Sensitization mechanism charge transfer state mediated

The photodissociation of an adsorbed molecule may occur directly or indirectly. Direct absorption of a photon of sufficient energy results in a Franck-Condon transition from the ground state to an electronically excited repulsive or predissociative state. Indirect photodissociation of adsorbates, involving absorption of photons by the substrate, can take place via two processes. The first one is analogous to the process of sensitized photolysis in gases. The second one, also substrate mediated, implies the phototransfer of an electron from the substrate to an antibonding orbital of the adsorbate, i.e. charge transfer photodissociation. The basic principles of these two excitation mechanisms will be discussed later in this part. 

The efficiency of a DSSC in the process for energy conversion depends on the relative energy levels and the kinetics of electron transfer processes at the sensitized semiconductorlelectrolyte interface. For efficient operation, the rate of electron injection (Fig. 10.1, equation 10.2) must be faster than the decay of the dye excited state. Also, the rate of re-reduction of the oxidized sensitizer (or dye cation) by the electron donor in the electrolyte (equation 10.4) must be faster than the rate of back reaction (recombination) of the injected electrons with the dye cation (equation 10.3), as well as the rate of reaction of injected electrons with the electron acceptor in the electrolyte (equation 10.6). This reaction, also called dark current , is the main loss mechanism for the DSSC. Finally, the kinetics of the reaction at the counter-electrode must also guarantee the fast regeneration of the charge mediator (equation 10.5), or this reaction could also become rate limiting in the overall cell performance. ... 

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