By Peter Brenner
Why Materials Matter
Metallization paste compositions have been used for more than 20 years to make economical, high-performance electrical contact to photovoltaic devices. In the case of crystalline silicon (c-Si) solar cells, new generations of metallizations have been developed to enable evolutionary and revolutionary changes in the technology. Advancements in metallization materials, newer materials such as silicon inks, and improvements in solar cell designs enable increased efficiency while reducing overall costs for cell and module makers, accelerating the drive towards grid parity.
The Efficiency Equation
Currently, the single most important factor that determines whether a solar system is going to be installed is the cost of the system, which is represented in dollars per watt of solar produced power (US$/W). This means that a solar system manufacturer can reduce the cost of a solar product in two ways: by reducing the cost of the production materials (wafers, metal contacts, glass, etc.) and related costs (labor, overhead, etc.) and/or by increasing the power output of the cell or module.
Solar manufacturers may consider many options if their sole objective is to produce higher efficiency solar cells. Unfortunately, many of the options available actually increase the manufacturing cost since the resulting increase in area-related costs does not offset the wattage-related cost savings (cost of ownership). This is the sole reason why these types of efficiency-enhancing solutions have not been implemented in mass production. The real objective for a solar manufacturer is to maximize the efficiency while minimizing the additional area-related costs, and this can be done most effectively by focusing on advancements in materials.
The advances made by DuPont in the field of front- and back-side metallization contacts have resulted in significant efficiency gains for solar cell technology. In 1998, the efficiency of a crystalline silicon solar cell was approximately 10%. Today, these cells operate at close to 16% for multicrystalline wafers, and at greater than 18% for monocrystalline wafers. Higher module efficiency delivers more power from the same module, so even a fraction of a percent efficiency improvement can substantially increase the return on investment from a photovoltaic system.
Frontside photovoltaic metallization pastes are typically made from silver and other materials, which collect electricity produced by the solar cell and transport it out of the cell. They play a critical role in how efficiently photovoltaic modules turn sunlight into electricity.
New frontside metallization technologies that lower contact resistance and grid line resistance allow for double printing and high-aspect ratio printing, and improve fine-line capability. DuPont™ Solamet® PV17x photovoltaic metallization pastes, for example, are dramatically increasing the efficiency of solar cells, while reducing material consumption, enabling new cell and module designs.
These new frontside silver photovoltaic metallization pastes utilize novel chemistry formulated to deliver advanced efficiency and adhesion for solar cells while enabling higher process reliability and yield for both cell and module manufacturers. Compatible with high-speed printing processes, and featuring improved contact and grid resistance, pastes like this will help enable the solar industry goal for conversion efficiency of c-Si solar cells beyond 20% in 2012.
One way the technology improves efficiency is that high aspect ratio printing allows for a thick print in one pass resulting in lower grid line resistance due to the additional metal. Meanwhile, the print speed is not compromised and the increased laydown allows for finer lines leading to equivalent usage and improved efficiency. Cell manufacturers can expect to achieve from 50 to 70 micron finger openings at high print speeds.
At the module level, solar cells metalized with Solamet® PV17x exhibit exceptional solderability and adhesion that enables reliable use in the highly automated stringing processes used by module makers. A wide processing latitude is necessary to meet the requirements for the various solder assembly techniques module makers employ. The excellent solderabilty and use of thicker ribbons, in particular, can contribute to higher productivity and a significant improvement in cell to module power.
With these breakthrough formulations, high efficiencies also can be achieved more cost-effectively, since about 10% less paste is required.
Next Generation Low-Silver PV Metallization Pastes
Next-generation pastes that actually have less silver content are also in development.
This will help offset some of the impact that rising silver prices have on the cost of producing solar cells and modules. The first generation of these products from DuPont is expected to have a reduced silver content of more than 10%. Ongoing research efforts should enable the development of subsequent generations of the products with the silver content reduced by as much as 20% compared to today’s existing products.
Lowering overall system costs is critical to the future of the PV industry, and the escalating price of silver has become a key concern for cell and module makers. If materials manufacturers can reduce the industry’s reliance on silver as a basic conductive material, it has the potential to reduce cell and module costs today and enable a more stable cost structure in the future.
The development of non-precious metal thick film compositions for low-cost applications began at DuPont in the early 1980s. Early efforts focused on copper material systems in the automotive industry, and have continued to successfully draw upon internal and external collaborative efforts to leverage technology to reduce costs for conductive materials in a range of industries including the automotive, passive component and plasma display panel markets. The history of component and material costs across all industries, and their affects on end-user equipment, is evidence of how much materials matter, which is why this area of development is so consequential to the evolution of the PV industry moving forward. It is critical, however, that cost reductions that come about through new material innovations do not compromise performance.
The lower-silver photovoltaic metallization paste system in development includes new frontside silver and tabbing pastes, which for many applications will be near drop-in replacements for current products. Development of products with that kind of flexibility is crucial to speed the broader adoption of solar energy.
Another key aspect of improving conversion efficiency is by changing the design of the crystalline silicon solar cells themselves, and there are several ways to do this. One fast-emerging technology is the selective emitter. According to industry estimates, selective emitter technology could represent 13% of crystalline silicon solar cell production by 2013 and up to 38% by 2020.
Enabled by specialized silicon ink and process technology developed by DuPont Innovalight, a selective emitter enables the cell to capture a broader range of the incident light especially at the blue end of the spectrum, and delivers an efficiency improvement of up to one percent.
A selective emitter is created on the front of the cell beneath where the metal contact lines will be printed to reduce electrical contact resistance and allow better flow of electricity. To heighten efficiency, silicon ink is applied to the front of the cell prior to the diffusion step and then the contact lines are printed in alignment with the silicon ink on top. This type of cell design will decrease the emitter sheet resistance under the front metal contacts while developing a very shallow emitter between the grid lines so more photons can be captured.
The process adds only one additional step to the conventional cell line in the standard crystalline manufacturing process but the ink process can be easily retrofitted on existing solar cell manufacturing lines.
The process begins with cleaning and texturization of the wafers in an alkaline bath to form random pyramids. This is followed by the added silicon ink screen print, and then the rest of the processing steps are the same as a traditional solar cell manufacturing line: phosphorous diffusion for P/N junction formation, removal of the PSG layer formed on the top surface using a wet bench, then passivation of the front surface with silicon nitride, and finally metallization with screen printers to pattern the wafer with silver and aluminum pastes.
By adding the silicon ink screen-printing step, solar cells can be produced with a higher conversion efficiency at lower cost per watt.
A Combination of Elements
Another area being investigated is how silicon inks and metallization pastes can be developed to achieve even greater efficiency working together than each material can achieve individually today. Right now, these two materials are being used in conjunction and are in contact with one another on the cell. Future developments at DuPont Innovalight are aimed at integrated development of the material sets in order to boost efficiency even further. This is being done for monocrystalline cells and the new cast-monocrystalline technology being broadly developed in the industry. These developments have the potential to significantly reduce the cost of ownership for PV systems by continuing to push the boundaries limiting efficiency today.
A More Sustainable Future
The World Energy Outlook predicts that in the next 20 years, energy consumption will increase by 35%?the generation and storage of renewable energy will be the fastest growing sector in the energy market for the next 20 years. With the PV market projected to achieve 15% growth in 2011 to approximately 19 GW annual installations worldwide, innovations in materials will continue to have a leading role in helping the industry achieve grid parity faster. Materials remain critical to the development of high-quality and high-reliability module and systems to further PV market growth, and reduce our dependence on fossil fuels for a more sustainable future.
Peter Brenner is Global Marketing Manager, Photovoltaics, for DuPont Microcircuit Materials, based in Bristol, U.K. He earned his degree in Applied Physics, and taught Math and Physics at DeHavilland Collage of Further Education before joining DuPont in 1986. Brenner has held a variety of leadership roles within DuPont Microcircuit Materials, in Quality Management, Applied Technologies, Sales and Marketing. He assumed responsibility for the European Photovoltaics market segment in 2003, and was appointed Global Marketing Manager in 2006.
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