With the advent of metal halide perovskites into the world of emerging sustainable semiconductors, a host of previously unprecedented applications has materialised. Aside from their raging success in photovoltaics, perovskites as applied to light emitting diodes (LEDs) and displays have gained particular interest due to their extremely facile bandgap tunability, directional and narrow-linewidth emission characteristics, high brightness, superior efficiencies and colour purity among other desirable features. However, most of the world records in terms of device efficiency and stability have been achieved on lab-scale pixels processed using solution-based techniques, primarily spin-coating. However, with its inherent advantages of scalability, reproducibility and precise thickness control, vacuum-based thermal evaporation provides an edge over solution-processing for all optoelectronic applications. Thus, as we move towards integrating sustainable semiconductors in consumer electronics, it is important to optimise thermal evaporation-based device fabrication, especially for large-area, flexible and niche applications for light sources and displays.

Following the recent advancements in vacuum-evaporated perovskite solar cells in our group, 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 unprecedented 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 fundamental 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) has been employed to obtain information on the chemical composition and electronic properties of the evaporated films. There is very little existing information on the structure-property relationships in evaporated films tailored to luminescence. To our knowledge, our work is the first of its kind to take into account various factors related to the unique morphology of these films and to study their concomittant effect on their optoelectronic behaviours. Finally, by extensive screening of the charge injection layers, we have been able to demonstrate proof-of-concept LEDs with external quantum efficiency >3% with turn-on voltage ~ 3V, which is a world record for evaporated red perovskite LEDs as of now.