  We have measured the ratio, r = $\sigma_S/\sigma_T$ of the formation cross
section, $\sigma$ of singlet ($\sigma_S$) and triplet ($\sigma_T$) excitons
from oppositely charged polarons in a large variety of $\pi$-conjugated
oligomer and polymer films, using the photoinduced absorption and optically
detected magnetic resonance spectroscopies. The ratio r is directly related to
the singlet exciton yield, which in turn determines the maximum
electroluminescence quantum efficiency in organic light emitting diodes (OLED).
We discovered that r increases with the conjugation length, CL; in fact a
universal dependence exists in which $r^{-1}$ depends linearly on $CL^{-1}$,
irrespective of the chain backbone structure. These results indicate that
$\pi$-conjugated polymers have a clear advantage over small molecules in OLED
applications.
