Photocurrent limit

As an illustration of this, the analysis of efficiency limits for triple junctions based on lower quality amorphous silicon materials were also reported by Rocheleau and Miller. In that analysis, a bottom cell, with a bandgap of 1.45 eV was assumed, and the middle and top cells were modeled based on the available range of bandgaps in thin-film silicon materials. The resulting current density and STH efficiency limits are shown in Fig. 7.25. Interestingly, even with the lower quality thin film materials, and despite the further photocurrent limits in the triple junction case, STH efficiencies in the 10-13% STH range appear readily attainable. 

It is difficult to measure metal/polymer Schottky energy barriers smaller than about 0.5 eV using internal pholoemission. Small Schotiky energy barriers lead to thermal emission currents produced by the absorption of light in the metal which are difficult to separate from true photocurrents 134]. If the structure is cooled to try to improve this contrast, it is often found that the significant decrease in the electrical transport properties of the polymer [27 [ makes it difficult to measure the internal photoemission current. To overcome this limitation, internal photoemission and built-in potential measurements are combined to measure small Schottky energy barriers, as described below. 

Since photoelectrochemistry is not limited to photocurrent measurements, it may at this point be useful to think about some general new research possibilities to be expected from the combination of electrochemical and microwave measurements. Table 1 shows obvious combination possibilities between electrochemical and microwave measurements. 

The schemes in Figs. 44 and 45 may serve to summarize the main results on photoinduced microwave conductivity in a semiconductor electrode (an n-type material is used as an example). Before a limiting photocurrent at positive potentials is reached, minority carriers tend to accumulate in the space charge layer [Fig. 44(a)], producing a PMC peak [Fig. 45(a)], the shape and height of which are controlled by interfacial rate constants. Near the flatband potential, where surface recombination... 

The first estimations of for photoinduced processes were reported by Dvorak et al. for the photoreaction in Eq. (40) [157,158]. In this work, the authors proposed that the impedance under illumination could be estimated from the ratio between the AC photopotential under chopped illumination and the AC photocurrent responses. Subsequently, the faradaic impedance was calculated following a treatment similar to that described in Eqs. (22) to (26), i.e., subtracting the impedance under illumination and in the dark. From this analysis, a pseudo-first-order photoinduced ET rate constant of the order of 10 to 10 ms was estimated, corresponding to a rather unrealistic ket > 10 M cms . Considering the nonactivated limit for adiabatic outer sphere heterogeneous ET at liquid-liquid interfaces given by Eq. (17) [5], the maximum bimolecular rate constant is approximately 1000 smaller than the values reported by these authors. 

It is clear that one of the major limitations of this analysis is the assumption of constant excited-state coverage. Deviations from the behavior described by Eq. (45) in the low frequency range have been observed at photocurrent densities higher than 10 Acm [50]. These deviations are expected to be connected to excited-state diffusion profiles similar to those considered by Dryfe et al. [see Eq. (38)] [127]. A more general expression for IMPS responses is undoubtedly required for a better understanding of the dynamics involved in back electron transfer as well as separation of the photoproducts. 

The efficiency of the photocurrent generation in practical photogalvanic cells is generally low, since it is limited by short lifetimes of the excited dyes, parasitic electron transfer reactions, etc. 

Typical photocurrent transients are shown in Fig. 6 for electrons and in Fig. 7 for holes. The shape of these curves is representative for all transients observed in the study and is characteristic of dispersive transport [64-68]. The carrier mobility p was determined from the inflection point in the double logarithmic plots (cf. Fig. 6b and Fig. 7b) [74]. TOF measurements were performed as a function of carrier type, applied field, and film thickness (Fig. 8). As can be seen from Fig. 8, the drift mobility is independent of L, demonstrating that the photocurrents are not range-limited but indeed reflect the drift of the carrier sheet across the entire sample. Both the independence of the mobility from L, and the fact that the slopes of the tangents used to determine the mobility (Fig. 6 and Fig. 7) do not add to -2 as predicted by the Scher-Montroll theory, indicate that the Scher-Montroll picture of dispersive transients does not adequately describe the transport in amorphous EHO-OPPE [69]. The dispersive nature of the transient is due to the high degree of disorder in the sample and its impact on car-... 

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