Structuring of transparent conductive oxide (TCO) films on glass by femtosecond laser pulses

Thin film solar modules are usually designed as 5-10 mm wide stripes electrically connected in series. The majority of structuring is currently done by the three laser patterning steps P1, P2 and P3 using nanosecond pulses to separate successively transparent conductive oxide (TCO), absorber and rear contact metal, respectively. Each scribe has a typical width of 30 ?m or more, and causes a heat-affected zone in the remaining material. Over all, a considerable part of the solar cell area is not used for electrical power production. Femtosecond lasers offer great potential to minimize the width and to optimize the quality of these scribes because the ultra-short pulses can be utilized for selective ablation via non-thermal physical mechanisms without deleterious effects like thermal and mechanical stresses in the glass substrate or contamination of the adjacent layer. Here, the P1 scribe, i.e. ablation of transparent conductive oxide (TCO) films was investigated in detail. The ablation craters and scribes, for single and multipulse ablation processes, were investigated by profilometry, optical microscopy and scanning electron microscopy. In addition, energy dispersive x-ray (EDX) analysis was performed to examine the influence of the laser structuring on the glass 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 film.