Highly excited photoluminescence of CsPbCl 3 , which is known to be one of the most photoluminescent semiconductors, has been measured for thin films prepared by crystallization from the amorphous phase into microcrystalline/polycrystalline states. With the increase of excitation intensity, the microcrystalline state shows successive jumps of the dominant emission band, from a free-exciton band to its phonon replica and finally to a lowest-energy band originating from exciton-exciton inelastic collision. For the exciton-exciton porcess stimulated emission occurs at very low threshold excitation intensities of the order of 10 kW/ cm 2 at 77 K. At higher excitation intensities above 50 kW/ cm 2 , single-path-light-amplification stimulated emission across the film thickness is observed suggesting a very large optical gain. The large-gain mechanism is attributable to giant oscillator strength effect characteristic of excitonic superradiance recently reported for films prepared in the same way.

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