Effect of dye doping on the charge carrier balance in PPV light emitting diodes as measured by admittance spectroscopy

Dye doping is a promising way to increase the spectral purity of polymer light-emitting diodes (LEDs). Here we analyze the frequency and field dependence of the complex admittance of Al–Ba–PPV–poly(3,4-ethylenedioxythiophene:polystyrene sulphonic acid)–indium tin oxide LEDs with and without dye. We compare the charge carrier mobilities of pristine and dye-doped double-carrier and hole-only (Au replacing Al–Ba) devices. Dye doping is shown to significantly influence the electron mobilities while the hole mobilities are left unchanged and thereby changing the carrier balance in a double carrier device towards that of a hole only device. The minimum in the LED capacitance as a function of voltage appears to be an excellent probe for the electron trapping phenomenon underlying the reduction of the mobility.