Determination of trap polarity in conjugated electroluminescent polymer by photoexcitation thermally stimulated current method

We succeed in using a photoexcitation thermally stimulated current (TSC) technique for a determination of trap states for poly[2-methoxy-5-(2'-ethylhexyloxy)-1,4-phenylene vinylene] (MEH-PPV). By use of this method, the trap polarity can be easily and clearly assigned. We provide direct evidence that molecular oxygen can form electron traps in MEH-PPV, while moisture cannot result in detectable hole traps. An unusual negative field-dependent TSC is observed for detrapping electron current, indicating a lowering of electron mobility under increased electric field. The state of observed hole trap is not affected by an exposure to ambient air and by a change of morphology, but can be attributed to extrinsic impurities or structure defects. (c) 2006 American Institute of Physics.