Dependence of hysteresis on the perovskite film thickness: Inverse behavior between TiO₂ and PCBM in a normal planar structure

The effect of perovskite film thickness on the current density (J)-voltage (V) hysteresis is investigated with a normal planar perovskite solar cell (PSC) having the FTO/ETL/MAPbI₃/spiro-MeOTAD/Au structure (ETL = electron transporting layer, MA = methylammonium, and spiro-MeOTAD = 2,2′,7,7′-tetrakis-(N,N-di-4-methoxyphenylamino)-9,9′-spirobifluorene). A compact TiO₂ (c-TiO₂) layer is used as an ETL, which is compared with a PCBM ETL and a c-TiO₂/PCBM bilayered ETL. The MAPbI₃ layer thickness is varied from 100 nm to 800 nm by controlling the precursor solution concentration. The hysteresis increases with perovskite layer thickness for the c-TiO₂ layer, while the hysteresis decreases with increasing the perovskite layer thickness in the presence of PCBM. Deep trap states are much more reduced upon inserting PCBM compared with those of the c-TiO₂ case, indicative of fewer traps for non-radiative recombination. The ideality factor obtained from the light-dependent open-circuit voltage (V_oc) increases for the c-TiO₂ layer but decreases for both the c-TiO₂/PCBM bilayer and the PCBM layer as the perovskite layer thickness increases, which again supports that the dependence of hysteresis on the perovskite thickness is related to trap states, that is, decreases in deep trap states can reduce hysteresis. From the capacitance-frequency studies, it is found that the low-frequency capacitance correlates with the observed hysteresis, where it increases for the c-TiO₂ layer but decreases for the PCBM containing ETLs with perovskite thickness. This work provides important insight into the hysteresis behavior of PSCs, where the hysteresis depends not only on the nature of the ETL but also on the degree of recombination in the bulk perovskite.