Evidence for a linear low-voltage space-charge-limited current in organic thin films. Film thickness and temperature dependence in alpha-conjugated sexithienyl

Abstract

The current-voltage (I-V) characteristics of Au/α-sexithienyl/Au sandwich structures have been measured. The I-V curves present two regimes : at high voltages, the square law corresponding to the space-charge-limited current (SCLC) is observed ; the linear variation occurring at low voltages is generally attributed to the ohmic current due to bulk free-carriers. A study of the linear current as a function of the sample thickness shows that two behaviors are encountered. The standard model is followed on thick films, where the conductivity calculated from the slope of the I-V curve does not depend on the thickness. In thin films (less than 2 μm), a thickness dependent conductivity is obtained. This is explained within the frame of the model developed by Bonham and co-workers, in which the diffusion of carriers thermally injected is taken into account. The bulk density of these injected carriers increases when the thickness is reduced, and may become higher than that of bulk free-carriers already present in the film. Experimental results are in good agreement with the predictions of the model. The effective mobility of α-sexithienyl (α-6T) could be calculated from the quadratic regime of the I-V curves. Measurements as a function of temperature were also carried out. The mobility is found to be thermally stimulated, which is interpreted as due to a shallow trap located 0.3 eV above valence band edge