Carrier - diffusion recombination

Relaxations in photoprocesses, which may be due to surface recombination, minority carrier diffusion, or capacitive discharges, are typically measured as transients of photocurrents or photoprocesses. An analysis of such processes in the time domain encounters some inherent problems. 

For not too low doped samples (D W), however, the contribution of 1SCR is usually negligible. If the surface recombination velocity at the illuminated front is low, IBPC then only depends on sample thickness D, illumination intensity eP, and minority charge carrier diffusion length ID. 

There have been many investigations of photoinduced effects in -Si H films linked to material parameters. Changes have been observed in the carrier diffusion length, unpaired spin density, density of states in the gap, and infrared transmission. The transition from state A to B seems to be induced by any process that creates free carriers, including x-ray radiation and injection (double) from the electrodes. Because degradation in a solar cell is accentuated at the open-circuit voltage conditions, the A to B transition occurs upon recombination of excess free carriers in which the eneigy involved is less than the band gap. It has been pointed out that this transition is a relatively inefficient one and the increase in spin density takes place at a rate of 10-8 spins per absorbed photon. 

Dynamic electron transfer is observed when the adsorption between the charge carrier acceptor species and the semiconductor particle is weak. As such, charge carrier acceptor molecules have to diffuse from the homogeneous solution bulk to the surface of the semiconductor particle in order to accept the photogenerated electrons/holes. As described in Section 9.2.1, photogenerated carriers typically recombine on the sub-nanosecond time scale thus, it is necessary to accumulate one of the charge carriers... 

What about the minority carrier injection process depicted in Figure 14 Here, contrasting with the process considered above, the hole injection step itself is usually very fast (see below). Then the current is limited by diffusion/recombination described by the Shockley equation [201] ... 

The kinetic competition between charge transport and recombination in DSSCs has been analysed in terms of an effective carrier diffusion length given by... 

The analysis of the spectral dependence of /p in the presence of surface recombination should yield the minority-carrier diffusion length Lp and S, the ratio of the surface recombination velocity to the bulk diffusion velocity. DeVore (1956) presented an analysis using a single carrier model in which local charge neutrality is maintained by fast dielectric relaxation. Schetzina... 

These experiments emphasize that an analysis of the photoconductivity requires consideration of surface recombination particularly when the film is thin and of high quality (large minority-carrier diffusion length). An example of the thickness dependence of the normalized photocurrent is shown in Fig. 14(Persans, 1980). 

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