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Exploiting Solar Paste to Increase Solar Cell Efficiency

published: 2011-09-14 8:39

An attractive solution to the energy conundrums faced by society is a natural resource that is all around us, and especially prevalent in countries located in the Sunbelt: solar energy. Harnessing this abundant resource gives rise to the strong possibility of reducing the dependence on polluting fossil fuels, with an equally feasible scenario of the total abandonment of using fossil fuels, helping to one day design future cities and towns free from damaging environmental concerns.

The main bugbear to realizing this vision of a solar-powered future is technology; more specifically, the electrical efficiency conversion rate. The efficiency of a solar cell virtually determines the electricity generated and is thus key to the viability of a solar energy project. Without doubt, the technology has improved greatly from traditional silicon-based solar cells to the current diverse crop that includes thin-film solar cells and carbon nanotubes integrated into the silicon mix. However, the efficiency rate still remains a concern.

From an industry point of view, the main issues are cost of manufacturing solar cells and engineering improvements to the electrical efficiency conversion rate. Of the two, the electrical efficiency conversion rate is the one that is problematic as this is controlled not by industry but by the constituents of the solar cell. On some occasions, improvements to the efficiency led to negligible cost reductions, as changes were required to the manufacturing set-up that negated any savings. Therefore, the trick would be to devise a way to maintain the manufacturing process but increase the efficiency conversion rate. And thankfully, such a trick has been devised by employing solar paste.

The Use of Thick-Film Processes for Manufacturing Solar Cells

A typical first-generation solar cell can be neatly divided into three sections: one sections concerns the semiconductor, the second section contains the semiconductor junction, and the final section will hold the front and back contacts. These contacts allow current to flow from the solar cell unit to the external circuit. As such, improving the rate of conductance (by reducing the rate of resistance) will greatly benefit the solar industry. By tackling this issue on the manufacturing level, this increase in the rate of conductance might be viable.

Thick-film technology focuses on devices for the electronics industry, from circuits to sensors. For the solar energy industry, thick-film techniques have been employed for over thirty years due to numerous advantages; one such example of the advantage of utilizing thick film technological processes centers on the fact that almost all of the components in a solar cell can be manufactured using thick-film processes. For example, the creation of the junction and the creation of the contacts (front and back) can be manufactured using the contact deposition technique (screen printing). This metalization brings a high degree of stability to manufacturers, allowing for reductions in cost and automations in the manufacturing process, making this technique appealing.

Unfortunately, using thick-film technological processes results in a lowering in the efficiency of the solar cell. By focusing on the back contact (usually made of silver (Ag) or aluminum (Al) based inks) and front contact (usually comprised of Ag-based ink), and by using thick-film processes, a creative solution was found to improve the efficiency conversion rate through the addition of an electrically conducting paste (also known as solar conducting paste).

Benefits of Applying Solar Paste on Solar Cell Improvement       

Solar conductive paste is a material that can be viewed as containing both organic and inorganic components dispersed evenly to form a viscous paste. Employing thick-film manufacturing techniques, such as screen-printing, the paste can be applied to the solar wafer; the organic component acts as the carrier, which, after evaporation (burnt off, a step in the manufacturing process), leaves a layer of inorganic-based inks on the solar cell.

Each inorganic component in the paste offers a different function: for example, in the case of the addition of Ag powder to the conduction paste, better conductivity rates will be obtained making this addition to a conductive paste for solar cell applications. This customization feature of the solar paste allows manufacturers to control the content – and efficiency – of either contact. Such bespoke solar pastes feature improvements such as lower resistivity / better conductivity, better line conductivity, better adhesion strength, better aspect ratios, and a better manufacturing processing phase window.

By applying a solar paste to the solar cell, certain features can be improved. Some improvements do not concern directly the efficiency of the electrical current transfer but nonetheless do affect the overall operation of the solar cell. In a general sense, the use of Ag solar paste has led to efficiency increases of between 0.2 – 0.4 % over standard monocrystalline silicon solar cell efficiency rates.

Lowering the resistivity makes for a better solar cell, as it will allow for better transfer of the current to the external circuit. In addition, improving the line conductivity will benefit the solar cell in a similar way to improving the resistivity. Line conductivity refers to the finger lines found on a screen-printed solar cell. Improving the lines, by making the lines less porous, will help in improving the conductivity. One such technique to improve the line conductivity is by employing double printing technology, a technique that will also help to improve the accuracy of the printer alignment and the metalization paste application; these three improvements by using the double printing technique will be very appealing to manufacturers of conventional silicon solar cells.

Silver Solar Paste in Enhancing a Solar Cell

The long-term warranty associated with manufactured solar cells is meant to provide peace of mind to consumers. With the addition of silver conducting paste, the reliability of the solar cell increases, helping the solar cell unit meet its lifespan warranty.

Using silver paste will improve the aspect ratio and this in turn will improve the efficiency of the solar cell. The aspect ratio (the width-to-height ratio of an object) for a solar cell will determine if the solar cell has an improved efficiency; the higher the aspect ratio, the better the efficiency. With a higher aspect ratio, more finger lines are present in the solar cell and each line would be taller and finer. This will result in less ‘shading’ (thicker finger lines) and reduce further the resistance of the solar cell. Applying customized silver solar paste will aid the production of finer finger lines, leading to a higher aspect ratio.

The processing phase is important to solar cell manufacturers as it allows for uniformity; for manufacturers, uniformity in areas such as temperature and material thickness provide stability, which in turn helps to reduce manufacturing costs. In a conventional solar cell, the manufacturing process involves a firing step and due to factors such as solar wafer thickness and even the variability of the furnace temperature, inefficiencies will creep in. By adding silver solar paste, these inefficiencies can be reduced significantly (or eliminated altogether, depending on the type of paste used).

The Trend in Using Solar Paste on Solar cells

The improvements listed above focus on first generation (monocrystalline) silicon solar cells. While the efficiency rates for monocrystalline solar cells lead the industry, advances are being made in developing and rolling out second and third generational solar cells. For example, thin film solar cells are becoming more widespread due to their lower manufacturing costs and flexibility.

Thin films, such as cadmium telluride (CdTe) and copper indium gallium selenide (CIGS), compared to monocrystalline solar cells, have lower efficiencies. Using bespoke solar paste on thin film solar cells will increase the efficiency of the solar cell whilst remaining insignificant in terms of additional cost.

Thin film solar cells, due to their dimensions, can sometimes be warped; that is, the material is at the technological extreme of width and thickness so components like the back contact or front contact might have an effect. An example of such an effect is a physical one, where deformation of the physical appearance of the solar cell will take place. For thin film solar cells, the front contact is usually a conductive oxide deposited on the surface. However, the back contact is usually either Ag or Al, with Al being the preferred material of choice as it is less costly. With an Al back contact, thin film solar cells have been known to warp.

Using Ag solar paste, this physical effect can be reduced and the loss in efficiency compensated. In addition, Ag silver paste will help to bring about better adhesion, strengthening the back contact component as well as improving the overall efficiency and operation the solar cell unit. Of course, another solution would be to convert the thick-film layer of aluminum (the customary back contact) into a thin-film layer, but research carried out by BP Solar indicated that more research would be required before employing aluminum thin-film layers as a feasible back contact.

Current Industrial Entities Producing Solar Paste for Solar cells

Numerous industrial players have manufactured solar paste for the solar industry, forming a vital part of the solar industry supply chain. With the adoption of thin film solar cells, and with the pace of development and advancement in all generations of solar cells, solar paste suppliers (both Ag and Al) will experience an uptick in their business.

Companies such as Giga Solar Material Corp., the world’s second-largest supplier of Al solar paste, can expect to see increased revenues from their customers increasing solar cell sales. For example, one important customer of Giga solar Material Corp. is Suntech Power Holdings Corp., the largest solar manufacturer in China. Other Taiwanese suppliers include Wellypower Optronics Corp. and Eternal Chemical Company Ltd., but the Al solar pastes these two firms offer are currently being certified.

Other solar paste suppliers include Heraeus. On its own, Heraeus offers a wide range of Ag solar pastes for different solar cell configurations, but on the whole, the range of pastes offer the same benefits: better contact resistivity, better current density, and better cell performance / cost ratio.

In partnership with Targray, Heraeus offers Ag solar paste for the front and back contacts. The HeraSol paste is rather interesting as it is both cadmium-free and lead-free. This utilization of this paste also led to better adhesion and better aspect ratios. Furthermore, for the front side contact, HeraSol paste resulted in better efficiencies and allowed for a wider processing phase window, features that are of particular interest to solar cell manufacturers.

DuPont, the large multinational science-based corporation, has also produced solar paste for use on solar cells. DuPont’s registered product Solamet offers photovoltaic metalization for both front and back contacts on both crystalline and thin film solar cells. Solamet offers an increase in the efficiency rate of 0.2 – 0.4 %, resulting in cost savings for manufacturers.

The Outlook for Solar Paste Utilization

The technology to fully harness the unlimited potential offered by solar energy has not yet arrived. Until that moment is realized, the rapidly advancing technology of the present will have to be creatively employed to continue further along this path of solar utilization. Using solar paste on crystalline and thin film solar cells is one such creative method, enhancing the overall solar cell; this can only be a good and positive feature for the solar energy industry.

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