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In recent years, there have been numerous reports on the inter-transformation between non-luminous Cs 4 PbX 6 nanocrystals (NCs) and luminous CsPbX 3 nanocrystals (NCs). However, most of these reports have focused on the transformation between Cs 4 PbBr 6 NCs and CsPbBr 3 NCs, with little attention given to Cs 4 PbCl 6 NCs. This paper aims to address this gap by investigating the formation of Cs 4 PbCl 6 through the addition of excess CsCl during thermal evaporation. The presence of Cs 4 PbCl 6 was found to passivate the CsPbCl 3 film and improve its luminescence. The highest photoluminescence (PL) intensity of the sample was observed when the ratio of PbCl 2 to CsCl was 1:1.17. This can be attributed to the enhanced crystal quality and reduced non-radiative recombination resulting from the presence of Cs 4 PbCl 6. Furthermore, rapid annealing led to a two-fold increase in the PL intensity of the film. Notably, the film containing Cs 4 PbCl 6 exhibited excellent thermal stability, maintaining a constant PL intensity within the annealing temperature range of 200°C–300 °C. This can be attributed to the prevention of grain size growth in the film due to the presence of Cs 4 PbCl 6. Finally, the electroluminescent device test results showed that the electroluminescence intensity reached the highest at 6 V driving voltage, which was caused by the effective carrier recombination, while the ratio of PbCl 2 to CsCl was 1:1.17 and the annealing temperature was 180 °C. This study provided a novel approach to enhancing the photoelectric performance of CsPbCl 3. • CsPbCl 3 perovskite film is prepared by thermal evaporation method, and CsPbCl 3 /Cs 4 PbCl 6 mixed film is obtained by adding excess CsCl. • The film maintains violet/blue emission at 409 nm, and the full width at half maximum is only 12 nm, showing excellent monochromaticity. • Photoluminescence intensity is nearly twice the original value at a PbCl 2 to CsCl ratio of 1:1.17. • Annealing can also improve the performance of the film, and the film has good stability under low temperature (200°C–300 °C) annealing conditions. • The presence of an appropriate quantity of Cs 4 PbCl 6 enhances the performance of photoinduced and electro-induced devices of CsPbCl 3 thin films. [ABSTRACT FROM AUTHOR]