Study on the Ion Substitution Mechanism of CsPbIBr₂ Films Prepared by a Drop-Coating Method

In this paper, the ion substitution mechanism of CsPbIBr2 films prepared by a drop-coating method is studied. CsI/CsBr methanol solution is drop-coated on a CsPbIBr2 precursor film to prepare the CsPbIBr2 perovskite film. After drop-coating, it is found that the color of the perovskite film changes with CsI/CsBr methanol solution drop-coated on the perovskite precursor film. When CsBr methanol solution is drop-coated on a CsPbIBr2 precursor film, the color of the obtained CsPbIBr2 film is orange. The color of the obtained CsPbIBr2 film is brown while CsI methanol solution is drop-coated on a CsPbIBr2 precursor film. Then, the hole-free, carbon-based CsPbIBr2 perovskite solar cells (FTO/ZnO/CsPbIBr2/C) are fabricated. For the solar cells fabricated by drop-coating with 30 μL of CsBr, without methanol solution, with 10 μL of CsI, with 50 μL of CsI, and with 90 μL of CsI, the power conversion efficiency (PCE) values are 2.98, 3.99, 6.04, 7.60, and 8.42%, respectively. The short-circuit current density (JSC) also increases with the changing of drop-coating solution. Therefore, we suppose that perhaps there is an ion substitution during the preparation of the CsPbIBr2 films. The ion substitution supposition is verified by the results of X-ray diffraction, ultraviolet–visible spectra, X-ray photoelectron spectroscopy, and secondary ion mass spectroscopy. Specifically, the I element is replaced by the Br element while drop-coating CsBr methanol solution. The Br element is substituted by the I element by drop-coating CsI methanol solution. Also, it is verified that the ion substitution occurs in the entire perovskite film.