Conjugation-length dependence of spin-dependent excition formations rates in ∏-conjugated oligomers and polymers

We have measured the ratio, r =ơ s/ơT of the formation cross section, ơ of singlet and triplet excitons from polarons in ∏-conjugated oligomer and polymer films, using a spectroscopic technique we developed recently. We discovered a universal relation between r and the conjugation length (CL): r-1 depends linearly on CL-1, irrespective of the chain structure. Since r is directly related to the maximum possible electroluminescence quantum efficiency in organic light emitting diodes (OLED), our results indicate that polymers have an advantage over small molecules in OLED applications.