Photoconductive systems

For convenience, the applications of tetrazolium salts to nonsilver photography will be divided into two sections photochemical systems and photoconductive systems. 

Photocurrent — Contribution to the current in a -+ semiconductor due to photon incidence. For both inorganic and organic photoconductors the light intensity II and the photocurrent Jph are related by Iph = l[y where y is characteristic of the photoconducting system. In photovoltaic devices photocurrent denotes the difference between the -> current generated by the device under illumination and the current with the device in the dark, the dark current. See also photoconductivity. 

This photoconductive system is a remarkable composition. Phase separation of the microcrystalline TPD occurs apparently with inclusion into the polycarbonate crystalline regions. The crystalline photoconductor exhibits a strongly red-shifted and substantially sharpened absorption band compared to the TPD simply dissolved in the polycarbonate (Fig. 8). Mom interesting are the changes in photoconductivity of the aggregated vs the unaggregated compositons the... 

Figure 5 Examples of (a) hole- and (b) electron-transporting materials for mono-and dual-layer photoconductive systems.

Before photocurrent excitation spectra can be normalised to allow for the wavelength dependence of the illumination intensity, it is essential to establish the relation between the photocurrent response and the incident photon flux. This can be done using calibrated neutral density fdters. The relationship is generally not linear for photoconductive systems or for systems in which processes such as surface recombination or photocurrent multiplication occur (Peter, 1990). The incident photon flux can be measured using a UV-enhanced silicon photodiode with known sensitivity. 

Liquid Crystalline Systems. Conventional photoconductive polymers are amphorous or systems withlow order. In the case of PVK, the hole moves by hoppiag between the pendent carba2ole groups. The hole mobilities are usuaUylow, cm /Vs, due to a trap-dominated hoppiag... 

The electrophotographic system (102,103) involves two key physicochemical elements a photoreceptor and a toner. The minimum requirements of the process are (/) to charge a photoconductive photoreceptor uniformly (2) to illuminate selectively the photoreceptor to form a latent electrostatic image and (J) to develop the image by applying charged toner. These steps are illustrated in Figure 17. 

Electron-hopping is the main charge-transport mechanism in ECHB materials. There is precedence in the photoconductivity Held for improved charge transport by incorporating a number of redox sites into the same molecule. A number of attempts to adapt this approach for ECHB materials have been documented. Many use the oxadiazole core as the electron-transport moiety and examples include radialene 40 and dendrimer 41. However, these newer systems do not offer significant improvements in electron injection over the parent PBD. 

Probe measurements in silane discharges have been reported [296,297]. Apparently, no difficulties were experienced, as the deposited amorphous silicon layer on the tip was sufficiently photoconductive. For typical silane discharge conditions values for are found to be between 2 and 2.5 eV. Electron densities are around 1 x 10 cm - [296]. Probe measurement in the ASTER system failed due to strong distortions of the probe current, even after following cleaning procedures. 

Madan et al. [515] have presented the effect of modulation on the properties of the material (dark conductivity and photoconductivity) and of solar cells. They also observe an increase in deposition rate as a function of modulation frequency (up to 100 kHz) at an excitation frequency of 13.56 MHz, in their PECVD system [159]. The optimum modulation frequency was 68 kHz, which they attribute to constraints in the matching networks. Increasing the deposition rate in cw operation of the plasma by increasing the RF power leads to worse material. Modulation with a frequency larger than 60 kHz results in improved material quality, for material deposited with equal deposition rates. This is also seen in the solar cell properties. The intrinsic a-Si H produced by RF modulation was included in standard p-i-n solar cells, without buffer or graded interface layers. For comparison, solar cells employing layers that were deposited under cw conditions were also made. At a low deposition rate of about 0.2 nm/s, the cw solar cell parameters... 

Prototype electrostatic loudspeakers where the graphite is replaced by a-Si H have been made, where a Mylar foil (area 10 x 10 cm-, thickness 6 /im) is used [657]. Deposition of the a-Si H layer was carried out in the ASTER deposition system. Uniform deposition (standard deviation of thickness, 1.5%) was achieved by diluting the SiHa with Ht with SiHa Hi = 1 2 [370]. The deposition was at room temperature. The hydrogen content amounted to 18 at.%, and the bandgap was 1.81 eV. The dark conductivity and AM 1.5 photoconductivity were 7.5 X 10 and 1.8 x 10 cm" , respectively. In practice the film would not... 

Photoconductivity in photographic systems. II. Some applications to the phenomena of supersensitization, desensitization and the Becquerel effect. J. chem. Physics 15, 539 (1947)-... 

A quite different approach to the 1,5-benzodiazodne system begins with 1,3-dipolar cycloaddition of a nitrile oxide and an indolenine (78MI51900). The spiroisoxazoline (303) is converted by irradiation in good yield to the 1,5-benzodiazocinone (304). By using TV,TV - alkylenediindoles and bifunctional nitrile oxides, the diazocine ring is incorporated as a repeating unit into a photoconducting polymer. 

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