Transient effects during laser processing of silicon for photovoltaic applications

With time-resolved microscopy this work reveals the physical processes on to the femtosecond to nanosecond scale, which are induced by intense pulses in silicon. To tailor a laser process towards applications in photovoltaics, such as pulsed laser annealing and laser ablation, a detailed knowledge of those mechanisms is required. Combining measurements and simulations, this work demonstrates how they determine the outcome and potential damage.

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Intense laser pulses deliver fine doses of highly concentrated energy to modify solid targets in an exceptionally well controllable manner. Two applications of this are treated herein: "laser annealing" as an approach to transfer amorphous silicon into the crystalline state and "laser ablation" to create local openings in dielectric layers for subsequent contacting. Both can be used in manufacturing of silicon solar cells as a cost-effective and environmentally friendly alternative to thermal and chemical processing. This work presents implementations of the pump-probe approach which enable the observation of the surface during and shortly after the arrival of a laser pulse. With an automated opto-mechanical setup concurrent physical processes happening in the material are made visible on the femtosecond to nanosecond scale. Using micro- and nano-characterization, modeling and simulations, this work shows how the absorption of intense laser pulses leads to phase transformations and material removal. It demonstrates that time-resolved observations are essential to investigate the origin of undesired side-effects, such as laser induced damage or incomplete layer removal.