Understanding the Impact of SAM Fermi Levels on High Efficiency p-i-n Perovskite Solar Cells

Completing the picture of the underlying physics of perovskite solar cell interfaces that incorporate self-assembled molecular layers (SAMs) will accelerate further progress in p-i-n devices. In this work, we modified the Fermi level of a nickel oxide-perovskite interface by utilizing SAM layers with a range of dipole strengths to establish the link between the resulting shift of the built-in potential of the solar cell and the device parameters. To achieve this, we fabricated a series of high-efficiency perovskite solar cells with no hysteresis and characterized them through stabilize and pulse (SaP), JV curve, and time-resolved photoluminescence (TRPL) measurements. Our results unambiguously show that the potential drop across the perovskite layer (in the range of 0.6-1 V) exceeds the work function difference at the device's electrodes. These extracted potential drop values directly correlate to work function differences in the adjacent transport layers, thus demonstrating that their Fermi level difference entirely drives the built-in potential in this device configuration. Additionally, we find that selecting a SAM with a deep HOMO level can result in charge accumulation at the interface, leading to reduced current flow. Our findings provide insights into the device physics of p-i-n perovskite solar cells, highlighting the importance of interfacial energetics on device performance.