By: P.Gečys, G. Račiukaitis, M.Gedvilas, A. Braun, S. Ragnow

Laboratory for Applied Research, Center for Physical Science and Technology, Lithuania
Solarion AG, Germany

Low material consumption, the possibility of deposition on large areas, use of cheap and flexible substrates make the thin-film photovoltaic elements the most promising technology to achieve a significant cost reduction in solar electricity. Interest in complex multilayered CuInxGa(1-x)Se2 solar cells has increased recently because of low production costs and scalability through a roll-to-roll process. CIGS has been established as the most efficient thin-film technology in converting sunlight into electricity with the theoretical limit as high as 27% and a record value of 20.2% achieved in laboratory. Flexible CIGS solar cells have several advantages compared to those fabricated on glass substrates. Their power to weight ratio as well as excellent resistance to radiation make them ideal candidates for space use and building-integrated applications beyond the capability of rigid, heavier PV products.

Efficiency of the thin-film solar cells with a large active area might be maintained if small segments are interconnected in series in order to reduce photocurrent in thin films and resistance losses. By alternating the layer deposition and layer patterning it is possible to produce integrated series interconnections. The complex absorber layer of copper-indium-gallium diselenide is a thermo-sensitive material, and laser processing might cause excessive melt formation close to edges of the laser scribe. Thermal degradation of the CIGS solar cells starts at temperatures above 350oC due to diffusion of the buffer layer metal (Cd, Zn) into the absorber layer. Therefore front side scribing processes of CIGS on flexible metal and polymer substrates are still challenging.

We demonstrated our new results in the P3 and P2 scribing of the films to open the molybdenum back-contact by selective removal of both the top-contact and CIGS layers. Selection of the right laser wavelength is important to keep the energy coupling in a well defined volume at the interlayer interface. There fore fundamental (1064 nm) and second harmonics (532 nm) of picosecond laser were applied for solar cell patterning. For laser scribing process optimization the top-hat shaped beam and parallel beam scribing were investigated. Scanning electron microscopy, electron-dispersion spectrometry and micro-Raman have been used to evaluate the processing results to show benefits of the used picosecond lasers in selective scribing on this type of photovoltaics structures.