MEASUREMENTS OF PHOTOLUMINESCENCE QUANTUM EFFICIENCIES IN CONJUGATED POLYMERS - IMPLICATIONS FOR POLYMER PHOTOPHYSICS AND FOR LIGHT-EMITTING-DIODES

The photoluminescence properties of conjugated polymers are currently of great interest, since the radiative decay of singlet excitons is th e process by which light emission occurs in polymer light-emitting dio des. We describe here accurate measurements of photoluminescence effic iencies in solid films of conjugated polymers, using an integrating sp here to collect the emitted light. Values in excess of 40% are measure d for cyano-substituted derivatives of poly(p-phenylenevinylene) (PPV) . In PPV itself, measured photoluminescence efficiencies of 27%, combi ned with time-resolved measurements of the luminescence decay, suggest that the species produced by photoexcitation is predominantly the sin glet exciton responsible for luminescence. The photoluminescence spect rum and the rate of radiative decay can be strongly affected by the pr esence of dielectric or metal interfaces. We model here the effects of optical interference in a number of multi-layer structures. We compar e these results with measured photoluminescence efficiencies in LED-li ke structures, and discuss the implications for the efficiency and emi ssion characteristics of polymer light-emitting diodes.