Researchers from the U.S. Department of Energy’s National Renewable Energy Laboratory (NREL; Golden, CO) and the University of New South Wales (Sydney, Australia) achieved a new world-record efficiency for two-junction solar cells, creating a cell with two light-absorbing layers that converts 32.9% of sunlight into electricity. Key to the cell’s design is the 150-layer 80-quantum-well structure in the cell’s bottom absorber, allowing it to capture energy from longer infrared (IR) wavelengths than what can usually be captured. While the new record only improves modestly on the previous 32.8% efficiency record, it is the first record-efficiency multijunction solar cell to use a strain-balanced structure—a design that holds promise for further improvements.
The solar cell has an inverted gallium arsenide (GaAs) and gallium indium phosphide (GaInP)/GaAs structure and strain-balanced GaInAs/GaAsP quantum wells. The top junction is GaInP and the bottom junction is GaAs with quantum wells. The inclusion of so many quantum wells in the bottom junction is what lowers that junction’s effective bandgap and increases the wavelength of light it can absorb beyond the absorption range of a traditional dual-junction GaInP/GaAs cell, making the new cell more efficient at converting light to electricity. As part of the development process, the NREL team produced a single-junction cell that demonstrated a very high external radiative efficiency (greater than 40%), which is the efficiency with which the cell converts electricity to light when run in reverse. While the team was not trying to build an LED device, the high-quality quantum wells demonstrated some potential in this area, too.
Previous work attempted to use quantum wells to adjust the bandgap of solar-cell junctions, but did not produce any record-efficiency cells, in part because it is difficult to grow so many layers of defect-free quantum-well material. For their world-record cell, the team alternated layers of GaInAs in compression and GaAsP under tension. A laser array was used to measure the curvature of the wafer throughout the growth process, allowing the researchers to detect and adjust for strain in the crystal lattice. Reference: M. A. Steiner et al., Adv. Energy Mater. (2020); https://doi.org/10.1002/aenm.202002874.
