Mixed solvent atmosphere induces the surface termination state transition of perovskite to achieve matched energy level alignment

Abstract

The optimal alignment of semiconductor's electronic energy level with respect to hole-transporting materials (HTMs and ETMs) determines the photovoltaic conversion efficiency of perovskite solar cells (PSCs). The energy level of perovskite can be greatly affected by the crystal surface termination state formed in the perovskite polycrystalline film preparation process. Perovskite featured with Pb-X termination always bears deep energy level which cannot match well HTM. Herein, to regulate the surface termination state of the perovskite crystals, a controlled solvent atmosphere dimethyl sulfoxide (DMSO) and N,N-dimethylformamide (DMF) was employed to induce the regrowth of perovskite crystals. Results prove that the mixed solvent of DMSO and DMF can induce perovskite crystals from Pb-X termination to A-X termination. Density functional theory (DFT) calculations show that the terminated conversion moves the valence and conduction band edge (VBE and CBE) of perovskite crystal to a shallower energy level, which does great favor to match the energy level of HTM, and then contributes to extracting charges. In addition, the defect density of the optimized perovskite polycrystalline film decreased from 1.258 × 1016 to 1.022 × 1016 cm -3, and the depletion width of PSCs increased from 48.9 to 56.1 nm. Profiting from the effective charge extraction and reduced non-radiative recombination, the open circuit voltage of the PSC increased from 0.99 to 1.09 V, and the conversion efficiency boosted from 17.8% to 20.3%..