  Extensive studies have focused on degradation of perovskite at low charge
carrier density (<10^16 cm^-3), but few have surveyed the degradation mechanism
at high charge carrier density (~10^18 cm^-3). Here, we investigate the
degradation mechanisms of perovskite under high charge carrier conditions.
Unlike the observations in previous works, we find that MAPbI3 degradation
starts at surface defects and progressing from the surface defects towards
neighboring regions under high charge carrier density condition. By using PbI2
passivation, the defect-initiated degradation is significantly suppressed and
the nanoplatelet degrades in a layer-by-layer way, enabling the MAPbI3 laser
sustain for 4500 s (2.7*10^7 pulses), which is almost 3 times longer than that
of the nanoplatelet laser without passivation. Meanwhile, the PbI2 passivated
MAPbI3 nanoplatelet laser with the nanoplatelet cavity displaying a maximum
quality factor up to ~7800, the highest reported for all MAPbI3 nanoplatelet
cavities. Furthermore, a high stability MAPbI3 nanoplatelet laser that can last
for 8500 s (5.1*10^7 pulses) is demonstrated based on a dual passivation
strategy, by retarding the defect-initiated degradation and surface-initiated
degradation, simultaneously. This work provides in-depth insights for
understanding the degradation of perovskite at high charge carrier density.
