Electroluminescence and Photoluminescence of Fluorene-Carbazole Copolymer
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"m Figure 1: Poly(DHF-co-NHK) 299 Mat. Res. Soc. Symp. Proc. Vol. 558 © 2000 Materials Research Society
Thin films were prepared from 20 mg.ml-1 chloroform solution (typical thickness 200nm) by spin coating onto fTO-coated glasses. Al contacts (thickness close to 150 nm), were deposited onto the polymer layer by vacuum evaporation using a mask to determine well-defined shape for the Al electrodes whose area is 9 mm2. PHOTOLUMINESCENCE AND ELECTROLUMINESCENCE Photoluminescence (PL) measurements were performed using the 360 nm line of a filtered mercury arc lamp for the excitation. For quantitative measurements, a calibrated Hamatsu silicon photodiode with a large surface collection was used. Figure 2 compares the spectra of EL and PL for freshly prepared films and chloroform solution of the same copolymer. Clearly the three spectra share the same spectral features, strongly indicating that PL and EL of films take their origin in electron-hole pair recombination, intra chain electronhole recombination dominating for fresh layers. An opposite conclusion is obtained for aged samples (spectra not shown here) for which spectral behavior are in favour of an interchain recombination, attributed to the crystallisation of the layer [1,4,51. This point will not be discussed here.
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Wavelength(nm) Figure 2 Photoluminescence (a) and electroluminescence spectra (b) of poly(DHF-coNHK) films compared to the photoluminescence spectrum (c) of the copolymer in chloroform solution. An other interesting point is the identical spectral shape of copolymer (this work) and spectra of fluorene homopolymer previously reported [4]. These results can only be explained if we assume that light comes from segments of poly(dihexylfluorene). We can expect, as previously reported for copolymer of poly(dihexylfluorene) and anthracene [4], that the composition of the copolymer tracks the starting molar monomer ratio (4:1 in our case). It is noteworthy that the small amount of N-alkylcarbazole within the main chain structure remains sufficient to allow hole injection improvement as discussed further.
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ELECTROLUMINESCENCE AND ELECTRICAL MEASUREMENTS When a bias of 25 V is applied between the two electrodes, the device rapidly turn on and emit a bright blue light. Figure 3 displays the light emission as a function of time over a period of 60 minutes under ambient air
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time (min) Figure 3: Light emission as a function of time for an ITO/poly(DHF-co-NHK)/AI device. We must point out that the emission is first only localised on the border of electrodes and progressively grow larger to concern the overall diode area within few minutes. After this transient state, which is not actually well understood, the emission slowly decreases but is rather stable. Typical lifetime for electroluminescence is 8 hours under 25 volts. Figure 4 compares I(V) curve and electroluminescence intensity versus voltage obtained during a single scan. 3.0 0.25 2.5
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