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

Abstract

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 μm, were initially charged (i.e., electrochemically p- and n-doped in situ) at T=85 °C and then cooled to room temperature under applied voltage. When operated at T=23 °C 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 °C. 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 ∼25 nm scale. The superyellow phase exhibits a glass transition at Tg∼180 °C, while the crystalline DCH18C6–LiCF3SO3 phase melts at Tm≈56 °C. Thus, we attribute the stabilization of charged Au/{superyellow+DCH18C6+LiCF3SO3}/Au devices in going from 85 to 23 °C as being directly related to the passage of Tm 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.