Optically detected magnetic resonance studies of pi-conjugated polymer-based light-emitting diodes

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Abstract
The X-band photoluminescence (PL)-, electroluminescence (EL)-, conductivity ((sigma) )-, and photoconductivity ((sigma) ph)-detected magnetic resonance of poly(p- phenylenevinylene) (PPV) and poly(p-phenyleneacetylene) (PPA)-based LEDs is described and discussed. A strong narrow PL-enhancing and EL-quenching resonance is observed at g approximately equals 2.0023 in all cases. However, while the (sigma) - and (sigma) ph-detected resonances are also quenching in the PPV-based diodes, the (sigma) -detected resonance of the PPA-based diodes is enhancing. The g-values and lineshapes of all of the resonances are identical in each type of diode, and they are attributed to polaron-polaron fusion to singlet excitons or bipolarons. The implications of this picture to the issues of polaron and bipolaron mobility in these diodes and a potential upper limit of their efficiency is discussed. In addition to the narrow resonance, half-field PL-, EL-, (sigma) - and (sigma) ph-detected resonances due to the (Delta) ms equals 2 transitions of triplet excitons are also observed. These resonances are believed to result from singlet exciton generation via triplet-triplet fusion.

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