Researchers combine PERC with perovskite cells for a promising tandem device

Researchers from Germany's Helmholtz-Zentrum Berlin (HZB) and Institute for Solar Energy Research (ISFH) have reported that passivated emitter and rear cell (PERC) tech can be suitable as a basis for tandem cells with perovskite top cells.

PERC cells are usually used in mass production of silicon solar cells, and while the efficiency of the study's tandem cells is still below that of optimized PERC cells alone, the team estimates that it could be increased to up to 29.5% through targeted optimization.

The two teams from HZB and ISFH have succeeded in combining a perovskite top cell with a so-called PERC/POLO silicon cell to form a tandem device. This is an important achievement, since PERC silicon cells on p-type silicon are seen as the "workhorse" of photovoltaics, with a market share of about 50% of all solar cells produced worldwide. They are largely optimized, long-term stable and temperature stable. Therefore, it is particularly interesting for the commercialization of a perovskite-silicon tandem technology to develop a "perovskite tandem upgrade" for PERC cells.

The team at ISFH used an industry-compatible PERC process for the backside contact of the silicon bottom cells. On the front side of the wafer, another technology was used, the so-called POLO contact, which was adapted for the small-area proof of concept cells.

The following process steps took place at HZB: A tin-doped indium oxide recombination layer was applied as a contact between the two subcells. On top of this, a perovskite cell was processed with a layer sequence similar to that in the current world record tandem cell on n-type silicon heterojunction cells, made by HZB. The first perovskite PERC/POLO tandem cells produced in this way achieved an efficiency of 21.3% on an active cell area of about 1 cm². This efficiency is still below the efficiency of optimized PERC cells in this feasibility study. "However, initial experimental results and optical simulations indicate that we can significantly improve the performance through process and layer optimization," explains Dr. Lars Korte, the corresponding author of the study.

The experts estimate the Power Conversion Efficiency (PCE) of these perovskite/silicon tandem solar cells with PERC-like sub-cell technology at 29.5 %. The next steps for further efficiency increases are clear: Dr. Silvia Mariotti from the HZB team had identified the coverage of the silicon surface by the perovskite as potential for improvement: "For this purpose, one could adapt the surface of the silicon wafers and thus quickly increase the efficiency to about 25%," says Mariotti.