Transient electroluminescence of polymer light emitting diodes using electrical pulses

Detailed experimental and theoretical analysis of the pulsed excitation of polymer light emitting diodes is presented. We find a set of universal tran sient features for a variety of device configurations (different polymers/c athodes) which can be reproduced using our phenomenological numerical model . We find that the temporal evolution of the electroluminescence in respons e to a step voltage pulse is characterized by: (i) a delay followed by; (ii ) a fast initial rise at turn-on followed by; (iii) a slow rise (slower by at least one order of magnitude). The large mobility mismatch between holes and electrons in conjugated polymers allows us to separately time resolve the motion of holes and electrons. We suggest a method for extracting mobil ity values that takes into account the possible field-induced broadening of carrier fronts, and which is found to be compatible with mobilities determ ined from constant wave measurements. By using appropriate device configura tions it is possible to determine the mobilities of both holes and electron s from a single device. Mobilities for holes and electrons are extracted fo r a poly(p-phenylenevinylene) copolymer and poly(di-octyl fluorene). (C) 19 99 American Institute of Physics. [S0021-8979(99)07621-5].