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Tandem solar cells made of silicon and metal halide perovskite compounds can convert to a large part of the solar spectrum into electrical energy. However, part of the light is reflected and thus lost for the purposes of energy conversion. Using nanostructures, the reflection can be reduced to much more than the solar cell captures more light. For example, pyramid-shaped microfeatures can be etched into silicon. However, these features cause microscopic roughness in the silicon surface, making it possible for the substrate to deposition of extremely thin perovskite layers. This is because of an extremely thin film, much thinner than the pyramidal features. A rough-etched silicon surface layer thus prevents formation of a uniform conformal layer.
Efficiency improved from 23.4% to 25.5%
A team headed by HZB physicist Steve Albrecht has investigated an alternative approach to light management with textures in tandem solar cells. The team fabricated an efficient perovskite / silicon tandem device whose silicon layer was etched on the back-side. The perovskite layer could be applied by spincoating onto the smooth front-side of the silicon. The team afterwards applied a polymer light management (LM) foil to the front-side of the device. This enabled processing of a high-quality perovskite film on a flat surface, while still benefiting from the front-side texture. "In this way, we succeed in improving the efficiency of a monolithic perovskite-silicon heterojunction tandem cell from 23.4% to 25.5%," says Marko Jošt, first author of the study and postdoctoral fellow in Albrecht's team.
Numerical model shows possibilities for up to 32.5%
In addition, Jošt and colleagues have developed a sophisticated numerical model for complex 3D features and their interaction with light. This enabled the team to calculate different devices with textures at various interfaces affect efficiency. "Based on these complex simulations and empirical data, we believe that an efficiency of 32.5% can realistically be achieved – if we succeed in integrating high quality perceptions with a band gap of 1.66 eV," Joet says.
Suitable for building integrated PV
And team leader Steve Albrecht adds: "Based on real weather data, we were able to calculate the energy yield over a year – for the different cell designs and for three different locations. In addition, the simulations show that the plasma ionization is favorable under light irradiation, i.e. not only under perpendicularly incident light. Tandem solar cells with the new LM foil could also be suitable for incorporation in building-integrated photovoltaics (BIPV), opening up large areas for energy generation from wide sky scraper facades.
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Materials provided by Helmholtz-Zentrum Berlin für Materialien und Energie. Note: Content can be edited for style and length.
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