The fabrication of TiO<Subscript>2</Subscript> nanoparticle/ZnO nanowire arrays bi-filmed photoanode and their effect on dye-sensitized solar cells

Porous TiO₂ nanoparticles coated on ZnO nanowire arrays (TiO₂ NP/ZnO NW) as photoanode for dye-sensitized solar cell (DSSC) has been fabricated and investigated to improve the power conversion efficiency. The TiO₂ NP/ZnO NW photoanode consists of single crystalline ZnO NWs synthesized via hydrothermal method and porous TiO₂ NP film covered on the surface of ZnO NW arrays by screen printing technique. The effect of TiO₂ NPs thickness of the bi-filmed photoanode on the cell performance has been investigated, and TiO₂ NP/ZnO NW DSSC with NP thickness of ~5 μm exhibits the best efficiency of 4.68%, higher than 1.16% of ZnO NW DSSC and 3.18% of TiO₂ NPs DSSC, prepared and tested under identical conditions. The efficiency increase is attributed to the enlarged photocurrent, due to the greatly enhanced surface area for dye absorption and light harvesting efficiency resulted from TiO₂ NPs, and improved open-circuit voltage, due to reduced electron recombination by providing direct conduction pathway along ZnO NWs.     

A double-layered photoanode made of ZnO/TiO<Subscript>2</Subscript> composite nanoflowers and TiO<Subscript>2</Subscript> nanorods for high efficiency dye-sensitized solar cells

We present a simple sol-gel hydrothermal process for the fabrication of a double-layered structure composed of a TiO₂ nanorod overlayer and TiO₂ nanoparticle-embedded ZnO nanoflower (ZNFs@TNPs-TNRs) underlayer. The ZNFs@TNPs-TNRs was used as a photoanode in dye-sensitized solar cells (DSSCs) and their photovoltaic performance was analyzed. The ZNFs@TNPs-TNRs can enhance the adsorption of N719 dyes, charge transport, and light scattering. The cell performances can be maximized by optimizing thickness ratio and total thickness of the double-layered photoanode, and the preliminary results demonstrate that a promising power conversion efficiency (PCE) of 8.01% is determined on the DSSC with ZNFs@TNPs-TNRs anode, yielding a 28.9% enhancement in the PCE in comparison to pristine TiO₂–P25 nanoparticle-based DSSC.     

Enhanced photovoltaic characterization and charge transport of TIO2 nanoparticles/nanotubes composite photoanode based on indigo carmine dye-sensitized solar cells

Dye-sensitized solar cells (DSSCs) have established themselves as an alternative to conventional solar cells owing to their remarkably high power conversion efficiency, longtime stability and low-cost production. DSSCs composed of a dyed oxide semiconductor photoanode, a redox electrolyte and a counter electrode. In these devices, conversion efficiency is achieved by ultra-fast injection of an electron from a photo excited dye into the conduction band of metal oxide followed by subsequent dye regeneration and holes transportation to the counter electrode. The energy conversion efficiency of DSSC is to be dependent on the morphology and structure of the dye adsorbed metal oxide photoanode. Worldwide considerable efforts of DSSCs have been invested in morphology control of photoanode film, synthesis of stable optical sensitizers and improved ionic conductivity electrolytes. In the present investigation, a new composite nano structured photoanodes were prepared using TiO₂ nano tubes (TNTs) with TiO₂ nano particles (TNPs). TNPs were synthesized by sol–gel method and TNTs were prepared through an alkali hydrothermal transformation. Working photoanodes were prepared using five pastes of TNTs concentrations of 0, 10, 50, 90, and 100 % with TNPs. The DSSCs were fabricated using Indigo carmine dye as photo sensitizer and PMII (1-propyl-3-methylimmidazolium iodide) ionic liquid as electrolyte. The counter electrode was prepared using Copper sulfide. The structure and morphology of TNPs and TNTs were characterized by X-ray diffraction and electron microscopes (TEM and SEM). The photocurrent efficiency is measured using a solar simulator (100 mW/cm²). The prepared composite TNTs/TNPs photoanode could significantly improve the efficiency of dye-sensitized solar cells owing to its synergic effects, i.e. effective dye adsorption mainly originated from TiO₂ nanoparticles and rapid electron transport in one-dimensional TiO₂ nanotubes. The results of the present investigation suggested that the DSSC based on 10 % TNTs/TNPs showed better photovoltaic performance than cell made pure TiO₂ nanoparticles. The highest energy-conversion efficiency of 2.80 % is achieved by composite TNTs (10 %)/TNPs film, which is 68 % higher than that pure TNPs film and far larger than that formed by bare TNTs film (94 %). The charge transport and charge recombination behaviors of DSSCs were investigated by electrochemical impedance spectra and the results showed that composite TNTs/TNPs film-based cell possessed the lowest transfer resistances and the longest electron lifetime. Hence, it could be concluded that the composite TNTs/TNPs photoanodes facilitate the charge transport and enhancing the efficiencies of 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.     

Hydrothermal Synthesis of a Crystalline Rutile TiO2 Nanorod Based Network for Efficient Dye‐Sensitized Solar Cells

One‐dimensional (1D) TiO₂ nanostructures are desirable as photoanodes in dye‐sensitized solar cells (DSSCs) due to their superior electron‐transport capability. However, making use of the DSSC performance of 1D rutile TiO₂ photoanodes remains challenging, mainly due to the small surface area and consequently low dye loading. Herein, a new type of photoanode with a three‐dimensional (3D) rutile‐nanorod‐based network structure directly grown on fluorine‐doped tin oxide (FTO) substrates was developed by using a facile two‐step hydrothermal process. The resultant photoanode possesses oriented rutile nanorod arrays for fast electron transport as the bottom layer and radially packed rutile head‐caps with an improved large surface area for efficient dye adsorption. The diffuse reflectance spectra showed that with the radially packed top layer, the light‐harvesting efficiency was increased due to an enhanced light‐scattering effect. A combination of electrochemical impedance spectroscopy (EIS), dark current, and open‐circuit voltage decay (OCVD) analyses confirmed that the electron‐recombiantion rate was reduced on formation of the nanorod‐based 3D network for fast electron transport. As a resut, a light‐to‐electricity conversion efficiency of 6.31 % was achieved with this photoanode in DSSCs, which is comparable to the best DSSC efficiencies that have been reported to date for 1D rutile TiO₂.     

Photovoltaic properties of oriented ZnO nanowires arrays decorated with TiO<Subscript>2</Subscript> shell layer for dye-sensitized solar cell application

Almost vertically aligned ZnO nanowires have been grown on Silicon substrates via a simple hydrothermal method. In order to improve the photoelectric conversion efficiency for fabricated dye-sensitized solar cells (DSSCs), an easily-operated immersing method was employed to fabricate a TiO₂/ZnO nanowires array heterojunction, which has advantage of high aspect ratio, low recombination rate and high absorption of visible light. The structure and surface morphology of the samples were characterized by X-ray powder diffraction (XRD) and scanning electron microscopy (SEM), respectively. The photovoltaic properties of TiO₂/ZnO based DSCCs were measured by considering the power efficiency (η), photocurrent density (J_sc), open-circuit voltage (V_oc), and fill factor (FF). An efficiency of 0.559% is achieved for the composite cell, increasing 0.426 and 0.185% for the ZnO nanowires cell and TiO₂ cell, respectively. The short-circuit current and open-circuit voltage are also enhancing. The improvements are because of high surface are of TiO₂ shell layer, as well as fast electron transport and light scattering effect of ZnO nanowires.     

Structural Control of Hierarchically‐Ordered TiO2 Films by Water for Dye‐Sensitized Solar Cells

A facile way of controlling the structure of TiO₂ by changing the amount of water to improve the efficiency of dye‐sensitized solar cells (DSSCs) is reported. Hierarchically ordered TiO₂ films with high porosity and good interconnectivity are synthesized in a well‐defined morphological confinement arising from a one‐step self‐assembly of preformed TiO₂ (pre‐TiO₂) nanocrystals and a graft copolymer, namely poly(vinyl chloride)‐g‐poly(oxyethylene methacrylate). The polymer–solvent interactions in solution, which are tuned by the amount of water, are shown to be a decisive factor in determining TiO₂ morphology and device performance. Systematic control of wall and pore size is achieved and enables the bifunctionality of excellent light scattering properties and easy electron transport through the film. These properties are characterized by reflectance spectroscopy, incident photon‐to‐electron conversion efficiency, and electrochemical impedance spectroscopy analyses. The TiO₂ photoanode that is prepared with a higher water ratio, [pre‐TiO₂]:[H₂O]=1:0.3, shows a larger surface area, greater light scattering, and better electron transport, which result in a high efficiency (7.7 %) DSSC with a solid polymerized ionic liquid. This efficiency is much greater than that of commercially available TiO₂ paste (4.0 %).     

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.     

Effect of TiCl4‐based TiO2 compact and blocking layers on efficiency of dye‐sensitized solar cells

The effect of the number and arrangement of TiO₂‐based photoanode layers on the efficiency of dye‐sensitized solar cells (DSSCs) was investigated. Compact, mesoporous, and blocking layers of TiO₂ were prepared to form monolayer, bilayer, and trilayer photoanodes. Compact and blocking TiO₂ layers were prepared using dip‐coating technique, whereas the doctor‐blade method was employed to prepare TiO₂ paste layers using nanoparticles prepared by the sol–gel method. The crystalline structure of photoanodes was characterized by X‐ray diffraction (XRD) measurements and their morphology and thickness were characterized by the scanning electron microscopy (SEM) technique. The photovoltaic performance of constructed DSSC devices was investigated and the optimum arrangement was identified and explained in terms of dye loading enhancement and recombination reduction at the fluorine‐doped tin oxide (FTO)/electrolyte interface.     

Environmentally Benign and Efficient Ag2S‐ZnO Nanowires as Photoanodes for Solar Cells: Comparison with CdS‐ZnO Nanowires

In this work, we develop a low‐temperature, facile solution reaction route for the fabrication of quantum‐dot‐sensitized solar cells (QDSSCs) containing Ag₂S‐ZnO nanowires (NWs), simultaneously ensuring low manufacturing costs and environmental safety. For comparison, a CdS‐ZnO NW photoanode was also prepared using the layer‐by‐layer growth method. Ultraviolet photoelectron spectroscopy analysis revealed type‐II band alignments for the band structures of both photoanodes which facilitate electron transfer/collection. Compared to CdS‐ZnO QDSSCs, Ag₂S‐ZnO QDSSCs exhibit a considerably higher short‐circuit current density (J_sc) and a strongly enhanced light‐harvesting efficiency, but lower open‐circuit voltages (V_oc), resulting in almost the same power‐conversion efficiency of 1.2 %. Through this work, we demonstrate Ag₂S as an efficient quantum‐dot‐sensitizing material that has the potential to replace Cd‐based sensitizers for eco‐friendly applications.     

Synthesis of TiO<Subscript>2</Subscript> nanotubes using different alkaline media and their applications in photocatalysis and DSSCs

TiO₂ nanotubes (TNTs) were successfully synthesized from different alkaline media (i.e., NaOH and KOH) by using a microwave hydrothermal process. The effects of different alkaline media on the formation of TiO₂ nanotubes and their physicochemical properties were investigated. The phases of different TiO₂ nanostructures were studied by using X-ray diffraction patterns. Morphologies of the nanostructures were observed with a transmission electron microscope. The optical properties of the nanostructures were evaluated through the absorption behavior using UV–Vis diffuse reflectance spectroscopy. The photocatalytic activities of the TiO₂ nanostructures were evaluated by the degradation of methylene blue aqueous dye solution under the simulated solar light irradiation. Similarly, the photovoltaic efficiencies of the prepared samples were investigated by making photo-anode layers in the Dye Sensitized Solar Cells (DSSCs). The results revealed that in comparison to the single layered TiO₂ nanostructures in the DSSC, creation of a double layer structure significantly enhanced the efficiency of DSSC.     

Efficiency improvement of flexible dye-sensitized solar cells by introducing mesoporous TiO2 microsphere

Mesoporous TiO2microsphere(MTM)was synthesized via a simple solution route and then mixed with commercial TiO2(P25)to form highly homogeneous and stable TiO2colloid by simple hydrothermal treatment.The TiO2colloid was coated onto the plastic conductive substrate to prepare mesoporous TiO2film for flexible dye-sensitized solar cells(DSSCs)by low-temperature heat treatment.The influence of MTM content on the physicochemical properties of the flexible TiO2film was characterized by scanning electron microscope,transmission electron microscopy,X-ray diffraction,energy-dispersive X-ray spectrometer,N2adsorption-desorption isotherms,UV–vis absorption and diffuse reflectance spectra.It is revealed that with increasing the MTM content,the dye-loading capability of TiO2film and light-harvesting efficiency of flexible DSSCs are improved due to MTM having high surface area and acting as a light scattering center,respectively,resulting in the enhancement of photocurrent of flexible DSSCs.However,more and larger cracks having negative effect on the performances of flexible DSSCs are formed simultaneously.Under the optimal condition with MTM content of 20%,a flexible DSSC with overall light-to-electric energy conversion efficiency of 2.74%is achieved under a simulated solar light irradiation of 100 mW cm 2(AM 1.5),with 26%improvement in comparison with DSSCs based on P25 alone.     

Well‐Dispersed CoS Nanoparticles on a Functionalized Graphene Nanosheet Surface: A Counter Electrode of Dye‐Sensitized Solar Cells

With a facile electrophoretic deposition and chemical bath process, CoS nanoparticles have been uniformly dispersed on the surface of the functionalized graphene nanosheets (FGNS). The composite was employed as a counter electrode of dye‐sensitized solar cells (DSSCs), which yielded a power conversion efficiency of 5.54 %. It is found that this efficiency is higher than those of DSSCs based on the non‐uniform CoS nanoparticles on FGNS (4.45 %) and built on the naked CoS nanoparticles (4.79 %). The achieved efficiency of our cost‐effective DSSC is also comparable to that of noble metal Pt‐based DSSC (5.90 %). Our studies have revealed that both the exceptional electrical conductivity of the FGNS and the excellent catalytic activity of the CoS nanoparticles improve the conversion efficiency of the uniformly FGNS‐CoS composite counter electrode. The electrochemical impedance spectra, cyclic voltammetry, and Tafel polarization have evidenced the best catalytic activity and the fastest electron transport. Additionally, the dispersion condition of CoS nanoparticles on FGNS plays an important role for catalytic reduction of I₃^?.     

Polyelectrolyte brushes as efficient platform for synthesis of Cu and Pt bimetallic nanocrystals onto TiO2 nanowires

A Cu–Pt nanoparticle catalyst supported on TiO₂ nanowires (NWs) was prepared through regenerative counterion exchange–reduction using polyelectrolyte brush as template. Cationic polydimethyl aminoethyl methacrylate brushes were grafted onto TiO₂ NWs. Cu–Pt nanocrystals were produced by anionic counterions CuCl₄^2? and PtCl₆^2? bound with the polymer brush through in situ reduction with NaBH₄ of high density and low polydispersity. The as‐prepared TiO₂ NWs/polymer brush/Cu–Pt was characterized by Fourier transform infrared spectroscopy (FT‐IR spectrometry), X‐ray photoelectron spectroscopy, transmission electron microscopy, and UV–Vis adsorption spectrometry analyses. Results showed that the highly dispersed Cu and Pt nanoparticles were present on the surface of the TiO₂ NWs/polymer brush. The resultant TiO₂ NWs/polymer brush/Cu–Pt exhibited extremely high catalytic activity and reduced p‐nitrophenol at room temperature. Copyright © 2017 John Wiley & Sons, Ltd.     

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.     

Highly ordered combined structure of anodic TiO2 nanotubes and TiO2 nanoparticles prepared by a novel route for dye-sensitized solar cells

Combined structure of anodic TiO₂ nanotubes and TiO₂ nanoparticles (TiNTs-TiNPs) has been synthesized by a facile combination of hydrothermal and chemical vapor deposition methods. Ordered TiO₂ nanotubes with smooth walls were fabricated by two step anodization method in ethylene glycol containing NH₄F at 50 V. This nanotubular array after annealing at 450 °C was subjected to the hydrothermally produced gaseous environment in an autoclave with diluted TiCl₄ solution at its bottom. Vapors of TiCl₄ were allowed to react chemically with water vapors for predefined time durations at 180 °C that resulted in the deposition of TiO₂ nanoparticles on tubes’ surface and side walls. Scanning electron microscopy (SEM) and transmission electron microscopy (TEM) confirmed that for one hour reaction duration, nanoparticles were evenly coated on the walls of nanotubes, whereas, longer durations tend to deteriorate the tubular structure. Consequently, the ordered TiNTs-TiNPs array produced after one hour coating has shown better performance for dye-sensitized solar cell DSSC) in back illumination mode with 130% increase in efficiency as compared to the device based on bare TiO₂ nanotubes. The same photoanode has higher reflective properties with higher scattering ability. The solar cell based on this photoanode exhibits higher external quantum efficiency and effective charge transport properties. This study shows that porous ordered 1D structures based on TiO₂ are of crucial importance for the high performance of DSSCs.     

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.     

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.     

Structure–Performance Correlations of Organic Dyes with an Electron‐Deficient Diphenylquinoxaline Moiety for Dye‐Sensitized Solar Cells

The high performances of dye‐sensitized solar cells (DSSCs) based on seven new dyes are disclosed. Herein, the synthesis and electrochemical and photophysical properties of a series of intentionally designed dipolar organic dyes and their application in DSSCs are reported. The molecular structures of the seven organic dyes are composed of a triphenylamine group as an electron donor, a cyanoacrylic acid as an electron acceptor, and an electron‐deficient diphenylquinoxaline moiety integrated in the π‐conjugated spacer between the electron donor and acceptor moieties. The DSSCs based on the dye DJ104 gave the best overall cell performance of 8.06 %; the efficiency of the DSSC based on the standard N719 dye under the same experimental conditions was 8.82 %. The spectral coverage of incident photon‐to‐electron conversion efficiencies extends to the onset at the near‐infrared region due to strong internal charge‐transfer transition as well as the effect of electron‐deficient diphenylquinoxaline to lower the energy gap in these organic dyes. A combined tetraphenyl segment as a hydrophobic barrier in these organic dyes effectively slows down the charge recombination from TiO₂ to the electrolyte and boosts the photovoltage, comparable to their Ru^II counterparts. Detailed spectroscopic studies have revealed the dye structure–cell performance correlations, to allow future design of efficient light‐harvesting organic dyes.     

Elementary Photoelectronic Processes at a Porphyrin Dye/Single‐Walled TiO2 Nanotube Hetero‐interface in Dye‐Sensitized Solar Cells: A First‐Principles Study

A unique one‐dimensional (1D) sandwich single‐walled TiO₂ nanotube (STNT) is proposed as a photoanode nanomaterial with perfect morphology and large specific surface area. We have thoroughly examined the elementary photoelectronic processes occurring at the porphyrin dye/STNT hetero‐interface in dye‐sensitized solar cells (DSSCs) by theoretical simulation. It is desirable to investigate the interfacial photoelectronic processes to elucidate the electron transfer and transport mechanism in 1D STNT‐based DSSCs. We have found that the photoexcitation and interfacial charge separation mechanism can be described as follows. A ground‐state electron of the dye molecule (localized around the electron donor) is first promoted to the excited state (distributed electron donor), and then undergoes ultrafast injection into the conduction band of the STNT, leaving a hole around the oxidized dye. Significantly, the injected electron in the conduction band is transported along the STNT by means of Ti 3d orbitals, offering a unidirectional electron pathway toward the electrode for massive collection without the observation of trap states. Our study not only provides theoretical guidelines for the modification of TiO₂ nanotubes as a photoanode material, but also opens a new perspective for the development of a novel class of TiO₂ nanotubes with high power‐generation efficiency.