Investigation On The Optoelectronic Characteristics And Full-Color Display Of Micro-Scale Light-Emitting Diodes

The applications of light-emitting diodes(LED)in the solid-state lighting and display have presented remarkable potential in energy saving.LED chips are not only used as light source in backlight of Liquid Crystal Display(LCD)in TV,computer,and mobile phone,but also as sub-pixels in LED displays.Driven by the demand from new applications,such as augmented reality(AR),virtual reality(VR),and flexible devices,micro-scale LED displays gradually become a promising candidate for the next generation technology of display.Blue and green LED chips based on GaN and red LED chips based on AlGaInP all have achieved large-scale production,constituting the industrial basis for the development of micro-scale LED displays.According to the size of LED chips,micro-scale LED displays are generally divided into two types,namely,mini-LED display and micro-LED display.The chip sizes of LEDs in mini-LED displays range from 100 ?m to 200 ?m,whereas less than 100 ?m for micro-LED displays.Although the apparent progress has been achieved,micro-scale LED displays still face many important problems to overcome.Full-color display is one of the key bottlenecks in the field of micro-LED display.In-depth study on basic scientific issues involved is of great significance for promoting its industrialization.The research on the key scientific problems of full-color micro-scale LED displays,based on red-green-blue(RGB)micro-scale LED array and nanocrystals-converted monochromatic micro-scale LED array,is carried out.The characteristics of microscale LED chips,color gamut optimization,stability of nanocrystal have been studied,respectively.The results will contribute to the industrialization of high-efficiency,widecolor-gumat,high-reliability full-color micro-scale LED displays.The main results and innovations are as follows:1.Study on temperature-current-voltage-luminescence(T-I-V-L)characteristics of RGB tri-color mini-LEDs.The characteristics of RGB mini-LED chips with the size of 100 ?m×200 ?m in the temperature range from 300 K to 380 K are studied.The effects of temperature and current on the external quantum efficiency(EQE),the centroid wavelength and the full-width at half-maximum(FWHM)of RGB mini-LED are studied,respectively.The study on nonradiative recombination mechanism reveals that Shockley-Read-Hall(SRH)nonradiative recombination dominates in blue and green mini-LED,whereas carrier leakage in red mini-LED is serious.The difference on distribution of injected carriers in RGB mini-LED has been further discussed.The main direction of performance improvement for different mini-LEDs has been analyzed,To improve the efficiency and luminous stability of full-color mini-LED displays,we need to optimize the process and the structure of red mini-LED to improve the distribution of injection current and eliminate the carrier leakage.2.Investigation on color gamut optimization of RGB tri-color mini-LED displays.The variation of chromaticity coordinates of RGB mini-LED chips with temperature and current has been studied separately,and the influences of RGB mini-LEDs on color gamut are mainly studied.Using the most widely used gamut standards,Rec.2020.DCI-P3 and NTSC,the optimization of color gamut value and color gamut coverage have been carried out.The influences of temperature and driving current on the chromaticity coordinates of mini-LEDs are determined.The color gamut of the tri-color mixed system is sensitive to the luminous properties of green mini-LED.It is impossible to reach the maxima of these three different color gamut standards at the same set of driving currents for RGB mini-LEDs.When currents of red and blue miniLEDs at those corresponding to the maxima of EQE,there exists an current for green mini-LED which enables the achievement of large color gamut and high power efficiency simultaneously for RGB tri-color mini-LED displays.A method for obtaining the optimized current combination of RGB Mini-LEDs to achieve the maximum color gamut value and color gamut coverage of full-color mini-LED display has been established.3.Research on current crowding in green micro-LED.Spatially resolved electroluminescence emitted through the sapphire substrate of GaN-based flip-chip green micro-LED chips with a mesa size of 20?m×20?m are collected by using microscopic hyperspectral imaging.The current crowding is identified from the spatial mappings of electroluminescence intensity,centroid wavelength and FWHM.On the basis of the theoretical model of current diffusion in the LED chip,the current crowding mechanism of the green-light micro-LED chip has been explained.The electroluminescence intensity across the mesa includes the bright region corresponding to the p-pad,the edge regions with bright spots,and the dim circular region between them.Although current crowding at the edges of the mesas has been reported in blue micro-LEDs,the bright region for the green micro-LED reveals current crowding in the region corresponding to the p-pad.The results not only provide a clear basis for improving the luminous efficiency of green-light micro-LED chips,but also contribute to the methodology for investigating the inhomogeneity of electroluminescence from micro-LEDs.4.Study on the stability of perovskite structure CH3NH3PbBr3 nanocrystals under heating and ultraviolet light irradiation.The aging characteristics of green CH3NH3PbBr3 nanocrystals embedded in polyvinylidene fluoride are characterized by simulating the conditions of temperature,ultraviolet light irradiation and atmosphere in the nanocrystals-converted full-color micro-scale LED displays.The mechanism of photoluminescence decay are studied systematically according to the change on the crystal structure and the photophysical parameters of CH3NH3PbBr3 nanocrystals.The results show that CH3NH3PbBr3 nanocrystals are partially decomposed and generate NH4Pb2Br5 during aging under the co-stresses of heating and ultraviolet light.This decomposition could not be prevented by removing the water and oxygen from the atmosphere,as it could also occur in pure N2 and even in vacuum.Therefore,to evaluate the performance of perovskite nanocrystals in optoelectronic applications,researchers should carry out aging tests under combined external stresses,rather than just under the individual one.The densities of interface trap states of the aged samples are far higher than that of the pristine sample,according to the fitting results from temperaturedependent PL spectra.Therefore,the co-stresses result in the generation of large amount of accompanying interface defects,which dissipate carriers and causes the PL quenching.It is necessary to develop new routes,besides encapsulation,to mitigate the intrinsic decomposition and enhance stability of perovskite nanocrystals.Therefore,this work reveals PL quenching mechanism of CH3NH3PbBr3 perovskite nanocrystals under combined external stresses that mimic operating conditions in full-color microLED displays.This thesis provides a direction for the performance improvement of Mini-LED chips,proposes a new method for color gamut optimization of full-color Mini-LED display,explains the current crowding mechanism of green micro-LED chips,and highlights a new PL quenching mechanism of CH3NH3PbBr3 perovskite nanocrystals.This thesis also contributes to the research methodology towards uniformity of light emission form micro-LED chips and aging tests for nanocrystals used in full-color micro-LED displays.