Effects of Defect on Work Function and Energy Alignment of PbI2: Implications for Solar Cell Applications

Two-dimensional (2D) layered lead iodide (PbI2) is an important precursor and common residual species during the synthesis of lead–halide perovskites. There are currently debates and uncertainties about the effect of excess PbI2 on the efficiency and stability of the solar cell with respect to its energy alignment and energetics of defects. Herein, by applying first-principles calculations, we investigate the energetics, changes of work function, and defective levels associated with the iodine vacancy (VI) and interstitial iodine (II) defects of monolayer PbI2 (ML-PbI2). We find that PbI2 has very low formation energies of VI of 0.77 and 0.19 eV for dilute and high concentrations, respectively, reflecting the coalescence tendency of isolated VI. Similar to VI, a low formation energy of II of 0.65 eV is found, implying a high population of such defects. Both defects generate in-gap defective levels which are mainly due to the unsaturated chemical bonds of the p orbitals of exposed Pb or inserted I. Such rich defective levels allow the VI and II to be the reservoirs or sinks of electron/hole carriers in PbI2. Our results suggest that the remnant PbI2 in perovskite MAPbI3 (or FAPbI3) play dual opposite roles in affecting the efficiency of the perovskite: (1) Forming a Schottky-type interface with MAPbI3 (or FAPbI3) in which the built-in potential would facilitate the electron–hole separation and prolong the carrier lifetime; (2) acting as the recombination centers due to the deep defective levels. To promote the efficiency by the Schottky effect, our work reveals that the II defect is favored, and to reduce the recombination centers, the VI defect should be suppressed. Our results provide a deep understanding of the effects of defect engineering in ML-PbI2, which shall be beneficial for the related optoelectronics applications.