  The maximum efficiency in organic light-emitting diodes (OLEDs) depends on
the ratio, $r=k_S/k_T$, where $k_S$ ($k_T$) is the singlet (triplet) exciton
formation rate. Several recent experiments found that r increases with
increasing oligomer length from a value $r \approx 1$ in monomers and short
oligomers. Here, we model exciton formation as a multi-phonon emission process.
Our model is based on two assertions: (i) More phonons are emitted in triplet
formation than in singlet formation. (ii) The Huang-Rhys parameter for this
phonon emission is smaller in long oligomers than in short ones. We justify
these assertions based on recent experimental and theoretical data.
