Ultrasonic spray coating as an approach for large-area polymer OLEDs: The influence of thin film processing and surface roughness on electrical performance

In this article we present a detailed comparison of ultrasonic spray coating and spin coating for the fabrication of polymer organic light-emitting diodes (OLEDs). Single-carrier devices of hole-transporting polymer poly[(9,9-dioctylfluorenyl-2,7-diyl)-co-(4,4’(N-(4-sec-butylphenyl))) diphenylamine] (TFB) were fabricated by ultrasonic spray coating. Uniform reference devices using spin coating were also made. We have shown, across a range of device thicknesses from 37 nm to 138 nm, typical of those used in OLED hole-transport layers, that there is no statistical difference in the hole-injection efficiency between ultrasonic spray coating and spin coating. We have also demonstrated the importance of controlling the roughness of the films and we determine a threshold of 10 nm average roughness below which injection efficiency is not controlled by roughness. However, above 10 nm roughness we find a reduction in injection efficiency up to an 86 % loss in performance for roughnesses of the order of 40 % the thickness of the film. By optimising the deposition parameters, in order to allow the wet films to start to equilibrate, we find a wide processing window for smooth uniform films with excellent injection efficiency. This work reinforces the importance of ultrasonic spray coating as a potential route to high volume manufacturing of OLED based technology.