We report an inverted and multilayer quantum dot light emitting diode (QLED) which boosts high efficiency by tuning the energy band alignment between charge transport and light emitting layers. The electron transport layer (ETL) was ZnO nanoparticles (NPs) with an optimized doping concentration of cesium azide (CsN₃) to effectively reduce electron flow and balance charge injection. This is by virtue of a 0.27 eV upshift of the ETL's conduction band edge, which inhibits the quenching of excitons and preserves the superior emissive properties of the quantum dots due to the insulating characteristics of CsN₃. The demonstrated QLED exhibits a peak current efficiency, power efficiency and external quantum efficiency of up to 13.5 cd A⁻¹, 10.6 lm W⁻¹ and 13.4% for the red QLED, and correspondingly 43.1 cd A⁻¹, 33.6 lm W⁻¹ and 9.1% for green, and 4.1 cd A⁻¹, 2.0 lm W⁻¹ and 6.6% for the blue counterparts. Compared with QLEDs without optimization, the performance of these modified devices shows drastic improvement by 95.6%, 39.4% and 36.7%, respectively. This novel device architecture with heterogeneous energy levels reported here offers a new design strategy for next-generation high efficiency QLED displays and solid-state lighting technologies.

A high-performance solution processed quantum dot light emitting diode (QLED) has been fabricated via modulating the hole transport by doping 1-bis[4-[N,N-di(4-tolyl)amino]phenyl]-cyclohexane (TAPC) into poly-N-vinylcarbazole (PVK) as the hole transport layer (HTL). With a low doping content, the hole transport ability of the HTL can be enhanced due to the extremely high hole mobility of TAPC. However, increasing the doping ratio of TAPC excessively results in degraded performance of the QLED due to the deteriorated film quality and increased energy barrier from the HTL to the quantum dots (QDs), which is attributed to the relatively shallow position of the HOMO level for TAPC. Using the optimized composition ratio of the doped HTL (3 : 1 vol% of PVK : TAPC), the best QLED performance was achieved with a low turn-on voltage of 3.3 V due to the charge balance, which facilitates exciton recombination in the emissive layer (EML). In addition, the highest current efficiency (26.2 cd A⁻¹), power efficiency (19.2 lm W⁻¹), external quantum efficiency (6.2%) and 60% enhancement in the stability of the device were achieved. The improvement in the device performance can be attributed to the suppression of Auger recombination due to the balance of carrier injection into the emissive layer composed of QDs and the separation of the carrier accumulation zone from the exciton formation interface by the modulation of hole transport. The lowered turn-on voltage and improved efficiency presented here offer potential for high throughput and practical use for commercialized QLED displays.

Surface plasmon-enhanced electroluminescence (EL) has been demonstrated by incorporating gold (Au) nanoparticles (NPs) in quantum dot light-emitting diode (QLED). Time-resolved photoluminescence (TRPL) spectroscopy reveals that the EL enhancement is ascribed to the near-field enhancement through an effective coupling between excitons of the quantum dot emitters and localized surface plasmons around Au NPs. It is found that the size of Au NPs and the distance between the Au NPs and the emissive layer have significant effects on the performance of QLED. The enhancement can be maximized as the SP resonance wavelength of Au NPs matches well with the PL emission wavelength of the QD film and the distance between Au NPs and the emissive layer maintains 15 nm. The photoluminance (PL) and EL intensity can be enhanced by 4.4 and 1.7 folds with the incorporation of Au NPs. The maximum current efficiency of 4.56 cd/A can be achieved for the resulting QLEDs by incorprating Au NPs with an enhancement factor of 2.0. In addition, the enhancement ratio of 2.2 can be achieved for the lifetime of resulting QLED.