Assessment of laser direct-scribing of a-Si:H solar cells with UV nanosecond and picosecond sources

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. Therefore a good characterization of the laser process, along with the assessment of different laser sources is compulsory. The current approach for laser scribing processes used in thin-film a-Si:H photovoltaic technology forces the use of transparent substrates [1]. In order to be able of keeping using laser for the module interconnection with other kind of substrates such as metal foil or plastic, the laser process must be performed in a direct scribing configuration, where the laser has to remove selectively the layers which make up the devices. In that sense, it is important to characterize the laser scribing process for different laser sources, owing to the different behaviour of the materials as a function of the laser parameters (wavelength, pulse width and pulse energy). In the current approach, the use of green wavelength (532 nm) is forced due to the strong dependency of the a-Si:H with this wavelength. This fact is leveraged for the two last laser steps of the monolithical interconnection of the cells, always in back-scribing configuration. On the other hand, for direct scribing configuration, the patterning must be obtained by means of selective ablation of the layers, and the morphologically affected area plays here an important role as well as the heat-affected zone. Thus, this work focuses on the use of UV laser sources working at two different temporal ranges of pulsewidth (nanosecond and picosecond) in order to eliminate the thermal affectation and improve the morphological quality of the groove.