Technical Papers

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.

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.

The PV manufacturing and technology hubs established over the past decade will change at an accelerated pace through the globalization of solar power installations. This development will be most pronounced in regions with high solar radiation, where grid parity can be achieved without subsidies. It can therefore be expected that parts of manufacturing within the PV added-value chain will also be established in new markets, such as South America, Africa, the Middle East and Asia. This trend will also stimulate these economies by the generation of new employment opportunities in the advanced technology sector. During the development of a new business plan, the key factors to resolve include the optimum manufacturing size and the extent to which upstream integration, from module manufacturing to poly Si, will be competitive. This paper addresses technology trends and strategic considerations for optimally selecting a PV manufacturer's strategy for each region, the determinants for centralized versus decentralized manufacturing, and the impact of these on fab and facilities concepts. Furthermore, the dependence of manufacturing capacity on fab and facility cost, as well as on the energy demand for individual manufacturing steps along the value chain, is discussed and compared.

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.

In PV power systems the choice of an appropriate location for the installation of the PV array box (or DC combiner box) is an important undertaking. It is essential that the box be placed so that the amount of DC cabling is minimized in order to not only save cable costs but also reduce voltage losses. This paper presents a fast solution to this problem, based on a mathematical model for the minisum location of the combiner according to the Manhattan metric between the PV array and the DC combiner box. The target function and its optimal solution (i.e. the most economical amount of cabling) for this particular model were obtained, and the optimality of the solution proved by contradiction. The application of this model is illustrated by means of two typical examples, involving an odd and an even number of strings in a PV array. The proposed model is efficient and easy to apply, and as such should be of interest to PV engineers and designers.

This paper discusses the role of wafer cleaning in solar cell processing, and addresses its increasing importance with the introduction of new process steps for manufacturing high-efficiency solar cells. The requirements for cleaning before several process steps, in relationship to the solar cell production sequence, are discussed: frontend- of-the-line (FEOL) cleaning needs to reduce metal surface concentrations by several orders of magnitude (residues from wafer sawing), while back-end-of-the-line (BEOL) cleaning needs to reduce mostly process induced contamination, which tends to be much lower. A ten-step roadmap for process integration and optimization of new cleaning processes from lab to fab is suggested, which is based on process analytics and simple bath-lifetime simulations. A number of advanced cleaning steps are identified and their suitability for solar cell mass production is examined. The influence of the different input variables is demonstrated, with a focus on feed and bleed settings. Finally, the need for constant monitoring of cleaning baths is highlighted, and a device developed by Metrohm for cost-effective on-site monitoring of metallic contamination is discussed.

This paper considers the relative technical and economic performance, for selected sites in Southeast Asia, of PV plants using crystalline and thin-film PV module technology. Technical performance estimates are based on a forensic analysis of in-field data for two grid-connected PV installations in Thailand using polycrystalline and thin-film PV modules. These two case studies help to validate the performance simulation approach for the other considered countries with similar environmental conditions. The case studies show that Mott MacDonald’s yield analysis approach demonstrates acceptable accuracy for energy yield assessment in a grid-connected PV plant, at least under the observed environmental conditions, which are most relevant to Southeast Asia plants with polycrystalline and thin-film PV modules installed. The findings presented in this paper are relevant to project developers and investors who have an interest in selecting solar PV technologies for Southeast Asian regional conditions.

Certain PV modules have begun showing signs of yellowing, a consequence of backsheet deterioration. This phenomenon can impact on power plant performance and safety, and is emerging as a potential problem waiting to happen with low-cost modules. This paper explores the key attributes of backsheets and assesses the relative benefits of the different types of backsheet on the market and the materials used in them. The different tests undertaken for backsheets are reviewed, and arguments are put forward for the requirement of a standardized testing regime for this crucial module component.

Minimizing the breakage rate of silicon wafers and cells during production has been one of the key issues for reliable and productive solar cell manufacturing. However, the root causes of damage or breakage, as well as the mechanical characteristics of manufacturing processes, are not completely understood. In the study described in this paper the change in mechanical strength and the damaging of wafers and cells was analyzed in an industrial cell manufacturing line in order to detect critical process steps and handling operations in certain processes such as etching, diffusion, screen printing and firing. An analysis and discussion of damage sources is presented which offers more insight than the conventional study of breakage rate that is mostly performed by cell manufacturers. In a systematic experimental study, 19 different locations in the production line were investigated. The mechanical strength of 800 wafers or cells at different points in the cell line was subsequently determined using the four-line bending test and the statistical parameters for the Weibull distribution. It was discovered that dramatic changes in strength occur at different process steps because of the change in defect structure; there were also found to be several positions at which no further damage was detected. This method of investigation can therefore be used as a fingerprint of a cell line in respect of yield and breakage rates. Individual processes can be identified that indicate high damage potential, although the actual breakage could occur in a subsequent process step.