Insights from Impedance Spectroscopy in Perovskite Solar Cells with Self-Assembled Monolayers: Decoding SAM's Tricks

Self-assembled monolayers (SAMs) are highly promising hole-transport layers for p-i-n perovskite solar cells, increasing photocurrent, reducing hysteresis and boosting photovoltage. However, the SAM's exact role in maintaining those benefits remains elusive. This work demonstrates that SAMs enhance open-circuit voltage (V_oc) and stability by suppressing surface recombination, as revealed by impedance spectroscopy. This is reflected in the time constants related to ionic dynamics, taking values from 10^-2 to 10^-1 s for PTAA-based samples and 10^-3 s for SAM devices. X-ray photoelectron spectroscopy shows that SAMs chemically bind with hydroxyl groups on metal oxide substrates such as indium tin oxide, reducing ionic accumulation and preventing ion-induced V_oc losses. With minimal ionic dynamics, SAM-based devices achieve outstanding photovoltage and stability, confirming SAMs as pivotal in advancing perovskite cell performance.