PERC cells are used in mass production of silicon solar cells, they are considered the workhorses of photo­voltaics, dominating the market. Now two teams from the Helmholtz Center Berlin HZB and the Institute for Solar Energy Research in Hamelin (ISFH) have shown that such standard silicon cells are also suitable as a basis for tandem cells with perovskite top cells. Currently, the efficiency of the tandem cell is still below that of optimised PERC cells alone, but could be increased to up to 29.5 % through targeted optimi­zation. The research was funded by the German Federal Ministry of Economics as part of a joint project.

Tandem cells made of silicon and perovs­kite are able to convert the broad energy spectrum of sunlight into electrical energy more effi­ciently than the respective single cells. Now, for the first time, the two teams have succeeded in combining a perovskite top cell with a PERC/POLO silicon cell to form a tandem device. This is an important achievement, since PERC silicon cells on p-type silicon are the workhorse of photo­voltaics, with a market share of about 50 % of all solar cells produced worldwide. They are largely optimised, 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 indus­trialisable techno­logy was used, the so-called POLO contact, which was adapted here 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. The first perovskite PERC/POLO tandem cells produced in this way achieve an efficiency of 21.3 % on an active cell area of about 1 cm². This efficiency is thus still below the effi­ciency of optimised 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 Lars Korte.

The experts estimate the Power Conversion Effi­ciency (POE) of these perovs­kite/silicon tandem solar cells with PERC-like sub-cell techno­logy at 29.5 %. The next steps for further effi­ciency increases are already clear: 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. This is then already signi­ficantly higher than the effi­ciency of PERC single cells. (Source: HZB)

Reference: S. Mariotti et al.: Monolithic Perovskite/Silicon Tandem Solar Cells fabricated using industrial p-type POLO/PERC Silicon Bottom Cell Technology, RRL Solar, online 8 February 2022; DOI: 10.1002/solr.202101066

Links: Perovskite Tandem Solar Cells, Helmholtz Zentrum Berlin HZB, Berlin, Germany • Perovskite-POLO-PERC tandem solar cells and modules, Institute for Solar Energy Research in Hamelin ISFH, Hamelin, Germany