The influence of noncovalent interactions in metal‐free organic dye molecules to augment the efficiency of dye sensitized solar cells: A computational study

The efficiency of dye sensitized solar cells (DSSCs) can be enhanced with achieving better planarity of metal‐free organic dye molecules and thinning of their aggregation on the semiconductor surface. We report that the subtle noncovalent N^…S interaction between the substituted phosphazene group and thiophene spacer unit in dye molecule which induces the desired planarity and avoid aggregation of such molecules on the TiO₂ surface using DFT calculations. DFT results show that phosphazene group increases the maximum absorption wavelength (λ_max), driving force for electrons injection (ΔG_injection), singlet excited state lifetime (τ), dipole moments (μ_normal), and number of electrons transferred from dye to TiO₂ surface (Δq), which are known to augment the efficiency of DSSCs. Further, the lower ΔG_regeneration value of phosphazene containing dyes (e.g., –.37 eV, dye 2 ) than the reported dyes (e.g., –.81 eV, dye 1 ) indicate the faster electron injection rate from the former dye to the semiconductor TiO₂. The role of phosphazene group to prevent the aggregation of dye molecules on the TiO₂ anatase surface was also examined with GGA‐PBE/DNP level of theory. The calculated results suggest that the dye molecules on 1 ‐(TiO₂)₃₈ and 2 ‐(TiO₂)₃₈ anatase clusters avoids the aggregation due to the steric congestion induced by phosphazene group. This work reports to accomplish dual properties with subtle noncovalent interactions in dye molecules to augment the efficiency in DSSCs.     

Unconventional Route to Oxygen‐Vacancy‐Enabled Highly Efficient Electron Extraction and Transport in Perovskite Solar Cells

The ability to effectively transfer photoexcited electrons and holes is an important endeavor toward achieving high‐efficiency solar energy conversion. Now, a simple yet robust acid‐treatment strategy is used to judiciously create an amorphous TiO₂ buffer layer intimately situated on the anatase TiO₂ surface as an electron‐transport layer (ETL) for efficient electron transport. The facile acid treatment is capable of weakening the bonding of zigzag octahedral chains in anatase TiO₂, thereby shortening staggered octahedron chains to form an amorphous buffer layer on the anatase TiO₂ surface. Such amorphous TiO₂‐coated ETL possesses an increased electron density owing to the presence of oxygen vacancies, leading to efficient electron transfer from perovskite to TiO₂. Compared to pristine TiO₂‐based devices, the perovskite solar cells (PSCs) with acid‐treated TiO₂ ETL exhibit an enhanced short‐circuit current and power conversion efficiency.     

Ionic Liquid Ordering at an Oxide Surface

The interaction of the ionic liquid [C₄C₁Im][BF₄] with anatase TiO₂, a model photoanode material, has been studied using a combination of synchrotron radiation photoelectron spectroscopy and near‐edge X‐ray absorption fine structure spectroscopy. The system is of interest as a model for fundamental electrolyte–electrode and dye‐sensitized solar cells. The initial interaction involves degradation of the [BF₄]^? anion, resulting in incorporation of F into O vacancies in the anatase surface. At low coverages, [C₄C₁Im][BF₄] is found to order at the anatase(101) surface via electrostatic attraction, with the imidazolium ring oriented 32±4° from the anatase TiO₂ surface. As the coverage of ionic liquid increases, the influence of the oxide surface on the topmost layers is reduced and the ordering is lost.     

The Origin of Higher Open‐Circuit Voltage in Zn‐Doped TiO2 Nanoparticle‐Based Dye‐Sensitized Solar Cells

Zn‐doped anatase TiO₂ nanoparticles are synthesized by a one‐step hydrothermal method. Detailed electrochemical measurements are undertaken to investigate the origin of the effect of Zn doping on the performance of dye‐sensitized solar cells (DSSCs). It is found that incorporation of Zn²⁺ into an anatase lattice elevates the edge of the conduction band (CB) of the photoanodes and the Fermi level is shifted toward the CB edge, which contributes to the improvement in open‐circuit voltage (V_OC). Charge‐density plots across the cell voltage further confirm the increase in the CB edge in DSSCs directly. Photocurrent and transient photovoltage measurements are employed to study transport and recombination dynamics. The electron recombination is accelerated at higher voltages close to the CB edge, thus leading to a negative effect on the V_OC.     

Dye‐Sensitized TiO2 Nanotube Solar Cells: Rational Structural and Surface Engineering on TiO2 Nanotubes

Owing to well‐defined structural parameters and enhanced electronic properties, highly ordered TiO₂ nanotube arrays have been employed to substitute TiO₂ nanoparticles for use in dye‐sensitized solar cells. To further improve the performance of dye‐sensitized TiO₂ nanotube solar cells, efforts have been directed toward the optimization of TiO₂ photoanodes, dyes, electrolytes, and counter electrodes. Herein, we highlight recent progress in rational structural and surface engineering on anodic TiO₂ nanotube arrays and their effects on improving the power conversion efficiency of dye‐sensitized TiO₂ nanotube solar cells.     

Charge Trapping at the Step Edges of TiO2 Anatase (101)

A combination of photoemission, atomic force, and scanning tunneling microscopy/spectroscopy measurements shows that excess electrons in the TiO₂ anatase (101) surface are trapped at step edges. Consequently, steps act as preferred adsorption sites for O₂. In density functional theory calculations electrons localize at clean step edges, this tendency is enhanced by O vacancies and hydroxylation. The results show the importance of defects for the wide‐ranging applications of titania.     

Dye‐sensitized solar cells based on zinc(II) phthalocyanines bearing 3‐pyridin‐3‐ylpropoxy anchoring groups

In this study, nonperipherally tetra‐substituted ( 2 ), peripherally tetra‐substituted ( 3 ), and peripherally octa‐substituted ( 4 ) zinc(II) phthalocyanines were synthesized as sensitizers for dye‐sensitized solar cells (DSSCs) in which 3‐pyridin‐3‐ylpropoxy substituent acts as anchoring unit to bind TiO₂ surface. The optical results indicated that there is an interaction between the dyes and the TiO₂ surface. The photovoltaic performances of the DSSCs based on these dyes were found to depend on both the position and number of the substituents. Despite the more red‐shifted absorption, the DSSC based on 2 showed the conversion efficiency of 0.68%, which is lower than 1.36% and 0.92% for 3 and 4 , respectively, under one sun (AM 1.5G). The vertical orientation of the dye on TiO₂ surface could be the main reason for the higher photovoltaic performance of complex 3 , which is beneficial for not only injecting the electrons into the conduction band of TiO₂ but also reducing the charge recombination. Overall, these results demonstrate that the peripherally tetra‐substituted 3‐pyridin‐3‐ylpropoxy zinc(II) phthalocyanine complex ( 3 ) as a sensitizer can more efficiently utilize the photons in the red/near‐infrared region with respect to the other complexes studied.     

Unconventional Route to Oxygen-Vacancy-Enabled Highly Efficient Electron Extraction and Transport in Perovskite Solar Cells

The ability to effectively transfer photoexcited electrons and holes is an important endeavor toward achieving high-efficiency solar energy conversion. Now, a simple yet robust acid-treatment strategy is used to judiciously create an amorphous TiO₂ buffer layer intimately situated on the anatase TiO₂ surface as an electron-transport layer (ETL) for efficient electron transport. The facile acid treatment is capable of weakening the bonding of zigzag octahedral chains in anatase TiO₂, thereby shortening staggered octahedron chains to form an amorphous buffer layer on the anatase TiO₂ surface. Such amorphous TiO₂-coated ETL possesses an increased electron density owing to the presence of oxygen vacancies, leading to efficient electron transfer from perovskite to TiO₂. Compared to pristine TiO₂-based devices, the perovskite solar cells (PSCs) with acid-treated TiO₂ ETL exhibit an enhanced short-circuit current and power conversion efficiency.     

TiO2 Nanospheres: A Facile Size‐Tunable Synthesis and Effective Light‐Harvesting Layer for Dye‐Sensitized Solar Cells

A facile route to synthesize amorphous TiO₂ nanospheres by a controlled oxidation and hydrolysis process without any structure‐directing agents or templates is presented. The size of the amorphous TiO₂ nanospheres can be easily turned from 20 to 1500 nm by adjusting either the Ti species or ethanol content in the reaction solution. The phase structure of nanospheres can be controlled by hydrothermal treatment. The TiO₂ nanospheres show excellent size‐dependent light‐scattering effects and can be structured into a light‐harvesting layer for dye‐sensitized solar cells with a quite high power conversion efficiency of 9.25 %.     

Photoanode Based on Chain‐Shaped Anatase TiO2 Nanorods for High‐Efficiency Dye‐Sensitized Solar Cells

Anatase TiO₂ nanorods with large specific surface areas and high crystallinity have been synthesized by surfactant‐free hydrothermal treatment of water‐soluble peroxotitanium acid (PTA). X‐ray diffraction and TEM analysis showed that all TiO₂ nanorods derived from PTA in different hydrothermal processes were in the anatase phase, and high aspect ratio TiO₂ nanorods with chain‐shaped structures were formed at 150 °C for 24 h by oriented growth. The nanorods were fabricated as photoanodes for high‐efficiency dye‐sensitized solar cells (DSSCs). DSSCs fabricated from the chain‐shaped TiO₂ nanorods gave a highest short‐circuit current density of 14.8 mA cm^?2 and a maximum energy conversion efficiency of 7.28 %, as a result of the presence of far fewer surface defects and grain boundaries than are present in commercial P25 TiO₂ nanoparticles. Electrochemical impedance spectroscopy also confirmed that DSSCs based on the TiO₂ nanorods have enhanced electron transport properties and a long electron lifetime.     

Effects of Dye‐Adsorption Solvent on the Performances of the Dye‐Sensitized Solar Cells Based on Black Dye

The effects of the dye‐adsorption solvent on the performances of the dye‐sensitized solar cells (DSSCs) based on black dye have been investigated. The highest conversion efficiency (10.6 %) was obtained in the cases for which 1‐PrOH and the mixed solvent of EtOH and tBuOH (3:1 v/v) were employed as dye‐adsorption solvents. The optimized value for the dielectric constant of the dye‐adsorption solvent was found to be around 20. The DSSCs that used MeOH as a dye‐adsorption solvent showed inferior solar‐cell performance relative to the DSSCs that used EtOH, 1‐PrOH, 2‐PrOH, and 1‐BuOH. Photo‐ and electrochemical measurements of black dye both in solution and adsorbed onto the TiO₂ surface revealed that black dye aggregates at the TiO₂ surface during the adsorption process in the case for MeOH. Both the shorter electron lifetime in the TiO₂ photoelectrode and the greater resistance in the TiO₂–dye–elecrolyte interface, attributed to the dye aggregation at the TiO₂ surface, cause the decrease in the solar‐cell performance of the DSSC that used MeOH as a dye adsorption solvent.     

Influence of sputter power on structural and electrical properties of TiO2 films for Al/TiO2/Si gate capacitors

Titanium dioxide (TiO₂) thin films were deposited onto p‐Si substrates held at room temperature by reactive Direct Current (DC) magnetron sputtering at various sputter powers in the range 80–200 W. The as‐deposited TiO₂ films were annealed at a temperature of 1023 K. The post‐annealed films were characterized for crystallographic structure, chemical binding configuration, surface morphology and optical absorption. The electrical and dielectric properties of Al/TiO₂/p‐Si structure were determined from the capacitance–voltage and current–voltage characteristics. X‐ray diffraction studies confirmed that the as‐deposited films were amorphous in nature. After post‐annealing at 1023 K, the films formed at lower powers exhibited anatase phase, where as those deposited at sputter powers > 160 W showed the mixed anatase and rutile phases of TiO₂. The surface morphology of the films varied significantly with the increase of sputter power. The electrical and dielectric properties on the air‐annealed Al/TiO₂/p‐Si structures were studied. The effect of sputter power on the electrical and dielectric characteristics of the structure of Al/TiO₂/p‐Si (metal‐insulator‐semiconductor) was systematically investigated. Copyright © 2014 John Wiley & Sons, Ltd.     

Nanoparticulate Hollow TiO2 Fibers as Light Scatterers in Dye‐Sensitized Solar Cells: Layer‐by‐Layer Self‐Assembly Parameters and Mechanism

Hollow structures show both light scattering and light trapping, which makes them promising for dye‐sensitized solar cell (DSSC) applications. In this work, nanoparticulate hollow TiO₂ fibers are prepared by layer‐by‐layer (LbL) self‐assembly deposition of TiO₂ nanoparticles on natural cellulose fibers as template, followed by thermal removal of the template. The effect of LbL parameters such as the type and molecular weight of polyelectrolyte, number of dip cycles, and the TiO₂ dispersion (amorphous or crystalline sol) are investigated. LbL deposition with weak polyelectrolytes (polyethylenimine, PEI) gives greater nanoparticle deposition yield compared to strong polyelectrolytes (poly(diallyldimethylammonium chloride), PDDA). Decreasing the molecular weight of the polyelectrolyte results in more deposition of nanoparticles in each dip cycle with narrower pore size distribution. Fibers prepared by the deposition of crystalline TiO₂ nanoparticles show higher surface area and higher pore volume than amorphous nanoparticles. Scattering coefficients and backscattering properties of fibers are investigated and compared with those of commercial P25 nanoparticles. Composite P25–fiber films are electrophoretically deposited and employed as the photoanode in DSSC. Photoelectrochemical measurements showed an increase of around 50 % in conversion efficiency. By employing the intensity‐modulated photovoltage and photocurrent spectroscopy methods, it is shown that the performance improvement due to addition of fibers is mostly due to the increase in light‐harvesting efficiency. The high surface area due to the nanoparticulate structure and strong light harvesting due to the hollow structure make these fibers promising scatterers in DSSCs.     

Structurally Stabilized Organosilane‐Templated Thermostable Mesoporous Titania

Structurally thermostable mesoporous anatase TiO₂ (m‐TiO₂) nanoparticles, uniquely decorated with atomically dispersed SiO₂, is reported for the first time. The inorganic Si portion of the novel organosilane template, used as a mesopores‐directing agent, is found to be incorporated in the pore walls of the titania aggregates, mainly as isolated sites. This is evident by transmission electron microscopy and high‐angle annular dark field scanning transmission electron microscopy, combined with electron dispersive X‐ray spectroscopy. This type of unique structure provides exceptional stability to this new material against thermal collapse of the mesoporous structure, which is reflected in its high surface area (the highest known for anatase titania), even after high‐temperature (550 °C) calcination. Control of crystallite size, pore diameter, and surface area is achieved by varying the molar ratios of the titanium precursor and the template during synthesis. These mesoporous materials retain their porosity and high surface area after template removal and further NaOH/HCl treatment to remove silica. We investigate their performance for dye‐sensitized solar cells (DSSCs) with bilayer TiO₂ electrodes, which are prepared by applying a coating of m‐TiO₂ onto a commercial titania (P25) film. The high surface area of the upper mesoporous layer in the P25–m‐TiO₂ DSSC significantly increases the dye loading ability of the photoanode. The photocurrent and fill factor for the DSSC with the bilayer TiO₂ electrode are greatly improved. The large increase in photocurrent current (ca. 56 %) in the P25–m‐TiO₂ DSSC is believed to play a significant role in achieving a remarkable increase in the photovoltaic efficiency (60 %) of the device, compared to DSSCs with a monolayer of P25 as the electrode.     

Photocatalytic Reduction of Nitro Compounds Using TiO2 Photocatalyst by UV and Vis Dye-sensitized Systems

Nitro‐aromatic compounds can be photocatalytically reduced into the corresponding amine‐aromatic compounds using TiO₂ as a photocatalyst in the UV/TiO₂/holes scavenger and Vis/TiO₂/dye‐sensitized systems. In the UV/TiO₂/holes scavenger system, reaction substrate alcohols such as methanol could be used as the holes scavengers, and in the Vis/TiO₂/dye‐sensitized system, substrate alcohols could be oxidized to the corresponding aldehydes with high selectivity. When methanol was used as the holes scavengers and the illumination time was 6 h, 87.2% of p‐nitrotoluene could be photocatalytically reduced into p‐toluidine. In the Vis/TiO₂/dye‐sensitized system, the effect of aromatic alcohols for the photocatalytic reduction of nitrobenzene was better than that of other alcohols. At the same time, aromatic alcohols can be easily oxidized, and the production efficiencies of the corresponding aldehydes were higher than those of other alcohols. The possible reaction mechanisms were also proposed.     

Photocatalytic Crystalline and Amorphous TiO2 Nanotubes Prepared by Electrospinning and Atomic Layer Deposition

In this work core/shell composite polymer/TiO₂ nanofibers and from those TiO₂ nanotubes were prepared. First, poly(vinyl alcohol) (PVA) and poly(vinylpyrrolidone) (PVP) fibers were synthetized by electrospinning. They were covered with a 100 nm thick amorphous TiO₂ layer by atomic layer deposition at 50 °C. Later the polymer core was removed by two different methods: dissolution and annealing. In the case of dissolution in water, the as-prepared TiO₂ nanotubes remained amorphous, while when annealing was used to remove the polymers, the TiO₂ crystallized in anatase form. Due to this, the properties of amorphous and crystalline TiO₂ nanotubes with exactly the same structure and morphology could be compared. The samples were investigated by SEM-EDX, ATR-IR, UV-Vis, XRD and TG/DTA-MS. Finally, the photocatalytic properties of the TiO₂ nanotubes were studied by decomposing methyl-orange dye under UV light. According to the results, crystalline anatase TiO₂ nanotubes reached the photocatalytic performance of P25, while amorphous TiO₂ nanotubes had observable photocatalytic activity.     

TiO2 nanotubes as recyclable catalyst for efficient photocatalytic degradation of indigo carmine dye

TiO₂ nanotubes have been synthesized in a hydrothermal system. The nanotubes were characterized by scanning electronic microscopy (SEM), FT-Raman spectroscopy and surface charge density by surface area analyzer. These nanocatalysts were applied to photocatalyse indigo carmine dye degradation. Photodegradation ability of TiO₂ nanotubes was compared to TiO₂ anatase photoactivity. Indigo carmine dye was completely degraded at 60 and 110 min of reaction catalysed by TiO₂ nanotubes and TiO₂ anatase, respectively. TiO₂ nanotubes presented high photodegradation activity at pH 2 and TiO₂ anatase at pH 4. TiO₂ nanotubes were easily recycled whereas the reuse of TiO₂ anatase was not effective. Nanotubes maintained 90% of activity after 10 catalytic cycles and TiO₂ anatase presented only 10% of its activity after 10 cycles.     

A Robust,Precious‐Metal‐Free Dye‐Sensitized Photoanode for Water Oxidation: A Nanosecond‐Long Excited‐State Lifetime through a Prussian Blue Analogue

Herein, we establish a simple synthetic strategy affording a heterogeneous, precious metal‐free, dye‐sensitized photoelectrode for water oxidation, which incorporates a Prussian blue (PB) structure for the sensitization of TiO₂ and water oxidation catalysis. Our approach involves the use of a Fe(CN)₅ bridging group not only as a cyanide precursor for the formation of a PB‐type structure but also as an electron shuttle between an organic chromophore and the catalytic center. The resulting hetero‐functional PB‐modified TiO₂ electrode demonstrates a low‐cost and easy‐to‐construct photoanode, which exhibits favorable electron transfers with a remarkable excited state lifetime on the order of nanoseconds and an extended light absorption capacity of up to 500 nm. Our approach paves the way for a new family of precious metal‐free robust dye‐sensitized photoelectrodes for water oxidation, in which a variety of common organic chromophores can be employed in conjunction with CoFe PB structures.     

Improvement of the Visible‐Light Photocatalytic Performance of TiO2 by Carbon Mesostructures

An improvement in the photodegradation performance for dyes due to interaction between carbon and titania in a self‐assembled mesoporous C? TiO₂ composite catalyst, even for the difficult degradation of azo dyes, is reported herein. The dye removal process involves adsorption of the dye from water by the mesoporous carbon–titania, followed by photodegradation on the separated dye‐loaded solid. Such adsorption–catalysis cycles can be carried out more than 80 times without discernible loss of photocatalytic activity or the anatase content of the composite. In each run, about 120 mg dye per g catalyst can be degraded. The mesoporous carbon–titania catalyst also exhibits a high capacity for converting methyl orange in aqueous solution under visible light. Characterization by XRD, TEM, and N₂ sorption techniques has revealed that the self‐assembled composite catalyst has an ordered mesostructure, uniform mesopores (4.3 nm), a large pore volume (0.30 cm³ g^?1), and a high surface area (348 m² g^?1). The pore walls are composed of amorphous carbon and anatase nanoparticles of size 4.2 nm, which are well dispersed and confined. X‐ray photoelectron spectroscopy (XPS), surface photovoltage spectroscopy (SPS), and UV/Vis absorption results indicate doping of carbon into the anatase lattice and a change in the bandgap of the semiconductor. The synergistic improvement in the composite catalyst can be attributed to the following features: (1) carbon doping of the anatase lattice modifies its bandgap and enhances its activity under visible light; (2) confinement within carbon pore walls prevents aggregation of tiny anatase nanoparticles, improving their activity and stability; (3) the mesopores provide a confined space for photocatalysis; and (4) the strong adsorption ability of porous carbon for organic substances ensures that large quantities can be processed and inhibits further diffusion of the adsorbed organic substances, thereby enhancing the mineralization on anatase.     

SrTiO3/TiO2 Hybridstructure as Photoanode in Dye‐Sensitized Solar Cell

In dye‐sensitized solar cells (DSSCs), the charge recombination at the TiO₂/dye/electrolyte interface greatly influences the photoelectron conversion efficiency. Hybrid semiconductor materials with matched band potentials are designed to reduce the charge recombination. In this study, SrTiO₃/TiO₂ hybridstructure was synthesized by using TiO₂ nanoparticles as template in a hydrothermal, showing a negative shift in the flat band potential. The DSSC with the SrTiO₃/TiO₂ anode exhibits an increased photovoltage and a reduced photocurrent. The suppression of charge recombination at the TiO₂/dye/electrolyte interface was observed in the electrochemical impedance spectroscopy, causing an improvement in the photovoltage. However, the SrTiO₃/TiO₂ system shows an obstructed electrons injection from the dye to SrTiO₃/TiO₂, limiting the photocurrent performance. The photoelectrochemical properties of the SrTiO₃/TiO₂ system are discussed in detail herein.