Anionic Ligand Assisted Synthesis of 3‑D Hollow TiO₂ Architecture with Enhanced Photoelectrochemical Performance

Hollow structured materials have shown great advantages for use in photoelectrochemical devices. However, their poor charge transport limits overall device performance. Here, we report a unique 3-D hollow architecture of TiO₂ that greatly improves charge transport properties. We found that citric acid (CA) plays crucial roles in the formation of the 3-D hollow architecture. First, CA controls the hydrolysis rate of Ti ions and facilitates surface hydrolysis on templates during hydrothermal synthesis. Second, CA suppresses the growth of the carbon template at the initial reaction stage, resulting in the formation of comparatively small hollow fibers. More importantly, a prolonged hydrothermal reaction with CA enables a hollow sphere to grow into entangled hollow fibers via biomimetic swallowing growth. To demonstrate advantages of the 3-D hollow architecture for photoelectrochemical devices, we evaluated its photoelectrochemical performance, specifically the electrolyte diffusion and electron dynamics, by employing dye-sensitized solar cells as a model device. A systemic analysis reveals that the 3-D hollow architecture greatly improves both the electrolyte diffusion and electron transport compared to those of the nanoparticle and hollow sphere due to the elongated porous hollow morphology as well as the densely interconnected nanoparticles at the wall layer

Roughness of Ti Substrates for Control of the Preferred Orientation of TiO₂ Nanotube Arrays as a New Orientation Factor

We report the surface roughness of a Ti substrate as a critical factor for controlling the degree of the preferred orientation of anatase TiO₂ nanotube arrays (NTAs) which are synthesized by anodization and a subsequent annealing process. The degree of the preferred orientation to the (004) plane of the anatase crystal structure has a strong dependency on the root-mean-square roughness (<i>S</i>_q) of the initial Ti substrate when the roughness-controlled substrates are anodized in an ethylene glycol-based electrolyte containing ∼2 wt % of water. Highly preferred oriented NTAs were obtained from low-<i>S</i>_q (<10 nm) substrates, which were accompanied by uniform pore distribution and low concentration of hydroxyl ions in as-anodized amorphous NTAs. The mechanism of the preferred oriented crystallization of nanometer-scaled tube walls is explained considering the microscopic geometrical uniformity of the oxide barrier and nanopores at the early stage of anodization, which affected the local electric field and thus the insertion of the hydroxyl group into the amorphous TiO₂ tube walls

Effect of Rubidium Incorporation on the Structural, Electrical, and Photovoltaic Properties of Methylammonium Lead Iodide-Based Perovskite Solar Cells

We report the electrical properties of rubidium-incorporated methylammonium lead iodide ((Rb_<i>x</i>MA_1–<i>x</i>)­PbI₃) films and the photovoltaic performance of (Rb_<i>x</i>MA_1–<i>x</i>)­PbI₃ film-based p–i–n-type perovskite solar cells (PSCs). The incorporation of a small amount of Rb⁺ (<i>x</i> = 0.05) increases both the open circuit voltage (<i>V</i>_oc) and the short circuit photocurrent density (<i>J</i>_sc) of the PSCs, leading to an improved power conversion efficiency (PCE). However, a high fraction of Rb⁺ incorporation (<i>x</i> = 0.1 and 0.2) decreases the <i>J</i>_sc and thus the PCE, which is attributed to the phase segregation of the single tetragonal perovskite phase to a MA-rich tetragonal perovskite phase and a RbPbI₃ orthorhombic phase at high Rb fractions. Conductive atomic force microscopic and admittance spectroscopic analyses reveal that the single-phase (Rb_0.05MA_0.95)­PbI₃ film has a high electrical conductivity because of a reduced deep-level trap density. We also found that Rb substitution enhances the diode characteristics of the PSC, as evidenced by the reduced reverse saturation current (<i>J</i>₀). The optimized (Rb_<i>x</i>MA_1–<i>x</i>)­PbI₃ PSCs exhibited a PCE of 18.8% with negligible hysteresis in the photocurrent–voltage curve. The results from this work enhance the understanding of the effect of Rb incorporation into organic–inorganic hybrid halide perovskites and enable the exploration of Rb-incorporated mixed perovskites for various applications, such as solar cells, photodetectors, and light-emitting diodes

1‑D Structured Flexible Supercapacitor Electrodes with Prominent Electronic/Ionic Transport Capabilities

A highly efficient 1-D flexible supercapacitor with a stainless steel mesh (SSM) substrate is demonstrated. Indium tin oxide (ITO) nanowires are prepared on the surface of the stainless steel fiber (SSF), and MnO₂ shell layers are coated onto the ITO/SSM electrode by means of electrodeposition. The ITO NWs, which grow radially on the SSF, are single-crystalline and conductive enough for use as a current collector for MnO₂-based supercapacitors. A flake-shaped, nanoporous, and uniform MnO₂ shell layer with a thickness of ∼130 nm and an average crystallite size of ∼2 nm is obtained by electrodeposition at a constant voltage. The effect of the electrode geometry on the supercapacitor properties was investigated using electrochemical impedance spectroscopy, cyclic voltammetry, and a galvanostatic charge/discharge study. The electrodes with ITO NWs exhibit higher specific capacitance levels and good rate capability owing to the superior electronic/ionic transport capabilities resulting from the open pore structure. Moreover, the use of a porous mesh substrate (SSM) increases the specific capacitance to 667 F g^–1 at 5 mV s^–1. In addition, the electrode with ITO NWs and the SSM shows very stable cycle performance (no decrease in the specific capacitance after 5000 cycles)

A Simple Method To Control Morphology of Hydroxyapatite Nano- and Microcrystals by Altering Phase Transition Route

Hydroxyapatite (HAp) particles with various morphologies such as sphere, rod, whisker, and platelet have attracted a great deal of scientific and technological interest for their broad utilization as reinforcing agents in bone cement, bone fillers, drug carriers, and adsorbents for chromatography. In this Article, a simple method to control the morphology of HAp particles by adjusting the initial pH of precursors and the amount of gelatin and urea additions is introduced. Initially formed calcium phosphate products such as octacalcium phosphate (OCP), hydroxyapatite (HAp), and amorphous calcium phosphate (ACP) are found to be altered by changing the pH of solutions, which induces variation of HAp morphology as well as phase transformation route to HAp. From the observation of HAp formation behavior, the addition of gelatin is revealed to retard HAp formation as well as to change the aspect ratio of HAp particles, which is ascribed to strong adsorption of gelatin on the surface of calcium phosphate. Also, urea is observed to boost HAp formation rate by enhancing hydrolysis reaction. Through the understanding of the influence of the aforementioned variables, the morphology of pure HAp particles is successfully controlled, and this enables the promotion of the applicability of HAp particles in various fields

Scalable Deposition of High-Efficiency Perovskite Solar Cells by Spray-Coating

Spray-deposition is a low-cost, roll-to-roll compatible technique that could potentially replace spin-coating for the deposition of highly efficient perovskite solar cells. Here, perovskite active layers were fabricated in air using an ultrasonic spray system and compared with equivalent spin-coated films. A chlorine-containing perovskite ink with a wide processing window coupled with an antisolvent extraction resulted in perovskite films with relatively rougher surfaces than those spin-coated. A power conversion efficiency (PCE) of 17.3% was achieved with an average of 16.3% from 24 devices. Despite observing differences in film roughness and structure, the performance of sprayed perovskite solar cells was comparable to that of the spin-coated cells processed in an inert atmosphere, showing the versatility of perovskite processing

Indium–Tin–Oxide Nanowire Array Based CdSe/CdS/TiO₂ One-Dimensional Heterojunction Photoelectrode for Enhanced Solar Hydrogen Production

For photoelectrochemical (PEC) hydrogen production, low charge transport efficiency of a photoelectrode is one of the key factors that largely limit PEC performance enhancement. Here, we report a tin-doped indium oxide (In₂O₃:Sn, ITO) nanowire array (NWs) based CdSe/CdS/TiO₂ multishelled heterojunction photoelectrode. This multishelled one-dimensional (1D) heterojunction photoelectrode shows superior charge transport efficiency due to the negligible carrier recombination in ITO NWs, leading to a greatly improved photocurrent density (∼16.2 mA/cm² at 1.0 V vs RHE). The ITO NWs with an average thickness of ∼12 μm are first grown on commercial ITO/glass substrate by a vapor–liquid–solid method. Subsequently, the TiO₂ and CdSe/CdS shell layers are deposited by an atomic layer deposition (ALD) and a chemical bath deposition method, respectively. The resultant CdSe/CdS/TiO₂/ITO NWs photoelectrode, compared to a planar structure with the same configuration, shows improved light absorption and much faster charge transport properties. More importantly, even though the CdSe/CdS/TiO₂/ITO NWs photoelectrode has lower CdSe/CdS loading (i.e., due to its lower surface area) than the mesoporous TiO₂ nanoparticle based photoelectrode, it shows 2.4 times higher saturation photocurrent density, which is attributed to the superior charge transport and better light absorption by the 1D ITO NWs

Highly Efficient Perovskite Solar Modules by Scalable Fabrication and Interconnection Optimization

To push perovskite solar cell (PSC) technology toward practical applications, large-area perovskite solar modules with multiple subcells need to be developed by fully scalable deposition approaches. Here, we demonstrate a deposition scheme for perovskite module fabrication with spray coating of a TiO₂ electron transport layer (ETL) and blade coating of both a perovskite absorber layer and a spiro-OMeTAD-based hole transport layer (HTL). The TiO₂ ETL remaining in the interconnection between subcells significantly affects the module performance. Reducing the TiO₂ thickness changes the interconnection contact from a Schottky diode to ohmic behavior. Owing to interconnection resistance reduction, the perovskite modules with a 10 nm TiO₂ layer show enhanced performance mainly associated with an improved fill factor. Finally, we demonstrate a four-cell MA_0.7FA_0.3PbI₃ perovskite module with a stabilized power conversion efficiency (PCE) of 15.6% measured from an aperture area of ∼10.36 cm², corresponding to an active-area module PCE of 17.9% with a geometric fill factor of ∼87.3%

Zn₂SnO₄‑Based Photoelectrodes for Organolead Halide Perovskite Solar Cells

We report a new ternary Zn₂SnO₄ (ZSO) electron-transporting electrode of a CH₃NH₃PbI₃ perovskite solar cell as an alternative to the conventional TiO₂ electrode. The ZSO-based perovskite solar cells have been prepared following a conventional procedure known as a sequential (or two-step) process with ZSO compact/mesoscopic layers instead of the conventional TiO₂ counterparts, and their solar cell properties have been investigated as a function of the thickness of either the ZSO compact layer or the ZSO mesoscopic layer. The presence of the ZSO compact layer has a negligible influence on the transmittance of the incident light regardless of its thickness, whereas the thickest compact layer blocks the back-electron transfer most efficiently. The open-circuit voltage and fill factor increase with the increasing thickness of the mesoscopic ZSO layer, whereas the short-circuit current density decreases with the increasing thickness except for the thinnest one (∼100 nm). As a result, the device with a 300 nm-thick mesoscopic ZSO layer shows the highest conversion efficiency of 7%. In addition, time-resolved and frequency-resolved measurements reveal that the ZSO-based perovskite solar cell exhibits faster electron transport (∼10 times) and superior charge-collection capability compared to the TiO₂-based counterpart with similar thickness and conversion efficiency

New Hybrid Hole Extraction Layer of Perovskite Solar Cells with a Planar p–i–n Geometry

We report a highly efficient p–i–n type planar perovskite solar cell with a hybrid PEDOT/NiO_<i>x</i> hole-extraction layer. It has been found that the perovskite solar cell with a NiO_<i>x</i> thin film as a hole-extraction layer generally exhibits lower fill factor compared to the conventionally used PEDOT:PSS thin film, whereas it shows higher photocurrent and photovoltage. The fill factor of the NiO_<i>x</i>-based perovskite solar cell can be significantly improved by treating the NiO_<i>x</i> surface with a dilute PEDOT solution. The photoluminescence quenching study and impedance spectroscopic (IS) analysis have revealed that the hole injection at the perovskite/NiO_<i>x</i> interface is significantly facilitated with the PEDOT treatment, which should lead to the increased fill factor. As a result, the p–i–n type planar perovskite solar cell with the new hybrid hole-extraction layer exhibits a high conversion efficiency of 15.1% without the hysteresis effect