Scientists at the Center for Solar Energy and Hydrogen Research Baden-Württemberg (ZSW) have developed what they believe to be a breakthrough approach for the continuous roll-to-roll processing of CIGS thin film solar cells on plastic film. The Stuttgart-based team has devised a manufacturing scheme in which the main coating steps take place in a single system, claiming 10.2% conversion efficiencies on cells made using this prototype tool.
Other CIGS deposition approaches require each film layer to be deposited individually in separate systems. With ZSW’s combination of these steps into one piece of equipment, the researchers’ goal is to produce fully integrated solar modules with even greater efficiency and at a lower cost.
The development of roll-based CIGS cells has been going on since 2010 at ZSW. A 12m web-coating system is deployed in one of its technical labs, which uses a temperature-resistant polyimide film, about 0.0025 centimetres thick (25um) and 30 centimetres wide, as the substrate material.
“The unique feature of this web-coating system is that all coating steps take place simultaneously in the same vacuum,” explained Michael Powalla, board member at ZSW and head of the photovoltaics division. “While the back contact is applied at one end of the system by means of cathode sputtering, the coevaporation of the CIGS absorber and the deposition of the transparent front-contact layer are located elsewhere in the system.”
In the current development stage, the molybdenum back contact, the four elements for the CIGS absorber (copper, indium, gallium, selenium), and the zinc-oxide window layers can be deposited, while the development and integration of a new buffer layer is an ongoing project. The full integration of a monolithic cell interconnection design is also a goal of the project.
The web-coating system was financially supported by the Federal Ministry for the Environment, Nature Conservation and Reactor Safety (BMU) through the CISROLL investment project.
Several commercial companies are developing roll-to-roll processed CIGS thin-film devices on polymer substrates, including Ascent Solar and Solarion.