Powering up PERC-perovskite tandem cells

From pv magazine Global

Tandem solar cells, where a perovskite layer is integrated on top of a silicon solar cell, have developed rapidly to demonstrate efficiencies close to 30%, and the first commercial devices will likely be seen within the next couple of years.

Silicon is already well established, accounting for around 95% of the solar cells on the market. So the bulk of the work on tandem cells has focused on the new, yet to be commercialised, perovskite element. On the silicon side, researchers have most often chosen heterojunction (HJT) cells as these offer the simplest route to high efficiency. Most of the silicon cells currently being produced, however, use passivated emitter rear cell (PERC) technology, so integrating perovskite with these would have much higher industrial reach, and has already been shown to work well.

“PERC technology as the current industrial standard is a very promising option to evolve into a low-cost tandem device,” said a group of scientists led by Germany’s Fraunhofer ISE. “However, the pathway towards an optimised tandem device based on PERC technology (Pero-PERC) needs to be further investigated.” The group’s latest work, a paper titled How to make PERC suitable for perovskite–silicon tandem solar cells: A simulation study, published in Progress in Photovoltaics, simulates the behaviour of five different PERC-like cell concepts in tandem devices, noting advantages and constraints for each, and guidelines for optimisation.

The group evaluated five variations on the PERC cell structure, focusing on potential upgrades to the front side of the silicon cell. They note that their integration into a tandem device will change both the light-absorbing and charge carrying characteristics of the silicon cell, allowing for optimisations that would improve the overall efficiency. “One aspect is the option to significantly reduce recombination and transport losses, because of the increased design freedom concerning blue response, the higher series resistance tolerance, and the fact that for monolithic 2-terminal tandems, no lateral transport at the front side is needed,” they explain. “This is an important factor to close the efficiency gap to SHJ bottom cells that are currently applied in record tandem devices but still lack industrial maturity.”

Several of the cell concepts investigated made use of tunnel-oxide passivation, making them more similar to the TOPCon cells currently being introduced in large-scale production. The group noted that such cells could reach efficiencies up to 30.5%, but would require development of new transparent electrode materials, while working with the full TOPCon cell structure could offer a solution to this.