| Quantum dots based light emitting diodes(QD-LEDs)take advantages of high purity of emission color,tunable emission wavelength,and solution processing,making it as a promising condidate in flat panel display.High-efficieny,long operation lifetime,and full-color integration are the key points for quantum dots based displays.Thus,the investigation of their working mechanism,degradation mechanism,and patterning technique is benificial for promoting the industrialization of quantum dots based display.The relationship between hole transport layer(HTL)characteristics and device performance in inverted device structure was systematically studied first.It was found that large injection barriers caused accumulatation of holes at the QD/HTL interface and exciton recombination zone moving to the interface,while the accumulated charges would induce exciton quenching and decay of device performance.A double-layer HTL was adopted to form a step-wise energy level,which could reduce the injection barrier and improve the carrier balance.The external quantum efficiency(EQE)of red QD-LED was increased from 4.0%to9.7%.On this basis,we proposed a blue-emitting polymer PFSO with deep-lying HOMO level as the HTL for inverted red QD-LED,achieving a peak current efficiency(CE)of 11.8 cd/A.Meanwhile,the hybrid device was innovatively designed and fabricated,in which PFSO was used as HTL in printed red QD-LED and light-emitting layer in OLED,respectively,realizing red and blue emission simultaneously.This work will enrich the device structure of full-color display.Inkjet printing has been considered an available way to achieve large size full-color QD-LED display,however,the QD ink formulation and the study of film drying process and film morphology are rarely reported.Here,we developed stably-printed quantum dot inks and systematically studied the influence of rheological properties of the ink and surface properties of the substrate on the film morphology.It was found that accelerating the droplet drying rate and enhancing the interaction between the quantum dots and the substrate can effectively suppress the‘coffee ring‘effect.Combined with metal oxide TFT backplane,2-inch,120 ppi full-color quantum dot based display was successfully achieved by inkjet printing technology,with gamut up to 109%NTSC standard,paving the road to manufature large-size,high-resolution AMQLED display with ink-jet printing technique.Inorganic ZnO and phosphomolybdic acid PMA was employed as interconnecting layer(ICL)in highly efficient solution-processed tandem white QD-LED.Due to the excellent solvent proof ability,high transmittance in the visible range,and efficient carrier injection and transport of ICL,the working voltage,current efficiency,and luminance of the tandem red QD-LED are all consistent with the theoretical values.White QD-LED by stacking RGB units in series exhibited a maximum CE of 60.4 cd/A and a maximum EQE of 27.3%at a luminance of10~5 cd/m~2.In addition,the tandem white QD-LED can exhibit an external quantum efficiency over 20%in the wide luminance range of 10~4 cd/m~2 to 10~5 cd/m~2.A high color gamut of 124%NTSC 1931 standard can be achieved when combined with color filters,demonstrating the great application potential as backlight for new-generation dispaly.Both ink-jet printed AMQLED display and tandem white QD-LED show great application potential for new-generation display,but the operationanl stability of the devices still needs to be improved further.The aging mechanism of the organic/inorganic hybrid red QD-LED device was systematically studied.It was found that the aging process of the QD-LED in constant current mode can be divided into two stages:the adhesion of the QD/HTL interface was enhanced in the first stage,resulting in enhancement of hole injection and carriers balance,hence increasing the brightness of the device;in the second stage,the decrease in brightness was attributed to the fact that the PLQY of quantum dot film was continuously reduced which may result from the debonding of surface ligands during operation of the device.A ligand exchange strategy was proposed to replace the organic ligands with low-binding energy,such as oleic acid,with a stronger ones,like thiol ligand,to improve the operational stability. |
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