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A hybrid system in which the counter electrode is exchanged with a redox couple dissolved in the electrolyte is an alternative to a “sandwich” type electrochromic (EC) device. We have developed a solid redox (tetramethylthiourea/tetramethylformaminium disulfide dication TMTU/[TMFDS]2+) electrolyte based on organically modified silane matrix. The EC device employing a solid electrolyte colors homogeneously, visible transmittance (Tvis) changes from 60% to 5%, at −1.5 V within less than 5 min when 240 nm cubic WO3 layer is employed. The results show that solidification of the electrolyte does not affect coloring kinetic, but the reduction of [TMFDS]2+ to TMTU at the Pt electrode significantly slows bleaching and reduces the self-bleaching of the device. Moreover, the EC device employing solid electrolyte colors homogeneously and is colorless in the bleached state (C, a and b values are below 10).

The transmittance of a photochromic (PC) window is reduced in sunlight but returns to its original value in the dark similar to photo-sensitive sun glasses. A 30 cm × 30 cm double glazed window containing a PC multi-layer coating on a single glass pane has been developed. A single PC glass pane can be used to build a window with a low heat transfer coefficient, e.g. double or triple glazed unit, using existing technology. To develop the PC glass with a multi-layered coating that combines chromogenic and photoactive materials we employed sol-gel chemistry approaches and up-scaled the synthesis. Double glazed windows have been assembled using PC glass with two different layer configurations. The transmittance (Tvis) was reduced from 76% to 35% under outdoor solar radiation (750 W/m2) within 15 min.

We report on the development of WO3 layers with increased symmetry induced by the incorporation of titanium into a crystalline WO3 matrix. The atomic ratio between W and Ti was varied stepwise from 100:1 to 100:20 with pure WO3 layers, made using peroxo sol–gel synthesis, as a reference. Layers were then formed by dip-coating from the corresponding sols. IR spectroscopy and XRD analysis of layers and powders heat-treated at 450 °C proved that the presence of titanium ions induces ordering and increases the symmetry of WO6 octahedra in the WO3 crystalline matrix leading to the transformation of the cell symmetry from monoclinic through tetragonal up to the cubic. Cyclic voltammetry confirmed that this increased symmetry leads to an up-shift in redox potential, which favors the coloration of the layers in photochromic devices. According to our knowledge, this is the first report of colorless cubic WO3 layers, which have potential applications in optoelectronic devices and sensors.

Photovoltaics is amongst the most important technologies for renewable energy sources, and plays a key role in the development of a society with a smaller environmental footprint. Key parameters for solar cells are their energy conversion efficiency, their operating lifetime, and the cost of the energy obtained from a photovoltaic system compared to other sources. The optimization of these aspects involves the exploitation of new materials and development of novel solar cell concepts and designs. Both theoretical modeling and characterization of such devices require a comprehensive view including all scales from the atomic to the macroscopic and industrial scale. The different length scales of the electronic and optical degrees of freedoms specifically lead to an intrinsic need for multiscale simulation, which is accentuated in many advanced photovoltaics concepts including nanostructured regions. Therefore, multiscale modeling has found particular interest in the photovoltaics community, as a tool to advance the field beyond its current limits. In this article, we review the field of multiscale techniques applied to photovoltaics, and we discuss opportunities and remaining challenges. © T. Abu Hamed et al., published by EDP Sciences, 2018., European Cooperation in Science and Technology: MP1406, The authors are grateful for the financial support by the COST Action MP1406 “MultiscaleSolar.”

High-temperature treatment steps in fabrication process of dye sensitized solar cell (DSSC) significantly contribute to the manufacturing costs and limit the use of temperature sensitive substrates. Therefore our aim was to develop a simple method for the preparation of water-based TiO2 paste. The paste is based on peroxotitanic acid (PTA) sol–gel matrix and commercial TiO2 nanoparticles (P25). Two fabrication processes to decompose/transform the PTA matrix in the printed TiO2 layer are explored and combined: annealing at temperatures up to 250 °C and/or oxygen plasma treatment. The results show that the PTA matrix in the paste converts to anatase phase and to some extent also attaches to the TiO2 nanoparticles P25 acting as an interconnecting network within TiO2 layer. The transformation of the PTA matrix occurs around 250 °C, but in the presence of TiO2 nanoparticles P25 it starts already at 120 °C. In addition the results reveal that the crystallization is achievable also solely with the oxygen plasma treatment. The efficiency of the TiO2 layers, exposed to different post-deposition treatments, is evaluated in DSSCs. The results show that oxygen plasma treatment of the TiO2 layers could efficiently replace temperature curing at 250 °C. Within this study the DSSCs with the efficiency up to 4.2 % measured under standard test conditions (1,000 W/m2, AM1.5, 25 °C) were realized.

Different approaches have been studied to evaluate the efficiency of the photoactive layer in a photochromic (PC) device. The studied device combines materials typically used in dye-sensitized solar cell (DSSC) and an electrochromic (EC) layer of WO3. First, we examine if the dye could be attached directly to the EC layer forming an efficient photoactive layer to colour the PC device. Further, different TiO2 layers have been coated on the EC layer to enhance dye loading and two different sol-gel TiO2 layers and the post-treatment of the EC layer with a titanium(IV) chloride tetrahydrofuran complex have been studied. The dye loading of different photoactive layers and their efficiencies in DSSC have been evaluated and discussed with regard to the performance of the PC devices. The results confirmed that the dye could be attached to the WO3 layer, but that the voltage of the DSSC is too low to colour the device. To realize a functioning PC device, the addition of TiO2 is necessary. The best performance (deep and fast colouration) is obtained for a PC device with a TiO2 sol-gel layer; the solar transmittance of the PC device decreases from 57% to 7.5% in under three minutes. According to our knowledge, this is the first study of the impact of the photoactive layer on the photochromic glazing's response.

The influence that the degree of polyesterification has on a titanium sol (Ti-sol) prepared via the Pechini method that acts as a matrix in TiO2 pastes used for dye sensitized solar cells is reported. The different content of the polyester in the Ti-sol was realized by varying the heating time of the Ti-sol. Titanium dioxide pastes were prepared by introducing a commercial TiO2 nanopowder into the Ti-sols. The TiO2 layers were tested as photoanodes in dye-sensitized solar cells (DSSCs). The most appropriate degree of polyesterification was achieved by heating the Ti-sol for 0.5 and 1 h, while longer heating deteriorates the TiO2 layer morphology. The highest efficiency of the DSSCs based on an ionic liquid electrolyte was 6.3% measured under standard test conditions (100 mW/cm2, AM 1.5, 25 °C).

A titanium oxide layer used in a dye-sensitized solar cell (DSSC) has to meet two opponent properties to enable high conversion efficiency: a large surface area (for high dye loading) and good connection between TiO2 grains (for efficient extraction of electrons toward the front contact). In order to meet a trade-off between these criteria a preparation method for TiO2 paste formulation based on Pechini sol–gel method and commercial nanocrystalline TiO2 powder has been developed. A series of TiO2 pastes with different molar ratios between titanium isopropoxide, citric acid and ethylene glycol (1:X:4X) in the paste have been examined. The structure and morphology as well as cross-cut tests of deposited and sintered TiO2 layers have been analyzed. Results reveal that the paste with X = 8 exhibits the best properties, resulting in an overall conversion efficiency of DSSC under standard test conditions (100 mW/cm2, 25 °C, AM 1.5G) up to 6.6% for ionic liquid based electrolyte.

The performance of dye-sensitized solar cells, based on pure and binary ionic liquid electrolyte with different iodine concentrations, have been evaluated under 1 sun illumination and different cells temperature between 5 and 55 °C. At lower temperatures a short-circuit current ( J SC ) may be limited by the diffusion of tri-iodide. In this situation J SC increases with increasing temperature, while at higher temperatures the J SC decreases due to more pronounced recombinations. We find a maximum for J SC at intermediate temperatures. This maximum shifts to lower temperatures by increasing iodine concentration. At a certain temperature different iodine concentration is optimal for different ionic liquids employed in the electrolyte. The open circuit voltage monotonically decreases with temperature while the conversion efficiency is basically dominated by temperature dependence of J SC .

A titanium oxide layer used for a dye-sensitized solar cell (DSSC) has to meet two opponent properties to assure a high efficiency DSSC: good connection between TiO2 grains and a large inner surface area. Three different paste formulations based on commercial nanocrystalline TiO2 powder (Degussa P25) are studied. Results confirm that modification of the TiO2 paste with the Pechini sol–gel method increases the surface area of the TiO2 layer while maintaining good connections between the nanocrystalline grains, consequently the efficiency of the DSSC increases from 1.8% to 5.3%. The structure and morphology of the TiO2 layers are described by scanning electron microscopy (SEM), high resolution transmission electron microscopy (HR-TEM) and X-ray diffraction (XRD).

A novel titanium oxide paste based on Pechini sol–gel method and nanocrystalline titanium oxide powder have been successfully developed. Titanium oxide layers possess high inner surface area assuring high dye loading and well-connected nanocrystalline grains assuring good electron transport within the layer. The dye-sensitized layers have been used to assemble dye-sensitized solar cells with acetonitrile- and ionic liquid-based electrolyte. Overall conversion efficiencies of dye-sensitized solar cells (DSSCs) determined under standard test conditions (100 mW/cm 2 , 25 °C and AM 1.5 G) are 10.2% for acetonitrile and 7.3% for ionic liquid-based electrolyte.

A simple manufacturing process based on screen-printing is crucial for a successful commercialization of dye-sensitized solar cells (DSSCs). We developed the sol–gel based TiO2 paste in such a way that solely a single step deposition is sufficient to realize a sponge-like structure of the layer assuring its high activity in DSSCs. For the first time the sol–gel based TiO2 paste was screen printed and tested in DSSC masterplates and PV mini-modules. Electroluminescence imaging of the mini-module proved layer homogeneity and no manufacturing defects. The conversion efficiency of the PV mini-module with the active area of 75 cm2 reaches 5.7% at STC.

Spatial characterization techniques are applied to dye-sensitized solar cells (DSSCs). A comparison between transmittance imaging (TI), light-beam-induced-current (LBIC) scan and electroluminescence imaging is carried out. Detected types of inhomogeneities have different fingerprints by each applied technique. Electroluminescence (EL) is advantageous over TI because the electrical activity of the inhomogeneities influences the result. EL is also advantageous over the LBIC scan due to shorter acquisition time.

Dye-sensitized solar cells (DSSCs) are recognized as one of the world's leading innovation in nanosciences and photovoltaic technology. In contrast to the conventional silicon-based solar cells the demand on purity of materials for DSSC is lower and forecasted manufacturing costs are approximately halved which make the DSSCs attractive alternative. Nowadays the conversion efficiency of DSSCs exceeds 10% which makes it competitive with other thin film solar cells. In this chapter the structure and the fundamental operation of the DSSC are presented. Afterwards the properties and requirements for each individual component used in a DSSC are given. In addition advanced hybrid DSSC systems also are presented. In overall, the emphasis of this chapter is given to the involvement of sol-gel chemistry in the preparation of the individual DSSCs components, primarily of TiO2 layers and electrolytes.

The performance of dye-sensitized solar cells (DSSC) based on the propyl-methyl-imidazolium iodide (PMII) ionic liquid (IL) with and without the addition of SiO 2 nanoparticles is studied. Results confirm that the presence of SiO 2 nanoparticles in PMII electrolyte improves the charge transport of iodide/tri-iodide redox couple in the electrolyte and consequently increases the efficiency of DSSC up to 20%, relatively. Short circuit current density ( J SC ) of the DSSC under illumination may be limited by the charge transport of the redox couple in the IL-based electrolytes and a theoretical maximum of J SC can be evaluated from the cyclic voltammetry measurements of simple symmetric cells (TCO-Pt electrolyte Pt-TCO). The results show a strong temperature dependence of the DSSC performance if the PMII/I 2 -based electrolytes are used.