Photoconductivity lifetimes

The charge transport in amorphous selenium (a-Se) and Se-based alloys has been the subject of much interest and research inasmuch as it produces charge-carrier drift mobility and the trapping time (or lifetime) usually termed as the range of the carriers, which determine the xerographic performance of a photoreceptor. The nature of charge transport in a-Se alloys has been extensively studied by the TOF transient photoconductivity technique (see, for example. Refs. [1-5] and references cited). This technique currently attracts considerable scientific interest when researchers try to perform such experiments on high-resistivity solids, particularly on commercially important amorphous semiconductors such as a-Si and on a variety of other materials... 

Properties dependent on adsorption are not confined to conductivity. Luminescence of materials may be affected, as Ewles and Heap (7) have shown for the case of silica, for which the luminescent peak at 4000 A. was shown to be associated with the adsorption of the OID radical. Many workers have demonstrated the dependence of the contact potential on the adsorption of gases. For example, Brattain and Bardeen (8) have shown that the contact potential of germanium varies with the adsorption of water vapor. Photoconductivity may be dependent on the adsorption. For example, Bube has shown (9) that the adsorption of water vapor has a marked effect on the photoconductivity of cadmium sulphide. He concluded (10) that the effect was indirect surface changes affect the lifetime of the excess carriers, thus affecting the photoconductivity. Melnick (11), however, working with zinc oxide, has produced evidence that part of the photoconductivity in this case is directly associated with excitation from adsorption levels. 

When a semiconductor is illuminated, electrons may be excited into the conduction band and/or holes into the valence band, producing photoconductivity. This excited condition is not generally permanent, and when the illumination ceases, the excess current carriers will decay, or recombine. The average time which a photoelectron remains in the conduction band is termed the lifetime. As the lifetime increases, the photocurrent, for a given intensity of illumination, increases. 

Metal chelates of 8-hydroxyquinoline such as (111) with photoconductive properties are reported to be useful in electrophotographic systems.233 The incorporation of a tin complex into a photo-conductive zinc oxide layer is stated to reduce dark decay . In other words, the electrostatic charge applied to the photoconductor has a longer lifetime. Two of the complexes disclosed for this application are (112) and (113). These compounds are prepared from dibutyltin oxide by reaction with 2-mercaptopropionic add and thioglycolic acid, respectively 234... 

Because of its long lifetime, the triplet state Ti is able to store energy, which may be essentially for the mechanism of photoconductivity or other photochemical processes, which proceed on a microsecond time scale. 

It is believed that surface localized electron-hole pairs produced under light in SC nanoparticles participate in photo-induced processes of charge transfer between nanoparticles. These processes most probably of quantum tunnel type determine photoconductivity of composite films containing SC nanoparticles in a dielectric matrix. The photocurrent response time in this case should correspond to the lifetime ip of such pairs, which is of the order nanosecond and even more [6]. This rather long ip makes photo-induced tunnel current in composite film possible. 

The pn-junction formed between the photo-conductive detector and the substrate enhances the photo-conductive signal by essentially isolating the photogenerated minority carriers in the photo-conductive detector from the majority carriers. The minority carriers are swept across the junction while the majority carriers are allowed to flow in the photoconductive detector. This inhibits the recombination rate and extends the lifetime of the majority carriers. 

Abstract. It is shown, that the photoconductivity of Cgo single crystal essentially depends on a spin state of the intermediate electron-hole pairs. The distance between components of electron-hole pairs in states with uncorrelated spins and their lifetime were estimated as R>3.4 nm and r 10 9 s. 

Where R is the reflectivity and d is the thickness. Very accurate values of R and T are needed when the absorptance, (id, is small. The technique of photothermal deflection spectroscopy (PDS) overcomes this problem by measuring the heat absorbed in the film, which is proportional to ad when ad 1. A laser beam passing just above the surface is deflected by the thermal change in refractive index of a liquid in which the sample is immersed. Another sensitive measurement of ad is from the speetral dependence of the photoconductivity. The constant photocurrent method (CPM) uses a background illumination to ensure that the recombination lifetime does not depend on the photon energy and intensity of the illumination. Both techniques are capable of measuring ad down to values of about 10 and provide a very sensitive measure of the absorption coefficient of thin films. 

Illumination creates excess electrons and holes which populate the extended and localized states at the band edges and give rise to photoconductivity. The ability to sustain a large excess mobile carrier concentration is crucial for efficient solar cells and light sensors and depends on the carriers having a long recombination lifetime. The carrier lifetime is a sensitive function of the density and distribution of localized gap states, so that the study of recombination in a-Si H gives much information about the nature of the gap states as well as about the recombination mechanisms. 

The second expression uses the experimental information about the conductivity prefactor derived in Eq. (7.19). The descriptions of the photoconductivity in terms of the recombination lifetimes or the quasi-Fermi energies are equivalent. 

The trapped holes which recombine slowly because of their low mobility are called safe hole traps . Their presence increases the electron lifetime and the photoconductivity and seems to account for the features of the photoconductivity not explained by the simple model of Eq. (8.69) (McMahon and Crandall 1989). Safe hole traps are most significant in low defect density material, when their concentration can exceed the defect density. A detailed analysis needs to take into account the full distribution of hole traps as well as the dispersive transport of holes. The role of transitions between the band edges in the recombination process also needs to be determined. 

The photoconductivity response of a-Si H nipi structures has an extremely long recombination lifetime. A brief exposure to illumination causes an increase in the conductivity which persists almost indefinitely at room temperature (Kakalios and Fritzsche 1984). An example of this persistent photoconductivity is shown in Fig. 9.32. The decay time exceeds 10 s at room temperature and decreases as the temperature is raised, with an activation energy of about 0.5 eV. 

E>efect equilibration provides a plausible explanation of the persistent photoconductivity (Kakalios and Street 1987). According to this model, defect generation is enhanced by the long lifetime of holes in the p-type region of the nipi structure. Holes in the p-type layers... 

Valerian and Nespurek (1993) determined values of the electron range (mobility-lifetime product) of vapor-deposited a-H2Pc from measurements of the photocurrent action spectra. The values were about 6 x 10-12 cm2/V, considerably lower than 10-9 cm2/V reported earlier by Popovic and Sharp (1977) for /J-H2Pc. For further discussions of photoconductivity in n-type phthalocyanies, see Schlettwein et al. (1994, 1994a), Meyer et al. (1995), and Karmann et al. (1996,1997). 

Fig. 7. Photoconductivity-decay lifetime, as a function of depth removed by etching, after fine polish. Note change in scale at 5p.

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