A group of scientists from the Tampere University in Finland and the Leibniz-Institut im Forschungsverbund Berlin e.V research center in Germany have designed a low bandgap solar cell based on a gallium-arsenide-nitrogen-bismuth (GaAsNBi) absorber.
“In terms of bandgap energy and solar cell performance achieved, these results are far from the expectation, reflecting the difficulty in synthesizing GaAs-based high quality compound semiconductor heterostructures incorporating Bi, and the early development stage of the field,” the researchers stated.
They explained that their device is intended to be used as a bottom device in a multi-junction PV cell. Their attempt to build a GaAsNBi solar cell follows scientists at Osaka University in Japan unveiling a 2.01%-efficient device with a 1.15 eV bandgap in 2021.
Challenges facing the efficacy of the new cell type include the “unconventional” growth conditions required for Bi and N incorporation, which may lead to a “large” number of defects, according to the paper. Incorporating Bi in GaAs needs low growth temperatures and carefully controlled V/III flux ratios. To solve this, the academics grew the GaAs layers using molecular beam epitaxy (MBE), which is an evaporation technique implemented in an ultra-high vacuum for decomposing compounds with extreme regularity of layer thickness and composition.
They fabricated three 6 × 6 mm2 solar cells with an active area of 0.25 cm2. The devices were processed from an As-grown wafer. “The pieces were selected to cover an area with As/Ga flux ratio between 0.82 and 1.20,” the scientists said. “Due to technical limitations, the orientation of the substrate was not controlled before growth of the GaAsNBi layer.”
The performance of the device was assessed through external quantum efficiency (EQE) and light-current-voltage (LIV) measurements under standard solar illumination conditions. EQE is the ratio of the number of energy carriers harvested by the solar cell to the number of photons of a given energy incident on the cell itself.
With this cell configuration, the group was reportedly able to minimize its energy bandgap to 0.86 eV, which in turn resulted in a minimized lattice mismatch.
“Increasing As/Ga over the stoichiometric ratio leads to a rapid increase in the tensile mismatch and bandgap energy, likely due to reduction in Bi incorporation,” the academics said. “Despite a slightly higher open-circuit voltage offset at high As/Ga, this also leads to an (sic) clear increase in the external quantum efficiency and short-circuit current, suggesting significant differences in the carrier collection efficiencies.”
The paper claimed that these findings will encourage the viability of GaAsNBi PV devices for use in multi-junction solar cells. The paper, titled “Low bandgap GaAsNBi solar cells,” was published in Solar Energy Materials and Solar Cells.
In 2021 the same Tampere University research group unveiled a solar cell made of gallium-indium-nitride-arsenide-antimony (GaInNAsSb) incorporating gold-based back-surface reflectors. The cell had a “p-i-n” layout and an energy bandgap close to 0.8 eV, and was grown through MBE.
That same year a different group of Tampere University researchers announced they developed a III-V multi-junction solar cell, which allegedly has the potential to reach near-50% power conversion efficiency.
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