Photogeneration photoacoustical measurements

Field-enhanced fluorescence quenching techniques have been widely used to study photogeneration during the past two decades (Menzel and Popovic, 1978 Popovic and Menzel, 1979 Popovic, 1982, 1983, 1984 Yokoyama and Mikawa, 1982 Popovic etal., 1985, 1987 Popovic andHor, 1988, 1995 Weiss and Bufbeny, 1988 Niimi and Umeda, 1993 Popovic and Mesbah, 1993 Hor and Popovic, 1994. Fluorescence quenching is not affected by trapping. As with photoacoustical measurements, a fundamental limitation of the technique is... 

Figure 23. Photogeneration quantum yield for H2Pc-based photoreeeptors with 33 wt.% hydrazone (DEH) CTM, as a function of applied field. Open-circle data were obtained by xerographic discharge and filled-circle data from photoacoustic measurements. (Reprinted with permission from Ref [36y].)...

The principal advantage of photoacoustical techniques is that the measurements are not affected by trapping. The limitation is that the method is based on the assumption that the absorption of radiation either produces heat or free carriers. In the presence of pther processes, it is difficult to measure photogeneration efficiencies on a quantitative basis. A final limitation is experimental complexity. Although of considerable potential relevance, the techniques have not been widely used. 

Figure 42 Photogeneration efficiencies for a double-layer photoreceptor comprised of a MSQ generation layer determined by photoacoustic and transient photocurrent measurements. The transport layer was a hydrazone doped polymer.

Another non-electrical measurement for < eh was reported by Tam using photoacoustic spectroscopy [293]. Tam assumed that if the major photophysical processes of the photoconductor are photogeneration of e-h pairs and charge recombination to generate heat, one can study the photogeneration process by monitoring the acoustic signal (S) of the photoconductive device as a function of E after optical excitation. The assumption is quite reasonable because the fluorescence quantum yield and the yield of photochemical reaction of the photoconductor are usually very small. Thus... 

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