Tandem solar cells made of silicon and metal halide perovskite compounds can transform a particularly large part of the solar spectrum into electrical energy. Part of the light is reflected and lost for energy conversion. With the aid of nanostructures, the reflection can be reduced considerably, so that the solar cell captures more light. For example, pyramid-shaped micro-properties can be etched in silicon. However, these properties cause microscopic roughness in the silicon surface, which makes it no longer suitable as a substrate for depositing extremely thin perovskite layers. This is due to the fact that perovskites are usually deposited on a polished disc using solvent treatment to form an extremely thin film, much thinner than the pyramid properties. A coarsely etched silicon layer therefore prevents formation of a uniformly conformal layer.
Efficiency improved from 23.4% to 25.5%
A team with HZB physicist Steve Albrecht has investigated an alternative way of managing light with textures in tandem solar cells. The team produced an efficient perovskite / silicon dental device whose silicon layer was etched on the backside. The perovskit layer can be applied by spin coating on the smooth front of the silicone. The team then applied a polymeric light handling film (LM) to the front of the device. This made it possible to process a high quality perovskit film on a flat surface while still benefiting from the front's consistency. "In this way, we managed to significantly improve the effectiveness of a monolithic perovskit-silicon heterojunktionstandem cell from 23.4% to 25.5%," said Marko Jošt, first author of the study and postdoctor of Albrecht's law.
Numerical model shows the possibility of 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 how different devices pattern with textures at different interfaces affect efficiency. "Because of these complex simulations and empirical data, we believe that 32.5% efficiency can be achieved realistically – if we succeed in incorporating high quality perovskites with a bandwidth of 1.66 eV," says Jošt.
Suitable for building integrated PV
And team leader Steve Albrecht adds: "Based on real weather data, we could calculate energy usage over a year – for the different cell designs and for three different locations." In addition, the simulations show that the LM film on the front of the solar cell unit is particularly advantageous in diffuse light irradiation, i.e. not just under perpendicular incident light. Therefore, tandem solar cells with the new LM film may also be suitable for integration into building-integrated photovoltaics (BIPV), which opens large new areas of energy production from large skyscrapers.
Materials provided by Helmholtz-Zentrum Berlin für Materialien und Energie. Note Content can be edited for style and length.