Planar polymer light-emitting device with fast kinetics at a low voltage

Polymer light-emitting electrochemical cells containing a ternary mixture of a soluble phenyl-substituted poly(para-phenylene vinylene) copolymer ("superyellow"), a dicyclohexano-18-crown-6 (DCH18C6) crown ether and a LiCF3SO3 salt as the active material have been assembled. Planar Au/{superyellow+DCH18C6+LiCF3SO3}/Au devices, with an interelectrode gap of 50 mum, were initially charged (i.e., electrochemically p- and n-doped in situ) at T=85 degreesC and then cooled to room temperature under applied voltage. When operated at T=23 degreesC charged devices show electroluminescence with fast response (< 1 s) at a low applied voltage (Vgreater than or equal to6 V). Charged devices could be stored under open-circuit conditions at room temperature for a prolonged time without detectable changes in device performance, and they can be completely (reversibly) discharged by raising the temperature to 85 degreesC. The active material mixtures were studied by atomic force microscopy and differential scanning calorimetry. The results demonstrate that superyellow phase separates from a crystalline DCH18C6-LiCF3SO3 complex on a similar to25 nm scale. The superyellow phase exhibits a glass transition at T(g)similar to180 degreesC, while the crystalline DCH18C6-LiCF3SO3 phase melts at T(m)approximate to56 degreesC. Thus, we attribute the stabilization of charged Au/{superyellow+DCH18C6+LiCF3SO3}/Au devices in going from 85 to 23 degreesC as being directly related to the passage of T-m of the DCH18C6-LiCF3SO3 phase. The ionic distribution related to the p- and n-doped regions is "frozen-in" by this crystallization allowing for the observed fast kinetics at low voltages at room temperature. (C) 2004 American Institute of Physics.