A research team from the Helmholtz-Zentrum-Berlin (HZB) have by using microstructured layers increased the efficiency of perovskite-silicon tandem solar cells to 25.5 %, which the researchers claim to be "the highest published value to date".

At the same time, computational simulations were utilised to investigate light conversion in various device designs with different nano-structured surfaces. This enabled optimisation of light management and detailed energy yield analyses.

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 is thus lost for purposes of energy conversion.

Using nano-structures, the reflection can be reduced significantly ensuring that 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 no longer suitable as a substrate for deposition of extremely thin perovskite layers. This is because perovskites are normally deposited to a polished wafer using solution processing to form an extremely thin film, much thinner than the pyramidal features. A rough-etched silicon surface layer therefore prevents formation of a uniform conformal layer.

A team headed by HZB physicist Steve Albrecht has investigated an alternative approach of 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 spin-coating 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.

The tandem cells were manufactured at HZB, the silicon cell being produced at HZB-Institute PVcomB and the perovskite cell at HySPRINT.

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 device designs with textures at various interfaces affect efficiency.

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.

"We succeeded in considerably improving the efficiency of a monolithic perovskite-silicon heterojunction tandem cell from 23.4 % to 25.5 %."

"Based on complex simulations and empirical data, we believe that an efficiency of 32.5 % can realistically be achieved – if we succeed to incorporate high quality perovskites with a band gap of 1.66 eV."

Marko Jošt, Postdoctoral fellow

"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."

Steve Albrecht, Team leader