Insights into the Explicit Mechanism and Dynamic Rate of Regeneration of Sensitizing Organic Dyes by Transition-Metal Redox Mediators in Solar Cells Using Ab Initio Molecular Dynamics

Ab initio molecular dynamics simulations were employed to investigate the regeneration of the oxidized organic dye LEG4+ by the reducing agents Fc0 and Co[(bpy)3]2+. Dynamical Mulliken spin population analyses suggest that the oxidized LEG4+ may be regenerated by Fc0 and Co[(bpy)3]2+ directly in specific configurations providing that the Fe2+ and Co2+ are in a low-spin state. An exponential coupling relation was found between the distance between the dye and the redox mediators. The rate of the LEG4+ regeneration by Fc0 and Co[(bpy)3]2+ ranges between 5.41 μs–1∼3.80 ps–1 and 0.58 μs–1∼0.04 ps–1, respectively, which spans the window of all experimentally reported rates

Understanding Charge Dynamics in TiO₂ Using Ultrafast Mid-infrared Spectroscopy: Trapping versus Recombination

We utilize herein ultrafast mid-infrared probe laser pulses to explore the mechanism for the charge recombination/trapping process of photogenerated charges within the band gap of TiO2 and across interfaces. Low-energy photons solely probe the free electrons present in the conduction band of TiO2 and those captured in shallow-trap states. We found that >70% of the photogenerated charges disappear from the conduction band in the first few nanoseconds due to electron trapping followed by charge recombination at longer time scales. Moreover, the behavior of the dynamics of the free electrons within the band gap of TiO2 and electrons generated at the interface of adsorbed organic dyes was investigated and compared. This comparison shows that the main driving force for the efficient charge trapping of photogenerated charges within the picosecond time scale is the presence of photogenerated holes, within the band gap of TiO2, or close to the interface of TiO2. If the hole is far from the TiO2 surface, the electron trapping process is hindered, and almost 100% of photogenerated charges can survive up to nanoseconds. This work offers a deeper understanding of the charge trapping and charge recombination processes, by knowing the spatial hole effect, in TiO2 and similar semiconductors upon utilization in photonic devices and photocatalysis

Polymer-Doped Molten Salt Mixtures as a New Concept for Electrolyte Systems in Dye-Sensitized Solar Cells

A conceptually new polymer electrolyte for dye-sensitized solar cells is reported and investigated. The benefits of using this type of electrolyte based on ionic liquid mixtures (ILMs) and room temperature ionic liquids are highlighted. Impedance spectroscopy and transient electron measurements have been used to elucidate the background of the photovoltaic performance. Even though larger recombination losses were noted, the high ion mobility and conductivity induced in the ILMs by the added polymer result in enhanced overall conversion efficiencies

Tuning of Molecular Water Organization in Water-in-Salt Electrolytes by Addition of Chaotropic Ionic Liquids

Water-in-salt electrolytes (WISEs) have expanded the useful electrochemical stability of water, making the development of functional aqueous lithium-ion batteries more accessible. The implementation of additives in the formulation of WISEs can further improve the electrochemical stability of water and avoid potential lithium-ion salt solubility issues. Here, we have used Gemini-type ionic liquids to suppress water activity by designing the structure of ionic-liquid cations. The different water-organizing effects of ionic-liquid cations have been investigated and correlated to battery performance in LTO/LMO full cells. The champion device, containing the most chaotropic ionic liquid, retained at least 99% of its Coulombic efficiency after 500 charging cycles, associated with a final specific discharge capacity of 85 mA h·g–1. These results indicated that water-rich Li+ solvation shells significantly contribute to the excellent device performance and long-term stability of the LTO/LMO-based full battery cells. This work shows that the fine-tuning of the Li+ solvation shell and water structure by the addition of chaotropic cations represents a promising strategy for generating more stable and effective lithium-ion-containing rechargeable aqueous batteries

Carrier Dynamics of Dye Sensitized-TiO₂ in Contact with Different Cobalt Complexes in the Presence of Tri(p-anisyl)amine Intermediates

Heterogeneous charge transfer processes at sensitized wide bandgap semiconductor surfaces are imperative for both fundamental knowledge and technical applications. Herein, we focus on the investigation of carrier dynamics of a triphenylamine-based dye, LEG4, sensitized TiO₂ (LEG4/TiO₂) in contact with two types of electrolyte systems: pure cobalt-based electrolytes and in combination with an organic donor, tri­(p-anisyl)­amine (TPAA). Four different cobalt redox systems with potentials spanning a 0.3 V range were studied, and the carrier recombination and regeneration kinetics were monitored both at low and at high TiO₂ (e^–) densities (1.3 × 10¹⁸ and 1.3 × 10¹⁹ cm^–3, respectively). The results reveal that the introduction of the TPAA intermediate more effectively suppress the recombination loss of TiO₂ (e^–) under high charge conditions, close to open-circuit, as compared to low charge conditions. As a result, the charge transfer from the cobalt complexes to the oxidized dyes is significantly improved by the addition of TPAA. Dye-sensitized solar cells fabricated with the TPAA-containing electrolytes demonstrate remarkable improvement in both <i>V</i>_OC and <i>J</i>_SC and lead to more than 25% increase of the light-to-electricity conversion efficiency. Furthermore, an unprecedented detrimental impact of TPAA on the device performance was identified when the redox potential of the TPAA donor and the cobalt complexes are close. This is ascribed to the formation of TPAA^•+ which can act as an active recombination centers and thus lower the solar cell performance. These insights point at a strategy to enhance the lifetimes of electrons generated in sensitized semiconductor electrodes by overcoming the charge recombination between TiO₂ and the oxidized dye under high carrier densities in the semiconductor substrate and offer practical guidance to the design of future efficient electrolyte systems for dye-sensitized solar cells

Aqueous Solvation and Surface Oxidation of the Cu₇ Nanoparticle: Insights from Theoretical Modeling

The current understanding on the behavior of nanoparticles in solution is limited. We have studied the effects of the aqueous environment on the anoxic oxidation of a Cu₇ nanoparticle using a range of different computational solvation models. On the basis of a series of hydroxylated Cu₇(H₂O)_<i>y</i>(OH)_<i>x</i> structures, the performance of standard continuum models have been compared to discrete models including up to, and beyond, the second solvation layer. Both full quantum chemical (DFT: PBE0-D3) and QM/MM (PBE0/EFP1) computations were employed in the analysis. The Cu₇ structures were solvated in water nanodroplets and studied by molecular dynamics simulations. On the basis of the simulations, we were able to identify new modes of H₂O interactions with the Cu₇ particle, modes that were previously considered unbeneficial. All solvation models favor the same equilibrium oxidation state corresponding to a Cu­(I)­OH surface species. However, for quantitative energy comparison of similar systems, our results suggest the use of a combined water cluster/continuum model including at least a first explicit solvation shell for energetic comparisons. Nevertheless, the second solvation shell is important for identifying representative inner solvation shell structures

Cation-Dependent Photostability of Co(II/III)-Mediated Dye-Sensitized Solar Cells

The electrolyte composition has a significant effect on the performance and stability of cobalt-based, dye-sensitized solar cells (DSSCs). The stability of DSSCs incorporating Co­(II/III) tris­(bipyridine) redox mediator has been investigated over 1000 h under full solar irradiation (with UV cutoff) at a temperature of 60 °C, the main focus being on monitoring the photovoltaic performance of the device and analyzing the internal charge-transfer dynamics in the presence of different cation coadditives (preferably added as tetracyanoborate salts). A clear cation-dependence is shown, not only of the early light-induced performance but also of the long-term photostability of the photovoltage of the device. These light-induced changes, which are attributed to the promotion of electron injection and less electron recombination loss, by transient spectral and electrochemical studies at the TiO₂/dye/electrolyte interface, indicate that the main cation effects involve the TiO₂ surface electric field and energy-state distribution. By examining the stability of adsorbed and solvated dye during aging, it has been found that the dye photodegradation is probably responsible for the decline in the photovoltage and that this is extremely dependent on the nature of the cation coadditives in the electrolyte. It is therefore suggested that optimization of the electrolyte cation composition is essential for improving the stability of cobalt-based DSSCs

Molecular Engineering of D–D−π–A-Based Organic Sensitizers for Enhanced Dye-Sensitized Solar Cell Performance

A series of molecularly engineered and novel dyes WS1, WS2, WS3, and WS4, based on the D35 donor, 1-(4-hexylphenyl)-2,5-di­(thiophen-2-yl)-1<i>H</i>-pyrrole and 4-(4-hexylphenyl)-4<i>H</i>-dithieno­[3,2-<i>b</i>:2′,3′-<i>d</i>]­pyrrole as π-conjugating linkers, were synthesized and compared to the well-known LEG4 dye. The performance of the dyes was investigated in combination with an electrolyte based on Co­(II/III) complexes as redox shuttles. The electron recombination between the redox mediators in the electrolyte and the TiO₂ interface decreases upon the introduction of 4-hexylybenzene entities on the 2,5-di­(thiophen-2-yl)-1<i>H</i>-pyrrole and 4<i>H</i>-dithieno­[3,2-<i>b</i>:2′,3′-<i>d</i>]­pyrrole linker units, probably because of steric hindrance. The open circuit photovoltage of WS1-, WS2-, WS3-, and WS4-based devices in combination with the Co­(II/III)-based electrolyte are consistently higher than those based on a I^–/I₃^– electrolyte by 105, 147, 167, and 75 mV, respectively. The WS3-based devices show the highest power conversion efficiency of 7.4% at AM 1.5 G 100 mW/cm² illumination mainly attributable to the high open-circuit voltage (<i>V</i>_OC)

EXAFS, ab Initio Molecular Dynamics, and NICIS Spectroscopy Studies on an Organic Dye Model at the Dye-Sensitized Solar Cell Photoelectrode Interface

The organization of dye molecules in the dye layer adsorbed on the semiconductor substrate in dye-sensitized solar cells has been studied using a combination of theoretical methods and experimental techniques. The model system is based on the simple D−π–A dye L0, which has been chemically modified by substituting the acceptor group CN with Br (L0Br) to offer better X-ray contrast. Experimental EXAFS data based on the Br K-edge backscattering show no obvious difference between dye-sensitized titania powder and titania film samples, thus allowing model systems to be based on powder slurries. Ab initio molecular dynamic (aiMD) calculations have been performed to extract less biased information from the experimental EXASF data. Using the aiMD calculation as input, the EXAFS structural models can be generated a priori that match the experimental data. Our study shows that the L0Br dye adsorbs in the trans-L0Br configuration and that adsorption involves both a proximity to other L0Br dye molecules and the titanium atoms in the TiO₂ substrate. These results indicate direct coordination of the dye molecules to the TiO₂ surface in contrast to previous results on metal–organic dyes. The molecular coverage of L0Br on mesoporous TiO₂ was also estimated using NICIS spectroscopy. The NICISS results emphasized that the L0Br dye on nanoporous titania mainly forms monolayers with a small contribution of multilayer coverage

Nanostructured Two-Component Liquid-Crystalline Electrolytes for High-Temperature Dye-Sensitized Solar Cells

Nanostructured liquid-crystalline (LC) ion transporters have been developed and applied as new electrolytes for dye-sensitized solar cells (DSSCs). The new electrolytes are two-component liquid crystals consisting of a carbonate-based mesogen and an ionic liquid that self-assemble into two-dimensional (2D) nanosegregated structures forming well-defined ionic pathways suitable for the I^–/I₃^– redox couple transportation. These electrolytes are nonvolatile and they show LC phases over wide temperature ranges. The DSSCs containing these electrolytes exhibit exceptional open-circuit voltages (<i>V</i>_oc) and improved power conversion efficiencies with increasing temperature. Remarkably, these solar cells operate at temperatures up to 120 °C, which is, to the best of our knowledge, the highest working temperature reported for a DSSC. The nature of the LC electrolyte and the interactions at the TiO₂ electrode/electrolyte interface lead to a partial suppression of electron recombination reactions, which is key in the exceptional features of these LC-DSSCs. Thus, this type of solar cells are of interest, because they can produce electricity efficiently from light at elevated temperatures