TiO2 Porous Electrodes Research Articles

Structure-Dependent Response Characteristics of Electrochromic Dye-Modified Transparent Conductive Oxide Porous Electrode

Fast-response electrochromic (EC) cell composed of EC dye-modified TiO2-nanoparticle-based transparent porous electrode was reported by R. Cinnsealach et al. (Sol. Energy Mater. Sol. Cells, 55, 215, 1998). From then, TiO2 porous electrodes modified with EC dyes have been regarded as a promising candidate for fast-response EC display (ECD) applications. Our research purpose is the realization of flexible fast-response ECD applications. However, TiO2 porous electrodes require high sintering temperature (>400 °C) to use as EC electrode because TiO2 has high resistivity. This high process temperature required by TiO2 porous electrode was an important obstacle for their application as transparent electrodes in plastic substrate-based flexible ECDs. To solve this problem, we substituted transparent conductive oxides (TCO) for TiO2 as the porous electrode material. TCO materials have high electrical conductivity than TiO2, so they have the potential to work as the EC electrode even if they are sintered at low temperature. Previously, we reported that the porous electrodes consisting of indium tin oxide (ITO) nanoparticles exhibit good optical transparency and high electric conductivity. Moreover, EC response characteristics of EC-dye modified ITO porous electrode sintered at 150 °C was improved than that of conventional TiO2 porous electrode due to its high conductivity (AIP Adv., 6, 065121, 2016). Therefore, TCO porous electrodes have potential for application in porous electrode for flexible ultrafast-response ECDs. To achieve such applications, it is necessary to analyze the factors controlling the EC response speed in TCO porous electrodes. In this study, we investigated the response characteristics of EC dye-modified ITO, antimony-doped tin oxide (ATO), and TiO2 porous electrodes. ITO and ATO porous films have low electric resistivity due to their high electric conductivity. As mentioned above, ITO porous electrode exhibited fast response. On the other hand, low-resistivity ATO porous electrode exhibited slower response than that of a conventional high-resistivity TiO2 porous electrode. This clearly indicates that the EC response speed of the ATO porous electrode is dependent on a factor other than the electric resistance of the porous film. From the structural comparisons, it is revealed that the pore size of the film consisting of small nanoparticles is one of the key factors controlling the EC response speed of porous electrode. The details about the effect of porous film structure on EC response characteristics will be discussed.

Electrochromic Response Characteristics of Dye-modified Porous Electrodes Affected by the Porous Film Structure

The electrochromic (EC) response characteristics of EC-dye-modified porous electrodes composed of degenerate transparent conductive oxides were investigated. Although the In2O3:Sn and SnO2:Sb porous electrodes exhibited low electrical resistivity, the SnO2:Sb porous electrode exhibited a response slower than that of a conventional high-resistivity TiO2 porous electrode, whereas the In2O3:Sn porous electrode exhibited a faster response. Structural comparisons revealed that the pore size of the film consisting of small nanoparticles is one of the key factors controlling the EC response speed.

Improvement of the electrochromic response of a low-temperature sintered dye-modified porous electrode using low-resistivity indium tin oxide nanoparticles

An indium tin oxide (ITO) nanoparticle-based porous electrode sintered at low temperatures was investigated as a transparent electrode for electrochromic displays (ECDs). The electrochromic (EC) response of the dye-modified ITO porous electrode sintered at 150 °C, which exhibited a generally low resistivity, was markedly superior to that of a conventional dye-modified TiO2 porous electrode sintered at the same temperature. Moreover, the EC characteristics of the dye-modified ITO porous electrode sintered at 150 °C were better than those of the high-temperature (450 °C) sintered conventional dye-modified TiO2 porous electrode. These improvements in the EC characteristics of the dye-modified ITO porous electrode are attributed to its lower resistivity than that of the TiO2 porous electrodes. In addition to its sufficiently low resistivity attained under the sintering conditions required for flexible ECD applications, the ITO porous film had superior visible-light transparency and dye adsorption capabilities. We conclude that the process temperature, resistivity, optical transmittance, and dye adsorption capability of the ITO porous electrode make it a promising transparent porous electrode for flexible ECD applications.

Effect of pore size/distribution in TiO2 films on agarose gel electrolyte-based dye-sensitized solar cells

This study develops a simple method to change the distribution of the pore size in a TiO2 layer, using polyethylene glycol (PEG), while maintaining nearly the same surface area and porosity to clarify how large pores affect the performance of dye-sensitized solar cells (DSSCs). Specifically, a heating step at 100 °C for a specific duration is added prior to PEG removal and TiO2 sintering at 400 °C. This process transforms the role of the PEG from a surfactant to a pore generator (porogen) and forms larger pores, depending on the loading and aggregation time for the PEG to gain larger pores. The effect of larger pores in TiO2 films under 30 % PEG loading, on the performance of an agarose gel electrolyte-based DSSC, was further investigated using the ionic liquid, 1-allyl-3-ethylimidazolium iodide (AEII). The I–V characteristic and the electrochemical impedance spectroscopy analysis show that larger pores readily improve redox couple diffusion in a TiO2 porous electrode and modify the interface between electrolyte and TiO2. Using the optimized TiO2 film with larger pores (30 % PEG loading, 100 °C/60 min), an efficiency of 7.43 % is achieved for the agarose gel electrolyte-based DSSC, which represents a 26.1 % improvement over TiO2 without the addition of PEG.

Light controlled assembling of iodine-free dye-sensitized solar cells with poly(3,4-ethylenedioxythiophene) as a hole conductor reaching 7.1% efficiency

We report the fabrication of indoline D205 dye-sensitized solar cells (DSSCs) with photoelectrochemically polymerized poly(3,4-ethylenedioxythiophene) (PEDOT) as a hole conductor. Different from conventional photoelectrochemical polymerization under continuous spectral light illumination (e.g. Xe lamp), we conduct the polymerization under monochromatic light. The device performance is found to be dependent on the wavelength of monochromatic light used for the polymerization of PEDOT. Under optimized conditions, the efficiency of DSSCs reaches 7.1%, which is better than that of devices fabricated via continuous spectral light illumination. Detailed characterization of these devices with photoelectrical and impedance measurements reveals that the wavelength of monochromatic light affects PEDOT penetration into TiO2 porous electrodes and photoelectron recombination at the FTO surface. The best device performance is obtained when penetration and recombination are optimized.

Effects of Homogenization Scheme ofTiO2Screen-Printing Paste for Dye-Sensitized Solar Cells

TiO2porous electrodes have been fabricated for photoelectrodes in dye-sensitized solar cells (DSCs) using TiO2screen-printing paste from nanocrystalline TiO2powder dried from the synthesized sol. We prepared the TiO2screen-printing paste by two different methods to disperse the nanocrystalline TiO2powder: a “ball-milling route” and a “mortal-grinding route.” The TiO2ball-milling (TiO2-BM) route gave monodisperse TiO2nanoparticles, resulting in high photocurrent density (14.2 mA cm−2) and high photoconversion efficiency (8.27%). On the other hand, the TiO2mortal-grinding (TiO2-MG) route gave large aggregate of TiO2nanoparticles, resulting in low photocurrent density (11.5 mA cm−2) and low photoconversion efficiency (6.43%). To analyze the photovoltaic characteristics, we measured the incident photon-to-current efficiency, light absorption spectroscopy, and electrical impedance spectroscopy of DSCs.

Development of Multiscale Simulator for Dye-Sensitized TiO2 Nanoporous Electrode Based on Quantum Chemical Calculation

In this study, we have developed a novel multiscale simulator for a dye-sensitized TiO2 porous electrode. In the simulator, we can estimate the properties of the dye-sensitized TiO2 porous electrode using the three-dimensional mesoscopic structure model constructed on the basis of our original porous structure simulator. The microscopic physical properties of the materials were estimated by quantum chemistry calculation using a tight-binding quantum chemical molecular dynamics program. From the calculation results, we determined the absorption coefficient and the diffusion coefficient of excited carriers used in the macroscopic simulation for photoelectrode characteristics. By using this multiscale simulator, we will be able to determine the best electrode system efficiently.

Investigation of the stability of solid-state dye-sensitized solar cells

The stability of the TiO2/ruthenium dye/CuI solid-state solar cell was investigated under continuous simulated sunlight illumination. The cells showed fast degradation under full-spectrum sunlight illumination, but showed rather good stability when the ultraviolet part of the illumination was removed. XPS measurements showed evidence that TiO2 could oxidize CuI in the presence of UV light. The photo-degradation mechanism of the cells is thus discussed on the basis of the photo-oxidative function of TiO2. The long-term stability of the solid-state dye-sensitized solar cell (DSSC) was found to be improved under simulated sunlight by coating the TiO2 porous electrode with an ultra-thin MgO layer, which was able to block the photo-oxidative activity of the TiO2.