Scientists at the National Renewable Energy Laboratory (NREL) in the U.S used gallium arsenide (GaAs) with quantum wells and invented a multijunction solar cell that exhibited a record efficiency of 39.5% under 1-sun global illumination.

Through the new research, the NREL team explored concepts of quantum well solar cells that function using ultra-thin layers to alter the properties of solar cells.

While in a previous study, NREL scientists achieved a record efficiency of 39.2% in a solar cell with six junctions using III-V materials, the new experiment enhanced the bandgap combination of the three-junction inverted metamorphic design.

The solar cell invented by the team is based on NREL’s invention of the inverted metamorphic multijunction (IMM) architecture.

The IMM devices are used as experimental test structures to support NREL’s research, which focuses on increasing photovoltaic power conversion efficiencies. NREL developed it to be compatible with small sample sizes to afford rapid turn-around times, providing accurate and timely feedback for researchers.

Triple-junction solar cells

III-V solar cells possess the highest efficiencies of any materials system, making them the dominant technology for satellites and space vehicles like the Perseverance and the Curiosity rovers sent to Mars by NASA. The triple-junction tandems are tolerant to high-energy particles, which become the key criteria for radiation environment during the launch of space vehicles.

The triple-junction tandems are named III-V because of where they fall on the periodic table. They span a wide range of energy bandgaps that allow them to target different parts of the solar spectrum. While gallium indium phosphide (GaInP) is the topmost junction, gallium arsenide (GaAs) is the middle layer with quantum wells (QW). The bottom layer is made of lattice-mismatched gallium indium arsenide (GaInAs).

“A key element is that while GaAs is an excellent material and generally used in III-V multijunction cells, it does not have the correct bandgap for a three-junction cell, meaning that the balance of photocurrents between the three cells is not optimal. Here we have modified the bandgap while maintaining excellent material quality by using quantum wells, enabling this device and potentially other applications,” observed senior scientist and cell designer Ryan France, who co-authored the research.

The NREL scientists introduced a middle cell with a lower bandgap than GaAs and proved better for global and space spectra. The team targeted the ideal three-junction bandgap combination using two metamorphic junctions and observed that even as the concentrator efficiency was excellent. The voltage loss in 1.35 Ev gallium indium arsenide resulted in a greater efficiency loss under 1-sun.

 

In contrast, a higher efficiency could be attained after the team implemented the improved bandgap combination. The NREL scientists pursued quantum wells to engineer the bandgap of materials. The QWs are planar nanostructures that sandwich a lower bandgap well layer between two higher bandgap-confining barrier layers. The team introduced an absorption edge (ionization threshold to continuum states) that depends on the low bandgap of the well layer combined with the effects of strain and quantum confinement— all of which act to raise the effective bandgap in the solar cells.

The scientists developed an optically thick GaInAs/ GaAsP strain balanced quantum well superlattices exhibiting excellent absorption and voltage for the first time by maintaining excellent material quality in multiple QW repeat procedures for the solar cells in the experiment. The optimal bandgap combination of the device, combined with high voltage and absorption in the QW solar cell, resulted in a record efficiency of 39.5% ± 0.5% under the AM1.5 global spectrum and 34.2% ± 0.6% efficiency under the AM0 space spectrum.

Researchers globally have been conducting experiments to enhance the properties of solar cells. Earlier this year, researchers from Stanford University claimed to have developed a solar photovoltaic cell that harvests energy from the environment during day and night, utilizing radiative cooling while avoiding the need for batteries.