Color Pure Green and Blue Electroluminescence Using Colloidal Quantum Confined Perovskites

Abstract

Hybrid lead halide perovskites, described by a formula ABX3, recently emerged as a new class of semiconducting materials for various optoelectronic devices. Especially, colloidal perovskites exhibit numerous favorable properties, such as high photoluminescence quantum yield (PLQY), tunable bandgap, defect tolerance, high charge carrier mobility, large exciton binding energy (EB ), outstanding color purity (FWHM < 25 nm) and low cost of production, for high efficiency solution-processed light-emitting diodes (LEDs).[1] Analogously to conventional, inorganic quantum dots (QDs), quantum confinement effects could be observed upon forming colloidal perovskite nanoplatelets (NPLs) with thicknesses below ~10 unit cells (n < 10). Taking MAPbBr3 for example, we demonstrate a facile synthetic route to obtain monodisperse colloidal solutions of layered perovskites with precise control from n = 1 to n = 7-10 layers.[2] These layer controlled perovskite NPLs not only offer a color-tunable emission, but also enhance the EB . The resultant high EB is crucial to enable efficient radiative recombination of excitons. Most interestingly, our layer controlled NPLs also showed record high solid-state PLQYs (50-85%) for blue and green emission. With this approach we were able to demonstrate efficient pure-blue electroluminescence at room temperature using colloidal 2D perovskites. On the other hand, in order to meet the requirement for ultrapure green emission which targets the Rec.2020 recommendation, FAPbBr3 perovskite NPLs were utilized. Through dielectric quantum well (DQW) engineering high exciton binding energy of 162 mV was achieved which further resulted in superior PLQY value of 92%. The optimized active layer LEDs exhibited a maximum current efficiency of 13 cd/A but most importantly with its CIE 1931 color coordinates (0.168, 0.773) it represents the “greenest” LED ever reported.[3] Motivated by harsh market requirements, we were able to further improve the performance of ultrapure green device over 2-fold through compositional engineering. Using colloidal, ultrapure green emitting perovskite nanocrystals such result has never been reached so far. This opens an avenue towards low-cost and high-throughput production of solution-processed LEDs. [1] J. Jagielski, S. Kumar, W.-Y. Yu, C.-J. Shih, Journal of Materials Chemistry C 2017, 5, 5610. [2] S. Kumar, J. Jagielski, S. Yakunin, P. Rice, Y.-C. Chiu, M. Wang, G. Nedelcu, Y. Kim, S. Lin, E. J. G. Santos, M. V. Kovalenko, C.-J. Shih, ACS Nano 2016, 10, 9720. [3] S. Kumar, J. Jagielski, N. Kallikounis, Y.-H. Kim, C. Wolf, F. Jenny, T. Tian, C. J. Hofer, Y.-C. Chiu, W. J. Stark, T.-W. Lee, C.-J. Shih, Nano Letters 2017, 17, 5277.