Transformation of the Excited State and Photovoltaic Efficiency of CH₃NH₃PbI₃ Perovskite upon Controlled Exposure to Humidified Air

TLDR

Humidity has been an important factor, in both negative and positive ways, in the development of perovskite solar cells and will prove critical in the push to commercialize this exciting new photovoltaic technology. The study investigates how CH₃NH₃PbI₃ interacts with H₂O vapor by characterizing optical absorption, morphology, and crystal structure, and examines humidity’s detrimental effects on solar cell efficiency and stability. The authors used optical absorption spectroscopy and femtosecond transient absorption to probe ground‑state, excited‑state, morphology, and crystal structure changes of CH₃NH₃PbI₃ upon H₂O exposure. H₂O exposure does not merely revert CH₃NH₃PbI₃ to PbI₂; instead, it complexes into a hydrate (CH₃NH₃)₄PbI₆·2H₂O, reducing visible‑range absorption and altering crystal structure.

Abstract

Humidity has been an important factor, in both negative and positive ways, in the development of perovskite solar cells and will prove critical in the push to commercialize this exciting new photovoltaic technology. The interaction between CH(3)NH(3)PbI(3) and H(2)O vapor is investigated by characterizing the ground-state and excited-state optical absorption properties and probing morphology and crystal structure. These undertakings reveal that H(2)O exposure does not simply cause CH(3)NH(3)PbI(3) to revert to PbI(2). It is shown that, in the dark, H(2)O is able to complex with the perovskite, forming a hydrate product similar to (CH(3)NH(3))(4)PbI(6)·2H(2)O. This causes a decrease in absorption across the visible region of the spectrum and a distinct change in the crystal structure of the material. Femtosecond transient absorption spectroscopic measurements show the effect that humidity has on the ultrafast excited state dynamics of CH(3)NH(3)PbI(3). More importantly, the deleterious effects of humidity on complete solar cells, specifically on photovoltaic efficiency and stability, are explored in the light of these spectroscopic understandings.