Research on perovskite solar cells has made progress, but there is still a long way to go for a highly efficient tandem with a conventional crystalline silicon cell.
At the European Photovoltaic Solar Energy Conference (EU PVSEC) in Munich last week, the PV-Lab at the Institute of Microengineering (IMT) in Neuchâtel, Switzerland presented a 1.4 cm² small, monolithically stacked silicon-perovskite tandem cell with an efficiency of 20.5%, a significant increase over the 16% value for a monolithic tandem that IMT showed at the previous EU PVSEC in Hamburg last September.
Within just a few years, the efficiency of Perovskite solar cells has skyrocketed; the record for a lab cell is currently held by a group at the Korean Research Institute of Chemical Technology with 22.1%.
Perovskite is an organic-inorganic compound; usually, methylammonium lead iodide is used for solar cells. Its bandgap of 1.56 eV is close to the ideal value of 1.7 eV for the top cell in a tandem with a wafer-based silicon cell in order to capture that part of the light spectrum that the silicon cell cannot absorb.
If the top and bottom cells are not monolithically stacked, but only optically coupled and efficiency is measured by four terminals, the potential of such a tandem becomes clear: Christophe Ballif, director of the PV-Lab at IMT and Vice President Photovoltaics at the Centre Suisse d'Electronique et de Microtechnique (CSEM), presented a tandem efficiency of 25.2% for a 0.25 cm² small perovskite cell on top of a silicon heterojunction cell. With further improvements, Ballif believes that 27 to 28% can be achieved.
The challenge, however, lies in upscaling perovskite cells to larger areas. A mini-module with an aperture area of 168 cm², produced by the Dutch-Belgian-German thin-film PV research consortium Solliance, has reached an efficiency of 10.0% – still far away from the record efficiencies of lab cells.
Another issue remains the long-term stability of perovskites. A Swiss research group at the École Polytechnique Fédérale de Lausanne (EPFL) has achieved some improvement here, as Anders Hagfeldt, head of the EPFL Laboratory of Photomolecular Science, reported at the EU PVSEC. By adding small amounts of inorganic cesium, the researchers could stabilize the efficiency of a perovskite cell for 250 hours at room temperature.
At 75°C, however, gold from the contact diffused into the cell and the efficiency degraded. Although an additional interface layer of chromium was able to prevent this effect, Hagfeldt admitted: “The stability of silicon is a challenge for perovskites.”