A technique to compare polythiophene solid-state dye sensitized TiO2 solar cells to liquid junction devices

In this communication, we report on a technique to fabricate solid-state polythiophene-based dye sensitized solar cells (DSSCs) that can be directly compared to analogous liquid junction devices. The device configuration is based on non-porous TiO₂ thin films and one of the three undoped polythiophene hole conductors: poly[3-(11 diethylphosphorylundecyl) thiophene], P3PUT, poly(4-undecyl-2,2′-bithiophene), P4UBT, or poly(3-undecyl-2,2′-bithiophene), P3UBT. These polymers were spin coated and cast from organic solutions onto the TiO₂ films. The dense TiO₂ thin films (ca. 30 nm) were deposited on conductive glass via facile spray pyrolysis and sol–gel techniques. After that, cis-(SCN)₂ Bis(2,2′ bipyridyl-4,4′-dicarboxylate) ruthenium(II) (a.k.a. Ru N3 dye) was adsorbed on the TiO₂ surface, and the polythiophenes were utilized as hole conductors in a simplified solar cell geometry. The results were compared to the control DSSC device made with dense TiO₂ and a liquid electrolyte, or 2,2′,7,7′-tetrakis(N,N-di-p-methoxyphenylamine)-9,9′-spirobifluorene (a.k.a. Spiro-MeOTAD). The polythiophenes exhibited bandgaps in the range 1.9–2.0 eV, and HOMO energy levels of approximately 5 eV (vs. vacuum). The P3PUT DSSC device exhibited an AM1.5 V_OC=0.8 V, a J_SC=0.1 mA/cm², as well as an IPCE=0.5–1%. The AM1.5 short-circuit photocurrents and quantum efficiencies for DSSCs made with the polythiophenes, the Spiro-MeOTAD and the standard liquid electrolyte (I⁻/I₃⁻) were found to be identical within the limits of experimental uncertainty and reproducibility. Our results indicate that a solid-state replacement to the liquid junction is not necessarily limited by the fundamental aspect of hole transfer, one of the three fundamental aspects that must be met for an efficient DSSC. Rather than suggest that P3UBT or P4UBT could be used to create efficient “organic solar cells” with the exclusion of the Ru dye, we suggest that transparent thiophene compounds could be attractive candidates for high-surface area solid-state DSSCs, and that the technique presented can be applied to other hole conductors. It can allow a verification of one of the things necessary for the DSSC, so that parallel studies using high-surface area materials can proceed with confidence.     

Tri-layer antireflection coatings (SiO2/SiO2–TiO2/TiO2) for silicon solar cells using a sol–gel technique

Antireflection coatings (ARCs) have become one of the key issues for mass production of Si solar cells. They are generally performed by vacuum processes such as thermal evaporation, reactive sputtering, and plasma-enhanced chemical vapor deposition. In this work, a sol–gel method has been demonstrated to prepare the ARCs for the non-textured monocrystalline Si solar cells. The spin-coated TiO₂ single-layer, SiO₂/TiO₂ double-layer and SiO₂/SiO₂–TiO₂/TiO₂ triple-layer ARCs were deposited on the Si solar cells and they showed good uniformity in thickness. The measured average optical reflectance (400–1000 nm) was about 9.3, 6.2 and 3.2% for the single-layer, double-layer and triple-layer ARCs, respectively. Good correlation between theoretical and experimental data was obtained. Under a triple-layer ARC condition, a 39% improvement in the efficiency of the monocrystalline Si solar cell was achieved. These indicate that the sol–gel ARC process has high potential for low-cost solar cell fabrication.     

TiO2 and TiO2–SiO2 thin films and powders by one-step soft-solution method: Synthesis and characterizations

Simple soft-solution method has been developed to synthesize films and powders of TiO₂ and mixed TiO₂–SiO₂ at relatively low temperatures. This method is simple and inexpensive. Furthermore, reactor can be designed for large-scale applications as well as to produce large quantities of composite powders in a single step. For the preparation of TiO₂, we used aqueous acidic medium containing TiOSO₄ and H₂O₂, which results in a peroxo-titanium precursor while colloidal SiO₂ has been added to the precursor for the formation of TiO₂–SiO₂. Post annealing at 500 °C is necessary to have anatase structure. Resulting films and powders were characterized by different techniques. TiO₂ (anatase) phase with (1 0 1) preferred orientation has been obtained. Also in TiO₂–SiO₂ mixed films and powders, TiO₂ (anatase) phase was found. Fourier transform infrared spectroscopy (FTIR) results for TiO₂ and mixed TiO₂–SiO₂ films have been presented and discussed. The method developed in this paper allowed obtaining compact and homogeneous TiO₂ films. These compact films are highly photoactive when TiO₂ is used as photo anode in an photoelectrochemical cell. Nanoporous morphology is obtained when SiO₂ colloids are added into the solution.     

Low-temperature synthesis of TiO2 nanoparticles and preparation of TiO2 thin films by spray deposition

Low-temperature (180–240 °C) synthesis of nanocrystalline titanium dioxide (TiO₂) by surfactant-free solvothermal route is investigated. Titanium iso-propoxide is used as the precursor and toluene as the solvent. Different precursors to solvent weight ratios have been used for the synthesis of TiO₂ nanoparticles. For the weight ratios 15/100, 25/100 and 35/100 the X-ray diffractograms show the formation of nanocrystalline TiO₂. The X-ray diffraction and transmission electron microscopy studies shows that the product has anatase crystal structure (for temperatures <200 °C) with average particle size below 15 nm. The films deposited by spray deposition method using these nanoparticles show the crystalline and porous nature of the films. The present method of deposition also avoids the post-treatment (sintering) of the films. The nanoparticles thus prepared and the films can be used for gas sensing and biological applications and also as photo-electrodes for dye-sensitized solar cells.     

Optical filters from SiO2 and TiO2 multi-layers using sol–gel spin coating method

Sol–gel spin coating process is used to produce optical filters from SiO₂ and TiO₂ multi-layers. By coating the films symmetrically on both sides of the glass substrates, we designed two types of three-layer anti-reflective (AR) filters for the near–infrared region, and a nine-layer reflective filter for the near–UV region. We develop a simple theoretical model for these filters, which incorporates sol–gel film densification during the coating process, and fit it to the experimental data to extract properties of the individual layers in the coatings.     

Synthesis, characterization and fabrication of solar cells making use of [Ru(dcbpy)(tptz)X]X (where X = Cl, SCN, CN) complexes

Dye-sensitized TiO₂ solar cells were fabricated using tridentate ligand ruthenium(II) complexes, [Ru(dcbpy)(tptz)X]X (where dcbpy = 4,4′-dicarboxy-2,2′-bipyridine, tptz = 2,4,6-Tris(2-pyridyl)-s-triazine and X = Cl⁻, SCN⁻, CN⁻) attached to sol–gel processed TiO₂ electrodes. The ligand tptz functions as spectator ligand and dcbpy functions as the anchoring ligand with sufficient visible light absorption. The synthesized complexes were characterized before using them in solar cells. The functioning of the solar cells fabricated using different conducting glasses was monitored and the current–voltage characteristics were measured. The efficiencies of different cells were calculated and compared.     

Oxidative photoelectrochemical technology with Ti/TiO2 anodes

Rutile and anatase TiO₂ films have been grown on Ti plates by thermal (500–800°C) and anodic oxidation followed by thermal annealing (400–500°C), respectively. The photoelectrochemical efficiency of these photoanodes, evaluated by current density measurements in the photooxidation of 4-methoxybenzyl alcohol in deaerated CH₃CN, has been determined. The photocurrent efficiency increases with the thickness of the TiO₂ rutile film up to 1 μm (the most efficient thickness). At the wavelengths furnished by the irradiation apparatus similar thicknesses of anatase and rutile films show nearly the same efficiencies. Anodic bias produces similar relative increases of current intensity in both crystalline forms.     

Morphological, optical and photocatalytic properties of TiO2–Fe2O3 multilayers

Morphological, optical and photocatalytic properties of TiO₂, Fe₂O₃ and TiO₂–Fe₂O₃ samples (formed by 1, 3 and 5 coatings) were studied. The layers were deposited on glass substrate by the sol–gel method. The catalytic activity of the samples was studied by the photodecomposition of methylene blue (MB) under visible light illumination. The FTIR results indicate that all samples present surface OH radicals that are bound either to the Ti or Fe atoms. This effect is better visualized at larger number of coatings in the TiO₂–Fe₂O₃/glass systems. Also, two mechanisms are observed during the photodecomposition of the MB.     

Electrochromic properties of heat-treated thin films of CeO2–TiO2–ZrO2 prepared by sol–gel route

CeO₂–TiO₂–ZrO₂ thin films were prepared using the sol–gel process and deposited on glass and ITO-coated glass substrates via dip-coating technique. The samples were heat treated between 100 and 500 °C. The heat treatment effects on the electrochromic performances of the films were determined by means of cyclic voltammetry measurements. The structural behavior of the film was characterized by atomic force microscopy and X-ray diffraction. Refractive index, extinction coefficient, and thickness of the films were determined in the 350–1000 nm wavelength, using nkd spectrophotometry analysis.Heat treatment temperature affects the electrochromic, optical, and structural properties of the film. The charge density of the samples increased from 8.8 to 14.8 mC/cm², with increasing heat-treatment temperatures from 100 to 500 °C. It was determined that the highest ratio between anodic and cathodic charge takes place with increase of temperature up to 500 °C.     

Effect of the sintering temperature on the photocatalytic activity of ZnO+Zn2TiO4 thin films

ZnO+Zn₂TiO₄ thin films were obtained by the sol–gel method, the precursor solutions were prepared using two Ti/Zn ratios: 0.49 and 0.69. The films were deposited on glass slide substrates and sintered at temperatures in the 200–600 °C range in increments of 50 °C, with the goal of studying the dependence of the photocatalytic activity (PA) on the annealing temperature. The films were characterized by X-ray diffraction and UV–Vis spectroscopy. The PA was evaluated by measuring the UV–Vis absorption spectra of the methylene blue in aqueous solution before and after photobleaching, using the Lambert–Beer's principle. The higher photocatalytic activities were obtained from the films with sintering temperature around 450 °C, for both Ti/Zn ratios studied.     

Fabrication of solid-state dye-sensitized TiO2 solar cells combined with polypyrrole

A solid-state solar cell was fabricated by photoelectrochemical polymerization of pyrrole on porous nanocrystalline TiO₂ electrode sensitized by the Grätzel dye, cis-di(thiocyanato)-N,N′-bis(2,2′-bipyridyl-4,4′-dicarboxylic acid)-ruthenium (II) dihydrate, [RuL₂(NCS)₂]), or a newly synthesized cis-Ru(dcb)₂(pmp)₂ (pmp=3-(pyrrole-1-ylmethyl)-pyridine). Polypyrrole successfully worked as a hole-transport layer with improvement of the cell characteristics when the TiO₂ cell with cis-Ru(dcb)₂(pmp)₂ was compared with the similarly fabricated cells using [RuL₂(NCS)₂]. The improvement by using Ru(dcb)₂(pmp)₂ can be explained as due to direct molecular wiring of the polymer-chain to the excited metal center of the complex.     

Sol–gel derived nanocrystalline CeO2–TiO2 coatings for electrochromic windows

Mixed CeO₂–TiO₂ coatings synthesized by sol–gel spin coating process using mixed organic–inorganic Ti(OC₃H₇)₄ and CeCl₃·7H₂O precursors with different Ce/Ti mole ratios were investigated by a wide range of characterization techniques. The attempts were directed towards achieving coatings with high transparency in the visible region and good electrochemical properties. Elucidation of the structural and optical features of the films yielded information on the aspects relevant to their usage in transmissive electrochromic devices. The films have been found to exhibit properties for counter electrode in electrochromic smart windows in which they are able to retain their transparency under charge insertion, high enough for practical uses. The high optical modulation and fastest switching for WO₃ film in the device configuration with the Ce/Ti (1:1) film is interpreted in terms of conducive microstructural changes induced by addition of TiO₂ in an amount equivalent to CeO₂.     

Effect of stabilizer on structural, optical and electrochemical properties of sol–gel derived spin coated TiO2 films

Thin films of TiO₂ were prepared using two different sol–gel routes. The two routes employed diethanolamine (DEA) and acetylacetone as stabilizing agents with titanium isopropoxide (Ti(OPrⁱ)₄) in ethanol as the deposition solution. The densification at 500 °C achieved the nanophase TiO₂ films, which were investigated by performing structural, optical and electrochemical studies. Ion storage capacity and transmission measurements showed superior response of the films derived from DEA. Between the films obtained from the two routes, the appearance of the rutile phase at lower temperature for the film synthesized using DEA was predicted on the basis of the thermal analysis of the corresponding xerogel. The nanocrystalline nature of the films was evident from the X-ray diffraction, atomic force microscopy, and scanning electron microscopy. The films deposited from both the stabilizers exhibited electrochromism in 1 M LiClO₄-propylene carbonate electrolyte on cathodic polarization.     

Electrochemical and photoelectrochemical investigation of new carboxylatobipyridine(bis-bipyridine)ruthenium(II) complexes for dye-sensitized TiO2 electrodes

Methods for the preparation and purification of new carboxylated 2,2′-bipyridine ligands, two of which contain the new anchoring functionality malonate, and a reliable method for the synthesis of the corresponding [Ru(bpy)₂(L)](PF₆)₂ complexes are described. Their suitability for fundamental studies of the processes in wet solar cell applications has been investigated. All complexes show stable voltammograms in acetonitrile solution and although the complexes were obtained as lithium carboxylate salts, they were soluble in non-polar organic solvents such as dichloromethane, which allowed for good reproducibility in the dye-coating step.The electrochemical and photoelectrochemical properties of dye–TiO₂ systems are discussed. The photoelectrochemical properties in monochromatic and white light are related to the attaching group that is varied in the series of complexes: A formally non-conjugated malonate group as anchoring group generally gives similar results to that of a conjugated carboxylic group, the second carboxylate compensating the absence of conjugation. Also, the complexes with malonate as attaching group are more efficient than a complex with a non-conjugated carboxylic group, especially in the red part of the action spectrum.The redox behavior of the adsorbed complexes is reversible at negative potentials and quasi-reversible at positive potentials. The latter feature is especially interesting since it is possible to electrochemically oxidize the surface-attached ruthenium complexes even at potentials in the mid-bandgap region of the metal oxide film.     

Structural and spectroelectrochemical (UV–vis and IR) studies of nanocrystalline sol–gel derived TiO2 films

Nanocrystalline TiO₂ films with a high surface roughness were prepared by the sol–gel route from titanium (IV) iso-propoxide and brij 56 surfactant. XRD and TEM showed that the films consisted predominantly of anatase nanoparticles embedded in an amorphous phase. Addition of the surfactant significantly increased the intensity of the surface hydroxyl modes between 3600 and 3750 cm⁻¹ in IR spectra. Spectroelectrochemical measurements of the films in 0.1 M LiOH electrolyte revealed optical modulation beyond 70%T. Slow-scan cyclovoltammetry showed that bleaching occurred in two steps, suggested to correspond to anatase and amorphous phase. The changes that occur during intercalation and deintercalation were followed by ex-situ IR transmission measurements in the regions of Ti–O, carbonate and surface hydroxyl bands.     

Photoelectric behavior of nanocrystalline TiO2 electrode with a novel terpyridyl ruthenium complex

The photoelectric behavior of a black dye, tris (isothiocyanato)-[N-(2,2′:6′,2″-terpyridine-4′-(4-carboxylic acid) phenyl)] ruthenium (II) complex, was examined under different conditions. The dye was adsorbed on nanocrystalline TiO₂ surface strongly and generated incident monochromatic photon-to-current conversion efficiency (IPCE) of about 90% at maximum absorption wavelength and greater than 20% in the near-IR region. A sandwich-type solar cell fabricated by this dye-sensitized nanocrystalline TiO₂ film generated 6.1 mAcm⁻² of short-circuit photocurrent, 0.58 V of open-circuit photovoltage and 2.9% of overall yield under irradiation of white light (78.0 mWcm⁻²) from a Xe lamp. Since the title dye shows better photoresponse than the N3 dye in the near-IR region, it would be a promising panchromatic sensitizer after optimization.     

Electrochemical evaluation of rutile TiO2 nanoparticles as negative electrode for Li-ion batteries

Nanosized rutile TiO₂ has been prepared by sol–gel chemistry from a glycerol-modified titanium precursor in the presence of an anionic surfactant. The sample has been characterized by X-ray diffraction, nitrogen sorption, scanning electron microscopy (SEM), high resolution transmission electron microscopy (HRTEM) and electrochemical tests. Nanosized rutile TiO₂ has been electrochemically investigated using two potential windows: 1.2–3 V and 1–3 V. It exhibits excellent high rates capabilities and good cycling stability.     

Comparative studies of “all sol–gel” electrochromic devices with optically passive counter-electrode films, ormolyte Li ion-conductor and WO3 or Nb2O5 electrochromic films

Laminated electrochromic (EC) devices are becoming increasingly important for making “smart” windows and switchable displays. Mostly, polymeric Li⁺ ionic conductors in combination with vacuum deposited active electrochromic and counter-electrode films are used. In this paper we report on the development of all sol–gel EC devices, that is, those where all three internal layers are prepared via the sol–gel route, including the ionically conductive inorganic–organic hybrid (ormolyte). The electrochemical and optical properties of EC devices are presented and the cycling stability and reversibility of their optical modulation assessed. The results show that WO₃/ormolyte/SnO₂ : Mo, WO₃/ormolyte/SnO₂ : Sb, WO₃/ormolyte/SnO₂ : Sb : Mo, Nb₂O₅/ormolyte/SnO₂ : Sb : Mo and WO₃/ormolyte/LiCo-oxide exhibit a transmission modulation dependent on the thickness of the active electrochromic and counter-electrode films and the thickness of the ormolyte layer. Electrochemical and optical properties of individual films are described and correlated with the stability of the all sol–gel EC devices.     

Highly efficient solid-state dye-sensitized TiO2 solar cells via control of retardation of recombination using novel donor-antenna dyes

Two series of heteroleptic tris(bipyridyl)Ru(II) and bis(bipyridyl)(NCS)₂Ru(II) complexes have been synthesized and characterized. This is a part of a new concept of covalent linkage of donor-antenna groups, e.g., triphenylamine or N,N′-bis(phenyl)-N,N′-bis(3-methylphenyl)-1,1′-biphenyl-4,4′-diamine (TPD) to Ru(II) dye center. For the covalent attachment of donor units, a multi-step synthesis was carried out starting from 4,4′-dimethyl-2,2′-bipyridine followed by chlorination and Wittig reaction with donor aldehydes. This was followed either by a metallation reaction using bis(4,4′-dicarboxy-2,2′-bipyridyl)Ru(II)dichloride ((bpy(COOH)₂Ru(II)2Cl₂ 2H₂O) as precursor to get tris(bipyridyl) dyes or by a one pot synthesis starting from dichloro(p-cymene)Ru(II) dimer resulting in bis(bipyridyl)(NCS)₂ dyes. The complexes (bpy(COOH)₂)₂(bpyMe₂)Ru(II) 2PF₆ and (bpy(COOH)₂)(bpyMe₂)(NCS)₂Ru(II) without donor-antenna groups were also prepared to study and compare the properties. The influence of donor-antenna groups in these complexes was studied using UV–Vis spectroscopy and cyclic voltammetry. The heteroleptic complexes carrying donor groups show appreciably broad absorption ranges and extraordinarily high extinction coefficients. These high extinction coefficients are explained as due to the extended delocalization of π-electrons in the donor-antenna ligands. The HOMO/LUMO energy values obtained from cyclic voltammetry support the multi-step charge transfer cascade possible in these donor-antenna dyes. Examples of solid-state dye-sensitized solar cell utilizing these novel donor-antenna dyes revealed spectacular performances of power conversion efficiencies of up to 3.4%, for the dye carrying a TPD donor group as measured under AM 1.5 spectral conditions. This is attributed to highly efficient light harvesting of these novel dyes and the improved charge transfer dynamics at TiO₂–dye and dye–hole conductor interfaces.     

Structure and photoelectrochemical properties of ruthenium(II) polypyridyl complexes as sensitizers for nanocrystalline TiO2 electrodes

The relationship between the structures and photoelectrochemical properties of two dyes, cis-dithiocyanato-N,N′-bis(4,4′-dicarboxyl-2,2′-bipyridyl) Ru(II) and cis-dithiocyanato-(4,4′-dicarboxyl-2,2′-bipyridyl)-(4,4′-di((N,N′-methylphenylamino)methylene)-2,2′-bipyridyl) Ru(II), was examined and compared under the same conditions. Data show that the photophysical properties (including molar extinction coefficients and excited-state lifetimes) and photoelectrochemical properties (including short-circuit photocurrent, open-circuit photovoltage, incident monochromatic photon to current conversion efficiency, overall energy conversion yield (η) and transient photocurrent) were changed greatly only due to an acceptor replaced by a donor in one of polypyridyl of the Ru(II) complex, suggesting that the molecular design in energy conversion is very sensitive.