This paper describes the degradation of sputtered aluminium-doped zinc oxide (ZnO:Al) layers which were exposed to damp heat (85°C/85% relative humidity). The ZnO:Al samples were characterized by electrical, compositional and optical measurements before, during and after damp heat exposure. Hall measurements showed that the carrier concentration stayed constant, while the mobility decreased and the overall resistivity thus increased. This mobility decrease can be explained by the enhancement of the potential barriers at the grain boundaries because of the occurrence of additional electron-trapping sites. X-ray diffraction (XRD) and optical measurements demonstrated that the crystal structure and transmission in the range 300 –1100nm did not change, thereby confirming that the bulk structure stayed constant. Depth profiling showed that the increase of the potential barriers was caused by the diffusion of H2O/OH- through the grain boundaries, leading to adsorption of these species or to the formation of Zn(OH)2 or similar species. Depth profiling also revealed the presence of carbon, chloride and sulphide in the top layer, which indicates the possible presence of Zn5(CO3)2(OH)6, Zn5(OH)8Cl2•H2O and Zn4SO4(OH)6•nH2O. Furthermore, white spots appeared on the ZnO:Al surface during damp heat exposure. The spots contained elements, such as silicon and calcium, which might have migrated from the glass and which reacted with species from the environment, including oxygen, carbon and chlorine.
The SPF solar glass certification was developed in 2002 to guarantee the quality of glazing for use as a transparent cover for solar thermal collectors. More than 200 glass types from leading manufacturers have been measured and certified to date. Despite the certification having been explicitly developed for solar thermal applications, it became widely used in the PV module industry, even though the results are not transferable and may lead to erroneous conclusions in some cases. In 2012 the certification was therefore adapted to the needs of the PV industry, and a dedicated PV solar glass certification has since been available. This paper explains the fundamentals of the certification process, which consists of three performance characterizations: 1) transmissivity, 2) incident angle modifier (IAM), and 3) UV degradation. Results are discussed for different representative glass types, including float glass, anti-reflective-coated glass and rolled glass with different structures. Furthermore, the performance of these glass types when used as covers of crystalline silicon PV
modules is compared. The examples presented also highlight the advantages of the adapted characterization methods compared with standard glass measurements.
Although considerable progress has been made in reducing the amount of Ag required per wafer in the classic screen-printing metallization of Si solar cells, the total cost of ownership of the metallization process today accounts for more than 50% of the total cell-process-related cost. There has been pressure on cell and module manufacturers to further reduce this cost, by either improving the metallization process or applying alternative contacting technologies. In this paper, the classic screen printing of standard Si-based solar cells, which has been the main metallization technique for many years, is described in detail. The required paste volume for providing the contacts in a state-of-the-art cell production process is calculated
on the basis of the contact dimensions (fingers and busbars on the front, Al layer and Al/Ag pads on the back). Taking into account today's paste prices, equipment investment, screen cost, energy, maintenance, yield, material utilization and necessary labour, the total cost of ownership of the cell metallization is also determined. The main cost drivers are discussed in detail. The cost reduction is estimated when improved printing processes – such as double, dual or stencil printing – are employed. Other promising alternative front-contact metallization technologies are listed and their potential is briefly discussed. To evaluate the competitiveness of these technologies, the limit of today's screen-printing method and its further cost reduction potential are estimated on the basis of the physical properties of cells and printing pastes.
The investment case for the establishment of PV manufacturing hubs in emerging regions became bleak as c-Si PV manufacturing capacities in China ballooned from 2004 to 2011/12. The resulting supply overhang, with dramatic price decreases throughout the PV value chain, led to severe margin compressions and ultimately to closures, insolvencies and postponement of expansion plans by incumbents across the board. A common misperception by private and public decision makers alike – reflected in the recent escalation in global trade disputes – is that products made outside China are, per se, not competitive. In contrast to this mind-set, and on the basis of experience in numerous development projects, the author argues that new entrants have multiple instruments available that can make local PV manufacturing plants commercially viable in many regions of the world.
Ethylene vinyl acetate (EVA) is still the dominant material used for encapsulation of solar cells. During PV module lamination, a three-dimensional network is formed by a chemical cross-linking of the polymer chains in order to increase the thermal stability of the material and to prevent the material from melting when exposed to application-relevant temperatures of up to 100°C. The cross-linking reaction, which is discontinuous and can take up to 30 min (depending on the EVA type), is the time-determining step in PV module lamination. The main objective of this paper is to gain a comprehensive understanding of the thermomechanical material behaviour during the PV module lamination process, and to develop a basis for the optimization of the PV module manufacturing process. The results presented will demonstrate that dynamic mechanical analysis (DMA) is a valuable and reliable characterization method for the investigation of the curing behaviour of EVA for solar cell encapsulation. DMA in shear mode allows a continuous measurement of the thermomechanical properties, even in the molten state, and therefore an in situ monitoring of the cross-linking reaction. Whereas it is possible to use temperature scans on partially cured EVA films to determine the state of cross-linking, isothermal scans on uncured samples allow the curing kinetics of EVA to be investigated. On the basis of an enhanced knowledge of the cross-linking reaction, the material-related process-parameter optimization potential of the PV module lamination process can be identified, and optimum processing temperature ranges and minimum cross-linking times can be derived.
After several years of crisis, the PV manufacturing industry is expected to pick up again from 2014 onwards, and cell and module producers will consequently expand their production capacities in the coming years. To obtain high margins, producers must introduce new products that are better performing in terms of electrical performance and lifetime, even under harsh climatic conditions (e.g. in desert regions). This requires the use of innovative technologies that not only allow low production costs (US$/Wp), but also guarantee at the same time high module efficiencies and – even more importantly – high energy yields in terms of kWh over the entire lifetime of the system. This means that the most promising advanced cell concepts will use a limited number of standard industrial process steps and proven standard equipment. For at least the next five (probably more) years, high efficiency (>20%) at a reasonable cost will still be achieved with crystalline silicon-based technology alone. The research and development at ISC Konstanz therefore concentrates mainly on cell concepts that can be implemented using standard tube furnace diffusions and screen-printed metallization, with a focus on n-type-based technologies. This paper gives an overview of ISC Konstanz's technology zoo, including BiSoN, PELICAN and ZEBRA cell concepts, which are ready for industrial implementation. In addition, the integration of these innovative cells into modules, along with the importance of various features – such as bifaciality – in increasing the energy yield, is discussed.
Over the last few years several technologies have been investigated with the aim of reducing recombination in emitters and at passivated surfaces. Because of its high efficiency potential, the passivated emitter and rear cell (PERC) design is of interest to both cell manufacturers and R&D institutes all over the world. Another cell design of interest is the metal wrap-through (MWT) solar cell, where the absence of front busbars leads to reduced shading. The MWT technology, especially when combined with rear-surface passivation, has the potential to significantly decrease the cost of ownership of today's solar cells. This paper gives an overview of the current status of the production technology for the fabrication of PERC and MWTPERC solar cells, as well as a summary of recently published papers in this field.
Encapsulants play a crucial role in ensuring the long-term stability of the power output of PV modules. For many years the most popular encapsulation material for crystalline silicon modules has been ethylene vinyl acetate (EVA), which leads the market because of its cost-effectiveness. Innovations in crystalline silicon cell and module technology, however, have changed the requirements that the encapsulants have to meet. A wide range of other encapsulation materials is also available; such alternatives offer improved outdoor
stability and gains in module performance. Furthermore, innovative module concepts that have new sets of requirements are under development. One attractive module concept in particular envisages the attachment of pieces of crystalline Si to the large module glass at an early stage, followed by the processing of the Si cell and the series interconnection at the module level using known processes from thin-film photovoltaics. This so-called thin-film/wafer hybrid silicon (HySi) approach relies heavily on module-level processing of Si solar cells, and is a new field of research. This paper discusses the applicability of silicone encapsulants for
module-level processing and compares their requirements with those of conventional EVA.
This paper presents the results of a study of the influence of silver powder particle size and inorganic additives on sintering and electrical performance of a PV front-side metallization paste. Three different silver powder grain sizes were used in sample front-side pastes. Also examined is the effect of using four different inorganic additives determined by their redox potential. Solar cells produced using the sample pastes were electrically characterized, and selective etch-backs and FESEM investigations were performed to correlate electrical
performance with the glassy interface between the metallization and the silicon wafer. In the absence of additives, the highest efficiencies were obtained with the medium silver grain size. If the inorganic species has an oxidizing nature, the mass transport of silver in the glass phase can be enhanced. However, the etch process at the wafer surface is also improved by a greater quantity of silver oxide in the flowing glass. It is shown that if the oxidizing capacity of the additive is too powerful, the electrical performance is negatively influenced.
Moreover, the impact of additives is highly dependent on the silver particle size.
A critical failure mechanism of PV modules is the degradation in performance as a result of exposure to temperature and humidity during a typical product lifetime of over 25 years. The time to failure of a PV module attributable to moisture ingress under given field conditions involves multiple factors, including encapsulant and edge seal moisture barrier performance as well as the degradation rate of particular solar cells when exposed to moisture. The aim of the work presented here is to establish a conservative estimate of field lifetime by examining the time to breakthrough of moisture across the edge seal. Establishing a lifetime model for the edge seal independent of the characteristics of the encapsulant and solar cells facilitates the design optimization of the cells and encapsulant. For the accelerated testing of edge seal materials in standard temperature- and humidity-controlled chambers, a novel test configuration is proposed that is amenable to varying dimensions of the edge seal and is decoupled from encapsulated components. A theoretical framework that accounts for the presence of desiccants is developed for analyzing the moisture ingress performance of the edge seal. Also developed is an approach to analyzing test data from accelerated testing which incorporates temperature dependence of the material properties of the edge seal. The proposed equations and functional forms have been validated by demonstrating fits to experimental test data. These functional forms and equations allow the prediction of edge seal performance in field conditions characterized by historical meteorological data. In the specific case of the edge seal used in certain MiaSolé glass–glass modules, this work has confirmed that the edge seal can prevent moisture ingress well beyond the intended service lifetime in the most aggressive climate conditions evaluated.
