Organic Hole Transport Material Ionization Potential Dictates Diffusion Kinetics of Iodine Species in Halide Perovskite Devices

Iodine-containing volatiles are major degradation products of halide perovskite materials under irradiation, yet iodine diffusion kinetics into and throughout organic hole transport materials (HTMs) and consequent reactions are largely unexplored. Here, we modify the Ca:O-2 corrosion test to Ag:I-2 to quantify I-2 transmission rates through common organic HTMs. We observe I-2 permeability to inversely correlate with HTM ionization energy, or the highest occupied molecular orbital (HOMO) energy. Tracking electronic conductance during exposure to I-2 confirms shallow HOMO HTMs are strongly oxidized (i.e., doped), leading to substantial I-2 uptake and increased transmission rates. Finally, relationships between HOMO level, doping, and transmission rate are maintained when methylammonium lead triiodide (MAPbI(3)) photolysis products are the only source of iodine. While HTM energetics influence the initial performance of halide perovskite devices by selective charge extraction, our results further suggest they will affect device stability; deeper HOMO energy HTMs will suppress iodine migration and associated degradation mechanisms.

Automated summary

Iodine-containing volatiles are major degradation products of halide perovskite materials under irradiation. The study examines the diffusion kinetics of iodine into organic hole transport materials (HTMs) and reactions, finding a correlation between I-2 permeability and HTM ionization energy. This suggests that HTM energetics influence the stability of halide perovskite devices by suppressing iodine migration.