Bandgap tuning is
a crucial characteristic of metal-halide perovskites,
with benchmark lead-iodide compounds having a bandgap of 1.6 eV. To
increase the bandgap up to 2.0 eV, a straightforward strategy is to
partially substitute iodide with bromide in so-called mixed-halide
lead perovskites. Such compounds are prone, however, to light-induced
halide segregation resulting in bandgap instability, which limits
their application in tandem solar cells and a variety of optoelectronic
devices. Crystallinity improvement and surface passivation strategies
can effectively slow down, but not completely stop, such light-induced
instability. Here we identify the defects and the intragap electronic
states that trigger the material transformation and bandgap shift.
Based on such knowledge, we engineer the perovskite band edge energetics
by replacing lead with tin and radically deactivate the photoactivity
of such defects. This leads to metal halide perovskites with a photostable
bandgap over a wide spectral range and associated solar cells with
photostable open circuit voltages.