A Review of Laser Processes Used in Solar Cell Fabrication

A Review of Laser Processes Used in solar kiosk Fabrication Abstract There atomic number 18 many different optical maser techniques that can be used in the production of solar electric cellular telephoneph mavens. By examining the research which the various solar cells were fictive with optical maser technique, it is possible to see the pros and cons of using optical maser for produce the cells. The following paragraphs lead list which exploit the laser technique have been used in this review. IntroductionThe interred contact solar cell was invented at University of New S come onh Wales by Green et al. in 1983. These solar cells have a relatively naughty faculty almost 25% and present a possibility of cost-reduction with applying this use science to the manufacturers production lines. The following are the general main steps of forming the hide contact solar cell 1. Texturing of straightens 2. Top come near dispersion 3. Oxide offshoot 4. Groove cutting and d iffusion 5. Aluminum deposition and sinter 6. metallic element plating 7. Edge isolationThe key parts of this bear upon, which result in the cells become more ability than the standard screen printing solar cells are the laser grooving and groove diffusion to humiliate the cell shading and contact resistance and the texturing which reduce the surface demonstration. A established of a buried contact solar cell is shown in the approach pattern below (Green 1995). picFigure 1 Cross-section of buried contact solar cell Research get acrosss working on the ways which could further improve the expertness of the buried contact solar cell.With the statistics obtained from the experiment, they can try to augur out the effects which using different methods and materials in solar cells would cause. These parts intromit different diffusion profiles to form the p-n junction, surface passivation using different materials, and the different methods of grooving of the silicon, rear surfac e treatment, metallization and so on. Laser texturing In mark of battle to reduce the reflection effect of the solar cells, move 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 cottier (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 unsmooth samples with different ablation pit reasonablenesss (0) 10mm, (? 20mm, (? ) 30 mm, (*) 40 mm, (x) 50mm with residual slag, (+) planar silicon and (line) random pyramid textured silicon. It is straightforward t hat we should texturing deeper pit, however, this will increase the surface recombination rate, which is detrimental to solar cells. up to now though the pit depths 50mm have the lowest reflection, it will leave virtually slag in the pits that acting like defects. These residual slags will elevate 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 demeanor 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 sensual vapour deposition, to control the doping profiles. Also there is a considerable choice method of forming doping areas in silicon solar cells by using laser-doping.With the NdYAG pulsed lase r, 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 summate 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 comely 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 exercise of thermal diffusion (TD).Laser groove The most common device which is used to form the laser grooved front surface contacts is the NdYAG 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 deadeningd 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. pic Figur e 4 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 write down cost of the IR laser is more affirmatory 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 engine room which usually used for rear contact passivation in solar cells is that highly doped region near the clog uprest 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 foregoing 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 lavish back surface field (16. %). Alternatively, aluminium deposition can be applied to the solar cell with the passivated emitter and rear contact, followed by fast see 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) exhibit 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 imputable 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 possess a passivating layer and depositing the aluminium, there is only one step require (laser firing) instead of several processes of photolithography.Hence by using NdYAG 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. unofficial 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 f ield 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 chinchy as possible so that public are willing to having it, hence minimizing the environment impact. Reference Abbott, M. and Cotter, J. 006, Optical and electrical properties of laser texturing for high-efficiency solar cells, senesce in Photovoltaics Research and Applications, b ook 14, Issue 3, pp. 225-235. Honsberg, C. and Bowden, S. 2009, Buried Contact Solar Cells, Photovoltaics CDROM, accessed 12 September 2009, . 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 susceptibility Conversion, conference Record of the 2006 IEEE fourth World Conference on, Volume 2, pp. 1338-1341. NdYAG laser, wiki article, 3 July 2009, accessed 14 September 2009, http//en. wikipedia. org/wiki/NdYAG_laser. Ogane, A. Hirata, K. Horiuchi, K. Nishihara, Y. Takahashi, Y. Kitiyanan, A. and Fuyuki, T. 2009, Laser-doping technique using ultraviolet light laser for shallow doping in crystalline silicon solar cell fabrication, Japanese Journal of Applied Physics, Volume 48, Issue 7, pp. 071201. Schneiderlochner, E. Preu, R. Ludemann, R. and Glunz, S. W. 2002, Laser-fired rear contacts for crystall ine silicon solar cells, Progress in Photovoltaics Research and Applications, Volume 10, Issue 1, pp. 9-34. Schneiderlochner, E. Grohe, A. Glunz, S. W. Preu, R. & Willeke, W. 2003, Scanning NdYAG laser system for industrially applicable processing in silicon solar cell manufacturing, Photovoltaic Energy Conversion, 2003. Proceedings of 3rd World Conference on, Volume 2, pp 1364-1367. Schoonderbeek, A. Stute, U. Ostendorf, A. Grischke, R. Engelhart, P. Meyer, R. and Brendel, R. 2007, Laser technology in silicon solar cell production, Proceedings of the 4th International WLT-Conference on Lasers in Manufacturing, pp 693698. pic