Charge injection and transport in tetra-phenyl-porphyrin

The analysis of charge injection and transport in 5,10,15,20-tetra-phenyl-phorphyrin (H-2-TPP) has been analysed by I(V, T) measurements in the 50-350 K range. Different metals have been used to study the influence of the metal/organic interface on the transport properties. Organic films have been deposited by spray coating and organic molecular beam deposition (OMBD). The observed results show ohmic transport at each temperature for thin films deposited by both the techniques. Two injection mechanisms have been evidenced: thermionic emission over a barrier independent of the metal contact for T > 250 K and tunnel effect at the lower temperatures. On the other hand, a different behaviour has been evidenced on thick films deposited by spray coating. Here, the dark current as a function of the applied voltage increases linearly at the low electric fields independent of the metal contact and shows the same temperature dependence observed for thinner films. At high voltage a non-linear field dependent transport mechanism has been seen. Moreover, the tunnelling contribution to the current increases with the metal work function. The observed trends have been analysed on the basis of injection limited transport in organic amorphous semiconductors. The resistance of interface states and deposition induced defects are considered to discuss the applicability of the Anderson principle for band alignment and justify the observed results. (C) 2003 Elsevier Science B.V. All rights reserved.