We have measured the ratio, $r={\sigma }_S/{\sigma }_T$ of the formation cross section, $\sigma$ of singlet and triplet excitons from polarons in $\pi$-conjugated oligomer and polymer films, using a spectroscopic technique we developed recently. We discovered a universal relation between $r$ and the conjugation length (CL): $r^{-1\mathrm{}}$ depends linearly on ${\mathrm{CL}}^{-1\mathrm{}}$, 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.

• Received 14 December 2001

DOI:https://doi.org/10.1103/PhysRevLett.88.197401