Charge Compensation by Iodine Covalent Bonding in Lead Iodide Perovskite Materials

Abstract

Metal halide perovskite materials (MHPs) are a family of next-generation semiconductors that are enabling low-cost, high-performance solar cells and optoelectronic devices. The most-used halogen in MHPs, iodine, can supplement its octet by covalent bonding resulting in atomic charges intermediate to I− and I0. Here, we examine theoretically stabilized defects of iodine using density functional theory (DFT); defect formation enthalpies and iodine Bader charges which illustrate how MHPs adapt to stoichiometry changes. Experimentally, X-ray photoelectron spectroscopy (XPS) is used to identify perovskite defects and their relative binding energies, and validate the predicted chemical environments of iodine defects. Examining MHP samples with excess iodine compared with near stoichiometric samples, we discern additional spectral intensity in the I 3d5/2 XPS data arising from defects, and support the presence of iodine trimers. I 3d5/2 defect peak areas reveal a ratio of 2:1, matching the number of atoms at the ends and middle of the trimer, whereas their binding energies agree with calculated Bader charges. Results suggest the iodine trimer is the preferred structural motif for incorporation of excess iodine into the perovskite lattice. Understanding these easily formed photoactive defects and how to identify their presence is essential for stabilizing MHPs against photodecomposition.

Stoichiometry dependent phase evolution of co-evaporated formamidinium and cesium lead halide thin films

“…Even though degradation mechanisms 10,11,[57][58][59] of FAPbI 3 α to δ phase are well explored, to the best of our knowledge, no calculations or theories explaining the simultaneous growth of α and δ phases of FAPbI 3 exist. In a stoichiometric absorber, the FAPbI 3 δ phase is favored at room temperature due to the oversized FA + cation, as has been shown by many groups 47,[60][61][62][63] . Yet, freshly grown layers often contain large shares of the α phase and this share is increased by including a larger amount of A-cations.…”

“…In a stoichiometric absorber, the FAPbI 3 d phase is favored at room temperature due to the oversized FA + cation, as has been shown by many groups. 47,[60][61][62][63] Yet, freshly grown layers often contain large shares of the a phase and this share is increased by including a larger amount of A-cations.…”

“…We speculate, that surface passivation and defect formation resulting from FAI excess and consequently including the FA + and I À ions could be less favorable in the d phase than in the a phase. These defects could include theoretically predicted I 3 À trimers, 61 interstitials FA i and I i or the antisite defect FA Pb , resulting in a stretched scaffold, large enough to hold FA + cations due to reduced surface stress. The decrease in lattice constant in Fig.…”

Effects of Surface Defects on Performance and Dynamics of CsPbI2Br Perovskite: First-Principles Nonadiabatic Molecular Dynamics Simulations

“…The defective I atom on the surface has strong interactions with the surrounding I atoms, leading to I-p and I-p orbital coupling and thus the formation of an iodine trimer. 81,82 This charge redistribution may significantly reduce the NAC and thus suppress the e−h recombination rate. 83 The e−p interactions can significantly affect the electronic structures.…”

“…The charge density of the CBM is predominantly localized at the Pb atoms on the subsurface. The defective I atom on the surface has strong interactions with the surrounding I atoms, leading to I-p and I-p orbital coupling and thus the formation of an iodine trimer. , This charge redistribution may significantly reduce the NAC and thus suppress the e–h recombination rate …”