Using plasma to enhance solar cell efficiency
Plasma technology is the core business of the company Roth & Rau AG, specifically the development and production of systems for the plasma coating of different substrates in a vacuum. Coating systems for the photovoltaics industry are a key part of their portfolio, and the company from the town of Hohenstein-Ernstthal in Saxony is one of the world’s leading suppliers in this field. But photovoltaics is not only a growth market and a dynamic creator of jobs. With global warming becoming an ever more prevalent concern, and the ready supply of fossil fuels diminishing, it is more important than ever to significantly expand the use of renewable energies. One of the key goals in the photovoltaics industry is to boost cell or module efficiency while simultaneously reducing manufacturing costs. To achieve this, Roth & Rau is working closely with various Fraunhofer Institutes.
Innovation has always been one of the driving forces at Roth & Rau. When the company was founded at the start of the 1990s, its main focus was the creation of systems for plasma technology based on the scientific know-how of the three founding members Silvia Roth, Dietmar Roth and Bernd Rau, but more recently the company’s portfolio has been expanded to include solutions for manufacturing both crystalline and thin film solar cells. Research and development are one of the key ways of maintaining a leading position in the international photovoltaics (PV) market, which is why Roth & Rau has boosted its in-house research capacity, investing more than 24 million euros in 2010. At the same time, it increased its global R&D workforce from 33 to 109 employees, most of whom work in the company’s technology center in Hohenstein-Ernstthal, which was inaugurated in April 2010. Fraunhofer research scientists and engineers are often to be found at this cutting-edge R&D facility – as an innovative company, Roth & Rau values cooperation and carries out joint projects with a variety of Fraunhofer Institutes in a number of technical fields.
Pilot line for crystalinne solar cells
One example of this cooperation is the production line that is operated in the technology center in collaboration with the Fraunhofer Institute for Ceramic Technologies and Systems IKTS. The pilot line features all the equipment required to process and characterize silicon wafer-based solar cells. With the support of the Fraunhofer researchers, Roth & Rau is aiming to achieve multiple objectives in this joint project, namely to produce new generations of equipment while simultaneously accelerating the development of the associated processes and technologies. The line is also intended as a means of demonstrating the capabilities of the company’s systems and processes to customers.
Here, too, the number one priority is to boost the efficiency of the solar cells while reducing manufacturing costs – which is why the main focus is on further enhancing crystalline standard cell technology and developing manufacturing technologies for high-efficiency cells. The research partners hope to improve the passivation and metallization layers, develop new systems and processes for forming selective emitter structures and create an entirely new manufacturing technology based on the heterojunction concept.
Another string to the company's bow: Thin film solar cells
At the start of 2009, Roth & Rau AG’s Management Board took the decision to enter the thin-film photovoltaics market, largely on the basis that thin film solar cells are cheaper to produce than crystalline silicon wafer-based solar cells. This is particularly true in the case of cadmium-telluride solar cells, which use glass as the substrate and cadmium-telluride as the photovoltaic active layer. A multi-stage process is used to apply different materials in thin films and arrange them in a way that establishes interconnections within the finished module. To help them carry out this challenging project, the company signed up the Fraunhofer Institute for Electron Beam and Plasma Technology FEP as a strategic partner. There was plenty to recommend this choice, including a well-established relationship and enough experience on joint projects to enable Roth & Rau to feel confident that Fraunhofer FEP had the resources to perform fast-paced research and development in this area. But the biggest draw was Fraunhofer FEP’s experience in the various ways of depositing contact layers, as well as its general expertise and knowledge of vacuum evaporation system technologies. The project is funded by the Saxon State Ministry for Science and the Arts.
Over the course of 2010, the Fraunhofer FEP team worked with Roth & Rau to construct a complete pilot line. The broad outlines of this production line are as follows: First, a transparent conductor is deposited onto the glass substrate; next comes the simultaneous vapor deposition of the photovoltaic active layers of cadmium sulfide and telluride; and, finally, the metal contact is positioned on the rear of the solar cell. This means that complete cells can be manufactured, analyzed and optimized on the pilot line. Since each layer influences the other layers, the team of researchers is experimenting with varying the parameters of the individual layers in order to evaluate the arrangement as a whole. Their ultimate aim is to find an ideal solution that gives the thin film cell the highest possible efficiency, and their ambitious target is to reach an efficiency of somewhere between 15 and 18 percent by 2015.
To foster optimum and continuous knowledge transfer, four of Roth & Rau’s employees are spending a period of time working directly on the pilot line at the Institute. This enables them to discuss progress with the Fraunhofer researchers on a daily basis and test out their own developments.