Technical Papers

The main objective of this study was to evaluate the suitability of thermal analysis for characterizing the stage of hydrolytic degradation of PV backsheets containing polyethylene terephthalate (PET) as a core layer. Additionally, the ageing behaviour of single backsheets was compared with that of backsheets incorporated within PV modules. Test modules using identical components (glass, encapsulant, solar cells, etc.), varying only in the type of backsheet used, were fabricated and artificially aged (damp heat:85°C / 85% relative humidity storage up to 2000h). The material characteristics of the single backsheets and module-incorporated backsheets before and after artificial ageing were determined by thermal analysis. It was shown that the most significant changes between unaged and aged sheets can be observed in the cooling curve of the differential scanning calorimetry (DSC) runs. For all materials, a significant increase in the crystallization temperature was found. Furthermore, the results revealed no influence of the PV module lamination procedure on the thermal characteristics of the polymeric backsheets. Even after artificial ageing under damp-heat conditions, differences between single and module-incorporated laminated backsheets were negligible. The ageinginduced degradation effects of PET could be detected by DSC for all the aged sheets. It is therefore proposed that the testing of single PET-based backsheets under accelerated ageing conditions may be a practicable way to investigate the applicability of a new backsheet material for use in reliable PV modules.

Actual solar cells are large-area, two-dimensional (2D) devices with lateral variations in internal voltage, but most of the time they are represented by simplistic equivalent circuits consisting of a few lumped elements. Griddler© is a finite-element-method (FEM) simulator that constructs and solves the full 2D distributed network representation of a metallized solar cell. Not only is this approach far more versatile and adaptable to real-world problems, accurate in predicting subtle device characteristics, and compatible with mapping data, but it can also be implemented in a way that is as easy and quick to use as a handy calculator. This paper covers a broad range of applications related to full-area 2D modelling and introduces Griddler 1.0 - a compact freeware computer program that places much of that power at the fingertips of any solar cell engineer with a PC.

Solar cell performance depends on material quality, as well as on the architecture of the cell. In the search for higher-performing cells, an ability to visualize the bulk and surface quality of the material is an advantage; to do this non-destructively, even in-line, is even better. It would be good to have X-ray vision to look inside, would it not? X-ray diffraction imaging (XRDI) does just that. Images are obtained of the distortions caused by crystal defects, and quantitative measures of the lattice deformation are available. In this paper the results obtained on a commercially available XRDI tool are compared with those from a largescale public research facility.

The PV industry is intensively evaluating technologies for further increasing conversion efficiency while maintaining, or even further reducing, production costs. Two promising technologies that meet these objectives are 1) the passivated emitter and rear cell (PERC), which reduces optical and recombination losses of the solar cell's rear side; and 2) multi-busbar/multi-wire module interconnection, which reduces optical and resistive losses of the front grid. This paper evaluates a combination of these two technologies, in particular industrial PERC solar cells with printed metal contacts employing a five-busbar (5BB) front grid instead of the typical three-busbar (3BB) design. The resulting 5BB PERC solar cells demonstrate an independently confirmed conversion efficiency of 21.2%, compared with the 20.6% efficiency for 3BB PERC cells. To the authors' knowledge, a value of 21.2% is the highest reported so far for typical industrial silicon solar cells with printed metal front and rear contacts. The higher conversion efficiency is primarily due to an increased short-circuit current, resulting from the reduced shadowing loss of the 5BB front-grid design, in combination with stencil-printed finger widths of only 46μm.

This paper presents the first 60-cell module results from a very simple process scheme for creating fully plated nickel-copper contacts on crystalline silicon solar cells. Standard Cz back-surface field (BSF) cells are processed in a completely analogous way to the standard process sequence up to and including rear-side screen printing. After a firing step for BSF formation, the front-grid positions are defined by picosecond pulse laser ablation and plated with nickel, copper and silver; this is followed by a short thermal anneal. Cell classification produces a very neat efficiency distribution of 19.6±0.1%. Solder and peel testing shows this approach to be competitive with standard screen-printed contacts in terms of adhesion. A batch of 60-cell modules were fabricated from the cells in a standard automated tabber-stringer system and subjected to thermal cycling and damp heat testing as part of the IEC 61215 reliability test sequence. The modules passed the test sequence without showing any signs of electrical degradation caused by, for example, copper diffusion.

The latest rounds of formal complaints against alleged breaches of trade agreements, the initiation of circumvention investigations, and preliminary announcements and rulings in various countries and trading zones all demonstrate that the multidimensional trade conflict in global PV markets is far from being resolved and is still simmering. The trade dispute is largely focused on the import of downstream products (c-Si wafer, cell and module) in current and prospective high-volume markets, such as the EU, the USA and potentially India. These nations or trading zones have implemented, or have proposed to implement, anti-dumping and countervailing duties, predominantly targeted against Chinese downstream producers. New rounds of investigations might lead to existing tariffs being extended to Taiwanese manufacturers that directly or indirectly import into the USA, while the EU might scrap a previous quota and minimum price system and revert to tariffs. This paper gives a brief historical review of the global PV trade dispute, and analyses the formal and legal grounding of anticircumvention actions, which in general increase the complexities of business planning. Because more than 70% of the global downstream manufacturing capacity is located in China and Taiwan, the manufacturers in these regions have no choice but to embrace an internationalization strategy that consists of production offshoring. The paper concludes with the introduction of potential strategies and recommendations which take account of increased complexities and uncertainties in business planning that arise from shifting trade barriers.

Project: Stillwater hybrid power plant; Location: Nevada, US; Project capacity: 26MW PV, 33MW geothermal, 2MW CSP.

Solar companies are always looking for new markets in an effort to remain competitive. Monica Wilson offers some advice on how developers and contractors should navigate the legal labyrinth that will face them as they go global.

Investors are popularly characterised as risk averse and fickle when it comes to new technologies. David Giordano, of the world’s largest asset management firm, BlackRock, tells John Parnell what has persuaded the investment community that solar is a safe bet.

Parallels are frequently drawn between the nascent energy storage business and PV 10 years ago – that it needs strong policy direction to take off. Andy Colthorpe profiles some of the areas that emerging as the world pioneers in supporting the deployment of storage.