OLEDs are seen as a promising lighting technology but their market penetration remains minute due to their high production costs and moderate light output compared to inorganic LEDs. But thanks to a CTI project (supported by the Swiss Confederation), CSEM and Sefar have found a way of utilising OLEDs for lighting, especially in residential, architectural, professional lighting segments, as well as consumer electronics.

Transparent, highly conductive substrates

CSEM and Sefar have developed low-cost, flexible, transparent, highly conductive electrodes made of fabric substrates comprising of flexible metallic wires (about 40μm in diameter) and polymeric fibres woven together in a highly transparent and flexible polymer.

 
CSEM OLED design (cr) Figure 1: OLED designed with Sefar's conductive fabric.  

The OLEDs are finalised by coating the substrate with a thin-film (tens of nanometres) layer of a solution-process conductive polymer. The high electrical conductivity of the metal wires in the fabric substrate ensures that the electrode displays high conductivity over large distances, even with an ultra-thin, and hence highly transparent, layer of the conductive polymer.

These fabric substrates (SEFAR TCS Planar) exhibit a light transmittance of around 90% throughout the visible and near infrared spectrum, a sheet resistance under 0.1Ω/sq, and can be flexed to a bending radius of 6mm with barely any decrease in conductivity. This makes it an interesting alternative to more expensive and brittle ITO layers typically used as transparent electrodes in many electronic applications, from touch displays to solar cells and OLEDs.

The fabrics can be further coated with a transparent polymer to make them impermeable to liquids and gases while remaining conductive from one side, providing a barrier for humidity and oxygen without altering noticeably their transmittance characteristics.

Eases large-area OLED production

 
CSEM Sefar fabric substrate diagram (cr) Figure 2: Micrometre-size conductive metal wires and transparent polymer fibres are woven together and embedded into an optically clear filling polymer. The substrate coated with the conductive polymer forms the electrode upon which the light-emitted polymer and the top electrode are processed to complete the OLED device.  

Sefar fabric-based electrodes simplify the production of large-area OLEDs by eliminating the evaporation, photolithography and electrical insulation of the supporting metal tracks, Peter Chabrecek, R&D manager at Sefar said.

The technology can be applied for OLEDs, as well as for many other products, including solar cells, EL devices, touch screens, electrochromic glasses, transparent heating elements, sensors, photo-detectors and transparent shielding elements, according to the company.