High Temperature Crystallization of Free‐Standing Anatase TiO2 Nanotube Membranes for High Efficiency Dye‐Sensitized Solar Cells

Despite the one‐dimensional ordering of anodic TiO₂ nanotube arrays (TNAs), the electron diffusion towards the substrate in TNA‐based dye‐sensitized solar cells (DSSCs) is comparably slow. The improvement of electron mobility by enhancing TNA crystallinity under high‐temperature annealing, however, is infeasible with the existence of Ti metal substrate. Herein, it is shown that, by high temperature (up to 700 °C) crystallization of high‐quality free‐standing TNA membranes, the TNAs can maintain their structure integrity and phase (anatase) stability as a result of the absence of the nucleation sites and the high quality of the membrane obtained by a self‐detachment method. The electron transport is much faster (≈4 times) in the 700 °C‐annealed TNA membranes than that in the 400 °C‐treated ones for 20 μm‐length nanotubes, which is mainly attributed to the improved crystallinity and reduced electron trap states. In spite of slightly reduced dye loading capacity (decreased by ≈30%) in the 700 °C‐annealed membranes, the superior electron transport leads to a significantly improved efficiency of 7.81% (enhanced by ≈50%). The strategy of manipulating the electron transport dynamics by high temperature treatment on high‐quality TNA membranes may open new route for further improvement in the performances of TNA‐based DSSCs.     

Room‐Temperature Synthesis of Porous Nanoparticulate TiO2 Films for Flexible Dye‐Sensitized Solar Cells

A novel room‐temperature method for the preparation of porous TiO₂ films with high performance in dye‐sensitized solar cells (DSSCs) has been developed. In this method a small amount of Ti^IV tetraisopropoxide (TTIP) is added to an ethanolic paste of TiO₂ nanoparticles, where it hydrolyzes in situ and connects the TiO₂ particles to form a homogenous and mechanically stable film of up to 10 μm thickness without crack formation. Residual organics originating from the TTIP were removed by UV–ozone treatment of the films, leading to a remarkable improvement of the cell efficiency. Intensity‐modulated photocurrent/voltage spectroscopy (IMPS/IMVS) showed that the main effect of the UV–ozone treatment is to suppress the recombination of photogenerated electrons, thereby extending their lifetime. The efficiency was further increased by preheating the TiO₂ nanoparticles before the paste preparation to remove contaminants originating from the preparation process of the particles. Solar‐to‐electric energy conversion efficiencies of 4.00 and 3.27 % have been achieved for cells with conductive glass and plastic film substrates, respectively, under illumination with AM 1.5 (100 mW cm^–2) simulated sunlight.     

Dependence of energy conversion efficiency of dye-sensitized solar cells on the annealing temperature of TiO2 nanoparticles

Dye-sensitized solar cells (DSSCs) were fabricated from porous electrodes derived from sol–gel-synthesized (SGS) nanoparticles (NPs) of TiO₂. Current–voltage measurements were performed to investigate performance characteristics of electrodes derived from SGS-NPs of TiO₂ annealed at different temperatures. Experimental results indicate that the effects of bulk traps and surface states within TiO₂ films on recombination of photo-injected electrons in DSSCs depend upon annealing temperature of SGS-TiO₂ NPs. Moreover, electrodes fabricated from SGS-TiO₂ showed higher photoelectric conversion efficiency than nonporous commercial (P25) TiO₂ NPs. Porous structures within SGS-TiO₂ NPs are of great benefit to sensitizer dye adsorption, and consequently to improvement of photo-electrochemical properties of DSSCs.     

Dye‐sensitized TiO2 solar cells based on nanocomposite photoanode containing plasma‐modified multi‐walled carbon nanotubes

A series of anatase TiO₂‐based nanocomposite incorporated with plasma‐modified multi‐walled carbon nanotubes (MWNTs) was prepared by physical blending and shows its capability for efficient electron transport when used as photoanode in dye‐sensitized solar cells (DSSCs). These MWNTs characterized with good dispersal performance were obtained by functionalization technique via in situ plasma treatment and subsequent grafting with maleic anhydride (MA) onto the external walls reported previously. Compared with the conventional DSSCs, the TiO₂ film with 1D carbon nanotubes possesses more outstanding ability to transport electrons injected from the excited dye within the device under illumination. As a result, at an optimum addition of 0.3 wt% MWNTs‐MA in TiO₂ matrix, the photocurrent–voltage (J–V) characteristics showed a significant increase in the short‐circuit photocurrent (J_sc) of 50%, leading to an increase in overall solar conversion efficiency by a factor of 1.5. Electrochemical impedance spectroscopy analyses reveal that the MWNTs‐MA/TiO₂ incur smaller resistances at the photoanode in assembled DSSCs when compared with those in the anatase titania DSSCs. These features suggest that the conducting properties of the MWNTs‐MA within the anodes are crucial for achieving a higher transport rate for photo‐induced electrons in TiO₂ layer by exhibiting lower resistance in the porous network and hence retard charge recombination that could result in poor conversion efficiency. Copyright © 2012 John Wiley & Sons, Ltd.     

Transparent conductive oxide‐less back contact dye‐sensitized solar cells using cobalt electrolyte

Transparent conductive oxide‐less (TCO‐less) dye‐sensitized solar cells (DSSCs) have been fabricated and characterized using nanoporous TiO₂‐coated stainless steel metal mesh as flexible photoanode and cobalt bipyridyl complex (Co(bpy))‐based one electron redox shuttle electrolyte. Attempts have been made towards enhancing the efficiency of TCO‐less DSSCs to match with their TCO‐based DSSC counterparts. It has been found that surface protection of metal mesh is highly required for enhancing the efficiency of TCO‐less DSSCs specially using cobalt electrolytes as confirmed by dark current–voltage characteristics. Photocurrent action spectra clearly reveal that TCO‐based DSSCs using (Co(bpy)) electrolyte exhibits photon harvesting (incident photon to current conversion efficiency (IPCE) 52%) in the 370–450 nm wavelength region as compared to photon harvesting at peak absorption of the dye (IPCE 56% at 550 nm), which is almost the same (IPCE 47%) in the 400–610 nm wavelength region for TCO‐less DSSCs. Under similar experimental conditions, replacing indoline dye D‐205 to porphyrin‐based dye YD2‐o‐C8 led to the enhancement in the photoconversion efficiency from 3.33% to 4.84% under simulated solar irradiation. Copyright © 2014 John Wiley & Sons, Ltd.     

Processing and characterization of a TiO2 paste based on small particle size powders for dye‐sensitized solar cell semi‐transparent photo‐electrodes

The formulation and the preparation of a TiO₂ paste for dye‐sensitized solar cell technology are proposed. The TiO₂ paste is characterized in terms of rheology, morphology, cross section, specific surface area, and average pore size. A conversion efficiency of 7.42% without any chemical treatment or scattering layer was obtained using the optimal thickness of 11 µm as carried out in this work. The results over different batches of the paste confirm the stability and the reproducibility. The characterization of the TiO₂ semi‐transparent film is completed by investigating the dye adsorption saturation time after cyclic dye dipping steps through UV–Vis spectra measurements. Copyright © 2011 John Wiley & Sons, Ltd.     

New Family of Ruthenium‐Dye‐ Sensitized Nanocrystalline TiO2 Solar Cells with a High Solar‐Energy‐Conversion Efficiency

A new type of ruthenium complexes 6 – 8 with tridentate bipyridine–pyrazolate ancillary ligands has been synthesized in an attempt to elongate the π‐conjugated system as well as to increase the optical extinction coefficient, possible dye uptake on TiO₂, and photostability. Structural characterization, photophysical studies, and corresponding theoretical approaches have been made to ensure their fundamental basis. As for dye‐sensitized solar cell applications, it was found that 6 – 8 possess a larger dye uptake of 2.4 × 10^–7 mol cm^–2, 1.5 × 10^–7 mol cm^–2, and 1.3 × 10^–7 mol cm^–2, respectively, on TiO₂ than that of the commercial N3 dye (1.1 × 10^–7 mol cm^–2). Compound 8 works as a highly efficient photosensitizer for the dye‐sensitized nanocrystalline TiO₂ solar cell, producing a 5.65 % solar‐light‐to‐electricity conversion efficiency (compare with 6.01 % for N3 in this study), a short‐circuit current density of 15.6 mA cm^–2, an open‐circuit photovoltage of 0.64 V, and a fill factor of 0.57 under standard AM 1.5 irradiation (100 mW cm^–2). These, in combination with its superior thermal and light‐soaking stability, lead to the conclusion that the concomitant tridentate binding properties offered by the bipyridine‐pyrazolate ligand render a more stable complexation, such that extended life spans of DSSCs may be expected.     

The use of Ti meshes with self‐organized TiO2 nanotubes as photoanodes of all‐Ti dye‐sensitized solar cells

This paper reports a simple and facile method for directly growing self‐organized TiO₂ nanotubular arrays around the whole Ti mesh by electrochemical anodization in organic electrolytes and their application in all‐Ti dye‐sensitized solar cells (DSSCs). Compared with the traditional fluorine‐doped tin oxide (FTO)‐based DSSC and the backside illuminated DSSC, this type of DSSC showed advantages such as low resistance, cheap fabrication cost and enhanced sunlight utilization. Different thicknesses of nanotubular array layers were investigated to find their influence on the photovoltaic parameters of the cell. We also considered three types of meshes as the substrates of anodes and found that the cell with 6 openings/mm² exhibited the highest conversion efficiency of 5.3%. The area of the cell had only a little impact on the photovoltaic performances. Copyright © 2010 John Wiley & Sons, Ltd.     

Fabrication of screen‐printing pastes from TiO2 powders for dye‐sensitised solar cells

A preparation technique of TiO₂ screen‐printing pastes from commercially‐available powders has been disclosed in order to fabricate the nanocrystalline layers without cracking and peeling‐off over 17 µm thickness for the photoactive electrodes of the dye‐sensitised solar cells. A conversion efficiency of 8·7% was obtained by using a single‐layer of a semi‐transparent‐TiO₂ film. A conversion efficiency of 9·2% was obtained by using double‐layers composed of transparent and light‐scattering TiO₂ films for a photon‐trapping system. Copyright © 2007 John Wiley & Sons, Ltd.     

Conformal Nano‐Sized Inorganic Coatings on Mesoporous TiO2 Films for Low‐Temperature Dye‐Sensitized Solar Cell Fabrication

Here, a new method based on sol–gel electrophoretic deposition to produce uniform high‐quality inorganic conformal coatings on mesoporous nano‐particulate films is presented. This novel sol preparation method allows for very fine control of the coating properties, thus inducing new adjustable functionalities to these electrodes. It is shown that the deposition of an amorphous TiO₂ and/or MgO shell onto photoanodes used in dye‐sensitized solar cells (DSSCs) improves their light‐to‐electric‐power conversion efficiency without the need for sintering. It is proposed that the amorphous TiO₂ coating improves the electronic inter‐particle connection and passivates the surface states. The insulating MgO coating further reduces the electron transfer from the conduction band into the electrolyte while the electron injection from the excited dye state remains unperturbed for thin coatings. Using a low‐temperature method for DSSC production on plastic substrates, a maximum efficiency of 6.2% applying pressure together with an optimized TiO₂ coating is achieved. For systems that cannot be pressed a conversion efficiency of 5.1% is achieved using a double shell TiO₂/MgO coating.     

Metal‐Free Sensitizers Containing Hydantoin Acceptor as High Performance Anchoring Group for Dye‐Sensitized Solar Cells

The anchoring group in dye‐sensitized solar cells (DSSCs) profoundly affects the electron injection and durability on TiO₂ films interface. Here, the hydantoin acceptor is introduced as anchoring group for DSSCs. The hydantoin based sensitizer achieves a photovoltaic efficiency of 7.66%, compared to 4.90% for sensitizer containing the conventional cyanoacrylic acid as anchoring group. Remarkably, the hydantoin anchoring group significantly enhances the electron‐injection efficiency (Φ_inj) and photocurrent (J_sc). The time dependent adsorption and desorption data indicate the strong binding strength and the superiority of stability for hydantoin based sensitizers. The Fourier transform infrared measurements investigate the adsorption mechanism of hydantoin on TiO₂ interface. These results strongly corroborate the advantages of incorporating hydantoin as acceptor and anchoring group. As a consequence, the sensitizer HY‐4 with hydantoin approaches the photovoltaic efficiency of 8.32% under 0.1 sunlight illumination. These observations offer a new route to design and develop efficient sensitizers for DSSCs.     

Improved‐Performance Dye‐Sensitized Solar Cells Using Nb‐Doped TiO2 Electrodes: Efficient Electron Injection and Transfer

Well‐crystallized Nb‐doped anatase TiO₂ nanoparticles are prepared by a novel synthetic route and successfully used as the photoanode of dye‐sensitized solar cells (DSSCs). The homogenous distribution of Nb in the TiO₂ lattice is confirmed by scanning transmission electron microscopy (STEM) elemental mapping and line‐scanning analyses. After Nb doping, the conductivity of the TiO₂ powder increases, and its flat‐band potential (V_fb) has a positive shift. The energy‐conversion efficiency of a cell based on 5.0 mol% Nb‐doped TiO₂ is significantly better, by about 18.2%, compared to that of a cell based on undoped TiO₂. The as‐prepared Nb‐doped TiO₂ material is proven in detail to be a better photoanode material than pure TiO₂, and this new synthetic approach using a water‐soluble precursor provides a simple and versatile way to prepare excellent photoanode materials.     

Nanosized Anatase TiO2 Single Crystals with Tunable Exposed (001) Facets for Enhanced Energy Conversion Efficiency of Dye‐Sensitized Solar Cells

This paper reports the synthesis of nanosized TiO₂ single crystals with different percentages of exposed (001) facets in the presence of HF solution. Various characterizations are conducted to understand the correlation between particle morphology, exposed (001) facets and photo‐conversion efficiency of the nanosized anatase TiO₂ single crystals. An enhancement in dye‐sensitized solar cells (DSSCs) overall conversion efficiency is observed for the photoanode consisting of nanosized TiO₂ single crystals with higher percentage of exposed (001) facets, increasing from 7.47%, 8.14% to 8.49% for the TiO₂ single crystals with ca. 10%, 38%, and 80% percentage of exposed (001) facets. Experimentally confirmed by dark current potential and open‐circuit voltage decay scans, such highly exposed (001) facets are not only favorable for more dye adsorption but also effectively retard the charge recombination process in DSSCs.     

TiO2 Microspheres with Controllable Surface Area and Porosity for Enhanced Light Harvesting and Electrolyte Diffusion in Dye‐Sensitized Solar Cells

An optimized configuration of TiO₂ microspheres in photoanodes is of great importance to prepare highly efficient dye‐sensitized solar cells (DSSCs). In this work, TiO₂ microspheres with tunable diameter, pore size, and porosity are synthesized by subtly adjusting the synthesizing conditions, including ratios of deionized water, ammonia, and ethanol, respectively. TiO₂ microspheres are obtained with large pore sizes and a high porosity without sacrificing specific surface areas. In addition, the effect of their porosity and pore size on the performance of DSSCs is investigated. As confirmed by the dye‐loading ability and electrolyte diffusion resistance, the large mesopores and the high porosity of the TiO₂ microspheres can improve dye adsorption and facilitate electrolyte diffusion, giving rise to a high light‐harvesting and electron collection efficiency. Consequently, the highest photocurrent of 19.21 mA cm^?2 and a power conversion efficiency of 9.98% are obtained by using the TiO₂ microspheres with the highest porosity, compared with a 9.29% efficiency demonstrated by the lowest porosity (an improvement of 7.4%). By modifying the interconnection and the external pores of the microspheres photoanode, a high efficiency of 11.67% is achieved for a DSSC based on the most potent TiO₂ microspheres.     

Enhanced Performance of I2‐Free Solid‐State Dye‐Sensitized Solar Cells with Conductive Polymer up to 6.8%

An iodine‐free solid‐state dye‐sensitized solar cell (ssDSSC) is reported here, with 6.8% energy conversion efficiency—one of the highest yet reported for N719 dye—as a result of enhanced light harvesting from the increased transmittance of an organized mesoporous TiO₂ interfacial layer and the good hole conductivity of the solid‐state‐polymerized material. The organized mesoporous TiO₂ (OM‐TiO₂) interfacial layer is prepared on large‐area substrates by a sol‐gel process, and is confirmed by scanning electron microscopy (SEM) and grazing incidence small‐angle X‐ray scattering (GISAXS). A 550‐nm‐thick OM‐TiO₂ film coated on fluorine‐doped tin oxide (FTO) glass is highly transparent, resulting in transmittance increases of 8 and 4% compared to those of the bare FTO and conventional compact TiO₂ film on FTO, respectively. The high cell performance is achieved through careful control of the electrode/hole transport material (HTM) and nanocrystalline TiO₂/conductive glass interfaces, which affect the interfacial resistance of the cell. Furthermore, the transparent OM‐TiO₂ film, with its high porosity and good connectivity, exhibits improved cell performance due to increased transmittance in the visible light region, decreased interfacial resistance ( Ω ), and enhanced electron lifetime ( τ ). The cell performance also depends on the conductivity of HTMs, which indicates that both highly conductive HTM and the transparent OM‐TiO₂ film interface are crucial for obtaining high‐energy conversion efficiencies in I₂‐free ssDSSCs.     

Hybrid Templated Synthesis of Crack‐Free,Organized Mesoporous TiO2 Electrodes for High Efficiency Solid‐State Dye‐Sensitized Solar Cells

Organic/inorganic hybrid templates, i.e., aluminium oxide (Al₂O₃) nanoparticles grafted with poly(oxyethylene) methacrylate, Al₂O₃‐POEM, are synthesized via surface‐initiated atom transfer radical polymerization (ATRP), as confirmed by Fourier transform‐infrared spectroscopy (FT‐IR) and thermogravimetric analysis (TGA). Upon combining the Al₂O₃‐POEM with titanium(IV) isopropoxide (TTIP), hydrophilic TTIP is selectively confined in the hydrophilic POEM chains through hydrogen bonding interactions. Following the calcination at 450 °C and the selective etching of Al₂O₃ with NaOH, the OM‐TiO₂ films with high surface areas, good interconnectivity, and anatase phase are obtained. The solid‐state dye‐sensitized solar cells (ssDSSCs) fabricated with OM‐TiO₂ photoelectrodes and a polymerized ionic liquid (PIL) show a high energy conversion efficiency of 7.3% at 100 mW cm^?2, which is one of the highest values for ssDSSCs. The high cell performance is due to the well‐organized structure, resulting in improved dye loading, excellent pore filling of electrolyte, enhanced light harvesting, and reduced charge recombination.     

Multi‐Shell Porous TiO2 Hollow Nanoparticles for Enhanced Light Harvesting in Dye‐sensitized Solar Cells

An optimized configuration for nanomaterials in working electrodes is vital to the high performance of dye‐sensitized solar cells (DSSCs). Here, a fabrication method is introduced for multi‐shell TiO₂ hollow nanoparticles (MS‐TiO₂‐HNPs) via a sol–gel reaction, calcination, and an etching process. The prepared uniform MS‐HNPs have a high surface area (ca. 171 m² g^?1), multireflection, and facile electrolyte circulation and diffusion. During the MS‐HNP fabrication process, the amount of SiO₂ precursor and H₂O under reaction has a significant effect on aggregation and side reactions. The etching process to obtain pure TiO₂ is influenced by anatase crystallinity. Additionally, single‐shell (SS)‐TiO₂‐HNPs and double‐shell (DS)‐TiO₂‐HNPs are synthesized as a control. The MS‐TiO₂‐HNPs exhibit a high surface area and enhance light reflectance, compared with the SS‐ and DS‐TiO₂‐HNPs of the same size. The power conversion efficiency of the optimized MS‐TiO₂‐HNP‐based DSSCs is 9.4%, compared with the 8.0% efficiency demonstrated by SS‐TiO₂‐HNP‐DSSCs (a 17.5% improvement). These results enable the utilization of multifunctional MS‐HNPs in energy material applications, such as lithium ion batteries, photocatalysts, water‐splitting, and supercapacitors.     

A Thiophene‐Based Anchoring Ligand and Its Heteroleptic Ru(II)‐Complex for Efficient Thin‐Film Dye‐Sensitized Solar Cells

A novel heteroleptic Ru^II complex (BTC‐2) employing 5,5′‐(2,2′‐bipyridine‐4,4′‐diyl)‐bis(thiophene‐2‐carboxylic acid) (BTC) as the anchoring group and 4,4′‐ dinonyl‐2,2′‐bipiridyl and two thiocyanates as ligands is prepared. The photovoltaic performance and device stability achieved with this sensitizer are compared to those of the Z‐907 dye, which lacks the thiophene moieties. For thin mesoporous TiO₂ films, the devices with BTC‐2 achieve higher power conversion efficiencies than those of Z‐907 but with a double‐layer thicker film the device performance is similar. Using a volatile electrolyte and a double layer 7 + 5 μm mesoporous TiO₂ film, BTC‐2 achieves a solar‐to‐electricity conversion efficiency of 9.1% under standard global AM 1.5 sunlight. Using this sensitizer in combination with a low volatile electrolyte, a photovoltaic efficiency of 8.3% is obtained under standard global AM 1.5 sunlight. These devices show excellent stability when subjected to light soaking at 60 °C for 1000 h. Electrochemical impedance spectroscopy and transient photovoltage decay measurements are performed to help understand the changes in the photovoltaic parameters during the aging process. In solid state dye‐sensitized solar cells (DSSCs) using an organic hole‐transporting material (spiro‐MeOTAD, 2,2′,7,7′‐tetrakis‐(N,N‐di‐p‐methoxyphenylamine)‐9,9′‐spirobifluorene), the BTC‐2 sensitizer exhibits an overall power conversion efficiency of 3.6% under AM 1.5 solar (100 mW cm^?2) irradiation.     

Large‐Scale Synthesis and Characterization of TiO2‐Based Nanostructures on Ti Substrates

Na₂Ti₆O₁₃ nanoplates, nanowires, and continuous nanowire network films are hydrothermally formed on a large scale directly on Ti substrates for the first time. The morphology of the formed Na₂Ti₆O₁₃ nanostructures can be easily tuned by varying the experimental parameters of temperature, reaction duration, and the NaOH concentration. Our study demonstrates that the synthesized Na₂Ti₆O₁₃ nanostructures are easily converted into H₂Ti₃O₇ nanostructures—a desirable precursor for the fabrication of various TiO₂‐based nanomaterials—with shape preservation, by an ion‐exchange process. Anatase, a mixture of anatase and rutile, and rutile TiO₂ nanowires are formed when the H₂Ti₃O₇ nanowires are annealed at 450, 600, and 750 °C, respectively. The optical properties and the photocatalytic activity of H₂Ti₃O₇ nanowires and of the TiO₂‐based nanomaterials are also addressed. The approach described in this study provides a simple and novel method for the large‐scale synthesis of various TiO₂‐based nanostructured materials that grow directly on Ti substrates and are ready for a wide range of practical applications, such as the photodegradation of wastewater.     

Control of Solid‐State Dye‐Sensitized Solar Cell Performance by Block‐Copolymer‐Directed TiO2 Synthesis

Hybrid dye‐sensitized solar cells are typically composed of mesoporous titania (TiO₂), light‐harvesting dyes, and organic molecular hole‐transporters. Correctly matching the electronic properties of the materials is critical to ensure efficient device operation. In this study, TiO₂ is synthesized in a well‐defined morphological confinement that arises from the self‐assembly of a diblock copolymer—poly(isoprene‐b‐ethylene oxide) (PI‐b‐PEO). The crystallization environment, tuned by the inorganic (TiO₂ mass) to organic (polymer) ratio, is shown to be a decisive factor in determining the distribution of sub‐bandgap electronic states and the associated electronic function in solid‐state dye‐sensitized solar cells. Interestingly, the tuning of the sub‐bandgap states does not appear to strongly influence the charge transport and recombination in the devices. However, increasing the depth and breadth of the density of sub‐bandgap states correlates well with an increase in photocurrent generation, suggesting that a high density of these sub‐bandgap states is critical for efficient photo‐induced electron transfer and charge separation.