Photogeneration efficients

X 10 electronic carriers must be generated for each square centimeter and for a photogeneration efficiency of unity this requkes an exposure of 1... 

For positive lit electrodes one can register the drift of holes, and for negative ones- the drift of the electrons. The photosensitizer (for example Se) may be used for carrier photoinjection in the polymer materials if the polymer has poor photosensitivity itself. The analysis of the electrical pulse shape permits direct measurement of the effective drift mobility and photogeneration efficiency. The transit time is defined when the carriers reach the opposite electrode and the photocurrent becomes zero. The condition RC < tlr and tr > t,r should be obeyed for correct transit time measurement. Here R - the load resistance, Tr -dielectric relaxation time. Usually ttras 0, 1-100 ms, RC < 0.1 ms and rr > 1 s. Effective drift mobility may be calculated from Eq. (4). The quantum yield (photogenerated charge carriers per absorbed photon) may be obtained from the photocurrent pulse shape analysis. 

Fig. 51. Spectral dependence of the photogeneration efficiency for polycarbonate film with concentration of isopropylcarbonate 40% [289], Electric field strength in Vcm- 7 - 5 x 104 2 - 103 3 - 1.5 x 103...

The theory predicts a strong dependence of photogeneration efficiencies on the field and it approaches unity at high field. The temperature sensitivity decreases with the increase in field. The theory has found satisfactory explanations in the photogeneration process in many organic disordered systems, such as PVK (Scheme la) [25], and triphenylamine doped in polycarbonate [26], Figure 4 shows an example of the field dependence of c() calculated from Eq. (22) (the solid lines) to fit the quantum efficiency data at room temperature for hole and electron generation in an amorphous material. The material consists of a sexithiophene covalently linked with a methine dye molecule (compound 1) (Scheme 2). 

A similar mechanism has been proposed15 to explain photogeneration data in the 1 1 charge transfer complex of PVK and 2,4,7-trinitro-9-fluorenone (TNF). The data points in Fig. 4 show the measured photogeneration efficiency and the solid line is calculated from the Onsager expression with 4>0 = 0.23 and r0= 35 A0. For a... 

Fig. 4. Photogeneration efficiency of 1 1 molar ratio TNF to PVK monomer as a function of applied field at 23 °C. Theoretical computation uses r0 = 35 A and 4>q = 0.23ls ...

Amorphous Se has very high sensitivity. Figure 1 shows the field dependencies of the photogeneration efficiency at different wavelengths (Pai and Enck, 1975). For clarity, the very low field data of the original reference has been deleted. The temperature was 296 K. In the near ultraviolet and blue, the efficiency approaches unity at high fields. Field-dependent photogeneration,... 

Figure 1 The field dependencies of the photogeneration efficiency of a-Se at different wavelengths. The thicknesses were 44.0 pm (open and solid circles) and 3.4 pm (crosses).

Figure 8 The field dependencies of the photogeneration efficiency of a-Si at different wavelengths.

There are two kinds of recombination of relevance to photoreceptors, geminate and bimolecular recombination. Geminate recombination is the recombination of a Coulombically bound electron-hole pair. The photogeneration efficiencies of most organic solids are believed controlled by geminate recombination. Bimolecular recombination is the recombination of a free electron and free hole. 

The conventional method for measuring photogeneration efficiencies is the time- of-flight photocurrent method, as first described by Kepler (1960). During... 

Figure 13 The hole photogeneration efficiency versus time delay between exposure and the application of the collection field for a-Se.

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