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.     

Stable Cocatalyst-Free BiVO4 Photoanodes with Passivated Surface States for Photocorrosion Inhibition

Improving charge transport and reducing bulk/surface recombination can increase the activity and stability of BiVO₄ for water oxidation. Herein we demonstrate that the photoelectrochemical (PEC) performance of BiVO₄ can be significantly improved by potentiostatic photopolarization. The resulting cocatalyst-free BiVO₄ photoanode exhibited a record-high photocurrent of 4.60 mA cm⁻² at 1.23 V_RHE with an outstanding onset potential of 0.23 V_RHE in borate buffer without a sacrificial agent under AM 1.5G illumination. The most striking characteristic was a strong “self-healing” property of the photoanode, with photostability observed over 100 h under intermittent testing. The synergistic effects of the generated oxygen vacancies and the passivated surface states at the semiconductor–electrolyte interface as a result of potentiostatic photopolarization reduced the substantial carrier recombination and enhanced the water oxidation kinetics, further inhibiting photocorrosion.     

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.     

Understanding the Roles of Oxygen Vacancies in Hematite‐Based Photoelectrochemical Processes

Oxygen vacancy (V_O) engineering is an effective method to tune the photoelectrochemical (PEC) performance, but the influence of V_O on photoelectrodes is not well understood. Using hematite as a prototype, we herein report that V_O functions in a more complicated way in PEC process than previously reported. Through a comprehensive analysis of the key charge transfer and surface reaction steps in PEC processes on a hematite photoanode, we clarify that V_O can facilitate surface electrocatalytic processes while leading to severe interfacial recombination at the semiconductor/electrolyte (S‐E) interface, in addition to the well‐reported improvements in bulk conductivity. The improved bulk conductivity and surface catalysis are beneficial for bulk charge transfer and surface charge consumption while interfacial charge transfer deteriorates because of recombination through V_O‐induced trap states at the S‐E interface.     

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.     

ZnS/SiO2协同包覆光阳极制备高效率Zn-Cu-In-Se量子点敏化太阳能电池

通过光阳极协同包覆的策略抑制Zn?Cu?In?Se(ZCISe)量子点敏化太阳能电池(QDSC)中光阳极/电解液界面上的电荷复合过程,提高电荷收集效率和电池光伏性能。采用溶液法在ZCISe量子点敏化的光阳极表面依次沉积包覆ZnS和SiO₂双钝化层,实现较单一ZnS包覆层更有效的界面电荷复合抑制作用,从而提高QDSC的性能。在包覆ZnS/SiO₂双钝化层后,所组装的ZCISe QDSC光电转换效率由传统单一ZnS包覆的12.17%提高到13.23%,这归因于双钝化层对光阳极/电解液界面电荷复合过程的有效抑制,电荷收集效率得到相应提升。     

Construction of high-efficient visible photoelectrocatalytic system for carbamazepine degradation: Kinetics,degradation pathway and mechanism

This study aimed to construct a photoelectrocatalytic (PEC) reaction system based on the self-made reduced TiO₂ NTAs (r-TNAs) photoanode and activated carbon/Polytetrafluoroethylene (AC/PTFE) cathode. It would be observed clearly that the degradation rate constant of carbamazepine (CBZ) over r-TNAs_(photoanode)-AC/PTFE_(cathode) PEC system (0.04961 min⁻¹) was even higher than that of r-TNAs_(photoanode)-Pt_(cathode) PEC system (0.04602 min⁻¹) with the assistance of visible light irradiation and +0.4 V external potential. Besides, in order to obtain optimized conditions, the influence of key parameters such as pH value, electric current density and electrolyte concentration were studied. Most importantly, photoelectrochemical (PECH) properties, reactive oxide species contribution, OH formation rate and CBZ degradation pathway were determined. The results illustrated that the excellent PEC degradation performance depended on the excellent photocatalytic property of r-TNAs photoanode and electron transfer property of photoelectrodes in r-TNAs_(photoanode)-AC/PTFE_(cathode) PEC system. Therefore, the study demonstrated that the r-TNAs_(photoanode)-AC/PTFE_(cathode) PEC system could be expected to replace metal-catalyzed cathodes depending on its excellent PEC performance activity and low cost as well as the reaction system possessed objective and practical application prospect.     

Performance of CeO2–TiO2-admixed photoelectrode for natural dye-sensitized solar cell

Extracts from roots of Beta vulgaris were used as natural sensitizers of a wide-bandgap semiconductor (CeO₂–TiO₂) in photoelectrochemical solar cells. The natural dye, adsorbed onto the semiconductor surface, absorbs visible light and promotes electron transfer across the dye/semiconductor interface. We have applied CeO₂–TiO₂ to natural dye sensitizer solar cells as a photoelectrode to reduce the charge recombination rate by providing energy barrier at the interface between the photoanode and electrolyte which offers an improvement of photovoltaic efficiency. Short-circuit current density (J _sc) and open-circuit voltages (V _oc) of 9.0 mA cm^?2 and 680 mV, respectively, were obtained, and an effective energy conversion efficiency of 3.5?% was achieved. This simple and cheap technique of cell preparation opens up a perspective of commercial feasibility for inexpensive and environment-friendly dye cells.     

An Efficient Strategy for Boosting Photogenerated Charge Separation by Using Porphyrins as Interfacial Charge Mediators

Surface recombination at the photoanode/electrolyte junction seriously impedes photoelectrochemical (PEC) performance. Through coating of photoanodes with oxygen evolution catalysts, the photocurrent can be enhanced; however, current systems for water splitting still suffer from high recombination. We describe herein a novel charge transfer system designed with BiVO₄ as a prototype. In this system, porphyrins act as an interfacial‐charge‐transfer mediator, like a volleyball setter, to efficiently suppress surface recombination through higher hole‐transfer kinetics rather than as a traditional photosensitizer. Furthermore, we found that the introduction of a “setter” can ensure a long lifetime of charge carriers at the photoanode/electrolyte interface. This simple interface charge‐modulation system exhibits increased photocurrent density from 0.68 to 4.75 mA cm^?2 and provides a promising design strategy for efficient photogenerated charge separation to improve PEC performance.     

Highly Efficient Photoelectrochemical Water Splitting with an Immobilized Molecular Co4O4 Cubane Catalyst

Molecular Co₄O₄ cubane water oxidation catalysts were combined with BiVO₄ electrodes for photoelectrochemical (PEC) water splitting. The results show that tuning the substituent groups on cobalt cubane allows the PEC properties of the final molecular catalyst/BiVO₄ hybrid photoanodes to be tailored. Upon loading a new cubane complex featuring alkoxy carboxylato bridging ligands ( 1 h ) on BiVO₄, an AM 1.5G photocurrent density of 5 mA cm⁻² at 1.23 V vs. RHE for water oxidation was obtained, the highest photocurrent for undoped BiVO₄ photoanodes. A high solar‐energy conversion efficiency of 1.84 % was obtained for the integrated photoanode, a sixfold enhancement over that of unmodified BiVO₄. These results and the high surface charge separation efficiency support the role of surface‐modified molecular catalysts in improving PEC performance and demonstrate the potential of molecule/semiconductor hybrids for efficient artificial photosynthesis.     

Photoelectrochemical Behavior of Sn‐Doped α‐Fe2O3 Photoanode with Different Reducer

Hematite has been considered as one of the most promising photoanode candidates for solar water‐splitting. However, its photoelectrochemical (PEC) efficiency is largely constrained by its sluggish oxygen evolution reaction. In this work, the photoelectrochemical performance of hematite was investigated in electrolytes containing different sacrificial agent. The photocurrent densities, onset potential, charge transfer resistance, Helmholtz capacitance at semiconductor liquid junctions (SCLJs), and their correlations were systematically studied. It was found that the onset potential is around the C_H peak potential and is related to the photovoltage. The surface states pinning the Fermi levels of the hematite photoanode are related to the adsorbed water molecules regardless of the sacrificial agents in the electrolyte.     

A photoelectrochemical study on the features of carbonate-catalyzed water oxidation at illuminated TiO2/Solution interface

Photoelectrochemical (PEC) measurements were performed to investigate the catalytic effect of carbonate on water splitting over TiO₂ nanotube photoanode at low applied potentials. The photocurrent measured with the presence of carbonate with a low concentration of 5?×?10^?3?M in 0.5 M NaClO₄ solution of pH?=?6–13 was ca. two to seven times of that measured in the absence of carbonate in solution. By adding a small amount of formate as a precursor of in situ generated carbonate in solution, the significant catalytic effect of carbonate was further evidenced. The features of the photocurrents measured under both potential dynamic and potentiostatic conditions indicated that the existence of carbonate in solution can effectively prevent the surface recombination of the photogenerated electron–hole pairs occurring at the TiO₂/solution interface, which was regarded as the main reason for the catalytic effect of carbonate on the PEC water splitting in this work. The related charge transfer processes during water photooxidation at the TiO₂/solution interface were discussed.     

CdSe量子点敏化纳米二氧化钛太阳电池的电化学交流阻抗谱

量子点敏化纳米TiO2太阳电池(QDSSCs)具有诱人的发展前景,但是与传统的染料敏化太阳电池(DSSCs)相比,其目前的光电转换效率还非常低(仅为3%左右).为了寻找QDSSCs光电转换效率低的原因,本文主要采用外加偏压下的交流阻抗谱技术对通常以S2-/S-x离子对为电解质的CdSe胶体量子点敏化纳米TiO2电极的准...     

Synthesis of N-doped TiO2 particles from aquaethylenediaminetitanium(IV) hydroxide complex and their optical properties on dye-sensitized solar cells

To promote the photoelectric conversion efficiency of solar cell, N-doped TiO₂ particles are introduced as working electrodes in dye-sensitized solar cells. The N-doped TiO₂ particles (N–en–TiO₂) are easily synthesized from [Ti(NH₂CH₂CH₂NH₂) _x (H₂O) _y ]zOH (aquaethylenediaminetitanium(IV) hydroxide) complex using a modified sol–gel process. The produced N–en–TiO₂ particles show rice-shapes of 25–50 nm and their band-gaps become to be shorter than that of TiO₂. The N–en–TiO₂ particles are applied to working electrode layers in dye-sensitized solar cells, and on comparing the performances of pure TiO₂ and N–en₂–TiO₂–DSCs, the latter shows good performance with a solar energy conversion efficiency of ~5.54 % versus the former of 4.21 % respectively with a notable photocurrent enhancement. Particularly, the N–en₂–TiO₂–DSCs exhibit relatively low charge transfer resistance at counter electrode and electron transfer resistance from dye/TiO₂/photoanode, slower recombination times, faster electron transport times, and higher electron diffusion coefficients than non-doped TiO₂–DSCs.     

Photoanodic and photocathodic behaviour of La5Ti2CuS5O7 electrodes in the water splitting reaction

The particulate semiconductor La₅Ti₂CuS₅O₇ (LTC) with a band gap energy of 1.9 eV functioned as either a photocathode or a photoanode when embedded onto Au or Ti metal layers, respectively. By applying an LTC/Au photocathode and LTC/Ti photoanode to, respectively, photoelectrochemical (PEC) water reduction and oxidation concurrently, zero-bias overall water splitting was accomplished under visible light irradiation. The band structures of LTC/Au and LTC/Ti calculated using a semiconductor device simulator (AFORS-HET) confirmed the critical role of the solid/solid junction of the metal back contact in the charge separation and PEC properties of LTC photoelectrodes. The prominently long lifetime of photoexcited charge carriers in LTC, confirmed by transient absorption spectroscopy, allowed the utilization of both photoexcited electrons and holes depending on the band structure at the solid/solid junction.     

Enhancement of electron transport in nano-porous TiO2 electrodes by dye adsorption

Influence of dye adsorption on electron transport in nano-porous TiO₂ electrodes filled with electrolytes is studied with various TiO₂ films and electrolytes. Electron diffusion coefficient in the electrodes is derived from pulsed laser induced current transients. It is found that dye adsorption increases the diffusion coefficients in the electrodes regardless of the difference of electrolytes and TiO₂ electrodes. The degree of the increases scales with the surface area of the nano-porous electrodes. Based on trapping model, these observations can be interpreted with that the charge traps located on the electrode surface are removed by the dye adsorption. However, dependency of electron diffusion coefficients on light intensity is not affected by dye adsorption, implying that the traps are located not only on the surface but also inside the electrodes.     

Enhanced electron collection in photoanode based on ultrafine TiO2 nanotubes by a rapid anodization process

The separated and ultrafine TiO₂ nanotubes are fabricated by a modified rapid anodization method, which cannot be achieved through conventional anodization. Then, model dye-sensitized solar cells based on the prepared TiO₂ nanotubes and commercial TiO₂ nanoparticles (P25) are investigated, and a discrepancy is discovered between the light-harvesting capability and the power conversion efficiency. The charge transport and recombination are studied by the electrochemical impedance spectroscopy and the open-circuit voltage decay technique. Results show that the nanotube photoanode owns a longer electron diffusion length and a larger electron lifetime than the nanoparticle one, which can compensate for the loss of light absorption. The enhanced electron collection efficiency observed is attributed to the facilitated charge carrier pathways in the photoanode composed by the separated TiO₂ nanotubes fabricated in this work. Therefore, the TiO₂ nanotubes synthesized by this method are verified to have good electronic properties, which might find applications not only in photovoltaic, but also in catalysis, sensors, and other areas.     

g-C_3N_4基多功能层光阳极在光电化学水分解中的应用（英文）

光电化学分解水制氢可以一并解决环境问题和能源危机,因而成为研究热点.由于TiO_2 禁带宽度较大,不能有效吸收太阳光中的可见光,使光电化学分解水制氢的应用受限.g-C_3N_4的禁带宽度约为2.7 e V,能有效吸收可见光,但g-C_3N_4薄膜制备研究较少.我们通过热聚缩合法直接在FTO导电玻璃上制备出g-C_3N_4薄膜,发现其光电化学分解水制氢稳定性不高,选择易制备的TiO_2 作为保护层可以提高g-C_3N_4的耐用性.此外,为提高g-C_3N_4光生电子空穴对的分离能力,依靠Co-Pi对光生空穴的捕获作用而将其覆盖在最外层.因此本文首次制备一种新型的g-C_3N_4/TiO_2 /Co-Pi光阳极用于光电化学分解水制氢,其中g-C_3N_4用作光吸收层,TiO_2 用作保护层,Co-Pi用作空穴捕获层.并在此基础上,通过扫描电子显微镜(SEM),X射线衍射(XRD),紫外可见光谱(UV-Vis)等手段研究了g-C_3N_4/TiO_2 /Co-Pi光阳极的形貌特征和光电化学性能.SEM、EDS和XRD结果表明,g-C_3N_4/TiO_2 /Co-Pi光阳极被成功制备在了FTO导电玻璃上,厚度约为3μm.UV-Vis测试表明,g-C_3N_4的光吸收边约为470 nm,可以有效地吸收可见光,并且g-C_3N_4的框架结构使光多次反射折射增加了光的捕获能力,由此可见,g-C_3N_4能够发挥很好的光吸收层作用.通过对g-C_3N_4光阳极,g-C_3N_4/TiO_2 光阳极和g-C_3N_4/TiO_2 /Co-Pi光阳极的电流电压测试发现,g-C_3N_4/TiO_2 光阳极的光电流密度小于g-C_3N_4光阳极,而g-C_3N_4/TiO_2 /Co-Pi光阳极的光电流密在可逆氢电极1.1 V下达到了0.346 mA?cm–2,约为单独g-C_3N_4光阳极的3.6倍.这说明Co-Pi是提升g-C_3N_4光电化学性能的主要因素.电化学阻抗测试结果发现,g-C_3N_4/TiO_2 /Co-Pi光阳极的界面电荷转移电阻小于g-C_3N_4光阳极的,这表明g-C_3N_4/TiO_2 /Co-Pi光阳极界面处载流子转移较快,同时也能促进内部光生电子空穴对的分离,整体性能的提高应该主要归因于Co-Pi对光生空穴的捕获作用.恒电压时间测试展示出g-C_3N_4/TiO_2 /Co-Pi光阳极的光电流密度在2 h测试过程中没有明显下降,表明g-C_3N_4/TiO_2 /Co-Pi光阳极是相当稳定的,具有良好的耐用性,归因于TiO_2 和Co-Pi的共同保护作用,主要归因于TiO_2 层对FTO导电玻璃上的g-C_3N_4薄膜保护,从电化学沉积Co-Pi到所有测试结束.总体而言,g-C_3N_4/TiO_2 /Co-Pi光阳极加强的光电化学性能归因于以下几个因素:(1)g-C_3N_4优异的光吸收能力;(2)TiO_2 稳定的保护提升了g-C_3N_4薄膜的耐用性;(3)Co–Pi对光生空穴的捕获有效促进了光生电子空穴对的分离.     

Enhancing the Performance of Dye‐Sensitized Solar Cells with a Gold‐Nanoflowers Box

To improve the electron collection, electron lifetime, and light‐harvesting efficiency of dye‐sensitized solar cells simultaneously, Au nanoflowers were prepared and used to cover the entire TiO₂ film. Deposition of Au nanoflowers around the TiO₂ film formed a light‐scattering “box” that covered the entire TiO₂ film. Compared with a light‐scattering layer that only covers the top surface of TiO₂, the Au‐nanoflowers box exhibited better light‐harvesting efficiency due to omnidirectional light scattering, faster electron transport (attributed to the formation of electron channels between the metallic Au nanoflowers and the electron‐collection electrode), and slower charge recombination. As a consequence, the short‐circuit photocurrent and open‐circuit photovoltage were both enhanced significantly, which improved the power conversion efficiency from 8.12 to 10.91 % (34 %) when an Au‐nanoflowers box was wrapped around the photoanode.     

偏压控制Cu2O和Cu在TiO2表面的生长及其光电化学性质

基于TiO₂/Ti 电极在含Cu²⁺溶液中的循环伏安图,调节电沉积的沉积电压,我们在TiO₂平整表面制备出Cu₂O和/或Cu颗粒. 通过扫描电镜（SEM）、X射线衍射（XRD）和X射线光电子能谱（XPS）表征,发现Cu₂O和Cu有不同的生长机制：Cu₂O颗粒在TiO₂表面分散结晶,而Cu颗粒是在已生长的颗粒上成核,从而形成堆积颗粒结构. 这是由于在Cu₂O/TiO₂界面和Cu/TiO₂界面形成不同的能带结构,使得电子的转移方式不同. 与纯TiO₂光阳极比较,可以观察到Cu₂O/TiO₂和Cu/TiO₂异质结构的光电流均有显著增强. 特别地,存在一个电压区间使得Cu₂O和Cu同时生长在TiO₂表面,此时对应的光电流比较稳定并且能达到最大. 紫外-可见（UV-Vis）漫反射光谱、电化学阻抗谱（EIS）和光电流-电压特性曲线均显示,Cu₂O和Cu明显有助于光的可见光吸收,同时Cu/TiO₂在光电转换过程中显示更宽波段的可见光利用率. 此外,开路电压的增加、有效的电荷分离和电极/电解质界面上载流子的快速迁移也增强了材料的光电化学性质.