Time-gated electroluminescence spectroscopy of polymer light-emitting diodes as a probe of carrier dynamics and trapping

We present time-gated electroluminescence (EL) spectroscopy of a polyfluorene-based conjugated polymer. The technique is shown to be sensitive enough to pick out impurity emission orders of magnitude weaker than the cw emission. By considering the temperature dependence of the delayed emission spectra and also the dependence on a constant-bias offset it is shown that both geminate pair formation and carrier trapping during operation result in a long EL decay tail. The technique also provides a direct probe of the validity of the Einstein law in conjugated polymers. The diffusion mobility is found to exceed the drift mobility by a factor of 12.