Wet chemical process equipment is widely used in industrial solar cell production, and inline etching systems in particular have attracted more and more attention since their introduction 10 years ago. The horizontal wafer transport within these systems has made it possible to think about single-side wafer treatments even for wet chemical process applications. Since its market introduction in 2004, the chemical edge isolation process based on the single-side removal of the parasitic emitter at the rear side of the solar cells has gained an increasing share of the market in comparison to competing technologies that use laser techniques. However, stabilization and control of such a process under mass production conditions remains challenging. The introduction of new high-efficiency cell concepts involving passivated rear sides will increase the importance of single-side wafer treatments, as the final solar cell performance is significantly affected not only by the complete removal of the parasitic emitter but also by an ideally polished surface on the rear side of the wafer.
This paper presents Calyxo’s recent advances in product design that have resulted in independently confirmed peak aperture-area efficiencies of 13.4% for modules and 16.2% for cells. Some insight is given into a suitable product design for achieving the highest reliability possible, even in hot climates such as Australia, with no signs of degradation during the first three years of deployment in the field. These technical advances and the midterm production-cost target of US$0.50/Wp allow a forecast levelized cost of electricity (LCOE) of under US$0.10/KWh, especially in sunny regions of the world.
A typical financial structure for a utility-scale (i.e. larger than a few MW) PV project is the so-called ‘non-recourse project financing’. Experience shows that lenders may occasionally refuse financing because they dislike a technology or even a certain supplier. This past behaviour has created the ‘myth of bankability’ and the perceived necessity of manufacturers to get onto the banks’ ‘bankability lists’. But there is no strictly defined process for doing this, and many of the experienced banks do not even work with such lists for good reason. Moreover, ‘bankability’ is not a feature that a manufacturer or a product can achieve or maintain forever.
Unidirectional solidification of large Si ingots from the melt phase is currently one of the most important technologies for producing mc-Si for PV cells. Si ingot furnaces began from casting equipment, and have been improved by DSS (directional solidification system) or DSS-like methods. To improve PV cell efficiency and reduce costs, intensive development has focused on increasing a single ingot’s volume, reducing impurities and controlling the growth speed and temperature gradient. One of the latest developments of Si ingot furnaces is mono-like crystalline silicon growth using a seed preservation method and more accurate control. The Si ingot furnaces are optimized with precise control of temperature gradients and growth speed for the formation of a large unit of quasi-monocrystalline Si. This optimization can further improve a PV cell’s efficiency by at least 1%. In order to obtain fundamental knowledge about the key process steps that determine the growth and electrical quality of mc-Si via directional solidification in an ingot furnace, a combined modelling-measuring approach is essential. Moreover, a mathematical model of the Si ingot casting process can be used for model-based process control.
Solar photovoltaic (PV) electricity continued its remarkable growth trend in 2011, even in the midst of a financial and economic crisis and despite the PV industry going through a difficult period. Once again PV markets grew faster than anyone had expected, just as they have done for the past decade, especially in Europe but also around the world. While such a rapid growth rate cannot be expected to last forever in Europe, prospects for growth around the world remain high. The results of 2011 – and indeed the outlook for the next several years – show that under the right policy conditions, PV can continue its progress towards competitiveness in key electricity markets and be a mainstream energy source. The major system-price decrease that was experienced in 2011, combined with measures taken in Germany and Italy after the Fukushima nuclear disaster, allowed the market to further develop in 2011, particularly in these two countries. However, the price decrease also helped weaken the policy support in many countries, with policymakers facing growing discontent with regard to the perceived cost of PV and the ailing PV industry in Europe.
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.
