Robustness to High Temperatures of Al2O3-Coated CsPbBr3 Nanocrystal Thin Films with High-Photoluminescence Quantum Yield for Light Emission

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.

Figure 1

(a) CsPbBr₃ NCs passivated with Cs-oleate ligands are obtained from synthesis and a ligand exchange to DDAB is performed in solution. (b) Thin films of NCs are fabricated by spin coating the ligand-exchanged solutions on glass substrates. (c) The films are coated with a thin Al₂O₃ layer via atomic layer deposition.

Figure 2

Bare and Al₂O₃-overcoated CsPbBr₃ DDAB-capped NC films. (a) Transmission electron microscopy image of the CsPbBr₃ NCs; scale bar is 100 nm. (b) Optical Absorption and PL spectra recorded from the NCs in solution. (c) Normalized PL emission of spin-coated CsPbBr₃ NC films, bare (red) and overcoated with a 13 nm thick layer of Al₂O₃ (blue). The inset shows an SEM image of the NC film demonstrating the homogeneity of the deposition. Scale bar is 1 μm. (d) PL decay traces of the bare and Al₂O₃-coated films.

Figure 3

PL emission during heating and cooling cycles of bare and Al₂O₃-coated CsPbBr₃ NC films. (a,b) PL spectra recorded during heating the bare (a) and Al₂O₃-coated (b) films up 100 °C (373 K) and cooling back to room temperature (RT). (c,d) Normalized PL intensity obtained by integrating the area under the PL peak for bare (c) and Al₂O₃-coated (d) films. The PLQY of the bare film drops from 75% to 44% after one heating/cooling cycle, while that of the Al₂O₃-coated film fully recovers. Here, unity in PL intensity corresponds to PLQY of 75%. (e) PL intensity at 80 °C (353 K) recorded over time. The emission intensity remains above 80% for up to 100 h, and the PL decay traces recorded after 1 and 100 h (plotted in the inset) do not show any significant changes, confirming the stability of the optical properties.

Figure 4

(a,b) Contour plots of the PL decay traces versus temperature of bare and Al₂O₃-coated films in a heating cycle up to 150 °C (423 K), and the related average PL life times (c,d) obtained from fitting with three exponentials for both heating (red) and cooling (blue) cycles.

Figure 5

(a-b) Amplified spontaneous emission under femtosecond-pulsed laser at 405 nm at a frequency of 1 kHz for bare (a) and Al₂O₃-coated (b) films. The emission is recorded at grazing angles to the sample surface and some selected ASE spectra for different pump fluence are shown. (c) Emission intensity versus pump fluence for the full data set, showing an ASE threshold around 60 μJ/cm².

Figure 6

Emission spectra (a) and ASE peak intensity (b) of an Al₂O₃ coated NC film recorded over time under constant optical pumping with a fluence of 430 μJ/cm².

Figure 7

(a) Distributed feedback lasing from DDAB-capped CsPbBr₃ NC films deposited on a silica substrate with a linear grating with 310 nm periodicity. Emission spectra for different excitation fluence show that above a threshold of 1 mJ/cm² lasing peaks appear. The inset shows the PL amplitude and that of the two lasing peaks (Peak1 @ 526 nm; Peak2 @ 529 nm) versus excitation fluence. (b) Stability of the DFB laser device over time under constant pumping.