Probing Ultrafast Interfacial Carrier Dynamics in Metal Halide Perovskite Films and Devices by Transient Reflection Spectroscopy

Abstract

Interfaces in metal halide perovskite (MHP) solar cells cause carrier recombination and thereby reduce their power conversion efficiency. Here, ultrafast (picosecond to nanosecond) transient reflection (TR) spectroscopy has been used to probe interfacial carrier dynamics in thin films of the reference MHP MAPbI3and state-of-the-art (Cs0.15MA0.15FA0.70)Pb(Br0.20I0.80)3(CsFAMA). First, MAPbI3films in contact with fullerene-based charge extraction layers (CTLs) in the presence and absence of LiF used as an interlayer (ITL) were studied. To quantify and discriminate between interface-induced and bulk carrier recombination, we employed a one-dimensional diffusion and recombination model. The interface-induced carrier recombination velocity was found to be 1229 ± 78 cm s-1in nonpassivated MAPbI3films, which was increased to 2248 ± 75 cm s-1when MAPbI3interfaced directly with C60, whereas it was reduced to 145 ± 63 cm s-1when inserting a 1 nm thin LiF interlayer between MAPbI3and C60, in turn improving the open-circuit voltage of devices by 33 mV. Second, the effect of surface and grain boundary passivation by PhenHCl in CsFAMA was revealed. Here, the recombination velocity decreased from 605 ± 52 to 0.16 ± 5.28 and 7.294 ± 34.5 cm s-1, respectively. The approach and data analysis presented here are immediately applicable to other perovskite/interlayer/CTL interfaces and passivation protocols, and they add to our understanding of the impact of surfaces and interfaces in MHP-based thin films on carrier recombination and device efficiency.