Study of Quantum Dot/Inorganic Layer/Dye Molecule Sandwich Structure for Electrochemical Solar Cells

A highly efficient quantum dot (QD)/inorganic layer/dye molecule sandwich structure was designed and applied in electrochemical QD-sensitized solar cells. The key component TiO₂/CdS/ZnS/N719 hybrid photoanode with ZnS insertion between the two types of sensitizers was demonstrated not only to efficiently extend the light absorption but also to suppress the charge recombination from either TiO₂ or CdS QDs to electrolyte redox species, yielding a photocurrent density of 11.04 mA cm^–2, an open-circuit voltage of 713 mV, a fill factor of 0.559, and an impressive overall energy conversion efficiency of 4.4%. More importantly, the cell exhibited enhanced photostability with the help of the synergistic stabilizing effect of both the organic and the inorganic passivation layers in the presence of a corrosive electrolyte

Kinetics versus Energetics in Dye-Sensitized Solar Cells Based on an Ethynyl-Linked Porphyrin Heterodimer

Out of the scientific concern on the kinetics versus energetics for rational understanding and optimization of near-IR dye-sensitized solar cells (DSCs), an <i>N</i>-fused carbazole-substituted ethynyl-linked porphyrin heterodimer (<b>DTBC</b>) reported previously by our group was focused upon in terms of photovoltaic, photoelectrochemical, and steady-state and time-resolved photophysical properties in varied electrolyte environments. A primitive attempt to balance the photocurrent against the photovoltage by varying the concentration of the common coadsorbent 4-<i>tert</i>-butylpyridine (TBP) revealed that TBP continuously suppressed injection but provided inadequate compensation in open-circuit voltage (<i>V</i>_oc). This further drew out the perspective of the widely ignored dye–electrolyte interaction in DSCs, specifically the axial coordination of TBP to the central zinc cation in porphyrin sensitizers that may retard electron injection. As an alternative, a TBP-free electrolyte containing guanidinium thiocyanate was developed to realize greatly promoted <i>V</i>_oc with little current sacrifice, thus significantly enhancing overall energy conversion efficiencies. The excited state was protracted to counteract the injection retardation caused by much reduced driving force, setting a successful example of bilateral compromise between kinetics and energetics in near-IR DSCs

Highly Accurate Excited-State Structure of [Os(bpy)₂dcbpy]²⁺ Determined by X‑ray Transient Absorption Spectroscopy

Determining the electronic and geometric structures of photoexcited transient species with high accuracy is crucial for understanding their fundamental photochemistry and controlling their photoreactivity. We have applied X-ray transient absorption spectroscopy to measure the XANES and EXAFS spectra of a dilute (submillimolar) solution of the osmium­(II) polypyridyl complex [Os­(bpy)₂dcbpy]­(PF₆)₂ (dcbpy = 4,4′-dicarboxy-2,2′-bipyridine) (OsL₂L′) in methanol at the Os L_III edge. We have obtained spectra of superb quality for both the ground state and the photoinduced ³MLCT excited state that have allowed us not only to extract detailed information about the Os 5d orbitals but also to resolve very small differences of 0.010 ± 0.008 Å in the average Os–N bond lengths of the ground and excited states. Theoretical calculations using a recently developed DFT-based approach support the measured electronic structures and further identify the nature of the molecular orbitals that contribute to the main absorption bands in the XANES spectra

Fe^II Hexa <i>N</i>‑Heterocyclic Carbene Complex with a 528 ps Metal-to-Ligand Charge-Transfer Excited-State Lifetime

The iron carbene complex [Fe^II(btz)₃]­(PF₆)₂ (where btz = 3,3′-dimethyl-1,1′-bis­(<i>p</i>-tolyl)-4,4′-bis­(1,2,3-triazol-5-ylidene)) has been synthesized, isolated, and characterized as a low-spin ferrous complex. It exhibits strong metal-to-ligand charge transfer (MLCT) absorption bands throughout the visible spectrum, and excitation of these bands gives rise to a ³MLCT state with a 528 ps excited-state lifetime in CH₃CN solution that is more than one order of magnitude longer compared with the MLCT lifetime of any previously reported Fe^II complex. The low potential of the [Fe­(btz)₃]³⁺/[Fe­(btz)₃]²⁺ redox couple makes the ³MLCT state of [Fe^II(btz)₃]²⁺ a potent photoreductant that can be generated by light absorption throughout the visible spectrum. Taken together with our recent results on the [Fe^III(btz)₃]³⁺ form of this complex, these results show that the Fe^II and Fe^III oxidation states of the same Fe­(btz)₃ complex feature long-lived MLCT and LMCT states, respectively, demonstrating the versatility of iron <i>N-</i>heterocyclic carbene complexes as promising light-harvesters for a broad range of oxidizing and reducing conditions

Probing the Anisotropic Distortion of Photoexcited Spin Crossover Complexes with Picosecond X‑ray Absorption Spectroscopy

For numerous spin crossover complexes, the anisotropic distortion of the first coordination shell around the transition metal center governs the dynamics of the high-spin/low-spin interconversion. However, this structural parameter remains elusive for samples that cannot be investigated with crystallography. The present work demonstrates how picosecond X-ray absorption spectroscopy is able to capture this specific deformation in the photoinduced high-spin state of solvated [Fe­(terpy)₂]²⁺, a complex which belongs to the prominent family of spin crossover building blocks with nonequivalent metal–ligand bonds. The correlated changes in Fe–N_Axial, Fe–N_Distal, and bite angle N_Distal–Fe–N_Axial extracted from the measurements are in very good agreement with those predicted by DFT calculations in <i>D</i>_2<i>d</i> symmetry. The outlined methodology is generally applicable to the characterization of ultrafast nuclear rearrangements around metal centers in photoactive molecular complexes and nanomaterials, including those that do not display long-range order

Toward Highlighting the Ultrafast Electron Transfer Dynamics at the Optically Dark Sites of Photocatalysts

Building a detailed understanding of the structure–function relationship is a crucial step in the optimization of molecular photocatalysts employed in water splitting schemes. The optically dark nature of their active sites usually prevents a complete mapping of the photoinduced dynamics. In this work, transient X-ray absorption spectroscopy highlights the electronic and geometric changes that affect such a center in a bimetallic model complex. Upon selective excitation of the ruthenium chromophore, the cobalt moiety is reduced through intramolecular electron transfer and undergoes a spin flip accompanied by an average bond elongation of 0.20 ± 0.03 Å. The analysis is supported by simulations based on density functional theory structures (B3LYP*/TZVP) and FEFF 9.0 multiple scattering calculations. More generally, these results exemplify the large potential of the technique for tracking elusive intermediates that impart unique functionalities in photochemical devices