New development in production of flexible thin-film solar cells

GERMANY: Thin-film solar cells can also use light, flexible substrates, which significantly increases their range of applications. Until now, however, such cells have been difficult and time-consuming to manufacture. The Center for Solar Energy and Hydrogen Research Baden-Württemberg (ZSW) has achieved a breakthrough in the development of efficient web coating of thin-film solar modules made of copper indium gallium diselenide (CIGS) on plastic film.

The researchers from Stuttgart have already completed the most important production steps in one go, using a continuously running system. The next phase is to complete all the steps in the same system, which will make the manufacturing process considerably more efficient. The solar cells have already achieved 10.2 percent efficiency.

Until now, the manufacture of flexible thin-film modules has been labour intensive and time consuming. Every single layer is deposited individually in a separate system. ZSW has now combined these steps in a single system. The researchers’ goal is to produce fully integrated solar modules with even greater efficiency. Mass production could help to create a new generation of affordable, flexible photovoltaic modules.

ZSW has been developing CIGS thin-film solar cells from the roll since 2010, using a twelve metre web-coating system in one of their technical labs. A temperature-resistant plastic polymer film called polyimide is used as the substrate. The film is 0.0025 centimetres thick and 30 centimetres wide.

“The unique feature of this web-coating system is that all coating steps take place simultaneously in the same vacuum,” explains Prof. 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 co-evaporation 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 and the zinc oxide window layers can be deposited. The development and integration of a new buffer layer is still in progress. The monolithic cell interconnection will also be fully integrated at a later point in time.

Flexible thin-film solar cells on plastic film have enormous potential: they enable many new areas of application where it is important for the cells to be both lightweight and flexible. Solar cells on T-shirts and jackets, modules on cars and light aircraft are just a few examples. It is also advantageous to bond the solar foil to a glass substrate. Such modules would weigh half as much as standard modules which are typically framed using two glass panels – making them ideal for roofs that aren’t capable of bearing heavy loads.

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.

ZSW is one of the leading institutes for applied research in the area of photovoltaics, renewable fuels, battery technology, fuel cells and energy system analysis. In collaboration with Würth Solar, the institute successfully developed CIGS thin-film photovoltaics to series production maturity. There are currently around 200 scientists, engineers and technicians employed at ZSW’s three locations in Stuttgart, Ulm and Widderstall.