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 ${\mathrm{LiCF}}_3{\mathrm{SO}}_3$ salt as the active material have been assembled. Planar $\mathrm{Au}\mathrm{/\{}\mathrm{superyellow}+\mathrm{DCH}18\mathrm{C}\text{6+}{\mathrm{LiCF}}_3{\mathrm{SO}}_3\mathrm{\}/}\mathrm{Au}$ devices, with an interelectrode gap of 50 μm, were initially charged (i.e., electrochemically p- and n-doped in situ) at $\text{T=85\hspace{0.05em}°}\mathrm{C}$ and then cooled to room temperature under applied voltage. When operated at $\text{T=23\hspace{0.05em}°}\mathrm{C}$ charged devices show electroluminescence with fast response (< 1 s) at a low applied voltage $\text{(V⩾6\hspace{0.17em}}\mathrm{V}\mathrm{).}$ 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 $\mathrm{DCH}18\mathrm{C}\text{6–}{\mathrm{LiCF}}_3{\mathrm{SO}}_3$ complex on a ∼25 nm scale. The superyellow phase exhibits a glass transition at $T_g\text{∼180\hspace{0.05em}°}\mathrm{C},$ while the crystalline $\mathrm{DCH}18\mathrm{C}\text{6–}{\mathrm{LiCF}}_3{\mathrm{SO}}_3$ phase melts at $T_m\text{≈56\hspace{0.05em}°}\mathrm{C}.$ Thus, we attribute the stabilization of charged $\mathrm{Au}\mathrm{/\{}\mathrm{superyellow}+\mathrm{DCH}18\mathrm{C}\text{6+}{\mathrm{LiCF}}_3{\mathrm{SO}}_3\mathrm{\}/}\mathrm{Au}$ devices in going from 85 to 23 °C as being directly related to the passage of $T_m$ of the $\mathrm{DCH}18\mathrm{C}\text{6–}{\mathrm{LiCF}}_3{\mathrm{SO}}_3$ 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.