German teams combine PV workhorse with perovskite
HZB and ISFH Hameln tandem device could have a Power Conversion Efficiency of 29.5 percent.
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, two teams from HZB (Helmholtz-Zentrum Berlin) and ISFH (Institute for Solar Energy Research Hameln) have combined a perovskite top cell with a so-called PERC/POLO silicon cell to form a tandem device.
The cooperation took place within the framework of the joint project P3T, which is funded by the German Federal Ministry of Economics and coordinated by HZB.
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.
HZB applied a tin-doped indium oxide recombination layer 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 achieve an efficiency of 21.3 percent 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 optimisation," explains Lars Korte, the corresponding author of the study.
The estimated Power Conversion Efficiency (PCE) of these perovskite/silicon tandem solar cells with PERC-like sub-cell technology is 29.5 percent. 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 percent," says Mariotti. This is then already significantly higher than the efficiency of PERC single cells.
'Monolithic Perovskite/Silicon Tandem Solar Cells Fabricated Using Industrial p-Type Polycrystalline Silicon on Oxide/Passivated Emitter and Rear Cell Silicon Bottom Cell Technology' by Silvia Mariottiet al; RRL Solar, 08 February 2022