High-speed laser drilling of silicon wafers for photovoltaics

Aachen, Germany – High-speed laser drilling is an important processing step in the production of high-efficiency solar cells, especially in the case of emitter-wrap-through (EWT) solar cells that require 10,000 and more vias to be drilled in each cell. Given that the typical duration of a single process step on a production line for solar cells is on the order of one or two seconds, a drilling rate of 10,000 vias per second is the minimum requirement for industrial-scale production.

The maximum drilling rate currently achievable with commercially available laser sources and galvanometer scanners is 4,000 vias per second. New optical concepts such as the combined use of high-performance galvanometer scanners and optical beam dividers offer a potential means of attaining the required drilling rate. This calls for laser sources with an average power output of several hundred watts and a pulse duration in the microsecond range. A number of different system solutions were set up and compared in order to assess their suitability.

Parameter studies indicate that pulse duration and pulse energy are critical factors for the efficiency of the drilling process. On the basis of experimental results, a numerical simulation of the ablation process was used to evaluate a number of different laser sources and optical concepts. By applying two coupled Jenoptik Jenlas IR 70 lasers, it was possible to achieve 9,600 holes/s in wafers with 200 micron thickness and 12,500 holes/s in 180 micron wafers.

The high-speed drilling process developed in this project by Fraunhofer Institute for Laser Technology ILT relates specifically to a processing step in the manufacture of EWT solar cells in which the back contacts are connected to the emitter layer through electrical feedthrough structures in the silicon. This increases the active surface area of the solar cell and hence its efficiency. The ability to drill holes of a similarly small diameter has other potential applications beyond that of photovoltaic cells, for instance in filter technology.

For more information on this process contact: [email protected]