Limits to accumulation of electric-field-stabilized geminate polaron pairs in an organic semiconductor thin film

We study the dynamics of geminate polaron-pair accumulation using time-dependent electroabsorption spectroscopy in a guest-host system composed of the laser dye, coumarin 6 (C6), doped into the host, 4, 4′ -bis (N -carbazolyl)biphenyl. Probing the Stark-shifted absorption of C6 at a wavelength of 496 nm, we determine the decrease of the internal electric field resulting from buildup of a polarizing charge distribution that accompanies optically induced geminate pair accumulation in an applied electric field. Our model suggests that the drop in internal field qualitatively leads to the observed time-dependent roll-off of accumulated geminate pair density. The results highlight the importance of both nongeminate recombination and reduced geminate pair stability resulting from the decrease in internal electric field in ultimately limiting geminate pair densities in doped organic semiconductors under continuous illumination.