Effect of Br content on phase stability and performance of H₂N=CHNH₂Pb(I_1−xBr_x)₃ perovskite thin films

Pristine and Br-doped H₂N = CHNH₂Pb(I_1−xBr_x)₃ (FAPb(I_1−xBr_x)₃, Br content x = 0, 0.05, 0.15, 0.2, 0.3, and 0.4) films were prepared. The effect of Br-doping on phase stability, defect density, and performance of FAPb(I_1−xBr_x)₃ was investigated by x-ray diffraction (XRD), scanning electron microscopy, ultraviolet–visible–near infrared absorbance spectroscopy, x-ray photoemission spectroscopy (XPS), Kelvin probe force microscopy (KPFM), positron annihilation spectroscopy, and current density–voltage (J–V) characteristics. The XRD measurements exhibit the enhancement of perovskite phase stability at x = 0.05. However, the phase stability decreases gradually with Br content (x) over 0.05. The increase of Br-doping content leads to the downshifting of both valence band (VB) position (indicated by XPS) and Fermi level (illustrated by KPFM). The energy level shifts are most probably due to the increase of Br 4p orbital content in VB and the change of self-doping levels. Doppler broadening spectra of positron annihilation radiation of the samples reveal that, the defect densities of Br-doped samples are much lower than that of pristine FAPbI₃. For FAPb(I_0.95Br_0.05)₃ sample, a high photoelectric conversion efficiency of 17.12% (25.7% higher than that of undoped sample) is successfully achieved. The significant enhancement of photoelectric conversion efficiency realized by Br-doping is attributed to the improvement of morphology, high phase stability, and low defect densities. (paper)