Electron and hole transport in a green-emitting alternating block copolymer: space-charge-limited conduction with traps

The transport characteristics of electrons and holes in an alternating block copolymer, poly(1,8-octanedioxy-2,6-dimethoxy-1,4-phenylene-1,2-ethenylene-1,4-phenylene-1,2-ethenylene-1,4-phenylene-1,2-ethenylene-3,5-dimethoxy-1,4-phenylene), have been investigated by current-voltage measurements in metal/polymer/metal sandwich structures. Strong evidence that the hole current is space-charge limited with a single discrete set of shallow traps and the electron current is space-charge limited with an exponential distribution of traps was obtained. A hole trap density of ~1 × 10 17 cm -3 and a hole trap energy of ~0.1 eV relative to the highest occupied molecular orbital level were determined. The characteristic energy of the trap distribution, ~0.05 eV, and the trap density <3.7 × 10 17 cm -3 for electrons were also estimated. It was found that the hole current became trap-free, space-charge limited, when using Au, Pd or Ni ( ϕ Au ≈ ϕ Pd ≈5.1 eV) instead of Cu as the anode, indicating that the hole trap energy must lie between ϕ Cu (≈4.65 eV) and ϕ Au . A hole mobility of ~10 -6 cm 2 V -1 s -1 in this copolymer was determined.