Increasing photovoltaic efficiency12 / 2010, Research & Development | By: Peter Oesterlin, Andreas Büchel
Selective emitters: Increasing cell efficiency remains an active and important aspect in the solar industry. Laser doping for selective emitters of crystalline solar cells holds the potential to break some boundaries, as Peter Oesterlin and Andreas Büchel of Jenoptik discuss.
Driven by falling prices and competition in the market, manufacturers of photovoltaic cells are seeking new methods and cell concepts to reduce production costs and increase cell efficiency. There has been substantial academic research in order to achieve mass production at reasonable costs. The thin film industry uses laser technology applications in a more comprehensive way in comparison with the silicon solar cell manufacturers. Nevertheless, laser applications show great promise to turn the ambitious goals of crystalline solar cell manufacturers for high efficiency cells into reality. There are six to seven methods available in relation to selective emitters in production. Some producers have already embraced one of the methods. Others are still dismissing it and not adopting.
The laser supply market is already equipped to fuse with the solar sector to achieve the goals of high efficiency, whether it is contact firing solutions or laser ablation. One of the most promising of these developments is laser doping or laser diffusion of selective emitters. Laser doping typically requires a phosphorous or boron bearing layer that is deposited on the surface of a solar wafer. Laser pulses or beams are then directed at this layer that melts the surface allowing for the diffusion of the dopant and silicon.
The selective doping process is one that is predominantly used. This process serves to enhance the efficiency of crystalline solar cells by applying local diffusion to the substrate for the implementation of selective emitters by doping from phosphosilicate glass (PSG). Using this method, an increase of absolute cell efficiency of 0.4 to 0.5 percent has been demonstrated by various groups.
Different laser types and wavelengths have been tested, from infrared (IR) to the ultraviolet (UV). Infrared lasers are available with very high power, but the penetration depth of the radiation in silicon is large (several hundred microns) so significant laser power is lost in heating the bulk material of the wafer with no contribution to the surface processes. Additionally there are indications that IR lasers generate more lattice defects than shorter-wavelength lasers.
At the other end of the spectral range, UV lasers are well suited for laser diffusion due to the high absorption in a very shallow surface layer. However, the UV laser power is typically limited to 20 watts, which is insufficient for production rate throughput. The green spectral range offers the best compromise. The penetration depth of roughly 1.0 micrometer is well suited and the available laser power ranges above 100 watts are sufficient for production rate throughput and minimized lattice defects. Both types of frequency doubled solid state lasers, pulsed and continuous wave, are available in the green spectral range.
Laser doping also offers an advantage for manufacturing costs. With an increase of 0.3 percent absolute efficiency for crystalline cells the additional laser process “pays for itself”, meaning that the depreciation and operating costs of the laser production tool would be compensated by the higher efficiency (and thus higher payback) of the cells.
Although the process itself is now well known, the debate of how to apply the laser beam in the most efficient and time saving way is still going on.
Handling of thin and fragile wafers (with a typical thickness 180 micrometers) is required as well as simple and reliable beam splitters and beam-shaping concepts that divide a single laser beam into as many spots as contact fingers are needed. The feeding of wafers into the machine must be optimized so that the loading and unloading time of the wafers is minimized. Additionally, the wafers must be aligned properly prior to the laser diffusion process because the laser treated lines must match with the contact fingers, which are applied in a subsequent process step. The Jenoptik-Votan Solas 1800 was the newly introduced tool platform for laser applications in wafer-based solar cell manufacturing like hole drilling and selective dielectric opening. It also has the proven capacity to run 1,800 wafers per hour and more. Selective emitter doping in a system is enabled by the integration of the JenLas Asama laser and the Innovavent Volcano.
Together with engineers from Jenoptik’s optical systems division, Innovavent developed a proprietary beam splitter technology based on patent pending diffractive optical elements (DOE) which opens the path to simultaneous processing of more than 100 contact fingers at the same time. Finger size, number and pitch can be defined by the design parameters of the DOE. It is possible to design DOEs for a smaller number of wide fingers or for a large number of narrow fingers as well as for the creation of bus bars. In combination with a green laser source with long pulses that reduce surface and bulk damage to a minimum, the technology for very fast laser doping of high quality solar cells is available. The beam characteristic of this laser, especially its small mode quality number M², is mandatory for combination with a DOE. Only in this combination can spots be created which are of the small dimension of 25 micrometers and simultaneously allow the processing of up to 156 lines at once. Another advantage is that no adjustment is requested. This is a crucial factor when looking at mass volume production.
There is immense potential in selective emitter doping for the increasing of cell efficiencies. Laser solutions enable the costs to remain low. With the developments happening at the moment, large scale production solutions for crystalline solar cells with selective laser doping is within the horizons.