Photogeneration geminate recombination

Muller JG, Lupton JM, Feldmann J, Lemmer U, Scharber MC, Sariciftci NS, Brabec CJ, Scherf U (2005) Ultrafast dynamics of charge carrier photogeneration and geminate recombination in conjugated polymer fullerene solar cells. Phys Rev B 72 195208... 

The Onsager theory of geminate recombination was qualitatively consistent for aryl-substituted thiapyrylium salt and dialkylamino-substituted triphenyl-methane dispersed in polycarbonate film [301]. The quantum yield of Hie photogeneration was equal to 0.5 at the electric field strength of 106 V cm-1, mobility of I0 12m2 V-1 s-1. Hole and electron conductivity was established. In a triphenylamine-lexan system doped with a boron diketone acceptor, the... 

Here an important definition for the rate constant of free carrier production, k, is given. The latter differs from the ionization rate constant by a multiplier equal to the charge separation quantum yield tpm, obtained in the Markovian approximation. This difference indicates that the number of photogenerated ions that avoid geminate recombination and become free is less than their total amount, cpm < 1. 

There are two kinds of recombination of relevance to photoreceptors, geminate and bimolecular recombination. Geminate recombination is the recombination of a Coulombically bound electron-hole pair. The photogeneration efficiencies of most organic solids are believed controlled by geminate recombination. Bimolecular recombination is the recombination of a free electron and free hole. 

In addition to surface-enhanced exciton dissociation and geminate recombination, direct photoexcitation (Northrop and Simpson, 1956 Heilmeier et al., 1963 Harima et al., 1989) and exciton-exciton annihilation (Silver et al., 1963 Jortner et al., 1963 Braun, 1968 Johnston and Lyons, 1968 Foumy et al., 1968 Swenberg, 1969 Braun and Dobbs, 1970 Orlowski and Scher, 1983) arguments have been proposed. In direct photoexcitation, a free electron and free hole are created without the involvement of intermediate states. With the exception of the work of Harima et al. and Orlowski and Scher, there have been few references to direct or exciton-exciton photogeneration processes in the past one and a half decades. 

The photogenerated radical anion in the DMA-Kapton film decays very rapidly, as in the model system, with a short lifetime of 150 ps, suggesting the presence in the back electron transfer or geminate recombination. The quantum efficiency of the photocurrent generation as a function of applied electric... 

The most important form of charge-charge interaction that influences DSSC performance is recombination. DSSCs work because geminate recombination, where the photogenerated exciton recombines before it can dissociate, is reduced by the much faster processes of charge injection and dye regeneration as shown in Fig. 4 [12]. Bimolecular recombination, where the electrons injected from the dye molecules reduce tri-iodide ions to iodide (electrolyte DSSCs) or recombine with a hole (SS-DSSCs), is a major source of loss since electron transport to the collector electrode is slow and takes place on similar timescales to these recombination processes. It is hence essential for the success of the dye-sensitized cell that this back reaction is rendered slow enough that electrons injected by the photoexcited dye can be collected efficiently. 

This set of data displays some important differences between the as-prepared and annealed samples. First, the extraction of charges from the annealed layers is faster and far more efficient. For example, half of the initially photogenerated carriers are swept out of the device at 0 V within 1 ms and non-geminate recombination is almost absent at short-circuit conditions (Qtot is essentially independent of fd). When raising the bias close to Voc, extraction is slowed down and the carrier density in the sample declines as a result of non-geminate recombination. For the as-prepared layer, non-geminate recombination is seen for both short-circuit and open-circuit conditions, rendering extraction inefficient for both bias conditions. Also, extraction is considerably slower and recombination is more efficient in these samples. 

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