Slow charge recombination

Figure 18. Schematic illustration of slow charge recombination via lateral diffusion of electrons and holes in the A and the D layers, respectively, in the A-S-D triad monolayer. Radical anions and cations on A and S moieties were created by photoexcitation of the S moieties followed by the charge separation.

The stabilization of the photogenerated redox products in the eosin-functionalized, Co(ll)-protoporphyrin IX-reconstituted myoglobin, Eo -Mb-Co(I), (slow charge recombination proceeds with the rate constant k = 1.4 x 10 ) allowed the tai-... 

The combination of CNT with donors or acceptors upon illumination seans to give a fast charge separation and a slow charge recombination, which is expected to open np opportunities to a new generation of donor-acceptor nanohybrids. The long lifetimes of the charge-separated species make these systems excellent candidates for the fabrication of photovoltaic devices. 

Whereas other experimental methods have been used to obtain values of kti no other method provides values of k-t or equilibrium data. There are, however, several important limitations of our method. First, the method is restricted to relatively fast hole transport processes that can compete with charge recombination of the Sa -G+ radical ion pair (Fig. 6). This precludes the use of strong acceptors which can oxidize A as well as G (Fig. 2a). We find that hole transport cannot compete with charge recombination in such systems, even when a charge gradient is constructed which should favor hole transport [35]. Second, the method is unable to resolve the dynamics of systems in which return hole transport, k t, is very slow (<104 s-1) or systems in which multiple hole transport processes occur. Third, since the guanine cation radical cannot be detected by transient spectroscopy, the method is dependent upon the analysis of the behavior of Sa-. In section 3.4 we de-... 

A typical example for current studies on the influence of the geometry and regio-chemical arrangement of linker groups on the ET properties is the comparison of meso-and ft-linked D-B-A systems. For example, the Zn(II)TPP donor has a relatively slow Si — , Sj IC rate and studies on the competitive ET from S2 and, Y states showed that efficient ET from the S2 state to the pyromellitimide acceptor occurs in both 10 and 11. However, charge separation from the, Sj state in 10 is 6 times faster than in the ft -linked 11, while the subsequent charge recombination is faster for 10142. The -positions have also been used to fuse aromatic systems onto the porphyrin to yield systems with extended conjugation110,143,144. 

This reaction has been studied in some detail [2,4,31,32] and will be considered only briefly here. It is a remarkably slow process (microseconds to milliseconds) at short circuit and, thus, does not limit the short-circuit photocurrent density, Jsc. However, the rate of reaction (3) [33] and of the other recombination reactions increases as the potential of the substrate electrode becomes more negative [e.g., as the cell voltage charges from short-circuit (0 V) to its open-circuit photovoltage, Voc, (usually between —0.6 V and —0.8 V versus the counterelectrode)]. At open circuit, no current flows and the rate of charge photogeneration equals the total rate of charge recombination. 

The decay of the trapped electron spectrum was very slow at 77° K in the dark. This fact suggests that cationic intermediates are trapped through the reactions 1 and 2, and the electrons are also trapped rather deeply by the electric dipole of 2-methyltetrahydrofuran molecules, so that the charge recombination reaction is suppressed. 

The second process that can reduce the yield of useful ions is charge recombination between any reduced electron acceptor and the original positive hole left in the initially excited molecule (shown as and in Figure 5.8). This can be prevented only by the spatial separation of the positive and negative charges. The reduction potentials of the successive electron acceptors decrease as their spatial separation increases, so that both the reverse electron transfer and the charge recombination become very slow. 

Recombination in the depletion layer can become important when the concentration of minority carriers at the interface exceeds the majority carrier concentration. Under illumination minority carrier buildup at the semiconductor-electrolyte interface can occur due to slow charge transfer. Thus surface inversion may occur and recombination in the depletion region can become the dominant mechanism accounting for loss in photocurrent. 

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