  Metal halide perovskites are attracting tremendous interest for a variety of
optoelectronic applications. The ability to tune the perovskite bandgap by
tweaking the chemical compositions opens up new applications as coloured
emitters and as components of tandem photovoltaics. Nevertheless, non-radiative
losses are still limiting performance, with luminescence yields in
state-of-the-art perovskite solar cells still far from 100% under solar
illumination conditions. Furthermore, in mixed halide perovskite systems
designed for continuous bandgap tunability (bandgaps ~1.7-1.9 eV),
photo-induced ion segregation leads to bandgap instabilities. Here, we
substantially mitigate both non-radiative losses and photo-induced ion
migration in perovskite structures by decorating the surfaces and grain
boundaries with passivating potassium-halide interlayers. We demonstrate
external photo-luminescence quantum yields of 66%, translating to internal
yields exceeding 95%. The high luminescence yields are achieved while
maintaining high mobilities over 40 cm2V-1s-1, giving the elusive combination
of both high luminescence and excellent charge transport. We find that the
external luminescence yield when interfaced with electrodes in a solar cell
device stack, a quantity that must be maximized to approach the efficiency
limits, remains as high as 15%, indicating very clean interfaces. We also
demonstrate the inhibition of photo-induced ion migration processes across a
wide range of mixed halide perovskite bandgaps that otherwise show bandgap
instabilities. We validate these results in full operating solar cells,
highlighting the importance of stabilising luminescence in device structures.
Our work represents a critical breakthrough in the construction of tunable
halide perovskite films and interfaces that can approach the efficiency limits
in tandem solar cells and coloured LEDs.
