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.     

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.     

CdSe Quantum Dots and N719‐Dye Decorated Hierarchical TiO2 Nanorods for the Construction of Efficient Co‐Sensitized Solar Cells

Three‐dimensional hierarchical TiO₂ nanorods (HTNs) decorated with the N719 dye and 3‐mercaptopropionic or oleic acid capped CdSe quantum dots (QDs) in photoanodes for the construction of TiO₂ nanorod‐based efficient co‐sensitized solar cells are reported. These HTN co‐sensitized solar cells showed a maximum power‐conversion efficiency of 3.93 %, and a higher open‐circuit voltage and fill factor for the photoanode with 3‐mercaptopropionic acid capped CdSe QDs due to the strong electronic interactions between CdSe QDs, N719 dye and HTNs, and the superior light‐harvesting features of the HTNs. An electrochemical impedance analysis indicated that the superior charge‐collection efficiency and electron diffusion length of the CdSe QD‐coated HTNs improved the photovoltaic performance of these HTN co‐sensitized solar cells.     

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.     

Enhancing Electron Collection Efficiency and Effective Diffusion Length in Dye‐Sensitized Solar Cells

Intensity‐modulated photocurrent spectroscopy and intensity‐modulated photovoltage spectroscopy are employed to measure the dynamics of electron transport and recombination in the ZnO nanowire (NW) array‐ZnO/layered basic zinc acetate (LBZA) nanoparticle (NP) composite dye‐sensitized solar cells (DSSCs). The roles of the vertical ZnO NWs and insulating LBZA in the electron collection and transport in DSSCs are investigated by comparing the results to those in the TiO₂–NP, horizontal TiO₂–NW and vertical ZnO–NW‐array DSSCs. The electron transport rate and electron lifetime in the ZnO NW/NP composite DSSC are superior to those in the conventional TiO₂–NP cell due to the existence of the vertical ZnO NWs and insulating LBZA. It indicates that the ZnO NW/NP composite anode is able to sustain efficient electron collection over much greater thickness than the TiO₂–NP cell does. Consequently, a larger effective electron diffusion length is available in the ZnO composite DSSC.     

Simultaneous growth of rutile TiO2 as 1D/3D nanorod/nanoflower on FTO in one-step process enhances electrochemical response of photoanode in DSSC

Simultaneous growth of 1D/3D-nanorod/nanoflower like structures of TiO₂ on fluorine doped tin oxide (FTO) glass substrates has been achieved in one-step hydrothermal process. X-ray diffraction (XRD) pattern confirms the formation of rutile phase whereas enhanced light scattering in 1D/3D nanorod/nanoflower TiO₂ photoanode was observed at longer wavelengths of 600–750 nm. Dye sensitized solar cell (DSSC) prepared with 1D/3D-nanorod/nanoflower structures had about 90% increases in efficiency as compared to 1D nanorod like structure. Thus, simultaneous assembly of TiO₂ as 1D/3D-nanorod/nanoflower like structure without significant change in their surface oxidation states (Ti^3 + and Ti^4 +) had better capabilities for light harvesting and efficient electron transportation for improving electrochemical responses of photo-electrode in DSSC to achieve higher sensitivity.     

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.     

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 %).     

Improved charge transport in dye-sensitized solar cells employing viscous non-volatile electrolytes

Dye-sensitized solar cells (DSSCs) employing a viscous non-volatile electrolyte were prepared by utilizing anatase TiO₂ nanorods (synthesized via oriented attachment) as a photoanode material. One promising way to enhance the photovoltaic performance of DSSCs employing viscous electrolytes is to increase ion conductivity by increasing the salt concentration. This is accompanied by an acceleration of the charge recombination reaction and the limiting of the overall conversion efficiency. The results showed that a TiO₂ nanorod electrode enables more favorable electron transport than a conventional nanoparticle-based electrode due to the improved electron diffusion length and the large intrinsic surface area.     

基于TiO2纳米管阵列的高效正面透光型染料敏化太阳能电池的制备及其光电性能

报道了一种基于TiO2纳米管(TNT)阵列正面透光型光阳极的高效染料敏化太阳能电池.将TNTs在450°C烧结后能避免其有序结构在HF处理过程中被破坏,使膜内高速电子传输通道被保留,有利于染料敏化太阳能电池(DSSC)实现高速电荷传输.再用HF、TiCl4、HF和TiCl4混合等溶剂对TNTs进行处理,提高其表面粗糙度以吸附更多染料.染料吸附量的增加能提高光阳极在300-570 nm波段光子捕获效率,该波段是染料吸收光子的主要区域.然而,在染料吸收光子较弱的长波段区域(570-800 nm)光子捕获效率的增加主要源于光阳极光散射率的提高.光阳极光子捕获效率的提高使DSSC的内外量子效率在全波段(300-800 nm)均有所增加,从而使短路电流明显提高.从电化学阻抗数据可知,与电子传输性能密切相关的电化学参数如电荷传输电阻、界面电荷复合电阻、电容、电子寿命、电子扩散长度和电子收集效率等在含处理过的TNTs光阳极DSSC中均有所改善,从而提高电池光电转换效率.含HF和TiCl4混合溶剂处理TNTs光阳极的DSSC最高光电转换效率能达到7.30%,比未处理的DSSC(5.38%)提高35.69%.     

New architectures for dye-sensitized solar cells

Modern dye-sensitized solar cell (DSSC) technology was built upon nanoparticle wide bandgap semiconductor photoanodes. While versatile and robust, the sintered nanoparticle architecture exhibits exceedingly slow electron transport that ultimately restricts the diversity of feasible redox mediators. The small collection of suitable mediators limits both our understanding of an intriguing heterogeneous system and the performance of these promising devices. Recently, a number of pseudo-1D photoanodes that exhibit accelerated charge transport and greater materials flexibility were fabricated. The potential of these alternative photoanode architectures for advancing, both directly and indirectly, the performance of DSSCs is explored.     

Performance of dye-sensitized solar cells based on various sensitizers applied on TiO2-Nb2O5 core/shell photoanode structure

The present investigation described the performance of dye-sensitized solar cells (DSSCs) based on various sensitizers applied on TiO₂-Nb₂O₅ core/shell photoanode film. The novel photoanodes were prepared using composite of TiO₂ nanoparticles (TNPs) and TiO₂ nanorods (TNRs) as core (TNPRs) layer with Nb₂O₅ shell coating. As well, tantalum pentoxide (Ta₂O₅), a blocking layer applied over the core/shell film. The DSSCs were fabricated based on various sensitizers namely zinc phthalocyanine, indoline, indigo carmine, zinc porphyrin, N719, coumarin NKX-2700, polymer dye, quantum dots (QDs), perylene and squaraine. The I–V characteristics of the DSSCs, photocurrent density (Jsc), open-circuit voltage (Voc), fill factor (FF), and photoconversion efficiency (PCE) were determined under illumination of AM 1.5 G. Electrochemical impedance spectroscopy (EIS) analysis is carried out to study the charge transport and life-time of charge carriers at photoanode/dye/electrolyte interface of the DSSCs. The I–V and EIS results explicated that the core/shell with blocking layers were able to alleviate the electron transport and suppressed charge recombination at photoanode/dye/electrolyte interface of the DSSCs. Concerning the sensitizers, PCE of the DSSCs exemplify the order N719 > zinc porphyrin > coumarin NKX-2700 > indoline > squaraine > QDs > zinc phthalocyanine > perylene > polymer dye > indigo carmine dye. The results of the present work demonstrated that among the sensitizers studied, N719 showed the highest PCE and fill factor. Besides, the metal-free organic sensitizers (coumarin NKX-2700 and indoline) exhibited comparable PCE as compared to N719.     

双层结构CeO2光阳极在染料敏化太阳能电池中的应用

利用水热法合成核壳结构Au@SiO₂@CeO₂纳米微球,制备了一系列双层结构复合光阳极并应用于染料敏化太阳能电池(DSSC)。研究表明：当CeO₂纳米微球和核壳结构Au@SiO₂@CeO₂纳米微球应用于DSSC光阳极散射层时,电池的光电转化效率有了显著提高。相对于纯TiO₂(P25)光阳极,P25/CeO₂纳米球光阳极电池的DSSC光电性能提高了15.3%,P25/Au@SiO₂@CeO₂纳米球光阳极电池的光电性能提高了27.9%。DSSC光电性能的提高主要归因于2个方面：一方面,Au纳米粒子的表面等离子体共振效应有效提高了光阳极薄膜的光散射效应。另一方面,CeO₂具有较高的染料负载能力,核壳球形结构具有较高的比表面积,增强了光的散射效应,提高了电子传输能力。     

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.     

双层结构CeO2光阳极在染料敏化太阳能电池中的应用

利用水热法合成核壳结构Au@SiO₂@CeO₂纳米微球,制备了一系列双层结构复合光阳极并应用于染料敏化太阳能电池(DSSC)。研究表明：当CeO₂纳米微球和核壳结构Au@SiO₂@CeO₂纳米微球应用于DSSC光阳极散射层时,电池的光电转化效率有了显著提高。相对于纯 TiO₂ (P25)光阳极,P25/CeO₂纳米球光阳极电池的 DSSC 光电性能提高了 15.3%,P25/Au@SiO₂@CeO₂纳米球光阳极电池的光电性能提高了27.9%。DSSC光电性能的提高主要归因于2个方面：一方面,Au纳米粒子的表面等离子体共振效应有效提高了光阳极薄膜的光散射效应。另一方面,CeO₂具有较高的染料负载能力,核壳球形结构具有较高的比表面积,增强了光的散射效应,提高了电子传输能力。     

Electron Transport in Quasi‐Two‐Dimensional Porous Network of Titania Nanoparticles,Incorporating Electrical and Optical Advantages in Dye‐Sensitized Solar Cells

The integration of fast electron transport and large effective surface area is critical to attaining higher gains in the nanostructured photovoltaic devices. Here, we report facilitated electron transport in the quasi‐two‐dimensional (Q2D) porous TiO₂. Liquid electrolyte dye‐sensitized solar cells were prepared by utilizing photoanodes based on the Q2D porous substructures. Due to electron confinement in a microscale porous medium, directional diffusion toward collecting electrode is induced into the electron transport. Our measurements based on the photocurrent and photovoltage time‐of‐flight transients show that at higher Fermi levels, the electron diffusion coefficient in the Q2D porous TiO₂ is about one order of magnitude higher when compared with the conventional layer of porous TiO₂. The results show that microstructuring of the porous TiO₂ leads to an approximately threefold improvement in the electron diffusion length. Such a modification may considerably affects the electrical functionality of moderate or low performance dye‐sensitized solar cells for which the internal gain or collection efficiency is typically low.     

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.     

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.     

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.