Slow Cation Transfer Follows Sensitizer Regeneration at Anatase TiO2 Interfaces

The BCL Method for DSSC: Basis and Applications

In this work, a comprehensive view of the route that led to the construction of a theoretical approach to the functioning of DSSC is presented. The model was developed based on the theoretical interpretation of experimental results obtained along the years for solar cells including different dyes. This allowed the authors to generate the Barrera, Crivelli, Loeb (BCL) model. The method is based on a system of equations that uses time-dependent density functional theory (TDDFT) calculations to obtain a theoretical index, the Global Efficiency Index (GEI), for the efficiency of a sensitized solar cell. The GEI is obtained through the product of three factors: the available energy for injection, the amount of charge injected, and the efficiency of regeneration. The results so far obtained show a promising correlation with the experimental index of photo conversion efficiency (PCE). Moreover, the method provides theoretical tools that allow us to obtain an understanding of the operation of the cell, and provide us with the keys to optimize it. Its application to other type of devices, as, e.g., the highly more efficient perovskite solar cells, emerges as a challenging future goal.

The band bending effect of LiI/NaI treated TiO2 photoanodes on the performance of dye-sensitized solar cells

A new method has been developed for surface treatment of TiO2 photoanodes with LiI/NaI to enhance the photocurrent and, subsequently, the performance efficiency of the fabricated dye-sensitized solar cells (DSSCs). Three different concentrations (0.1, 0.25, and 0.5 mmol%) of LiI and NaI solutions were used to investigate the effect of this surface treatment on the device performance of DSSCs. A positive shift in the energy level of TiO2 has been experienced by surface treated devices, which is predominantly supported by the decrease in VOC. Furthermore, the introduction of LiI/NaI onto the TiO2 surface resulted in a reduction in the crystallite size, indicating an increase in the surface area which helps in more dye adsorption leading to higher JSC values of the devices. Besides, modification of the conduction band energy level, it also allows a fast electron injection process by shifting the density of states. Thus, this approach offers a simple but efficient route to enhance the photocurrent and efficiency of DSSCs.

A key discovery at the TiO2/dye/electrolyte interface: slow local charge compensation and a reversible electric field

Photo-induced absorption spectroscopy on dye-sensitized solar cells reveals reversible electron-induced cation adsorption at the TiO₂ surface, resulting in changes of the surface electric field.

Intermolecular Interactions in Dye-Sensitized Solar Cells: A Computational Modeling Perspective

We present a unified overview of our recent activity on the modeling of relevant intermolecular interactions occurring in dye-sensitized solar cells (DSCs). The DSC is an inherent complex system, whose efficiency is essentially determined by the interrelated phenomena occurring at the multiple molecular-semiconductor-electrolyte heterointerfaces. In this Perspective, we illustrate the basic methodology and selected applications of computational modeling of dye-dye and dye-coadsorbent intermolecular interactions taking place at the dye-sensitized interface. We show that the proposed methodology offers a realistic picture of aggregation phenomena among surface-adsorbed dyes and nicely describes semiconductor surfaces cosensitized by different dyes. The information acquired from this type of studies might constitute the basis for an integrated multiscale computational description of the device functioning, including all of the possible interdependencies among the device constituents, which may further boost the DSCs efficiency. We believe that this direction should be the target of future computational research in the DSC field.

Synthesis and characterization of tris(heteroleptic) Ru(II) complexes bearing styryl subunits

We have developed and optimized a well-controlled and refined methodology for the synthesis of substituted π-conjugated 4,4'-styryl-2,2'-bipyridine ligands and also adapted the tris(heteroleptic) synthetic approach developed by Mann and co-workers to produce two new representative Ru(II)-based complexes bearing the metal oxide surface-anchoring precursor 4,4'-bis[E-(p-methylcarboxy-styryl)]-2,2'-bipyridine. The two targeted Ru(II) complexes, (4,4'-dimethyl-2,2'-bipyridine)(4,4'-di-tert-butyl-2,2'-bipyridine)(4,4'-bis[E-(p-methylcarboxy-styryl)]-2,2'-bipyridine) ruthenium(II) hexafluorophosphate, [Ru(dmbpy)(dtbbpy)(p-COOMe-styryl-bpy)](PF(6))(2) (1) and (4,4'-dimethyl-2,2'-bipyridine)(4,4'-dinonyl-2,2'-bipyridine)(4,4'-bis[E-(p-methylcarboxy-styryl)]-2,2'-bipyridine) ruthenium(II) hexafluorophosphate, [Ru(dmbpy)(dnbpy)(p-COOMe-styryl-bpy)](PF(6))(2) (2) were obtained as analytically pure compounds in high overall yields (>50% after 5 steps) and were isolated without significant purification effort. In these tris(heteroleptic) molecules, NMR-based structural characterization became nontrivial as the coordinated ligand sets each sense profoundly distinct magnetic environments greatly complicating traditional 1D spectra. However, rational two-dimensional approaches based on both homo- and heteronuclear couplings were readily applied to these structures producing quite definitive analytical characterization and the associated methodology is described in detail. Preliminary photoluminescence and photochemical characterization of 1 and 2 strongly suggests that both molecules are energetically and kinetically suitable to serve as sensitizers in energy-relevant applications.

Computational Modeling of Stark Effects in Organic Dye-Sensitized TiO2 Heterointerfaces

We report a computational modeling study, based on DFT and time-dependent DFT techniques, to investigate the origin and the effect of local electric fields on the optical properties of organic dye-sensitized heterointerfaces, examining the case of the indoline D149 sensitizer on TiO2. On the one hand, we give precise information about the anchoring mode of D149 and its orientation with respect to the TiO2 surface, and on the other hand, we provide the computational framework model to interpret the Stark shifts experimentally observed by PIA spectroscopy. Our results show that the presence of oxidized dye molecules induces major spectral changes on the adjacent neutral dyes, which, along with the simulated effect of injected charge into TiO2, provide Stark shifts nicely reproducing the experimental observations.

Viable alternative to N719 for dye-sensitized solar cells

A new synthetically facile heteroleptic ruthenium(II) sensitizer (NBu(4))[Ru(4,7-dpp)(dcbpyH)(NCS)(2)], coded as YS5, where NBu(4) is tetrabutylammonium, 4,7-dpp is 4,7-diphenyl-1,10-phenanthroline, and dcbpyH is the singly deprotonated surface anchoring derivative of 4,4'-dicarboxy-2,2'-bipyridine (dcbpyH(2)), was designed, synthesized, and incorporated into regenerative mesoscopic titania-based dye-sensitized solar cells. The sensitizer has characteristic broad, high extinction coefficient MLCT bands spanning the visible spectrum. The compound was fully characterized by 1D and 2D (1)H NMR, MALDI-TOF-MS, UV-vis, photoluminescence, Raman, IR, and electrochemistry. YS5 exhibits strong visible absorption properties with a molar extinction coefficient of 1.71 x 10(4) M(-1) cm(-1) at its 522 nm maximum. In operational liquid junction-based DSSCs under simulated AM 1.5G one-sun excitation (100 mW/cm(2)), the photovoltaic performance of YS5 compares almost equally against the current benchmark sensitizer N719 in side-by-side comparisons, producing a power conversion efficiency of 6.05% with a maximum IPCE of 65% at 540 nm. The data presented in this manuscript strongly suggest that YS5 is indeed a viable sensitizer for nanocrystalline TiO(2)-based DSSCs, seemingly poised for widespread adaptation.

Dye-sensitized solar cells: sensitizer-dependent injection into ZnO nanotube electrodes

Two porphyrin-based dyes with carboxylic acid tethers of differing acidity in both protonated and deprotonated forms were examined on ZnO nanotube electrodes. All of the dyes have similar surface coverage, but only the more acidic dye in the acid form injects electrons well; this dye is the only one that corrodes the ZnO. In control experiments on TiO(2) nanoparticle electrodes, both dyes load and inject in protonated and deprotonated forms. These results are consistent with a requirement that the dye must partially corrode the ZnO surface in order for efficient injection to occur. Alternatively, it may possibly point to a coupling of electron injection to proton uptake.

Photodriven heterogeneous charge transfer with transition-metal compounds anchored to TiO2 semiconductor surfaces

A critical review of light-driven interfacial charge-transfer reactions of transition-metal compounds anchored to mesoporous, nanocrystalline TiO2 (anatase) thin films is described. The review highlights molecular insights into metal-to-ligand charge transfer (MLCT) excited states, mechanisms of interfacial charge separation, inter- and intra-molecular electron transfer, and interfacial charge-recombination processes that have been garnered through various spectroscopic and electrochemical techniques. The relevance of these processes to optimization of solar-energy-conversion efficiencies is discussed (483 references).