PV Modules

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.

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 a comparison of different characterization methods used for determining the relative degree of cross-linking of samples of PV-type EVA films, obtained under three different process conditions in a vacuum PV laminator. The methods investigated are gel content measurements, rheological measurements and differential scanning calorimetry (DSC). For the latter, two distinct procedures are employed – the residual enthalpy method and the melt/freeze method.

As part of the European FP7 R&D project ‘Cu-PV’, the compatibility of copper-electroplated metal wrapthrough (MWT) cells with conductive adhesives has been investigated. The objectives of this project include to reduce, by the use of copper plating, the amount of silver utilized in cell manufacturing, and to demonstrate the compatibility of high-power n-type back-contact module technology with copper-plated cells. The overall goal is to reduce the impact on the environment of cell and module manufacture. MWT module technology as developed by ECN uses conductive adhesive to make the interconnection between cells and a conductive backsheet foil. These adhesives have been proved to result in very reliable modules in the case of cells with fired silver metallization. To determine the compatibility of conductive adhesive with copper-plated cells, component tests were performed, followed by the manufacture of modules with copperplated cells and conductive adhesive interconnections. Climate chamber testing of these modules showed that the adhesive is compatible with the copper-plated cells. The next steps include further optimization of the plating process and additional testing at the module level.

The aim of this paper is to shed some light on what difference the quality of a PV product makes to the customer and how much effort is required to deliver it. From the customer’s point of view, the quality of a PV product is key to a worthwhile investment, since the value of a PV system is defined by its cost compared with its yield over the entire lifetime, or the levelized cost of electricity (LCOE). But while many manufacturers make more or less the same promises, in this paper a closer look is taken at what is really involved in living up to those promises. If quality is understood to be a fundamental attitude that is reflected in every single process along the entire value chain, only then will this eventually lead to high-quality products and services. The paper discusses in detail the principles, methods, tests and processes required to secure a superior quality brand.

The potential for PV modules to fail before the end of their intended service life increases the perceived risk, and therefore the cost, of funding PV installations. While current IEC and UL certification testing standards for PV modules have helped to reduce the risk of early field (infant mortality) failures, they are a necessary, but not sufficient, part of determining PV module service life. The goal of the PV Durability Initiative is to establish a baseline PV durability assessment programme. PV modules are rated according to their likelihood of performing reliably over their expected service life. Modules are subjected to accelerated stress testing intended to reach the wear-out regime for a given set of environmental conditions. In parallel with the accelerated tests, modules are subjected to long-term outdoor exposure; the correlation between the accelerated tests and actual operation in the field is an ultimate goal of the programme. As understanding of PV module durability grows, the test protocols will be revised as necessary. The regular publication of durability ratings for leading PV modules will enable PV system developers and financiers to make informed deployment decisions.

The rapid growth of the PV market during the last five to seven years entailed a considerable expansion of the encapsulation material market, which temporarily led to shortages in the supply chain. Simultaneously, module prices decreased significantly, which resulted in intense pressure on production costs and the cost of PV module components, inducing changes in the encapsulation material market towards new materials and suppliers. This pressure – together with the huge impact of the encapsulation material on module efficiency, stability and reliability – makes the selection of encapsulation technologies and materials a very important and critical decision in the module design process. This paper presents an overview of the different materials currently on the market, the general requirements of PV module encapsulation materials, and the interactions of these materials with other module components.

Electroluminescence (EL) imaging for photovoltaic applications has been widely discussed over the last few years. This paper presents the results of a thorough evaluation of this technique in regard to defect detection in photovoltaic modules, as well as for quality assessment. The ability of an EL system to detect failures and deficiencies in both crystalline Si and thin-film PV modules (CdTe and CIGS) is thoroughly analyzed, and a comprehensive catalogue of defects is established. For crystalline silicon devices, cell breakages resulting from micro-cracks were shown to pose the main problem and to significantly affect the module performance. A linear correlation between the size of the breakages and the power drop in the module was established. Moreover, mechanical stress and temperature change were identified as the major causes of the proliferation of cracks and breakages. For thin-film modules, EL imaging proved the existence of an impressive reduction in the size of localized shunts under the effect of light-soaking (together with a performance improvement of up to 8%). Aside from that, the system voltage was applied in order to monitor transparent conductive oxide (TCO) corrosion effects and laser-scribing-induced failures, as well as several problems related to the module junction box in respect of its sealing and the quality of its electric connectors.

Because potential-induced degradation (PID) can cause power losses of more than 30% for modules out in the field, there has already been an extensive effort placed on avoiding this adverse phenomenon. A key feature at the cell level is the silicon nitride (SiNx) anti-reflective coating (ARC). Apart from the known dependency of PID susceptibility on the refractive index, the impact of the deposition parameters has also been under investigation. This paper illustrates the influence of different silicon nitrite layers and their ability to prevent PID. A large number of cells and modules were therefore manufactured, differing only in the type of ARC. The modules were subsequently PID tested under three different climatic conditions, and acceleration factors and activation energies were determined from these tests. In addition this paper presents the results of addressing the weak-light performance and the hot-spot risk of panels after PID exposure. Finally, the reversibility of PID was also investigated in relation to the state of degradation of these samples.

There has been recent interest in the use of thermoplastic encapsulant materials in photovoltaic modules to replace chemically cross-linked materials, for example ethylene-vinyl acetate. The related motivations include the desire to reduce lamination time or temperature, to use less moisture-permeable materials, and to use materials with better corrosion characteristics or improved electrical resistance. However, the use of any thermoplastic material in a hightemperature environment raises safety and performance concerns, as the standardized tests do not currently include exposure of the modules to temperatures in excess of 85°C, even though fielded modules may experience temperatures above 100°C. Eight pairs of crystalline silicon modules and eight pairs of glass/encapsulation/glass thin-film mock modules were constructed using different encapsulant materials, of which only two were designed to chemically cross-link. One module set with insulation on the back side was exposed outdoors in Arizona in the summer, and an identical set was exposed in environmental chambers. High-precision creep measurements (±20μm) and performance measurements indicated that, despite many of these polymeric materials being in the melt state during outdoor deployment, there was very little creep because of the high viscosity of the materials, the temperature heterogeneity across the modules, and the formation of chemical cross-links in many of the encapsulants as they aged. In the case of the crystalline silicon modules, the physical restraint of the backsheet reduced the creep further.