The dynamics of electron transfer at the dye-titania and titania-electrolyte interfaces is investigated in two post-sensitization processes: (i) atomic layer deposition of blocking alumina coating and (ii) hierarchical molecular multicapping. To measure the electron transfer dynamics, time-resolved spectroscopic methods (femtosecond transient absorption on the time scale from femtoseconds to nanoseconds and electrochemical impedance spectroscopy on the time scale from milliseconds to seconds) are applied to the complete dye-sensitized solar cells with cobalt-based electrolyte and champion ADEKA-1 dye (with silyl-anchor unit) or its popular carboxyl-anchor analogue, MK-2 dye. Both molecular capping and alumina blocking layers slow down the electron injection process (the average rate constant decreases from 1.1 ps to 0.4 ps) and partial sub-nanosecond back electron transfer from titania to the dye (from ca. 10 ns to 5 ns). Very small alumina layers (of 0.1 nm thickness) have the highest impact on reducing the rate constants of these electron transfer processes, and for the thicknesses greater than 0.3 nm the rate constants hardly change. In contrast, the electron recombination between titania and electrolyte, occurring on the millisecond time scale, starts to be significantly suppressed for the blocking layers of 0.3 nm or more in thickness (up to ca. 20 times for 0.5 nm thickness with respect to that for untreated sample), improving open circuit voltage and fill factor of the cells. The amplitude of the relative photocurrent (short circuit current per number of absorbed photons) is found to depend almost exclusively on the ultrafast and fast processes taking place in the first nanoseconds after dye excitation. The positive impact of coadsorbents on the solar cells performance for both ADEKA-1 and MK-2 is also studied.

The substitution of iodide electrolytes with cobalt ones has led to the current champion laboratory efficiencies for dye-sensitized solar cells (DSSCs). However, unlike with organic dyes, this strategy does not work with classical ruthenium dyes. Therefore, we compare DSSCs sensitized with a popular Ru dye (N719) using both types of electrolytes by exploring the electron dynamics occurring from sub-ps to seconds. An important limitation in the photocurrent of cobalt-based cells is revealed to be due to electron recombination between titania and oxidized Ru dyes, which is much higher than that in iodide-based cells and occurs on the time scale of tens and hundreds of ps. Electron recombination between titania and the electrolyte, taking place on the millisecond time scale, is responsible for further lowering of the photovoltage and fill factor of cobalt-based cells. Ruthenium dyes also exhibit lower absorption coefficients with respect to their organic counterparts. For this reason, we also investigate the effect of the changes in the titania layer thickness, addition of scattering nanoparticles and modifications in the TiCl treatment on DSSC performance.

The photovoltaic performance of solar cells sensitized with indoline D205 dye and its new derivative comprising an alkoxysilyl anchoring unit (D205Si) in the [Cu(tmby) 2 ](TFSI) 2/1 (tmby = 4,4′,6,6′tetramethyl-2,2′-bipyridine, TFSI = bis(trifluoromethane)sulfonimide) redox couple mediated systems was studied in the presence of various titania/dye/electrolyte interface modifications. Cucurbit[7]uril (CB7) was employed to encapsulate dye molecules, creating an electronically insulating layer, suppressing electron interception by redox mediator, and leading to the increase in the electron lifetime in the titania conduction band. For example, the electron lifetime increased from 2.2 to 6.5 ms upon CB7 encapsulation of D205 cells at 0.9 V voltage. Further, molecular multicapping was optimized to minimize dye desorption and prevent electron recombination. As a result, photovoltaic performance was found to be enhanced by the interface modifications in most cases, especially when applied to the alkoxysilyl anchoring derivative. The charge transfer processes (dye regeneration, titania-dye and titania-redox mediator recombination) in the above-mentioned system and in the reference [Co(bpy) 3 ](TFSI) 3/2 (bpy = 2,2′-bipyridine) redox couple mediated systems were investigated by means of small light perturbation electron lifetime measurements, electrochemical impedance spectroscopy, and nanosecond and femtosecond transient absorption spectroscopies. Indoline dyes were also found to be outstandingly fast regenerated by the Cu-based mediator (time constant shorter than 100 ns), which may open new opportunities for sensitizer improvements.

The photophysical properties of 5-deazaalloxazine and 1,3-dimethyl-5-deazaalloxazine were studied in different solvents. These compounds have higher values of fluorescence quantum yields and longer fluorescence lifetimes, compared to those obtained for their alloxazine analogs. Electronic structure and S0 -Si transitions were investigated using the ab initio methods [MP2, CIS(D), EOM-CCSD] with the correlation-consistent basis sets. Also the time-dependent density functional theory (TD-DFT) has been employed. The lowest singlet excited states of 5-deazaalloxazine and 1,3-dimethyl-5-deazaalloxazine are predicted to have the π, π* character, whereas similar alloxazines have two close-lying π, π* and n, π* transitions. Experimental steady-state and time-resolved spectral studies indicate formation of an isoalloxazinic excited state via excited-state double-proton transfer (ESDPT) catalyzed by an acetic acid molecule that forms a hydrogen bond complex with the 5-deazaalloxazine molecule. Solvatochromism of both 5-deazaalloxazine and its 1,3-dimethyl substituted derivative was analyzed using the Kamlet-Taft scale and four-parameter Catalán solvent scale. The most significant result of our studies is that the both scales show a strong influence of solvent acidity (hydrogen bond donating ability) on the emission properties of these compounds, indicating the importance of intermolecular solute-solvent hydrogen-bonding interactions in their excited state.