Geminate recombination energetics

According to the pioneering work of Weller and coworkers, back electron transfer in radical ion pairs can proceed by two different mechanisms [53 a], Le. a slow homogeneous recombination (second order) and a fast geminate recombination (first order). Both may lead to triplets of either the donor or the acceptor, depending on the energetics. 

This process is quite akin to geminate recombination in radiation chemistry and indeed many experimental results show excellent parallelism between the data of Ps formation and radiation chemistry (7,8). The main part of Ps we observe in polymers appears to come from reaction (3), but if the epithermal Ps via reaction (1) survive the oxidation reaction (2) it will also show up. The energetics of Ps formation via the spur process are given by. 

This section will review KMC findings relating to geminate and non-geminate recombination in OPVs, and, in doing so, will discuss the effect that energetic and morphological structure has on OPV devices. 

First, injection occurs from the photoexcited dye into the tin oxide conduction band, but is followed by very rapid trapping at a site that is energetically close to the conduction band and physically close to the dye. Trapping is accompanied by rapid, charge-compensating uptake of a proton—either from a hydronium ion or from a water molecule. Perhaps because of the proton uptake, the trapped electron remains proximal to the dye for at least a few hundred nanoseconds. The proximity enables each electron to return precisely to the dye that initially injected it. In other words, the recombination is geminate and the process is first... 

Of particular relevance to the present discussion is the observation that the CSS, which is a biradical cation, is formed with essentially pure triplet spin correlation. For energetic reasons, this triplet radical pair cannot recombine to form the MLCT state and can only form the singlet ground state. Therefore, direct recombination is spin forbidden. Moreover, because the radical pair which constitute the CSS product can separate only to a limited distance, essentially every CSS recombination event is between the same geminate radical pair—in other words, every reduced acceptor is ultimately oxidized by the donor radical cation that was formed from the same initial photochemical event. The spin behavior of the DC A triad CSS can be effectively explained by application of the relaxation mechanism of Hayashi and Nagakura. ... 

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