Technical Papers

With current state-of-the-art PV module tests stipulating only a static mechanical load test in accordance with IEC 61215 and IEC 61646 standards, hardly any fatigue stressing is carried out on cells, cell connectors or rigid component parts such as the glass or framing. This paper presents research on dynamic load testing of PV modules and discusses reliability aspects of these essential requirements that must be considered in future standardization work.

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.

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.

This paper presents examples of recent process developments at ECN in silicon solar cells on n-type monocrystalline base material. For all PV manufacturers, the challenge is to increase module efficiencies while maintaining low production cost. An effective way to move to higher and more stable efficiencies, using low-cost industrial-type processing, is n-type solar cell technology. The solar cell considered in this paper is the n-pasha cell – a bifacial solar cell with homogeneous diffusions and screen-printed metallization. The n-pasha cell is currently produced on an industrial scale by Yingli Solar; in 2011 a maximum solar cell conversion efficiency of 19.97% was obtained using this cell concept on 239cm2 n-type Cz at the ECN laboratory. The focus of the paper will be increasing efficiency by optimization of the cell process, in particular the front-side metallization, and by improvements to the rear-surface passivation. These two steps have contributed an increase in efficiency of 0.8%, allowing cell efficiencies of 20% to be reached.

As a relative newcomer to the industrial world compared to more mature manufacturing sectors, the PV industry has not yet been subject to consistent environmental regulatory standards internationally. Like all industries that have preceded it, PV manufacturing is seeing its regulatory future evolve as PV producers migrate to different regions of the world. With this global expansion come significantly different levels of regulatory stringency, reflective of local conditions and cultures.

Non-destructive methods for measuring photovoltaic modules are discussed in this paper, with the aim of comparing different quality-assurance methods for different module technologies (e.g. crystalline and thin-film technologies: a-Si, CdTe, CIS). For a complete quality control of PV modules, a combination of fast and non-destructive methods was investigated. In particular, camera-based measurements, such as electroluminescence (EL) and infrared (IR) technologies, offer excellent possibilities for determining production failures or defects in solar modules, which cannot be detected by means of standard power measurements. These methods are applied effectively in quality control and development support, and EL is already an important characterization tool in industry and research. Most short circuits reduce the voltage in their surrounding area and appear dark in EL images. However, as this failure is not always critical and apparent, short circuits are only precisely identifiable in combination with IR measurements. Therefore, to quickly detect at high resolution the most common defects in a PV module, a combination of EL and IR measurements is advisable.

Most high-efficiency solar cells are fabricated from monocrystalline Czochralski silicon (Cz-Si) wafers because the material quality is higher than multicrystalline silicon (mc-Si) wafers. However, the material study presented in this paper reveals strong variations in the material quality of commercially available Cz-Si wafers, leading to a loss in solar cell efficiency of 4% absolute. The reason for this is the presence of defects, which appear as dark rings in photoluminescence (PL) images of the finished solar cells. It is shown that these efficiency-limiting defects originate from oxygen precipitation during emitter diffusion. It is demonstrated that an incoming inspection in the as-cut state is difficult, as strong ring structures in as-cut wafers turn out to originate most often from thermal donors. These are dissolved during high-temperature treatments and are therefore harmless, whereas moderate ring structures in the as-cut state may become severe. That is why critical wafers can be identified and sorted out reliably only after emitter diffusion, by using QSSPC-based lifetime measurements or PL imaging. The two-year statistics gathered from the research line at Fraunhofer ISE on the occurrence of ring defects in Cz-Si wafers indicate that ring defects are highly relevant in terms of material yield.

Advances in nanofabrication for enhancing the efficiency of optical devices, such as solar cells and photo-detectors, via nanostructuring have attracted a great deal of interest. A photoconversion strategy employing nanorods (NRs) has emerged as a powerful way of overcoming the limitations of planar wafer-based or thin-film solar cells. But there is also a broad spectrum of challenges to be tackled when it comes to putting into practice cost-effective NR solar cell concepts. ROD-SOL is a 10-partner, ‘nanotechnology for energy’ project with end-users, equipment manufacturers and institutes from six countries forming the consortium. The aim of the project is to provide the photovoltaic market with a highly efficient (> 10%), potentially low-cost, thin-film solar cell concept on glass, based on silicon nanorods. This paper presents the project’s achievements and discusses what the future might hold for nanotech-based solar energy production.

Today, crystalline-Si photovoltaics (PV) dominate the market, accounting for more than 85% of market share in 2010. A large scientific community made up of academic as well as industrial stakeholders strives to find solutions to improve device efficiencies and to drive down costs. One of the important cost elements of a module is the c-Si wafer itself. This paper discusses the fabrication of a carpet of c-Si foils on glass, either by layer transfer of an epitaxially-grown layer or by bonding of a very thin wafer, and processing this c-Si thin-foil device into a photovoltaic module. This could constitute an advantageous meet-in-the-middle strategy that benefits not only from c-Si material quality but also from thin-film processing developments.