Electrophosphorescence and Delayed Electroluminescence from Pristine Polyfluorene Thin-Film Devices at Low Temperature

Intrinsic long-lived electrophosphorescence and delayed electroluminescence from a conjugated polymer (polyfluorene) thin film is observed for the first time at low temperature. From bias offset voltage dependent measurements, it is concluded that the delayed fluorescence is generated via triplet-triplet annihilation. A fast and efficient triplet exciton quenching by charge carriers is found to occur in the active polymer layer of the working devices.

Figure 1

Prompt fluorescence spectra (dashed line: $\mathrm{\text{bias}}=11\mathrm{V}$) and delayed emission spectra (solid line: $\mathrm{\text{bias}}=11\mathrm{V}$, pulse $\mathrm{\text{width}}=5\mathrm{m}\mathrm{s}$, $\mathrm{\text{delay}}=5\mathrm{m}\mathrm{s}$, gate $\mathrm{\text{width}}=70\mathrm{m}\mathrm{s}$) of PF2/6am4 thin film device at 28 K. The inset shows the chemical structure of PF2/6am4.

Figure 2

Delayed emission spectra of PF2/6am4 thin film device at 28 K ($\mathrm{\text{bias}}=12\mathrm{V}$, pulse $\mathrm{\text{width}}=5\mathrm{m}\mathrm{s}$, $\mathrm{\text{delay}}=5\mathrm{m}\mathrm{s}$, gate $\mathrm{\text{width}}=70\mathrm{m}\mathrm{s}$) with different bias offset voltages (from top to bottom: $+3.0\mathrm{V}$, $+1.0\mathrm{V}$, 0 V, $-0.5\mathrm{V}$, $-1.0\mathrm{V}$, $-1.5\mathrm{V}$). The inset shows the bias offset dependence of the integral EL intensities of the delayed emission bands (●: Ph; ○: DF).

Figure 3

Decay curves of Ph (●) and DF (○) of PF2/6am4 thin film device at 28 K for delay times $>60\mathrm{m}\mathrm{s}$ ($\mathrm{\text{bias}}=12\mathrm{V}$, pulse $\mathrm{\text{width}}=1\mathrm{m}\mathrm{s}$, gate $\mathrm{\text{width}}=40\mathrm{m}\mathrm{s}$). The inset shows the decay curve of DF for delay times $<30\mathrm{m}\mathrm{s}$ ($\mathrm{\text{bias}}=12\mathrm{V}$, pulse $\mathrm{\text{width}}=400\mu \mathrm{s}$, gate $\mathrm{\text{width}}=100\mathrm{m}\mathrm{s}$).