Nano-textured surfaces are an interesting approach for optimizing the optical characteristics for monolithic perovskite/silicon tandem solar cells. Scientists from Germany’s Helmholtz-Zentrum Berlin (HZB) have examined the development of different textures of silicon surfaces using various commercial additives and their performance in silicon heterojunction (SHJ) and SHJ–perovskite tandem solar cells.
The team performed optical and electrical characterization and found that nano-textured surfaces can compete with standard textured surfaces, yielding higher average efficiencies in single junctions. In addition, their compatibility with solution-processed perovskite top cells was demonstrated in the recent study, yielding a perovskite/silicon tandem solar cell efficiency of >28% on a bottom cell with nano-texture on both sides.
In silicon wafer and cell production, the surface of the wafer is usually given a texture of tiny pyramid shapes, which serve to reduce reflection and trap more light in the cell. This texture is usually achieved by dipping wafers into liquid chemicals, which remove some silicon and leave behind the textured surface. Other processes, usually involving firing a plasma substance at the silicon surface, are known to offer better control over the size and uniformity of the bumps that make up the texture. Most manufacturers, however, view these as too complex and costly, with only a limited benefit compared to the wet chemical process.
Placing a perovskite solar cell on top, however, adds new requirements for the silicon surface. Many of the achievements made so far with tandem cells have been done on a silicon cell with no texturing, which makes it easier to integrate the perovskite but could be seen as a lost opportunity for the silicon cell efficiency. The group at HZB investigated some of the more complex texturing processes and found that the increased control and smaller surface textures they afford could solve the perovskite integration challenge, or even be of interest again for single-junction silicon cells.
“For tandem cells, a uniform and small-sized Si texture is crucial in order to be able to wet process the perovskite top cell,” the group explained. “We showed that sub-micrometer sized random pyramids by an adjusted wet etching process are an interesting approach not only for such tandem solar cells but also for SHJ single junction solar cells.”
In their paper, the researchers described experiments with different etching solutions. They found that varying the composition of the solution enabled them to control the size of the pyramids on the surface, maintaining the desirable anti-reflective features of the etched surface, while making it easier to place the perovskite layer on top.
“All JV parameters were similar or even slightly better as compared to those of cells with our standard micro-texture,” said the team.
Using these new nano-texturing processes, the group demonstrated a 28.9% efficient tandem cell. The researchers said they are confident that such a process could be adopted at scale.
“We expect that the presented processes will be easy to be implemented in industrial production and, hence, will contribute to the development of a production process for highly efficient tandem solar cells,” they said.