Charge carrier transport in poly(p-phenylenevinylene) light-emitting devices

The role of doping and trap states for charge carrier transport in light-emitting devices based on unsubstituted poly(p-phenylenevinylene) (PPV) was investigated and charge carrier mobilities were determined by different techniques. Using temperature dependent impedance spectroscopy and thermally stimulated currents, the energetic depth and density of states created by doping of PPV during device fabrication on different substrate materials were determined. It was found that the conversion of PPV on indium–tin oxide (ITO) substrates creates shallow traps with a depth of about 0.1–0.2 eV, which are responsible for the p-type doping of PPV and govern the room temperature device characteristics. The total density of ionized acceptors at room temperature is of the order of 1016–1017 cm-3. The temperature dependent behaviour of electrical transport quantities such as conductivity and mobility is dominated by deeper states with energies in the range 0.6–1 eV. Their density can be varied by applying a vacuum to the devices. The charge carrier mobility in PPV was determined by the time-of-flight (TOF) method on devices with the configuration ITO/PPV/Al, which is typically used in light-emitting diodes. Hole mobilities in the region of 10-5 cm2 V-1 s-1 at room temperature for an electric field of about 105 V cm-1 were obtained. Field and temperature dependent TOF measurements yielded an exponential increase in the mobility with increase in the applied field and thermally activated behaviour with activation energies between 0.4 and 0.7 eV on different samples. The values of the mobility at room temperature are consistent with space-charge limited currents, but considerably larger than the values from transient electroluminescence measurements. This indicates that the transit times obtained by the latter method are dominated by the much lower electron mobility rather than by the hole mobility. Assuming luminescence quenching within a distance of 20 nm of the Al contact, an electron mobility of about 10-8 cm2 V-1 s-1 can be estimated at room temperature and fields in the region of 105 V cm-1.