Negative effects of oxygen and halogen species on charge extraction in perovskite solar cells with fullerene electron transport layers

In this work, we explore the effect of small molecular species such as air, iodide, iodine, and triiodide species on the charge transport behavior of fullerene electron transport layers and the concomitant effects on performance and stability of perovskite solar cells. First, we show that without SnOx buffer layers, two-step vapor deposited perovskites have notable losses upon air exposure while spin-coated perovskites have minor losses. We hypothesize that these effects are caused by intrinsic organohalide excess in the CSS perovskite film, leading to an enrichment of mobile halide species such as iodide, iodine, and polyiodides, coupled with a deterioration of C60 charge transport properties. We show that the fill factor losses found in two-step vapor-deposited devices upon air exposure can be induced in spin-coated perovskites by treating their surface with potassium triiodide solutions, further implicating polyhalide species in these losses. Drift-diffusion simulations support the hypothesis that this may be explained by deep electron traps in the C60 formed by polyhalides, while O2 may form shallower traps that have more minor effects on device performance in isolation. Critically, we identify that common ammonium halide surface passivation treatments can lead to performance losses with similar current-voltage signatures during operation in time periods as short as 30 minutes. This may be caused by in situ iodine and polyhalide generation from photolysis of residual lead iodide reducing charge extraction in these solar cells. This work has important implications for the design of stable perovskite solar cells with fullerene charge transport layers.