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

Abstract

Pristine and Br-doped H 2 N = CHNH 2 Pb(I 1−x Br x ) 3 (FAPb(I 1−x Br 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−x Br 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 FAPbI 3 . For FAPb(I 0.95 Br 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.

Stable High-Efficiency Two-Dimensional Perovskite Solar Cells Via Bromine Incorporation

“…The bandgap of the BA 2 MA 4 Pb 5 I 16-10 x Br 10 x increases with the increasing PbBr 2 ratio, as evidenced by Fig. 3 a [ 49 ]. Thus, the 15 mol% PbBr 2 -contained device shows the highest V oc .…”

“…All these films show distinctive exciton absorption peaks in the absorption spectra, which are assigned to 2D phases with n = 2, 3, and 4, although nominally prepared as “ n = 5.” The perovskite-10% exhibits the enhanced absorbance intensity, resulting from a dense and uniform nature of the resultant film, as evidenced by the SEM and AFM images. Besides, the absorption edge of BA 2 MA 4 Pb 5 I 16-10 x Br 10 x has a blue shift with the increase of x value, which proves the widening of bandgap [ 49 ]. Figure 3 b presents the steady-state PL spectra of the 2D perovskite films deposited on glass substrates.…”

Efficient and Stable FA‐Rich Perovskite Photovoltaics: From Material Properties to Device Optimization

“…There are most probably because of the increase of Br 4p orbital content in the valence band (VB) and the change of self-doping levels and results in the shift down of both VB position and Fermi level (Figure 10b). [131] Furthermore, a lowed content Br − would slightly reduce the lattice content (Figure 10d), resulting in a more stable corner shared PbX 6 frame by the Reproduced with permission. [117] Copyright 2016, The Authors, published by American Association for the Advancement of Science.…”

“…There are most probably because of the increase of Br 4p orbital content in the valence band (VB) and the change of self‐doping levels and results in the shift down of both VB position and Fermi level (Figure 10b). [ 131 ] Furthermore, a lowed content Br − would slightly reduce the lattice content (Figure 10d), resulting in a more stable corner shared PbX 6 frame by the enhanced electrostatic forces between ions, while excess Br − would lead to PbX 6 octahedral serious tilting and increase the repelling forces between halide ions, which combine the expansion of PbX 6 framework containing FA + ions induced the rearrangement of PbX 6 octahedral (Figure 10c). Thus, the α‐phase stability is markedly reduced with high Br‐doping content.…”

“…d) The lattice constants of FAPb(I 1−x Br x ) 3 films. Reproduced with permission [131]. Copyright 2019, IOP Publishing.…”