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Abstract
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.
Highlights
We have 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
Gilissen et al deposited the emissive layer of a polymer organic light-emitting diodes (OLEDs) via ultrasonic spray coating and achieved a power efficacy of 9.71 Lm W-1 compared to 12 Lm W-1 via spin coating
We have compared the injection efficiency of holes into films of poly[(9,9-dioctylfluorenyl-2,7-diyl)-co-(4,4’(N-(4-secbutylphenyl))) diphenylamine], cast by ultrasonic spray coating with those cast by spin coating
Summary
Spray coating is a promising candidate for the low cost and large scale processing of polymer semiconductors for use in optoelectronic devices such as area lighting, solar cells, electrochromic devices and transistors. Ultrasonic spray coating has the benefit over other spray coating techniques, such as airbrush spraying, of increased uniformity of droplet size, leading to increased spray and film uniformity. Ultrasonic spray coating has been used widely in polymer organic photovoltaics (OPVs), spraying single and multilayer devices but the use for polymer organic lightemitting diodes (OLEDs) it has so far been limited. Gilissen et al deposited the emissive layer of a polymer OLED via ultrasonic spray coating and achieved a power efficacy of 9.71 Lm W-1 compared to 12 Lm W-1 via spin coating.. Gilissen et al deposited the emissive layer of a polymer OLED via ultrasonic spray coating and achieved a power efficacy of 9.71 Lm W-1 compared to 12 Lm W-1 via spin coating.. We conclude that the surface roughness plays an important role in controlling injection efficiency but that for devices fabricated using spray or spin coating, that have comparable surface roughness, no difference in charge-injection efficiency can be measured. These results suggest that there is nothing intrinsic in spray coating that limits the overall device performance. The morphology of the as-formed layer does not limit the injection efficiency and transport in our experimental devices
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