Fraunhofer Institute for Solar Energy Systems ISE researchers have examined the question of which silicon bottom cell will be most suitable for use in tandem cells. The team evaluated multiple silicon cell concepts based on both cost and efficiency in serving as the bottom layer in a perovskite-silicon tandem cell.

Investigated perovskite silicon tandem concepts imageInvestigated perovskite silicon tandem concepts featuring four different silicon bottom cells (P E RC, TOPerc, TOPCon2, and SHJ) and two different interconnection concepts (ReCO and SiT). Image from article

The study, based on both simulation and experimental work, details advantages to various approaches with the silicon cell and concludes that in almost every case, perovskite-silicon tandem cells have the potential to bring solar costs down below what could be achieved with silicon alone.

As the first commercial iteration of perovskite solar technology has good chances of being in the form of a tandem cell with a silicon technology sitting at the bottom, the Fraunhofer Institute for Solar Energy Systems (ISE) decided to look into these silicon cells, evaluating the performance of various cell types for their compatibility with a perovskite top cell, aiming to identify ways to achieve higher energy yields from solar, at lower costs for manufacturing and deployment.

The research examines four silicon cell types, taking in PERC, TOPCon and heterojunction designs, and two different strategies for coupling the two cells. It also evaluates both planar and textured silicon surfaces, as well as both mono- and bifacial cell designs.

Each approach was evaluated using an experimentally validated ray-tracing simulation model, giving a calculated efficiency potential. Then, manufacturing costs were calculated using a model based on existing silicon production, and LCOE calculations based on a fixed-tilt installation in southern Europe.



In every case, the tandem cell was shown to improve LCOE, and differences between the silicon cell types investigated all fell within the model’s uncertainty parameters. “…all tandem concepts have the potential to yield a significantly lower LCOE compared with the conventional PERC concept for both residential and utility installation promising an LCOE reduction of about 11%,” the researchers state. “…to our best knowledge, the ‘race' for the best silicon bottom cell in terms of industrial application is still open to all the investigated bottom cell technologies.”

It should also be noted that the cost models assume a 30-year lifetime for the modules, which is not a given for perovskite technology. “…it is important to underline that the presented LCOE potential of perovskite‐silicon tandem modules depends on achieving a comparable lifetime, degradation rates, and financing conditions (i.e., bankability) for the tandem configuration to the reference PERC modules,” the paper explains.

To account for this, the group calculated an LCOE ‘breakeven’ point, noting that assuming a 30-year project lifetime, a tandem module would have to perform for at least 21 years to have a lower LCOE than current generation PERC technology. And, with very few doubts surrounding the efficiency, ensuring this long-term performance and quality should be the direction of new research. “attention should be directed to technological feasibility, upscaling and the production environment, as well as high module lifetimes and field performance,” the Fraunhofer ISE researchers concluded. “It will be most essential for the Pero‐Si tandem technology to overcome these fundamental prerequisites in order to actually achieve lower LCOE with respect to conventional PERC stand‐alone devices and other competing technologies like all‐perovskite tandems.”

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