LIMITS TO QUANTUM EFFICIENCY IN ELECTROLUMINESCENT DEVICES BASED ON CONJUGATED POLYMERS

Excited state dynamics in poly(p-phenylenevinylene) (PPV) have been st udied by transient absorption and photoluminescence measurements. The quantum yield of emissive singlet exciton photogeneration is estimated not to exceed 20 %. This is mostly limited by efficient formation of non-emissive interchain ''spatially indirect'' singlet excitons analog ous to those in type II semiconductor heterostructures which decay non radiatively. We also show that exciton quenching by oxidation defects is an additional process which can dramatically reduce photoluminescen ce quantum yield.