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New Tandem Solar Cells achieve improved efficiencies

Solar energy is one of the driving forces when it comes to achieving more sustainable energy production and enabling the energy transition. A reliable method to boost the efficiency of solar cells is to adapt the cells to different spectrums of light using tandem solar cells. Tandem solar cells consist of a series of connected cells which are grown as monolithic layers on a substrate. The individual cells are joined via junctions. They are capable of overcoming the common efficiency limits by combining different construction materials.

Now (2021), scientists at Fraunhofer ISE have managed to improve efficiencies in tandem solar cells by designing a cell consisting of a new compound III-V silicon semiconductor (GaInAsP) for the middle cell. This has improved the life of the charge carriers and receives a higher cell voltage.The triple-junction solar cell was able to transform 35.9% of the sunlight into power. Connecting the III-V semiconductor layers to the silicon sub-cell was another fact that contributed to increased efficiencies.

Previous research (2017-2020) into triple-junction solar cells consisting of AlGaInP/AlGaAs//Si having an area of 4cm² and an efficiency of 34.1% under the AM1.5g spectrum had already been demonstrated successfully at Fraunhofer ISE. The III-V semiconductor layers had been5μm thick and wafer bonding had been used to transfer them from the GaAs substrate to silicon. This tandem cell had shown that monolithic structures with silicon having only 2 contacts could have efficiency higher than the theoretical limit of single-junction solar cells. Also, the absorption had been improved through optical structures for light guidance on the rear side.

Scientific efforts have long been directed towards making tandem solar cells more efficient. In 2014, scientists created a triple-junction hybrid tandem solar cell combining a double-junction amorphous silicon cell and an organic photovoltaic rear cell. In order to achieve the best possible result, computer simulations were conducted, including optical absorption and internal quantum efficiency. The simulations showed that the high-energy photons were used more efficiently, which lead to a significantly increased voltage gain. Also, an enhanced interface using tin-doped In2O3 deposition as interlayer and its UV-ozone treatment enhanced the performance the triple-junction solar cells. As a result, the power conversion efficiency reached 7.81% with an open-circuit voltage of 2.35 V.

In 2020¸ a two-step solution process for high-quality 1.73 eV wide-, 1.57 eV mid-, and 1.23 eV narrow-bandgap perovskite films was designed. The concept included the development of robust and low-resistivity interconnecting layers, which could reach power conversion efficiencies of over 19% for monolithic all-perovskite tandem solar cells having only little loss of potential energy and fill factor. When 1.73 eV, 1.57 eV, and 1.23 eV perovskite sub-cells were combined, a power conversion efficiency of 16.8% for monolithic all-perovskite triple-junction solar cells was achieved.

The advantages of the GaInAsPcell over standard cells are numerous: the use of a new compound semiconductor for the middle cellis is an important step towards gaining improved efficiencies. This material is responsible for an increased lifetime of the charge carriers as well as achieving a higher cell voltage. In addition, these higher efficiencies enable significant reduction in cell and module components, including semiconductor material, metals, glass, frames, or connectors, which contributes to higher sustainability and lower resource consumption.

Despite these promising results, much research still has to be carried out before this type of solar cell can be launched onto the market. The research at Fraunhofer, however, forms an important contribution to enhancing photovoltaic technologies, which is crucial for the green energy transition.