| Work Detail |
Danish scientists have attempted to build a selenium-silicon tandem solar cell for the first time and have discovered that the device is capable of immediately generating a notable open-circuit voltage. Despite its current low efficiency levels, the new cell concept promises to achieve efficiencies greater than 40%, according to its creators.
A group of scientists led by the Technical University of Denmark has developed a tandem solar cell based on an upper cell with a selenium (Se) absorber and a lower silicon (Si) device.
The novelty of the research consisted of integrating for the first time a broadband energy Se cell in a photovoltaic device with a tandem configuration. This simple tandem device is unique in that it is based on single-element absorbent materials.
“The Shockley-Queisser limit for a tandem solar cell of this type exceeds 40% efficiency,” Rasmus Nielsen, corresponding author of the research, told pv magazine . “Therefore, this tandem solar cell has the potential to increase the power conversion efficiency of silicon solar cells at a very low cost, given that selenium must be processed at temperatures below 200°C.”
According to its creators, the new cell is a potential alternative to perovskite-silicon tandem solar cells, since the semiconductor is inorganic, consists of a single element and absorbs light very well, in addition to having low toxicity and being abundant. on earth.
Using the SCAPS-1D solar cell capacitance software, developed by Ghent University, the researchers identified a critical transport barrier for this type of tandem cells at the interface between the electron transport layer (ETL) and the of recombination, which, according to them, restricts the flow of charge carriers and reduces the energy conversion efficiency of the cell.
They showed that this problem can be solved by replacing zinc magnesium oxide (ZnMgO) with a layer of titanium dioxide (TiO2) for the n-type contact in the upper selenium cell.
“The integrated short circuit current densities of the individual subcells show that the upper selenium cell limits the overall current of the tandem device,” they explain. “By applying a reverse bias to the upper cell through the third accessible terminal, the collection efficiency is significantly improved.”
For the bottom silicon cell, the scientists used a custom n-type TOPCon device with an indium tin oxide (ITO) recombination layer and a silver (Ag) metal contact on the back. The upper device was fabricated directly on the ITO layer, using an n-type contact based on zinc magnesium oxide (ZnMgO) or titanium dioxide (TiO2).
The scientists found that the ZnMgO-based tandem device achieved an open-circuit voltage of 1.68 V, but with low efficiency. The best overall performance was obtained with the TiO2 n-type contact, which achieved an efficiency of 2.7%, 10 times higher than that of ZnMgO. The still significant losses of the upper selenium cell were analyzed in depth and strategies were proposed to further improve the tandem solar cell.
“By replacing the selective electron transport material, we achieved a promising tenfold increase in efficiency, although compromising the ideal band alignment in the pn heterojunction,” they stated. “This highlights the importance of optimizing the interfaces of the heterostructure, where the inclusion of surface modification layers should be considered.”
They presented the new cell concept in the study “ Monolithic Selenium/Silicon Tandem Solar Cells ,” published in PRX Energy .
Looking ahead, the research group says it intends to reverse the structure of the device, so that the n-type and p-type contacts of the top and bottom cells are swapped. “We also suggest replacing the ITO layer with another recombination layer that is more compatible with ZnMgO thin films,” Nielsen explained. “Finally, many series resistance losses would have to be solved.” |
