Laser Technology in Photovoltaics

Solar cells produce electrical current through a photoelectric effect in semiconducting materials. In the case of conventional silicon solar cells, a thin conductive layer of metal is applied for contacting the doped silicon. In modern organic thin-film solar cells, not only contacting, but also semiconducting layers are
applied to a transparent film. In addition, thin anti-reflection and passivation layers are used for both thin-film and silicon solar cells, which lead to an improvement in the optical and electrical properties of the solar cells and thus to higher efficiencies. For both types of solar cells, these thin layers must be selectively removed at defined points with high precision. This can be achieved with high throughput at speeds of up to 10 m/s and high precision with ultrashort pulsed laser radiation. With an adapted temporal and local shaping of the laser
radiation, ultrashort pulse lasers permit a complete removal of the layers with a line width of a few micrometers without damaging the substrate or the underlying layer

Layers with thicknesses fewer than one hundred nanometers are extremely sensitive and can easily degrade or lose their functionality. The reasons for this can be particle debris or delamination of irradiated layers, thermal damage of neighboring areas and other layers, as well as the generation of bulges in the outer areas of the ablations. In order to achieve a sufficiently high quality, parameters – such as gas atmosphere, spatial and temporal pulse shape, and subsequent cleaning – have to be taken into account while the process
is developed. Alternatively, one can fall back on hybrid processes such as thermochemical ablation. In particular,
during structuring of the transparent conductive Indium tin oxide (ITO) – commonly used in organic electronics – bulges can form, which are problematic for subsequent layering. Fraunhofer ILT is developing processes which can significantly reduce these sources of defects according to the requirements of the application, for example by using adapted wavelengths, pulse durations and ablation strategies.

Anti-reflection layers on silicon solar cells lead to reduced reflections on the cell surface, thus increasing their efficiency. To exploit the energy potential of solar radiation, reflections must be further minimized and absorption maximized. In order to achieve this in silicon solar cells, their surfaces are processed by means of laser radiation and plasma etching. Processing with laser radiation enables a defined periodic microscale structuring of the surface, which facilitates the absorption of the most energy-intensive part of the solar spectrum. In the subsequent plasma etching process, a nanostructure is applied to this microstructure, which reduces the reflection even more. It is a combination of surface structures that will increase the efficiency of silicon solar cells.

In the manufacturing process of wafer-based Silicon solar cells the raw wafer is doped with Phosphor to create the emitter and thus the essential pn-junction. In today’s production a trade-off is necessary between conductivity of the Silicon and the life-time of the free carriers, the former requiring a high, the latter a low doping concentration. Using selectively controllable doping profiles of the emitter the cell efficiency can
be improved. Starting with a weakly doped emitter the doping concentration is increased locally at the positions of the metal contacts. For this selective doping process the laser is an ideal tool, with which the Silicon is locally heated respectively molten. The use of different dopant sources (gaseous, liquid or solid in the form of thin phosphor glass layers) allows the variation of the profiles, different laser wavelengths with suitable penetration depths and process adapted temporal irradiation profiles are used. With proper selection of process strategies, the doping process can be combined with the structuring process, which simplifies subsequent metallization.

For the production of solar modules from individual solar cells, several cells are electrically connected with long-term stability. Currently the contacting is done with infrared or selective thermode soldering. Thermal and mechanical stress created during these processes leads to the formation of cracks, which is especially dangerous for thin wafers. The laser soldering process allows strongly localized energy deposition without heating the whole wafer. In addition the process is controlled with a pyrometer, so that the laser power can be adapted in real-time during soldering to achieve a constant temperature even at varying material conditions. This allows an ideal temperature profile on the cell and a minimum of heat input. Using a setup with two beam paths simultaneous soldering of the front and back contacts is possible, which enables process times below three seconds per cell. For future cell concepts laser beam welding allows a decrease of the processing times by a factor of ten compared to soldering.