Direct Laser Doping for High Efficiency Solar Cells

By: Malte Schulz-Ruhtenberg

To make the generation of electricity from sunlight competitive to conventional means (like coal and nuclear power) the major goal at the moment is to reach the so-called grid parity, which marks the point where electricity from the receptacle costs the same as electricity generated by solar cells on the roof. This can only be achieved by the reduction of production costs and the increase of the efficiency of the cells. Both production costs and cell efficiency can be improved by using lasers as machining tools. Only a few processes within the production of solar cells are currently done with lasers, but many more can harvest the benefits of these devices, such as contactless processing, highly localized energy insertion and flexibility.

As for any semiconductor device the central part responsible for the function of a solar cell is the p-n junction, at which positively and negatively doped semiconductor materials meet. This junction allows the sunlight to create free carriers that can flow to the terminals, which results in electrical current. Improving the properties of the emitter, the thin negatively doped region at the top of the cell, is currently a high priority in research, since the standard concept uses a trade-off:

a high amount of doping is desired for a good conductivity of the material while a low amount ensures a long lifetime of free carriers which allows them to reach the terminals. Thus an intermediate amount is necessary which limits the efficiency of the solar cells.

This trade-off can be circumvented by a concept called selective emitters. Here, a low doped emitter is created in a first production step to ensure a high carrier lifetime. In a second step the doping is locally increased at those positions where the metal contacts are later formed. This allows a good electrical contact between semiconductor and metal and thus a low series resistance of the solar cell. Overall efficiency improvements of 0.5% can be obtained with this technique.

In our approach a thin layer containing phosphorus glass, which remains after the standard doping process, is used as a dopant source. With a laser beam it is melted together with the underlying silicon. Both elements mix and during cooling recrystallize. This results in an increase of the doping. By careful selection of the laser parameters the doping profile can be controlled so that junctions as deep as several micrometers or as shallow as several hundred nanometers are possible.

Shulz-Ruhtenberg- Image-Direct Laser Doping for High Efficiency Solar Cells

With the help of fast galvanometric laser scanners it is possible to process one silicon wafer in about 2.5 seconds. Since the goal is one second per production step there is still room for improvement here.

The above brief overview was extracted from its original abstract and paper presented at The International Congress on Applications of Lasers & Electro-Optics (ICALEO) in Orlando, FL. To order a copy of the complete proceedings from this conference click here