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Solar cells made of a perovskite absorber are a hot candidate for the scientific community to boost the efficiency of traditional crystalline silicon cells further through a tandem structure.

At the European Photovoltaic Solar Energy Conference (EU PVSEC) in Hamburg this week, the PV-Lab at the Institute of Microengineering (IMT) in Neuchâtel, Switzerland presented a tiny silicon-perovskite tandem cell with an efficiency of 22.8%, higher than the silicon cell’s stand-alone efficiency of 21.7%. However, the long-term stability of perovskite cells remains an issue.

Perovskite solar cells have raised the interest of many scientists since the efficiency of lab cells has skyrocketed to slightly over 20% within just a few years of research after the discovery of the material for photovoltaics. Perovskite is an organic-inorganic compound; frequently, methylammonium lead iodide is used for solar cells.

In IMT’s 22.8% tandem structure, the 0.25cm² small perovskite top cell is only optically coupled with the silicon bottom cell; both cells have separate electrical contacts in a four-terminal configuration.

The goal, however, is a monolithic tandem, where the layers of the perovskite cell are directly deposited on the surface of the silicon cell and there is just one electrical contact each on the front and the backside of the tandem.

With such a monolithic structure, IMT has only reached a cell efficiency of 16% so far. The record is held by the British start-up Oxford PV with 21.3%.

The efficiency for mini-modules is even lower: IMT achieved 11.5% on an area of 12cm², the Belgian Interuniversity Microelectronics Centre (imec) 12.5% on 16cm².

However, research is still in the beginning. At the EU PVSEC, Christophe Ballif, director of the PV-Lab at IMT, called a tandem efficiency of 27% a realistic target.

“The big challenge,” said Ballif, “is long-term stability.” The efficiency results measured by IMT “stabilized for a few seconds, not for thousands of hours.”

Ricky Dunbar from the PV Performance Laboratory of the Commonwealth Scientific and Industrial Research Organisation (CSIRO) in Newcastle, Australia confirmed the problems with perovskite cells: “It is difficult to measure the efficiency accurately. It’s degrading as you measure.”

Ballif admitted: “At this point, I cannot guarantee that the stability will ever reach 25 years.” If the issue of degradation cannot be solved, perovskite cells will remain a playground for scientists, but not become the hoped-for turbo charger for industrial silicon solar cells.

Dr. Jef Poortmans, Scientific Director photovoltaics at imec, told PV Tech that it was too early in the study analysis of perovskite cell stability issues to comment on the outcome. 

According to Dr. Poortmans, attention is being placed on the active layer and issues such as pin holes to establish perovskite cells manufacturability. 

Additional reporting by Mark Osborne