Zinc-oxide (ZnO) is widely used as an n-type electron transporting layer (ETL) for quantum dot (QD) light-emitting diode (QLED) because various metal doping can be possible and ZnO nanoparticle can be processed at low temperatures. We report here a Li- and Mg-doped ZnO, MLZO, which is used for ETL of highly efficient and long lifetime QLEDs. Co-doping, Mg and Li, in ZnO increases its band gap and electrical resistivity and thus can enhance charge balance in emission layer (EML). It is found also that the O-H concentration at the oxide surface decreases and exciton decay time of QDs on the metal oxide increases by co-doping in ZnO. The inverted green QLEDs with MLZO ETL exhibits the maximum current efficiency (CE) of 69.1 cd/A, power efficiency (PE) of 73.8 lm/W, and external quantum efficiency (EQE) of 18.4%. This is at least two times higher compared with the efficiencies of the QLEDs with Mg-doped ZnO ETL. The optimum Li and Mg concentrations are found to be 10% each. The deep-red, red, light-blue, and deep-blue QLEDs with MLZO ETLs exhibit the CE of 6.0, 22.3, 1.9, and 0.5 cd/A, respectively. The MLZO introduced here can be widely used as ETL of highly efficient QLEDs.
Perovskite quantum dots have recently emerged as a promising light source for optoelectronic applications. However, integrating them into devices while preserving their outstanding optical properties remains challenging. Due to their ionic nature, perovskite quantum dots are extremely sensitive and degrade on applying the simplest processes. To maintain their colloidal stability, they are surrounded by organic ligands; these prevent efficient charge carrier injection in devices and have to be removed. Here we report on a simple method, where a moderate thermal process followed by exposure to UV in air can efficiently remove ligands and increase the photo-luminescence of the room temperature synthesized perovskite quantum dot thin films. Annealing is accompanied by a red shift of the emission wavelength, usually attributed to the coalescence and irreversible degradation of the quantum dots. We show that it is actually related to the relaxation of the quantum dots upon the ligand removal, without the creation of non-radiative recombining defects. The quantum dot surface, as devoid of ligands, is subsequently photo-oxidized and smoothened upon exposure to UV in air, which drastically enhances their photo-luminescence. This adequate combination of treatments improves by more than an order of magnitude the performances of perovskite quantum dot light emitting diodes.
Incorporation of rubidium (Rb) into mixed lead halide perovskites has recently achieved record power conversion efficiency and excellent stability in perovskite solar cells. Inspired by these tremendous advances in photovoltaics, this study demonstrates the impact of Rb incorporation into MAPbBr-based light emitters. Rb partially substitutes MA (methyl ammonium), resulting in a mixed cation perovskite with the formula MARb PbBr. Pure MAPbBr crystallizes into a polycrystalline layer with highly defective sub-micrometer grains. However, the addition of a small amount of Rb forms MARb PbBr nanocrystals (10 nm) embedded in an amorphous matrix of MA/Rb Br. These nanocrystals grow into defect-free sub-micrometer-sized crystallites with further addition of Rb, resulting in a 3-fold increase in exciton lifetime when the molar ratio of MABr/RbBr is 1:1. A thin film fabricated with a 1:1 molar ratio of MABr/RbBr showed the best electroluminescent properties with a current efficiency (CE) of 9.45 cd/A and a luminance of 7694 cd/m. These values of CE and luminance are, respectively, 19 and 10 times larger than those achieved by pure MAPbBr devices (0.5 cd/A and 790 cd/m). We believe this work provides important information on the future compositional optimization of Rb-based mixed cation perovskites for obtaining high-performance light-emitting diodes.
A new transparent p-type oxide semiconductor (POS) is reported, Cu 2 SnS 3 -Ga 2 O 3 , having high Hall mobility of 36.22 cm 2 V −1 s −1 , and high work function of 5.17 eV. The existence of Cu 2 SnS 3 and Ga 2 O 3 phases in the film is confirmed by X-ray photoelectron spectroscopy results and the Cu 2 SnS 3 shows polycrystalline structure according to Raman spectrum and X-ray diffraction analysis. The transparent Cu 2 SnS 3 -Ga 2 O 3 exhibits the carrier concentration of 5.86 × 10 16 cm −3 , and electrical resistivity of 1.94 Ω·cm. The transparent POS is applied to green quantum light-emitting diodes (QLEDs) as a hole injection layer (HIL) because of its high work function. The QLED exhibits the maximum current efficiency of 51.72 cd A −1 , power efficiency of 31.97 lm W −1 , and external quantum efficiency (EQE) of 14.93%, which are much higher than the QLED using polyethylene dioxythophene:poly(styrenesulfonate) HIL exhibiting current efficiency of 42.66 cd A −1 , power efficiency of 20.33 lm W −1 , and EQE of 12.36%. The Cu 2 SnS 3 -Ga 2 O 3 developed in this work can be widely used as a transparent and conductive p-type oxide for thin-film devices.
We report solution-processed metal-oxide p-n junction, Li-doped CuO (Li:CuO) and Li-doped ZnO (Li:ZnO), as a charge generation junction (CGJ) in quantum-dot light-emitting diode (QLED) at reverse bias. Efficient charge generation is demonstrated in a stack of air-annealed Li:CuO and Li:ZnO layers in QLEDs. Air annealing of Li:ZnO on Li:CuO turns out to be a key process to decrease oxygen vacancy (V) and increase the copper (II) oxide (CuO) fraction at the Li:CuO/Li:ZnO interface for efficient charge generation. Green QLEDs incorporating Li:CuO/Li:ZnO CGJ show the maximum current and power efficiencies of 35.4 cd/A and 33.5 lm/W, respectively.
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