Light-induced degradation of mixed-cation, mixed-halide perovskite: observed rates and influence of oxygen

Abstract

Formamidinium-rich lead halide perovskite semiconductors comprise the absorber layer in the most efficient single-junction perovskite solar cells (PSCs) but suffer from chemical instability when exposed to high temperatures, moisture, oxygen, and light. Light-induced degradation (LID) is unavoidable in PSCs and can be slowed only by limiting the escape of decomposition products. Here, we study the LID of FA_0.8Cs_0.2Pb(I_0.83Br_0.17)₃ thin films using in situ and ex situ optical spectroscopy, microscopy, and X-ray diffraction. The results reveal that the primary decomposition products under LID conditions are reduced lead-containing species that have broadband optical absorption. XPS reveals the presence of Pb⁰, but it may coexist with partially reduced lead-containing species. We use in situ sub-bandgap optical absorbance measurements to selectively probe and quantitatively measure the formation rate of reduced lead species. We derive a rate law for reduced-Pb formation (r_Pb⁰ estimated at ∼3 × 10⁻¹⁰ mol m⁻² s⁻¹ at 25 °C in N₂ under 1 sun photon flux that would result in complete conversion of a 300 nm film in ∼78 days), determine an activation energy (∼0.61 eV), determine an effective reaction order with respect to the flux of above bandgap photons (r_Pb⁰ ∝ I_in^0.72), and find that the wavelength of above bandgap photons minimally affects the rate, suggesting that PbI₂ photolysis is not the mechanism for the formation of decomposition products. These observations represent the first quantitative measurements of the rate of formation of reduced lead species in perovskites and emphasize a unique interplay among environmental stressors and degradation pathways for commercially relevant perovskite materials.