Thermally Evaporated All-Inorganic Perovskites for Pure-Red Light Emiting Diodes and Quantum-Well Heterostructures

With its inherent advantages of scalability, reproducibility and precise control of thickness, vacuum thermal evaporation provides an efficient processing route for halide perovskite films for optoelectronic applications. Recently, we have employed this technique to fabricate highly luminescent all-inorganic CsPbI₂Br perovskite films for red light emitting diodes (LEDs). By optimizing the growth conditions, we have been able to achieve photoluminescence quantum efficiencies (PLQE) close to 20% under 1-sun equivalent conditions. Moreover, no sign of unwanted halide segregation has been observed under continuous illumination, thereby resulting in a stable PL emission in the wavelength range of 630-640 nm (pure-red emission). To understand the effect of deposition conditions on the resulting optoelectronic properties of evaporated perovskites, a range of characterization including intensity-dependent PLQE, fluence-dependent TRPL, widefield hyperspectral imaging, temperature-dependent PL, THz spectroscopy and transient photoconductivity measurements have also been conducted. Furthermore, X-ray photoelectron spectroscopy (XPS) and ultraviolet photoelectron spectroscopy (UPS) is employed to obtain information on the chemical composition and electronic properties of the evaporated films. Finally, by extensive screening of the charge injection layers, we have been able to demonstrate LEDs with external quantum efficiency >3% with turn-on voltage ~ 3V, which is a record for evaporated red perovskite LEDs as of now.

Finally, we have also employed these evaporated CsPbI₂Br perovskite films (as wells) in fabricating heterostructures with different solution-processed Ruddlesden-Popper and Dion-Jacobsson 2D perovskites (as barriers). By exposing these to prolonged light and heat stressing, we investigate the stability of these heterostructures under these conditions, which have important ramifications for their applicability in a variety of perovskite optoelectronic devices.