  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-PEDOT-ITO 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 function of voltage appears to be an excellent probe for the electron
trapping phenomenon underlying the reduction of the mobility.

Materials Science;Soft Condensed Matter