A Review of Laser Processes Used in Solar Cell Fabrication
There are many different laser techniques that can be used in the production of solar cells. By examining the research which the various solar cells were fabricated with laser technique, it is possible to understand the pros and cons of using laser for produce the cells. The following paragraphs will list which process the laser technique have been used in this review.
The buried contact solar cell was invented at University of New South Wales by Green et al. in 1983. These solar cells have a relatively high efficiency approximately 25% and present a possibility of cost-reduction with applying this technology to the manufacturers’ production lines.
The following are the general main steps of forming the buried contact solar cell: 1. Texturing of surfaces
2. Top surface diffusion
3. Oxide growth
4. Groove cutting and diffusion
5. Aluminum deposition and sinter
6. Metal plating
7. Edge isolation
The key parts of this process, which result in the cells become more efficiency than the standard screen printing solar cells are the laser grooving and groove diffusion to reduce the cell shading and contact resistance and the texturing which reduce the surface reflection. A schematic of a buried contact solar cell is shown in the figure below (Green 1995). [pic]Figure 1: Cross-section of buried contact solar cell
Research continues working on the ways which could further improve the efficiency of the buried contact solar cell. With the statistics obtained from the experiment, they can try to figure out the effects which using different methods and materials in solar cells would cause. These parts include different diffusion profiles to form the p-n junction, surface passivation using different materials, and the different methods of grooving of the silicon, rear surface treatment, metallization and so on.
In order to reduce the reflection effect of the solar cells, front surface texturing is one solution. There have many methods to increase the light trapping, such as mechanical scribing and reactive ion etching. However, laser texturing could effectively texture the multicrystalline surface, providing isotropic etching that other techniques cannot do. Abbott and Cotter (2006) revealed that with deeper laser texturing, the less the front surface reflection is. More detailed results are shown in figure 2 (adapted from Abbott and Cotter 2006). Note that with very shallow texturing (10mm), they cannot trapping very well, as a result behaving like the planar one.
Figure 2: Front surface reflection of laser textured samples with different ablation pit depths (○) 10mm, (Δ) 20mm, () 30 mm, (*) 40 mm, (x) 50mm with residual slag, (+) planar silicon and (line) random pyramid textured silicon.
It is straightforward that we should texturing deeper pit, however, this will increase the surface recombination rate, which is detrimental to solar cells. Even though the pit depths 50mm have the lowest reflection, it will leave some slag in the pits that acting like defects. These residual slags will enhance the surface recombination rate, reducing the open-circuit voltage as well as the efficiency of the solar cells. Finding better parameters of operation to texture the wafer properly without the appearance of slag is therefore becomes the main issue for the manufacturers.
Top Surface diffusion
The conventional method for doping materials is the thermal diffusion which performed at high temperature (over 800℃). The process is so-called solid state diffusion and has various methods, for instance physical vapour deposition, to control the doping profiles.
Also there is a considerable alternative method of forming doping areas in silicon solar cells by using laser-doping. With the Nd:YAG pulsed laser, the doping profiles can be controlled with the desirable doping area. Ogane et al. (2009)...
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