Research On Suppression Of Efficiency Droop And System-Level Thermal Management Of High-Power LEDs

Light-Emitting Diodes(LEDs)offer numerous advantages,including low operating voltage,long lifetime,high luminous quality,and environmental friendliness.In particular,the high electro-optical conversion efficiency of LEDs is significant for energy conservation and efficient energy use,making them a green lighting solution that aligns with the goals of "carbon peak" and "carbon neutrality." Furthermore,the development of LED technology has played a positive role in addressing basic and common scientific problems in the fields of semiconductors and chips,which contributes to the national strategic development plan.Although the luminous efficiency of LEDs has surpassed that of most traditional lighting tools,the actual efficiency of LEDs is still far less than the expected value under ideal conditions due to efficiency attenuation.High-power LEDs experience two main types of light attenuation.One is the "J-droop" which relates to the drop in efficiency under high current injections.And the other is the "T-droop" which relates to the drop in efficiency under high-temperature conditions.Researchers have developed many effective methods for suppressing J-droop,but further consideration is necessary to address some remaining problems.For the T-droop,there is no unified consensus on its mechanism,and existing research mostly focuses on the micro level,discussions on macro-level influencing factors(different from microparticles)are relatively rare.As the LED has been developing towards high power,integration,and miniaturization,the temperature rise caused by heat production and accumulation becomes more significant,leading to efficiency drops,spectral offset,shortened life,and even damage to LEDs.And it has become a major problem for LED development.Therefore,thermal management of high-power LED systems has become a focus of research,where faster and more accurate detection of junction temperature becomes the primary task.Coupling studies of multi-physics domains contribute to reflecting physical processes in LEDs more accurately,which is important for the thermal management of highpower LED systems.However,due to the complexity of physical processes and the huge number of variables involved in coupling research,there is still ample room for improvement and development of existing models and theories.To address the above issues,this dissertation focuses on two types of droop problems in power LEDs,as well as thermal management issues associated with multiphysics coupling in LED systems.The research work has successfully resolved a number of key problems,making it of significant theoretical importance and guiding significance.The specific works and innovations of the dissertation can be summarized as follows:1.The influence of the step-type quantum well in suppressing the J-droop problem has been studied,and the advantages and disadvantages of the step-type structure have also been clarified.In addition,a symmetric step-type quantum barrier has been proposed,which can improve the internal quantum efficiency and effectively suppress the J-droop issue.Comprehensive studies have been conducted on the influences of step-type quantum wells on energy bands,radiative recombination rates,and internal quantum efficiency across the entire current range.By analyzing both the radiative recombination intensity and effective recombination region within the quantum well.it can be explained why the two different step settings(In component from low to high vs.In component from high to low)have similar effects on lighting output,but the latter is relatively better.Additionally,the differences in internal quantum efficiency between step-ty pe and conventional quantum well LEDs under different current conditions have been compared.While step-type stepped quantum wells can enhance radiative recombination,they cannot fully suppress luminous attenuation throughout the full current range due to some regions not effectively participating in the radiative recombination process.Furthermore.a symmetric step-type structure of quantum barriers is suggested to enhance internal quantum efficiency and effectively suppress Jdroop.It can be achieved by increasing the In component at both ends of the barrier region to reduce the lattice mismatch at the heterojunction between the well and barrier.The approach not only weakens the polarization field and inhibits energy band tilting but also maintains the bandgap of quantum wells and avoids spectrum shift.The lower barrier height between the wells enhances hole injection efficiency and ensures a relatively uniform hole distribution in each well.Thus,the proposed structure improves electron utilization efficiency and then enhances radiation recombination,and it also avoids an excessively high hole concentration in the quantum well near the p-GaN and then weakens Auger recombination.For input currents of 350 and 500 mA respectively,the internal quantum efficiency of symmetric step-type quantum barrier LEDs is 5.92%and 5.86%higher than that of conventional quantum barrier LEDs.2.For the first time.the mechanism of the T-droop effect has been studied from the perspective of LED voltage changes caused by temperature rise.The results reveal that the cause of the T-droop effect is not only related to microscale factors such as carrier leakage and Auger recombination but also influenced by the LED’s own parameter characteristics at the macro level.Currently,research on the T-droop effect in LEDs has primarily focused on microlevel factors such as Shockley Read Hall(SRH)recombination,carrier leakage,and Auger recombination.However,there has been less discussion of macro-scale factors.Our study examines the mechanism of the T-droop effect from the perspective of LED voltage changes caused by temperature increases.It has been found that the T-droop effect can also be influenced by the LED’s own parameter characteristics at the macro level.Specifically,under the same current conditions,the decrease in voltage caused by temperature rise leads to lower efficiency of hole injections and a change in the radiative recombination intensity within each quantum well,resulting in the T-droop.It leads to an increase in the difference and imbalance in radiative recombination rates among various wells,causing some wells to have a decreased radiative recombination rate and others to have an increased Auger recombination rate.As a result,the total radiative recombination rate of the entire active region decreases.3.For the first time,a laminated structure LED system model of arbitrary layers is proposed for analyzing the thermal management of power LED systems,which covers the equivalent modeling methods of some system components of special structures.An analytical solution to the temperature field of any layer in the proposed model is also derived.Additionally,an improved equivalent thermal resistance network model that describes the heat dissipation process in high-power LED systems is introduced.With these developments,a direct and efficient method for calculating the LED junction temperature and the thermal resistance of the heat transfer path is developed.Based on the analysis of the heat transfer processes at both chip and system levels,a laminated structure model of arbitrary layers is developed to simulate high-power LED systems.It includes equivalent methods for modeling components with special structures such as heatsinks and thermal interface materials.The model is not limited by a fixed number of layers and can be flexibly applied to different LED systems.Therefore,it solves the problems such as the basic model cannot be directly used and the system components are insufficient.Furthermore,the analytical solution for the temperature field of the model is also extended to arbitrary layers.The derivation process is detailed,eliminating issues such as being limited to models with a fixed number of layers and using empirical approximations.Furthermore,by introducing the spreading thermal resistance,the equivalent thermal resistance network model is improved.It can be used to analyze the influence of LED chip size,quantity,distribution,and mutual interaction,as well as the materials and structural parameters of each system component on heat transfer.Based on the above works,direct and accurate calculations of junction temperature and pathway thermal resistance are achieved.By comparing with the numerical method,the proposed model and the analytical calculation method are verified to be effective and correct,and the high accuracy and efficiency of the methods are particularly highlighted.Because it can realize the direct calculation of LED junction temperature without solving the complete temperature distribution of the entire system model,the calculation time is always less than 1 s,making it an ideal choice for rapid parameter scanning research of power LED systems.4.Based on the proposed LED system model and the analytical solutions,the influences of LED chips and other system components on junction temperature and various thermal resistances can be systematically studied,which can provide useful assistance for the design and optimization of high-power LED systems.The effects of LED chips and other common components in LED systems on junction temperature and various thermal resistances have been analyzed in detail.The conclusions are as follows:For a single LED chip,there is a linear relationship between junction temperature and both thermal resistance and LED heat power.Additionally,there is no evidence that thermal resistance increases with input power.The decrease in power density inside LEDs and the spreading thermal resistance are the primary reasons for a decrease in junction temperature as LED size increases.For multi-chip arrays,an increase in the number of chips does not lead to a parallel effect on thermal resistance,and heat superposition can increase the spreading thermal resistance.For the printed circuit board(PCB),the large spreading thermal resistance is primarily caused by the size difference between the PCB and LED chips as well as the low thermal conductivity of the insulating dielectric material.For the thermal interface material,the non-ideal surface and the surface air gap do not significantly hinder heat transfer.For the heatsink,it shows that the different metal materials have no significant impact on junction temperature or thermal resistance.5.Since the existing LED multi-domain model is relatively simple and cannot accurately reflect the electro-thermal coupling of LEDs,an improved model is proposed to solve the above problems and enrich the LED electro-thermal coupling theory.Based on that,a rapid coupling calculation method for the system-level electrical and thermal characteristics of LEDs is developed.An improved and innovative model has been proposed to address the limitations of the existing LED multi-domain model,which seems to be relatively simple as it can not establish a clear relationship between electrical and thermal characteristics at the chip level,and also can not reflect the electro-thermal coupling at the system level.The proposed model suggests that electro-thermal coupling can be seen as a process where the thermal conditions and electrical response adapt to each other under a specific system environment.Based on that,a rapid coupling calculation method for the systemlevel electrical and thermal characteristics of LEDs has been developed.Firstly,the method begins with the construction of an I-V-Tj(current-voltage-junction temperature)function through multi-point measurements and data processing.The function completely records the chip-level electrical and thermal characteristics of LEDs across a full current and wide junction temperature range and establishes the relationship between them.Secondly,the LED system is modeled based on its specific system configuration and the environment to reflect the various influencing factors as comprehensively as possible.Thirdly,the electro-thermal coupling is reflected by the iterative calculation rather than taking the LED as the constant heat source.