Lithography process optimization for high efficiency thin film solar cells

In this work, photolithography-assisted chemical etching is explored as an alternative method, with respect to mechanical scribing, for sub-cell division in Copper Indium Gallium Selenide (CIGS) solar cells. This technique is indeed expected to selectively remove only the buffer and top contact layers of the cells, leaving the substrate unaltered. The separation of sub-cells through photolithography was successful: the intended layers were properly and selectively removed, while the absorber layer was preserved. Opto-electronic characterization was then carried out on cells divided into sub-cells either through photolithography or by mechanical scribing, in order to compare their performances. Higher efficiencies were consistently registered in the samples processed by photolithography, with an average increase in efficiency between the two methods of 30\% for standard (1.18 eV) band-gap CIGS. The selective nature of photolithography-assisted etching is therefore found to have a positive impact on the charge collection in the cells. Photolithography is additionally tested as a technique for the creation of a pattern for the deposition of top contact grids for wider-area sub-cells. The grids were successfully fabricated and their performance was compared to the ones obtained for scribed sub-cells with evaporated contacts.