Photoelectrochromic Research Articles

Rational design of the electrolyte systems for the photochromic device

A photochromic (PC) device, a simplified version of the photoelectrochromic (PEC) device without the transparent conducting oxide (TCO) layer, can modulate the intensity of transmitted light by automatically changing its transmittance. To effectively regulate the transmittance of sunlight, the PC device should possess a high optical contrast and reasonable transition time. However, these properties cannot be achieved simultaneously due to the competitive coloring and bleaching reactions. In this study, the effects of the concentration ratio of redox couples (tetramethylthiourea/tetramethylformaminium disulfide (TMTU/TMFDS2+)) and the concentration of lithium source on the performance of the PC device were systematically investigated. The results confirmed that the intrinsic limitation can be overcome by the rational design of the electrolyte system. The PC device containing an optimized TMTU/TMFDS2+ (0.05 M/0.1 M) redox couple and 1 M LiClO4 provided excellent coloration depth (~51% transmission change at 670 nm) and fast bleaching speed (80% recovery in under 1.5 h). In addition, long-term durability (>100 h) tests of the PC device revealed the high stability of the investigated electrolyte system. The in-depth discussion in combination with the experimental results provided in this work can pave the way for the future rational design of PC devices.

(Invited) Investigation on Coloring/Self-Bleaching Mechanism in Photochromic Device Using Raman Spectroscopy

Smart windows have been of great interest for a wide variety of energy saving application to control the penetration of solar energy into buildings by switching between a coloring and bleaching states. For dynamic transmittance control, various smart devices are developed, such as electrochromic (EC), photochromic (PC), and photoelectrochromic (PEC) devices. Although, the coloring/bleaching mechanisms of EC and PEC devices are well-known to achieve better smart window performance, it is still lacking information about coloration/decoloration mechanism in PC device. In this regards, unrevelling photochromic mechanism during coloring/self-bleaching could help to improve their performance to advance smart window application.In this talk, we investigate the coloring/self-bleaching mechanism using Raman spectroscopy. The Raman spectroscopy help to understand the photochromic mechanism upon light irradiation and self-bleaching stage. Our approach is based on the PC device, which is composed of the tungsten oxide (WO3) and titanium oxide (TiO2), and 5-methyl salicylic acid (5-MSA) as a cost effective ligand. The Raman spectroscopy allows us to understand the correlation between coloring/bleaching process and intercalation/deintercalation of Li ion. Figure 1

Novel photoelectrochromic devices incorporating carbon-based perovskite solar cells

Abstract In the present work, we propose for the first time, the use of a carbon-based perovskite solar cell (PSC) in a “partly covered” photoelectrochromic (PEC) device, with a three-electrode architecture that de-couples the photovoltaic (PV) and electrochromic (EC) operation, thus enabling better control of the device optical modulation. The proposed configuration also permits solar power production when the EC window remains idle at a given coloration state. The PSC unit of the devices consists of an FTO/ZnO nanowire/ZrO2/Carbon stack infiltrated with the MAPbI3 perovskite and covering a mere 4% of the device area. The remaining 86% consists of a redox type electrochromic in the form of an FTO/WO3/Liquid Iodine redox electrolyte/Pt/FTO sandwich. The two units, PV and EC, share a common anode and have different cathodes. The fabricated devices with dimensions of 3.0 cm × 4.0 cm, exhibit initial visible transmittance around 50%, fast and reversible transmittance modulation (9:1 within 2 min), charge storage of 20 mC/cm2, with the PSC having a power conversion efficiency of nearly 3%. They can withstand more than 40 continuous coloration-bleaching cycles. With combination of optical and electrical measurements, some unique features of these devices have been revealed.

Photoelectrochromic Devices with Enhanced Power Conversion Efficiency.

In the present work, we propose a new architecture for partly covered photoelectrochromic devices with a modified anode layout, so that the TiO2 film is deposited first on the substrate, covering a small part of its surface, followed by the WO3 film that covers the remaining device area. As a result, the TiO2 film can be subjected to the proper thermal and chemical treatment without affecting the electrochromic performance of the WO3 film. The proposed design led to photoelectrochromic (PEC) devices with a power conversion efficiency (PCE) four times higher than that of typical partly covered devices, with a measured maximum of 4.9%. This, in turn, enabled a reduction in the total area covered by the photovoltaic unit of the devices by four times (to 5% from 20%), thus reducing its visual obstruction, without affecting the depth, uniformity and speed of coloration. A detailed study of the parameters affecting the performance of the new devices revealed that, with the cover ratio decreasing, PCE was increasing. The photocoloration efficiency also exhibited the same trend for cover ratio values below 15%. Storage of the devices in short circuit conditions was found to accelerate optical reversibility without affecting their photovoltaic and optical performance.

Scalable photoelectrochromic glass of high performance powered by ligand attached TiO2 photoactive layer

A scalable photoelectrochromic (PEC) glass which can be darkened on exposure to sunlight and return to its original clear state in the absence of sunlight without any expensive materials like ruthenium dye or transparent conductive oxide was fabricated. The PEC glass is composed of light absorbing layer of ligand attached titanium oxide, color changing layer of tungsten oxide and the electrolyte containing redox mediator. This work reports, for the first time, on an application of salicylic acid attached TiO2 photoactive layer for PEC devices. The applicability of various salicylic acid derivatives attached TiO2 films in PEC device is individually investigated in dye solar cells. Among various salicylic acid derivatives studied, 5-methylsalicylic acid showed the best performance in terms of charge generation for coloration of WO3. By using 5-methylsalicylic acid instead of ruthenium dye, lower cost, higher stability, and simpler electrolyte composition have been realized. The changes in transmittance for coloration (ΔTC) depend on the thickness of WO3 and TiO2 in this device. Thus, we changed the thickness of WO3 and TiO2 to get the highest variation of transmittance for coloration. The preliminary optimization of PEC device exhibits ΔTC of 40.8% at 550 nm, the wavelength most sensitive to the human eye, and of 64.3% at 700 nm.

Assessment of the long-term performance of partly covered photoelectrochromic devices under insolation and in storage

This work comprises an experimental study of the long-term performance and stability of partly covered photoelectrochromic (PEC) devices under various operating conditions.The most durable PEC device was tested for nearly one year in open circuit storage in the dark exhibiting a 26% reduction in the power conversion efficiency of the photovoltaic (PV) unit. As regards optical performance, it was found to gradually converge to a contrast ratio of about 1.5:1, having passed through a transitional period of about 80 days, that caused a decrease in the coloration depth, without affecting reversibility. This gradual loss of coloration depth was attributed to the reaction of photoelectrons with triiodide ions at the electrochromic WO3 film-electrolyte interface, that takes place prior to the formation of the colored LixWO3 tungsten bronze, and is enhanced while the liquid electrolyte wets the WO3 film. This phenomenon was also observed in “redox” type electrochromic devices, verifying the proposed loss mechanism.PEC devices were also exposed to insolation in short (SC) and open (OC) circuit conditions. The PEC device tested in SC operation for more than four months, exhibited a contrast ratio comparable to that of the devices kept in storage, although an 80% reduction in the power conversion efficiency of the PV element has been observed. On the other hand, OC operation of 3 months did not affect the performance of the PV element, but, resulted in irreversible coloration of the device tested, due to trapping of Li ions within the WO3 matrix.Having identified the causes for the observed adverse phenomena, remedies can be sought, such as thin film barrier layers to prevent the loss of photoelectrons to the electrolyte, modified electrolytes to enhance the PV element stability, avoidance of prolonged OC operation to prevent irreversible coloration. Indeed, a 60 nm thick ZnS barrier deposited on top of the WO3 film was found to effectively suppress the loss current reactions, doubling the contrast ratio of the stabilized PEC devices to 2.8:1, at the expense of coloration speed.

Photoelectrochromic devices based on sputtered WO3 and TiO2 films

The photoelectrochromic (PEC) device, a combination of a dye solar cell and an electrochromic film, can be used for the dynamic solar control of buildings under illumination or with an external voltage. Typically, titanium oxide (TiO2) and tungsten oxide (WO3) films are being prepared from colloids or sol-gel chemistry with high porosity and high surface area as functional layers for a photoelectrochromic device in lab scale. We have, for the first time, successfully developed a PEC device with conventionally sputter deposited TiO2 and WO3 films. This coating technique is attractive due to the well documented upscaling capability and industrial viability for window applications. The functional layers WO3 and TiO2 were deposited onto fluorine doped oxide (F:SnO2) coated glass . Characterization of WO3 and TiO2 was performed in an electrochromic cell and in a dye solar cell, respectively. Three types of PEC devices with different layer configuration were constructed and their performance based on visual transmissions was compared. In best case, the visual transmission could be switched from 61 down to 15% in 30min under 1.5 A.M. illumination.

Large area self-powered semitransparent trifunctional device combining photovoltaic energy production, lighting and dynamic shading control

We present a large area trifunctional glass prototype combining a photo-electrochromic (PEC) device and an organic light-emitting diode (OLED), interfaced through a properly designed electronic control system. A 12×17cm2 PEC glass-on-glass module was realized, containing four dyesensitized solar cells (DSSCs) and a central electrochromic (EC) section deposited on the same glass panel. All PEC layers are screen-printed, including the mesoporous electrochromic layer, obtained from a custommade tungsten paste. DSSCs show an efficiency of 2.4%, while the coloration efficiency of the EC section reaches a value of 40 cm2C−1 at 700nm. A 10×8cm2 transparent white OLED was also realized, designed and tailored in order to unbalance the emission of light, i.e. maximizing the bottom emission. The efficiency of large area OLED section reaches 8cd A-1 in the operative conditions and without light outcoupling enhancement systems. The OLED device is clamped on the back of the PEC module and all sections are electrically connected to an external electronic control system. The energy collected by the DSSCs is stored in supercapacitors and used when requested, either applied to the EC section to produce a light shading effect in the daytime, or to the OLED for illumination at night.

Photoelectrochromic cell with a CdS quantum dots/graphitic-nanoparticles sensitized anode and a molybdenum oxide cathode

Low cost photoelectrochromic (PEC) cells with a cadmium sulfide (CdS) quantum dots/graphitic-nanoparticles (g-NPs) sensitized titanium oxide (TiO2) film as the photoanode, an electrodeposited molybdenum oxide (MoO3) film as the electrochromic counter electrode and an aqueous electrolyte were assembled for the first time. The oversimplified but surprisingly less used approach of incorporating g-NPs (10–50nm in dimensions) in the photovoltaic film increases the light harvesting ability of the TiO2/CdS film, by the virtue of their high electrical conductivity and a suitably poised work function at 4eV, which maximized electron transfer and transport to the current collector. The proportion of the g-NPs in the film was optimized on the basis of solar cell performances, and the PEC cell with the following configuration: TiO2/g-NPs (90s)/CdS–S2−–MoO3 delivered a power conversion efficiency (PCE) of 4.59% and a short circuit current density of ~17mAcm−2, which were 1.35 times higher than the values obtained for a similar cell with a TiO2/CdS photoanode. Under 1sun illumination and short-circuit conditions, the electrons from the photoanode reach MoO3 via external circuit and it colors deep blue. The transmittance of the device underwent a change from ~58% to ~27% at 630nm and bleached under dark. The transmission modulation varied between 24% and 31% over a wide wavelength range of 500–840nm. The optical change was reversible, demonstrating that energy conversion and saving can be combined efficiently using an inexpensive cell, and thus it has the potential to function as a highly affordable self-powered window.

Electrochromic and photoelectrochromic properties of sol–gel derived tungsten trioxide/titania composite thin films

Tungsten trixoide/titania (WO3-titania) composite thin films with W/Ti molar ratios of 100/0, 98/2, 96/4, 94/6 92/8 and 90/10 were prepared on fluorine-doped tin oxide conducting glass, and their electrochromic (EC) and photoelectrochromic (PEC) performances were investigated in this study. The composite thin films were synthesized by sol–gel process using peroxotungstic acid and titanium (IV) n-butoxide as the precursors. The surface morphology and composition of the composite thin films were characterized using scanning electron microscope with energy dispersive spectrometer. Electrochemical experiments with in situ spectroscopic measurement were employed to study the EC properties of the composite thin films. It was found that the presence of titania in the WO3 matrix might slightly decreases its EC performance. PEC cells using the composite thin films as the working electrode and a sputtered semitransparent platinum thin film on ITO as the counter electrode were fabricated and their PEC performances were investigated. The device using composite thin film prepared from sol solution with a W/Ti molar ratio of 96/4 exhibited the best PEC performance.

Performance and stability of “partly covered” photoelectrochromic devices for energy saving and power production

This work comprises an experimental study on various factors affecting the stability of “partly covered” photoelectrochromic (PEC) devices. The fabricated devices were stored in the dark at open circuit conditions and at regular time intervals, transmittance spectra for coloration and bleaching were taken and I–V measurements were conducted. This methodology revealed that the PEC devices, after passing a transitional period of about 10days post fabrication, seem to stabilize or to decay at a lower rate.The annealing temperature of the anode and the composition of the electrolyte affect drastically the operation and stability of the devices. By annealing the anode at 350°C for 40min and using a commercial electrolyte, developed for high performance dye sensitized solar cells, stable devices were fabricated (nearly 10months of shelf life), but with reduced reversibility.In order to improve the reversibility two different strategies were tried. The first one was the electrochemical treatment of the anode, before incorporation into the device, and the other was to alter the composition of the electrolyte. Reversible devices with shelf life exceeding 5months were fabricated using an anode annealed at 100°C and an electrolyte consisting of 0.5M LiI/0.005M I2/0.5M 4-TBP/ACN. Electrolytes specifically designed for PEC devices need to be developed for optimum operation.

“Partly covered” photoelectrochromic devices with enhanced coloration speed and efficiency

This work comprises an experimental study of photoelectrochromic (PEC) devices for dynamic solar control in buildings. In an effort to overcome the performance limitations of “fully covered” PEC devices in which a thin, transparent TiO 2 layer covers the entire device area, we have developed the “partly covered” PEC type, with an opaque 5 μm thick TiO 2 film covering only a fraction of WO 3 area on one end of the device. It was found that the partly covered devices are transparent in the visible (with luminous transmittance up to 51%), exhibit rapid coloration (∼3 min) upon solar irradiation in open circuit conditions and achieve a considerable transmittance modulation (up to 10.2:1). Various TiO 2 to WO 3 area ratios were tried ranging from 0.23 to 0.48. Reduction of the area ratio decreases the speed of coloration but it does not affect neither the coloration homogeneity nor the end coloration result. Use of the N3 Ruthenium dye has led to substantial photo-coloration efficiencies (up to 4 times more than those achieved with Rhodamine B) and improved the durability of the devices.

Photoelectrochromic performance of tungsten oxide based devices with PEG–titanium complex as solvent-free electrolytes

Photoelectrochromic (PEC) devices were assembled with an electrochemically deposited tungsten oxide (WO3) thin film on transparent conductive glass (ITO) substrates, a composite polymeric electrolyte (PE), and a Ruthenium 535 sensitized TiO2 sol–gel film deposited on ITO substrates. The polymeric electrolyte was prepared by the sol–gel method using a viscous polyethylene glycol (PEG, MW 600) and titanium isopropoxide as precursors in an acidic medium. A lithium salt (LiI) was added to the PEG–Ti complex for photoelectrochemical purposes. The color change of WO3 based PEC devices under 100mW/cm2 illumination at short circuit conditions indicates that the introduction of titanium complex inside the polymeric electrolyte accelerates the coloring speed from 26 to 9s. Electrochemical impedance spectroscopy analyses of PEC devices show that the presence of titanium complex in PE reduces the charge transfer resistance and increases the charge storage capacity of the cells. This improvement could come from the oxidation of iodides and reduction of titanium ions inside the composite electrolytes that increase the lithium ion's mobility.

Development of photoelectrochromic devices for dynamic solar control in buildings

An experimental study of photoelectrochromic (PEC) devices for dynamic solar control in buildings is presented. The fabricated devices consist of a 400 nm thick WO 3 film, a 300 nm thick dye-sensitized nano-structured TiO 2 layer, a 0.5 M LiI/PC electrolyte and an electrodeposited Pt counter electrode. All layers were developed aiming to increase the device transparency without reducing its functionality. The PEC devices thus prepared exhibit fast coloration (∼12 min) under sunlight, considerable transmittance modulation ( T lum varying from 50% to 14% and contrast ratio of 3.6:1) and a haze factor in the bleached state equal to 27.8%. They can withstand more than 100 continuous coloration–bleaching cycles. Distinct differences were observed between PEC devices and electrochromics, regarding the change in color coordinates during coloration. The former take a green color as opposed to the deep blue color of the latter. Furthermore, in PEC devices the optical interference between the WO 3 and TiO 2 films can be used advantageously to tailor their color. A novel efficiency index, suitable for PEC devices, termed “photocoloration efficiency, PhCE” was introduced and it was used for their characterization. It was found that the PEC devices presented in this work have high photocoloration efficiencies during the initial coloration stages (∼6 min) and that they compare well in terms of PhCE with similar devices developed by others.

Preparation and Characterisation of Nano-Structured WO3-TiO2 Layers for Photoelectrochromic Devices

Nano-structured WO3-TiO2 layers were prepared by the sol-gel route. To obtain transparent, porous and crack free layers up to 0.8 μ m with a single dipping cycle a templating strategy was used. As a template three-dimensionally network based on organically modified silane was introduced to the WO3 and TiO2 sols. The WO3 layers were dip-coated onto the conductive glass substrate (TCO) and the TiO2 layers on the top of the WO3 layer. The morphology and the structure of the layers were determined by Scanning Electron Microscopy (SEM), High Resolution Transmission Electron Microscopy (HR-TEM), Energy Dispersive X-Ray Spectroscopy (EDXS), Auger and Infrared spectroscopy. SEM image of the WO3-TiO2 layer confirmed the nano-porosity of the layers and give the size of the particles of about 10 nm for TiO2 and 30 nm for WO3 layer. Further analysis indicated that the titanium sol penetrates the WO3 layer. Particles in the WO3 layer consist of a crystalline monoclinic WO3 core surrounded by a 5–10 nm amorphous phase consisting of WO3, TiO2 and SiO2. The WO3-TiO2 layers were used to assemble all solid state photoelectrochromic (PE) devices. Under 1 sun irradiation (1000 W/m2) the visible transmittance of the PE device changes from 62% to 1.6%. The colouring and bleaching processes last about 10 minutes.

Performance of a solid-state photoelectrochromic device

Abstract The photoelectrochromic (PEC) device reported in this paper is a new type of switchable system, which combines a dye-sensitised solar cell with an additional electrochromic layer. The colouring process takes place under open-circuit conditions and illumination, and bleaching is achieved under short-circuit conditions. Solid PEC devices in a size up to 10×10 cm 2 employing a solid Li + ion conductor, based on organically modified silane (ormolyte) with redox couple, were built and characterised. The addition of the cationic surfactant to the electrolyte improves the stability, colouration and memory effect of the PEC devices. The photopic transmittance of the solid PEC device is varied between 62% (bleached state) and 1.6% (coloured state). The corresponding colouring time under 1 sun illumination is 15 min and 15 min are needed for bleaching.

Novel photoelectrochromic cells containing a polyaniline layer and a dye-sensitized nanocrystalline TiO 2 photovoltaic cell

We present a novel photoelectrochromic (PEC) cell containing a polyaniline layer and a dye-sensitized nanocrystalline TiO 2 layer. Electrochromic thin films of polyaniline layers have been prepared by electrochemical deposition and by spin-coating a dispersion of a polyaniline lacquer. The influences of the preparing parameters on the micromorphologies of the polyaniline layers were investigated. Under illumination of about one sunlight intensity, this self-powered PEC cell can modulate its transmission, averaged over the whole visible spectral region. The structure-property relations of the photoelectrochromic cells are discussed.