Photoconduction in monolayer assemblies with functional units of sensitizing and conducting molecular components

Abstract

Vectorial charge separation is studied in assemblies where a mixed monolayer of a cyanine dye with the chromophores in the layer plane and a chainlike π-electron system oriented perpendicular to the layer plane are sandwiched between fatty acid monolayers and metal electrodes. The cyanine dye is excited by light and the excited electrons move through the π-electron system which acts as the conducting element. Conduction takes place according to different mechanisms depending on the temperature. In the low temperature mode the logarithm of the photocurrent decreases linearly with (1/temperature)1/2, while in the high temperature mode it decreases linearly with (1/temperature); in both modes the photocurrent is proportional to the light intensity and its logarithm increases linearly with the bias voltage. If the conducting π-electron system is absent, the photocurrent is about an order of magnitude smaller but again proportional to the light intensity. Its logarithm increases linearly with the square root of the bias voltage. The results in the complex assembly can be interpreted by assuming that the excited electron is transfered from the cyanine dye to the π-electron system by tunneling or by thermal activation over a barrier of 0.25 eV; from there it tunnels through the next fatty acid layer to an interface state, and then hops to the positively biased electrode. This model can be checked by specifically altering the thickness of the tunneling barrier (by exchanging arachidic acid for fatty acids with shorter chain lengths). In the arrangment where the conducting element is absent the results are interpreted by assuming that the excited electron either tunnels through or is thermally activated over the potential barrier of the hydrocarbon matrix (1 eV). The different voltage dependence in the two arrangements with and without conducting π-electron system can be quantitatively explained as being due to the fact that the chromophore of the cyanine dye is perpendicular to the applied field, while the chain of the π-electron system is parallel to this field.