Electrolyte/Dye/TiO2 Interfacial Structures of Dye-Sensitized Solar Cells Revealed by In Situ Neutron Reflectometry with Contrast Matching.

The nature of an interfacial structure buried within a device assembly is often critical to its function. For example, the dye/TiO2 interfacial structure that comprises the working electrode of a dye-sensitized solar cell (DSC) governs its photovoltaic output. These structures have been determined outside of the DSC device, using ex situ characterization methods; yet, they really should be probed while held within a DSC since they are modulated by the device environment. Dye/TiO2 structures will be particularly influenced by a layer of electrolyte ions that lies above the dye self-assembly. We show that electrolyte/dye/TiO2 interfacial structures can be resolved using in situ neutron reflectometry with contrast matching. We find that electrolyte constituents ingress into the self-assembled monolayer of dye molecules that anchor onto TiO2. Some dye/TiO2 anchoring configurations are modulated by the formation of electrolyte/dye intermolecular interactions. These electrolyte-influencing structural changes will affect dye-regeneration and electron-injection DSC operational processes. This underpins the importance of this in situ structural determination of electrolyte/dye/TiO2 interfaces within representative DSC device environments.

Cosensitization in Dye-Sensitized Solar Cells.

Dye-sensitized solar cells (DSCs) are a next-generation photovoltaic technology, whose natural transparency and good photovoltaic output under ambient light conditions afford them niche applications in solar-powered windows and interior design for energy-sustainable buildings. Their ability to be fabricated on flexible substrates, or as fibers, also makes them attractive as passive energy harvesters in wearable devices and textiles. Cosensitization has emerged as a method that affords efficiency gains in DSCs, being most celebrated via its role in nudging power conversion efficiencies of DSCs to reach world-record values; yet, cosensitization has a much wider potential for applications, as this review will show. Cosensitization is a chemical fabrication method that produces DSC working electrodes that contain two or more different dyes with complementary optical absorption characteristics. Dye combinations that collectively afford a panchromatic absorption spectrum emulating that of the solar emission spectrum are ideal, given that such combinations use all available sunlight. This review classifies existing cosensitization efforts into seven distinct ways that dyes have been combined in order to generate panchromatic DSCs. Seven cognate molecular-engineering strategies for cosensitization are thereby developed, which tailor optical absorption toward optimal DSC-device function.

Electrochemically activated solid synthesis: an alternative solid-state synthetic method.

Solid-state synthesis is one of the most common synthetic methods in chemistry and is extensively used in lab-scale syntheses of advanced functional materials to ton-scale production of chemical compounds. It generally requires at least one or several high temperature and/or high-pressure steps, which makes production of compounds via solid-state methods very energy and time intensive. Consequently, there is a persistent economic and environmental incentive to identify less energy and time consuming synthetic pathways. Here, we present an alternative solid-state synthetic method, which utilizes structural changes, induced by an electrochemical "activation" step followed by a thermal treatment step. The method has been used to synthesize a Sc0.67WO4-type phase where Sc0.67WO4 has previously only been obtained at 1400 °C and 4 GPa for 1 h. Through our method the Sc0.67WO4-type phase has been prepared at only 600 °C and ambient pressure. Experimental factors that influence phase formation from the electrochemical perspective are detailed. Overall, the method presented in this work appears to be able to generate the conditions for unusual and new phases to form and thus becomes another tool for synthetic solid-state chemists. This in turn permits the exploration of a larger synthetic parameter space.

Structural evolution and stability of Sc2(WO4)3 after discharge in a sodium-based electrochemical cell.

Sc2(WO4)3, prepared by solid state synthesis and constructed as an electrode, is discharged to different states in half-cell batteries, versus a Na negative electrode. The structural evolution of the Na-containing electrodes is studied with synchrotron powder X-ray diffraction (PXRD) revealing an increase in microstrain and a gradual amorphization taking place with increasing Na content in the electrode. This indicates that a conversion reaction takes place in the electrochemical cell. X-ray absorption spectroscopy (XAS) at the tungsten L3 absorption edge shows a reduction in the tungsten oxidation state. Variable temperature (VT) PXRD shows that the Sc2(WO4)3 electrode remains relatively stable at higher temperatures, while the Na-containing samples undergo a number of phase transitions and/or turn amorphous above ∼400 °C. Although, Sc2(WO4)3 is a negative thermal expansion (NTE) material only a subtle change of the thermal expansion is found below 400 °C for the Na-containing electrodes. This work shows the complexity in employing an electrochemical cell to produce Na-containing Sc2(WO4)3 and the subsequent phase transitions.