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

摘要

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 LiCF_(3)SO_(3) salt as the active material have been assembled. Planar Au/{superyellow+DCH18C6+LiCF_(3)SO_(3)}/Au devices, with an interelectrode gap of 50 μm, were initially charged (i.e., electrochemically p- and n-doped in situ) at T=85℃ and then cooled to room temperature under applied voltage. When operated at T=23℃ charged devices show electroluminescence with fast response (<1 s) at a low applied voltage (V≥6 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℃. 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-LiCF_(3)SO_(3) complex on a ～25 nm scale. The superyellow phase exhibits a glass transition at T_(g)～180℃, while the crystalline DCH18C6-LiCF_(3)SO_(3) phase melts at T_(m)≈56℃. Thus, we attribute the stabilization of charged Au/{superyellow+DCH18C6+LiCF_(3)SO_(3)}/Au devices in going from 85 to 23℃ as being directly related to the passage of T_(m) of the DCH18C6-LiCF_(3)SO_(3) 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.