Evolution of laser-fired aluminum-silicon contact geometry in photovoltaic devices

Abstract

The evolution of temperature and velocity fields during laser processing of solar cells to produce an ohmic contact between an aluminum thin film and a silicon wafer is studied using a transient numerical heat transfer and liquid metal flow model. Since small changes in pulse duration, power, and power density can result in significant damage to the substrate and, in extreme cases, expulsion of droplets from the molten zone, the selection of optimal laser processing parameters is critical. The model considers the unusually large heat of fusion of the Al-Si alloy formed during processing and the large composition-dependent two phase region. The calculated size and shape of the fusion zone were in good agreement with the corresponding experimental data, indicating the validity of the model and providing a basis for using the model to develop a better understanding of the laser-assisted fabrication of contacts for solar cell devices. The transient changes in the composition of the Al-Si molten region are found to have a major impact on the heat transfer during the formation of the contact. Consideration of the time-dependent concentration of Al in the molten region is also essential to achieve good agreement between the experimental and computed molten pool sizes. Process maps showing peak temperatures and the depth and width of the molten pool are presented in order to assist users in the selection of safe process parameters for the rapid fabrication of these silicon-based photovoltaic devices.