Experimental analysis of degradation of multi-quantum well GaN based solar cells under current stress

Abstract

GaN-based multi-quantum wells solar cells could be breakthrough devices for extreme applications like space environment and harsh photovoltaics. We performed a forward-bias stress on samples with 30 quantum wells with 15% indium content to better understand degradation mechanisms. DUTs were characterized by means of dark and illuminated IV, CV and Steady-State Photocapacitance measurements. First, we performed a step-stress experiment, by increasing current in 30 minutes steps up to 500 mA, when the device failed. We observed a variation in series resistance, a decrease in shunt resistance and a strong decrease in EQE, conversion efficiency and open-circuit voltage, especially at low excitation intensities. CV measurements showed an increase and then a decrease in free charge density in the device, that was correlated to the variation in trap-states density evaluated by SSPC. Based on the results of this step-stress experiment, we carried out a 100 hours constant-current stress at 40 mA. This stress showed a moderate decrease in series resistance and a decrease in EQE and conversion efficiency. An increase in charge density was observed and correlated with the decrease in conversion efficiency. The degradation of the device was related to the generation of defects, that may create a nearmidgap states, detected by SSPC measurement, and a shallow donor state that generates a change in free carrier density. These defects possibly migrate through the devices, as they are detected at different times by SSPC and CV measurements.