Selective ablation with UV lasers of a-Si:H thin film solar cells in direct scribing configuration

Monolithical series connection of silicon thin-film solar cells modules performed by laser scribing plays a very important role in the entire production of these devices. In the current laser process interconnection the two last steps are developed for a configuration of modules where the glass is essential as transparent substrate. In addition, the change of wavelength in the employed laser sources is sometimes enforced due to the nature of the different materials of the multilayer structure which make up the device. The aim of this work is to characterize the laser patterning involved in the monolithic interconnection process in a different configurations of processing than the usually performed with visible laser sources. To carry out this study, we use nanosecond and picosecond laser sources working at 355 nm of wavelength in order to achieve the selective ablation of the material from the film side. To assess this selective removal of material has been used EDX (energy dispersive using X-ray) analysis, electrical measurements and confocal profiles. In order to evaluate the damage in the silicon layer, Raman spectroscopy has been used for the last laser process step. Raman spectra gives information about the heat affected zone in the amorphous silicon structure through the crystalline fraction calculation. The use of ultrafast sources, such as picoseconds lasers, coupled with UV wavelength gives the possibility to consider materials and substrates different than currently used, making the process more efficient and easy to implement in production lines. This approach with UV laser sources working from the film side offers no restriction in the choice of materials which make up the devices and the possibility to opt for opaque substrates.

Research highlights▶ Ablation thresholds and scribing process parameters calculated for the materials involved in the a-Si:H thin film technology. ▶ Morphology study of the laser patterning at 355 nm of wavelength and two different pulsewidth (picosecond and nanosecond). ▶ Consecution of the selective ablation from the film side of the thin films which make up the device. ▶ The study of the Raman crystalline fraction of the silicon layer shows good results for direct scribing ablation in the last laser step, with low affectation of the silicon structure. ▶ Potential use of alternative materials in the substrate configuration with full laser interconnection.