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Optimized scribing of TCO layers on glass by selective femtosecond laser ablation

Stephan Krause1, Paul-Tiberiu Miclea1,2, Stefan Schweizer1,3, Gerhard Seifert1,4 1Fraunhofer Center for Silicon Photovoltaics CSP, Halle (Saale), Germany /2Institute of Physics, Martin Luther University of Halle-Wittenberg, Halle (Saale), Germany /3Department of Electrical Engineering, South Westphalia University of Applied Sciences, Soest, Germany /4Centre for Innovation Competence SiLi-nano, Martin Luther University of Halle-Wittenberg, Halle (Saale), Germany

Today’s thin film solar modules typically consist of many separated cell stripes of 5-10 mm width. To guarantee their electrical serial connection, the layers (front contact, absorber, and rear metal contact) are usually structured directly after being deposited by nanosecond laser patterning steps (P1, P2, P3) through the glass substrate. Since “thermal ablation” by nanosecond laser pulses causes unavoidably heat-affected zones (HAZ) in the layer materials during each scribe processing, reasonable electrical functionality is only achieved with sufficient displacement between the scribes of each layer. The resulting, optically inactive zone of at least 100 µm in width, can potentially be strongly reduced using ultrashort laser pulses. In particular, when pulses in the real “cold ablation” range (<1 ps) are applied, HAZ can be avoided, and one can minimize the width and optimize the quality of the trenches. Here, we want to demonstrate this advantageous processing on the example of different transparent conductive oxide (TCO) films (P1 scribes) by comparing the results of scribing with nano- and femtosecond pulses of ~1 µm wavelength. The obtained ablation craters and scribes were analyzed by profilometry and optical and scanning electron microscopy. Additionally, energy dispersive x-ray (EDX) analysis was performed to examine the influence of the laser structuring on the substrate and the direct environment of the laser scribe, for instance, residual TCO. Electrical conductivity measurements were done to analyze the influence of the scribe width on the electrical properties of the TCO films. The results clearly show that femtosecond laser processing has the potential to reduce the total width of the scribe region (inactive zone) to below 50 µm, and, in particular, the width of the P1-scribe below 10 µm without thermal and mechanical stresses in the substrate or adjacent material.