The Role of Donor Species and Heteroatom Electron Delocalization on Additive Interactions with Methylammonium Lead Iodide

Additive engineering─the incorporation of small organic molecules during film deposition or as a postfabrication step─is a common strategy for regulating crystallization kinetics or passivating defects during the fabrication of metal halide perovskite films. However, much of these efforts have been based on chemical intuition, leaving a limited understanding of how molecular structure influences interactions with perovskite surfaces. This paper describes using heterocyclic molecules as a model system to probe the effects of heteroatomic species (N, O, S, Se, and P) and heteroatom electron delocalization on additive interactions with the PbI₂-rich (100) surface of MAPbI₃ using density functional theory (DFT) calculations. For all heteroatom species studied, we observe that adsorption energies (or interaction strength) with the perovskite surfaces increase as the heteroatom electron delocalization (i.e., degree of unsaturation) decreases. We observe that adsorption energies are strongest with N-donors and weakest with O-donors, with P-, S-, and Se-donors having adsorption energies in between, describable by correlation of adsorption energies to heteroatom charge in two different regimes. The electronegativity of the heteroatom plays a critical role, dictating the extent of surface charge transfer from the Pb to the adsorbate, with increasing electronegativity correlated with a lower extent of Pb reduction (or possibly oxidation in the case of O-donors). Heteroatom electronegativity is also predictive of surface band gap shifting, with more electronegative donors increasing the surface band gaps; conversely, adsorption of low electronegativity P-donors typically results in surface band gap reductions.