Degradation of GaN-Based Multiple Quantum Wells Solar Cells Under Forward Bias: Investigation Based on Optical Measurements and Steady-State Photocapacitance

Abstract

Gallium nitride (GaN) multiple quantum well (MQW) solar cells proved to have very good performance in high-temperature conditions and under intense excitation. However, the long-term reliability under harsh conditions has not been investigated in the literature. The aim of this article is to fill this gap, by investigating the degradation mechanisms of GaN solar cells, submitted to forward current stress. The cells were characterized by means of dark and illuminated current–voltage ( <italic xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">I</i> – <italic xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">V</i> ) measurements, capacitance–voltage ( <italic xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">C</i> – <italic xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">V</i> ) measurements, and steady-state photocapacitance (SSPC). The current step-stress experiment showed an initial decrease in the main parameters of the devices (open-circuit voltage, external quantum efficiency (EQE), and optical-to-electrical power conversion efficiency). <italic xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">C</i> – <italic xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">V</i> and SSPC showed a correlation between the charge inside the active region of the device and the concentration of trap states. Also, a relation was found between the decrease in power conversion efficiency and the amount of charge in the active region of the devices. Degradation was ascribed to a redistribution of the charge in the active region, related to an increase in the density of midgap states ( <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$\textit{E}_{\textit{C}} -\text{1.6}$</tex-math> </inline-formula> eV), resulting in the lowering of the efficiency of the devices.