  Two-dimensional (2D) layered lead iodide (PbI2) is an important precursor and
common residual species during the synthesis of lead-halide perovskites. There
currently exist some 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 the first-principles
calculations, we investigate the energetics, changes of work function and the
defective levels associated with the iodine vacancy (VI) and interstitial
iodine (II) defects of monolayer PbI2 (ML-PbI2). We find that the PbI2 has a
very low formation energy of VI of 0.77 and 0.19 eV for dilute and high
concentration, respectively, reflecting coalescence tendency of isolated VI,
much lower than that of vacancies in other 2D materials like phosphorene.
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 p-orbitals of exposed
Pb or inserted I. Such rich defective levels allow the VI and II as the
reservoir or sinks of electron/hole carriers in PbI2. Our results suggest that
the remnant PbI2 in perovskite MAPbI3 (or FAPbI3) would play dual opposite
roles in affecting the efficiency of the perovskite: (1) Forming 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 shall be beneficial in improving strategies for the
related optoelectronics applications.
