Current limiting mechanisms in indium-tin-oxide/poly3-hexylthiophene/aluminum thin film devices

We studied the temperature dependent current-voltage characteristics of regioregular poly (3-hexylthiophene 2.5-diyl) (P3HT) thin films sandwiched between indium tin oxide (ITO) and aluminum (Al) electrodes (ITO/P3HT/Al devices), with the aim of determining the current limiting mechanism(s) in these devices, and the temperature and/or applied electric field range(s) in which these mechanisms are valid. The current-voltage characteristics of the ITO/P3HT/Al devices showed that current flow across the device is limited by hole injection at the Al/P3HT interfaces at temperatures below 240 K, when the device is biased with high potential on Al. Above this temperature, the bulk transport properties control the characteristics. For the reverse bias, the ITO/P3HT contact does not limit the current; instead it is controlled by a space charge that accumulates due to the low charge carrier mobility in the polymer. An expression that provides a criterion to determine the validity of applying either the Richardson-Schottky thermionic emission model or the Fowler-Nordheim field emission model was deduced. It can be employed to determine the electrical field at which the transition from charge injection by thermionic emission to that by field emission for a given temperature and interface potential barrier height takes place. Our experimental data fit to the deduced expression. Theoretical limits of the model are also discussed. By considering the regions of the current-voltage curves where field emission or thermionic emission was applicable, the interface potential barriers were estimated, respectively. Hence, conclusions on whether the current-voltage behavior of the devices was contact limited or bulk limited could be drawn. (C) 2003 American Institute of Physics.