Lasers help make Photovoltaics more competetive

At present, the photovoltaic market is not really competitive, because the production costs of so-lar cells are relatively high, and the effectivity of indus¬trial solar cells has much room for improve¬ment. Cost reduction and increase in effectivity – these are the reasons that laser technology has been used for several years for making and improving solar cells - with promising results.
At the international conference "Laser Techno¬logy in Photovoltaics" during the trade fair SOLTEC in Hamelin, Germany, about 130 participants from the USA, the Netherlands, Great Britain, Norway, Lithuania, Korea and Germany discussed progress based on research and new concepts from laser manu¬facturers. Invitations were sent by PhotonicNet (competency network of optical technologies in Lower Saxony) with their partners ISFH (Institute for Solar Energy Research Hamelin) and the LZH (Laser Zentrum Hannover).

Multitalented Lasers
Lasers can be used in many ways in producing solar modules. They can be used to structure and give texture to the crystalline silicon structures, or to drill or mark them or use less expensive substrate material such as glass or a metal foil.
The two-day event focussed on laser applications for various cell technologies such as thin-film technologies and silicon-based wafer technology. The lectures discussed analyses of the interac-tion of laser radiation with different materials commonly used in photovoltaics. Lectures also cov-ered both established and new process technologies for highly-efficient solar cell concepts, which should make serial production more economic.

A cell a second
The central problem in manufacturing solar modules is the trend to thinner and thinner wafers (about 100 µm), at the same time aiming at a production rate of one cell per second. The laser, which works non-tactilely, offers definite advantages. Discussions centered around which laser type should be used for the processes cutting, drilling and structuring the cells best. Pulse dura-tion, intensity and the laser wavelength are decisive factors for using one or the other laser in each processing step.
"Our experience shows," says Peter Engelhart of ISFH, " that picosecond lasers are best suited for near-surface ablation to open the solar cell contacts, since these lasers have both a short pulse length and high intensity." To drill through the extremely sensitive semiconductor material, the ISFH prefers disc lasers in the infrared range. For cutting monocrystalline silicon, the LZH has achieved the best results with a cw-laser, making it unnecessary to post-process the cut edges.

Trend: Contacts on the reverse side
Different concepts to improve the effectivity based on a special construction of the solar cells were also presented in Hamelin. One trend aims at placing contacts on the reverse side of the cell, such as Emitter-Wrap-Through-Cells (EWT), which are already being mass-produced (Ad-ventSolar). Emitter contacts can be placed on the reverse side via laser-drilled holes, thus de-creasing the losses due to shadows caused by the metal strips on the front side. Such cell con-cepts make it possible to make highly efficient solar cells with effectivity rates of over 20%, such as those reached at the ISFH using RISE-EWT Solar Cells (Rear Interdigited Single Evapora-tion).

Another concept was explained by Alex Cole from NaREC (New and Renewable Energy Centre, Northumberland), that of concentrator solar cells, which are radiated with more solar energy by using parabolic mirrors or lenses. The concentrator cells also need low-shadow contacts, which can nonetheless transport higher electrical currents of several amperes. These cells use the so-called Laser Grooved Buried Contact Concept (LGBC), which "hides" the contacts on the front side in grooves which were made by the laser, in order to decrease shadow-based losses.

Lasers in thin-film technologies
Thin-film technologies have been using lasers for several years. Thin films less than 50 nanome-ters thick can be selectively cut and ablated. Thus, circuits can be made from the deposited lay-ers on a glass plate, and flexible module circuits are possible. In this way, the parameters of the solar module, electricity and voltage can be adjusted individually to customer demands. Cost-intensive, and difficult to adjust photolithographic processes or masking processes are not nec-essary if the laser is used. Scaling the processing to larger areas is well-known from display tech-nologies, and can also be used for module circuiting.

By bringing science and industry together, this workshop could make an important contribution to supporting mutual understanding of the subject. The laser manufacturers could on the one hand elucidate the expected development potential of laser units, and on the other hand, laser users could give clear input to the direction they would like laser development to go, in order to be able to meet a continually increasing operational capacity for solar cell production lines.

An exchange of ideas between photovoltaic experts and laser experts should continue in future workshops. "Supporting the flow of information in this young application field of laser technology should help us make more progress on using solar energy more efficiently and competitively," said Dr. Hartmann, CEO of PhotonicNet, at the end of the meeting. 

Abstracts:
http://www.photonicnet.de/veranstaltungen/angebote-photonicnet/events-2007/workshop-photovoltaics/abstracts

Gallery:
http://www.photonicnet.de/veranstaltungen/angebote-photonicnet/resolveUid/478d04c13de18deb0fca081a270b0811

Contact:
PhotonicNet GmbH
Anja Nieselt-Achilles
Marketing & Communications
Tel. (0511) 277 16 43
Fax (0511) 277 16 50
Mail: anja.nieselt@photonicnet.de
URL: www.photonicnet.de

CEO: Dr. Hans-Jürgen Hartmann
County court: Amtsgericht Hannover: HRB 59 301