Abstract
The use of lead halide perovskites in optoelectronic and photonic devices is mainly limited by insufficient long-term stability of these materials. This issue is receiving growing attention, mainly owing to the operational stability improvement of lead halide perosvkites solar cells. On the contrary, fewer efforts are devoted to the stability improvement of light amplification and lasing. In this report we demonstrate that a simple hydrophobic functionalization of the substrates with hexamethyldisilazane (HMDS) allows to strongly improve the Amplified Spontaneous Emission (ASE) properties of drop cast CsPbBr3 nanocrystal (NC) thin films. In particular we observe an ASE threshold decrease down to 45% of the value without treatment, an optical gain increase of up to 1.5 times and an ASE operational stability increase of up to 14 times. These results are ascribed to a closer NC packing in the films on HMDS treated substrate, allowing an improved energy transfer towards the larger NCs within the NC ensemble, and to the reduction of the film interaction with moisture. Our results propose hydrophobic functionalization of the substrates as an easy approach to lower the ASE and lasing thresholds, while simultaneously increasing the active material stability.
Highlights
Lead halide perovskites are receiving an enormous interest in the last few years, mainly driven by their extremely promising photovoltaic properties, ascribed to a combination of large absorption coefficient, low defect density and high charge carrier mobility[1,2,3], that recently allowed the demonstration of a certified power conversion efficiency of 25.2%4
Remembering that the net gain linearly increases with the excitation density, up to gain saturation, and that the net gain is 0 at the Amplified Spontaneous Emission (ASE) threshold excitation density (0.68 mJcm−2 for NC1HMDS sample), we evaluated a lower limit for the net gain of NC1HMDS
In order to rationalize the origin of the observed improvement of ASE threshold, intensity and stability, due to the HMDS functionalization of the substrate, we start by observing that the final ASE properties can be affected by several extrinsic factors, like waveguide thickness[70,71] and active film uniformity[72,73], beyond the intrinsic gain cross section of the active layer
Summary
Lead halide perovskites are receiving an enormous interest in the last few years, mainly driven by their extremely promising photovoltaic properties, ascribed to a combination of large absorption coefficient, low defect density and high charge carrier mobility[1,2,3], that recently allowed the demonstration of a certified power conversion efficiency of 25.2%4. Optical gain has been demonstrated both in bulk polycrystalline thin films[9,10,11,12] and in perovskite NC thin films[13,14,15,16,17,18,19] both under single and multiphoton absorption[20,21,22] and laser demonstrators have been reported with different cavity geometries, including dielectric-metal vertical microcavity[23], distributed feedback lasers (DFB)[11,24,25] and whispering gallery mode microlasers[26,27,28,29,30] and even butterfly wings microstructures[31] Despite these promising results and the fast improvements of the performances of the various device demonstrators, the perspectives for applications of lead halide perovskites in diverse optoelectronic devices are currently limited by the poor chemical and morphological stability of these materials. In order to improve the operational stability of solar cells based on bulk polycrystalline perovskites films the deposition of a protecting layer between the hole transport layer and the active layer[42,43,44] and the employment of a water-resistant electron transport layer[45], have been proposed
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