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SUMMARY:Charge transport in conducting polymers - Paul Blom\, University o
 f Groningen\,The Netherlands
DTSTART:20070316T143000Z
DTEND:20070316T153000Z
UID:TALK6113@talks.cam.ac.uk
CONTACT:Chris McNeill
DESCRIPTION:Poly(dialkoxy-p-phenylene vinylene) (PPV) derivatives have bee
 n widely studied for their potential use in organic semiconducting devices
  like light-emitting diodes (LEDs)\, field-effect transistors or solar cel
 ls. The hole transport in PPV-based diodes is known to be space-charge lim
 ited current (SCLC) for Ohmic contacts. At room temperature the SCL curren
 t 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 wit
 h the third power on device thickness\, as expected for SCL currents. We d
 emonstrate that for thicknesses smaller than 100 nm this scaling law does 
 not apply. In stead\, the current in these thin PLEDs is strongly enhanced
  as compared to the SCL model. We show that this strong increase of the ho
 le transport properties in these thin devices originates from the presence
  of the Ohmic hole contact. In order to align the Fermi-level holes diffus
 e 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 tran
 sport in this accumulation region is strongly enhanced. For the analysis o
 f thin PLEDs it is therefore essential that both drift and diffusion of ch
 arge carriers are taken into account. \n	The electron transport in diodes 
 of poly(dialkoxy-p-phenylene vinylene) (PPV) derivatives is strongly reduc
 ed as compared to the hole transport. The room-temperature electron curren
 t 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 existi
 ng trap-limited models. We demonstrate that the presence of a Gaussian den
 sity of states (DOS) for the mobile carriers\, being characteristic for di
 sordered semiconductors\, reduces the temperature dependence of the trap-l
 imited charge transport. The reduction is governed by the width of the Gau
 ssian DOS and originates from the equilibrium concentrations of the mobile
  and trapped carriers.   \n\n
LOCATION:Small Lecture Theatre\, Cavendish Laboratory\, Department of Phys
 ics
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