TiO2 Scaffold Research Articles

Improved performance and stability of perovskite solar cells by crystal crosslinking with alkylphosphonic acid ω-ammonium chlorides.

In the past few years, organic-inorganic halide perovskites have rapidly emerged as promising materials for photovoltaic applications, but simultaneously achieving high performance and long-term stability has proved challenging. Here, we show a one-step solution-processing strategy using phosphonic acid ammonium additives that results in efficient perovskite solar cells with enhanced stability. We modify the surface of methylammonium lead triiodide (CH3NH3PbI3) perovskite by spin-coating its precursor solution in the presence of butylphosphonic acid 4-ammonium chloride. Morphological, structural and elemental analyses show that the phosphonic acid ammonium additive acts as a crosslink between neighbouring grains in the perovskite structure, through strong hydrogen bonding of the -PO(OH)2 and -NH3(+) terminal groups to the perovskite surface. The additives facilitate the incorporation of the perovskite within a mesoporous TiO2 scaffold, as well as the growth of a uniform perovskite layer at the surface, enhancing the material's photovoltaic performance from 8.8 to 16.7% as well as its resistance to moisture.

Light stability tests of methylammonium and formamidinium Pb-halide perovskites for solar cell applications

We have prepared perovskite [CH3NH3PbI3 (MALI), CH3NH3PbBr3 (MALB), NH2CH=NH2PbI3 (FALI), and NH2CH=NH2PbBr3 (FALB)] thin films by a one-step process on glass/TiO2 and glass/Al2O3 substrates and studied the stability of the perovskite under UV/visible light radiation up to 24 h at 1.5AM in air. After irradiation, the films were characterized by UV–vis absorption and X-ray diffraction measurements. In addition, photovoltaic performance characteristics in air were studied using different perovskites before (0 h) and after 24 h irradiation. The results revealed that Al2O3 protected the perovskite crystal from degradation. However, the perovskites were unstable except for NH2CH=NH2PbI3 under the same conditions using a TiO2 scaffold layer.

The effect of TiO2 nanopowder coating on in vitro bioactivity of porous TiO2 scaffolds

Porous TiO2 ceramic has attracted considerable research interest as a scaffold material for bone tissue engineering. However, no special considerations are given to the in vitro bioactivity of highly porous TiO2 ceramic scaffolds and they are assumed as bioinert ceramic. The main goal of the present work was to fabricate novel highly porous and bioactive TiO2 ceramic scaffolds with interconnected pore structure for bone tissue engineering. TiO2 ceramic scaffolds with pore size in the range from 300 to 700μm and porosity >90% were produced via polymer foam replica method. Scaffolds were coated with TiO2 nanopowder and the effect of TiO2 nanopowder coating on in vitro bioactivity in simulated body fluid was studied. Uncoated TiO2 scaffolds showed no in vitro bioactivity after immersion in SBF for 21 days, but sphere-like particles were observed on the surface of coated TiO2 scaffolds that were considered being B type CO32− substituted or bonelike apatite.

Crystal Morphologies of Organolead Trihalide in Mesoscopic/Planar Perovskite Solar Cells.

The crystal morphology of organolead trihalide perovskite (OTP) light absorbers can have profound influence on the perovskite solar cells (PSCs) performance. Here we have used a combination of conventional transmission electron microscopy (TEM) and high-resolution TEM (HRTEM), in cross-section and plan-view, to characterize the morphologies of a solution-processed OTP (CH3NH3PbI3 or MAPbI3) within mesoporous TiO2 scaffolds and within capping and planar layers. Studies of TEM specimens prepared with and without the use of focused ion beam (FIB) show that FIBing is a viable method for preparing TEM specimens. HRTEM studies, in conjunction with quantitative X-ray diffraction, show that MAPbI3 perovskite within mesoporous TiO2 scaffold has equiaxed grains of size 10-20 nm and relatively low crystallinity. In contrast, the grain size of MAPbI3 perovskite in the capping and the planar layers can be larger than 100 nm in our PSCs, and the grains can be elongated and textured, with relatively high crystallinity. The observed differences in the performance of planar and mesoscopic-planar hybrid PSCs can be attributed in part to the striking differences in their perovskite-grain morphologies.

A scalable electrodeposition route to the low-cost, versatile and controllable fabrication of perovskite solar cells

Hybrid organic/inorganic perovskite solar cells (PSCs) have emerged as a highly promising alternative renewable energy source because of their high efficiency and low-cost solution processable manufacturing technology. However, the commonly used spin coating process limits the large-scale manufacturing of perovskite layers for commercialization. Here we report on the development of an electrodeposition technique for fabricating perovskite layers and demonstrate its simplicity, versatility, scalability and roll-to-roll manufacturing compatibility. The key step is the electrodeposition of a PbO2 layer on TiO2 scaffold, which is then subjected to chemical bath conversion to sequentially generate PbI2 and CH3NH3PbI3 perovskite. Clearly demonstrated is the controllability of morphology and optical properties of the CH3NH3PbI3 layer, leading to a higher power conversion efficiency (PCE) reproducibility and a higher average PCE when incorporated into carbon-based PSCs than with the spin coating technique. Remarkably, the cell area of electrodeposited PSCs could be easily scaled up to 4cm2 with an excellent perovskite film uniformity, rendering a PCE gain of 36.3% over the spin-coated counterpart. We further demonstrate the deposition of perovskite layers on complex shape substrates (e.g., stainless steel net), which would be rather difficult or impossible with other competing film deposition techniques. These results establish electrodeposition as a versatile and controllable route toward low-cost and large scalable manufacturing of high efficiency PSCs.

A redox-flow electrochromic window.

A low-cost electrochromic (EC) window based on a redox-flow system that does not require expensive transparent conductive oxide (TCO) substrates is introduced and demonstrated for the first time. An aqueous I3–/I– redox electrolyte is used in place of a TCO to oxidize/reduce a molecular layer of an EC triphenylamine derivative that is anchored to a mesoporous TiO2 scaffold on the inner faces of a double-paned window. The redox electrolyte is electrochemically oxidized/reduced in an external two-compartment cell and circulated through the window cavity using an inexpensive peristaltic pump, resulting in coloration or decoloration of the window due to reaction of the redox solution with the triphenylamine derivative. The absorption characteristics, coloration/decoloration times, and cycling stability of the prototype EC window are evaluated, and prospects for further development are discussed.

Influence of Structure and Morphology of Perovskite Films on the Performance of Perovskite Solar Cells

Perovskite solar cells based on the inorganic/organic hybrid perovskite have attracted increasing attention over the past 3 years. Many studies have been done in this area. Controling the morphology of the perovskite film is an effective way to improve the photoelectric conversion efficiency of the devices. In our reserch, we studied the influence of structure and morphology of perovskite films on the performance of the organic-inorganic hybrid perovskite solar cells which prepared by a sequential deposition method. Mesoporous TiO2 scaffold were introduced as electron collecting layer. Lead iodide (PbI2) was then spin cast on the TiO2 scaffold. The PbI2 subsequently transformed into the perovskite (CH3NH3PbI3) by dipping the TiO2/PbI2 film into a solution of CH3NH3I. We studied the difference between the PbI2 film with or without drying under room temperature after spin-coating. Through drying under room temperature, larger pores formed in the PbI2 film. While without drying under room temperature, smaller and shallower pores formed in the PbI2 film. The results show that larger pores in PbI2 film leads to more complete transformation of PbI2 to CH3NH3PbI3 and larger CH3NH3PbI3 particles. CH3NH3PbI3 films were prepared with three different processes: (a) direct dipping the PbI2 film with smaller pores into the CH3NH3I solution; (b) direct dipping the PbI2 with larger pores into the CH3NH3I solution; (c) dipping the PbI2 with larger pores into the CH3NH3I solution after pre-wetting.The resulting CH3NH3PbI3 films were studied with SEM, UV-vis absorption spectrum and XRD. The particles size of the CH3NH3PbI3 are 150, 250 and 350 nm for process (a), (b) and (c) respectively. CH3NH3PbI3 films fabricated through process (a) show insufficient absorption due to the insufficient transformation of the PbI2. The pre-wetting procedure leads to slower reaction result in larger CH3NH3PbI3 particle size. Devices with proper size of CH3NH3PbI3 particles show the highest photoelectric conversion efficiency. An efficiency of 13.5% was achieved with a Jsc of 17.8 mA/cm, a Voc of 1.05 V and a FF of 72.5%.

Alginate hydrogel enriched with enamel matrix derivative to target osteogenic cell differentiation in TiO2 scaffolds

The purpose of bone tissue engineering is to employ scaffolds, cells, and growth factors to facilitate healing of bone defects. The aim of this study was to assess the viability and osteogenic differentiation of primary human osteoblasts and adipose tissue–derived mesenchymal stem cells from various donors on titanium dioxide (TiO2) scaffolds coated with an alginate hydrogel enriched with enamel matrix derivative. Cells were harvested for quantitative reverse transcription polymerase chain reaction on days 14 and 21, and medium was collected on days 2, 14, and 21 for protein analyses. Neither coating with alginate hydrogel nor alginate hydrogel enriched with enamel matrix derivative induced a cytotoxic response. Enamel matrix derivative–enriched alginate hydrogel significantly increased the expression of osteoblast markers COL1A1, TNFRSF11B, and BGLAP and secretion of osteopontin in human osteoblasts, whereas osteogenic differentiation of human adipose tissue–derived mesenchymal stem cells seemed unaffected by enamel matrix derivative. The alginate hydrogel coating procedure may have potential for local delivery of enamel matrix derivative and other stimulatory factors for use in bone tissue engineering.

Loading of mesoporous titania films by CH3NH3PbI3 perovskite, single step vs. sequential deposition.

Infiltration of mesoporous TiO2 scaffolds by CH3NH3PbI3 is more complete when using sequential compared to single step deposition processing and avoids formation of disordered capping layers affording greatly improved performance of perovskite based photovoltaics.

Ultrafast photoinduced dynamics of the organolead trihalide perovskite CH3NH3PbI3 on mesoporous TiO2 scaffolds in the 320-920 nm range.

We present femtosecond broadband transient absorption experiments for the investigation of the carrier dynamics in the organolead trihalide perovskite CH3NH3PbI3. The perovskite was prepared on a mesoporous TiO2 scaffold either by 1-step deposition from solution or by 2-step methods employing deposition of lead iodide followed by an on-surface reaction with methylammonium iodide. The thin films were characterized by XRD and FTIR chemical mapping. After pumping with an ultrashort laser pulse at 400 or 500 nm, the dynamics were monitored by a broadband supercontinuum reaching from the near IR (920 nm) into the UV. Specifically, the usage of quartz substrates and thin perovskite/TiO2 films enabled us to cover the spectral development down to 320 nm. The charge carrier dynamics were largely independent from the specific route of perovskite preparation: initial ultrafast carrier relaxation steps with time constants τCC and τCOP of <0.08, 0.2 and 2.6 ps were assigned to carrier-carrier and carrier-optical phonon scattering. Pronounced sub-band-gap absorption was found in the near IR at early times. Transient carrier temperatures were extracted from a Boltzmann fit to the blue wing of the photobleach band in the time range 0.2-700 ps, allowing us to distinguish between the decay of acoustic phonons (τAP = 50 and >1000 ps) and Auger recombination (τAR = 9, 75 and 450 ps). Carrier relaxation was accompanied by formation of an absorption band around 550 nm, with a characteristic structure assignable to a transient Stark effect, i.e. a red-shift of the perovskite spectrum due to the appearance of a directed electric field in the material and possibly additional influence of lattice heating. We observed a substantial Stokes shift between the relaxed photobleach and photoluminescence bands. Contributions of unreacted PbI2 to the transient absorption features appear to be negligible.

Predicting the Open‐Circuit Voltage of CH3NH3PbI3 Perovskite Solar Cells Using Electroluminescence and Photovoltaic Quantum Efficiency Spectra: the Role of Radiative and Non‐Radiative Recombination

The remarkably high open-circuit voltage of methylammonium lead iodide perov­skite solar cells is investigated. Both the theoretical maximum and the real open-circuit voltage are predicted from electroluminescence and photovoltaic external quantum efficiency spectra. Radiative and non-radiative recombination are quantified, where a source of non-radiative recombination is found in the mesoscopic structure, independent of the Al2O3 or TiO2 scaffold. Without a hole-transport layer, non-radiative recombination is strongly enhanced, which reduces the open-circuit voltage.

Mesh‐Shaped Nanopatterning of Pt Counter Electrodes for Dye‐Sensitized Solar Cells with Enhanced Light Harvesting

A facile process to produce large‐area platinum (Pt) counter electrode platforms with well‐arrayed, mesh‐shaped nanopatterns using commercially available TiO2 paste and poly(dimethyl siloxane) (PDMS) nanostamps is presented. The process involves mesh‐shaped (200 nm × 200 nm) nanopatterning of a TiO2 scaffold onto a fluorine‐doped tin oxide (FTO) substrate, followed by Pt sputtering. The structure and morphology of the counter electrodes are characterized by a field emission scanning electron microscope (FE‐SEM) and an atomic force microscope (AFM). Solid‐state dye‐sensitized solar cells (ssDSSCs) fabricated with these mesh‐shaped Pt counter electrodes showed an efficiency of 7.0%. This is one of the highest efficiencies observed for N719 dye and is much higher than that of devices with non‐patterned, thermally deposited electrodes (5.4%) or non‐patterned, sputtering deposited electrodes (5.7%). This improvement is attributed to enhanced light harvesting and a greater surface area and has been confirmed by incident photon‐to current efficiency (IPCE), electrochemical impedance spectroscopy (EIS), and cyclic voltammetry (CV) measurements.

First-Principles Investigation of the TiO2/Organohalide Perovskites Interface: The Role of Interfacial Chlorine.

We investigate the prototypical interface between organohalide perovskites and TiO2 by first-principles electronic structure calculations. The investigated heterointerface is representative of conventional dye-sensitized solar cells based on a mesoporous TiO2 scaffold and of flat devices in which a compact TiO2 film is used as electron selective layer. We find that the MAPbI3 and MAPbI3-xClx perovskites tend to grow in (110)-oriented films on TiO2, due to the better structural matching between rows of adjacent perovskite surface halides and TiO2 undercoordinated titanium atoms. Interfacial chlorine atoms further stabilize the (110) surface, due to an enhanced binding energy. We find that the stronger interaction of MAPbI3-xClx with TiO2 modifies the interface electronic structure, leading to a stronger interfacial coupling and to a slight TiO2 conduction band energy upshift. Our modeling study may constitute the basis for a further exploitation of perovskite solar cells.

Unraveling the Nanoscale Morphologies of Mesoporous Perovskite Solar Cells and Their Correlation to Device Performance

Hybrid solar cells based on organometal halide perovskite absorbers have recently emerged as promising class for cost- and energy-efficient photovoltaics. So far, unraveling the morphology of the different materials within the nanostructured absorber layer has not been accomplished. Here, we present the first visualization of the mesoporous absorber layer in a perovskite solar cell from analytical transmission electron microscopy studies. Material contrast is achieved by electron spectroscopic imaging. We found that infiltration of the hole transport material into the scaffold is low and inhomogeneous. Furthermore, our data suggest that the device performance is strongly affected by the morphology of the TiO2 scaffold with a fine grained structure being disadvantageous.

Simvastatin coating of TiO2 scaffold induces osteogenic differentiation of human adipose tissue-derived mesenchymal stem cells

Bone tissue engineering requires an osteoconductive scaffold, multipotent cells with regenerative capacity and bioactive molecules. In this study we investigated the osteogenic differentiation of human adipose tissue-derived mesenchymal stem cells (hAD-MSCs) on titanium dioxide (TiO2) scaffold coated with alginate hydrogel containing various concentrations of simvastatin (SIM). The mRNA expression of osteoblast-related genes such as collagen type I alpha 1 (COL1A1), alkaline phosphatase (ALPL), osteopontin (SPP1), osteocalcin (BGLAP) and vascular endothelial growth factor A (VEGFA) was enhanced in hAD-MSCs cultured on scaffolds with SIM in comparison to scaffolds without SIM. Furthermore, the secretion of osteoprotegerin (OPG), vascular endothelial growth factor A (VEGFA), osteopontin (OPN) and osteocalcin (OC) to the cell culture medium was higher from hAD-MSCs cultured on scaffolds with SIM compared to scaffolds without SIM. The TiO2 scaffold coated with alginate hydrogel containing SIM promote osteogenic differentiation of hAD-MSCs in vitro, and demonstrate feasibility as scaffold for hAD-MSC based bone tissue engineering.

The Importance of Perovskite Pore Filling in Organometal Mixed Halide Sensitized TiO2-Based Solar Cells.

Emerging from the field of dye-sensitized solar cells, organometal halide perovskite-based solar cells have recently attracted considerable attention. In these devices, the perovskite light absorbers can also be used as charge transporting materials, changing the requirements for efficient device architectures. The perovskite deposition can vary from merely sensitizing the TiO2 electron transporting scaffold as an endowment of small nanoparticles, to completely filling the pores where it acts as both light absorber and hole transporting material in one. By decreasing the TiO2 scaffold layer thickness, we change the solar cell architecture from perovskite-sensitized to completely perovskite-filled. We find that the latter case leads to improvements in device performance because higher electron densities can be sustained in the TiO2, improving electron transport rates and photovoltage. Importantly, the primary recombination pathway between the TiO2 and the hole transporting material is blocked by the perovskite itself. This understanding helps to rationalize the high voltages attainable on mesoporous TiO2-based perovskite solar cells.

Crystalline IrO2-decorated TiO2 nanofiber scaffolds for robust and sustainable solar water oxidation

Crystalline IrO2 nanoparticles immobilized on TiO2 nanofiber scaffolds as robust and sustainable oxygen evolving catalysts showed a high turnover number (TON: 322) and excellent recyclability (90% O2 evolving after 10 cycles). The effects of the decoration position, crystallite size, loading amount of IrO2, and TiO2 scaffolds were investigated.

Bioinspired Scaffolds with Varying Pore Architectures and Mechanical Properties

Scaffolds with potential biological applications having a variety of microstructural and mechanical properties can be fabricated by freezing colloidal solutions into porous solids. In this work, the structural and mechanical properties of TiO2 freeze cast with different soluble additives, including polyethylene glycol, NaOH or HCl, and isopropanol alcohol, are characterized to determine the effects of slurry viscosity, pH, and alcohol concentration on the freezing process. TiO2 powders mixed with water and these different additives are directionally frozen in a mold, then sublimated and sintered to create the porous scaffolds. The different scaffolds are characterized to compare the compressive strength, modulus, porosity, and pore morphology. For all scaffolds, the overall porosity remains constant (80–85%). By changing the concentration of each additive, the lamellar thickness, pore area, and aspect ratio vary significantly, showing inverse relationships to both the compressive strength and modulus. The strength is predicted from the pore aspect ratio of the scaffolds when subjected to compressive loading with the primary failure mode identified as Euler buckling. TiO2 scaffolds freeze cast with different soluble additives are suitable for biomedical applications, such as bone replacements, requiring high porosity and specific pore morphologies.

Optimisation of Ruthenium Dye Sensitised Solar Cells Efficiency via Sn Diffusion into the TiO2 Mesoporous Layer

Dye sensitised solar cells (DSCs) typically include a mesoporous titanium dioxide (TiO2) scaffold, sensitised with an adsorbed dye, as the main active element responsible for the photon absorption and charge separation functionalities. The sintering process employed in the TiO2 active layer fabrication plays a crucial role in the formation of the nanoparticle (NP) scaffold and hence in the performance of a dye sensitised solar cell, as it allows the particles to form efficient inter-crystalline electric contacts providing high electron conductivity. Furthermore, the DSC design requires a conductive transparent top electrode which is typically made of fluorinated stannic oxide. Here we report on a highly spatially resolved scanning electron microscopy study in conjunction with focussed ion beam milling and energy dispersive X-ray (EDX) mapping of the distribution of all relevant elements within a DSC subsequent to a classical sintering process in the range of 350°C–550°C. Additionally, the article provides quantitative results regarding the found Sn diffusion and its effect on efficiency confirmed via J-V measurements. The effective spatial resolution of the EDX studies was calculated by Monte Carlo simulations of the electron trajectories and X-ray emission region. This permits to construct a model for the migration of Sn from the transparent conductive oxide into the TiO2 scaffold, resulting in alterations in the composition of the complex system which has a direct effect on the DSC performance. J-V measurements conclude that sintering temperature of 500°C is close to the optimum regarding Sn diffusion enhancement of DSCs. Sintering temperatures above 500°C were causing a drop in the DSC efficiency and are therefore not recommended. In order to optimize the DSC efficiency, the results are summarized by a model that explains how the efficiency varies with the Sn diffusion process.

Spray-assisted silar deposition of cadmium sulphide quantum dots on metal oxide films for excitonic solar cells

The proof of principle of the successful application of spray deposition to the SILAR (successive ionic layer absorption and reaction) technique, one of the most effective strategies to sensitized TiO2 scaffold with QDs, is demonstrated. Systematically improved optical features of the materials (higher optical density together with reduced nanocrystal sizes) as well as of the functional performances of QD solar cells (photoconversion efficiency, fill factor, short circuit current, open circuit voltage) sensitized via SD-SILAR, with respect to traditional SILAR sensitization based on impregnation, are demonstrated.