Exciton geminate recombination

The Coulomb potential model in Fig. 8.7 does not take into account quantum effects. The binding potential does not increase indefinitely as the electron and hole difiuse together, but instead an exciton is formed of binding energy E. The exciton can either dissociate thermally with rate exp (— E x/ T), or else can recombine with rate fV. An additional condition for geminate recombination is therefore. 

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. 

Geminate Recombination of Interfacial Charge-Transfer States into Triplet Excitons... 

This exciton then diffuses to a donor/acceptor interface where it is dissociated, producing an interfacial electron-hole pair. Dissociation of this interfadal electron-hole pair can also be problematic (see Section 14.5.1) with their sogeminate recombination thought to be a dominant loss mechanism in all-polymer cells. Under favorable conditions, the electron and hole are separated from the interface and are collected at the relevant electrode (electrons at the top, low-work-function electrode and holes at the bottom, high-work-function electrode) avoiding the bimolecular recombination of charges. 

In view of the binding energies of thermalised CT excitons, their predominant mode of decay is expected to be charge recombination (rate constant fe ec in Fig. 7.3). The observed photocurrent is unlikely to originate from such excitons. It has been proposed that CT excitons can avoid geminate recombination by separating into free charge carriers immediately after the electron transfer at the BHJ... 

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. 

In an all-polymer system, the excitons generate in the donor phase after the absorption of photons in active layer and then transform to the charge-transfer state, which will dissociate into free charge or recombine geminately. Those two processes compete with each other. A promoted efficiency of OSCs requires the inhibition of geminated recombination and the transformation of more excitons into free charges [134,135]. 

In accordance with the model it is assumed that the rate of geminate recombination of the CT state to the first excited singlet state follows Eq. (5) under the constraint that E >E. This implies that the CT state is always above the singlet exciton state. In single component molecular systems this condition is usually fulfilled as evidenced by the observations that (1) the fluorescence spectra are mirror-symmetric with absorption and (2) fluorescence quenching requires a high electric field [53]. Obviously, the present theory is limited to cases in which E y > Ecoui-... 

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