White PeLED with the composite emission layer of organic small molecule and perovskite materials preparedby thermal evaporation

Metal halide perovskite material has received considerable attention in recent years because of its unique properties, such as high photoluminescence quantum yield, adjustable bandgap, high carrier mobility, and long diffusion length, which provide promising research and application potential in the following optoelectronic fields: Photovoltaic, light-emitting diode, and photodetector. By improving emission layer morphology and structure quality as well as controlling emission materials’ dimensional size, the external quantum efficiency (EQE) of the sole color light-emitting PeLED has already been over 20%, which is comparable to the performance of the optimal organic light-emitting diode. However, the development of PeLED still faces serious challenges, such as poor temperature, long-term stability, and hard reproducibility of the fabrication. However, metal halide PeLED demonstrates superior stability compared with routine hybrid PeLED owing to the replacement of organic cations like MA⁺ and FA⁺ at the A site of the ABX₃ molecular structure by monovalent metal cations such as Cs⁺, Ag⁺, and Na⁺, which provides a promising substitute to the routine hybrid PeLED. There are few reports regarding PeLED fabrication using physical vapor deposition methods such as thermal evaporation, which can be attributed to its typically higher cost and lower light-emitting efficiency. However, the thermal evaporation procedure has distinct advantages, such as precise control of the element content and layer thickness, which is especially important for achieving a superthin layer. In this study, the fabrication and EL performance of the white PeLED with the structure of ITO/MoO₃/TAPC/TCTA/CsPbBr₃/mCP/TmPyPb/LiF/Al have been investigated, in which the CsPbBr₃/mCP is employed as emission layer. Consequently, all functional layers are fabricated through thermal evaporation, while the CsPbBr₃ layer is formed through the dual-source coevaporation of CsBr and PbBr₂. The passivation effects of the organic small molecule material mCP on the perovskite emitter are observed with considerably reduced trap densities in the emission layer. Meanwhile, as the mCP thickness increases, the trap density decreases. Furthermore, the thickness of the mCP strongly affects the ratio of the different emitting colors in white light and the shifting of the exciton recombination zone. Therefore, the white PeLED can be realized while the thicknesses of CsPbBr₃ and mCP are kept at 20 nm and within ~10–30 nm, respectively, in which the 20-nm mCP will yield the highest quality white light with the highest color rending index (CRI) of 89 and optimal CIE coordinates of (0.33, 0.34) due to the balanced emission of blue, green, and red light. The optimal EL performance can be obtained at the mCP thickness of 30 nm, with the maximum current efficiency, brightness, EQE, and CRI of 0.35 cd/A, 341 cd/m², 0.42%, and 82, respectively. Therefore, the improvement of the EL performance with the increasing mCP layers thickness (~10–30 nm) can be demonstrated. The blue-purple and red light at 380 and 620-nm wavelengths are attributed to the mCP, whereas the green light at 520 nm is attributed to CsPbBr₃. The research demonstrates a practical method for developing white PeLEDs with high-quality white light emission and a simple structure.