SPATIALLY INDIRECT EXCITONS AS PRIMARY PHOTOEXCITATIONS IN CONJUGATEDPOLYMERS

We observe competition between picosecond photoinduced absorption and stimulated emission in poly(paraphenylenevinylene) which demonstrates that they derive from two different pathways. We conclude that the pho toinduced absorption originates from nonemissive ''spatially indirect' ' singlet excitons which are formed with quantum yield as high as 0.9. Previous arguments about maximum possible electroluminescence device efficiencies and about lasers in conjugated polymer films are reexamin ed in light of the prevalence of this species.