CIGS-perovskite tandem cell achieves record efficiency of 25.5 %

A Berlin-based team from HZB and Center for the Science of Materials Berlin (CSMB) at the Humboldt-Universität zu Berlin has set a new record for a tandem solar cell. Using a combination of a CIGS semiconductor layer and perovskite, along with several optimised intermediate layers, they were able to convert 25.5% of sunlight into electrical energy. The previous record for this combination of materials and this size of cell stood at 24.6%. The new record has been certified and is visible in the prestigious Solar Cell Efficiency Tables (the "Green Tables"), which serve as the definitive ledger for the global photovoltaic community.

To be included in this special ‘record table’, not only is a high efficiency required, but also an area of more than 1 cm². The well-known NLR-table (formerly NREL), by contrast, only lists the maximum efficiency per technology, even if the cell has an area of 0.001 cm².

'To push past our previous milestone within the framework of the European project SOLMATES, we employed CIGSe-bottom cells with different band gaps (i.e. 1.05 eV and 1.1 eV) and two different thicknesses of aluminium doped zink oxides with similar characteristics. We also tested different cell architectures, added to the continuous improvements we had achieved with our previous record,' says Dr. Guillermo Farias Basulto.

By aiming to reduce interfacial recombination losses and improve device stability, chemist Wuai Zhang screened multiple combinations of nickel oxide (NiOx) and self-assembled monolayers (SAMs) as hole transport material. Zhang also refined the electron-selective contact processing by regulating the initial thermal evaporation rate of Buckminsterfullerene (C60) onto an ultra-thin (1 nm) lithium fluoride (LiF) passivation layer.

The cell has an area of 1.081 cm², which, of course, is still small. However, within the SOLMATES project, Nicolas Otto from the University of applied sciences in Berlin (HTW) together with Thede Mehlhop from HZB were able to able to fabricate a mini- module with a similar stack of materials with about 19.7 % efficiency with an area of 2.25cm².

‘The physics embedded in our current cell architecture suggests that 25.5% is merely a steppingstone, given that our in-house testing of similar architectures have already reached efficiencies of 27.5%’, Farias-Basulto points out.