Thermal Processing of Thin Films Using Ultra-short Laser Pulses Applied to Photovoltaic Materials

Ultra-short pulsed laser sources, with pulse durations in the ps and fs regime, arecommonly exploited for cold ablation. However, operating ultra-short pulsed lasersources at fluence levels well below the ablation threshold allows for fast and selectivethermal processing. The latter is especially advantageous for the processing of thinfilms. A precise control of the heat affected zone, as small as tens of nanometers,depending on the material and laser conditions, can be achieved. It enables thetreatment of the upper section of thin films with negligible effects on the bulk of thefilm and no thermal damage of sensitive substrates below. By applying picosecondlaser pulses, the optical and electrical properties of 900 nm thick SnO2 films, grownby an industrial CVD process on borofloat R©-glass, were modified. The treated filmsshowed a higher transmittance of light in the visible and near infra-red range, aswell as a slightly increased electrical sheet resistance. Changes in optical propertiesare attributed to thermal annealing, as well as to the occurrence of LaserInducedPeriodic Surface Structures (LIPSSs) superimposed on the surface of the SnO2 film.The small increase of electrical resistance is attributed to the generation of laserinduced defects introduced during the fast heating-quenching cycle of the film. Theseresults can be used to further improve the performance of SnO2-based electrodes forsolar cells and/or electronic devices. Published as: D. Scorticati et al., Optical and electrical properties of SnO2 thin films afterultra-short pulsed laser annealing, Proceedings of SPIE The International Society for OpticalEngineering, 8826:88260I-1-12 (2013).