  We demonstrate the critical role of surface recombination in mixed-cation,
mixed-halide perovskite, FA0.83Cs0.17Pb(I0.85Br0.15)3. By passivating
non-radiative defects with the polymerizable Lewis base
(3-aminopropyl)trimethoxysilane (APTMS) we transform these thin films. We
demonstrate average minority carrier lifetimes > 4 {\mu}s, nearly single
exponential monomolecular PL decays, and concomitantly high external
photoluminescence quantum efficiencies (>20%, corresponding to ~97% of the
maximum theoretical quasi-Fermi-level splitting) at low excitation fluence. We
confirm both the composition and valence band edge position of the
FA0.83Cs0.17Pb(I0.85Br0.15)3 perovskite using multi-institution,
cross-validated, XPS and UPS measurements. We extend the APTMS surface
passivation to higher bandgap double cation (FA,Cs) compositions (1.7 eV, 1.75
eV and 1.8 eV) as well as the widely used triple cation (FA,MA,Cs) composition
and observe significant PL and PL lifetime improvements after surface
passivation. Finally, we demonstrate that the average surface recombination
velocity (SRV) decreases from ~1000 cm/s to ~10 cm/s post APTMS passivation for
FA0.83Cs0.17Pb(I0.85Br0.15)3. Our results demonstrate that surface-mediated
recombination is the primary non-radiative loss pathway in MA-free mixed-cation
mixed-halide films with a range of different bandgaps, which is a problem
observed for a wide range of perovskite active layers and reactive electrical
contacts. This study indicates that surface passivation and contact engineering
will enable near-theoretical device efficiencies with these materials.
