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

Two-dimensional (2D) layered lead iodide (PbI 2 ) 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 PbI 2 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 (V I ) and interstitial iodine (I I ) defects of monolayer PbI 2 (ML-PbI 2 ). We find that PbI 2 has very low formation energies of V I of 0.77 and 0.19 eV for dilute and high concentrations, respectively, reflecting the coalescence tendency of isolated V I . Similar to V I, a low formation energy of I I 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 V I and I I to be the reservoirs or sinks of electron/hole carriers in PbI 2 . Our results suggest that the remnant PbI 2 in perovskite MAPbI 3 (or FAPbI 3 ) play dual opposite roles in affecting the efficiency of the perovskite: (1) Forming a Schottky-type interface with MAPbI 3 (or FAPbI 3 ) 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 I I defect is favored, and to reduce the recombination centers, the V I defect should be suppressed. Our results provide a deep understanding of the effects of defect engineering in ML-PbI 2, which shall be beneficial for the related optoelectronics applications.