Light-Induced Metallic Lead Formation in Mixed-Cation, Mixed-Halide Perovskite— Observed Rates and Effects of Oxygen

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 in the presence of high temperatures, moisture, oxygen, and photoexcited charge carriers. While so-called ‘extrinsic stressors’ such as water and oxygen can be excluded from a PSC for some time with rigorous encapsulation, photons that produce chemically reactive electrons and holes, as well as elevated temperatures, are inherent to practical device operation. Thus, studies of perovskite degradation in inert environments represent the limiting case of strong encapsulation, where only light and heat stress the perovskite absorber. Light-induced degradation (LID), if it occurs, is unavoidable and can only be slowed by limiting the escape of decomposition products. Here, we investigate the light-induced degradation (LID) of FA_0.8Cs_0.2Pb(I_0.83Br_0.17)₃ thin films through in-situ and ex-situ optical spectroscopy, microscopy, and x-ray diffraction, identifying metallic lead (Pb⁰) as the primary decomposition product. In addition to studying LID in oxygen-free environments, we explore the role of oxygen, hypothesized to scavenge photoexcited electrons for photooxidation reactions and thereby inhibit Pb⁰ formation. Carrier transport losses are measured under illumination regardless of oxygen presence, with samples in inert environments where Pb⁰ forms showing a notable reduction in carrier lifetime, which drives the decline in optoelectronic performance. Given the critical role of Pb⁰ defect formation during LID, understanding its formation rate is essential. To do so, we used in-situ sub-bandgap optical absorbance measurements to selectively probe and quantitatively measure the formation rate of Pb⁰. We derive a rate law for Pb⁰ formation (r_Pb⁰ predicted to be ~3x10^-10 mol/(m²s) at 25 °C in N_{<font size="1">2</font>} 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 PbI_{<font size="1">2</font>} photolysis is not the mechanism of Pb⁰ formation. These observations represent the first quantitative measurements of Pb⁰ formation in halide perovskite absorbers and emphasize the complex interplay of environmental stressors and degradation pathways for commercially relevant perovskite materials.