## 5 Conclusions  
EL spectra of blue and green LEDs at various junction temperatures and forward currents were investigated. The \(T_{j}\) was measured by a built-in sensor on the LED chip, enabling accurate real-time \(T_{j}\) measurements and EL spectrum measurements for the LEDs under a high forward current and in a wide range of \(T_{j}\). To the best of our knowledge, this work is the first investigation of the changes in the EL spectrum under high current density in a large temperature range, particularly in the range of 120-220 K. Both blue and green LEDs first showed a blueshift and then a redshift of the emission peak as the temperature increased from \(\sim\)120 K to \(\sim\)350 K. Green LED showed two maxima in the plot of the emission peak energy versus temperature, which can be explained by the transition of the main luminescence mechanism of the green LED from DX recombination to FXA recombination. No change in the main luminescence mechanism was observed in the blue LED in the similar temperature range. We attribute this to the higher degree of localization and lower exciton mobility in the green LED compared to those in the blue LED. The transition of the luminescence mechanism from FXA-dominated to DX-dominated occurred at approximately 180 K in the green LED, and it is possible that in the blue LED, this transition occurs at a temperature that is lower than the temperature range examined in this work, which needs to be investigated in the future work. According to the band-tail model, in the low-temperature region, excitons are mainly bonded around the potential minima. Therefore, in the green LED with strong potential fluctuation and lower exciton mobility, an increase in the current supplied a greater number of excitons to be bound at the potential minima at different positions and different energy levels, so that the emission peak energy, FWHM, and radiation power increased rapidly with increasing current at low temperatures (below 150 K). By contrast, in the blue LED, the potential fluctuation was weaker and exciton mobility was higher, leading to only a slight change of the emission peak energy, the FWHM, and radiation power of blue LED at low temperatures (below 150 K). The threshold voltages of the blue and green LEDs were higher at low temperatures, and the increased voltage was most likely due to the combined effect of the band gap, dynamic resistance, piezoelectric polarization, and EBL.  
Conceptualization, Y.Z. ([PERSON]); methodology, Y.Z. ([PERSON]), S.P., and C.S.; validation, C.S. and W.C.; formal analysis, C.S., W.C., and S.P.; investigation, S.P. and C.S.; resources, Y.Z. ([PERSON]); data curation, C.S.; writing--original draft preparation, S.P. and C.S.; writing--review and editing, Y.Z. ([PERSON]); supervision, Y.Z. ([PERSON]); funding acquisition, Y.Z. ([PERSON]), R.Z., and Y.Z. ([PERSON]). All authors have read and agreed to the published version of the manuscript. This research was funded by the National Natural Science Foundation of China under Grant (Grant Nos. 61634005), National Key R&D Program of China (Grant No. 2016 YFB0400904). This work was supported by Collaborative Innovation Center of Solid-State Lighting and Energy-Saving Electronics and a project funded by the Priority Academic Program Development of Jiangsu Higher Education Institutions (PAPD).  
Figure 11: (**a**) Relationship between the threshold voltage and \(T_{j}\). (**b**) Relationship between the dynamic resistance and \(T_{j}\).