Electrospray deposition of polymer thin films for organic light-emitting diodes
© Hwang et al; licensee Springer. 2012
Received: 6 September 2011
Accepted: 5 January 2012
Published: 5 January 2012
Electrospray process was developed for organic layer deposition onto polymer organic light-emitting diode [PLED] devices in this work. An electrospray can be used to produce nanometer-scale thin films by electric repulsion of microscale fine droplets. PLED devices made by an electrospray process were compared with spin-coated ones. The PLED device fabricated by the electrospray process showed maximum current efficiency of 24 cd/A, which was comparable with that of the spin-coating process. The electrospray process required a higher concentration of hole and electron transport materials in the inks than spin-coating processes to achieve PLED maximum performance. Photoluminescence [PL] at 407 nm was observed using electrosprayed poly(N-vinyl carbazole) films, whereas a peak at 410 nm was observed with the spin-coated ones. Similar difference in peak position was observed between aromatic and nonaromatic solvents in the spin-coating process. PLED devices made by the electrospray process showed lower current density than that of spin-coated ones. The PL peak shift and reduced current of electrosprayed films can therefore be attributed to the conformation of the polymer.
During the last two decades, intense academic and industrial research has been devoted on organic light-emitting devices [OLEDs] due to their potential applicability to flat panel displays and solid-state lighting [1–3]. Small areas of less than 5-in. devices have been commercialized recently. Currently, all commercial processes adopt vacuum evaporation processes for both organic and metal layer formations. However, vacuum evaporation processes are significantly limited in large-area processing as well as hardware cost and require a considerable material. These limitations and drawbacks prevent OLEDs from being applicable to large-area device fabrication. Much effort has been made for solution process development that can form nanoscale-thin organic films with a large area while minimizing material wastes. A laboratory-scale spin-coating process is one of the most well known and widely used solution processes. Various solution printing techniques have been developed such as ink-jet printing, nozzle printing, screen printing, gravure printing, and so on. Thin-film transfer process, organic vapor deposition, and blade coating are also being developed as large-area, cost-effective alternatives [4–10]. However, challenges remain relating to good uniformity over a large-area and multilayer formation without buffer or cross-linking materials for commercial development of organic displays and lighting devices.
Recently, electrospray process has gained much attention as a solution process for organic and inorganic thin films, and a few research groups have reported applications to organic device fabrications [11–13]. In the electrospray process, a liquid flow is injected into the nozzle with an electric field applied between the nozzle tip and ground plate, and microscale monodisperse fine droplets are generated due to repulsion forces between like charges in the drops. The size of droplets can be controlled by adjusting the flow rate and electric field applied to the injection nozzles and substrates, and the diameter of the droplets can be as small as several hundred nanometers in scale [14–16]. The electrospray process with vapor treatment has been applied to organic photovoltaic fabrication, while comparable power conversion efficiency has been reported with the spin-coating process . Additionally, applicability of the electrospray process to organic thin films in OLEDs was demonstrated in small-scale devices .
In this work, we demonstrated nanoscale-thick organic thin films using the electrospray process as a solution process alternative. Polymer LEDs [PLEDs] were fabricated and compared with those made by the spin-coating process. The electrospray process can be considered as an effective process for patterning, multilayer stacking, and continuous processing of organic thin films.
Ratio of the PLED ink
All electrical measurements were performed under ambient conditions. Device performance was measured using a source measure unit (2400, Keithley Instruments, Inc., Cleveland, OH, USA) and a luminance meter (CS100, Konica Minolta Sensing, Inc., Sakai, Osaka, Japan). Photoluminescence [PL] spectra were collected with a monochromatized 150-W Xe light source (FP-6200, Jasco International Co. Ltd., Hachioji, Tokyo, Japan), and a wavelength of 335 nm was used for analysis of PL excitation. The thickness and roughness of PVK films were determined by a surface profiler (Alpha-Step, KLA-Tencor Corporation, Milpitas, CA, USA) and by atomic force microscopy (Digital Instruments, Santa Barbara, CA, USA), respectively.
Results and discussion
Summary of device performances
We have demonstrated polymer organic LEDs using an electrospray process as a solution process alternative. Device performance comparable to spin-coating was achieved with the electrospray process, which can be considered as a scalable large-area process alternative. However, the electrospray process requires elaborate choices involving solvents and higher concentrations of hole and electron transport materials in active materials. A high dielectric constant and a high-boiling point solvent like DCB are preferred in electrospray processing. Accordingly, the device based on the electrospray process had better performance when we used the ink having higher concentrations of hole and electron transport materials. The electrospray process can be considered as a viable solution for large-area organic thin-film formation technology in the future.
This research was supported by the Basic Science Research Program through the National Research Foundation of Korea (NRF) funded by the Ministry of Education, Science and Technology (2011-0012279) and (2011-0006268).
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