Photovoltaics International Papers

Global PV end-market demand for PV modules is expected to reach around 50GW in 2014, which has prompted the need for manufacturers to expand capacity to meet demand. With effective module capacity standing at around 45GW at the end of 2013, Photovoltaics International (PVI) has analysed solar cell, c-Si and thin-film capacity expansion announcements that were extensively reported by sister website, PV Tech, from the beginning of 2014 through to the end of November to establish key trends.

Modules based on p-type multi c-Si technologies look set to dominate the PV industry over the next five years, continuing a trend that has developed over the past two years. This paper explores why high-efficiency p-type multi seems destined to remain the workhorse of the global PV industry.

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.

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.

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.

Using a prescribed test protocol to compare the measured performance of a solar PV power plant relative to its expected performance is often a means by which the value of the facility is determined. Performance testing is used contractually to determine matters such as the fee paid to a constructor, the price paid to a seller, and the cost of capital from a lender or investor. To ensure that performance testing produces consistent and independently verifiable results, it is essential that accurate and repeatable test methods be used. This paper outlines critical deficiencies in older solar PV performance testing protocols, and how the methods prescribed in ASTM E2848 and E2939 eliminate these deficiencies and enable test practitioners to produce consistent, verifiable results with a high degree of confidence.

Why change a product which can be sold in high quantities with a large margin? This is one of the reasons why crystalline silicon modules look the same today as they did 30 years ago. In addition, a module has to last for more than 20 years; to change the technology, or even just the material, many complicated, long-lasting and costly tests are necessary. And even after a series of successful tests there is no guarantee of a long-lasting product. Moreover, during the PV crisis starting in 2009, module manufacturers did not have the manpower and budget for introducing novelties into the module market. All the above are reasons why module architecture and materials did not significantly change with time and did not adapt to the introduction of powerful, highly efficient solar cells. After the crisis, however, many module manufacturers became aware that in order to be able to sell modules on the market with a high margin, their products not only have to be cost effective but also must differentiate themselves from the mass product. Consequently high-power, optically nice, colourful, backcontact, transparent, bifacial, light and highly durable modules are now being developed and are gradually being introduced into today’s market. This paper reports on current trends and discusses future developments.

Cracking of solar cells is a serious issue for product safety and module performance. Cracks may result in power loss, hot spots or arcing, and are caused by exceeding the strength limit of silicon. During the last few years, various studies have shown that fracture of encapsulated solar cells can be influenced by the manufacturing processes, which lead to residual stresses in solar cells. The results presented in this paper will give insights into the stresses generated by soldering and lamination. Furthermore, mechanisms of stress generation will be explained. On the basis of these findings, recommendations are made as to how to mitigate stresses, for example by means of alternative soldering processes, different soldering parameters or material optimization of the copper ribbon or the encapsulant.

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.

A detailed analysis of state-of-the-art CIGS technology has resulted in a direct cost of ownership (CoO) of €0.44/Wp for this PV module type. However, the reduction in production costs, although impressive, is not sufficient for CIGS to become competitive with today’s c-Si technology. In order to answer the question as to whether CIGS will ever be able to challenge c-Si, the cost-reduction potential of CIGS is investigated. The impact of savings is evaluated in respect of the material segment, production equipment, energy and labour, production yield, device efficiency and absorber thickness. A total cost-reduction potential of around €0.21/ Wp is identified, which would be enough to put CIGS back into the game (the direct CoO will continue to be dominated by material and equipment depreciation, adding up to 68%). These cost reductions, however, cannot be realized immediately: within the next two years, €0.03/Wp is expected to be feasible, while it will take two to four years for the next €0.107/Wp. For the final €0.073/Wp, a time frame of at least five years is predicted, with corresponding costs for the technology developments. Provided that someone is willing to spend the necessary amount of time and money, the second part of the answer regarding CIGS’ competitiveness will depend on how c-Si evolves within this time period.