A Review of Laser Processes Used in Solar Cell Fabrication

 Abstract

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. Introduction

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:

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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. Laser texturing 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 (0) 10mm, (? 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) asserted that by using the laser doping technique with 0. 5W output, it can fabricate the solar cells with comparable efficiency to those fabricated by thermal diffusion methods. Some statistics of the result from their research are shown in figure 3 (adapted from Ogane et al. 2009). Note that a large number of the oxygen atoms are induced to the solar cells owing to the operation is in air. This increase the sheet resistance and as a result, reducing the efficiency of the solar cells.

The advantages of the laser doping process are that it can be operated at room temperature, in the atmosphere, as well as the easier process forming a selective doping area without any lithography pre-set processes. With these advantages, even though the properties of these solar cells are slightly worse than the cells handled by thermal diffusion, it is acceptable to some manufacturers and worthy to work on it. [pic] Figure 3: The average of six solar cells properties of cells with emitters fabricated by laser doping as a function of laser output power, compared with the case of thermal diffusion (TD).

Laser groove. The most common device which is used to form the laser grooved front surface contacts is the Nd:YAG pulsed laser, operating at a high frequency. The depth of the laser groove after lasing is typically 30 microns deep and approximately 20 microns wide. During the process, laser will vaporises silicon through the oxidation layer, and this will induced the thermally damaged to silicon. Such this damage will acts as recombination sites of carriers when operating the solar cells, minimised this defect is therefore necessary in order to improve the efficiency of the solar cells.

Studies on this part (Schoonderbeek et al. 2007) have revealed that using the shorter wavelength of the laser, i. e. the higher power output can reduce the thermal damage induce by the laser.  Increased lifetime ratios (decreased laser damage) are obtained when scribing lines using short-wavelength lasers. Figure 4 (adapted from Schoonderbeek et al. 2007) above shows that by using lasers with short-wavelength at either 532nm (green) or 355nm (UV) will have better properties in comparison to 1064nm (IR) laser. Nevertheless, the lower cost of the IR laser is more favourable to the manufacturer.

Thus the solar cell research of this part is undertaken at institutions and companies in order to find out the better trade-off point. Back surface field and laser-fired contacts The technology which usually used for rear contact passivation in solar cells is that highly doped region near the back contact. This is called back surface field. Normally aluminium is applied to it by evaporation, following by sintering to make aluminium atoms diffuse into the silicon. For reducing the cost per watts generated by solar cell, some different methods were tested for the feasibility analysis.

One of the ways for reducing the cost is doping aluminium locally (Meemongkolkiat et al. 2006). It is obviously that doping less metal could reduce the cost, however the efficiency should not be diminish too much. In this process the oxide on the back surface is partially etched prior to aluminium were screen-printed on it. In their research, with choosing the aluminium screen-printing pastes properly (the composition is not provided in their paper), the efficiency of the solar cells are adequately (16%) compared to the normally full back surface field (16. %). Alternatively, aluminium deposition can be applied to the solar cell with the passivated emitter and rear contact, followed by fast scanning laser firing to reduce the rear surface recombination rate as well as improve light trapping. With this so-called laser-fired contact technique, Schneiderlochner et al. (2002, p. 32) demonstrated that the efficiency of the laser-fired contact solar cell can reach up to 21. 3%. Some results from their research can be seen in figure 5, which are adapted from Schneiderlochner et al. 002. Note that the silicon nitride passivated layer has a lower efficiency maybe due to degradation in passivation quality during laser firing. [pic] Figure 5: The solar cell results with the laser-fired contact technique compared with conventionally processed passivated emitter and rear cells The main advantage of the laser-fired contact is that after possessed a passivating layer and depositing the aluminium, there is only one step needed (laser firing) instead of several processes of photolithography.

Hence by using Nd:YAG pulsed laser system for laser-fired contact processing, the cost per watts can be reduced due to less handling steps, as well as the expensive chemicals. Summary There are some different techniques with respect to laser that applied to the fabrication of the solar cells are discussed above. With the laser system applied to the back surface field and laser-fired contacts, the cost can be reduced by using less costly chemicals, whereas shrinking the cost of instruments and operation for the case that laser system induced to the top surface diffusion.

The aims of both grooving and texturing using laser technique are improving the properties of the solar cells. The laser system provides the possibilities of reducing the cost per watts without decreasing too many characteristics of the solar cells, which is the most important issue for manufacturers. Therefore, studies focus on these techniques will continue while some of them are already used in fabricating solar cells with other techniques. The ultimate aim of these is make the solar cells as cheap as possible so that public are willing to having it, thus minimizing the environment impact.

Reference

1. Abbott, M. and Cotter, J. 006, ‘Optical and electrical properties of laser texturing for high-efficiency solar cells’, Progress in Photovoltaics: Research and Applications, Volume 14, Issue 3, pp. 225-235.
2. Honsberg, C. and Bowden, S. 2009, Buried Contact Solar Cells, Photovoltaics CDROM, accessed 12 September 2009, .
3. Meemongkolkiat, V. ; Nakayashiki, K. ; Dong Seop, K. ; Kim, S. ; Shaikh A. ; Kuebelbeck, A. ; Stockum, W. and Rohatgi, A. 2006, ‘Investigation of modified screen-printing Al pastes for local back surface field formation’, Photovoltaic Energy Conversion, Conference Record of the 2006 IEEE 4th World Conference on, Volume 2, pp. 1338-1341.
4. Nd:YAG laser’, wiki article, 3 July 2009, accessed 14 September 2009, < http://en. wikipedia. org/wiki/Nd:YAG_laser>. Ogane, A. ; Hirata, K. ; Horiuchi, K. ; Nishihara, Y. ; Takahashi, Y. ; Kitiyanan, A. and Fuyuki, T. 2009, ‘Laser-doping technique using ultraviolet laser for shallow doping in crystalline silicon solar cell fabrication’, Japanese Journal of Applied Physics, Volume 48, Issue 7, pp. 071201. Schneiderlochner, E. ;

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