Spin-conserving carrier recombination in conjugated polymers.

The ultimate efficiency of polymer light-emitting diodes is limited by the fraction of charges recombining in the molecular singlet manifold. We address the question of whether this fraction can principally exceed the fundamental limit set down by spin statistics, which requires the possibility of spin changes during exciton formation. Sensitized phosphorescence at 4-300 K enables a direct quantification of spin conversion in coulombically bound electron-hole pairs, the precursors to exciton formation.