Performance of 2-bromoterephthalic acid passivated all-inorganic perovskite cells

All-inorganic perovskite cesium lead iodine (CsPbI3) without any volatile organic components has attracted much attention due to its superior stability, high absorption efficiency and suitable band gap. However, the power-conversion efficiencies of CsPbI3 based perovskite solar cells (PSCs) are substantially low compared with those of the organic-inorganic hybrid lead halide PSCs. The surface passivation of the CsPbI3 film by long-chain halide salts has been found to be an effective method of improving the performance. In this paper, we report the concentration effect of an inexpensive 2-bromoterephthalic acid (BBr) as passivation material on the performance of CsPbI3 perovskite solar cells. The experimental results show that the conversion efficiency of perovskite solar cells first increases and then decreases as the concentration of BBr increases from 0 to 2 mg/mL. The best conversion efficiency of CsPbI3 perovskite solar cells reaches 13.5% at 0.2 mg/mL BBr. The results from X-ray diffraction and scanning electron microscopy suggest that there is no change in the phase or microstructure of the CsPbI3 perovskite film after surface passivation by BBr. By further analyzing the photoluminescence data of the CsPbI3 film with and without capping hole transport layer, it can be found that the passivation of BBr with the concentration of 0.2 mg/mL can enhance the fluorescence excitation intensity of the CsPbI3 film and accelerate the exciton separation at the interface between CsPbI3 film and hole transport layer. Based on the electrochemical impedance spectroscopy data, we find that the electron transport ability at the interface between TiO2 and CsPbI3 can be significantly improved after surface passivation, which is induced by the acceleration of the exciton separation at the interface between CsPbI3 film and hole transport layer. The decrease of the PSCs performance when the concentration of the BBr precursor increases from 0.5 mg/mL to 2 mg/mL can be attributed to the local agglomeration of the BBr material, resulting in the block of charge transportation. This research is expected to provide basic support for the low-cost development of the passivation materials for perovskite solar cells.