White-Light-Emitting Diodes Based on Iridium Complexes via Efficient Energy Transfer from a Conjugated Polymer

Efficient white‐light‐emitting diodes (WLEDs) have been developed using a polyfluorene‐type blue‐emitting conjugated polymer doped with green and red phosphorescent dyes. The emission spectrum of the conjugated polymer, which has a very high luminescent efficiency, shows a large spectral overlap with the absorbance of green and red iridium complexes. Also, efficient energy transfer from the conjugated polymer to the iridium complexes is observed. Poly(N‐vinyl carbazole) is used to improve the miscibility between conjugated polymer and iridium complexes because of their poor chemical compatibility and phase separation. A white emission spectrum is easily obtained by varying the contents of the three materials and controlling the phase morphology. Moreover, these WLEDs show a voltage‐independent electroluminescence owing to the threshold and driving voltage of the three materials being similar as a result of energy transfer. Efficient polymer‐based electrophosphorescent devices for stable white‐light emission (see Figure) take advantage of efficient energy transfer from a conjugated polymer to iridium complexes. Poly(N‐vinylcarbazole) increases energy transfer from the conjugated polymer to the phosphorescent dyes by improving the chemical compatibility; hence, the microphase‐separated morphology of the blend allows blue‐light emission from the polymer, and green and red from the iridium complexes.