In the Sharc25 European research project, which was successfully completed in 2018 after 3.5 years, ZSW coordinated the preparation, simulation and characterisation activities of eleven European partners from eight countries. The aim was to investigate highly efficient CIGS thin-film solar cells at world-record level in detail, identify loss mechanisms as well as develop and implement new concepts to increase efficiency. The efficiency of CIGS solar cells was successfully increased by about one percentage point (see figure below), mainly by using improved absorber material as well as optimised surfaces and interfaces.
A major role was played by the efficiency-enhancing post-deposition treatment (PDT) with the alkali element rubidium (Rb). This PDT process is typically performed after the CIGS coating and before the buffer deposition. Supplementary theoretical calculations on diffusion behaviour and phase formation in the presence of different alkali metals helped to better understand the effects of Rb and to utilise the interaction with other alkali elements such as sodium and potassium which are already present in the film for optimised PDT processes. The Rb introduced by PDT accumulates at the boundary between the CIGS and the chemical-bath-deposited buffer, and it is very likely that an AlkInSe2 phase such as RbInSe2forms there. A record efficiency of 22.6% (see figure below) was achieved with a CIGS solar cell containing a CIGS absorber with RbF-PDT and a very thin chemical-bath-deposited CdS buffer in combination with a sputtered (Zn,Mg)O-layer.
Project homepage: http://sharc25.eu/