Air-Induced Conductivity\nLoss in Fullerene ETLs Can\nDrive Charge Extraction Losses in Vapor-Deposited Perovskite Solar\nCells

The effect of air exposure on all-vapor processed perovskite\nsolar\ncells using C<sub>60</sub> fullerene electron transport layers (ETLs)\nwas investigated. C<sub>60</sub> is used in lead halide perovskite\nsolar cells as an ETL to decrease hysteresis and improve stabilized\npower output. However, air exposure to n-i-p solar cells using C<sub>60</sub> ETLs without encapsulation or doping can result in performance\ndegradation due to FF loss and the onset of s-shaped <i>J–V</i> curves. This is correlated to orders of magnitude increase in C<sub>60</sub> resistivity upon air exposure. Drift-diffusion simulations\nprovide evidence that a change in the C<sub>60</sub> carrier concentration\nor mobility can lead to the FF loss and s-shaped <i>J–V</i> curve. The degradation does not occur when using inorganic ETLs\nbut does occur in p-i-n architecture using C<sub>60</sub> ETLs, further\nconfirming that the C<sub>60</sub> layer is the source of the degradation.\nThis is an additional pathway for perovskite solar cell degradation\nupon air exposure beyond the instability of the perovskite itself.\nThe loss of efficiency can be reduced in p-i-n solar cells using a\nLiF interlayer, and a better combination of hysteresis and air stability\ncan be achieved in n-i-p solar cells using a C<sub>60</sub>/SnO<sub>2</sub> bilayer ETL.