Side-chain engineering of benzotriazole-based polymers as hole transport material enables high-efficiency CsPbI2Br single-junction and tandem perovskite solar cells

Abstract

Rational design and synthesis of high-performance doping-free hole transport polymers is critical to achieve high-efficiency and stable perovskite solar cells (PSCs). Side chain engineering is a widely employed strategy for modulating the photoelectronic properties of polymers. In this study, based on dithieno[2,3-d;2′,3′-d′]benzo[1,2-b;4,5-b′]dithiophenes (DTBDT) as D unit, benzo[d][1,2,3]triazole (BTA) as A segment, and thiophene as a π bridge, three D-π-A type polymeric materials with different length of alkyl side chain (PE51, PE52, and PE53) were utilized as hole transport materials in CsPbI2Br-based PSCs and perovskite/organic tandem solar cells (TSCs). With the increase of alkyl chain length from 2-butyloctyl (PE51) to 2-hexyldecyl (PE52) and to 2-octyldodecyl (PE53), the highest occupied molecular orbital energy level of the polymer gradually decreases, and the molecular stacking of the material gradually shifts from edge-on stacking to face-on stacking. Moreover, among three polymers, the PE53 exhibits the most efficient hole extraction at the CsPbI2Br/hole transport layer (HTL) interface. Hence, the CsPbI2Br-based PSCs and perovskite/organic TSCs employing PE53 as HTL realize the highest power conversion efficiency (PCE) of 17.65% and 23.07%, respectively. Our results indicate that side chain engineering of HTM is an effective strategy to modulate their molecular orientation and improve the photovoltaic performance of PSCs.