The Intrinsic Charge Carrier Behaviors and Applications of Polyoxometalate Clusters Based Materials

Polyoxometalates (POMs) are a series of molecular metal oxide clusters, which span the two domains of solutes and solid metal oxides. The unique characters of POMs in structure, geometry, and adjustable redox properties have attracted widespread attention in functional material synthesis, catalysis, electronic devices, and electrochemical energy storage and conversion. This review is focused on the links between the intrinsic charge carrier behaviors of POMs from a chemistry-oriented view and their recent ground-breaking developments in related areas. First, the advantageous charge transfer behaviors of POMs in molecular-level electronic devices are summarized. Solar-driven, thermal-driven, and electrochemical-driven charge carrier behaviors of POMs in energy generation, conversion and storage systems are also discussed. Finally, present challenges and fundamental insights are discussed as to the advanced design of functional systems based upon POM building blocks for their possible emerging application areas.

Polyoxometalates in dye-sensitized solar cells

Dye-sensitized solar cells (DSSCs) are the third generation of photovoltaic cells developed by Grätzel and O'Regan. They have the characteristics of low cost, simple manufacturing process, tunable optical properties, and higher photoelectric conversion efficiency (PCE). With an ever increasing energy crisis, there is an urgent need to develop highly efficient, environmentally benign, and energy-saving cell materials. Polyoxometalates (POMs), a kind of molecular inorganic quasi-semiconductor, are promising candidates for use in different parts of DSSCs due to their excellent photosensitivity, redox, and catalytic properties, as well as their relative stability. Following a brief introduction to the development of DSSCs and the potential virtues of POMs in DSSCs, we attempt to make some generalizations about the energy level regulation of POMs that is the underlying theoretical basis for their application in DSSCs, and then we summarize the research progress of POMs in DSSCs in recent years. This is organized in terms of the properties of POMs, namely, electron acceptor, photosensitivity, redox and catalysis, based on the accumulation of our research into POMs over many years. Meanwhile, in view of the fact that the properties of POMs depend primarily on their electronic structural diversity, we keep this point in mind throughout the article with a view to revealing their structure-property relationships. Finally we provide a short summary and remarks on the future outlook. This review may be of interest to synthetic chemists devoted to designing POMs with specific structures, and researchers engaged in the extension of POMs to photoelectric materials.

A Strategy to Enhance the Efficiency of Quantum Dot-Sensitized Solar Cells by Decreasing Electron Recombination with Polyoxometalate/TiO2 as the Electronic Interface Layer

Electron recombination occurring at the TiO₂ /quantum dot sensitizer/electrolyte interface is the key reason for hindering further efficiency improvements to quantum dot sensitized solar cells (QDSCs). Polyoxometalate (POM) can act as an electron-transfer medium to decrease electron recombination in a photoelectric device owing to its excellent oxidation/reduction properties and thermostability. A POM/TiO₂ electronic interface layer prepared by a simple layer-by-layer self-assembly method was added between fluorine-doped tin oxide (FTO) and mesoporous TiO₂ in the photoanode of QDSCs, and the effect on the photovoltaic performance was systematically investigated. Photovoltaic experimental results and the electron transmission mechanism show that the POM/TiO₂ electronic interface layer in the QDSCs can clearly suppress electron recombination, increase the electron lifetime, and result in smoother electron transmission. In summary, the best conversion efficiency of QDSCs with POM/TiO₂ electronic interface layers increases to 8.02 %, which is an improvement of 25.1 % compared with QDSCs without POM/TiO₂ . This work first builds an electron-transfer bridge between FTO and the quantum dot sensitizer and paves the way for further improved efficiency of QDSCs.

Low-cost p-type dye-sensitized solar cells based on Dawson-type transition metal-substituted polyoxometalate inorganic co-sensitizers

α₂-K₈P₂W₁₇O₆₁(Co²⁺·OH₂)·16H₂O(P₂W₁₇Co) and α₂-K₇P₂W₁₇O₆₁(Mn³⁺·OH₂)·12H₂O(P₂W₁₇Mn) are employed to construct a new inorganic co-sensitizer.

Phosphomolybdic acid immobilized on graphite as an environmental photoelectrocatalyst

A new phosphomolybdic acid (PMA)/Graphite surface was prepared based on electrostatic interactions between phosphomolybdic acid and graphite surface. The PMA/Graphite was characterized by cyclic voltammetry (CV) analysis and scanning electron microscope (SEM). SEM images showed that the phosphomolybdic acid particles were well stabilized on the graphite surface and they were evidenced the size of particles (approximately 10 nm). The CV results not only showed that the modified surface has good electrochemical activity toward the removal of the dyestuff, but also exhibits long term stability. The PMA/Graphite was used as a photoanode for decolorization of Reactive Yellow 39 by photoelectrocatalytic system under UV irradiation. The effects of parameters such as the amount of phosphomolybdic acid used in preparation of PMA/Graphite surface, applied potential on anode electrode and solution pH were studied by response surface methodology. The optimum conditions were obtained as follows: dye solution pH 3, 1.5 g of immobilized PMA on graphite surface and applied potential on anode electrode 1 V. Under optimum conditions after 90 min of reaction time, the decolorization efficiency was 95%.

Recent Advances of Cobalt(II/III) Redox Couples for Dye-Sensitized Solar Cell Applications

In recent years dye-sensitized solar cells (DSSCs) have emerged as one of the alternatives for the global energy crisis. DSSCs have achieved a certified efficiency of >11% by using the I(-) /I3 (-) redox couple. In order to commercialize the technology almost all components of the device have to be improved. Among the various components of DSSCs, the redox couple that regenerates the oxidized sensitizer plays a crucial role in achieving high efficiency and durability of the cell. However, the I(-) /I3 (-) redox couple has certain limitations such as the absorption of triiodide up to 430 nm and the volatile nature of iodine, which also corrodes the silver-based current collectors. These limitations are obstructing the commercialization of this technology. For this reason, one has to identify alternative redox couples. In this regard, the Co(II/III) redox couple is found to be the best alternative to the existing I(-) /I3 (-) redox couple. Recently, DSSC test cell efficiency has risen up to 13% by using the cobalt redox couple. This review emphasizes the recent development of Co(II/III) redox couples for DSSC applications.

NiO-decorated mesoporous TiO2 flowers for an improved photovoltaic dye sensitized solar cell

The flower-like porous TiO₂–NiO nanoparticles, possessing a high BET surface area, are first used as a photoanode in a DSSC.

Transition metal complex redox shuttles for dye-sensitized solar cells

This review provides an in-depth investigation into exciting alternative electrolyte shuttles in DSSCs and the various advantages that they provide, such as high conversion efficiency and non-corrosive properties.