Revealing the interfacial properties of halide ions for efficient and stable flexible perovskite solar cells.

The severe non-radiative recombination loss caused by SnO/perovskite interface defects greatly hinders the further improvement of the performance and stability of flexible perovskite solar cells (PSCs). Herein, a series of halides KM (M = F, Cl, Br, I) were inserted as interface layers to modulate the SnO/perovskite interface. Both experimental and theoretical calculation demonstrated that F exhibited stronger interface coupling effect between the SnO and perovskite than other halide ions (Cl, Br, I). The F had unique effects in effectively depressing SnO/perovskite interface defects ascribed to the simultaneous formation of SnF bonds and hydrogen bonds, resulting in reduced non-radiative recombination and improved interfacial contact. In consequence, the champion KF-modified flexible and rigid PSCs delivered outstanding PCE of 18.16 % and 21.36 %, accompanied by the enhanced V of 1.14 V and 1.16 V, respectively. Meanwhile, the stability of flexible PSCs was significantly ameliorated after KF modification, and about 84 % of the original efficiency could be retained even after 3000 bending cycles. This work provides a facile and efficient strategy for interface modulation to achieve high-performance and stable flexible PSCs.