Halide perovskite semiconductors made a great impact on the field of solar cells with efficienies soaring beyond 26% [1]. On the other hand, halide perovskites are attractive materials for light emitting devices, i.e. LEDs and lasers. Perovskite lasers can be prepared from solution at low temperatures on a wide range of substrates, which provides exciting opportunities in the field of integrated optoelectronics.[2]

Amplified spontaneous emission (ASE) and optically driven lasing were achieved in so-called hybrid organic-inorganic lead halide perovskites, where the A-site cation is based on an organic compound, such as methylammonium or formamidinium. [3,4] Intriguingly, these crystalline compounds are soft and photonic nanostructures can be directly patterned into them by thermal imprint at moderate temperatures.[5]

All-inorganic halide perovskites, such as those where the A-site cation is Cs⁺, are frequently claimed to provide improved thermodynamic stability.[6] Among them, CsPbBr₃ has been shown to be an excellent gain medium for perovskite lasers in the green spectral region. [7] More recently, we could show that b-CsPbI₃ stabilized with 2.5wt% of PEO demonstrates a low lasing threshold of 45 μJ cm^-2 at room temperature with tunable emission from 714.1 nm to 723.4 nm [8]_.

For the Cl-based representative, i.e. CsPbCl₃, deposition of thin films from solution is essentially impossible due to the poor concomitant solubility of the precursor salts PbCl₂ and CsCl.[9]

Here, we will show two concepts to achieve CsPbCl₃ thin films as gain medium. As a first strategy, we use superlattice structures of PbCl₂ and CsCl with a thickness of the sublayers on the order of 3-5 nm by thermal evaporation. We evidence the formation of CsPbCl₃ at the interface of PbCl₂ and CsCl. Already a 1 nm thick CsCl layer deposited on top of PbCl₂ gives rise to a notable photoluminescence at 409 nm with a narrow line width of 8 nm (FWHM), which agrees with reports of CsPbCl₃ single crystals. The superlattices show amplified spontaneous emission (ASE) in the deep blue spectral region at 427 nm above a threshold energy density of 190 µJ/cm² at room temperature (RT).

In a second approach, we subject thin-films of CsPbBr₃ to halide exchange. Upon pulsed exposure to TiCl₄ gas in an atomic layer deposition system, the CsPbBr₃ film is step-wise converted to CsPbCl₃. Depending on the number of TiCl₄ pulses, the emission of the resulting material can be tuned between 523 nm (CsPbBr₃) to 413 nm (CsPbCl₃). Thermal imprint is shown to significantly improve the material quality affording a narrow luminescence linewidth of 8 nm (FWHM). The resulting CsPbCl₃ show ASE at 427.6 nm with a low threshold of 70 µJ/cm². Our work states the first report of CsPbCl₃ thin films showing ASE at room temperature.