Lasers as manufacturing tools offer several advantages, ones especially relevant for the production of solar cells. The contactless and localized nature of the energy deposition allows for new processes, such as laser selective emitter doping, laser ablation of dielectric coatings and via drilling for back contact cell concepts. A critical factor is the correct selection of suitable laser sources and processing parameters in a manner that adapts the laser process to the requirements of the material, the process nature and the solar cell properties.
Researchers all over the world are working on new and improved laser processes that benefit solar cells in several ways: new high efficiency cells concepts become feasible by integration of laser processes, e.g. laser doped selective emitters, laser drilled back contact cells and rear-passivated cell types. Laser processes allow lower production costs by reducing expenses for machines, maintenance and resources. And the high throughput of laser-based production processes can help increasing the output of production lines.
Of course the integration of new production processes, laser-based or not, requires at least the adaptation of existing equipment, but in most cases completely new production lines have to be bought and ramped up. This means that newly developed laser processes have to be thoroughly benchmarked against competing approaches, often by proving their performance on an industrial-like basis to show competitive productivity, reproducibility and reliability.
Ultimately, a production line making use of new laser processes has to show improvements in yield and up-time of the equipment, production throughput, required maintenance, and most importantly, conversion efficiency of the solar cells.
The Solasys project
Up to now lasers are used for only a few processes in commercial production. These include laser edge isolation, which however is more and more replaced by chemical etching, and laser cutting for some types of wafering processes. Laser drilling for metal wrap-through cells and laser doping for selective emitters are currently entering the solar cell market with considerable force. But this short list already covers more or less the laser-based processes used in solar cell manufacturing.
It is the goal of the project Solasys (Next Generation Solar Cell and Module Laser Processing Systems), a multi-national demonstration activity partly funded by the European Commission, to strengthen lasers as manufacturing tools for crystalline solar cells and to demonstrate five laser processes in such a way that their use in existing or new production lines becomes attractive and profitable.
The project is coordinated by the Fraunhofer Institute for Laser Technology (ILT) in Aachen, Germany. The consortium consists of researchers from Belgium (Imec) and France (CNRS-LP3: Laboratoire Lasers, Plasmas et Procédés Photoniques), machine and component integrators from Germany (Manz Automation and Scanlab), laser developers from Germany (Laserline and Trumpf) and solar cell manufacturers from Spain (BP Solar) and the Netherlands (Solland Solar). The project with a duration of 36 months started in September 2008. The final year is currently running and the demonstration takes place on equipment located at Fraunhofer ILT.
To achieve the project goals the following laser processes are developed, optimized and finally demonstrated on prototype equipment under near-industrial conditions:
Laser doping for selective emitters using pulsed laser sources,
High-speed laser drilling for emitter wrap-through cells,
Laser-based soldering for string production,
Laser ablation of thin passivation layers on the front side of solar cells, and
Laser edge isolation using ultra-short pulse (i.e. pulse durations of the order of picoseconds) laser sources.
In addition, laser-based texturization is investigated. In the following, the project results related to two of these processes will be described.
Laser doping for selective emitters
In standard doping processes a trade-off between conductivity of the emitter and the life-time of the minority carriers is necessary, the former requiring a high, the latter a low doping concentration. By using selectively controllable doping profiles for the emitter instead, 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 a one step laser doping process the residual phosphor silicate glass layer (PSG) is used as dopant source. The silicon is locally molten using laser irradiation, allowing the phosphor atoms to diffuse into the silicon and enter the silicon lattice. A big advantage of this concept is the possibility of including it in existing manufacturing lines without increasing the complexity of the fabrication process significantly.
Most laser doping approaches are carried out at a wavelength of 532 nanometers (nm). The pulse length is usually in the nanosecond (ns) range, which allows melting of the silicon without evaporation. The influence of the pulse duration on the doping profile was investigated in a scientific study for the range between 10 and 400 ns. It was found that laser doping is possible for all pulse durations. However the pulse energy has to be selected carefully to avoid evaporation of the material. For longer pulses a larger process window makes this much simpler, thus enabling a more stable and reliable laser process.
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Laser drilling for EWT solar cells
Drilling holes in silicon wafers is one fundamental processing step for emitter wrap-through (EWT) solar cells. This cell concept features a front side emitter, connected by vias to the rear side where both the emitter and the base metallization are located. Even though high cell efficiencies have been proven in the lab, the complex structure of the EWT solar cell inhibits the progress from a concept to an industrial product.
Approximately 25,000 holes are required for a six inch emitter wrap-through solar cell. Laser drilling potentially allows rates of over 10,000 vias per second, which are needed for industrial scale production. However, current approaches do not reach this number and new laser sources and optical concepts are required to increase the drilling speed.
Based on a simple model which allows the calculation of drilling rates for different pulse durations and pulse energies, the selection of the proper laser source and optical setup can be simplified. A process adapted version of a commercial laser by Trumpf has been developed in the project Solasys on the basis of experiments and calculations to achieve the required drilling rates.
Process development
These results and experiments in progress allow a selection of the most suitable laser source for each process. It is obvious that each of the processes requires a different type of laser source with different parameters, mainly in the form of pulse duration and wavelength. This is due to the fact that ablation of nanometer thin layers and drilling of comparatively thick wafers require different physical effects and mechanisms. The physics behind the interaction of material systems and the incident laser light is a highly complex matter and for solar cells this has not been investigated thoroughly up to now. The complexity of the material systems, which often consist of a multitude of different elements, and the high flexibility of laser sources make this a challenging scientific field.
But process development does not only concern the selection of the suitable laser source: for laser processing, a different system technology is necessary than for established processes in solar cell production. Optical systems have to be combined with handling, vision and metrology equipment into stable and reliable machine platforms to allow both a high productivity and a high quality of the result in terms of cell efficiency and longevity. This can be achieved by combining the proper laser source with fast galvanometric scanner systems, which allow a movement of the laser beam at speeds of ten meters per second and more across the wafer surface.
Over the past 50 years, lasers have proven their potential for contact-free and selective processing of many materials. It is the next logical step to establish lasers also in the production of solar cells on a broader basis. Many experts see photonics, the manipulation and application of light, as the next breakthrough after the information age and lasers play the central role in this field. It is only a matter of time and effort before laser processing becomes as established in solar cell production as it already is in many other industries.
More details and current results of the project Solasys will be presented on May 25, 2011, in Munich, during the workshop Lasers in Photovoltaics. This will take place in parallel to the Laser 2011 exhibition and the World of Photonics Congress.
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