for Inhibiting Phase Separation of Cs0.15FA0.85PbI3

Abstract To inhibit phase separation of Cs x FA 1− x PbI 3 perovskite into optically inactive δ‐CsPbI 3 and δ‐FAPbI 3 under high humidity and light conditions, herein two polymers, poly[3‐methyl‐6‐(2‐methylspiro[cyclopenta[1,2‐b:5,4‐b′]dithiophene‐4,2′‐[1,3]dithiolan]‐6‐yl)‐9‐octyl‐9H‐carbazole] (TM3) and poly[3‐methyl‐6‐(2‐methylspiro[cyclopenta[1,2‐b:5,4‐b′]dithiophene‐4,2′‐[1,3]dioxolan]‐6‐yl)‐9‐octyl‐9H‐carbazole] (TM4), are designed and synthesized to dope the Cs 0.15 FA 0.85 PbI 3 perovskite film for perovskite solar cells (PSCs). The results of X‐ray photoelectron spectroscopy and nuclear magnetic resonance confirm the existence of coordination and hydrogen bonds between the polymer and Cs 0.15 FA 0.85 PbI 3 perovskite. Ion migration in the perovskite film is thus markedly suppressed by the TM doping, as verified by the electrical poling measurement. Benefiting from these interactions, the TM doping is able to inhibit phase separation of Cs 0.15 FA 0.85 PbI 3 markedly at a relative humidity of 55–65% and under light. Furthermore, the TM doping has other positive effects such as enhancing perovskite crystallinity, improving crystal growth orientation, reducing trap density, increasing hole extraction, and suppressing charge recombination. Consequently, the TM3‐ and TM4‐doped PSCs achieve a power conversion efficiency of 21.96% and 20.01%, respectively, against 17.82% for the undoped PSC. The humidity, environmental, illumination, and thermal stabilities of unencapsulated devices are also improved significantly by the TM3‐ and TM4‐doping, respectively. By comparison, TM3 with the S substitution shows abetter effect than TM4 with the O substation on photovoltaic performance and device stability.