Technical Papers - Solar Media Store (PVI)

Deconstructing solar photovoltaic energy: Part 2

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This is the second and concluding part of a study on the solar photovoltaic market. In the first part, photovoltaic energy was contrasted with other energy sources used to generate electricity, and cost points necessary to produce a sustainable photovoltaic market were identified. In this second part, learning rates required to attain those cost points are provided. The paper concludes by examining a scenario in which 15% of the world’s electricity in 2035 is generated using photovoltaic energy, and frames the challenge from both global investment and profitability perspectives. Learn More

Achieving higher efficiencies with a low-cost etch for in-line-diffused silicon wafer cells

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Emitter formation is one of the most critical processes in the fabrication of silicon wafer solar cells. The process for standard emitter formation adopted in the photovoltaic industry is tube-based diffusion, using phosphorus oxychloride as the dopant source. A potentially low-cost alternative that typically results in lower solar cell efficiencies is in-line diffusion, using phosphoric acid as the dopant source. The Solar Energy Research Institute of Singapore (SERIS) recently developed a technique called the ‘SERIS etch’, a non-acidic etch-back process technology that provides a controllable, uniform and substantially conformal etch-back suitable for solar cell processing. By using the SERIS etch, efficiencies of up to 18.7% have been demonstrated for omogeneous-emitter silicon wafer cells; a 0.4%abs efficiency improvement has also been achieved for a unique selective-emitter approach exploiting this novel etch. All work was carried out on industrial-grade p-type Cz wafers with conventional screen-printed metallization and a full-area aluminium back-surface field (Al-BSF). With Al local BSF (LBSF) homogeneous-emitter solar cells, efficiencies of 19.0% were achieved using in-line emitter diffusion and the SERIS etch, a 0.7%abs efficiency increase over the baseline efficiency at the time. To the authors’ knowledge, these are the highest solar cell efficiencies ever reported for in-line-diffused silicon solar cells. Moreover, the SERIS etch is a costeffective alternative to generating pyramid-textured surfaces without using conventional metal-assisted siliconetching processes. Learn More

Potential for mono-cast material to achieve high efficiencies in mass production

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Despite the drop in price of silicon wafers, they are still one of the main factors influencing the cost and performance of Si-based solar cells. These two consequences have initiated a growing commercial interest in mono-cast (castmono, mono-like or quasi-mono) Si wafers, supported by R&D in the areas of material characterization, correlation with cell efficiencies, and mono-cast material use in advanced cell technologies. This paper gives a broad overview and comparison of commercially available grades of mono-cast material from different suppliers. The performance of the material from production in high-throughput screen-printing lines, as well as an analysis of the main material characteristics influencing these results, is presented. A characterization using a lifetime tester and a photoluminescence (PL) imaging tool has shown that not only grain boundaries but also dislocations could cause a drop in cell Voc of more than 15mV. Wafers with large surface areas of <100> Si lattice planes, when processed with anisotropic texturing, could yield an increase in Isc greater than 400mA for 6" substrates, as compared to the isotropic-textured equivalents. Furthermore, when a high-grade mono-cast material processed in anisotropic texturing was compared with CZ mono material from the same supplier and of the same resistivity, light-induced degradation (LID), presented as combined Voc and Isc degradation, was only one-third of that in CZ material. However, although mono-cast material has the potential to increase cell line performance to the same level as that gained by important process and technological improvements, it imposes very high requirements for better material sorting in order to achieve stable cell electrical performance and module aesthetics acceptable to the market. Learn More

When capacity buys are not an option: Technical trends in c-Si cell manufacturing and their implications

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Economics will always play a crucial role in the way PV technology advances. However, the current generation of products is facing substantial business challenges in the attempt to scale the product technologies. This paper is the fifth in a series covering business analysis for PV processes. The methods applied in these papers fall into two categories: cost of ownership (COO) modelling and cost and resource modelling. Both methods examine the business considerations associated with the adoption of new processes, tools or materials. This is more critical than ever. Nearterm issues – in some cases the survival of the business – heavily influence today’s decision processes. This paper tries to identify the areas that it is thought will produce the largest near-term paybacks. The areas identified are n-type wafers, Al2O3 passivation and copper metallization. Learn More

Overview of PV module encapsulation

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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. Learn More

Cavities observed in PV modules induced by the tabbing and stringing process

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A major cause of failure in PV modules is related to the penetration of the module by moisture and its retention within. The presence of moisture results in corrosion of metallic contacts or accelerates the molecular degradation of the encapsulant, causing a loss of transparency and in some cases the development of yellowing. The moisture penetration may be intrinsic to the resin itself, but most often it will occur at the interfaces. As a consequence, the adhesion of the resin to glass, metallization, cell and backsheet surfaces may be affected. Engineers involved in the assembly of PV modules used to link adhesion degradation issues to poor conditions for storing polymeric materials, especially the encapsulation resin and the backsheet. In this paper another cause, which has not yet been studied by specialists, is discussed. It is shown that the welding of copper strips can induce residues which prevent the satisfactory adhesion of the resin, resulting in elamination. This phenomenon is identified by ‘spots’ along the busbars after lamination. The study highlights the possible consequences of these defects for a module’s performance, after consecutive thermal cycling, damp-heat and humidity-freeze testing. Recommendations are also given for choosing a suitable solder flux and optimizing the soldering process, in order to maintain satisfactory control over potential delamination problems. Learn More

i-PERC technology enables Si solar cell efficiencies beyond 20%

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A cost-effective and industrial version of the well-known passivated-emitter and rear cell (PERC) concept has been developed by imec. The imec i-PERC technology comprises a large-area p-type monocrystalline Si solar cell with, on its front, a homogeneous emitter, a thin thermal oxide layer and fine-line Ag screen-printed contacts; on its rear the cell has a chemically polished surface, low-cost rear dielectric stack layers and local Al contacts. Yielding certified efficiencies of up to 20% and fill factors of 80%, these cells clearly outperform aluminium back-surface field (Al-BSF) cells. During the development stages, process complexity and additional tool investment were kept to a minimum. It is therefore believed that this technology can be picked up by companies in a straightforward way as the next-generation industrial solar cell technology. Learn More

Modelling for PV plant optimization

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Because most of the costs of developing a PV power plant are paid before any energy is generated, optimizing the energy production from the plant is critical during plant design. Lost energy and increased operations costs due to non-optimal site characterization, technology choice, plant design, installation and other factors result in lower energy production and a higher levelized cost of energy (LCOE). Many design decisions are based on results from PV performance models. Current PV performance models can represent only some of the differences between sites, technologies, designs and operations choices. This paper provides a description of what is currently known about some of the performance tradeoffs faced by PV plant designers and operators. It presents a vision for improving PV performance models so that in the near future a full optimization can be carried out to improve the performance and lower the costs of PV plants. This will hasten the adoption of clean energy production from the sun. Learn More

The next big step – Optimizing the performance of PV power assets

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To achieve project cash flow expectations, it is necessary to operate, maintain and optimize the performance of a PV power asset to meet or exceed the pro forma operating assumptions. To assume as given the achievement of these model assumptions is both naive and risky. Experience in operating the largest fleet of solar PV power plants in the world has demonstrated that project financial hurdle rates can be missed by as much as 25% if the plant is not well maintained and its performance is not optimized. Conversely, an optimized PV asset can generate cash flows 2–10% higher than expected if the optimization approach described in this paper is implemented. Learn More

Development of POCl3 emitters which enable Ag reduction while increasing solar cell efficiency: Assessing the impact on manufacturing and system costs

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The market price of Ag has fluctuated considerably over the past ten years and has impacted the manufacturing cost of Si solar cells and the price of Si PV. Reducing Ag consumption can decrease this cost; however, such reduction may come at the expense of cell performance. In order to address the issue of Ag cost reduction while maintaining high cell efficiency, phosphorus emitter profiles are tailored via POCl3 diffusion to create solar cell emitters displaying low saturation current density (J0e), variable electrically active surface phosphorus concentration ([Psurface]), and variable sheet resistance with the aim of reducing Ag consumption. By optimizing emitter diffusion conditions, it is possible to reduce screen-printed Ag paste consumption by 33% with no loss in cell performance. Using a screen-printable Ag conductor paste designed to contact low [Psurface] emitters, the performance of cells with screen-printed Ag paste dry masses of 200, 120 and 80mg is compared. By using a tailored low-J0e 55Ω/sq emitter, it is possible to achieve a high open-circuit voltage (Voc) and short-circuit current (Jsc) to yield average cell efficiencies of 18.64% and 18.73% for 120mg and 80mg Ag paste dry mass, respectively. This is compared with efficiencies of 18.52% for cells using state-of-the-art technology (industrial high [Psurface] 65Ω/sq emitter with 120mg Ag paste dry mass). On the basis of a Ag market price of US$32/troy oz and an 85% by weight thick-film paste Ag metal content, a Ag front-side metallization cost of US¢2.11/W can be achieved by using 80mg Ag paste dry mass, which translates to a Ag cost saving of US$5.4M per year for a 500MW production line when compared with the Ag cost for state-of-the-art technology. Further cost analysis shows a 1.2% area-related balance of system (BOS) cost reduction and a US¢0.1/kWh reduction when comparing low-J0e 55Ω/sq modules and state-of-the-art modules. Calculations show that an additional 0.5% absolute efficiency for state-of-the-art modules is required, to compensate the efficiency gains and Ag cost reduction afforded by low-J0e 55Ω/sq modules. Learn More