By: Alexander Schell and Keming Du
EdgeWave GmbH, Germany
In order to reduce production costs and to increase throughput, great efforts have been put to explore efficient processing in full-automated manufacturing lines. Because of non-contact processing and high flexibility the use of lasers in photovoltaic production has grown in popularity, for example laser fired contact, drilling contact holes, edge isolation, scribing, edge deletion, etc. Different application needs different laser parameters such as wavelength and beam profiles.
For ablation processes (ablation of coatings, e.g. scribing or edge deletion) the effective energy efficiency (EEE) is significant different for a Gaussian beam and for a 2D Top-Hat beam. As shown in figure 1 the intensity of a Gaussian beam in the outer region is below the ablation threshold (Ith, rth). Therefore it does not contribute the ablation process. Even worse it could have thermal influence to the coating. The thermal effected zone decreases the efficiency of the solar cell. The intensity in the beam centre is higher than required. This energy is lost or in worst case it can damage the material below. In total the maximum effective energy efficiency of a Gaussian laser beam is about 36.7%. A 2D Top-Hat beam without modulation has 100% efficiency.
Another benefit of using rectangular beams instead of round beams is that the required overlap between two laser shots is lower so that the process energy can be reduced and production speed can be increased (figure 2).
2D Top-Hat beams with best beam quality offer the possibility working with standard scanning systems as shown in figure 3. This enables remote ablation processing on big panels with small spot (down to several ten microns) at high speed.
The applications shown in figure 4 are made by INNOSLAB lasers with tailored beam profile, wavelength, pulse energy and pulse width. The picture shows a CIGS Thin Film Module with three different laser processes.
For best drilling results of the shown contact through slots a green laser with Gaussian beam profile was used. The process time for each slot (2mm x 4mm) is below five seconds.
The scribing of the thin films were done using an infrared or green 2D square Top-Hat laser with beam quality M² < 2 which can be imaged down to 35µm by 35µm to reach a homogenous isolation without zigzag.
The edge deletion process is one of the most important quality issues for thin film solar modules. Bad isolation will cause a significant power loss over time by surface discharges on not completely removed materials. For homogenous and efficient deletion a 2D rectangular Top-Hat INNOSLAB laser was used.
Among other things EdgeWave develops laser systems with tailored beam parameters for the photovoltaic industry having in mind how the energy requirements can be reduced and how production system solutions can be simplified through suitable laser beam engineering.