Effect of Zn(TFSI)₂ on the performance-aging time of perovskite solar cells

Abstract

Hole transport layers (HTLs) are one of the essential layers of perovskite solar cells (PSCs). Generally, 2,2ʹ,7,7ʹ-Tetrakis [N,N-di(4-methoxyphenyl)amino]-9,9ʹ-spirobifluorene (spiro-MeOTAD) doped by lithium bis(trifluoromethanesulfonyl)imide (LiTFSI) is used as the HTL in PSCs. PSCs employing spiro-MeOTAD require an additional aging process to reach an optimized point of photovoltaic performance due to doping and energy alignment. However, LiTFSI is responsible for low thermal stability and has a hygroscopic nature; therefore, Zinc(II) bis(trifluoromethanesulfonyl)imide (Zn(TFSI)2) has been reported as an outstanding candidate to replace LiTFSI. Nevertheless, utilization of Zn(TFSI)2 as a dopant for PSCs has rarely been reported, which is likely due to the difficulty in achieving high device performances comparable to that with LiTFSI. Herein, we investigate the effect of Zn(TFSI)2 on the doping kinetics of spiro-MeOTAD and correlate it with the time-dependent photovoltaic performance of PSCs employing Zn(TFSI)2. Devices with Zn(TFSI)2 require a considerably longer aging time (∼270 h) to reach the optimized performance, while LiTFSI takes only ∼20 h due to the different doping kinetics of spiro-MeOTAD depending on the dopant. Remarkably, engineering at the interface of the perovskite/HTL can effectively shorten the device aging time by manipulating the recombination rate, leading to a comparable aging time to LiTFSI.