Transient photoconductivity

A. Pecchia, O.R. Lozman, B. Movaghar, N. Boden, R.J. Bushby, Photoconductive transients and one-dimentional charge carrier dynamics in discotic liquid crystals. Phys. Rev. B 65, 104204-1-10 (2002)... 

Sokel R, Hughes RC (1982) Numerical-analysis of transient photoconductivity in insulators. J Appl Phys 53 7414... 

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... 

Figure 4.1 Illustration of the principle of the conventional TOP transient photoconductivity technique.

Fig. 4. Energy below the conduction band of levels reported in the literature for GaP. States are arranged from top to bottom chronologically, then by author. At the left is an indication of the method of sample growth or preparation liquid phase epitaxy (LPE), liquid encapsulated Czochralski (LEC), irradiated with 1-MeV electrons (1-MeV e), and vapor phase epitaxy (VPE). Next to this the experimental method is listed photoluminescence (PL), photoluminescence decay time (PLD), junction photocurrent (PCUR), photocapacitance (PCAP), transient capacitance (TCAP), thermally stimulated current (TSC), transient junction dark current (TC), deep level transient spectroscopy (DLTS), photoconductivity (PC), and optical absorption (OA).

Transient Photoconductivity. A solution of neutral molecules in a polar solvent shows only ohmic conductivity, but if ions are formed by the action of the photolytic flash these charge carriers generate an additional current which is proportional to the ion concentration. The observation of such transient photocurrents is the most direct experimental evidence for the formation of free, solvated ions in electron transfer reactions. The quantum yield of ion formation can be obtained through proper calibration procedures and the kinetics of ion recombination can be determined. Figure 7.37 gives an example of such transient photocurrent rise and decay. 

Photoconductivity (17,170) and transient absorption spectroscopy (36) studies indicate that (S+Au)-sensitization causes electron trap formation or deepens existing traps, but does not provide evidence that the traps are deeper than those produced by S-sensitization alone. Low-temperature luminescence studies likewise indicate that the gold does not enhance the electron trapping (172). 

The photoinduced ionization of benzophenon in acetonitrile has also been reported to proceed via triplet-triplet annihilation at very low laser pulse intensities [272], The biexcitonic ionization in this system has been studied by applying the transient photoconductivity technique and described with the conventional (Markovian) rate equations, with the time-independent rate constants [273], Such equations can be represented as follows... 

Fig. 1. Schematic of the experimental arrangement used for measuring transient photoconductivity...

The photocyanation and the photohydrolysis of 4-chloroanisole and 4-fluoroanisole have been studied with time-resolved spectroscopy and by measurement of the photoconductivity in anhydrous and aqueous acetonitrile and tert-butyl alcohol solutions632. The mechanism is depicted in equation 167. The transient species which have been... 

A3.5 Time-resolved photoluminescence studies of GaN A3.6 Persistent photoconductivity in GaN A3.7 Electrical transport in wurtzite and zincblende GaN A3.8 Characterisation of III-V nitrides by capacitance transient spectroscopy... 

Figure 2 (a) Schematic representation of measuring cells in photoconductivity experiments (b) typical experimental arrangement for transient photoconductivity studies (TOF) (R is the load resistance) (c) 7ph versus time transient as a result of the drifting charge in the external circuit and registered by the scope. 

The depletion layer profile on undoped a-Si H can be obtained by transient photoconductivity. A pulse of light excites electron-hole pairs very near the contact. As in the time-of-fiight experiment, holes are immediately collected by the contact and electrons drift down the internal field of the depletion layer, giving transient conductivity of... 

Fig. 9.5. The depletion potential profile of a-Si H with platinum and chromium contacts measured by transient photoconductivity. The inset plots the dependence of the charge collection, Q., on applied bias which shows the shrinking of the depletion width in forward bias (Street 1983).

Transient photoconductivity measurements of the depletion width, as described in Section 9.1.3, show that there is an electron accumulation layer at the interface with SijN4 (Street et al. 1985b). In contrast, an oxide interface (either a native or deposited oxide) has a depletion layer (Aker, Peng, Cai and Fritzsche 1983). The band bending causes similar changes in the conductance of the films as is described for adsorbed molecules in Section 9.2.2. 

Fig. 9.19. Transient photoconductivity measurements of the band bending at the interface of a-Si H and a-SiN H, showing the effects of growth order (Street and Thompson 1984).

Recent measurements of fast transient photoconductivity (11) in trans-fCm have demonstrated that the photogenerated soiitons are mobile and contribute to the electrical conductivity. Figure 2 shows the transient photoconductivity following a 1 pJ pulse at 2.1 eV with a bias voltage of 300 V. The charge carriers are produced within picoseconds of optical excitation. The fast rise is foiiowed by (approximateiy exponentiai) decay with a time constant of - 300 ps. The magnitude and time decay of Oph(t) are temperature independent... 

They also observed that the species absorbing at 720 nm which decays on the microsecond time-scale by first-order kinetics is a long-lived SSIP. Evolution of the SSIP to free-radical ions was ruled out, because free-radical ions should decay by second-order kinetics. Later observations by Mataga [159, 160] and by Haselbach [161], on the basis of photoconductivity measurements and transient absorption spectroscopy of the benzophenone-DABCO system, showed that the species absorbing at 710 nm decays not by first-order kinetics but by second-order kinetics this is consistent with the formation of free-radical ions. 

Donor acceptor charge transfer complex based photoreceptors continue to be described in the literature and studied using modern spectroscopic techniques but none has been commercialized. For example, the photoconducting charge transfer complex between poly(V-epoxypropylcarbazole) and TNF has been studied with transient absorption and time-resolved fluorescence. On the basis of Monte Carlo simulations, the results were interpreted in terms of a heterogeneity of charge transfer complexes with different radiative probabilities and a distribution of initial charge pair separation distances [30c]. 

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