Molecular Switch Cobalt Redox Shuttle with a Tunable Hexadentate Ligand.

Strong-field hexadentate ligands were synthesized and coordinated to cobalt metal centers to result in three new low-spin to low-spin Co(III/II) redox couples. The ligand backbone has been modified with dimethyl amine groups to result in redox potential tuning of the Co(III/II) redox couples from -200 to -430 mV versus Fc+/0. The redox couples surprisingly undergo a reversible molecular switch rearrangement from five-coordinate Co(II) to six-coordinate Co(III) despite the ligands being hexadentate. The complexes exhibit modestly faster electron self-exchange rate constants of 2.2-4.2 M-1 s-1 compared to the high-spin to low-spin redox couple [Co(bpy)3]3+/2+ at 0.27 M-1 s-1, which is attributed to the change in spin state being somewhat offset by this coordination switching behavior. The complexes were utilized as redox shuttles in dye-sensitized solar cells with the near-IR AP25 + D35 dye system and exhibited improved photocurrents over the [Co(bpy)3]3+/2+ redox shuttle (19.8 vs 18.0 mA/cm2). Future directions point toward pairing the low-spin to low-spin Co(II/III) tunable series to dyes with significantly more negative highest occupied molecular orbital potentials that absorb into the near-IR where outer sphere redox shuttles have failed to produce efficient dye regeneration.

Interfacial Engineering at Quantum Dot-Sensitized TiO2 Photoelectrodes for Ultrahigh Photocurrent Generation.

Metal oxide semiconductor/chalcogenide quantum dot (QD) heterostructured photoanodes show photocurrent densities >30 mA/cm2 with ZnO, approaching the theoretical limits in photovoltaic (PV) cells. However, comparative performance has not been achieved with TiO2. Here, we applied a TiO2(B) surface passivation layer (SPL) on TiO2/QD (PbS and CdS) and achieved a photocurrent density of 34.59 mA/cm2 under AM 1.5G illumination for PV cells, the highest recorded to date. The SPL improves electron conductivity by increasing the density of surface states, facilitating multiple trapping/detrapping transport, and increasing the coordination number of TiO2 nanoparticles. This, along with impeded electron recombination, led to enhanced collection efficiency, which is a major factor for performance. Furthermore, SPL-treated TiO2/QD photoanodes were successfully exploited in photoelectrochemical water splitting cells, showing an excellent photocurrent density of 14.43 mA/cm2 at 0.82 V versus the Reversible Hydrogen Electrode (RHE). These results suggest a new promising strategy for the development of high-performance photoelectrochemical devices.

Imidazolium Iodide-Doped PEDOT Nanofibers as Conductive Catalysts for Highly Efficient Solid-State Dye-Sensitized Solar Cells Employing Polymer Electrolyte.

The electrical conductivity and catalytic activity of nanofibrous poly(3,4-ethylenedioxythiophene)s (PEDOT NFs) was improved by redoping with dimethyl imidazolium iodide (DMII) as a charge transfer facilitator. Addition of the new DMII dopant into the PEDOT NFs reduced the concentration of dodecyl sulfate anions (DS-) predoped during the polymerization process and concomitantly enhanced the doping concentration of I- by ion exchange. Redoping with DMII increased the mobility of the PEDOT NFs by up to 18-fold and improved the conductivity due to the enhanced linearization, suppressed aggregation, and improved crystallinity of the PEDOT chains. The catalytic activity was also improved, primarily due to the increase in the compatibility and the effective surface area upon replacement of sticky DS- with the more basic and smaller I- of DMII on the surface of the PEDOT NFs. The charge-transfer resistance across the interface between the poly(ethylene oxide)-based solid polymer electrolyte and PEDOT NF counter electrode (CE) was thus reduced to a large extent, giving an energy conversion efficiency (ECE) of 8.52% for solid-state dye-sensitized solar cells (DSCs), which is even better than that achieved with Pt CE (8.25%). This is the highest ECE reported for solid-state DSCs with conductive polymer CEs under 1 sun conditions.

Triumphing over Charge Transfer Limitations of PEDOT Nanofiber Reduction Catalyst by 1,2-Ethanedithiol Doping for Quantum Dot Solar Cells.

Charge transfer between a conducting polymer-based counter electrode (CE) and a polysulfide (S2-/Sn2-) electrolyte mediator is a key limitation to improvements of solar energy conversion efficiency (ECE) in quantum-dot-sensitized solar cells (QDSCs). In this paper, 1,2-ethanedithiol (EDT) was doped into nanofibrous poly(3,4-ethylenedioxythiophene) (PEDOT NF) to overcome the charge transfer limitation between PEDOT NF and S2-/Sn2-. EDT not only helps to reduce the aggregation and thus enhance the linearization of the PEDOT chains but also changes the molecular conformation of the PEDOT chains from a benzoid to a quinoid structure. EDT-doped PEDOT NF-based CEs showed almost 3.7 times higher conductivity, better electrocatalytic activity, and improved compatibility with S2-/Sn2- in an aqueous electrolyte. As a result, the charge transfer resistance between the polymer-based CE and the S2-/Sn2- electrolyte was significantly reduced, resulting in over 3% ECE in QDSCs, more than double that of a bare PEDOT NF-based CE.

Exploring Interfacial Events in Gold-Nanocluster-Sensitized Solar Cells: Insights into the Effects of the Cluster Size and Electrolyte on Solar Cell Performance.

Gold nanoclusters (Au NCs) with molecule-like behavior have emerged as a new light harvester in various energy conversion systems. Despite several important strides made recently, efforts toward the utilization of NCs as a light harvester have been primarily restricted to proving their potency and feasibility. In solar cell applications, ground-breaking research with a power conversion efficiency (PCE) of more than 2% has recently been reported. Because of the lack of complete characterization of metal cluster-sensitized solar cells (MCSSCs), however, comprehensive understanding of the interfacial events and limiting factors which dictate their performance remains elusive. In this regard, we provide deep insight into MCSSCs for the first time by performing in-depth electrochemical impedance spectroscopy (EIS) analysis combined with physical characterization and density functional theory (DFT) calculations of Au NCs. In particular, we focused on the effect of the size of the Au NCs and electrolytes on the performance of MCSSCs and reveal that they are significantly influential on important solar cell characteristics such as the light absorption capability, charge injection kinetics, interfacial charge recombination, and charge transport. Besides offering comprehensive insights, this work represents an important stepping stone toward the development of MCSSCs by accomplishing a new PCE record of 3.8%.

Surface Modification of TiO2 Photoanodes with Fluorinated Self-Assembled Monolayers for Highly Efficient Dye-Sensitized Solar Cells.

Dye aggregation and electron recombination in TiO2 photoanodes are the two major phenomena lowering the energy conversion efficiency of dye-sensitized solar cells (DSCs). Herein, we introduce a novel surface modification strategy of TiO2 photoanodes by the fluorinated self-assembled monolayer (F-SAM) formation with 1H,1H,2H,2H-perfluorooctyltriethoxysilane (PFTS), blocking the vacant sites of the TiO2 surface after dye adsorption. The F-SAM helps to efficiently lower the surface tension, resulting in efficient repelling ions, e.g., I3(-), in the electrolyte to decrease the electron recombination rate, and the role of F-SAM is characterized in detail by impedance spectroscopy using a diffusion-recombination model. In addition, the dye aggregates on the TiO2 surface are relaxed by the F-SAM with large conformational perturbation (i.e., helix structure) seemingly because of steric hindrance developed during the SAM formation. Such multifunctional effects suppress the electron recombination as well as the intermolecular interactions of dye aggregates without the loss of adsorbed dyes, enhancing both the photocurrent density (11.9 → 13.5 mA cm(-2)) and open-circuit voltage (0.67 → 0.72 V). Moreover, the combined surface modification with the F-SAM and the classical coadsorbent further improves the photovoltaic performance in DSCs.

Excellent optical and interfacial performance of a PEDOT-b-PEG block copolymer counter electrode for polymer electrolyte-based solid-state dye-sensitized solar cells.

A poly(3,4-ethylenedioxythiophene)-b-poly(ethylene glycol) (PEDOT-b-PEG) block copolymer doped with perchlorate on FTO shows excellent optical and interfacial performance as a counter electrode (CE), such as low charge transfer resistance and low reflectivity for polymer electrolyte-based solid-state dye-sensitized solar cells (DSCs), resulting in 8.45% energy conversion efficiency, greater than the common Pt CE, via a facile room-temperature process.