Photo-EMF measurements on highly ordered layers of a cyanine dye

Abstract

A new type of thin layers of 1,7-bis-(N,N-dimethylamino)heptamethineperchlorate was prepared in which the molecules are oriented uniformly in extended areas, like in single crystals. By modifying the preparation process, two different crystal faces (010) and (201) could be developed. On these layers time-resolved measurements of the photo-electromotive force (PEMF) were made using the Dember effect. The number of flash-generated charge carriers in the subsurface, and in the bulk could be controlled simply by changing the angle α between the optical transition in the layer and the vibration direction of the polarised incident light. This results in a different depth of light penetration, followed by an angle-dependent PEMF. The biexponential PEMF decay results from two different deactivation processes. The fast process, caused by a high recombination rate of carriers in the subsurface, depends on the angle α whereas the slow process taking place in the bulk material is almost independent of this angle. The PEMF depends also on which crystal face is illuminated. This may be explained by the higher charge carrier mobility along the closely stacked dye molecules or by the different energy of the exciton states involved.