Texturization of (100) monocrystalline silicon (mono-Si) for solar cells is still an issue in the industrial production of standard screen-printed mono-Si solar cells. This fact is due to the properties of isopropyl alcohol (IPA), which is used together with potassium hydroxide (KOH) in the standard etching solution KOH-IPA (or used with sodium hydroxide NaOH in NaOH-IPA). The low boiling point of IPA (82.4°C) limits the etching temperature and thus the processing speed. Furthermore, KOH-IPA etching solution is very sensitive to the wafer pre-treatment characteristics of as-cut mono-Si wafers. Two ways to overcome these disadvantages are presented in this paper. The first approach involves the use of a high boiling alcohol (HBA) instead of IPA in the standard KOH-IPA etching solution. This allows higher etching temperatures to be used, without evaporation losses of the alcohol, but with reduced etching times. The second approach consists of using a closed etching bath in which vacuum (low-pressure) steps (i.e. pressure oscillations between atmospheric and below-atmospheric pressure) are achievable; in addition, a cooling system located on top of the etching bath allows the liquefaction of the evaporated IPA. The second texturing approach considerably decreases the etching time of mono-Si wafers. Examples of mono-Si wafers were textured using the new KOH-HBA etching solution and then processed into solar cells; the current-voltage results of the processed solar cells are presented.
Solar-grade silicon (SoG-Si) based on metallurgical refining processes, often called upgraded metallurgical-grade silicon (UMG-Si), is expected to play an important role in achieving the solar industry’s necessary cost targets per Wp in order to compete with other energy sources. The broad term ‘UMG-Si’ currently embraces types of silicon feedstock that differ quite substantially in product quality and performance. This paper presents a summary of the work carried out by Elkem on low-cost production of silicon feedstock via a flexible, recycling metallurgical processing route with the lowest carbon footprint on the market. Results are given that qualify Elkem Solar Silicon® (ESS™) as a SoG-Si, with comparable efficiencies to polysilicon (poly-Si) from the traditional Siemens process. The latest results on the performance of modules based on ESS are reported. An indication of the stability of older modules based on SoG-Si feedstock from Elkem is also considered. On the basis of the results, there is no reason to expect modules based on ESS to differ from other commercial modules based on poly-Si. ESS is therefore shown to be a viable alternative to conventional poly-Si, but with the additional benefit of lowering specific energy use and cost per Wp.
Several PV module producers have performed a carbon footprint analysis and published a sustainability report as part of their corporate social responsibility policy. Comparison of carbon footprint results is difficult because several international standards and life cycle assessment (LCA) databases are used. No product footprint category rules (PFCR) or product category rules (PCRs) for photovoltaics exist, so LCAs are performed with varying underlying assumptions. Furthermore, a fair comparison can only be made when all environmental footprints of a product are taken into account.
In the photovoltaics industry, contacts to crystalline silicon are typically formed by the firing of screen-printed metal pastes. However, the stability of dielectric surface passivation layers during the high-temperature contact formation has turned out to be a major challenge for some of the best passivating layers, such as intrinsic amorphous silicon. Capping of well-passivating dielectric layers by hydrogen-rich silicon nitride (SiNx), however, has been demonstrated to improve the thermal stability, an effect which can be attributed to the atomic hydrogen (H) diffusing out of the interface during firing, and passivating dangling bonds. This paper presents the results of investigations into the influence of two different dielectric passivation stacks on the firing stability, namely SiNy/SiNx (y < x) and Al2O3/SiNx stacks. Excellent firing stability was demonstrated for both stack systems. Effective surface recombination velocities of < 10cm/s were measured after a conventional co firing process on 1.5Ωcm p-type float-zone silicon wafers for both passivation schemes. On the solar cell level, however, better results were obtained using the Al2O3/SiNx stack, where an efficiency of 19.5% was achieved for a large-area screen-printed solar cell fabricated on conventional Czochralski-grown silicon.
The solar photovoltaics market in the United Kingdom was virtually non-existent until April 2010, when the long-awaited feed-in tariff scheme was implemented. Yet, despite coming late to the game, the UK’s solar industry took off immediately, installing more than 80MW in the first 12 months alone. Now, just two years down the line, the market is placed as the world’s eighth largest. This paper will take a look back at how the UK got to this point as well as considering just how bright the future of this fast-paced market will realistically be.
Predicting what will happen to the global PV market is very nearly an impossible task. Its underlying principles are very similar to the dozens of other electronics markets that IMS Research studies, but the key difference in the PV industry is the very close link to, and ultimate dependence on, government policy. In a few years’ time, the introduction, halting or change (or rumoured change) of a single government’s PV policy will have little effect on the global industry, and the huge swings in demand will be less common and less severe. The reasons for this are clear. First, because of geographic diversification in the industry, a single country will account for a smaller portion of the global total (unlike in 2011, when Germany and Italy accounted for more than half of global demand) and thus individual governments’ policy changes will have a smaller impact. Second, if system prices continue to drop rapidly (and IMS Research believes they will), a growing number of regions will achieve the ‘holy grail’ of grid parity and will thus no longer depend solely on government policy to drive their markets.
The purpose of this paper is to give an overview of the use and potential of diamond wire for the silicon-shaping process in the PV industry. The current market and future prospects for helping to meet the goals of 2020’s roadmap of thinner wafers and reduced $/W are described.
The 15th edition of Photovoltaics International hopes to bring some optimism to 2012 by tackling the key factors on how to cope with the current situation. Contributions come from MIT on using TCAD as a viable method for modelling metal impurity evolution; Alternative Energy Investing presents a comprehensive look at materials cost; and efficiency improvements are on offer by ECN. REC Solar and Tata BP Solar report on module lifetime and IMS Research gives us a brief rundown of 2012 global market expectations.
Crystalline silicon solar modules installed in the field are exposed to atmospheric conditions and experience stress, which induces a wear-out phenomenon in various parts of the modules and degrades performance over time. The performance eventually reaches a point where the output power falls below an acceptable level. Thermal cycling (TC) and damp heat (DH) are two important reliability tests for estimating infant failures related to materials and the manufacturing process, as well as providing the information on performance degradation with respect to time. In this study, modules composed of 156mm × 156mm multicrystalline silicon cells were subjected to TC and DH tests. By applying acceleration models, such as the Norris-Landzberg model for TC and the Hallberg-Peck model for DH, the minimum guaranteed life was calculated. The electrical and reliability results were interpreted and explained on the basis of the respective models.
Coupled device and process simulation tools, collectively known as technology computer-aided design (TCAD), have been used in the integrated circuit industry for over 30 years. These tools allow researchers to quickly home in on optimized device designs and manufacturing processes with minimal experimental expenditures.
The PV industry has been slower to adopt these tools, but is quickly developing competency in using them. This paper introduces a predictive defect engineering paradigm and simulation tool, while demonstrating its effectiveness at increasing the performance and throughput of current industrial processes. The impurity-to-efficiency (I2E) simulator is a coupled process and device simulation tool that links wafer material purity, processing parameters and cell design to device performance. The tool has been validated with experimental data and used successfully with partners in industry. The simulator has also been deployed in a free web-accessible applet, which is available for use by the industrial and academic communities.
