HZB Sets New Record For Perovskite Tandem Efficiency
Tandem solar cells made of silicon and metal halide perovskite compounds can convert a particularly large portion of the solar spectrum into electrical energy. However, part of the light is reflected and lost for purposes of energy conversion.
Now a team headed by physicist Steve Albrecht from the Helmholtz-Zentrum Berlin (HZB) has investigated an alternative approach of light management using textures and in this way has improved efficiency from 23.4 percent to 25.5 percent.
There are various approaches can help tandem cells capture more light. For example, pyramid-shaped microfeatures can be etched into silicon. However, these features cause microscopic roughness in the silicon surface, making it no longer suitable as a substrate for deposition of extremely thin perovskite layers.
The HZB team instead etched the silicon layer on the back-side. The perovskite layer could then 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 succeeded in considerably improving the efficiency of a monolithic perovskite-silicon heterojunction tandem cell from 23.4 percent to 25.5 percent", says Marko Jost, first author of the study and postdoctoral fellow in Albrecht's team.
Numerical model shows possibility for up to 32.5 percent
In addition, Jost 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 device designs with textures at various interfaces affect efficiency. "Based on these complex simulations and empirical data, we believe that an efficiency of 32.5 percent can realistically be achieved - if we succeed to incorporate high quality perovskites with a band gap of 1.66 eV", says Jost.
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 the course of a year - for the different cell designs and for three different locations." In addition, the simulations show that the LM foil on the front-side of the solar cell device is particularly advantageous under diffuse light irradiation, i.e. not only under perpendicularly incident light. Tandem solar cells with the new LM foil could therefore also be suitable for incorporation in building-integrated photovoltaics (BIPV), opening up huge new areas for energy generation from large sky scraper facades.
'Textured interfaces in monolithic perovskite/silicon tandem solar cells: Advanced light management for improved efficiency and energy yield'¸ by Marko Jost et al: Energy & Environmental Sciences (2018) 25 Oct 2018