Bipolar charge transport, injection, and trapping studies in a model green-emitting polyfluorene copolymer

Experimental studies of charge injection and transport of holes and electrons in LUMATION (TM) Green 1300 Series light-emitting polymer (LEP) by a combination of experimental techniques are reported. It is found that hole mobility is lower than electron mobility and the former exhibits steeper electric-field dependence thus reducing the misbalance between charge mobilities at higher device operating voltages. An approach to quantitatively analyze charge injection and trapping in organic semiconductors is proposed. Based on our analysis, hole current is limited by injection from. the anode and trapping in the bulk of the polymer. Further, we found that hole trapping is approximately independent of electric field and injection efficiency increases with increasing electric field. Injection limitation of holes from poly(3,4-ethylenedioxythiophene)/polystyrenesulphonic acid (PEDOT:PSS) electrode is believed to be the result of nonuniform contact between the PEDOT:PSS and LEP rather than the energy barrier. On the other hand, electron injection is close to ohmic and the steady-state electron current is affected by trapping, mainly due to deep traps prevailing at low electric fields and with an estimated concentration of 10(16) cm(-3). Electron trapping is found to be significantly reduced in dual-carrier devices, which is believed to be the effect of faster exciton formation and recombination rates, compared to electron trapping processes. (c) 2005 American Institute of Physics.