Nonpolar III-Nitride Solar Cell Device for High-Temperature Applications
High-performance solar cells that operate at temperatures above 300°C are desirable for applications such as space missions near the sun, terrestrial photovoltaic thermal (PVT) hybrid solar collector systems, and concentrating solar power (CSP) systems. Current high-temperature electronic technologies often employ active cooling processes which consume power and reduce total efficiency. This has led research into new photovoltaic (PV) materials that enable efficient operation at high temperatures. Conventional III-V semiconductors (including GaAs, GaP/AlGaP, AlGaInP, and SiC) have been investigated but exhibit sharp decreases in intrinsic PV conversion efficiency with increasing temperatures.
Wide-bandgap III-nitride InGaN materials have emerged as a promising candidate for high-temperature solar cells. Single-junction InGaN quantum well (QW) solar cells have demonstrated high external quantum efficiencies (EQEs) of ~50% and sustained operation at 300°C. Despite these advantages, InGaN solar cells suffer from polarization-related effects from the adoption of c-plane sapphire substrates. These effects limit efficiency at room temperature (RT) and are increasingly problematic at high temperatures.
Researchers at Arizona State University have developed a novel III-nitride solar cell structure featuring a nonpolar m-plane GaN substrate. The nonpolar InGaN solar cells have demonstrated a working temperature range from RT to 500°C, with positive temperature coefficients up to 350°C. Peak EQE values of the devices increase continuously from ~32% at RT to ~81% at 500°C, which is distinct from all other solar cells reported thus far. This can be attributed to the over 70% increase in the carrier lifetime as obtained from time-resolved photoluminescence (TRPL) measurements. The nonpolar structure can comprise (from bottom to top): (1) an n-type III-nitride layer on or above a nonpolar GaN substrate, (2) a III-nitride active region, and (3) a p-type III-nitride layer. The device can be further coated with a transparent conductive layer such as Ni/Au or indium-tin-oxide (ITO) to enhance carrier collection and reduce light reflection.
• High-temperature photovoltaics
• Concentrating solar power (CSP) systems
• Terrestrial photovoltaic thermal (PVT) hybrid solar collectors
Benefits and Advantages
• Nonpolar m-plane substrate eliminates detrimental polarization effects in energy band profile of device
• Use of GaN substrate improves device structural integrity, allowing thicker active regions to be grown
• GaN substrate provides a unique and pronounced self-cooling effect for high-temperature operation