  We demonstrate reduced surface recombination velocity (SRV) and enhanced
power-conversion efficiency (PCE) in mixed-cation mixed-halide perovskite solar
cells by using (3-aminopropyl)trimethoxysilane (APTMS) as a surface passivator.
We show the APTMS serves to passivate defects at the perovskite surface, while
also decoupling the perovskite from detrimental interactions at the C60
interface. We measure a SRV of ~125 + 14 cm/s, and a concomitant increase of
~100 meV in quasi-Fermi level splitting in passivated devices compared to the
controls. We use time-resolved photoluminescence and excitation-correlation
photoluminescence spectroscopy to show that APTMS passivation effectively
suppresses non-radiative recombination. We show that APTMS improves both the
fill factor and open-circuit voltage (VOC), increasing VOC from 1.03 V for
control devices to 1.09 V for APTMS-passivated devices, which leads to PCE
increasing from 15.90% to 18.03%. We attribute enhanced performance to reduced
defect density or suppressed nonradiative recombination and low SRV at the
perovskite/transporting layers interface.
