November 13 (Solar) – Scientists at Helmholtz Zentrum Berlin (HZB) announced on Monday 25.5% efficiency for a monolithic perovskite-silicon heterojunction tandem cell and said reaching 32.5% is a realistic target.
Dealing with reflection, which limits energy conversion within the cell, is one of the challenges for tandem solar cells made of silicon and metal halide perovskite compounds. With the help of nanostructures, such as pyramid-shaped microfeatures etched into silicon, the reflection can be cut significantly. However, the microscopic roughness in the silicon surface resulting from this method could prevent the deposition of extremely thin perovskite layers.
The HZB team, led by physicist Steve Albrecht, explored an alternative approach to light management with textures in tandem solar photovoltaic (PV) cells. The silicon layer was etched on the back-side of their cell and the perovskite layer could be applied by spincoating onto the smooth front-side of the silicon. Then a polymer light management (LM) foil was applied to the front-side of the device. Thus the team could still get a high-quality perovskite film on the flat surface, while benefiting from the front-side texture.
Marko Jošt, first author of the study, and colleagues have developed a sophisticated numerical model for complex 3D features and their interaction with light. With that they can 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% can realistically be achieved – if we succeed to incorporate high quality perovskites with a band gap of 1.66 eV,” Jošt pointed out.
The simulations done by the team show that the LM foil on the front-side does a great job under diffuse light irradiation, which means that tandem PV cells with the foil could also be suitable for building-integrated photovoltaic (BIPV) applications.