Hot band emission and energy transfer in organic electrophosphorescent devices

The evolution of the electroluminescence spectra of platinum porphyrin doped polyfluorene light-emitting diodes is investigated after turn-off of the driving pulse. In the delayed spectra a new emission band is identified, which is of comparable intensity to the delayed polyfluorene emission. Through temperature dependent photoluminescence measurements an assignment of this emission is made to a hot level of the platinum porphyrin with a thermal activation energy of 0.42 eV. The evolution of the delayed emission spectra as well as the observation that the hot band at 540 nm overlaps the polyfluorene triplet level suggest that energy transfer can occur between the host and guest.