The performance of perovskite solar cells is limited by non-radiative recombination which occurs both in the bulk and at the interfaces with other layers in solar cell devices. These losses are due to structural and chemical defects and heterogeneities, and/or improper energetic alignment [1]. A commonly used strategy to overcome defects at interfaces and improve energetic alignment is molecular passivation. Recently, amino-silane molecules have been demonstrated to be effective molecular passivation for perovskite solar cells with a range of bandgaps (~1.6-1.8 eV). These molecules work by passivating defects subsequently resulting in solar cells with significantly boosted efficiency [2,3,4].

Scanning probe microscopy is a valuable tool for understanding the nano- and micro-scale properties of perovskite semiconductors and their solar cells. Kelvin Probe Force Microscopy (KPFM) can be particularly powerful due to its capability to map the surface photovoltage temporally and spatially which can provide valuable information about defect densities, chemical stabilisation, and electrical heterogeneities [5]. 
In this work we use KPFM to investigate the interaction of AEAPTMS (C₉H₂₃NO₃Si) with a 1.6 eV Cs_0.13FA_0.87Pb(I_0.9Br_0.1)₃ perovskite correlating its impact on structural and electrical heterogeneity spatially. Using temporal KPFM measurements under white light illumination we examine the impact of AEAPTMS passivation and show that it works to reduce both the heterogeneity in the surface photovoltage and the time taken for it to stabilise. We also examine this passivation molecule on the surface potential at the grain boundaries and interiors. In unpassivated perovskite thin films there is often a significant difference between the surface photovoltage at grain boundaries compared to that measured within the bulk of the grain. Prior to illumination we observe that the AEAPTMS passivation results in a significant decrease in the difference in surface photovoltage between the grain boundaries and interior (relative to an unpassivated perovskite). Under illumination this effect is maintained, indicating that the passivation reduces heterogeneity subsequently decreasing band bending at grain boundaries. Combining temporal and spatial measurements we show the dual passivation effect of this amino silane molecule in both reducing defect densities and homogenising the energetic landscape. Furthermore we will highlight some areas of best practise when undertaking KPFM measurements to ensure the extraction of quantitative data at high resolution.