Mechanisms dark limiting current

Compton etal. (1990a) examined the mediated reduction of r-butyl bromide ( BuBr) by the photochemically excited radical anion of tetrachlorobenzo-quinone (TCBQ) in acetonitrile solution using a channel electrode. Under dark conditions, the reduction of TCBQ proceeded via a simple reversible one-electron transfer process in the presence of BuBr. On photo-excitation of the radical anion of TCBQ, the limiting current associated with its formation was enhanced suggestive of the EC mechanism (88). 

The photovoltage is esentially determined by the ratio of the photo- and saturation current. Since io oomrs as a pre-exponential factor in Eq. 1 it determines also the dark current. Actually this is the main reason that it limits the photovoltage via Eq. 2, The value of io depends on the mechanism of charge transfer at the interface under forward bias and is normally different for a pn-junction and a metal-semiconductor contact. In the first case electrons are injected into the p-region and holes into the n-region. These minority carriers recombine somewhere in the bulk as illustrated in Fig. 1 c. In such a minority carrier device the forward current is essentially determined... 

The diffusion/recombination mechanism results in considerable overpotential for (cathodic) current flow in the dark (again assuming an n-type semiconductor for illustration). Such a rate-limiting process was found to describe the charge transfer at n-GaAs in 6 M HCl containing Cu as the hole-injecting species [159, 176]. 

Here, we assume that electron transfer only occurs via the CB and not via surface states. As in a Schottky diode, j generally increases exponentially with (decreasing) potential (Fig. 3a). The form of the dark current-potential curve, however, depends on the mechanism and kinetics of the charge-transfer reaction. At high overpotential, corresponding to a large deviation from equilibrium, the reaction expressed by Eq. (4) may become limited by mass transport in solution, that is, the cathodic current becomes potential-independent (this is not shown in Fig. 3). 

The efficiency of a DSSC in the process for energy conversion depends on the relative energy levels and the kinetics of electron transfer processes at the sensitized semiconductorlelectrolyte interface. For efficient operation, the rate of electron injection (Fig. 10.1, equation 10.2) must be faster than the decay of the dye excited state. Also, the rate of re-reduction of the oxidized sensitizer (or dye cation) by the electron donor in the electrolyte (equation 10.4) must be faster than the rate of back reaction (recombination) of the injected electrons with the dye cation (equation 10.3), as well as the rate of reaction of injected electrons with the electron acceptor in the electrolyte (equation 10.6). This reaction, also called dark current , is the main loss mechanism for the DSSC. Finally, the kinetics of the reaction at the counter-electrode must also guarantee the fast regeneration of the charge mediator (equation 10.5), or this reaction could also become rate limiting in the overall cell performance. ... 

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