Toward Electrically Pumped Organic Diode Lasers: Electroluminescence of Proton Transfer Polymers
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inhibit the proton transfer in such polymers [13,14]. The results also showed that molecular size does not inhibit ESIPT and that the barrier to ESIPT can be lower than the barrier to excimer formation in polymers exhibiting ESIPT [ 13,14]. The successful development of semiconducting polymer light emitting diodes (LED) [12] and recent demonstration of optically-pumped lasers [15] in the same class of materials have now focused interest in the prospects of making electrically-pumped laser diodes [1517]. The question of whether electrical pumping can produce the necessary population inversion in organic semiconductors has become of central importance to this quest. The recent report of electrically-generated intramolecular proton transfer (EGIPT) and stimulated emission from proton transfer polymers has provided the first affirmative answer to this question [17]. In this paper we report investigation of the EGIPT process in several proton transfer polymers with different structures shown in Figure 1. The investigated polymers include poly(l,4-(2-hydroxy)phenyleive benzobisthiazole-co-decamethylene benzobisthiazole) (HPBT-co-PBTC1O), poly(styrene-co-3-hydroxy-4'-ethenylflavone) (PS-co-V3HF), and poly(2,5-benzoxalediyl(2,2,2-trifluoro-1-(trifluoromethyl)ethylidene)-5,2-benzoxazodiyl-4hydroxy-1,3-phenylene) (mH6FPBO). ,H-O N
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mH6FPBO Figure 1. Molecular structures of proton transfer polymers and a schematic the LED. EXPERIMENTS The synthesis and characterization of the proton transfer polmers shown in Figure 1 have previously been reported [14,17,18]. The structure of the electroluminescence (EL) devices is also shown in Figure 1. First, a layer of 50 nm poly(vinyl carbazole) (PVK) was deposited onto the ITO (indium tin oxide) coated glass substrate by spin coating from dichloroethane solution. The PVK layer functions as the hole transport layer and electron
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blocking barrierpolymer, to confine layer. Next, a 50acid nm (HPBT-colayer of a proton transfer the electrons emissive within layer, the was emissive spin coated from formic PBTC10 and mH6FPBO) or toluene (PS-co-V3HF) solutions. After drying the films in vacuum oven at 80 °C for 12 hours, an aluminum electrode (electron injecting electrode) of 50-100 nm thick was thermally evaporated at high vacuum (below 10.5 torr). Optical absorption measurements were done using a Perkin-Elmer Lambda-9 UV/vis/near-IR spectrophotometer. Photoluminescence (PL) and electroluminescence spectra were taken by using a Spex Fluorolog-2 spectrofluorimeter. Current-luminance-voltage curves were recorded simultaneously by hooking up an HP4155A semiconductor parameter analyzer together with a Grasby $370 optometer equipped with a calibrated luminance sensor head. All measurements were performed under ambient conditions. RESULTS Figure 2 shows the optical absorption and PL spectra of 5 and 15 % HPTB-coPBTC10
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