Degradation of GaN-based InGaN-GaN MQWs solar cells caused by Thermally-Activated Diffusion

We investigate the degradation of high-periodicity GaN-based InGaN-GaN multiple quantum wells (MQWs) solar cells submitted to stress under high excitation intensity and high temperature; stress conditions are chosen to investigate cell behavior in a harsh scenario, such as wireless power transfer systems, space applications and concentrator harvesting systems. By examining the decrease in the short-circuit current and electroluminescence of these devices and the increase in the forward current at low bias, we suggest the presence of a thermally-activated diffusion process of impurities from the p-side of the device toward the active region. This process favors the increase in the Shockley-Read-Hall (SRH) recombination rate. By employing the van Opdorp and t'Hooft model, we analyzed the time-variation of non-radiative Shockley-Read-Hall lifetime during aging, extracting the diffusion coefficient of the defect involved in the degradation; we also extracted the related activation energy by an appropriate fitting of the degradation kinetics according to Fick's second law of diffusion. The obtained values suggest that degradation originates from the diffusion of hydrogen, whose severity depends also on the thickness of the p-GaN layer of these devices. The proposed analysis methods and the obtained results are useful for understanding the physics of multiple quantum wells (MQWs) solar cells during degradation. The results can be used to increase the performance and reliability in novel applications where these devices are proposed, such us additional layer in multi-junction (MJ) solar cells, and the application in harsh environments.