Air-Induced Conductivity Loss in Fullerene ETLs Can Drive Charge Extraction Losses in Vapor-Deposited Perovskite Solar Cells

The effect of air exposure on all-vapor processed perovskite solar cells using C60 fullerene electron transport layers (ETLs) was investigated. C60 is used in lead halide perovskite solar cells as an ETL to decrease hysteresis and improve stabilized power output. However, air exposure to n-i-p solar cells using C60 ETLs without encapsulation or doping can result in performance degradation due to FF loss and the onset of s-shaped J–V curves. This is correlated to orders of magnitude increase in C60 resistivity upon air exposure. Drift-diffusion simulations provide evidence that a change in the C60 carrier concentration or mobility can lead to the FF loss and s-shaped J–V curve. The degradation does not occur when using inorganic ETLs but does occur in p-i-n architecture using C60 ETLs, further confirming that the C60 layer is the source of the degradation. This is an additional pathway for perovskite solar cell degradation upon air exposure beyond the instability of the perovskite itself. The loss of efficiency can be reduced in p-i-n solar cells using a LiF interlayer, and a better combination of hysteresis and air stability can be achieved in n-i-p solar cells using a C60/SnO2 bilayer ETL.