“Selenium (Se) is a suitable wide-bandgap absorber with a reported direct bandgap of 1.83–2 electronvolts for its crystallin trigonal allotrope,” the researchers explained, adding that the semiconductor has low toxicity and the potential for a low-cost fabrication process.
For the proposed cell, the Danish group used trigonal selenium that has a higher bandgap of 1.92 electronvolts and is semitransparent on both sides.
The cell was developed in four variants: Two cells both with a thickness of 300 nm and with a monofacial and bifacial design, respectively; and two devices cells both with a thickness of 500 nm and with a monofacial and bifacial design, respectively. For the backside contact, the academics used indium tin oxide (ITO) instead of gold.
The 300 nm bifacial device, which delivered the best performance among the four cell types, showed a power conversion efficiency of 5,2% on the front side through an n-type contact and 2.7% on the rear side through a p-type contact. Its short-circuit current was 10.96 mA for the front side and 7.40 mA on the backside, and the respective open-circuit voltage values were 0.91 and 0.88 V. The 300 nm thickness is claimed to enable an optimal fill factor of 52.6% on the front side and 41.4% on the rear side.
“It was shown how the performance from back-side illumination is more strongly dependent on the architecture of the device and also in particular on the selenium thickness,” the scientists further explained. “This is an important step toward a future Se-incorporated tandem, where it will be advantageous to invert the typical single-junction device structure before/when incorporating it into the tandem device.”
The cell is presented in the study Semitransparent Selenium Solar Cells as a Top Cell for Tandem Photovoltaics, published in RRL Solar.
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