Charge transfer state recombination

The work on PCBMipolythiophene blends underlines that, in order to increase the efficiency of organic solar cells, the kinetics of the charge-transfer state are a key factor. To improve device efficiency the rate of charge-transfer state recombination should be reduced and the rate of separation of charges from the charge-transfer state should be increased. In order to modify these rates the mechanisms by which the processes occur must be understood in greater detail. 

Fig. 17 Detailed balance approach for determining open circuit voltage based on the energy of the D/A charge-transfer state and its coupling to recombination loss modes. Reproduced with permission from [195]. Copyright 2009 Macmillan Publishers Limited...

The accumulated surface charge Qs t) oscillates periodically, and the time dependent charging, transfer and recombination terms are replaced by the sums of their steady state and periodic equivalents. 

Geminate recombination is the recombination of an electron with its parent cation. Geminate recombination models are premised on the assumption that the formation of a free electron-hole pair involves the dissociation of an intermediate charge-transfer state. Early models were based on the Poole-Frenkel effect. Most recent models have been based on theories due to Onsager. 

Demonstration of the photorelease has been done in particular with Sr + [46]. This process was monitored on several time scales providing evidence for (1) the delayed formation in 9 ps of the charge transfer state of the merocyanine chromophore following ultrafast photodisruption of the nitrogen - cation interaction, (2) the cation movement away from the excited chromophore into the bulk in 400 ps, (3) recombination of the complex in the ground in about 120 ns. These three steps are respectively illustrated in Fig. 7.17a, b, c (see caption for details). Similar transient absorption studies have been carried out on a PDS-crown-Ca + complex, where PDS is an aza-crown derivative of a substituted stilbene [47]. The spectrodynamics observed on the short time scale are very similar to those found in step (1) of the above description, with in particular a delayed rise of a stimulated emission band attributed to a solvent-separated cation-probe pair. Although the full scenario of the cation photoejection from the DCM-crown-Sr, is complex [46], the spectra shown in Fig. 7.17 demonstrate that at least part of the photoexcited complexes does eject the ion into the bulk. 

A further aspect, not yet treated in detail for organic solar cells, is the role of intermediate charge-transfer states, such as bound polaron pairs, in mediating charge separation and recombination rates (Morteani et al, 2004). 

The competition between charge transfer and recombination via surface states can be treated exactly by Hall-Shockley-Read statistics, taking proper account of... 

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