Changes in the extraction and collection efficiency of GaN-based MQW solar cells under optical step-stress

Abstract

InGaN/GaN multiple quantum well (MQWs) solar cells are promising devices for application in harsh environments. However, understanding their degradation kinetics can be complicated by the high periodicity of the active region (AR). To overcome this issue, we carried out an experiment on structures with only two quantum wells, having different indium concentrations, that were submitted to an optical power step stress at 55 °C. Firstly, illuminated current-voltage (I-V) characterizations indicate that the QW near the p-side of the device strongly contributes to carrier collection. This is explained by the enhanced hole extraction and collection efficiency. Notably, a non-monotonic ‘hump effect’ emerges when the device design is suboptimal. Secondly, during optical power step stress, two key phenomena were observed: a) a reduction in short-circuit current (Isc), especially at high excitation intensities; b) an increase in current conduction below the main diode turn-on voltage, which is ascribed to the increased amount of traps in the active region of the devices. The degradation leads to a reduction in the extraction and collection efficiency of photogenerated carriers, as evidenced by the decrease in the open circuit voltage (Voc), the parameter most affected by degradation. Results give insight for the optimization of InGaN/GaN-based solar cells structure, which can be used to improve performance and reliability.