Effect of Posttreatment of Titania Mesoscopic Films by TiCl4 in Solid- State Dye-Sensitized Solar Cells: A Time-Resolved Spectroscopy Study

Post-treatment of mesoporous titanium dioxide films by TiCl4 solutions is commonly applied during the fabrication of solid-state dye-sensitized solar cells (ssDSCs), as this operation markedly improves the performance of the photovoltaic device. The effect of the post-treatement upon the charge carrier dynamics was scrutinized in ssDSC aiming at unraveling its mechanism. Kinetic studies carried out using femtosecond and nanosecond transient absorption spectroscopy, showed that a biphasic electron injection from the dye excited state is observed, for both treated and non-treated films, which kinetics is not significantly affected by the surface modification step. However, hole injection in the hole transport material (HTM) spiro-OMeTAD and charge recombination were found to be markedly slower in TiCl4-treated films. These findings are rationalized by a model describing the interaction at the interface between TiO2, the dye-sensitizer and spiro-OMeTAD. Rather than resulting from a modification of the energetics of the conduction band of the oxide, the effect of the TiCl4 post-treatment appears to be associated with a subtle change of the film morphology. Results emphasize the importance of controlling the contact at the heterojunction between the HTM and the sensitized semiconductor oxide network

Butyronitrile-based electrolyte for dye-sensitized solar cells

We elaborated a new electrolyte composition, based on butyronitrile solvent, that exhibits low volatility for use in dye-sensitized solar cells. The strong point of this new class of electrolyte is that it combines high efficiency and excellent stability properties, while having all the physical characteristics needed to pass the IEC 61646 stability test protocol. In this work, we also reveal a successful approach to control, in a sub-Nernstian way, the energetics of the distribution of the trap states without harming cell stability by means of incorporating NaI in the electrolyte, which shows good compatibility with butyronitrile. These excellent features, in conjunction with the recently developed thiophene-based C106 sensitizer, have enabled us to achieve a champion cell exhibiting 10.0% and even 10.2% power conversion efficiency (PCE) under 100 and 51.2 mW cm-2 incident solar radiation intensity, respectively. We reached >95% retention of PCE while displaying as high as 9.1% PCE after 1000 h of 100 mW cm-2 light-soaking exposure at 60 °C

Photoinduced processes in lead iodide solid-state solar cells

Organic-inorganic hybrid systems based on lead halide compounds have recently encountered considerable success as light absorbers in solid-state solar cells. Herein we show how fundamental mechanistic processes in mesoporous oxide films impregnated with CH3NH3PbI3 can be investigated by time resolved techniques. In particular, charge separation reactions such as electron injection into the titanium dioxide film and hole injection into the hole transporting material spiro-OMeTAD as well as the corresponding charge recombination reactions were scrutinized. Femtosecond transient absorption spectroscopy and time-resolved terahertz spectroscopy were applied to CH3NH3PbI3 deposited either on TiO2 or Al2O3 mesoporous films and infiltrated with the hole transporting material spiro-OMeTAD

Dynamics of Interfacial Charge Transfer States and Carriers Separation in Dye-Sensitized Solar Cells: A Time-Resolved Terahertz Spectroscopy Study

Electron injection from a photoexcited molecular sensitizer into a wide-bandgap semiconductor is the primary step toward charge separation in dye-sensitized solar cells (DSSCs). According to the current understanding of DSSCs functioning mechanism, charges are separated directly during this primary electron transfer process, yielding hot conduction band electrons in the semiconductor and positive holes localized on oxidized dye molecules at the surface. Comparing results of ultrafast transient absorption and time-resolved terahertz measurements, we show here that intermediate interfacial charge transfer states (CTSs) are rather formed upon ultrafast injection from photoexcited Ru(II)− bipyridyl dye-sensitizer molecules into mesoporous TiO2 films. Formation and dissociation of these CTSs were found to strongly depend on their ionic environment and excess excitation energy. This finding establishes a new mechanism for charge separation in DSSCs. It also offers a rationale for the effect of electrolyte composition in liquid-based devices and of ion doping in solid-state solar cells under working conditions

Dynamics of Interfacial Electron Transfer from Betanin to Nanocrystalline TiO2: The Pursuit of Two-Electron Injection

We report spectroelectrochemical and transient absorption spectroscopic studies of electron injection from the plant pigment betanin (Bt) to nanocrystalline TiO2. Spectroelectrochemical experiments and density functional theory (DFT) calculations are used to interpret transient absorption data in terms of excited state absorption of Bt and ground state absorption of oxidation intermediates and products. Comparison of the amplitudes of transient signals of Bt on TiO2 and on ZrO2, for which no electron injection takes place, reveals the signature of two-electron injection from electronically excited Bt to TiO2. Transient signals observed for Bt on TiO2 (in contrast to ZrO2) on the nanosecond time scale reveal the spectral signatures of photo-oxidation products of Bt absorbing in the red and the blue. These are assigned to a one-electron oxidation product formed by recombination of injected electrons with the two-electron oxidation product. We conclude that whereas electron injection is a simultaneous two-electron process, recombination is a one-electron process. The formation of a semiquinone radical through recombination limits the efficiency and long-term stability of the Bt-based dye-sensitized solar cell. Strategies are suggested for enhancing photocurrents of dye-sensitized solar cells by harnessing the two-electron oxidation of organic dye sensitizers

Two-electron photo-oxidation of betanin on titanium dioxide and potential for improved dye-sensitized solar energy conversion

The plant pigment betanin is investigated as a dye-sensitizer on TiO2 with regard to its potential to undergo two- electron oxidation following one-photon excitation. Electrochemical, spectroelectrochemical and transient absorption measurements provide evidence for two-electron proton-coupled photo-oxidation leading to a quinone methide interme- diate which rearranges to 2-decarboxy-2,3-dehydrobetanin. Time-resolved spectroscopy measurements of betanin on nanocrystalline TiO2 and ZrO2 films were performed on femtosecond and nanosecond time-scales and provide evidence for transient species with absorption bands in the blue and the red. The results shed light on previous reports of high quantum efficiencies for electron injection and point the way to improved solar conversion efficiency of organic dye- sensitized solar cells

Dynamics of Interfacial Charge Transfer States and Carriers Separation in Dye-Sensitized Solar Cells: A Time-Resolved Terahertz Spectroscopy Study

Electron injection from a photoexcited molecular sensitizer into a wide-bandgap semiconductor is the primary step toward charge separation in dye-sensitized solar cells (DSSCs). According to the current understanding of DSSCs functioning mechanism, charges are separated directly during this primary electron transfer process, yielding hot conduction band electrons in the semiconductor and positive holes localized on oxidized dye molecules at the surface. Comparing results of ultrafast transient absorption and time-resolved terahertz measurements, we show here that intermediate interfacial charge transfer states (CTSs) are rather formed upon ultrafast injection from photoexcited Ru(II)− bipyridyl dye-sensitizer molecules into mesoporous TiO2 films. Formation and dissociation of these CTSs were found to strongly depend on their ionic environment and excess excitation energy. This finding establishes a new mechanism for charge separation in DSSCs. It also offers a rationale for the effect of electrolyte composition in liquid-based devices and of ion doping in solid-state solar cells under working conditions

Photoinduced interfacial electron transfer and lateral charge transport in molecular donor–acceptor photovoltaic systems

Nanostructured liquid|solid and solid|solid bulk heterojunctions designed for the conversion of solar energy offer ideal models for the investigation of light-induced ET dynamics at surfaces. Despite significant study of processes leading to charge generation in third-generation solar cells, a conclusive picture of the photophysics of these photovoltaic converters is still missing. More specifically searched is the link between the molecular structure of the interface and the kinetics of surface photoredox reactions. Fundamental scientific issues in this field are addressed by the research project undertaken in the frame of the NCCR-MUST endeavor, an outline of which is given here

Kinetics of the Regeneration by Iodide of Dye Sensitizers Adsorbed on Mesoporous Titania

Regeneration of dye sensitizer molecules by reducing species contained in the electrolyte is a key mechanism in liquid dye-sensitized solar cells because it competes kinetically with a detrimental charge recombination process. Kinetics of the reduction by iodide ions of the oxidized states (S+) of two RuII complex dyes and four organic π-conjugated bridged donor−acceptor sensitizers were examined as a function of the electrolyte concentration. Results show that two different cases can be distinguished. A sublinear behavior of the regeneration rate and a plateau value reached at high bulk iodide concentrations were found for N820 ruthenium dye and interpreted as being due to an associative interaction involving the formation of (S+, I−)···I− surface complexes prior to the reaction. On the other hand, feeble reaction rates at low electrolyte concentrations and a superlinear behavior are observed predominantly for the organic dyes, pointing to a repulsive interaction between the dyed surface and iodide anions. At higher iodide bulk concentration, a linear behavior is reached, providing an estimate of a second-order rate constant. A correlation of these two opposite behaviors with the structure of the dye is observed, emphasizing the role of sulfur atoms in the association of I− anions in the dye-sensitized layer. These findings allow for a better understanding of the dye−electrolyte interaction and of the effect of the iodide concentration on the photovoltaic performances of dye-sensitized solar cells

Dynamics and mechanisms of interfacial photoinduced electron transfer processes of third generation photovoltaics and photocatalysis

Photoinduced electron transfer (PET) across molecular/bulk interfaces has gained attention only recently and is still poorly understood. These interfaces offer an excellent case study, pertinent to a variety of photovoltaic systems, photo- and electrochemistry, molecular electronics, analytical detection, photography, and quantum confinement devices. They play in particular a key role in the emerging fields of third-generation photovoltaic energy converters and artificial photosynthetic systems aiming at the production of solar fuels, creating a need for a better understanding and theoretical treatment of the dynamics and mechanisms of interfacial PET processes. We aim at achieving fundamental understanding of these phenomena by designing experiments that can be used to test and alter modern theory and computational modeling. One example illustrating recent investigations into the details of the ultrafast processes that form the basis for photoinduced charge separation at a molecular/bulk interface relevant to dye-sensitized solar cells is briefly presented here. Kinetics of interfacial PET and charge recombination processes were measured by fs and ns transient spectroscopy in a heterogeneous donor-bridge-acceptor (D-B-A) system, where D is a RuII(terpyridyl-PO3)(NCS)3 complex, B an oligo-p-phenylene bridge, and A nanocrystalline TiO2. The forward ET reaction was found to be faster than the vibrational relaxation of the vibronically excited state of the donor. Instead, the back ET occurred on the μs time scale and involved fully thermalized species. The D-A distance dependence of the electron transfer rate was studied by varying the number of p-phenylene units contained in the bridge moiety. The remarkably low damping factor β = 0.16 Å–1 observed for the ultrafast charge injection from the dye excited state into the conduction band of TiO2 is attributed to the coupling of electron tunneling with non-equilibrium vibrations redistributed on the bridge, giving rise to polaronic transport of charges from the donor ligand to the acceptor solid oxide surface