Photoconductive curves

It was found that preliminary ultraviolet irradiation alters the photoelectrical sensitivity. Figure 23 represents the spectral response curves for the photoconductivity (curve 1) and the photoelectromotive force (curve 3) of poly (PiP -diethynylazobenzene). 

The photoconductive gain G as measured from saturated current-voltage curves is of the order of unity in several dyes 3,50,51) .g. in malachite green G =0.2, pinacyanol G=0.37 and merocyanine A 10 7 G=0.6. 

The photoconductivity spectrum after preliminary irradiation of polymer is given by curve 2. The observed redistribution of the peaks is partly reversed on prolonged exposure to air. The bathochromic shift of the shorter wavelength peak depends on the exposure time. Ultraviolet irradiation produces a slight change in the polymer colour. Such irradiation increases, likewise, the photo-electromotive force a 1.5 h irradiation increases it 10 times. The photoconductivity spectrum is situated at longer wavelengths than the photo-electromotive force spectrum. 

The presence of a photoconductivity peak at 610 nm at the threshold of the absorption spectrum (curve 4) is a common phenomenon in inorganic semiconductors and is explained by competition between surface and volume recombination processes of the charge carriers. The optical activation energy determined from the spectral photoconductivity threshold is equal to 1.82 + 0.02 eV. The thresholds of the photoelectromotive force and the absorption spectra are likewise in agreement with this value. It is remarkable that the same value has been found for the activation energy of the dark conductivity in this polymer... 

Typical results are shown in Fig. 44. The spectral threshold of the proper photoconductivity and the photo-emf of PAC is situated at 520 nm. The spectral response for the photo emf of PAC itself is shown by curve 1. After PAC has been immersed in an ethanol solution of methylene blue and dried its spectral response is represented by curves 2 and 2. The photo-response appears in the range of the absorption maximum of the dye at 680 nm characteristic of the monomolecular form in the dilute initial solution (curve 3). The observed enhancement of the second maximum at 620 nm in comparison to the solution spectrum is obviously connected with the presence of dye dimers. The shift of the maximum photoresponse to the longer wavelength by 15 nm relatively to the solution is usually the case for the adsorbed state. The sign of the charge carriers both in the proper and sensitized spectra ranges is positive. As seen in Fig. 44 the adsorption of the dye also markedly changes the proper photosensitivity of the PAC. When the monomolecular form of the adsorbed dye dominates, the... 

Fig. 7.15 Detectivities D of some photoconductive and pyroelectric detector materials curve 1, PbS curve 2, InAs curve 3, HgCdTe curve 4, PbSe curve 5, InSb curve 6, L-alanine-doped TGS curve 7, LiTa03 curve 8, PZ curve 9, PVDF. (Curves 1-5 measured at 200 K curves 6-9 measured at 300 K.)...

FIGURE 1 (a) A typical behaviour of persistent photoconductivity (PPC) in Mg-doped p-GaN grown by reactive MBE. (b) The dark conductivity as a function of temperature. The bottom curve (solid circles) represents data taken with the sample cooling down in the dark, while the top curve (open triangles) is for data taken with the sample illuminated at 10 K for about ten minutes and then warming up in the dark. After [5],... 

The origin of these effects is not at all clear. The bias effects and the room temperature persistent photoconductivity have similar annealing properties. There is also an obvious similarity between the annealing curves and those for the frozen-in excess conductivity of bulk doped a-Si H (e.g. Fig. 6.3). It is therefore probable that carrier-induced defect creation is the origin of the changes in conductivity and that annealing to the equilibration temperature restores the initial state. However there is as yet no complete explanation for the non-ohmic behavior and why it depends on the applied bias. 

Of particular interest are the optical spectra. Chclikow.sky and Schluter calculated the Joint density of states for direct transitions (which would be proportional to C2 were the dipole matrix elements all equal). sec Section 4-A -with the result shown at the bottom of Fig. 11-12. It bears little resemblance to the experimental Cj curve (uppermost in the figure), for a number of reasons. Tlie prominent peak at 10.4 eV appears to be an cxciton peak (See Section 6-H), as had been stiggested earlier by Platzoder (1968) on the basis of observed temperature dependence. Pantelidcs and Harrison took this peak to result from interband transitions, since it lay at an enci gy above the photoconductivity threshold of 9 eV (DiStephano and Eastman, 1971b) that would rule out the possibility that the peak represents a simple exciton, but not that it represents an excitonlikc... 

Fig. 4.10. LHeT spectra of the transition of donors in InSb samples obtained (a) at a laser wavelength of 890 pm (11.24 cm-1 or 1.393 meV) as a function of the magnetic held. The broken curve gives the transmittance and the full curve the photoconductivity. The features labelled A, B, C and D are due to electronic transitions of different chemical donors (Nd- Na = 8 x 1013cm-3). (b) FTS photoconductivity spectra at a resolution of 0.2 cm-1 ( 25 peV) for different values of the magnetic field of a sample with AT - Na = 5 x 1013 cm-3 after [23]. Reproduced with permission from the Institute of Physics...

Fig. 8.24. Shift with stress of the photoconductivity threshold energy of the Al acceptor in germanium at 2K. This threshold represents the ionization energy of Al. The solid lines are for F// [100], The vertical scale is different for each curve. Reproduced with permission from [77], Copyright 1977, with permission from Elsevier...

Figure 8 shows the growth and decay curves (the kinetics) of these three phenomena—photoconductivity, light induced polarization and the electron spin resonance signal. All of them have the same unimole-cular time constants at 25°C. When the system is cooled to —100°, which has been done for the photoconductivity and the spin signal, they decay faster at the lower temperature but again they are parallel they have the same kinetic behavior. 

Since the transport in chromia is by electron transfer, chromia does not exhibit the Hall effect, photoconductivity, or carrier injection. Inconsistent results have been obtained from direct measurements of the diffusion of chromic ions in chromia 191,192). The dielectric constant was measured by Fang and Brower 193). The energy distribution curve of the photoelectrons emitted by chromia shows a spread of many electron volts 194), as is typical of insulators. The adsorption of gases such as ethanol shifts the limit of the photoeffect to longer wavelengths 195). Photoconductivity was not detected in chromia 196). Chemisorption of oxygen on chromia was not influenced by a previous nuclear irradiation in vacuo 189). 

The photoconductivity of the polymers was measured by using a sandwich cell composed of ITO / polymer / Au. Both current-potential (i-E) curves of poly-(b) and poly-(c) show that each contact between the polymer and the electrode is ohmic. In these polymers, it is confirmed that a Schottky junction was not formed at the interface with either the ITO or Au. 

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