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On a basic level, solar or “photovoltaic” cells work by using solar energy in the form of photons to knock electrons loose from atoms such that they can form an electric current. Conventional solar cell technologies are based around the semiconductor silicon, however decades of research have pushed silicon cells close to their theoretical maximum efficiency. This means that, to get more bang for your buck when it comes to making electrical power from each watt of incoming solar radiation, the use of new absorber materials is needed.
In their new study, electrical engineer Professor Thomas Riedl of the University of Wuppertal in Germany and his colleagues combined two such alternative materials.
The first was an organic semiconductor, a carbon-based material that can conduct electricity under certain conditions, which was paired with a lead–halogen-based perovskite which also had excellent semiconducting properties.
A perovskite is any material with a crystalline structure following the formula ABX₃, such as the mineral of the same name, which is made of calcium titanate, CaTiO₃.
Both of the materials used, the team explained, are less material- and energy-intensive to produce than conventional silicon solar cells, meaning that the new cell design might be produced more sustainably.
Different semiconductor materials absorb different parts of the electromagnetic spectrum.
Because of this, combining two different materials into a “tandem” solar cell as the researchers did allows the completed photovoltaic device to absorb a greater percentage of the incoming radiation and thus work more efficiently.
In Prof. Riedl and his colleagues’ design, for example, the organic semiconductors were used to absorb light in the ultraviolet and visible parts of the spectrum, while the lead–halogen perovskite works better in the near-infrared.
While similar combinations of materials have been tested in the past, the team were able to increase their efficiency from 20 percent, measured in terms of the fraction of photons converted into electrons, up to a record-breaking 24 percent.
For comparsion, the world record for solar cell efficiency of any design is 47.1 percent, which was achieved in 2019 in the US using so-called multi-junction concentrator solar cells.
In their paper, the team wrote: “Our devices are shown to be the most efficient monolithic perovskite–organic tandem cells achieved so far”
Paper author and organic semiconductor expert Dr Selina Olthof of the University of Cologne added: “To achieve such high efficiency, the losses at the interfaces between the materials within the solar cells had to be minimised.
“To solve this problem, the group in Wuppertal developed a so-called interconnect that couples the organic sub-cell and the perovskite sub-cell electronically and optically.”
This interconnect came in the form of a sheet of indium oxide just 1.5 nanometres in thickness — 60,000 times thinner than a single strand of human hair.
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The team said that they were able to optimise the performance of their new solar cell design by studying the electrical and energetic properties of the interconnect and semiconductor interfaces.
This allowed them to identify and combat energy losses that might otherwise lower the cell’s conversion efficiency.
The researchers added that, according to their simulations, these approaches should allow them in the future to manufacture tandem solar cells that have energy conversion efficiencies of more than 30 percent.
The full findings of the study were published in the journal Nature.
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