Scientists at the U.S. Department of Energy’s National Renewable Energy Laboratory (NREL) enhanced solar cell efficiency through precise material design. By combining computational and experimental methods, they cultivated a gallium arsenide (GaAs) heterojunction solar cell using dynamic hydride vapor phase epitaxy (D-HVPE), achieving a record-breaking 27% efficiency. This research aims to make III-V solar cells more cost-effective for Earth-based applications. III-V cells, named after their periodic table position, are commonly used in space technology.
D-HVPE offers a promising, cost-efficient approach to synthesizing these cells. The study also outlines a strategy to enhance solar cell performance by optimizing the doping and bandgap of the “emitter” device layer to reduce defects’ impact on efficiency. These findings have potential applications in materials beyond III-Vs, including silicon, cadmium telluride, and perovskites using heterojunctions.
The paper titled “Modeling and Design of III-V Heterojunction Solar Cells for Enhanced Performance” is a collaborative effort by NREL researchers John Simon, Myles Steiner, and Aaron Ptak.
In this study, the solar cell incorporated a gallium indium arsenide phosphide (GaInAsP) emitter layer in addition to the GaAs base layer, forming a heterojunction. The researchers conducted simulations to assess the impact of varying zinc doping density and the bandgap of the emitter layer, achieved through altering the concentrations of gallium, indium, arsenic, and phosphorus during layer growth, on cell efficiency. These simulations pinpointed the ideal parameters to maximize device efficiency. Subsequently, the researchers synthesized cells following these modeling guidelines and observed the predicted efficiency improvements.
The baseline rear heterojunction solar cell used GaInP for the emitter layer and had an efficiency of 26%. By reducing doping and transitioning to the lower bandgap GaInAsP for the emitter layer while keeping the rest of the device unchanged, the efficiency increased to 27%.
Although the advantages of heterojunctions are widely recognized, the practical demonstrations of III-V heterojunctions are currently limited to only a few combinations, as noted by the researchers.
“No matter how hard you try, with whatever method you choose to make them, solar cells will always contain some defects thanks to entropy. By using a heterojunction structure, with carefully designed emitter properties, you can minimize the adverse impact of these defects on efficiency, even though you haven’t done anything to reduce their concentration,” said Kevin Schulte, a scientist in NREL’s High-Efficiency Crystalline Photovoltaics group and lead author of the new paper published in the journal Cell Reports Physical Science. “Furthermore, the relative efficiency improvement scales with defect concentration. While the baseline D-HVPE cell already had a high efficiency, a device that had a higher defect concentration would receive a higher relative efficiency boost using the methods described in the paper.”
“We took this concept that was known but not quantified this way and mapped it out,” he further added. “We showed the modeling matches what we see experimentally, showing that it is a powerful tool for solar cell design.”