  Metal halide perovskites (MHPs) have shown an incredible rise in efficiency,
reaching as high as 25.7%, which now competes with traditional photovoltaic
technologies. Herein, we excluded CsX and RbX, the most commonly used cations
to stabilize FAPbI3, from the bulk of perovskite thin films and applied them on
the surface, as passivation agents. Extensive device optimization led to a
power conversion efficiency (PCE) of 24.1% with a high fill factor (FF) of
82.2% upon passivation with CsI. We investigated in-depth the effect of CsI
passivation on structural and optoelectronic properties using X-ray diffraction
(XRD), angle resolved X-ray photoelectron spectroscopy (ARXPS), Kelvin Probe
Force (KPFM) microscopy, time-resolved photoluminescence (TRPL),
photoluminescence quantum yield (PLQY) and electroabsorption spectroscopy
(TREAS). Furthermore, passivated devices exhibit enhanced operational
stability, with optimized passivation with CsI leading to a retention of ~90%
of initial PCE under 1 Sun illumination with maximum power point tracking for
600 h.
