Stable and Efficient Perovskite Solar Cells Based on Titania Nanotube Arrays

Emerging Trends in Electron Transport Layer Development for Stable and Efficient Perovskite Solar Cells

Perovskite solar cells (PSCs) stand at the forefront of photovoltaic research, with current efficiencies surpassing 26.1%. This review critically examines the role of electron transport materials (ETMs) in enhancing the performance and longevity of PSCs. It presents an integrated overview of recent advancements in ETMs, like TiO2, ZnO, SnO2, fullerenes, non-fullerene polymers, and small molecules. Critical challenges are regulated grain structure, defect passivation techniques, energy level alignment, and interfacial engineering. Furthermore, the review highlights innovative materials that promise to redefine charge transport in PSCs. A detailed comparison of state-of-the-art ETMs elucidates their effectiveness in different perovskite systems. This review endeavors to inform the strategic enhancement and development of n-type electron transport layers (ETLs), delineating a pathway toward the realization of PSCs with superior efficiency and stability for potential commercial deployment.

Champion Device Architectures for Low-Cost and Stable Single-Junction Perovskite Solar Cells

High power conversion efficiencies (PCE), low energy payback time (EPBT), and low manufacturing costs render perovskite solar cells (PSCs) competitive; however, a relatively low operational stability impedes their large-scale deployment. In addition, state-of-the-art PSCs are made of expensive materials, including the organic hole transport materials (HTMs) and the noble metals used as the charge collection electrode, which induce degradation in PSCs. Thus, developing inexpensive alternatives is crucial to fostering the transition from academic research to industrial development. Combining a carbon-based electrode with an inorganic HTM has shown the highest potential and should replace noble metals and organic HTMs. In this review, we illustrate the incorporation of a carbon layer as a back contact instead of noble metals and inorganic HTMs instead of organic ones as two cornerstones for achieving optimal stability and economic viability for PSCs. We discuss the primary considerations for the selection of the absorbing layer as well as the electron-transporting layer to be compatible with the champion designs and ultimate architecture for single-junction PSCs. More studies regarding the long-term stability are still required. Using the recommended device architecture presented in this work would pave the way toward constructing low-cost and stable PSCs.

Cube-like anatase TiO2 mesocrystals as effective electron-transporting materials toward high-performance perovskite solar cells

Electron-transporting materials (ETMs) with higher carrier mobility and a suitable band gap structure play a significant role in determining the photovoltaic performance of perovskite solar cells (PSCs). Herein, cube-like mesoporous single-crystal anatase TiO₂ (Meso-TiO₂) nanoparticles synthesized by using a facile hydrothermal method were utilized as an efficient ETM for PSCs. The superior semiconducting properties of the Meso-TiO₂ based ETM enabled the best power conversion efficiency (PCE) of 20.05% for a PSC. Moreover, the device retained 80% of its initial PCE after being stored in ambient conditions for 20 days under 25 ± 5% relative humidity. In contrast to the commercial TiO₂ ETM, the Meso-TiO₂ ETM based PSC showed a distinguished interface with better interfacial conditions and improved carrier extraction originating from the cube-like mesoporous single-crystal anatase TiO₂ ETM.

Boosting the Photoelectrochemical Water Oxidation Performance of TiO2 Nanotubes by Surface Modification Using Silver Phosphate

Photoelectrocatalytic approaches are fascinating options for long-lasting energy storage through the transformation of solar energy into electrical energy or hydrogen fuel. Herein, we report a facile method of fabricating a composite electrode of well-aligned TiO2 nanotubes (TNTs) decorated with photodeposited silver phosphate (Ag3PO4) nanoparticles. Assessment of the optical, physiochemical and photoelectrochemical features demonstrated that the fabricated TNTs/Ag3PO4 films showed a substantially boosted photocurrent response of 0.74 mA/cm2, almost a 3-fold enrichment in comparison with the pure TNTs. Specifically, the applied bias photon-to-current efficiency of the fabricated TNTs/Ag3PO4 composite electrode was 2.4-fold superior to that of the pure TNTs electrode. In these TNTs/Ag3PO4 photoanodes, the introduction of Ag3PO4 over TNTs enhanced light absorption and improved charge transfer and surface conductivity. The developed process can be generally applied to designing and developing efficient contact interfaces between photoanodes and numerous cocatalysts.

Preparation of low-cost perovskite solar cells with high-quality perovskite films in an ambient atmosphere

High-quality perovskite films are extremely crucial to obtain perovskite solar cells with excellent photovoltaic performance, especially for carbon-based hole transport materials (HTM)-free perovskite solar cells. In this work, a facile and low-cost double two-step method (DT-method) is developed to prepare uniform and pinhole-free CH₃NH₃PbI₃perovskite films in an ambient atmosphere by utilizing the dissolution-recrystallization of PbI₂in DMF. That is to spin-coat PbI₂and CH₃NH₃I solution sequentially onto pristine perovskite films prepared by the conventional two-step method. The solar cells fabricated by the DT-method show a dramatic performance improvement, includingV_oc,J_sc, and fill factor reach 0.85 V, 15.56 mA cm^-2, and 0.58 respectively, which increase power conversion efficiency from 3.93% to 7.58% compared with the conventional two-step method. The improvement in performance and stability of solar cells is mainly due to the higher coverage of perovskite films onto the underlying mesoporous TiO₂layer and a negligible amount of PbI₂residue, which can effectively reduce charge recombination and promote the rapid transfer of charge carriers. In summary, this work presents a process for preparing carbon-based HTM-free perovskite solar cells (PSCs) in a high-humidity atmospheric environment (60%-85%). This simple device structure and preparation condition can greatly reduce the production threshold and cost of PSCs.

TiO2 Mesocrystals Processed at Low Temperature as the Electron-Transport Material in Perovskite Solar Cells

TiO₂ is the most widely used material for preparing the electron-transporting layer (ETL) in perovskite solar cells (PSCs). However, it requires a high-temperature sintering process. Moreover, the intrinsic defects and low electron mobility of TiO₂ ETLs cause instability and hysteresis effects in PSCs. In this study, a mesoporous film composed of anatase TiO₂ mesocrystals was facilely fabricated by a low-temperature route and then used as an ETL in PSCs for the first time. A satisfactory efficiency of 20.26 % can be achieved through delicate control of the entire device fabrication procedure. The optimal device, with an area of 1 cm² , achieves an efficiency of 17.07 %. In comparison to the common TiO₂ ETLs, those composed of TiO₂ mesocrystals show the enhanced electron extraction and suppression of charge accumulation at the perovskite/ETL interface, resulting in improved photovoltaic performance and reduced hysteresis.

Zr4+-Doped Anatase TiO2 Nanotube Array Electrode for Electrocatalytic Reduction of L-cystine

A Zr⁴⁺-doped anatase TiO₂ nanotube array electrode was prepared using a process that included Ti anodizing, chemical immersion, and heat treatment. The compositions, microstructure, and electrochemical properties of the prepared electrodes were characterized. The results show that Zr⁴⁺ was successfully introduced into the TiO₂ nanotube array electrodes. Because Zr⁴⁺ was doped into the crystal structure of the TiO₂and replaced a part of Ti⁴⁺ to form more oxygen vacancies and Ti³⁺, the electrocatalytic activity of the prepared electrodes, for the reduction of L-cystine, was significantly improved.

Improving efficiency and stability of colorful perovskite solar cells with two-dimensional photonic crystals

Colorful solar cells have been much sought after because they can generate electricity and concurrently satisfy ornamentation purposes. Owing to their outstanding power conversion efficiency and flexibility in processing, perovskite solar cells (PSCs) have the great potential to become both efficient and aesthetically appealing. Here, we specially devise and fabricate two novel electron transport layers (ETLs) for PSCs with two-dimensional (2D) photonic crystal structures, namely the 2D inverse opal (IO) structured SnO₂ (IOS) and SnO₂-TiO₂ composite (IOST), using the template-assisted spin-coating method. The synergistic structure and material modifications to the ETLs lead to a number of unique features, including the remarkable electron transfer ability, vivid colors and good protection to the infiltrated perovskite films. Furthermore, the IOS and IOST ETLs are effectively incorporated into the CH₃NH₃PbI₃-based PSC devices that deliver the best efficiency of 16.8% with structural colors.

TiO2 Nanocolumn Arrays for More Efficient and Stable Perovskite Solar Cells

Organic-inorganic hybrid perovskite solar cells have attracted much attention due to their high power conversion efficiency (>25%) and low-cost fabrication. Yet, improvements are still needed for more stable and higher-performing solar cells. In this work, a series of TiO₂ nanocolumn photonic structures have been intentionally fabricated on half of the compact TiO₂-coated fluorine-doped tin oxide substrate by glancing angle deposition with magnetron sputtering, a method particularly suitable for industrial applications due to its high reliability and reduced cost when coating large areas. These vertically aligned nanocolumn arrays were then applied as the electron transport layer into triple-cation lead halide perovskite solar cells based on Cs_0.05(FA_0.83MA_0.17)_0.95Pb(I_0.83Br_0.17)₃. By comparison to solar cells built onto the same substrate without nanocolumns, the use of TiO₂ nanocolumns can significantly enhance the power conversion efficiency of the perovskite solar cells by 7% and prolong their shelf life. Here, detailed characterizations on the morphology and the spectroscopic aspects of the nanocolumns, their near-field and far-field optical properties, solar cells characteristics, as well as the charge transport properties provide mechanistic insights on how one-dimensional TiO₂ nanocolumns affect the performance of perovskite halide solar cells in terms of charge transport, light harvesting, and stability, knowledge necessary for the future design of higher-performing and more stable perovskite solar cells.

Vapor growth of binary and ternary phosphorus-based semiconductors into TiO2 nanotube arrays and application in visible light driven water splitting

We report successful synthesis of low band gap inorganic polyphosphide and TiO2 heterostructures with the aid of short-way transport reactions. Binary and ternary polyphosphides (NaP7, SnIP, and (CuI)3P12) were successfully reacted and deposited into electrochemically fabricated TiO2 nanotubes. Employing vapor phase reaction deposition, the cavities of 100 μm long TiO2 nanotubes were infiltrated; approximately 50% of the nanotube arrays were estimated to be infiltrated in the case of NaP7. Intensive characterization of the hybrid materials with techniques including SEM, FIB, HR-TEM, Raman spectroscopy, XRD, and XPS proved the successful vapor phase deposition and synthesis of the substances on and inside the nanotubes. The polyphosphide@TiO2 hybrids exhibited superior water splitting performance compared to pristine materials and were found to be more active at higher wavelengths. SnIP@TiO2 emerged to be the most active among the polyphosphide@TiO2 materials. The improved photocatalytic performance might be due to Fermi level re-alignment and a lower charge transfer resistance which facilitated better charge separation from inorganic phosphides to TiO2.

Bifacial Contact Junction Engineering for High-Performance Perovskite Solar Cells with Efficiency Exceeding 21

Ordered 1D metal oxide structure is desirable in thin film solar cells owing to its excellent charge collection capability. However, the electron transfer in 1D electron transporting layer (ETL)-based devices is still limited to a submicrometer-long pathway that is vertical to the substrate. Here, an innovative closely packed rutile TiO₂ nanowire (CRTNW) network parallel to the facet of fluorine-doped tin oxide (FTO) substrate is reported, which can serve as a 1D nanoscale electron transport pathway for efficient perovskite solar cells (PSCs). The PSC constructed using newly prepared CRTNW ETL achieves an impressive power conversion efficiency of 21.10%, which can be attributed to the facilitated electron extraction induced by the favorable junctions formed at FTO/ETL and ETL/perovskite interfaces and also the suppressed charge recombination originating from improved perovskite morphology with large grains, flat surface, and good surface coverage. The bifacial contact junctions engineering also enables large-area device fabrication. The PSC with 1 cm² aperture yields an efficiency of 19.50% under one sun illumination. This work highlights the significance of controlling the orientation and packing density of the ordered 1D oxide nanostructured thin films for highly efficient optoelectronic devices in a large-scale manner.

Distinguishing between Deep Trapping Transients of Electrons and Holes in TiO2 Nanotube Arrays Using Planar Microwave Resonator Sensor

A large signal direct current (DC) bias and a small signal microwave bias were simultaneously applied to TiO₂ nanotube membranes mounted on a planar microwave resonator. The DC bias modulated the electron concentration in the TiO₂ nanotubes and was varied between 0 and 120 V in this study. Transients immediately following the application and removal of DC bias were measured by monitoring the S-parameters of the resonator as a function of time. The DC bias stimulated Poole-Frenkel-type trap-mediated electrical injection of excess carriers into TiO₂ nanotubes, which resulted in a near-constant resonant frequency but a pronounced decrease in the microwave amplitude due to free electron absorption. When ultraviolet illumination and DC bias were both present and then stepwise removed, the resonant frequency shifted due to trapping-mediated change in the dielectric constant of the nanotube membranes. Characteristic lifetimes of 60-80, 300-800, and ∼3000 s were present regardless of whether light or bias was applied and were also observed in the presence of a hole scavenger, which we attributed to oxygen adsorption and deep electron traps, whereas another characteristic lifetime >8000 s was only present when illumination was applied, and is attributed to the presence of hole traps.

Observation of a low temperature n-p transition in individual titania nanotubes

Manipulating the transport properties of titania nanotubes (NTs) is paramount in guaranteeing the material's successful implementation in various solid state applications. Here we present the unique semiconducting properties of individual titania NTs as revealed from thermoelectric and structural studies performed on the same individual NTs. The NTs were in the anatase phase fabricated by anodic oxidation and doped with intrinsic defects created by reducing the lattice thermally. Despite their polycrystalline nature and nanoscale walls, the doped NTs were found to be 4-5 orders of magnitude more electrically conducting than TiO₂ nanowires and thin films, with values approaching the bulk single crystal conductivity. The reason for the high conductivity was found to be the high carrier concentration on the order of 10²² cm^-3, which counteracted the low mobility values ∼0.006 cm² V^-1 s^-1. Furthermore, this high level of carrier concentration transitioned the NTs to a degenerate state, which is the first such example in thermally doped titania NTs. More importantly, our study showed the creation of acceptor states along with donor states in individual nanotubes upon lattice reduction. These acceptor levels were found to be active at low temperatures when donor states were not ionized, shifting the Fermi level (E_f) from the conduction band to the valence band.

Improved efficient perovskite solar cells based on Ta-doped TiO2 nanorod arrays

Organometal halide perovskite solar cells (PSCs) are nowadays regarded as a rising star in photovoltaics. In particular, PSCs incorporating oriented TiO₂ nanorod (NR) arrays as the electron transport layer (ETL) have attracted significant attention owing to TiO₂ NR's superior electron transport abilities and its potential in long-term stable PSCs. In addition to improve the electron-transport ability of TiO₂ NRs, the tuning of the band alignments between the TiO₂ NR array and the perovskite layer is also crucial for achieving efficient solar cells. This work describes a facile, one-step, solvothermal method for the preparation of tantalum (Ta) doped TiO₂ NR arrays for efficient PSCs. It is shown that the trace doping with Ta tunes the electronic structure of TiO₂ NRs by a synergistic effect involving the lower 5d orbitals of the doped Ta⁵⁺ ions and the reduced oxygen vacancies. The synergistic tuning of the electronic structure improves the band alignment at the TiO₂ NR/perovskite interface and boosts the short-circuit current and the fill factor. By using the optimized doped TiO₂ NR array as the ETL, a record efficiency of 19.11% was achieved, which is the highest among one-dimensional-array based PSCs.

A novel perovskite solar cell design using aligned TiO2 nano-bundles grown on a sputtered Ti layer and a benzothiadiazole-based, dopant-free hole-transporting material

This work highlights the utilization of a novel hole-transporting material (HTM) derived from benzothiadiazole: 4-(3,5-bis(trifluoromethyl)phenyl)-7-(5'-hexyl-[2,2'-bithiophen]-5-yl)benzo[c][1,2,5]thiadiazole (CF-BTz-ThR) and aligned TiO₂ nano-bundles (TiO₂ NBs) as the electron transporting layer (ETL) for perovskite solar cells (PSCs). The aligned TiO₂ NBs were grown on titanium (Ti)-coated FTO substrates using a facile hydrothermal method. The newly designed CF-BTz-ThR molecule with suitable highest occupied molecular orbital (HOMO) favored the effective hole injection from perovskite deposited aligned TiO₂ NBs thin film. The PSCs demonstrated a power conversion efficiency (PCE) of ∼15.4% with a short circuit current density (J_sc) of ∼22.42 mA cm^-2 and an open circuit voltage (V_oc) of ∼1.02 V. The efficiency data show the importance of proper molecular engineering whilst highlighting the advantages of dopant-free HTMs in PSCs.

Optical anisotropy in vertically oriented TiO2 nanotube arrays

Nanofabricated optically anisotropic uniaxial thin films with deep submicron feature sizes are emerging as potential platforms for low-loss all-dielectric metamaterials, and for Dyakonov surface wave-based subwavelength optical confinement and guiding at interfaces with isotropic media. In this context, we investigate the optical properties of one such uniaxial platform, namely self-organized titania nanotube arrays (TNTAs) grown by the bottom-up nanofabrication process of electrochemical anodization on silicon wafer substrates, and subsequently annealed at different temperatures, i.e. 500 °C and 750 °C. We performed detailed quantitative analysis of the structure of the TNTAs using x-ray diffraction and Raman spectroscopy, which revealed a measurable phonon confinement in TNTAs annealed at 500 °C. Variable angle spectroscopic ellipsometry was used to investigate the optical anisotropy in two kinds of TNTAs-those constituted by anatase-phase and those containing a mixture of anatase and rutile phases. Both kinds of TNTAs were found to have positive birefringence (Δn) exceeding 0.06 in the spectral region of interest while mixed phase TNTAs exhibited Δn as high as 0.15. The experimentally measured anisotropy in the refractive index of the TNTAs was compared with the predictions of two different effective medium approximations incorporating the uniaxial geometry. The measured value of Δn for TNTAs exceeded that of bulk anatase single crystals, indicating the potential of nanostructured dielectrics to outperform dielectric crystals of the same material with respect to the magnitude of the achievable directional refractive index contrast.

Multidimensional Anodized Titanium Foam Photoelectrode for Efficient Utilization of Photons in Mesoscopic Solar Cells

Mesoscopic solar cells based on nanostructured oxide semiconductors are considered as a promising candidates to replace conventional photovoltaics employing costly materials. However, their overall performances are below the sufficient level required for practical usages. Herein, this study proposes an anodized Ti foam (ATF) with multidimensional and hierarchical architecture as a highly efficient photoelectrode for the generation of a large photocurrent. ATF photoelectrodes prepared by electrochemical anodization of freeze-cast Ti foams have three favorable characteristics: (i) large surface area for enhanced light harvesting, (ii) 1D semiconductor structure for facilitated charge collection, and (iii) 3D highly conductive metallic current collector that enables exclusion of transparent conducting oxide substrate. Based on these advantages, when ATF is utilized in dye-sensitized solar cells, short-circuit photocurrent density up to 22.0 mA cm^-2 is achieved in the conventional N719 dye-I₃^- /I^- redox electrolyte system even with an intrinsically inferior quasi-solid electrolyte.

Effect of the Microstructure of the Functional Layers on the Efficiency of Perovskite Solar Cells

The efficiencies of the hybrid organic-inorganic perovskite solar cells have been rapidly approaching the benchmarks held by the leading thin-film photovoltaic technologies. Arguably, one of the most important factors leading to this rapid advancement is the ability to manipulate the microstructure of the perovskite layer and the adjacent functional layers within the device. Here, an analysis of the nucleation and growth models relevant to the formation of perovskite films is provided, along with the effect of the perovskite microstructure (grain sizes and voids) on device performance. In addition, the effect of a compact or mesoporous electron-transport-layer (ETL) microstructure on the perovskite film formation and the optical/photoelectric properties at the ETL/perovskite interface are overviewed. Insight into the formation of the functional layers within a perovskite solar cell is provided, and potential avenues for further development of the perovskite microstructure are identified.

One-Dimensional Electron Transport Layers for Perovskite Solar Cells

The electron diffusion length (Ln) is smaller than the hole diffusion length (Lp) in many halide perovskite semiconductors meaning that the use of ordered one-dimensional (1D) structures such as nanowires (NWs) and nanotubes (NTs) as electron transport layers (ETLs) is a promising method of achieving high performance halide perovskite solar cells (HPSCs). ETLs consisting of oriented and aligned NWs and NTs offer the potential not merely for improved directional charge transport but also for the enhanced absorption of incoming light and thermodynamically efficient management of photogenerated carrier populations. The ordered architecture of NW/NT arrays affords superior infiltration of a deposited material making them ideal for use in HPSCs. Photoconversion efficiencies (PCEs) as high as 18% have been demonstrated for HPSCs using 1D ETLs. Despite the advantages of 1D ETLs, there are still challenges that need to be overcome to achieve even higher PCEs, such as better methods to eliminate or passivate surface traps, improved understanding of the hetero-interface and optimization of the morphology (i.e., length, diameter, and spacing of NWs/NTs). This review introduces the general considerations of ETLs for HPSCs, deposition techniques used, and the current research and challenges in the field of 1D ETLs for perovskite solar cells.

Morphology Analysis and Optimization: Crucial Factor Determining the Performance of Perovskite Solar Cells

This review presents an overall discussion on the morphology analysis and optimization for perovskite (PVSK) solar cells. Surface morphology and energy alignment have been proven to play a dominant role in determining the device performance. The effect of the key parameters such as solution condition and preparation atmosphere on the crystallization of PVSK, the characterization of surface morphology and interface distribution in the perovskite layer is discussed in detail. Furthermore, the analysis of interface energy level alignment by using X-ray photoelectron spectroscopy and ultraviolet photoelectron spectroscopy is presented to reveals the correlation between morphology and charge generation and collection within the perovskite layer, and its influence on the device performance. The techniques including architecture modification, solvent annealing, etc. were reviewed as an efficient approach to improve the morphology of PVSK. It is expected that further progress will be achieved with more efforts devoted to the insight of the mechanism of surface engineering in the field of PVSK solar cells.

Anodically Grown Titania Nanotube Induced Cytotoxicity has Genotoxic Origins

Nanoarchitectures of titania (TiO₂) have been widely investigated for a number of medical applications including implants and drug delivery. Although titania is extensively used in the food, drug and cosmetic industries, biocompatibility of nanoscale titania is still under careful scrutiny due to the conflicting reports on its interaction with cellular matter. For an accurate insight, we performed in vitro studies on the response of human dermal fibroblast cells toward pristine titania nanotubes fabricated by anodic oxidation. The nanotubes at low concentrations were seen to induce toxicity to the cells, whereas at higher concentrations the cell vitality remained on par with controls. Further investigations revealed an increase in the G₀ phase cell population depicting that majority of cells were in the resting rather than active phase. Though the mitochondrial set-up did not exhibit any signs of stress, significantly enhanced reactive oxygen species production in the nuclear compartment was noted. The TiO₂ nanotubes were believed to have gained access to the nuclear machinery and caused increased stress leading to genotoxicity. This interesting property of the nanotubes could be utilized to kill cancer cells, especially if the nanotubes are functionalized for a specific target, thus eliminating the need for any chemotherapeutic agents.

One-dimensional TiO2 Nanotube Photocatalysts for Solar Water Splitting

Hydrogen production from water splitting by photo/photoelectron-catalytic process is a promising route to solve both fossil fuel depletion and environmental pollution at the same time. Titanium dioxide (TiO2) nanotubes have attracted much interest due to their large specific surface area and highly ordered structure, which has led to promising potential applications in photocatalytic degradation, photoreduction of CO2, water splitting, supercapacitors, dye-sensitized solar cells, lithium-ion batteries and biomedical devices. Nanotubes can be fabricated via facile hydrothermal method, solvothermal method, template technique and electrochemical anodic oxidation. In this report, we provide a comprehensive review on recent progress of the synthesis and modification of TiO2 nanotubes to be used for photo/photoelectro-catalytic water splitting. The future development of TiO2 nanotubes is also discussed.

Efficient Perovskite Solar Cells Depending on TiO2 Nanorod Arrays

Perovskite solar cells (PSCs) with TiO2 materials have attracted much attention due to their high photovoltaic performance. Aligned TiO2 nanorods have long been used for potential application in highly efficient perovskite solar cells, but the previously reported efficiencies of perovskite solar cells based on TiO2 nanorod arrays were underrated. Here we show a solvothermal method based on a modified ketone-HCl system with the addition of organic acids suitable for modulation of the TiO2 nanorod array films to fabricate highly efficient perovskite solar cells. Photovoltaic measurements indicated that efficient nanorod-structured perovskite solar cells can be achieved with the length of the nanorods as long as approximately 200 nm. A record efficiency of 18.22% under the reverse scan direction has been optimized by avoiding direct contact between the TiO2 nanorods and the hole transport materials, eliminating the organic residues on the nanorod surfaces using UV-ozone treatment and tuning the nanorod array morphologies through addition of different organic acids in the solvothermal process.

Tuning the Structural Color of a 2D Photonic Crystal Using a Bowl-like Nanostructure

Structural colors of the ordered photonic nanostructures are widely used as an effective platform for manipulating the propagation of light. Although several approaches have been explored in attempts to mimic the structural colors, improving the reproducibility, mechanical stability, and the economic feasibility of sophisticated photonic crystals prepared by complicated processes continues to pose a challenge. In this study, we report on an alternative, simple method for fabricating a tunable photonic crystal at room temperature. A bowl-like nanostructure of TiO2 was periodically arranged on a thin Ti sheet through a two-step anodization process where its diameters were systemically controlled by changing the applied voltage. Consequently, they displayed a broad color distribution, ranging from red to indigo, and the principal reason for color generation followed the Bragg diffraction theory. This noncolorant method was capable of reproducing a Mondrian painting on a centimeter scale without the need to employ complex architectures, where the generated structural colors were highly stable under mechanical or chemical influence. Such a color printing technique represents a potentially promising platform for practical applications for anticounterfeit trademarks, wearable sensors, and displays.

Improved photovoltaic performance of mesoporous perovskite solar cells with hydrogenated TiO2: prolonged photoelectron lifetime and high separation efficiency of photoinduced charge

Hydrogenated titanium dioxide (H-TiO₂) nanocrystals and nanorods (H-TNRs) are successfully synthesized and employed as electron transfer materials in mesoscopic perovskite solar cells (PSCs).

An efficient titanium foil based perovskite solar cell: using a titanium dioxide nanowire array anode and transparent poly(3,4-ethylenedioxythiophene) electrode

The light-weight perovskite solar cell is prepared by employing TiO₂ nanowire (TNW) arrays on the Ti foil substrate as the electron collection layer and using a highly transparent PEDOT on the ITO/PEN substrate as the hole transporting layer.