Temperature dependent efficiency droop in GaInN light-emitting diodes with different current densities

Abstract

The effect of chip area on the temperature-dependent light-output power (LOP) in GaInN-based light-emitting diodes (LEDs) is investigated. The larger the chip size, the faster the reduction in LOP with increasing temperature becomes, indicating that increasing the size of LED chips, a technology trend for reducing the efficiency droop at high currents, is detrimental for high temperature-tolerant LEDs. In addition, it is found that regardless of chip size, the temperature-dependent LOP is identical for the LEDs operating at the same current density.GaN-based high-power light-emitting diodes (LEDs) have become increasingly prevalent in illumination applications such as interior/exterior lighting and automotive headlights. However, a long standing problem called "efficiency droop" has been dimming the future prospects of LEDs as the ultimate illumination sources. The efficiency droop can be categorized using two classifications: current-density droop and temperature droop. First, the conventional definition of efficiency droop describes the decrease in radiative efficiency with increasing operating current, which we call the current-density droop, J-droop, in this study. Several explanations have been proposed for the causes of the J-droop, including electron leakage due to polarization mismatch 1 and poor hole injection caused by asymmetry of carrier-transport properties, 2 delocalization of carriers, 3,4 density-activated defect recombination, 5,6 and Auger recombination. 7 These have led to possible solutions such as polarization matched multiple quantum well (MQW) structures, double heterostructure designs, and large-size (large junction area) devices for reducing the current density. Second, GaNbased LEDs also suffer from a strong decrease in radiative efficiency with increasing temperature, 8,9 which we call the temperature droop, T-droop, in this study. Figure 1 shows the external quantum efficiency (EQE) of a commercial high-power LED as a function of driving current measured at several ambient temperatures. The EQE peaks at a low forward current and then drops with increasing current, showing typical J-droop behavior. In addition, the EQE decreases significantly as the ambient temperature increases; increasing temperature from 300 to 450 K results in the reduction of the EQE by about 30% of its peak value, indicating that the T-droop can be more detrimental than the J-droop. High temperature-tolerant LEDs are becoming increasingly important in applications where a weak-temperature-dependence of the EQE is highly desirable, for example, automotive headlights for which the ambient temperatures can be as high as 90 C. Until now, however, T-droop has been less focused on than the J-droop.There has been a great deal of research to understand the mechanisms of efficiency droop caused by high current densities; however, the understanding of T-droop, originating from high temperature, is not comprehensive. One of the technology trends in current state-of-the-art LEDs is to increase the chip size, thus decreasing t...

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“…A local high temperature of C was observed in high power LEDs in our other experiments. Such a high chip temperature caused by the poor CS performance in the LEDs with long will lead to other issues such as the "thermal droop" [15], [40], which will further decreases the LEDs' efficiency and reliability, resulting in more remarkable efficiency droop in LEDs.…”