Unbalanced carrier transport is known to strongly affect the efficiency of polymer light-emitting diodes. Here, we report the results of time-of-flight (TOF), current density–voltage, and electroluminescence (EL) quantum efficiency measurements on single-layer poly(9,9-dioctylfluorene) (PFO) devices doped with the red-emitter tetraphenylporphyrin (TPP). TOF shows that PFO is a unipolar conductor, with hole transport much better than electron transport. At a field of 5×105 V/cm, a nondispersive hole mobility of 4×10−5–5×10−4 cm2/V s, dependent on sample morphology, is obtained. Upon the addition of 5% by weight TPP, hole transport becomes as highly dispersive as electron transport, having no measurable average mobility. This results in a decrease in the current for a given applied bias but an increase in the external EL quantum efficiency. TPP acts as a strong hole trap, reducing the dominant hole current and producing more balanced carrier transport. At TPP concentrations above 6%, the device characteristics start to revert to those found at lower TPP concentrations. This is due to the onset of efficient hole transport between the dopant molecules that reestablishes a transport imbalance. © 2001 American Institute of Physics.