Roll-to-roll (R2R) manufacturingMarkus Hösel
Organic solar cells have the advantage to be fully solution processable using printing and coating methods. The processing has been shown in labs on a variety of different substrates other than rigid glass, such as plastic foil (PET, PEN, barrier) or metal foil. The fastest fabrication can be achieved using full roll-to-roll (R2R) processes in a machine as shown in Figure 1. Companies such as infinityPV.com employing ambient R2R printing and coating methods for the fabrication of flexible organic solar cells as shown in the video below.
Different coating and printing processes can be used and will be described in more detail in the following sections
- Blade coating
- Spray coating
- Slot die coating
- Screen printing
- Flexographic printing
- Gravure printing
Further important processing units for the fabrication of functional devices on foil are pre-treatment tools such as corona or plasma to increase the surface energy of the substrate. It is used to improve wetting and adhesion of the ink (e.g. silver ink) to the bare substrate surface. To remove dust particles and impurities from the surface prior printing a web cleaning station is used.
After the actual deposition of the liquid ink the wet layer has to be dry before getting in contact with the roller. Hot air ovens are typically used and can be supported by infrared dryers. The maximum drying temperature is limited by the substrate material (e.g. 130-140°C for PET). The drying time is ultimately defined by web speed and dryer length.
R2R fabrication strategies
Organic solar cells are multilayer devices where each layer has a specific function such as conductive electrode or light absorbing active layer for charge generation. Furthermore, each of the functional layers and inks have different process requirements in the final device such layer thickness and drying time. Each ink has to be optimized for a specific processing method and vice versa. The final process parameters are often very different with respect to speed and drying time or temperature.
In a discrete R2R processing workflow each functional layer is printed or coated in a separate run. All process parameters (fabrication method, ink, speed, drying) are optimized to each of the layers. A machine can be optimized to a specific layer and the required conditions or vice versa. Discrete processing is the most favorable workflow for research purposes.
The ultimate fabrication scenario is inline processing of all layers in one machine and at the same time. The order of printing unit and dryer length would be tailored to a specific device stack while the maximum processing speed is limited by the slowest single process. A process failure in one of the layers can lead to a malfunction of the full device stack. The advantage of full inline processing is the minimized bending stress inside the R2R machine, reduced handling, and potentially much faster device completion.
A special case is an intermediate process workflow with a combination of some inline processes in an overall discrete process. Subsequent printing and coating steps with similar speed ranges and drying requirements can be combined. It has been shown to work very well for the fabrication of ITO-free organic solar cells, where front silver grids and PEDOT:PSS was inline printed, and ZnO and active layer was inline slot-die coated in one run.DOI:10.1002/ente.201200029