Exciton diffusion and charge-carrier generation in two-layer organic photoreceptors with azo and phthalocyanine sensitizer

Two-layer organic photoreceptors consisting of a dye sensitizer acting as the charge generation layer (CGL) and a hole transport layer (HTL) have been investigated by steady- state photoconduction measurements and time-of-flight experiments. With these techniques effects of exciton diffusion, charge-carrier generation, charge injection and charge transport can be studied. The measurements were carried out with (i) single generation layers consisting of azo-pigment dispersion and titanylphthalocyanine pigment dispersion and (ii) with two-layered photoreceptors using azo and TiOPc as charge generation layer and TDAPB and hexa(hexylthio)triphenylene (HHTT) as charge transport materials. We found that in the Phthalocyanine/HTL-system the efficiency for charge injection into the HTL is transport-limited due to the hole mobility of the Titanylphthalocyanine dispersion, whereas the efficiency in the azo/HTL receptor is limited by the diffusion of excitons and (or) holes towards the CGL/HTL-interface. We draw to the conclusion that illumination of the Phthalocyanine sensitizer leads to the formation of charge-transfer excitons which subsequently dissociate into free charges. In contrast to this, the azo-pigment as a sensitizer seems to form strongly bound excitons. Hence no dissociation in free charge-carriers occurs except on the CGL/HTL-interface where the hole transport molecules act as electron donors. From steady-state photocurrent measurements we calculate diffusion lengths belonging to the diffusion of excitons and monitory charge-carriers respectively.