MARCOS INSTITUCIONALES, MODELOS DE GOBIERNO Y MODELOS DE EMPRESA

The importance of lateral electron transfer between surface-immobilized molecules at the semiconductor interface is becoming increasingly more apparent in dye-sensitized technologies. In DSSCs, recent studies published by the Barnes and Nelson groups have shown that lateral self-exchange electron transfer between surface anchored indoline chromophores (14, Scheme 1.4) is instrumental for regeneration when solid hole-transport materials are used, i.e. solid-state DSSCs.106 In these devices, a solid hole-transport material is used to replace the liquid electrolyte containing the redox mediator to shuttle electrons from the counter-electrode to the oxidized chromophores. Traditional hole-transport material deposition methods are often limited by partial pore-filling of the mesoporous thin film. Using transient absorption spectroscopy and transient anisotropy measurements, Moia and co-workers showed that lateral self-exchange electron transfer contributed significantly to the regeneration efficiency of a solar cell when the fraction of chromophores in contact with the hole-transport medium was small.106

Figure 1.12. A depiction of the “dry cell” dye-sensitized solar cell which used lateral self- exchange electron transfer between the oxidized chromophore to complete the circuit rather than a redox mediator in solution. Photoexcitation leads to electron injection on the sensitized TiO2 interface, and the electrons are collected at the FTO. Lateral self-exchange electron transfer shuttle the oxidizing equivalent to the Pt counter electrode.

In a proof-of-concept example, lateral self-exchange electron transfer was used to shuttle oxidizing equivalents to the counter-electrode instead of a redox mediator.107 _This process is highly desirable because it allows for charge transport without a loss of free energy and hence could lead to more efficient devices. Indeed, Moia et al. demonstrated that lateral self-exchange electron transfer was sufficient to shuttle a small fraction of the oxidizing equivalents to the counter electrode to complete the circuit.107 The structure of such a solar cell, which they referred to as a “dry cell,” is shown in Figure 1.12. In this device, photoexcitation of 14 anchored to the TiO2 interface resulted in electron injection. The electrons in TiO2 move through the TiO2 substrate and are collected at the FTO. The oxidized chromophore undergoes sequential self-exchange electron transfer reactions to shuttle the oxidizing equivalent to the Pt counter electrode. To prevent a short-circuit in the cell, Al2O3 is used as the conduction band potential is too negative to allow for electron injection from 14

and electron transfer from the TiO2 to the Al2O3.This resulted in solar cells with photovoltages of ~1 V; however, only a small fraction of the photogenerated charge-separated states were

collected due to high rates of charge recombination resulting in small photovoltages. The incident photon-to-current efficiency in these solar cells was ~13%.107

Back-electron transfer between an electron in TiO2 and the oxidized chromophore occurs with kinetics that generally do not follow first- or second-order kinetic models.30,102 These kinetics are typically fit to the Koulrausch-Williams-Watts function, which accounts for back-electron transfer occurring with a distribution of rates.30,102 _{Such dispersive kinetics are} often attributed to electron mobility in TiO2, which is rate-limited by a trapping-detrapping model, allowing back-electron transfer to occur from many different sites on the surface.156 However, it has been demonstrated that the self-exchange rate constant is proportional to the back-electron transfer rates.110-112 Moia and co-workers showed that that when the surface coverage of the chromophore was below the percolation threshold, back-electron transfer rates were slower than at high surface coverages.112 These results were corroborated by two reports from Sampaio et al. where both the surface coverage and the inherent self-exchange rate constant were used to control back-electron transfer kinetics.110,111 Again, when sensitizer self- exchange rate was sluggish, back-electron transfer was also found to be slow. Through temperature dependent studies, Sampaio and DiMarco concluded that rapid lateral electron transfer rates lowered the barrier for the back-electron transfer.110 These results indicated that self-exchange on the surface contributes to the dispersive kinetics measured.

In DSPECs, lateral electron transfer provides a mechanism for shuttling redox equivalents to a co-adsorbed model catalyst, Scheme 1.3.45,108,109 Several recent studies have demonstrated that the lateral electron transfer process could compete with back-electron transfer reactions and shuttle an oxidizing equivalent to a co-adsorbed electron donor to localize the charge on a single molecule.109 _{However, these authors report surface coverage}

dependent back-electron transfer rates from the oxidized donor. At a 25:1 chromophore to electron donor ratio, back-electron transfer rates from the donor were small. Higher electron donor coverages provide lateral self-exchange electron transfer pathways between the donors that increase back-electron transfer rates which were in agreement with the results of Moia and Sampaio.110-112,115

Multi-electron accumulation onto a model catalyst was recently reported by Chen and Ardo.108 Kinetic control of lateral self-exchange electron enabled accumulation of two oxidizing equivalents onto a single model catalyst. Furthermore, they found the largest yield of the doubly-oxidized model catalyst occurred at low catalyst surface coverages. Presumably at higher proxy catalyst coverages, comproportionation reactions resulted in the formation of two singly-oxidized proxy catalysts. These results present proof-of-concept that lateral electron transfer can accumulate multiple oxidizing equivalents onto a catalyst. Therefore, lateral electron transfer kinetics is paramount for the optimization of solar cells and is the focus of this thesis.