Photogeneration of charge carriers in a methyl-substituted ladder-poly(para-phenylene) (LPPP), which possesses a high intrachain order, has been studied by xerographic experiments. The time-of-flight technique was used to study charge carrier transport in LPPP. The quantum efficiency of charge carrier generation was calculated by evaluating the photoinduced-discharge curves and found to be independent of both excitation wavelength and temperature. The results can be described by a modified Braun theory with a simple exponential distribution of radii of thermalized electrons. LPPP shows a high charge carrier mobility up to 10-3 cm2/V s, a high primary quantum efficiency of 80 percent, a low residual potential and a small dark decay. THus, LPPP represents a promising material to build single component single-layer photoreceptors. For a comparison, another polymer ladder-type poly(para-phenylene thienylene) (LPPPT), which possesses low intrachain order, is also studied. The quantum efficiency of free charge generation in LPPPT is about one order of magnitude smaller as compared to that in LPPP.
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.
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