Accumulation of electric-field-stabilized geminate polaron pairs in an organic semiconductor to attain high excitation density under low intensity pumping

The recombination dynamics of geminate polaron pair (PP) states are investigated by monitoring electric-field-induced delayed fluorescence in thin films consisting of the green laser dye, Coumarin-6 (C6) doped at 1wt% into 4,4′-bis(N-carbazolyl)biphenyl. We find that the PP decay follows τ−m (with m∼0.1), where τ is the time that the PPs are held in the field. This sublinear decay suggests the possibility for accumulation of PPs over time that can then be reconverted into excitons upon field removal. We demonstrate the generation of short (∼50ns full width at half maximum) bursts of C6 fluorescence with peak intensities >20 times the steady-state fluorescence intensity (corresponding to a C6 singlet exciton density NS>4×1015cm3) when pumped continuously by a low intensity (<1W∕cm2) laser in the presence of a pulsed electric field.