Vertically aligned nanostructured TiO2 photoelectrodes for high efficiency perovskite solar cells via a block copolymer template approach

A Stable Perovskite Sensitized Photonic Crystal P-N Junction with Enhanced Photoelectrochemical Hydrogen Production

The slow photon effect in inverse opal photonic crystals represents a promising approach to manipulate the interactions between light and matter through the design of material structures. This study introduces a novel ordered inverse opal photonic crystal (IOPC) sensitized with perovskite quantum dots (PQDs), demonstrating its efficacy for efficient visible-light-driven H2 generation via water splitting. The rational structural design contributes to enhanced light harvesting. The sensitization of the IOPC with PQDs improves optical response performance and enhances photocatalytic H2 generation under visible light irradiation compared to the IOPC alone. The designed photoanode exhibits a photocurrent density of 3.42 mA cm-2 at 1.23 V vs RHE. This work advances the rational design of visible light-responsive photocatalytic heterostructure materials based on wide band gap metal oxides for photoelectrochemical applications.

γ-Ray Irradiation Enables Annealing- and Light-Soaking-Free Solution Processable SnO2 Electron Transport Layer for Inverted Organic Solar Cells

The electrode buffer layer is crucial for high-performance and stable OSCs, optimizing charge transport and energy level alignment at the interface between the polymer active layer and electrode. Recently, SnO2 has emerged as a promising material for the cathode buffer layer due to its desirable properties, such as high electron mobility, transparency, and stability. Typically, SnO2 nanoparticle layers require a postannealing treatment above 150°C in an air environment to remove the surfactant ligands and obtain high-quality thin films. However, this poses challenges for flexible electronics as flexible substrates can't tolerate temperatures exceeding 100°C. This study presents solution-processable and annealing-free SnO2 nanoparticles by employing y-ray irradiation to disrupt the bonding between surfactant ligands and SnO2 nanoparticles. The SnO2 layer treated with y-ray irradiation is used as an electron transport layer in OSCs based on PTB7-Th:IEICO-4F. Compared to the conventional SnO2 nanoparticles that required high-temperature annealing, the y-SnO2 nanoparticle-based devices exhibit an 11% comparable efficiency without postannealing at a high temperature. Additionally, y-ray treatment has been observed to eliminate the light-soaking effect of SnO2. By eliminating the high-temperature postannealing and light-soaking effect, y-SnO2 nanoparticles offer a promising, cost-effective solution for future flexible solar cells fabricated using roll-to-roll mass processing.

Sub-30 nm 2D Perovskites Patterns via Block Copolymer Guided Self-Assembly for Color Conversion Optical Polarizer

Despite the remarkable advances made in the development of 2D perovskites suitable for various high-performance devices, the development of sub-30 nm nanopatterns of 2D perovskites with anisotropic photoelectronic properties remains challenging. Herein, a simple but robust route for fabricating sub-30 nm 1D nanopatterns of 2D perovskites over a large area is presented. This method is based on nanoimprinting a thin precursor film of a 2D perovskite with a topographically pre-patterned hard poly(dimethylsiloxane) mold replicated from a block copolymer nanopattern consisting of guided self-assembled monolayered in-plane cylinders. 1D nanopatterns of various 2D perovskites (A'2 MAn -1 Pbn X3 n +1 ,A' = BA, PEA, X = Br, I) are developed; their enhanced photoluminescence (PL) quantum yields are approximately four times greater than those of the corresponding control flat films. Anisotropic photocurrent is observed because 2D perovskite nanocrystals are embedded in a topological 1D nanopattern. Furthermore, this 1D metal-coated nanopattern of a 2D perovskite is employed as a color conversion optical polarizer, in which polarized PL is developed. This is due to its capability of polarization of an incident light arising from the sub-30 nm line pattern, as well as the PL of the confined 2D perovskite nanocrystals in the pattern.

Block Copolymer Nanopatterning for Nonsemiconductor Device Applications

Block copolymer (BCP) nanopatterning has emerged as a versatile nanoscale fabrication tool for semiconductor devices and other applications, because of its ability to organize well-defined, periodic nanostructures with a critical dimension of 5-100 nm. While the most promising application field of BCP nanopatterning has been semiconductor devices, the versatility of BCPs has also led to enormous interest from a broad spectrum of other application areas. In particular, the intrinsically low cost and straightforward processing of BCP nanopatterning have been widely recognized for their large-area parallel formation of dense nanoscale features, which clearly contrasts that of sophisticated processing steps of the typical photolithographic process, including EUV lithography. In this Review, we highlight the recent progress in the field of BCP nanopatterning for various nonsemiconductor applications. Notable examples relying on BCP nanopatterning, including nanocatalysts, sensors, optics, energy devices, membranes, surface modifications and other emerging applications, are summarized. We further discuss the current limitations of BCP nanopatterning and suggest future research directions to open up new potential application fields.

Fabrication of Nanodevices Through Block Copolymer Self-Assembly

Block copolymer (BCP) self-assembly, as a novel bottom-up patterning technique, has received increasing attention in the manufacture of nanodevices because of its significant advantages of high resolution, high throughput, low cost, and simple processing. BCP self-assembly provides a very powerful approach to constructing diverse nanoscale templates and patterns that meet large-scale manufacturing practices. For the past 20 years, the self-assembly of BCPs has been extensively employed to produce a range of nanodevices, such as nonvolatile memory, bit-patterned media (BPM), fin field-effect transistors (FinFETs), photonic nanodevices, solar cells, biological and chemical sensors, and ultrafiltration membranes, providing a variety of configurations for high-density integration and cost-efficient manufacturing. In this review, we summarize the recent progress in the fabrication of nanodevices using the templates of BCP self-assembly, and present current challenges and future opportunities.

Rectangular Cylinders Formed by Compositionally Bidisperse ABC Triblock Terpolymer Blends: A Self-Consistent Field Theory Study

Compared with traditional cylinders that have circular cross-sections, cylinders with rectangular cross-sections can endow nanomaterials with various novel optical properties and functions. In this work, the formation of the rectangular cylinders self-assembled by compositionally bidisperse ABC triblock terpolymer blends has been investigated via numerical simulations based on self-consistent field theory. The specially designed blending systems are composed of two types of linear ABC triblock terpolymers that have the same total chain lengths and the middle B block chain lengths, but different chain lengths of the side A/C blocks. By tuning the chain length fractions and the interactions between different blocks, rectangular cylinders with a fourfold symmetry pattern were successfully obtained in our simulations. Each rectangular phase domain is self-assembled together by the short and long side blocks of the same species. The simulation results indicate that the selective aggregation of the short side blocks determines the formation of the rectangular cylindrical phase, i.e., the short side blocks prefer to aggregate at the four corners within a rectangular cylindrical phase domain. This simulation result reveals a formation mechanism that is different from the mechanism proposed in previous experiments [Asai ACS Macro Lett., 2014, 3, 166-169]. Moreover, under different middle B block chain length fractions, phase diagrams as a function of the interaction parameter between different blocks and the short side block chain length fraction have been constructed. The phase diagrams show that the parameter window of the rectangular cylinders is considerably expanded by increasing the chain length fraction of the middle B blocks. Our simulation works can provide a theoretical basis for molecular design to regulate and fabricate nanomaterials with nontraditional phase domains in future experiments.

Flame Synthesized Blue TiO2- x with Tunable Oxygen Vacancies from Surface to Grain Boundary to Bulk

Fabrication of nonstoichiometric metal oxides containing oxygen vacancies (OVs) has been an effective strategy to modulate their (photo)catalytic or (photo)electrochemical performances which are all affected by charge transfer at the interface and in the bulk. Considerable efforts are still needed to achieve tunability of OVs, as well as their quantitative characterization. Herein, a one-step flame synthesis method is reported for the first time for fast fabrication of blue TiO_{2- x} with controllable defect content and location. Temperature-programmed oxidation (TPO) analysis is applied for the first time and found to be an excellent technique in both differentiating and quantifying OVs at the surface, grain boundary (GB), and bulk of TiO_{2- x} . The results indicate that a moderate level of OVs can greatly enhance the charge transfer. Importantly, the OVs locked at GBs due to the thermal sintering of nanoparticles during the synthesis can facilitate the anchoring and reduction of Pt species.

Soft-template-assisted synthesis: a promising approach for the fabrication of transition metal oxides

The past few decades have witnessed transition metal oxides (TMOs) as promising candidates for a plethora of applications in numerous fields. The exceptional properties retained by these materials have rendered them of paramount emphasis as functional materials. Thus, the controlled and scalable synthesis of transition metal oxides with desired properties has received enormous attention. Out of different top-down and bottom-up approaches, template-assisted synthesis predominates as an adept approach for the facile synthesis of transition metal oxides, owing to its phenomenal ability for morphological and physicochemical tuning. This review presents a comprehensive examination of the recent advances in the soft-template-assisted synthesis of TMOs, focusing on the morphological and physicochemical tuning aided by different soft-templates. The promising applications of TMOs are explained in detail, emphasizing those with excellent performances.

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.

Growth of Doped SrTiO3 Ferroelectric Nanoporous Thin Films and Tuning of Photoelectrochemical Properties with Switchable Ferroelectric Polarization

Ferroelectric polarization is an intriguing physical phenomenon for tuning charge-transport properties and finds application in a wide range of optoelectronic devices. So far, ferroelectric materials in a planar geometry or chemically grown nanostructures have been used. However, these structural architectures possess serious disadvantages such as small surface areas and structural defects, respectively, leading to reduced performance. Herein, the growth of room-temperature ferroelectric nanoporous/nanocolumnar structure of Ag,Nb-codoped SrTiO₃ (Ag/Nb:STO) using pulsed laser deposition is reported and demonstrated to have enhanced photoelectrochemical (PEC) properties using ferroelectric polarization. By manipulating the external electrical bias, ∼3-fold enhancement in the photocurrent from 40 to 130 μA·cm^-2 of film area is obtained. Concurrently, the flat-band potential is decreased from -0.55 to -1.13 V, revealing a giant ferroelectric tuning of the band alignment at the semiconductor surface and enhanced charge transfer. In addition, an electrochemical impedance spectroscopy study confirmed the tuning of the charge transfer with ferroelectric polarization. Our nanoporous ferroelectric-semiconductor approach offers a new platform with great potential for achieving highly efficient PEC devices for renewable energy applications.

Recent progress and perspectives of gas sensors based on vertically oriented ZnO nanomaterials

Vertically oriented zinc oxide (ZnO) nanomaterials, such as nanorods (NRs), nanowires (NWs), nanotubes (NTs), nanoneedles (NNs), and nanosheets (NSs), are highly ordered architectures that provide remarkable properties for sensors. Furthermore, these nanostructures have fascinating features, including high surface-area-to-volume ratios, high charge carrier concentrations, and many surface-active sites. These features make vertically oriented ZnO nanomaterials exciting candidates for gas sensor fabrication. The development of efficient methods for the production of vertically oriented nanomaterial electrode surfaces has resulted in improved stability, high reproducibility, and gas sensing performance. Moving beyond conventional fabrication processes that include binders and nanomaterial deposition steps has been crucial, as the materials from these processes suffer from poor stability, low reproducibility, and marginal sensing performance. In this feature article, we comprehensively describe vertically oriented ZnO nanomaterials for gas sensing applications. The uses of such nanomaterials for gas sensor fabrication are discussed in the context of ease of growth, stability on an electrode surface, growth reproducibility, and enhancements in device efficiency as a result of their unique and advantageous features. In addition, we summarize applications of gas sensors for a variety of toxic and volatile organic compound (VOC) gases, and we discuss future directions of the vertically oriented ZnO nanomaterials.

Improved photovoltaic performance of perovskite solar cells based on three-dimensional rutile TiO2 nanodendrite array film

In order to explore high performance and stable perovskite solar cells (PSCs), the design and optimization of electron transport layer (ETL) have been paid more and more attention. Vertically oriented, one-dimensional (1D) TiO2 nanostructured array films are considered superior ETLs because of their rapid electron transporting property and open pore architectures. In this study, a three-dimensional (3D) rutile TiO2 nanodendrite array (RTNDA) film containing 1D trunks and branches was fabricated through second hydrothermal treatment of 1D rutile TiO2 nanorod array (RTNRA) film hydrothermally grown on a fluorine tin oxide (FTO) conductive glass. The resulting 3D-RTNDA film not only facilitates close contact with mixed-ion perovskite (Cs0.05(FA0.83MA0.17)0.95Pb(I0.9Br0.1)3) film, but also promotes the formation of a perovskite layer with larger crystal grain sizes. Both can efficiently retard the interface charge recombination, and thus result in a significantly improved power conversion efficiency (PCE) of 18.0%, improved by 20% as compared to that (15.0%) of the device fabricated with the 1D-RTNRA film. Spectroscopic, electrochemical and photoelectrochemical measurements indicate that the improved photovolatic performance can be mainly ascribed to the largely suppressed hysteresis effect, the increased open-circuit voltage and fill factor stemming from the more effective hole blocking and electron transport. The results presented here demonstrate that 3D-RTNDA film with 3D rutile TiO2 hierarchical nanoarchitecture is a promising ETL selection in designing high-performance PSCs.

Tunable photovoltaic performance of preferentially oriented rutile TiO2 nanorod photoanode based dye sensitized solar cells with quasi-state electrolyte

Photoanodes made of highly oriented TiO₂ nanorod (NR) arrays with different aspect ratios were synthesized via a one-step hydrothermal technique. Preferentially oriented single crystalline rutile TiO₂ was confirmed by the single peak in an XRD pattern (2θ = 63°, (0 0 2)). FESEM images evidenced the growth of an array of NRss having different geometries with respect to reaction time and solution refreshment rate. The length, diameter and aspect ratio of the NRs increased with reaction time as 4 h (1.98 μm, 121 nm, 15.32), 8 h (4 μm, 185 nm, 22.70), 12 h (5.6 μm, 242 nm, 27.24) and 16 h (8 μm, 254 nm, 38.02), respectively. Unlike a conventional dye-sensitized solar cell (DSSC) with a liquid electrolyte, DSSCs were fabricated here using one-dimensional rutile TiO₂ NR based photoanodes, N719 dye and a quasi-state electrolyte. The charge transport properties were investigated using current-voltage curves and fitted using the one-diode model. Interestingly the photovoltaic performance of the DSSCs increased exponentially with the length of the NR and was attributed to a higher surface to volume ratio, more dye anchoring, and channelized electron transport. The higher photovoltaic performance (J_sc = 5.99 mA cm^-2, V_oc = 750 mV, η = 3.08%) was observed with photoanodes (16 h) made with the longer, densely packed TiO₂ NRs (8 μm, 254 nm).

Moldable biomimetic nanoscale optoelectronic platforms for simultaneous enhancement in optical absorption and charge transport

Nano-scale patterns such as those found on the exterior surface of the eyes of certain nocturnal insects have far-reaching implications in terms of optoelectronic device design. The advantage of using these patterns for optoelectronic enhancement in photovoltaic light harvesting has been less explored due to the lack of suitable engineered materials to easily fabricate such nanostructures. Here, an attempt is made to realize these complex patterns using a self-assembly based molding process on hitherto unexplored robust structural epoxies with excellent repeatability and scalability to a larger area. The incorporation of these patterns in the substrate shows nearly a 50% broadband drop in the specular reflectance of the nanostructured substrate. Furthermore, it is demonstrated that by tweaking the bio-inspired patterns on the interior side of a light harvesting device, it is possible to obtain a broadband improvement in the external quantum efficiency in the spectral window between 350 and 650 nm leading to a significant improvement of up to 49% in the photocurrent density in the structured devices. From our experiment and simulations, it is observed that this enhancement stems from a combination of two effects: first, a broadband drop in the specular reflectance exceeding 70%, arising from trapped surface plasmon-polariton modes, and second, an improved charge separation in the structured device arising due to perturbed built-in electric fields. Furthermore, the simulations which take into account the interfacial nano-scale morphology show that for absorbers with low carrier mobilities, a significant improvement in the photocurrent and in the fill factor is simultaneously possible. Overall, this work demonstrates a combination of tweaked bio-mimetic design and the use of unconventional robust structural materials as nanostructured optoelectronic substrates. This effort can bridge the gap between naturally evolved designs and practical optoelectronics to enhance the performance.

Enhanced crystallinity of CH3NH3PbI3 by the pre-coordination of PbI2–DMSO powders for highly reproducible and efficient planar heterojunction perovskite solar cells

The pre-coordinated PbI₂–DMSO powders improved efficiency and reproducibility of perovskite solar cells with enhanced crystallinity of the CH₃NH₃PbI₃ film.

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.

Tungsten-doped TiO2/reduced Graphene Oxide nano-composite photocatalyst for degradation of phenol: A system to reduce surface and bulk electron-hole recombination

Recombination of photogenerated charges is the main factor affecting the photocatalytic activity of TiO₂. Here, we report a combined strategy of suppressing both the bulk as well as the surface recombination processes by doping TiO₂ with tungsten and forming a nanocomposite with reduced graphene oxide (rGO), respectively. Sol-gel method was used to dope and optimize the concentration of W in TiO₂ powder. UV-Vis, XPS, PL and time resolved PL spectra along with DFT calculations indicate that W⁶⁺ in TiO₂ lattice creates an impurity level just below the conduction band of TiO₂ to act as a trapping site of electrons, which causes to improve the lifetime of the photo-generated charges. Maximum reduction in the PL intensity and the improvement in charge carrier lifetime was observed for TiO₂ doped with 1 at.% W (1W-TiO₂), which also displayed the highest photo-activity for the degradation of p-nitro phenol pollutant in water. Tuning of rGO/TiO₂ ratio (weight) disclosed that the highest activity can be achieved with the composite formed by taking equal amounts of TiO₂ and rGO (1:1), in which the strong interaction between TiO₂ and rGO causes an effective charge transfer via bonds formed near the interface as indicated by XPS. Both these optimized concentrations were utilized to form the composite rGO/1W-TiO₂, which showed the highest activity in photo-degradation of p-nitro phenol (87%) as compared to rGO/TiO₂ (42%), 1W-TiO₂ (62%) and pure TiO₂ (29%) in 180 min. XPS and PL results revealed that in the present nanocomposite, tungsten species traps the excited electron to reduce the interband recombination in the bulk, while the interaction between TiO₂ and rGO creates a channel for fast transfer of excited electrons towards the latter before being recombined on the surface defect sites.

Solution-processed indium oxide electron transporting layers for high-performance and photo-stable perovskite and organic solar cells

Interface engineering is considered the key to improving the device performance and stability of solar cells. In particular, TiO₂ nanostructures, when used as electron transporting layers (ETLs) in metal halide perovskite solar cells (PSCs), led to excellent power conversion efficiencies (PCEs) of over 20%. They effectively transferred charge carriers from the perovskite and suppressed charge recombination at the interfaces. However, the photocatalytic effect of TiO₂ on the perovskite can significantly degrade the device performance under ultraviolet illumination. Therefore, other classes of n-type metal oxides with a wide band gap should be developed to improve their photo-stability. Herein, we demonstrate the development of In₂O₃ thin films by a solution process and their application as ETLs in PSCs and organic solar cells (OSCs). Pin hole-free In₂O₃ ETLs obtained by the thermal decomposition of an In precursor thin film exhibit high conductivity (2.49 × 10^-4 S cm^-1) and low surface roughness (7.33 nm). This leads to impressive PCEs of 14.63% and 3.03% for the PSC and the inverted OSC, respectively. Furthermore, the In₂O₃-PSC shows better photo-stability than the TiO₂-PSC by virtue of the wider band gap of In₂O₃, which leads to a PCE retention of 74% and 46%, relative to the initial PCE values of the PSC and the inverted OSC, respectively.

TiO2 Electron Transport Bilayer for Highly Efficient Planar Perovskite Solar Cell

In planar perovskite solar cells, it is vital to engineer the extraction and recombination of electron-hole pairs at the electron transport layer/perovskite interface for obtaining high performance. This study reports a novel titanium oxide (TiO₂ ) bilayer with different Fermi energy levels by combing atomic layer deposition and spin-coating technique. Energy band alignments of TiO₂ bilayer can be modulated by controlling the deposition order of layers. The TiO₂ bilayer based perovskite solar cells are highly efficient in carrier extraction, recombination suppression, and defect passivation, and thus demonstrate champion efficiencies up to 16.5%, presenting almost 50% enhancement compared to the TiO₂ single layer based counterparts. The results suggest that the bilayer with type II band alignment as electron transport layers provides an efficient approach for constructing high-performance planar perovskite solar cells.

Block Copolymer-Tuned Fullerene Electron Transport Layer Enhances the Efficiency of Perovskite Photovoltaics

In this study, we enhanced the power conversion efficiency (PCE) of perovskite solar cells by employing an electron transfer layer (ETL) comprising [6,6]phenyl-C61-butyric acid methyl ester (PC61BM) and, to optimize its morphology, a small amount of the block copolymer polystyrene-b-poly(ethylene oxide) (PS-b-PEO), positioned on the perovskite active layer. When incorporating 0.375 wt % PS-b-PEO into PC61BM, the PCE of the perovskite photovoltaic device increased from 9.4% to 13.4%, a relative increase of 43%, because of a large enhancement in the fill factor of the device. To decipher the intricate morphology of the ETL, we used synchrotron grazing-incidence small-angle X-ray scattering for determining the PC61BM cluster size, atomic force microscopy and scanning electron microscopy for probing the surface, and transmission electron microscopy for observing the aggregation of PC61BM in the ETL. We found that the interaction between PS-b-PEO and PC61BM resulted in smaller PC61BM clusters that further aggregated into dendritic structures in some domains, a result of the similar polarities of the PS block and PC61BM; this behavior could be used to tune the morphology of the ETL. The optimal PS-b-PEO-mediated PC61BM cluster size in the ETL was 17 nm, a large reduction from 59 nm for the pristine PC61BM layer. This approach of incorporating a small amount of nanostructured block copolymer into a fullerene allowed us to effectively tune the morphology of the ETL on the perovskite active layer and resulted in enhanced fill factors of the devices and thus their device efficiency.

Enhanced perovskite morphology and crystallinity for high performance perovskite solar cells using a porous hole transport layer from polystyrene nanospheres

(1) Porous-PEDOT:PSS from PS nanospheres. (2) The perovskite quality is improved, with the improved crystallinity and enhanced grain sizes. (3) High-performance perovskite solar cells.