GaN-based solar cells degradation kinetics investigated at high temperature under high-intensity 405 nm optical stress

GaN-based solar cells are promising devices for application in space environment, concentrator solar systems and wireless power transmission. Thus, it is essential to understand their degradation kinetics when submitted to high-temperature, high-intensity stress. We submitted GaN-based multiple-quantum-well solar cells with AlGaN electron-blocking-layer to two step-stress experiments at 35 degrees C and 175 degrees C in short-circuit condition under 405 nm monochromatic excitation by increasing optical power from 47 W/cm2 to 375 W/cm2. We found almost no degradation in the dark-IV, light-IV, electroluminescence and photocurrent characteristics after low-temperature stress, whereas the degradation after high-temperature stress was significant: we observed a lowering in power-conversion efficiency, a decrease in open-circuit voltage and an increase in low forward bias current. We then submitted the device to several constant power stress at 180 W/cm(2) for 100 hours at 95 degrees C, 135 degrees C and 175 degrees C. We found that, by increasing the temperature, the short-circuit current during the stress decreased of 7%, 9% and 12.5% respectively. Dark IV characteristics showed an increase in low-forward bias current stronger at 175 degrees C. We also found a higher decrease in open-circuit voltage, external quantum efficiency, power conversion efficiency and electroluminescence with higher stress temperature. The causes of degradation are possibly diffusion mechanisms, which increase defect density in the p-GaN bulk region and/or in the GaN barrier region, promoting trap-assisted tunneling mechanisms, leading to the decrease in open-circuit voltage, and non-radiative recombination mechanisms, that cause the drop in quantum efficiency.