Mesoporous Photoanode Research Articles

Effect of Different Sensitization Technique on the Photoconversion Efficiency of CdS Quantum Dot and CdSe Quantum Rod Sensitized TiO2 Solar Cells

The procedure employed for the sensitization of mesoporous photoanodes affects strongly the final performance of sensitized devices, especially when semiconductor quantum dots and quantum rods are used as sensitizers. In this work the effect of three different sensitizing methods in the final cell performance was analyzed. The TiO2 films were sensitized with CdS QDs grown by successive ionic layer adsorption and reaction, SILAR, and with CdSe quantum rods deposited by electrophoretic and pipetting methods. Several configurations of the sensitizers and combinations of sensitization methods were tested. 4% photoconversion efficiencies were obtained for TiO2 electrodes sensitized with CdS and CdSe by electrophoretic and pipetting respectively, while for the sensitizer with both techniques the efficiency was 4.7%. This high efficiency is mainly due to the high fill factor (60%) and the photocurrents (13.1 mA/cm2) obtained by the correct combination of near-infrared and visible light photoabsorption, the bette...

Development of Hybrid Tungsten Oxide Photoanodes Admixed with Borododecatungstate-Polyanion Modified-Hematite: Enhancement of Water Oxidation upon Irradiation with Visible Light

A typical procedure based on sol-gel aggregation for the fabrication of mesoporous WO3 photoanodes was modified and used produce modified tungsten oxide films by utilizing the tungstic acid sol admixed with Keggin-type borotungstic acid (H5BW12O40) or the borotungstic acid–stabilized hematite (Fe2O3). Physicochemical properties (structure, morphology and spectroscopic identity) of the resulting hybrid (composite) WO3 films were assessed using X-ray diffraction, scanning electron microscopy, as well as UV-Vis and Raman spectroscopies. During photoelectrochemical experiments with a solar cell system operating in 0.5moldm−3 H2SO4 in three-electrode configuration, the composite WO3 films acting as photoanodes yielded (following illumination with visible light) significantly (up to 80%) larger water oxidation photocurrents (at potentials higher than 0.75V vs. RHE), relative to the performance of analogous bare (pristine) mesoporous tungsten oxide films. Independent voltammetric diagnostic experiments with Pt microdisk electrode implied the appearance of oxygen during photooxidation of water. The enhancement effect was the most pronounced for WO3 films admixed with borotungstic acid–stabilized hematite. The observed phenomena could be rationalized either in terms of the presence of new conduction band structures in composite WO3 films or a marked increase in the degree of hydration of the composite oxide structures regardless of the annealing at high temperatures (450°C). It was also apparent from our experiments in 0.5moldm−3 NaCl, the systems were also applicable to sea-water-type environments.

Hyperbranched quasi-1D TiO2 nanostructure for hybrid organic-inorganic solar cells.

The performance of hybrid solar cells is strongly affected by the device morphology. In this work, we demonstrate a poly(3-hexylthiophene-2,5-diyl)/TiO2 hybrid solar cell where the TiO2 photoanode comprises an array of tree-like hyperbranched quasi-1D nanostructures self-assembled from the gas phase. This advanced architecture enables us to increase the power conversion efficiency to over 1%, doubling the efficiency with respect to state of the art devices employing standard mesoporous titania photoanodes. This improvement is attributed to several peculiar features of this array of nanostructures: high interfacial area; increased optical density thanks to the enhanced light scattering; and enhanced crystallization of poly(3-hexylthiophene-2,5-diyl) inside the quasi-1D nanostructure.

Synthesis of Thin Titania Photoanodes with Large Mesopores for Electricity-generating Windows

Abstract Highly transparent mesoporous titania photoanodes with large mesopores and crystallized framework are synthesized by surfactant-assisted sol–gel method. The conversion efficiency of DSC with rare-metal-free dye (MK-2) reaches 2.6%, though the film thickness of photoanode is of sub-micron order (around 445 nm). The thinness of the photoanodes and the red-light transmission of the dye maintain the high transmittance of the dye-adsorbed photoanodes (around 80% for 700-nm light), demonstrates that obtained photoanodes are promising for electricity-generating windows.

Formation and Characterization of Pb<sub>x</sub>Cd<sub>1-x</sub>S Interlayer for PbS/CdS/ZnS Quantum Dots Sensitized Solar Cells

PbxCd1-xS quantum dots (QDs) was successfully deposited on TiO2 mesoporous film as TiO2 mesoporous photoanode using successive ionic layer adsorption and reaction (SILAR) method for quantum dot sensitized solar cells (QDSSCs). Quantum dot sensitized solar cells (QDSSCs) were prepared by sandwiching the TiO2 mesoporous photoanode with Cu2S counter electrode. Single layer of PbxCd1-xS, PbS and CdS and multilayer of PbS/CdS/ZnS as well as multilayer PbS/PbxCd1-xS/CdS/ZnS were prepared for characterizations. The characterizations including X-ray Diffraction Spectroscopy (XRD), Field Emission Scanning Electron Microscopy (FESEM), UV-visible spectrophotometer and Current-density voltage (J-V) measurement were carried out to study the effects of number of SILAR cycles of PbxCd1-xS interlayer for molar fraction, x of 0.2 in QDSSCs. PbxCd1-xS interlayer with four SILAR cycles is incorporated between PbS and CdS layer will increase the efficiency in QDSSCS.

Cation adsorption by mesoporous titania photoanodes and its effect on the current-voltage characteristics of photoelectrochemical cells

Current-voltage curves recorded during the study of photoelectrochemical cells functioning in the presence of aqueous electrolytes contain a characteristic anodic current feature, which may interfere in the interpretation of the results. This anodic current is observed at negative potentials and it derives from the adsorption of cations into the mesoporous structure of nanoparticulate titania photoanodes. The shape and the height of the peak depends on the size of the cation, its concentration, the thickness of the titania film and the nature of the counter ion and the corresponding pH. This anodic current is similar to the one observed in other works during ion storage and electrochromism studies with mesoporous titania. In the presence of an organic sacrificial agent, like ethanol, adsorption of cationic species is still strong but it decreases at higher organic content.

Highly efficient dye sensitized solar cells based on ordered and disordered mesoporous titania thick templated films

Highly efficient DSSCs were achieved by worm-like and grid-like mesoporous thick templated photoanodes prepared by spin and dip coating methods.

On the response time of dye-sensitized solar cells to pulsed monochromatic illumination

It is common knowledge that the slow photocurrent response time of dye-sensitized solar cells (DSSCs) is due to trap-limited diffusion of photogenerated carriers in the mesoporous photoanode; therefore this quantity should depend on the carrier generation profile and on the carrier density, which are determined by the light wavelength and intensity, respectively. However, the dependence of the response time of DSSCs on these two parameters has scarcely been investigated. Here, we systematically measure the response time of two devices with different photoanode thickness to square-wave modulated illumination as a function of incident wavelength and of the intensity of the background bias light and we show that a simple transport model in the small-modulation approximation can account for the observed dependence. This information is relevant for the characterization of carrier transport in the device and for the design of correct photon conversion efficiency measurement setups when using lock-in techniques.

Invited: iCVD Synthesis of Polymer Electrolytes for Application in Dye Sensitized Solar Cells (DSSCs)

Initiated chemical vapor deposition (iCVD) is used for the synthesis and integration of polymer electrolytes, such as poly(2‐hydroxyethyl methacrylate) (PHEMA), poly(glycidyl methacrylate) (PGMA), poly(glycidol) (PGL), poly(4-vinylpyridine) (P4VP) and polyvinylpyrrolidone (PVP), within the mesoporous TiO2 photoanode of dye sensitized solar cells (DSSCs). DSSCs with the conventional liquid-electrolyte are prone to leakage and evaporation—hindering DSSC applications, durability, and thermal stability. Polymer electrolytes do not suffer from these disadvantages and can also enhance the cell I-V behavior. However, spin coating and drop casting to deposit the polymer electrolytes leads to incomplete pore filling of the polymer inside the mesoporous TiO2photoanode, leading to poor electrical contact and lower efficiency; this is one of the main factors limiting solid-state DSSC performance.To alleviate these pore filling concerns, we fabricated polymer electrolytes using the novel, solvent-free technique of initiated chemical vapor deposition (iCVD)1,2. iCVD is generally an adsorption limited technique where the reagents for polymerization, the monomer and initiator, are heated to a vapor that can easily penetrate into the mesoscale voids of the TiO2 photoanode. The initiator is activated by a hot filament (250-350 C) and polymerization occurs inside small crevices and on surfaces (Figure 1). By understanding the competition between surface reaction kinetics and mass transport into the mesovoids, iCVD polymerization allows for uniform coatings for nanoscale geometries and is an exceptional technique for allowing polymerization in the mesoporous structure of TiO2. The surface reaction kinetics and mass transport have been found to depend on the relative pressure of monomer, z=PM/PM,sat, which is an adsorption parameter that provides a measure of the surface availability of the monomer. By carefully controlling , nearly 100% pore filling of the TiO2 structure has been achieved as estimated by thermogravimetric analysis (TGA)3. This is significantly higher than that achievable with liquid techniques like spin coating or drop casting of polymer solutions and allows pore penetration for layer thicknesses up to 12 μm, which is much greater than the 2 μm limit with liquid‐based methods. FTIR, NMR, and XPS confirmed the polymers formed are stoichiometric in composition expected of linear homopolymers and identical to ones formed with liquid solution methods.In this paper, we show that iCVD polymer electrolytes can be effectively integrated within TiO2 mesoporous photoanodes to produce enhanced DSSCs. By varying the polymer electrolyte chemistry, we show that DSSC cell characteristics, including open circuit voltage, short circuit current density and fill factor, can be tuned (Figure 2). To gain a better understanding on the photochemical processes inside the DSSC, we have integrated our experimental work with first principles macroscopic modeling4 to examine the influence of different polymer chemistries. TiO2 exhibits a Nernstian response to variation in pH, leading to a 0.059V positive shift (vs NHE) of the conduction band energy per pH unit decrease5. As suggested by our first principles model, we believe a similar mechanism is occurring when the TiO2 surface is in intimate contact with these polymer electrolytes. For instance, PHEMA and PGMA have relatively more acidic character compared to the more basic P4VP with its pyridine group, therefore PHEMA and PGMA have higher V oc than P4VP. The mathematical model also suggests that the primary improvements of J sc and minor improvements of V oc are due to the suppression of the back electron transfer at the dye-TiO2-electrolyte interface. For example, the basic pyridine group of P4VP absorbs readily onto the bare TiO2 surface due to its Lewis acidity, thus preventing the invasion of triodide and decreasing the rate of undesirable electron transfer from the TiO2 to triiodide. PGMA and PHEMA do not have this basic character, and therefore have a lower J sc than P4VP.

Photoanode Based on (001)-Oriented Anatase Nanoplatelets for Organic–Inorganic Lead Iodide Perovskite Solar Cell

The methodology adopted for the preparation of perovskites plays a critical role in achieving high efficiency. Toward this direction, various strategies have been adopted to achieve a rational control over the growth of the perovskite layer for the fabrication of highly efficient devices. In the beginning, a single step was generally employed for the deposition of the perovskite layer. Apart from the technique for the preparation of perovskites, the modification of the photoanode has also garnered significant attention, with reports on how the replacement of the mesoporous photoanode with a planar architecture leads to devices that do not compromise much on performance. Although the performance in dye sensitized solar cells (DSC) is strongly dictated by the porosity and surface properties of mesoporous photoanode, the aim of the approach presented here is specifically to reduce the thickness of a relatively less porous layer of photoanode by exploiting the planar nature of nanoplatelets.

Influence of structural and chemical properties on electron transport in mesoporous ZnO-based dye-sensitized solar cell

Two chemical bath deposition (CBD) solutions were prepared at two different temperatures of 60 and 80 °C for dye-sensitized solar cell (DSSC) application. The deposition time was varied from 12, 18, and 24 h. The X-ray diffraction (XRD) and Fourier transform infrared spectroscopy (FTIR) analysis confirmed a transformation of hydrozincite into ZnO after calcined at 300 °C. The flower-like structure was observed by scanning electron microscopy (SEM) analysis. The ZnO photoanode deposited at 80 °C for 24 h showed the highest incident photon to charge carrier efficiency (IPCE) value of 5.25 %. The power conversion efficiency, η increased from 1.81 to 2.45 % for the photoanode deposited at 60 and 80 °C, respectively. The effective electron lifetimes, τeff of 6.36 ms was measured as much shorter than the TiO2-based DSSC (τeff of 25 to 160 ms). The effective electron diffusion coefficient, Deff, increased from 1.7 × 10−3 to 4.7 × 10−3 cm2.s−1 when the deposition temperature increases from 60 to 80 °C. Effective rate constant for recombination, keff of 157 s−1 was calculated for both samples. The results and analysis of the film’s surface structure, morphology, electron diffusion, and recombination inside the mesoporous ZnO photoanode proved the applicability of ZnO for the DSSC’s application.

Well-Organized Mesoporous TiO2 Photoelectrodes by Block Copolymer-Induced Sol–Gel Assembly for Inorganic–Organic Hybrid Perovskite Solar Cells

A well-aligned mesoporous thin layer was formed by spin-coating followed by the burn-out of its organic components at 500 °C. This film, prepared by an inorganic sol–gel process using block copolymers as a sacrificial template, showed mesoporous (mp) and crack-free layers with various pore sizes depending on the amount of 1,3,5-trimethylbenzene (TMB), which acted as a swelling agent. The mesoporous photoanodes were used in the fabrication of CH3NH3Pb(I0.9Br0.1)3 perovskite-based solar cells. The cells showed overall power conversion efficiencies of 11.7% and 12.8% with the 10 and 15 nm mp-layers, respectively. The superior performance shown in the 15 nm mp-layers may be attributed to the easier filling of CH3NH3Pb(I0.9Br0.1)3 by bigger pore sizes, and the efficiency is comparable with that of the reference cell fabricated by TiO2 paste. This template-induced self-organizing sol–gel process was shown to be a useful technique for depositing mesoporous photoanodes in the fabrication of perovskite-based solar...

Toward Higher Energy Conversion Efficiency for Solid Polymer Electrolyte Dye-Sensitized Solar Cells: Ionic Conductivity and TiO2 Pore-Filling.

Even though the solid polymer electrolyte has many intrinsic advantages over the liquid electrolyte, its ionic conductivity and mesopore-filling are much poorer than those of the liquid electrolyte, limiting its practical application to electrochemical devices such as dye-sensitized solar cells (DSCs). Two major shortcomings associated with utilizing solid polymer electrolytes in DSCs are first discussed, low ionic conductivity and poor pore-filling in mesoporous photoanodes for DSCs. In addition, future directions for the successful utilization of solid polymer electrolytes toward improving the performance of DSCs are proposed. For instance, the facilitated mass-transport concept could be applied to increase the ionic conductivity. Modified biphasic and triple-phasic structures for the photoanode are suggested to take advantage of both the liquid- and solid-state properties of electrolytes.

(Invited) Enhancement of Photoelectrochemical Water Splitting and Solar Energy Induced Electroreduction of Carbon Dioxide through Utilization of Plasmonic and Electrocatalytic Metal Nanoparticles

In the present study, we utilize phosphomolybdate (PMo12O40 3-) modified gold nanoparticle (diameters, 30-40 nm) deposits on mesoporous tungsten oxide photoanodes to study plasmon-assisted photoelectrochemical water splitting. Such n-type semiconducting oxide as WO3 (band gap, 2.5 eV) suffers from inadequate solar light absorption and, therefore, there is a need to use of a thin plasmon active layer of nanostructured Au of proper architecture to enhance the interfacial light trapping and conversion. A remarkable increase in the solar energy conversion efficiency as well as in the photooxidation current (oxygen evolution) are believed to be induced by the plasmonic excitation of Au nanoparticles occurring within WO3 absorption range. The polyoxometallate (PMo12O40 3-) monolayer-type nanostructures may play electrocatalytic role, improve charge distribution, in addition to the stabilization effect. In the other study, the PMo12O40 3- modified Au nanoparticles, when deposited on step-bunched silicon Si(111) surface have also acted as electrocatalytic centers during photoelectrochemically induced reduction of carbon oxide (IV). The applicability of phosphomolybdate-protected gold nanoparticles to create porous matrix for photoelectrochemically assisted reduction of carbon dioxide is even more pronounced when the system is further decorated with copper. We also present the result of decorating multi-walled carbon nanotubes with gold and copper nanoparticles (by chemical reduction method and through electrodeposition) acting as catalytic systems active towards CO2 electroreduction under voltammetric conditions. Carbon nanotubes have been chemically modified here with surface oxygen complexes using HNO3 and H3PMo12O40 solutions.

Enhanced Charge Transport Kinetics in Anisotropic, Stratified Photoanodes

The kinetics of charge transport in mesoporous photoanodes strongly constrains the design and power conversion efficiencies of dye sensitized solar cells (DSSCs). Here, we report a stratified photoanode design with enhanced kinetics achieved through the incorporation of a fast charge transport intermediary between the titania and charge collector. Proof of concept photoanodes demonstrate that the inclusion of the intermediary not only enhances effective diffusion coefficients but also significantly suppresses charge recombination, leading to diffusion lengths two orders of magnitude greater than in standard mesoporous titania photoanodes. The intermediary concept holds promise for higher-efficiency DSSCs.

ZnS-Passivated CdSe/CdS Co-sensitized Mesoporous Zn2SnO4 Based Solar Cells

CdS and CdSe/CdS quantum dot (QD) sensitizers were assembled onto mesoporous ternary Zn2SnO4 photoanodes using a two-step sequential process of successive ionic layer absorption and reaction (SILAR) and chemical bath deposition (CBD) for QD-cosensitized solar cell applications. The assembled CdS and CdSe QDs were observed using high-angle annular dark-field scanning transmission electron microscopy (HAADF-STEM), high-resolution TEM (HRTEM), and STEM/energy-dispersive X-ray spectroscopy (EDS) elemental mapping. CdSe/CdS cosensitized Zn2SnO4-based cells showed a higher absorption onset position, a stepwise band-edge level alignment, and, thereby, an improved power conversion efficiency (PCE) of 1.628% under one-sun conditions, which is the best result obtained using mesoporous Zn2SnO4 reported to date. In addition, the effect of a ZnS passivation layer on the photovoltaic performance and aging behavior has been investigated. Electrochemical impedance spectroscopy (EIS) showed that the main role of the ZnS layer is to enhance the aging behavior of the CdSe/CdS/Zn2SnO4 cells by improving the electron lifetime and charge recombination.

The effect of photoanode structure on the performances of quantum-dot-sensitized solar cells: a case study of the anatase TiO2nanocrystals and polydisperse mesoporous spheres hybrid photoanodes

Four types of photoanodes, made from TiO2nanocrystals and PMSs, were presented to shed light on their structural influence over the performance of QDSSCs.

Thick titania films with hierarchical porosity assembled from ultrasmall titania nanoparticles as photoanodes for dye-sensitized solar cells

Thin mesoporous titania films prepared by surfactant templating feature some of the highest light conversion efficiencies per thickness ratios as anodes in dye-sensitized solar cells (DSCs). However, the fabrication of thicker films required for sufficient light absorption is very challenging using this approach, often resulting in cracking and delamination of the films. Here we present a simple and scalable method to prepare thick mesoporous titania photoanodes via a surfactant-directed assembly of crystalline ultra-small TiO2 nanoparticles in combination with phase separation due to ethyl cellulose added to the coating solutions. Along with increasing film thickness, the ethyl cellulose introduces an interpenetrating macropore network into the films, leading to the formation of hierarchical porous films with bimodal porosity, with the smaller mesopores resulting from the structure-directing agent, Pluronic F127. In this way, films of up to 2 μm per layer without delamination can be produced, exhibiting a high surface area of 130 m2 g−1, about twice the value of films based on standard TiO2 nanoparticle paste. The preparation of multilayer films by a sequential spin-coating and calcination procedure enables the production of films with an overall thickness of up to 10 μm in only 5 steps, which showed high efficiencies of 7.7% in dye-sensitized solar cells.

Development of selective, ultra-fast multiple co-sensitization to control dye loading in dye-sensitized solar cells

Enhancing the spectral response of dye-sensitized solar cells (DSC) is essential to increasing device efficiency and a key approach to achieve this is co-sensitization (i.e. the use of multiple dyes to absorb light from different parts of the solar spectrum). However, precise control of dye loading within DSC mesoporous metal oxide photo-anodes is non-trivial especially for very rapid processing (minutes). This is further complicated by dyes having very different partition (Kd) and molar extinction (e) coefficients which strongly influence dye uptake and spectral response, respectively. Here, we present a highly versatile, ultra-fast (ca. 5 min) desorption and re-dyeing method for dye-sensitized solar cells which can be used to precisely control dye loading in photo-electrode films. This method has been successfully applied to re-dye, partially desorb and re-dye and selectively desorb and re-dye photo-electrodes using examples of a Ru-bipy dye (N719) and also organic dyes (SQ1 and D149) giving η up to 8.1% for a device containing the organic dye D149 and re-dyed with the Ru dye N719. The paper also illustrates how this method can be used to rapidly screen large numbers of dyes (and/or dye combinations) and also illustrates how it can also be used to selectively study dye loading.

Improvement of Dye Solar Cell Efficiency by Photoanode Posttreatment

The basic concept for efficiency improvement in dye-sensitized solar cells (DSSC) is limiting the electron-hole recombination. One way to approach the problem is to improve the photogenerated charge carriers lifetime and consequently reduce their recombination probability. We are reporting on a facile posttreatment of the mesoporous photoanode by using a colloidal solution of TiO2nanoparticles. We have investigated the outcome of the different sintering temperature of the posttreated photoanodes on their morphology as well as on the conversion efficiency of the DSSC. The DSSCs composed of posttreated photoanodes at 450°C showed an increase inJSCand consequently an increase in efficiency of 10%. Investigations were made to determine the electron recombination via the electrolyte by the OCVD technique. We found that the posttreatment has the effect of reducing the surface trap states and thus increases the electron lifetime, which is responsible for the increase of the overall cell efficiency.