Rheological and Drying Considerations for Uniformly Gravure‐Printed Layers: Towards Large‐Area Flexible Organic Light‐Emitting Diodes

Printing organic semiconductor inks by means of roll‐to‐roll compatible techniques will allow a continuous, high‐volume fabrication of large‐area flexible optoelectronic devices. The gravure printing technique is set to become a widespread process for the high throughput fabrication of functional layers. The gravure printing process of a poly‐phenylvinylene derivative light‐emitting polymer dissolved in a two solvent mixture on poly(3,4‐ethylenedioxythiophene):poly(styrene sulfonate) (PEDOT:PSS) is studied. The surface tensions, contact angles, viscosities, and drying times of the formulations are investigated as a function of the solvent volume fraction and polymer concentration. The properties of the ink grant a homogeneous printed layer, suitable for device fabrication, when the calculated film leveling time is shorter than a critical time, at which the film has been frozen due to loss of solvent via evaporation. The knowledge obtained from the printing process is applied to fabricate organic light‐emitting diodes (OLEDs) on flexible substrates, yielding a luminance of ≈5000 cd m^?2.