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Charge transport in conducting polymers

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Poly(dialkoxy-p-phenylene vinylene) (PPV) derivatives have been widely studied for their potential use in organic semiconducting devices like light-emitting diodes (LEDs), field-effect transistors or solar cells. The hole transport in PPV -based diodes is known to be space-charge limited current (SCLC) for Ohmic contacts. At room temperature the SCL current in PPV -derivatives is governed by the dependence of the hole mobility on charge carrier density, whereas at low temperatures the field-dependence prevails. For layer thicknesses larger than 100 nm the current scales with the third power on device thickness, as expected for SCL currents. We demonstrate that for thicknesses smaller than 100 nm this scaling law does not apply. In stead, the current in these thin PLE Ds is strongly enhanced as compared to the SCL model. We show that this strong increase of the hole transport properties in these thin devices originates from the presence of the Ohmic hole contact. In order to align the Fermi-level holes diffuse into the PPV , forming an accumulation layer with a width of a few tens of nanometers. Due to the density dependence of the mobility the hole transport in this accumulation region is strongly enhanced. For the analysis of thin PLE Ds it is therefore essential that both drift and diffusion of charge carriers are taken into account. The electron transport in diodes of poly(dialkoxy-p-phenylene vinylene) (PPV) derivatives is strongly reduced as compared to the hole transport. The room-temperature electron current shows the fingerprints of trap-limited transport with a distribution of traps in energy. However, the weak temperature dependence of the electron current in these PPV derivatives seems to be in contradiction with existing trap-limited models. We demonstrate that the presence of a Gaussian density of states (DOS) for the mobile carriers, being characteristic for disordered semiconductors, reduces the temperature dependence of the trap-limited charge transport. The reduction is governed by the width of the Gaussian DOS and originates from the equilibrium concentrations of the mobile and trapped carriers.

This talk is part of the Optoelectronics Group series.

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