The aim of the “NovaZOlar” Swiss project within the European SOLAR-ERA.NET program was to develop an innovative, low-cost process of solution deposition of the ZnO-based transparent electrical contacts for CIGS solar cells and photovoltaic mini-modules. Together with solution-grown ZnS-based buffers that were investigated by German and French national sub-projects, the novelty was to employ a single deposition technique – chemical bath deposition – for producing the Cd-free window and buffer layers.
The solution process with post-deposition UV annealing yielded transparent and conductive ZnO layers with a sheet resistance of 20 ? for a thickness of 2.7 µm. An absolute efficiency of 18.3 % was achieved with the solution-grown ZnO electrodes, on par with the sputtered reference. The conductive doped ZnO layers were directly grown on Zn(O,S) buffers achieving an efficiency of 16 %, proving the feasibility of manufacturing the buffer-TCO bilayer using a single solution technique. Finally, solution electrodes were successfully implemented into flexible CIGS cells on polymer substrates with efficiency up to 13.8 % and mini-modules of 5x5 cm2 were manufactured.
Insufficient homogeneity, the need of thicker layers and a rather long deposition time have been identified as main deficiencies of the developed process. As a consequence, immediate industrial implementation and scale-up of the current laboratory process is predicted to result in ca. 40% cost increase as compared to reference sputter process. As outlook, further research activities on solution electrodes should focus on accelerating the deposition rate as well as improving the long term stability of ZnO with capping layers or extrinsic dopants.
