Mechanism exciton-electron transfer

Our interest in quantum dot-sensitized solar cells (QDSSC) is motivated by recent experiments in the Parkinson group (UW), where a two-electron transfer from excitonic states of a QD to a semiconductor was observed [32]. The main goal of this section is to understand a fundamental mechanism of electron transfer in solar cells. An electron transfer scheme in a QDSSC is illustrated in Figure 5.22. As discussed in introduction, quantum correlations play a crucial role in electron transfer. Thus, we briefly describe the theory [99] in which different correlation mechanisms such as e-ph and e-e interactions in a QD and e-ph interactions in a SM are considered. A time-dependent electric field of an arbitrary shape interacting with QD electrons is described in a dipole approximation. The interaction between a SM and a QD is presented in terms of the tunneling Hamiltonian, that is, in... 

Proposed intermediates in the above reaction include atomic hydrogen [27, 28], hydride ions [29, 30], metal hydroxides [31], metaphosphites [32, 33], and excitons [34]. In general, the postulated mechanisms are not supported by direct independent evidence for these intermediates. Some authors [35] maintain that the mechanism is entirely electrochemical (i.e. it is controlled by electron transfer across the metal-electrolyte interface), but others [26] advocate a process involving a surface-catalyzed redox reaction without interfacial electron transfer. 

Bimolecular electron transfer is an important mechanism in determining the singlet exciton yield in light emitting polymers, as discussed in the next section. 

The different rates for singlet and triplet exciton formation predicted in the literature for interchain recombination (Ye et al. 2002 Tandon et al. 2003) arise largely from the assumption that an interchain density-dependent electron transfer term is an important factor in the recombination mechanism. This term couples states of the same ionicity. Since the interchain charge transfer states are predominately ionic, while the intrachain triplet exciton has more covalent character than the intrachain singlet exciton, the rate for the singlet exciton formation is correspondingly greater. 

---