Photovoltaics International Papers

One question to emerge in recent years is whether monocrystalline silicon (mono-Si) or multicrystalline silicon (mc-Si) will become the dominant mainstream technology in the future PV industry. However, despite all the arguments, the market share of mc-Si seems barely changed, while the market share of mono-Si has not increased significantly. The reasons why mono-Si has not made progress have been extensively mentioned in the literature and will therefore not be covered here; rather, the objective of this paper is to discuss several benefits of mono-Si.

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.

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.

This paper reports on the progress of R&D in two n-type cell and module concepts: the n-Pasha solar cell and bifacial module, and the n-MWT (metal wrap-through) cell and module. Both are part of ECN's technology platform, acting as a roadmap for research in n-type Cz cells and modules. The technology platform also encompasses low-cost IBC solar cells. In the case of n-Pasha, recent developments involve improved stencilprinted metallization, resulting in an increased Isc and Voc and efficiencies of up to 20.5%. For the bifacial module aspect, research has been done on the effect of different albedo on the module output. A gain of 20%rel in module output power was obtained with an optimized background, increasing the module power from 314W to 376W. As regards n-MWT cells, the front-side metallization pattern has been changed significantly. The number of vias for conducting the emitter current to the rear has been increased from 16 to 36, resulting in reduced lengths of busbars and fingers and consequently an increase in FF. At the same time, the metal coverage on the front side has been reduced from 5% to 3% of the total area, leading to a gain in Isc and Voc and a significant reduction in Ag consumption. All these factors will result in a lower cost/Wp. For the improved n-MWT design, average efficiencies of 20.8% over a large batch (134) of cells have been obtained, with the highest recorded efficiencies being 21.0%.

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 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.