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.

Standard ruthenium components of dye-sensitized solar cells (sensitizer N719) and dye-sensitized photoelectrochemical cells (sensitizer RuP and water oxidation catalyst RuOEC) are investigated in the same solar cell configuration to compare their photodynamics and charge separation efficiency. The samples are studied on time scales from femtoseconds to seconds by means of transient absorption, time-resolved emission and electrochemical impedance measurements. RuP shows significantly slower electron injection into a mesoporous titania electrode and enhanced fast (sub-ns) electron recombination with respect to those of N719. Moreover, RuOEC is found to be responsible for partial light absorption and electron injection with low efficiency. The obtained results reveal new insights into the reasons for the lower charge separation efficiency in water splitting systems with respect to that in solar cells. The important role of the initial processes occurring at the dye-titania interface within the first nanoseconds in this efficiency is emphasized.

Basic photovoltaic parameters of the cells sensitized with a popular indoline D149 dye, containing the carboxyl anchoring group, were compared with those of the cells sensitized with the dye modified by the addition of the alkoxysilyl anchoring moiety. Desorption measurements of both compounds showed that the time constant of this process was longer for the modified dye. Two types of electrolytes were used, and the long-time stability and open-circuit voltage were significantly improved when the iodide electrolyte was replaced by the cobalt−bipyridine one. The dynamics of electron injection and titania−electrolyte recombination processes that occur in different time scales, from femtoseconds to seconds, were monitored by femtosecond transient absorption and electrochemical impedance spectroscopy. Additionally, the effect of molecular passivation by post-sensitization capping was checked. The cells after molecular capping showed better relative photocurrent and longer lifetime of the excited state, which was explained as resulting from the suppressed self-quenching of the dyes.