Fig. 5.

a Schematic illustration of the gradient FASnI₃-based inverted PSCs and b the corresponding device cross-sectional SEM image. c Forward and reverse J–V curves of the best-performing FASnI₃ control and gradient PSCs. d IPCE spectra and the integrated current density of the FASnI₃ control and gradient devices. e Forward and reverse J–V curves of the HTL-free FASnI₃ gradient device (the inset shows the cross-section of the SEM image). f Operational stability test of the encapsulated FASnI₃ gradient PSC under simulated AM 1.5G light in air