Identification and characterization of a new intermediate to obtain high quality perovskite films with hydrogen halides as additives

Abstract

High quality perovskite films were fabricated from different precursor solutions containing a certain proportion of different hydrogen halides. An anti-solvent, toluene, was used here to isolate an intermediate from the perovskite precursor solution to reveal the impact of the hydrogen halides on the crystallization of perovskite films. From the Fourier transform infrared spectra we found that the stretch vibration of CO for pure N,N-dimethylformamide (DMF) appeared at 1663 cm⁻¹, which was shifted to a smaller wavenumber while reacting DMF with CH₃NH₃I (MAI) + PbI₂ and MAI + PbI₂ + HX (X = Cl, Br, I). Moreover, the appearance of X-ray diffraction (XRD) peaks at low angles (near 8 degrees) as well as some small angle shift showed that the MAI–PbI₂–DMF–xHX (X = Cl, Br, I) intermediate was formed, which benefits perovskite crystallization because the formation of the intermediate will avoid a rapid reaction between MAI and PbI₂. What's more, the solubility of the precursor can be improved by adding hydrogen halides. By adding a certain amount of hydrogen halide into the one-step perovskite precursor solution we can obtain pin-hole free and almost no defect films. Transient absorption (TA) was carried out to investigate the charge recombination in intrinsic perovskite films and perovskite devices, giving some reasonable interpretations.