It is well known that the Dember (or photo diffusion) effect can be observed in semiconductors when the electrons and holes have different mobilities and the light-illumination of the sample leads to a non-uniform distribution of charge carriers . The Dember effect (if the electrons and holes have different mobilities) is caused by the excitation light intensity and photogenerated charge (electron-hole) pairs concentration gradient in the film sample volume, respectively . As a rule, the sign of the non-illuminated sample surface potential (with respect to illuminated surface) becomes the same as the charge sign of the photogenerated current carriers which have the larger mobility . Under the exciting light illumination of the different sides of investigated samples the direction of this gradient becomes inverse. Therefore, under these conditions, the sign of the Dember's photovoltage also becomes inverse . To check the above-proposed assumption, the additional photovoltage measurements were carried out for symmetrical sandwich structure samples of investigated PPCs [glass substrate -ITO -PPC film -ITO -glass substrate].