The Dielectric Response Of 8-Hydroxyquinoline Aluminum used in Multilayer Organic Devices
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controls the emission color [5]. For Alq 3, thin films, such as those incorporated into LED's, were investigated in the UV-Vis to near-IR spectral region in transmission and reflectance in order to determine the dielectric function (e) & complex refractive index (i) = n + iK) of the material. We also present internal field distribution measurements of multilayers of Alq 3 and m-LPPP devices, which also allows us to determine the dielectric coefficients of these two materials. EXPERIMENTAL UV-VIS spectroscopy In order to determine the complex dielectric function of Alq 3, thin films produced by vacuum deposition onto quartz substrates were investigated by optical spectroscopy. Quartz is an ideal substrate for this purpose due to its transparency in the full UV-Vis region. UV-Vis spectroscopy was done using a Lambda-9 spectrophotometer, in the wavelength range 200-2500 nm at normal incidence for transmission (7), and near normal (50) in reflectance (R) using a specially designed reflectance sample holder. This requires the calibration of the mirrors making up the sample holder beforehand. Both measurements were done on the same portion of the thin film, so that the spectra are homologous. For the characterization, a bare quartz spectrum was also measured, as we require the optical constants of the substrate for the calculations 713 Mat. Res. Soc. Symp. Proc. Vol. 488 01998 Materials Research Society
Shown below are the two spectra for a thin film (160 nm thickness) of Alq3 on quartz. What is immediately apparent is the clear interference pattern in both R and T spectra, where the minima and maxima in R & T show the same spacing. This is a good indication that the film surface is homogeneous, indicating good film quality. Below 250nm, the films do not transmit any light, which means any characterization below this wavelength is problematic. 20-
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Figure 1: UV-Vis transmission and reflectance spectra of a 160 nm thick Alq 3 film With the two measurements, we can solve Fresnel equations for a thin film on substrate model in order to determine the optical coefficients of a layer, as long as we have knowledge of the film thickness and the refractive index of the substrate [6, 7]. The substrate data was determined from solving Fresnel formulae for a film substrate (quartz has negligible absorption, and a spectrally nearly constant refractive index n,u = 1.48). The film thickness was determined in-situ during the vacuum deposition process, and confirmed by the Tolansky method. Internal Field distribution measurements The internal field distribution of LED's in a multilayer structure of ITO/m-LPPPP/Alq 3/AI:Mg on glass was investigated by electromodulation transmission/absorption spectroscopy [4]. Devices with the same thickness of the m-LPPP layer but varying thickness of transport layer Alq 3 were characterized. These devices show a strong dependency of onset voltage, emission color and quantum efficiency with Alq 3 film thickness [8]. By applying a su
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