Photoconductivity generation efficiency

Recently photorefractivity in photoconductive polymers has been demonstrated (92—94). The second-order nonlinearity is obtained by poling the polymer doped with a nonlinear chromophore. Such a polymer may or may not be a good photoconductor. Usually sensitizers have to be added to enhance the charge-generation efficiency. The sensitizer function of fuUerene in a photorefractive polymer has been demonstrated (93). 

The photoconductivity of poly[2-(N-carbazolyl)ethyl vinylether]91,92) and particularly of poly(N-acryloylcarbazole)91) is much inferior to that of PVK. In the case of the acrylic polymer the reported photocurrents are at least two orders of magnitude lower. The poor charge carrier generating efficiency is blamed for low photocurrents91 The relatively poor performance of the vinylether polymer is how ever attributed to charge carrier mobility92). 

Loutfy et al. (1983) measured the photoconductivity of a series of squaraines dispersed in a styrene butylmethacrylate copolymer. The generation efficiencies were strongly field dependent. In most materials, a quadratic field dependence was observed. For MSQ, the results were in agreement with Tam. At high fields, the efficiency was approximately 0.70. 

Recently, several photoactive polymers have been examined that fit these criteria. They include JV-polyvinylcarbazole (PVK) [101,102], (phenyl-methyljpolysilane (PMPS) [103], and amine-doped polycarbonate [104], All are known hole-transporting polymers. A large number of semiconductor nanoclusters can be doped into these polymers and interact with the polymer and facilitate carrier injection into the polymer [101-104]. Dramatic enhancement in charge generation efficiency has been observed. The availability of these semiconductor nanocluster/polymer composites opens the doorway for exploring transport-related applications. In the following sections, I review their photoconductive properties and discuss possibilities in other related areas. 

With the addition of 2.7 wt% of fullerene, the photoinduced discharge rate of PVK shows a dramatic enhancement [101]. Wavelength dependence of the charge generation efficiency shows that fullerene acts as the sensitizer for the enhanced photoconductivity [101]. Figure 21 shows the field... 

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