Abstract
Lead-halide perovskite nanocrystals are a promising material in optical devices due to their high photoluminescence (PL) quantum yield, excellent color purity, and low stimulated emission threshold. However, one problem is the stability of the nanocrystal films under different environmental conditions and under high temperatures. The latter is particularly relevant for device fabrication if further processes that require elevated temperatures are needed after the deposition of the nanocrystal film. In this work, we study the impact of a thin oxide layer of Al2O3 on the light emission properties of thin nanocrystal films. We find that nanocrystals passivated with quaternary ammonium bromide ligands maintain their advantageous optical properties in alumina-coated films and do not suffer from degradation at temperatures up to 100 °C. This is manifested by conservation of the PL peak position and line width, PL decay dynamics, and low threshold for amplified spontaneous emission. The PL remains stable for up to 100 h at a temperature of 80 °C, and the ASE intensity decreases by less than 30% under constant pumping at high fluence for 1 h. Our approach outlines that the combination of tailored surface chemistry with additional protective coating of the nanocrystal film is a feasible approach to obtain stable emission at elevated temperatures and under extended operational time scales.
Highlights
Perovskite nanocrystals (NCs)[1−5] currently show record performance in various optoelectronic devices
Contrary to the more commonly used primary alkyl ammonium or alkyl carboxylate not affected by the alumina overcoating, and we obtained a photoluminescence quantum yield (PLQY) of 75 ± 8% at room temperature from both bare and Al2O3-coated films
Ambient air, and temperature are known to influence the emission properties of perovskite NC films.[38−43] The temperature cycling in Figure 3b,d demonboth in solution and in films, we focus on the following on the strates that the film coating with the alumina layer provides a didodecyl dimethylammonium bromide (DDAB)-passivated samples for the temperature and stability sufficient protection that suppresses NC degradation for characterization
Summary
Perovskite nanocrystals (NCs)[1−5] currently show record performance in various optoelectronic devices. Light-emitting diodes already present external quantum efficiencies exceeding 16% in the green spectral region,[6,7] and very recently amplified spontaneous emission under continuous-wave excitation has been reported.[8] Such optoelectronic performance has been reached after only a few years of intense research following the seminal work from Protesescu et al.[9] Yet, a major hindrance of perovskite NCs is their limited stability,[10−13] which leads to relatively short operational lifetimes of the respective devices notwithstanding the performance.[14] In order to tackle this stability issue,[1] various computational studies have identified the NC surface as the main culprit.[15−18] In addition, experimental studies on NCs synthesized via different methods[19−21] have pointed out that the nature of the surface ligands plays a major role in the material stability In this context, postsynthetic treatments are an interesting venue to increase the stability of perovskite NCs as well as to improve photoluminescence quantum yield (PLQY);[22] such methods include ligand exchange procedures,[23] amines addition,[24] cross-linking,[25] MnCl2 doping,[26] potassium incorporation,[27] and more. Atomic layer deposition (ALD) of a thin Al2O3 layer on NC films provides an additional pathway to improve stability and performance of thin films of NCs.[28−33]
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