Solution-Processed Extremely Efficient Multicolor Perovskite Light-Emitting Diodes Utilizing Doped Electron Transport Layer

A specially designed n-type semiconductor consisting of Ca-doped ZnO (CZO) nanoparticles is used as the electron transport layer (ETL) in high-performance multicolor perovskite light-emitting diodes (PeLEDs) fabricated using an all-solution process. The band structure of the ZnO is tailored via Ca doping to create a cascade of conduction energy levels from the cathode to the perovskite. This energy band alignment significantly enhances conductivity and carrier mobility in the CZO ETL and enables controlled electron injection, giving rise to sub-bandgap turn-on voltages of 1.65 V for red emission, 1.8 V for yellow, and 2.2 V for green. The devices exhibit significantly improved luminance yields and external quantum efficiencies of, respectively, 19 cd A(-1) and 5.8% for red emission, 16 cd A(-1) and 4.2% for yellow, and 21 cd A(-1) and 6.2% for green. The power efficiencies of these multicolor devices demonstrated in this study, 30 lm W-1 for green light-emitting PeLED, 28 lm W-1 for yellow, and 36 lm W-1 for red are the highest to date reported. In addition, the perovskite layers are fabricated using a two-step hot-casting technique that affords highly continuous (>95% coverage) and pinhole-free thin films. By virtue of the efficiency of the ETL and the uniformity of the perovskite film, high brightnesses of 10 100, 4200, and 16,060 cd m(-2) are demonstrated for red, yellow, and green PeLEDs, respectively. The strategy of using a tunable ETL in combination with a solution process pushes perovskite-based materials a step closer to practical application in multicolor light-emitting devices.