Tandem solar cells made of silicon and metal halide perovskite compounds can transform a particularly large part of the solar spectrum into electrical energy. However, some of the light is reflected and is therefore lost to convert energy. Thanks to nanostructures, the reflection can be significantly reduced, thanks to which the solar cell captures more light. For example, pyramid-shaped microtubes can be etched in silicon. However, these features cause a microscopic roughness on the surface of the silicon, which makes it no longer suitable as a substrate for the deposition of extremely thin layers of perovskite. This is because perovskites are usually deposited on a polished wafer using solution processing, creating an extremely thin film, much thinner than the pyramid features. The roughly etched silicone surface layer thus prevents the formation of a homogeneous conformal layer.
Productivity improved from 23.4% to 25.5%
The team led by HZB physicist Steve Albrecht investigated an alternative approach to light management with textures in tandem solar cells. The team produced an efficient tandem perovskite / silicon device whose silicone layer was etched on the back side. The perovskite layer could be applied by spin coating to the smooth side of the silicon. Next, the team used a polymer light management film (LM) on the front of the device. This enabled the processing of a high-quality perovskite film on a flat surface, while using textures at the front. "In this way, we have been able to significantly improve the performance of the monolithic tandem perovskite-silicon cell from 23.4% to 25.5%," says Marko Jošt, the first author of the study and a postdoctoral doctor in the Albrecht team.
The numerical model shows the possibility of up to 32.5%
In addition, Jošt and his colleagues have developed a sophisticated numerical model for complex 3D functions and their interaction with light. This allowed the team to calculate how different designs of devices with textures on different interfaces affect performance. "Based on these complex simulations and empirical data, we believe that a 32.5% yield can be realistic – if we can introduce high quality perovskites with a 1.66 eV band," says Jošt.
Suitable for embedding integrated PV
Team leader Steve Albrecht adds: "Based on actual weather data, we were able to calculate energy efficiency over a year – for different cell designs and for three different locations." Furthermore, the simulations show that the LM film on the front side of the solar cell device is particularly advantageous for scattered irradiation with light, i.e. not only under the perpendicular incident light. Tandem solar cells with a new LM foil can therefore also be suitable for building in the building's integrated photovoltaics (BIPV), opening up huge new areas for energy production from large shaft facades.
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Materials provided by Helmholtz-Zentrum Berlin for Materialien und Energie. Note: The content can be edited due to style and length.