Photoconduction regime

Since it is important to address this issue at the earliest times following photoexcitation, measurements of transient photoconductivity in the picosecond to nanosecond regime were carried out [145,146,201,202], In response to an ultrafast light pulse (duration 25 ps), there is an initial fast photocurrent response with decay time of about 100 ps followed by a slower component with... 

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 similarity of the temperature dependence of steady-state photocurrent in thin films to that of the transient photoconductivity in the sub-ns regime implies that, in thin films, carrier sweep-out occurs prior to deep trapping. The weak residual T-dependence above 80 K in the thinnest sample is again similar to that observed in sub-ns time-resolved experiments. In the time domain, this corresponds to the temperature dependent tail characteristic of the transient photocurrent [199]. This weak T-dependence arises from the effect of shallow traps with multiple release and retrapping during carrier sweep-out. Note that the... 

The question whether amorphous semiconductors fulfil the condition for relaxation-case semiconductors was raised (Fagen (1972)) because the photoconductive decay time is found to be much longer than the lifetime assumed by van Roosbroeck and coworkers (about 10 sec). However, Ryvkin (1964) and also van Roosbroeck pointed out that in the relaxation regime the photoconductive decay is not governed by the lifetime but by the much longer relaxation time. 

While the field-dependent hopping conductivity at low temperatures was always a challenge for theoretical description, the theories for the temperature dependence of the hopping conductivity at low electric fields were successfully developed for all transport regimes for the dark conductivity [28, 43], for the drift mobility [29], and for the photoconductivity [30]. In all these theories, hopping transitions of electrons between localised states in the exponential band-tails play a decisive role, as described above. 

We have found that the photoconductivity of stretched films is only measureable below about 180K, the apparent photocurrent at higher temperature being due to thermal modulation of the dark current [20]. At low temperatures the photocurrent is dominated by a short-lived component, as is found for Shirakawa material [43], our measurements indicate that this component is relatively isotropic (similar results have recently been obtained by Bleier et al [44]). The intensity dependence of the PC also indicates that a fraction of the total photocurrent measured is due to the S states observed in PA experi-ments.We find that at 170K, for laser intensities below the saturation regime, this fraction is around 50% [14]. Work is presently underway to obtain more information on the PC. 

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