Large-scale Molybdenum Electrodes for Manufacturing CIGS Solar Cells

There is a growing demand for cost-effective and high-performance thin-film photovoltaic solutions, thanks to the falling prices of their crystalline silicon (c-Si) counterparts. Copper-indium-(gallium)-diselenide (or/and sulfide) (CIGS) technology is regarded by some as the most promising thin-film PV contender to the c-Si due to its high conversion efficiencies, potential cost effectiveness and incrementally improving manufacturability.

In order to stay successful and remain profitable, CIGS manufacturers must further increase the ratio of conversion efficiency to manufacturing cost of their modules. The former relies primarily on new technological advances, such as the advanced engineering solutions for the absorber layer, interface adjustment and use of better and purer materials. The latter is intimately related to the improved production throughput and yield, better material utilization and improved logistics.

In a typical CIGS device, fabrication starts with the deposition of a sputtered molybdenum (Mo) electrode on a substrate, such as soda-lime glass. The use of Mo as the back electrode has remained unchanged since the inception of the CIGS technology. In fact, molybdenum is one of the few metals that remain relatively inert during the CIGS high-temperature (>500 °C) selenization process. Despite the fact that CIGS manufacturing recipes vary significantly between individual module producers, the number of “knobs” available in tuning sputter-deposited Mo electrodes is always quite limited. Recipe differences include sputtering pressure and power, the levels of intentionally added impurities, compositional grading profile, and the number and position of sputtering targets in the coater, etc. All of these variables are crucial in achieving high conductivity, proper electrical matching to the CIGS, good adhesion and long-term stability.

The Mo electrode should be about 400-800 nm thick to achieve 0.2 – 0.5 Ohm/sq sheet resistance. Depending on the CIGS fabrication method, the back electrode represents 7-20% of the manufacturing cost of the complete module. There are a number of strong economical and technological reasons for CIGS companies to outsource the back Mo electrode from a large-scale glass manufacturer.

Benefits of Outsourcing

The economic benefits of outsourcing include high throughput on up to jumbo formats (about 3200mm x 6000mm) at low operating costs, reduced downtime and energy consumption and increased production capacities. Another important advantage of outsourcing this process is reducing the capital needed for the facility. CIGS manufacturing facilities that outsource have a smaller footprint, which is reflected by their reduced total cost of ownership per unit module area.

Glass manufacturers can also ensure a high material utilization through optimizations in glass processing and use of consumable reclamation and recycling programs, not to mention the cost reduction advantages gained by purchasing large quantities of targets and other consumables. Another benefit is that yields of the large scale production are much higher due to the use of advanced automation, improved layer uniformity and reproducibility of the coating, higher glass cutting yield, and the benefits of the strict pre-sputter and post-deposition inspection and qualification. Last, but not least, the glass manufacturer can be a one-stop shop for both Mo-coated back glass and low-iron high-transmission front cover glass.

The technical benefits of outsourcing include the fact that the large scale sputter deposition can be done in vacuum coaters in the same location where the float glass is made. This inhibits a long-term degradation of glass/electrode interface due to the shorter time between the Mo deposition and glass production. The need for uncoated substrate glass inventory at the PV module manufacturing site is also eliminated. Large scale sputtering greatly reduces the negative impact of back-side deposition or wraparound, when sputtering material ends up on the back side of the glass substrate. Other technological benefits include the use of advanced robotics, upgraded in-line defect detection systems, flaking suppression measures, advanced power supplies and sputtering magnets, etc. Employment of state of the art cutting and grinding lines allows handling of bare and coated glass without touching the front surface and ensures the desirable glass edge quality.

Innovation on Large Scale

Besides significant cost saving potential, coated glass manufacturers can also offer a great deal of expertise in customizing the back electrode design through existing and innovative engineering solutions. For example, each of the different types of CIGS absorbers (co-evaporated, sputtered, electroplated, etc.) requires specific Mo surface morphology, material density, surface energy, work function and controlled susceptibility to the formation of the transitional MoSe2 layer. These characteristics are meticulously tuned first on R&D and then on production scale to optimize the CIGS growth, enhance its charge-generating properties and reduce the potential barrier at the Mo/CIGS interface.

Coated glass manufacturers also have extensive experience mitigating mechanical stress to prevent warping of large glass sheets and peel-off of the coating during substrate cutting as well as to insure compatibility of the Mo layer with interconnecting laser scribes. A perfect scribe must be free of residue material at the bottom of the trough, have abrupt “shoulder-free” edges and consistently provide an ideal electrical isolation between the separated areas. All these requirements are achieved by layer grading during the Mo deposition and by applying proprietary buffer layers at the bottom and the top of the Mo layer.

The success of CIGS technology using outsourced large-scale glass manufacturers depends on the innovations these manufacturers can come up with today. Examples include the development of thinner and better performing back electrodes based on new materials and stack designs, innovative ways to control element diffusion through the electrode, development of new generations of glass with improved characteristics and much more.

Large scale coated glass manufacturing is ready to meet ever growing needs of CIGS technology. 

Alexey Krasnov is a Staff Scientist with Guardian Industries Corporation – Science & Technology Center. 

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Dr. Alex Krasnov is a Staff Scientist at Guardian Industries Corp., Science&Technology Center. His international experience is in R&D and manufacturing of a wide range of products for high-end defense and commercial customers. Dr. Krasnov's expertise includes high-contrast TFEL and OLED FPDs, photovoltaic devices, large-area coatings for energy and electronics, process and product development.

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