Employing microsecond pulses to form laser-fired contacts in photovoltaic devices

Laser-fired contacts (LFCs) are typically fabricated with nanosecond pulse durations despite the fact that extremely precise and costly control of the process is necessary to prevent significant ablation of the aluminum metallization layer. Microsecond pulse durations offer the advantage of reduced metal expulsion and can be implemented with diffractive optics to process multiple contacts simultaneously and meet production demands. In this work, the influence of changes in laser processing parameters on contact morphology, resistance, and composition when using microsecond pulses has been fully evaluated. Simulated and experimental results indicate that contacts are hemispherical or half-ellipsoidal in shape. In addition, the resolidified contact region is composed of a two-phase aluminum-silicon microstructure that grows from the single-crystal silicon wafer during resolidification. As a result, the total contact resistance is governed by the interfacial contact area for a three-dimensional contact geometry rather than the planar contact area at the aluminum-silicon interface in the passivation layer opening. The results also suggest that for two LFCs with the same size top surface diameter, the contact produced with a smaller beam size will have a 25-37% lower contact resistance, depending on the LFC diameter, because of a larger contact area at the LFC/wafer interface.