Moisture Effect on Polythiophene Electroluminescence
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599 Mat. Res. Soc. Symp. Proc. Vol. 488 01998 Materials Research Society
polymer is reduced by adding an electron into the chain, a carboanion will form. This extra electron tends to combine one of the 7r-electrons in the backbone and forms a lone electronpair which occupies an sp3 orbital. The stronger interaction between the lone pair and the saturated o-bond produces a much larger out-of-plane electron-phonon interaction for the electron-polarons. The electron mobility is suppressed strongly. One can assume that the majority of charge carriers in the device are hole polarons. Therefore, it is reasonable to consider only two forms of the polythiophenes, neutral and positively charged, in the studies of the interaction with water molecules. METHOD AND EXPERIMENT We studied the effect of moisture on the PT electroluminescence by carrying out quantumchemical studies on model systems composed of neutral (or positively charged) thiophene
oligomers and water molecules. To minimize the chain-end effect, we chose the tert-thiophene which contains only three thiophene rings in the present study. The quantum-chemical calculations were carried out by several steps in order to minimize computational cost. The geometry of a water/tert-thiophene complex was first optimized using AM1 semiempirical method in order to get a preliminary molecular conformation. Thereafter, a precise geometry optimization using Hartree-Fock (HF) and density functional theory methods with 6-31G* basis set built in the Gaussian94 program package[4] was carried out. The functionals we used were Becke's three parameter hybrid method with the Lee-Yang-Parr correlation functional. All geometries were optimized without using any constraint. The LED devices for the electroluminescence experiment consist of an ITO glass electrode, a subsequently spin coated polyhexylthiophene film, and an Al vapor evaporated cathode. These samples were kept inside a closed box under ambient atmosphere with chemicals as drying agents, which are corresponding to different relative humidity. The relative humidities inside closed box for P205, KOH, and NaOH are 0.0001%, 0.1%, and 3%, respectively. The electroluminescence spectra were measured by Shimadsu RF-5000 fluorophotometer at room temperature (see Figure 1). The luminance were measured by Minolta LS-100 luminance meter. The measured luminances of PT LED devices at 4.5 voltage are 0.08, 0.047, and 0.006 (cd/mr2 ). RESULTS AND DISCUSSION The bond lengths of the optimized geometries of neutral and positively charged water/tertthiophene complexes found in these calculations are listed in the table 1. The backbone structure of an undoped tert-thiophene is actually a cis-transnoid polyene with linked sulfur atoms. The bond length alternation pattern is similar to that of polyenes but with a smaller dimerization amplitude. The bond lengths of carbon-carbon single bonds and double bonds are 1.42 and 1.38 A, respectively. The inter-ring carbon-carbon single bonds ("-, 1.45 A) are longer than single bonds in the thiophene rings.
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