Photoconduction transit time

The poly-TV-vinylcarbarzole trinitrofluorenone (PVK TNF) charge-transfer complex [21-23] was the first commercial organic photoreceptor used in electrophotography by IBM. The photoconductivity of this material is comparable to that of amorphous selenium, but its utilization in practical devices was limited, owing to its toxicity and long transit-time value, comparable, as for most single-layer photoreceptors, to the development process time. 

Using this thin sample sweep-out approach, the np product obtained from steady-state photoconductivity exhibits a temperature dependence in agreement with fast transient photoconductivity data obtained in the sub-ns time regime [203]. As the film thickness is increased, an activated temperature dependence emerges. The crossover from T-independent p to activated p occurs when the transit time across the film is comparable to the time required for deep trapping. At longer times (thicker films), the mobility becomes trap-dominated with an activated T-dependence. 

The photoconductive gain is given by the ratio of free carrier lifetime to transit time. 

In summary it appears fair to conclude that there is no need to invoke a mobility greater than about 100 cm (Vs) "l for carrier transport along a defect-free PDA chain, compatible with optic po-laron transport, yet experiments available to date cannot rule out existence of an ultra-high mobility either. The experiment that clarifies whether or not the drift velocity of a free carrier is saturated with field as predicted by the acoustic polaron model (54,59) needs still to be done. If performed in the time domain it requires ps-photoconduction work under conditions where the transit time of a carrier along an individual chain exceeds the response time of the circuit. Experiments done on a ns-time scale will always reveal barrier- or trap-controlled transport with pronounced ID-features. High frequency ac-photoconduction studies would be extremely useful to answer the fundamental question about the nature of the transport process of an excess carrier on a conjugated perfect ID chain. 

Reimer, B. and Baueisler, H., Trauisient photoconduction in a polydiacetylene single crystal. Determination of transit time, free lifetime amd recombination time of chaurge caurriers. Phys.stat.solidi. (b) (1978), 85(1), 145. 

The time of transit between the contact pads of a photoconductive device is determined as the ratio between the inter-electrode distance L and the product of electric field and the mobility of the considered carrier type, T = UE i. If we divide the detector length into infinitesimally small elements dy, the differential transit time across each of these elements will be At = AylE(y) t. Thus the total transit time across the detector will be... 

Figure 20 (a-c) Drift and diffusion of optically excited carriers in P-rhombohedral boron. Densities of electrons, electron-bole pairs, and boles at different distances from tbe illuminated surface versus transit time related to the onset of steady-state optical excitation (112). (d) Temperature dependence of the hole drift mobility in P-rhombohedral boron derived from transient photoconduction (113). 

From these relationships it can be seen that, e.g. in low-resistivity photoconductors with equal response times, or by introduction of deep traps that increase the transition voltage Gscl, photoconductive gains greater than unity may be obtained. For example, in merocyanine dyes doped with electron-acceptor compounds a quantum yield G=2.3 has been measured 14>53>. 

Identical methods for investigation of photoconductivity can be used for inorganic and organic semiconductors. Polymer semiconductors as a rule have very high resistance. For such materials the main information about the photoconductive mechanisms and properties may be obtained by two methods electrophotographic (or discharge method) and time of flight (or transit method). Both methods are successfully applied for materials with low mobilities, less than lO-4m2 V-1 s-1, which are the usual values for polymer semiconductors. 

The provocative idea Donovan and Wilson (DW) (3,A) introduced into the discussion is that even for times tfree chain motion prevails the transport velocity is still field-saturated because of formation of an acustic polaron which becomes bigger as it moves in an electric field (5). A transition between defect controlled to free chain motion will therefore not be revealed by a change in the current voltage relation measured in course of a transient photoconductivity experiment. We briefly recall the experiment (3,4) that led to the above conclusion ... 

PbN(,. Dedman and Lewis [144] studied the photoconductivity of PbN crystals as a function of Ught intensity, temperature, and time. A prominent feature of their spectra is a peak at 407.0 nm in the region of strong absorption. Based on temperature coefficient measurements, the peak was interpreted to arise from transitions to an exciton level 0.86 eV below the bottom of the conduction band. Application of the Mott relation between thermal and optical band gaps (via the ratio of dielectric constants) led to a band-gap value of 3.9 eV. Cook and coworkers [145] also observed a peak in photocurrent at 406.0 nm, and, based on its enhancement with thermal decomposition, attributed it to the presence of interstitial nitrogen. 

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