Three-Color Doping of Polymer Oleds by Masked Dye Diffusion
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Figure 1: Experimental set-up: a shadow mask is sandwiched between the diffusion source which provides the dye and the device substrate with the polymer film 5
The experimental set-up is shown in figure 1. The device substrate with the organic film to be patterned were deposited on a glass substrate coated with sputtered ITO ( 30 Q/0). A polymer blend layer with poly(9-vinylcarbazole) (PVK; Mw ca. 1,100,000 g/mole; 71.5 % by weight in the final film) as a hole transport polymer and 2-(4-Biphenylyl)-5-(4-tert-butylphenyl)-1,3,4-oxadiazole (PBD; 28.5 % by weight) as electron transport molecule was created by spin coating 6.Some blends also contained 9, 1O-Dioxa-syn-dimethylbimane 7,8 (bimane; wt 0.2 513 Mat. Res. Soc. Symp. Proc. Vol. 558 ©2000 Materials Research Society
%) as an emissive dye (emission peak at X= 435 nm). The resulting thickness of the organic film after spin coating was approximately 100 nm. The dye used during the diffusion process was either coumarin 6 (C6, green) or Nile red. The diffusion source consists of a flat plate with a highly doped polymer layer as the dye source. The layer was deposited by spin coating using a solution of equal amounts by weight of PVK and C6 or Nile Red dissolved in chloroform. The shadow mask is a patterned stainless steel foil (200 lam thick). The diffusion source layer, the shadow mask and the polymer film of the device substrate were then placed in close contact as shown in figure 1 to perform the diffusion. Figure 2 shows the photoluminescence (PL) spectra after different diffusion times in which C6 was diffused into a layer of pure PVK (i.e. the film does not contain PBD or bimane) in an oven at 100 °C in air. In this case, the dye source was a film of pure C6 evaporated on a glass substrate. The PL excitation source was a ultraviolet lamp (X=254 nm). Before the diffusion, one only observes the emission of PVK with a peak at 407 nm. After 15 min, the C6 emission peak at -500 nm can be detected. After about 50 min the C6 peak dominated the spectrum.
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Figure 2: PL spectra of a pure PVK film in which C6 was diffused for different times
This simple experiment demonstrates the feasibility of this method. However, since the quantum efficiency strongly depends on the dye concentration it is imperative to investigate the dye transfer under different conditions in order to be able to control the diffusion process. The patterning process consists basically of three steps: The dye must diffuse out of the dye source film, travel to the surface of the device substrate and diffuse into the polymer film. The last step appears to be the rate limiting one. After a very long diffusion time, the color of the PL emission of C6 from the top of the target film was shifted from green to orange. Since C6 in its pure form em
