Stacked graphene–TiO2 photoanode via electrospray deposition for highly efficient dye-sensitized solar cells

A series of TiO₂–graphene stacked photoanodes for dye-sensitized solar cells (DSSCs) were fabricated by electrospray (E-spray) deposition. Among devices incorporating single graphene layer with different deposition times, device with 1 min graphene deposition gave the best performance. For multi-graphene-layer involved devices, best result was obtained with 3 layers of graphene. The working principles were analyzed by scanning electron microscopy, transmittance spectra, electrochemical impedance spectroscopy and incident-photo-conversion efficiency data. We found that although graphene layers incorporated in TiO₂ photoanode slightly decreased dye adsorption, they were able to significantly improve the electron transport, and the charge recombination at the interfaces of TiO₂/dye and TiO₂/electrolyte were greatly suppressed, leading to dramatic improvement in power conversion efficiency. When inserting three layers of pure graphene into the TiO₂ photoanode, high efficiency of 8.9% was obtained, constituting an over 23.6% improvement. Further increasing graphene layers to five, although electron lifetimes is the longest, both the largest charge transfer resistance and the least amount of the dye loading lead to the lowest device efficiency. Our work demonstrated, that pure graphene layer can be successfully incorporated into TiO₂ photoanode by E-spray method with easiness of thickness control and the photoanode with graphene/TiO₂ alternatively layered structure is an excellent candidate for DSSCs.     

Honeycomb‐Like Organized TiO2 Photoanodes with Dual Pores for Solid‐State Dye‐Sensitized Solar Cells

A solid‐state dye‐sensitized solar cell (ssDSSC) with 7.4% efficiency at 100 mW/cm² is reported. This efficiency is one of the highest observed for N719 dye. High performance is achieved via a honeycomb‐like, organized mesoporous TiO₂ photoanode with dual pores, high porosity, good interconnectivity, and excellent light scattering properties. The TiO₂ photoanodes are prepared without any TiCl₄ treatment via a one‐step, direct self‐assembly of hydrophilically preformed TiO₂ nanocrystals and poly(vinyl chloride)‐g‐poly(oxyethylene methacrylate) (PVC‐g‐POEM) graft copolymer as a titania source and a structure‐directing agent, respectively. Upon controlling the secondary forces between the polymer/TiO₂ hybrid and the solvent by varying the amounts of HCl/H₂O mixture or toluene, honeycomb‐like structures are generated to improve light scattering properties. Such multifunctional nanostructures with dual pores provide good pore‐filling of solid polymer electrolyte with large volume, enhanced light harvesting and reduced charge recombination, as confirmed by reflectance spectroscopy, incident photon‐to‐electron conversion efficiency (IPCE), and electrochemical impedance spectroscopy (EIS) analysis.     

Hematite Photoanodes: Synergetic Enhancement of Light Harvesting and Charge Management by Sandwiched with Fe2TiO5/Fe2O3/Pt Structures

Efficient charge separation and transport as well as high light absorption are key factors that determine the efficiency of photoelectrochemical (PEC) water splitting devices. Here, a PEC device consisting of a hematite nanoporous film deposited on Pt nanopillars, followed by the decoration with an Fe₂TiO₅ passivation layer, is designed and fabricated. This structure can largely improve the light absorption in the composite materials, and significantly enhance the water oxidation performance of hematite photoanodes. The Fe₂TiO₅ thin shell and Pt underlayer significantly improve the interfacial charge transfer while minimizing the hole‐migration length in Fe₂O₃ photoanodes, leading to a drastically increased photocurrent density. Specially, the Fe₂TiO₅/Fe₂O₃/Pt photoanode yields an excellent photoresponse for PEC water splitting reactions with 1.0 and 2.4 mA cm^?2 obtained at 1.23 and 1.6 V_RHE under AM 1.5G illumination in 1 m KOH. The resulting photocurrents are 2.5 times enhanced compared to a pristine Fe₂O₃ photoanode of the same geometry. These results demonstrate a synergistic charge transfer effect of Fe₂TiO₅ and Pt layers on hematite for the improvement of PEC water oxidation.     

Metal–Organic Framework Derived Narrow Bandgap Cobalt Carbide Sensitized Titanium Dioxide Nanocage for Superior Photo‐Electrochemical Water Oxidation Performance

Despite recent progress in photo‐electrochemical (PEC) water oxidation systems for TiO₂‐based photoanodes, PEC performance improvement is still seriously hampered due to poor carrier transport efficiency and sluggish surface water oxidation kinetics of pristine TiO₂. Herein, for the first time a brand new metal–organic framework (MOF)‐derived Co₃C nanosheet with narrow bandgap energy is demonstrated, to effectively sensitize TiO₂ hollow cages as a heterostructure photoanode for PEC water oxidation. It is found that MOF‐derived Co₃C nanosheet with narrow bandgap characteristic can simultaneously accelerate the surface water oxidation kinetics and extend the light harvesting range of pristine TiO₂. Meanwhile, a uniquely matched type‐II heterojunction constructed between MOF‐derived Co₃C and TiO₂ results in an evidently spontaneous e^?/h⁺ separation. MOF‐derived Co₃C/TiO₂ heterostructure photoanodes bring about drastically improved PEC water oxidation performance. Specifically, MOF‐derived Co₃C‐3/TiO₂ photoanode with an optimized content of Co₃C achieves the highest photocurrent density and charge separation efficiency of 2.6 mA cm^?2 and 92.6% at 1.23 V versus reversible hydrogen electrode, corresponding to 201% and 152% improvement compared with pristine TiO₂ nanocages. The ingeniously prepared MOF‐derived Co₃C carbide with narrow bandgap energy as a cocatalyst paves new way to construct potentially high performance solar‐energy conversion system.     

A multifunctional photoanode made of titania submicrospheres and titania nanoparticles modified titania nanotube arrays on FTO glass for dye-sensitized solar cells

We have proposed a novel engineered multifunctional photoanode which consists of one-dimensional (1D) TiO₂ nanotube arrays modified by TiO₂ nanoparticles as a bottom layer and three-dimensional (3D) TiO₂ submicrospheres as a top layer by a simple one-time chemical bath deposition method. This 1D-3D bilayer photoanode simultaneously possesses the incompatible features such as high specific surface area, pronounced light-scattering ability, and fast electron transport. The novel 1D-3D bilayer photoanode demonstrates a power conversion efficiency (PCE) of 6.93%, leading to a 26.5% increment of PCE compared with that of TiO₂ bare nanotube arrays (5.48%).     

An Effective Approach for High‐Efficiency Photoelectrochemical Solar Cells by Using Bifunctional DNA Molecules Modified Photoanode

This paper firstly reports the effect of deoxyribonucleic acid (DNA) molecules extracted from chickpea and wheat plants on the injection/recombination of photogenerated electrons and sensitizing ability of dye‐sensitized solar cells (DSSCs). These high‐yield DNA molecules are applied as both linker bridging unit as well as thin tunneling barrier (TTB) at titanium dioxide (TiO₂ )/dye interface, to build up high‐efficient DSSCs. With its favorable energy levels, effective linker bridging role, and double helix structure, bifunctional DNA modifier shows an efficient electron injection, suppressed charge recombination, longer electron lifetime, and higher light harvesting efficiency, which leads to higher photovoltaic performance. In particular, a photoconversion efficiency (PCE) of 9.23% is achieved by the binary chickpea and wheat DNA‐modified TiO₂ (CW@TiO₂) photoanode. Furthermore, time‐resolved fluorescence spectroscopy measurements confirm a better electron transfer kinetics for DNA‐modified TiO₂ photoanodes, implying a higher electron transfer rate (k_ET). This work highlights a great contribution for the photoanodes that are linked with DNA molecule, which act as both bridging unit and TTB to control the charge recombination and injection dynamics, and hence, boost the photovoltaic performance in the DSSCs.     

Simultaneous Power Conversion Efficiency and Stability Enhancement of Cs2AgBiBr6 Lead‐Free Inorganic Perovskite Solar Cell through Adopting a Multifunctional Dye Interlayer

Perovskite solar cells (PSCs) are highly promising next‐generation photovoltaic devices because of the cheap raw materials, ideal band gap of ≈1.5 eV, broad absorption range, and high absorption coefficient. Although lead‐based inorganic‐organic PSC has achieved the highest power conversion efficiency (PCE) of 25.2%, the toxic nature of lead and poor stability strongly limits the commercialization. Lead‐free inorganic PSCs are potential alternatives to toxic and unstable organic‐inorganic PSCs. Particularly, double‐perovskite Cs₂AgBiBr₆‐based PSC has received interests for its all inorganic and lead‐free features. However, the PCE is limited by the inherent and extrinsic defects of Cs₂AgBiBr₆ films. Herein, an effective and facile strategy is reported for improving the PCE and stability by introducing an N719 dye interlayer, which plays multifunctional roles such as broadening the absorption spectrum, suppressing the charge carrier recombination, accelerating the hole extraction, and constructing an appropriate energy level alignment. Consequently, the optimizing cell delivers an outstanding PCE of 2.84%, much improved as compared with other Cs₂AgBiBr₆‐based PSCs reported so far in the literature. Moreover, the N719 interlayer greatly enhances the stability of PSCs under ambient conditions. This work highlights a useful strategy to boost the PCE and stability of lead‐free Cs₂AgBiBr₆‐based PSCs simultaneously, accelerating the commercialization of PSC technology.     

Effectively Utilizing NIR Light Using Direct Electron Injection from Up‐Conversion Nanoparticles to the TiO2 Photoanode in Dye‐Sensitized Solar Cells

TiO₂/NaYF₄:Yb³⁺,Er³⁺ nano‐heterostructures are prepared in situ on the TiO₂ photoanode of dye‐sensitized solar cells (DSCs). Transmission electron microscopy (TEM) and high‐resolution (HR)‐TEM confirm the formation of TiO₂/NaYF₄:Yb³⁺,Er³⁺ nano‐heterostructures. The up‐converted fluorescence spectrum of the photoanode containing the nano‐heterostructure confirms electron injection from NaYF₄:Yb³⁺,Er³⁺ to the condution band (CB) of TiO₂. When using a photoanode containing the nano‐heterostructure in a DSC, the overall efficiency (η) of the device is 17% higher than that of a device without the up‐conversion nanoparticles (UCNPs) and 13% higher than that of a device containing mixed TiO₂ and UCNPs. Nano‐heterostructures of TiO₂/NaYF₄:Yb³⁺,Tm³⁺ and TiO₂/NaYF₄:Yb³⁺,Ho³⁺ can also be prepared in situ on TiO₂ photoanodes. The overall efficiency of the device containing TiO₂/NaYF₄:Yb³⁺,Ho³⁺ nano‐heterostructures is 15% higher than the control device without UCNPs. When nano‐heterostructures of TiO₂/NaYF₄:Yb³⁺,Tm³⁺ are used, the open‐circuit voltage (V_oc) and the short‐circuit current density (J_sc) are all slightly decreased. The effect of the different UCNPs results from the different energy levels of Er³⁺, Tm³⁺, and Ho³⁺. These results demonstrate that utilizing the UCNPs with the apporpriate energy levels can lead to effective electron injection from the UCNPs to the CB of TiO₂, effectively improving the photocurrent and overall efficiency of DSCs while using NIR light.     

TiO2 Microspheres with Controllable Surface Area and Porosity for Enhanced Light Harvesting and Electrolyte Diffusion in Dye‐Sensitized Solar Cells

An optimized configuration of TiO₂ microspheres in photoanodes is of great importance to prepare highly efficient dye‐sensitized solar cells (DSSCs). In this work, TiO₂ microspheres with tunable diameter, pore size, and porosity are synthesized by subtly adjusting the synthesizing conditions, including ratios of deionized water, ammonia, and ethanol, respectively. TiO₂ microspheres are obtained with large pore sizes and a high porosity without sacrificing specific surface areas. In addition, the effect of their porosity and pore size on the performance of DSSCs is investigated. As confirmed by the dye‐loading ability and electrolyte diffusion resistance, the large mesopores and the high porosity of the TiO₂ microspheres can improve dye adsorption and facilitate electrolyte diffusion, giving rise to a high light‐harvesting and electron collection efficiency. Consequently, the highest photocurrent of 19.21 mA cm^?2 and a power conversion efficiency of 9.98% are obtained by using the TiO₂ microspheres with the highest porosity, compared with a 9.29% efficiency demonstrated by the lowest porosity (an improvement of 7.4%). By modifying the interconnection and the external pores of the microspheres photoanode, a high efficiency of 11.67% is achieved for a DSSC based on the most potent TiO₂ microspheres.     

Nickel Phosphide Clusters Sensitized TiO2 Nanotube Arrays as Highly Efficient Photoanode for Photoelectrocatalytic Urea Oxidation

The photoelectrocatalytic urea oxidation reaction (PEC-UOR) holds a great promise for the wastewater remediation and energy production. However, the low efficiency of semiconductor/cocatalysts type photoanodes for UOR restricts their applications in photoelectrocatalytic system. Herein, a new semiconductor/cocatalyst, Ni₂P clusters sensitized TiO₂ nanotube arrays photoanode (Ni₂P/TiO₂-NTAs) for PEC-UOR with high efficiency are developed. The 1D TiO₂-NTAs structure accelerates urea molecules diffusion and promotes CO₂ gas release at the electrode interface. Meanwhile, Ni₂P is also beneficial to urea molecule absorption and CO₂ desorption and enable to lower the energy barrier for amine (N H) dehydrogenation. Furthermore, the robust interfacial charge transfer pathway between Ni₂P and TiO₂ interface promotes the separation of photogenerated electrons and holes and the transfer of photogenerated electrons from Ni₂P to TiO₂. Therefore, this photoanode shows excellent PEC-UOR performance with the potential of 1.43 V versus reversible hydrogen electrode (RHE) when the current density reaches 10 mA cm⁻², which is much lower than that of 2.24 V versus RHE and 1.58 V versus RHE for TiO₂-NTAs and Ni(OH)₂/TiO₂-NTAs, respectively.     

The TiO<Subscript>2</Subscript> Hierarchical Structure with Nanosheet Spheres for Improved Photoelectric Performance in Dye-Sensitized Solar Cells

A bi-layer photoanode is successfully fabricated for dye-sensitized solar cells (DSSCs) composed of P25/TiO₂ nanorod (P25/TNR) as the underlayer and TiO₂ nanosheet spheres (TNSs) as the light-scattering layer. Notably, the P25-TNR provides multiple functions, including more dye loading, more efficient charge transport and a lower electron recombination rate for the photoanode. Besides, the unique structure of TNS can significantly improve the light-harvesting capacity, boosting the light-harvesting efficiency. Therefore, an enhanced short-circuit current and power conversion efficiency of 18.04 mA cm^?2 and 5.99%, respectively, were achieved for the P25/TNR-TNS-based DSSC, which was better than that of the P25-TNS-based (15.17 mA cm^?2, 5.36%) and bare TNS-based (11.43 mA cm^?2, 4.14%) DSSCs. This indicates that this bi-layer structure effectively combines the advantages of the one-dimensional (1D) nanostructure and three-dimensional (3D) hierarchical structure. In short, this work demonstrates the possibility of fabricating desirable photoanodes for high-performance DSSCs by rational design of nanostructures and effective combination of multi-functional components.     

Pulsed Laser Ablation Based Synthesis of PbS‐Quantum Dots‐Decorated One‐Dimensional Nanostructures and Their Direct Integration into Highly Efficient Nanohybrid Heterojunction‐Based Solar Cells

The pulsed laser deposition (PLD) technique is used for the direct fabrication of nanohybrid heterojunctions (NH‐HJs) solar cells exhibiting high PCE and excellent stability in air without any encapsulation and/or resorting to any surface treatment, ligand engineering and/or post‐synthesis processing. The NH‐HJs are achieved through the PLD‐based decoration of hydrothermally‐grown one‐dimensional TiO₂ nanorods (TiO₂‐NRs) by PbS quantum dots (PbS‐QDs). By optimizing both the amount of PbS‐QDs (via the number of laser ablation pulses) and the length of the TiO₂‐NRs, it is possible to achieve optimal NH‐HJs based PV devices with high power conversion efficiency (PCE) of 4.85%. This high PCE is found to occur for an optimal length of the NRs (≈290 nm) which coincides with the average penetration depth of PbS‐QDs into the porous TiO₂‐NRs matrix, leading thereby to the formation of the largest extent of NH‐HJs. Most importantly, the PCE of these novel devices is found to be fairly stable for several months under ambient air. The addition of single‐wall carbon nanotubes (SWCNTs) onto the TiO₂‐NRs prior to their decoration by PbS‐QDs is shown to further enhance their PCE to a value as high as 5.3%, because of additional light absorption and improved charge collection ensured by SWCNTs.     

Co-sensitization of N719 and RhCL dyes on carboxylic acid treated TiO2 for enhancement of light harvesting and reduced recombination

We report herein the findings from a systematic study conducted on the effect of co-sensitization of two dyes i.e. N719 and rhodamine 19 perchlorate (RhCL) onto TiO₂ electrodes (thickness: 4 μm), which were treated by three different carboxylic acids namely, formic acid (FA), oxalic acid (OA) and citric acid (CA). Co-sensitization was carried out by first loading the carboxylic acid treated TiO₂ electrodes with RhCL dye followed by N719. The amount of dye loading was estimated from the UV/vis spectroscopy and the results show that: (i) the amount of individual dye loading (RhCL or N719) onto carboxylic acid treated TiO₂ electrodes is, in general, higher than that onto untreated TiO₂ electrodes; (ii) the amount of individual dye loading is highest for OA-treated and lowest for CA-treated TiO₂ electrodes; and (iii) the co-sensitization leads to highest loading of N719 onto FA-treated and lowest on CA-treated TiO₂ electrodes. The results of dye sensitized solar cells fabricated using these acid treated TiO₂ electrodes revealed that the efficiency (η) is higher for electrodes having higher loading of N719 dye. For single N719 dye loading, the highest η of 4.5% is observed for OA-treated TiO₂; while upon co-sensitization the highest η of ∼5.3% is observed for FA treated TiO₂. A detailed analyses of Fourier transform infrared spectroscopy (FTIR), UV–Visible, impendence, incident photon to current efficiency (IPCE) results show that η enhancement occurs due to the following factors: (i) increased short circuit current density (J_SC) owing to high N719 dye loading which enhances light harvesting; (ii) improved IPCE; (iii) increased open circuit voltage (V_OC) due to an upward shift of the conduction band edge (CBE) and quasi Fermi level; and (iv) suppressed back electron transfer.     

Tungsten Trioxide Nanostructures for Photoelectrochemical Water Splitting: Material Engineering and Charge Carrier Dynamic Manipulation

To address the energy crisis and environmental problems, the applications of solar energy have received intensive attention. Converting solar energy to hydrogen using a photoelectrochemical (PEC) cell is one of the most promising approaches to meet future energy demands. As an earth abundant metal oxide, tungsten trioxide (WO₃), which has a moderate band gap (2.5–2.7 eV), ideal valence band position, and high resistance to photocorrosion, has been widely utilized in PEC photoanodes. To obtain a WO₃ photoanode with high PEC efficiency, tremendous efforts have been made to improve the light absorption capacity, charge carrier dynamics, and oxygen evolution activity. In this report, the recent advances in WO₃ photoanode optimization, including morphology design, dopants doping, heterojunction fabrication, and surface modification are summarized. In this review, these developments and representative applications of WO₃ photoanodes in unassisted water splitting devices are also discussed. Finally, perspectives on the significant challenges and future prospects for the development of WO₃ photoanodes for PEC water splitting are provided.     

Increasing the Efficiency of Organic Dye‐Sensitized Solar Cells over 10.3% Using Locally Ordered Inverse Opal Nanostructures in the Photoelectrode

3D inverse opal (3D‐IO) oxides are very appealing nanostructures to be integrated into the photoelectrodes of dye‐sensitized solar cells (DSSCs). Due to their periodic interconnected pore network with a high pore volume fraction, they facilitate electrolyte infiltration and enhance light scattering. Nonetheless, preparing 3D‐IO structures directly on nonflat DSSC electrodes is challenging. Herein, 3D‐IO TiO₂ structures are prepared by templating with self‐assembled polymethyl methacrylate spheres on glass substrates, impregnation with a mixed TiO₂:SiO₂ precursor and calcination. The specific surface increases from 20.9 to 30.7 m² g^?1 after SiO₂ removal via etching, which leads to the formation of mesopores. The obtained nanostructures are scraped from the substrate, processed as a paste, and deposited on photoelectrodes containing a mesoporous TiO₂ layer. This procedure maintains locally the 3D‐IO order. When sensitized with the novel benzothiadiazole dye YKP‐88, DSSCs containing the modified photoelectrodes exhibit an efficiency of 10.35% versus 9.26% for the same devices with conventional photoelectrodes. Similarly, using the ruthenium dye N719 as sensitizer an efficiency increase from 5.31% to 6.23% is obtained. These improvements originate mainly from an increase in the photocurrent density, which is attributed to an enhanced dye loading obtained with the mesoporous 3D‐IO structures due to SiO₂ removal.     

Trifunctional TiO2 Nanoparticles with Exposed {001} Facets as Additives in Cobalt‐Based Porphyrin‐Sensitized Solar Cells

In this study, highly mesoporous TiO₂ composite photoanodes composed of functional {001}‐faceted TiO₂ nanoparticles (NPs) and commercially available 20 nm TiO₂ NPs are employed in efficient porphyrin‐sensitized solar cells together with cobalt polypyridyl‐based mediators. Large TiO₂ NPs (approximately 50 nm) with exposed {001} facets are prepared using a fast microwave‐assisted hydrothermal (FMAH) method. These unique composite photoanodes favorably mitigate the aggregation of porphyrin on the surface of TiO₂ NPs and strongly facilitate the mass transport of cobalt‐polypyridyl‐based electrolytes in the mesoporous structure. Linear sweep voltammetry reveals that the transportation of Co(polypyridyl) redox is a diffusion‐controlled process, which is highly dependent on the porosity of TiO₂ films. Electrochemical impedance spectroscopy confirms that the FMAH TiO₂ NPs effectively suppress the interfacial charge recombination toward [Co(bpy)₃]³⁺ because of their oxidative {001} facets. In an optimal condition of 40 wt% addition of FMAH TiO₂ NPs in the final formula, the power conversion efficiency of the dye‐sensitized cells improves from 8.28% to 9.53% under AM1.5 (1 sun) conditions.     

Improved‐Performance Dye‐Sensitized Solar Cells Using Nb‐Doped TiO2 Electrodes: Efficient Electron Injection and Transfer

Well‐crystallized Nb‐doped anatase TiO₂ nanoparticles are prepared by a novel synthetic route and successfully used as the photoanode of dye‐sensitized solar cells (DSSCs). The homogenous distribution of Nb in the TiO₂ lattice is confirmed by scanning transmission electron microscopy (STEM) elemental mapping and line‐scanning analyses. After Nb doping, the conductivity of the TiO₂ powder increases, and its flat‐band potential (V_fb) has a positive shift. The energy‐conversion efficiency of a cell based on 5.0 mol% Nb‐doped TiO₂ is significantly better, by about 18.2%, compared to that of a cell based on undoped TiO₂. The as‐prepared Nb‐doped TiO₂ material is proven in detail to be a better photoanode material than pure TiO₂, and this new synthetic approach using a water‐soluble precursor provides a simple and versatile way to prepare excellent photoanode materials.     

Interface Functionalization of Photoelectrodes with Graphene for High Performance Dye‐Sensitized Solar Cells

The microstructures of photo‐ and counter‐electrodes play critical roles in the performance of dye‐sensitized solar cells (DSSCs). In particular, various interfaces, such as fluorinated‐tin oxide (FTO)/TiO₂, TiO₂/TiO₂, and TiO₂/electrolyte, in DSSCs significantly affect the final power conversion efficiency (PCE). However, research has generally focused more on the design of various nanostructured semiconducting materials with emphasis on optimizing chemical or/and physical properties, and less on these interface functionalizations for performance improvement. This work explores a new application of graphene to modify the interface of FTO/TiO₂ to suppress charge recombination. In combination with interfaces functionalization of TiO₂/TiO₂ for low charge‐transport resistance and high charge‐transfer rate, the final PCE of DSSC is remarkably improved from 5.80% to 8.13%, achieving the highest efficiency in comparison to reported graphene/TiO₂‐based DSSCs. The method of using graphene to functionalize the surface of FTO substrate provides a better alternative method to the conventional pre‐treatment through hydrolyzing TiCl₄ and an approach to reduce the adverse effect of microstructural defect of conducting glass substrate for electronic devices.     

Hematite Films Decorated with Nanostructured Ferric Oxyhydroxide as Photoanodes for Efficient and Stable Photoelectrochemical Water Splitting

Hematite photoanodes are decorated with nanostructured FeOOH by photoelectrodeposition. An obvious cathodic shift in the photocurrent onset potential is observed, while four‐times enhancement of photocurrent density enhancement is acheived with FeOOH present. This can be ascribed to the high reaction area for the structure and high electrocatalytic activity of nanostructured FeOOH, which increases the amount of photogenerated holes involved in the water oxidation reaction and accelerates the kinetics of water oxidation. Furthermore, the obtained Fe₂O₃/FeOOH photoanode achieves considerable O₂ evolution rate (10.1 μmol h^?1 cm^?2) under AM 1.5 G illumination and is maintained for as long as 70 h. The Fe₂O₃/FeOOH films show visible light response, high photocurrent density, and long‐term stability, and they are well qualified photoanode materials and a promising candidate for photoelectrochemical water splitting.     

Influence of the Relative Humidity on the Performance of Polymer/TiO2 Photovoltaic Cells

Hydrolysis of titanium(IV ) isopropoxide (TTIP) is a well‐known method for the fabrication of TiO₂. Normally it is made via a sol–gel reaction in the presence of water. In this paper we report on the preparation of flat TiO₂ films for conjugated polymer/TiO₂ photovoltaic cells, from a TTIP/isopropanol solution. It is shown that the morphological structure of the TiO₂ film is strongly dependent on the relative humidity during spin‐coating of the TTIP/isopropanol solution. In bilayer devices consisting of TiO₂/poly[2‐methoxy‐5‐(3′,7′‐dimethyloctyloxy)‐1,4‐phenylene vinylene] (MDMO‐PPV), a low relative humidity (< 25 %, room temperature) is needed in order to form smooth, transparent TiO₂ films. Increasing the relative humidity results in porous TiO₂ films with a high surface roughness, which leads to shunted devices. Apart from bilayer devices, bulk‐heterojunction (BHJ) hybrid TiO₂:MDMO‐PPV photovoltaic cells have been made, by spin‐coating a mixture of TTIP and MDMO‐PPV in toluene. Again a strong relation was found between the relative humidity during spin‐coating and the current–voltage characteristics of the devices. However, in contrast to the bilayer devices, the best BHJ devices were made at higher relative humidity. The observed performance dependence on relative humidity is discussed in relation to the TiO₂ morphology.