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Lasers set to augment production of organic photovoltaic cells

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The ablation of nm-thin layers on a moving film with USP laser scribing in the fs regime. (Image: Fraunhofer ILT)

A new project is set to boost the mass production of organic photovoltaic cells using a number of different laser technologies.

Both short- and ultrashort-pulse lasers will be used within the EffiLayers project, launched in September, to increase the efficiency of roll-to-roll processes, which can be used to manufacture the cells on an industrial scale.

Organic photovoltaic cells (OPV cells), while less efficient than traditional silicon-based solar cells, are flexible and transparent. This enables them to be functionally and decoratively integrated, for example into building facades.

The individual layers of the OPV cells are only a few nanometres thick, meaning the technology only requires small amounts of material to coat large areas of flexible substrates – an area of 10m2 can be produced using only three grams of organic active material. This way, producers save on resources. Manufacturing them using industrial-scale roll-to-roll processes also removes the need for energy-intensive and costly process steps – as are required in classical silicon photovoltaic production.

‘We would like to implement the process in a manner oriented to the industry’, confirmed Ludwig Pongratz, a researcher at Fraunhofer ILT – one of the five project partners of EffiLayers. ‘Our approach is to replace expensive, energy-intensive sputtering processes with wet-chemical coating processes.’

The project partners are using lasers to improve the roll-to-roll production of organic photovoltaics. (Image: Fraunhofer ILT)

In the new approach, the functional layers of the cells are applied on top of each other via wet-chemical solutions by means of heated slot-die coating. The 10 to 250nm-thick layers are then processed using multiple laser sources wielding both short and ultrashort pulses.

For example, an ultrashort pulse laser in the femtosecond regime is used to separate the individual layers so that individual cells can be connected in series. ‘With laser scribing, we guide eleven partial beams onto the surface while the film moves’, explained Pongratz. ‘The laser beams selectively separate the composite layers so that, at the end, twelve serially connected subcells are produced on a single film. The challenge is to selectively ablate the individual nanometre-thick layers without damaging the underlying layers or causing short circuits.’

The OPV cells are later sealed by laser encapsulation with a barrier film to protect them from environmental factors.

A high-resolution line scan camera is used to control the laser scribing with eleven laser beams in the new EffiLayers process. (Image: Fraunhofer ILT)

In order to map the entire process, Fraunhofer ILT is cooperating with the Chair of Applied Laser Technologies (LAT) at the Ruhr-Universität Bochum. The two German research institutes are working closely with the plant manufacturer Coatema Coating Machinery from Dormagen, the communication experts of Ortmann Digitaltechnik from Attendorn and LIMO from Dortmund, the latter of which provides the optical components for the eleven partial beams.

The individual complex steps involved in the roll-to-roll production of OPV cells were originally developed in two predecessor projects conducted by the partners, FlexLas and PhotonFlex. EffiLayers has since been launched with the aim of implementing and integrating innovative analysis and process technologies into the production process. The individual process steps will be monitored by high-resolution sensors and implemented in a process control system.

‘Because we were able to successfully implement many innovative process steps into our plant in the first two projects, the funding of a third project was approved with funds from the European Regional Development Fund (ERDF),’ Pongratz remarked.

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