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

The role of doping and trap states for charge carrier transport in light-em itting devices based on unsubstituted poly(p-phenylenevinylene) (PPV) was i nvestigated and charge carrier mobilities were determined by different tech niques. Using temperature dependent impedance spectroscopy and thermally st imulated currents, the energetic depth and density of states created by dop ing of PPV during device fabrication on different substrate materials were determined. It was found that the conversion of PPV on indium-tin oxide OTO ) 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 temperatur e device characteristics. The total density of ionized accepters at room te mperature is of the order of 10(16)-10(17) cm(-3). The temperature dependen t behaviour of electrical transport quantities such as conductivity and mob ility is dominated by deeper states with energies in the range 0.6-1 eV. Th eir density can be varied by applying a vacuum to the devices. The charge c arrier 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 ligh t-emitting diodes. Hole mobilities in the region of 10(-5) cm(2) V-1 s(-1) at room temperature for an electric held of about 10(5) V cm(-1) were obtai ned. Field and temperature dependent TOF measurements yielded an exponentia l increase in the mobility with increase in the applied held and thermally activated behaviour with activation energies between 0.4 and 0.7 eV on diff erent samples. The values of the mobility at room temperature are consisten t with space-charge limited currents, but considerably larger than the valu es from transient electroluminescence measurements. This indicates that the transit times obtained by the latter method are dominated by the much lowe r electron mobility rather than by the hole mobility. Assuming luminescence quenching within a distance of 20 nm of the Al contact, an electron mobili ty of about 10(-8) cm(2) V-1 s(-1) can be estimated at room temperature and fields in the region of 10(5) V cm(-1)