Space-Charge-Mediated Anomalous Ferroelectric Switching in P(VDF–TrEE) Polymer Films

We report on the switching dynamics of P­(VDF–TrEE) copolymer devices and the realization of additional substable ferroelectric states via modulation of the coupling between polarizations and space charges. The space-charge-limited current is revealed to be the dominant leakage mechanism in such organic ferroelectric devices, and electrostatic interactions due to space charges lead to the emergence of anomalous ferroelectric loops. The reliable control of ferroelectric switching in P­(VDF–TrEE) copolymers opens doors toward engineering advanced organic memories with tailored switching characteristics

Perovskite Oxide SrTiO₃ as an Efficient Electron Transporter for Hybrid Perovskite Solar Cells

In this work, we explored perovskite oxide SrTiO₃ (STO) for the first time as the electron-transporting layer in organolead trihalide perovskite solar cells. The steady-state photoluminescence (PL) quenching and transient absorption experiments revealed efficient photoelectron transfer from CH₃NH₃PbI_3–<i>x</i>Cl_<i>x</i> to STO. Perovskite solar cells with <i>meso</i>-STO exhibit an open circuit voltage of 1.01 V, which is 25% higher than the value of 0.81 V achieved in the control device with the conventional <i>meso</i>-TiO₂. In addition, an increase of 17% in the fill factor was achieved by tailoring the thickness of the <i>meso</i>-STO layer. We found that the application of STO leads to uniform perovskite layers with large grains and complete surface coverage, leading to a high shunt resistance and improved performance. These findings suggest STO as a competitive candidate as electron transport material in organometal perovskite solar cells

Emerging chalcohalide materials for energy applications

Semiconductors with multiple anions currently provide a new materials platform from which improved functionality emerges, posing new challenges and opportunities in material science. This review has endeavored to emphasize the versatility of the emerging family of semiconductors consisting of mixed chalcogen and halogen anions, known as “chalcohalides”. As they are multifunctional, these materials are of general interest to the wider research community, ranging from theoretical/computational scientists to experimental materials scientists.This review provides a comprehensive overview of the development of emerging Bi- and Sb-based as well as a new Cu, Sn, Pb, Ag, and hybrid organic−inorganic perovskite-based chalcohalides. We first highlight the high-throughput computational techniques to design and develop these chalcohalide materials.We then proceed to discuss their optoelectronic properties,band structures, stability, and structural chemistry employing theoretical and experimental underpinning toward high-performance devices. Next, we present an overview of recent advancements in the synthesis and their wide range of applications in energy conversion and storage devices. Finally, we conclude the review by outlining the impediments and important aspects in this field as well as offering perspectives on future research directions to further promote the development of chalcohalide materials in practical applications in the future.</p

Epitaxy-Enabled Vapor–Liquid–Solid Growth of Tin-Doped Indium Oxide Nanowires with Controlled Orientations

Controlling the morphology of nanowires in bottom-up synthesis and assembling them on planar substrates is of tremendous importance for device applications in electronics, photonics, sensing and energy conversion. To date, however, there remain challenges in reliably achieving these goals of orientation-controlled nanowire synthesis and assembly. Here we report that growth of planar, vertical and randomly oriented tin-doped indium oxide (ITO) nanowires can be realized on yttria-stabilized zirconia (YSZ) substrates via the epitaxy-assisted vapor–liquid–solid (VLS) mechanism, by simply regulating the growth conditions, in particular the growth temperature. This robust control on nanowire orientation is facilitated by the small lattice mismatch of 1.6% between ITO and YSZ. Further control of the orientation, symmetry and shape of the nanowires can be achieved by using YSZ substrates with (110) and (111), in addition to (100) surfaces. Based on these insights, we succeed in growing regular arrays of planar ITO nanowires from patterned catalyst nanoparticles. Overall, our discovery of unprecedented orientation control in ITO nanowires advances the general VLS synthesis, providing a robust epitaxy-based approach toward rational synthesis of nanowires

General Strategy for Fabricating Thoroughly Mesoporous Nanofibers

Recently, preparation of mesoporous fibers has attracted extensive attentions because of their unique and broad applications in photocatalysis, optoelectronics, and biomaterials. However, it remains a great challenge to fabricate thoroughly mesoporous nanofibers with high purity and uniformity. Here, we report a general, simple and cost-effective strategy, namely, foaming-assisted electrospinning, for producing mesoporous nanofibers with high purity and enhanced specific surface areas. As a proof of concept, the as-fabricated mesoporous TiO₂ fibers exhibit much higher photocatalytic activity and stability than both the conventional solid counterparts and the commercially available P25. The abundant vapors released from the introduced foaming agents are responsible for the creation of pores with uniform spatial distribution in the spun precursor fibers. The present work represents a critically important step in advancing the electrospinning technique for generating mesoporous fibers in a facile and universal manner

Mechanism of Polarization Fatigue in BiFeO₃

Fatigue in ferroelectric oxides has been a long lasting research topic since the development of ferroelectric memory in the late 1980s. Over the years, different models have been proposed to explain the fatigue phenomena. However, there is still debate on the roles of oxygen vacancies and injected charges. The main difficulty in the study of fatigue in ferroelectric films is that the conventional vertical sandwich structure prevents direct observation of the microscopic evolution through the film thickness during the electric field cycling. To circumvent this problem, we take advantage of the large in-plane polarization of BiFeO₃ and conduct direct domain and local electrical characterizations using a planar device structure. The combination of piezoresponse force microscopy and scanning kelvin probe microscopy allows us to study the local polarization and space charges simultaneously. It is observed that charged domain walls are formed during the electrical cycling, but they do not cause polarization fatigue. After prolonged cycling, injected charges appear at the electrode/film interfaces, where domains are pinned. When the pinned domains grow across the channel, macroscopic fatigue appears. The role of injected charges in polarization fatigue of BiFeO₃ is clearly demonstrated

Growing Crystalline Chalcogenidoarsenates in Surfactants: From Zero-Dimensional Cluster to Three-Dimensional Framework

Although surfactants have been widely used to tailor the size, shape, and surface properties of nanocrystals and control the pore size and phases of mesoporous frameworks, the use of surfactants as reaction media to grow chalcogenide crystals is unprecedented. In addition, compared with ionic liquids, surfactants are much cheaper and can have multifunctional properties such as acidic, basic, neutral, cationic, anionic, or even block. These features suggest that surfactants could be promising reaction platforms for the development of novel chalcogenide crystals. In this work, we used chalcogenidoarsenates as a model system to demonstrate our strategy. By using three different surfactants as reaction media, we obtained a series of novel thioarsenates ranging from a zero-dimensional (0D) cluster to a three-dimensional (3D) framework, namely, [NH₄]₈[Mn₂As₄S₁₆] (<b>1</b>), [Mn­(NH₃)₆]­[Mn₂As₂S₈(N₂H₄)₂] (<b>2</b>), [enH]­[Cu₃As₂S₅] (<b>3</b>), and [NH₄]­[MnAs₃S₆] (<b>4).</b> The band gaps (estimated from the steep absorption edges) were found to be 2.31 eV for <b>1</b> (0D), 2.46 eV for <b>2</b> (1D), 1.91 eV for <b>3</b> (2D), and 2.08 eV for <b>4</b> (3D). The magnetic study of <b>4</b> indicated weak antiferromagnetic behavior. Our strategy of growing crystalline materials in surfactants could offer exciting opportunities for preparing novel crystalline materials with diverse structures and interesting properties

A Photodetector Based on p‑Si/n-ZnO Nanotube Heterojunctions with High Ultraviolet Responsivity

Enhanced ultraviolet (UV) photodetectors (PDs) with high responsivity comparable to that of visible and infrared photodetectors are needed for commercial applications. n-Type ZnO nanotubes (NTs) with high-quality optical, structural, and electrical properties on a p-type Si(100) substrate are successfully fabricated by pulsed laser deposition (PLD) to produce a UV PD with high responsivity, for the first time. We measure the current–voltage characteristics of the device under dark and illuminated conditions and demonstrated the high stability and responsivity (that reaches ∼101.2 A W^–1) of the fabricated UV PD. Time-resolved spectroscopy is employed to identify exciton confinement, indicating that the high PD performance is due to optical confinement, the high surface-to-volume ratio, the high structural quality of the NTs, and the high photoinduced carrier density. The superior detectivity and responsivity of our NT-based PD clearly demonstrate that fabrication of high-performance UV detection devices for commercial applications is possible

Robust Room-Temperature Ferromagnetism with Giant Anisotropy in Nd-Doped ZnO Nanowire Arrays

As an important class of spintronic material, ferromagnetic oxide semiconductors are characterized with both charge and spin degrees of freedom, but they often show weak magnetism and small coercivity, which limit their applications. In this work, we synthesized Nd-doped ZnO nanowire arrays which exhibit stable room temperature ferromagnetism with a large saturation magnetic moment of 4.1 μ_B/Nd as well as a high coercivity of 780 Oe, indicating giant magnetic anisotropy. First-principles calculations reveal that the remarkable magnetic properties in Nd-doped ZnO nanowires can be ascribed to the intricate interplay between the spin moments and the Nd-derived orbital moments. Our complementary experimental and theoretical results suggest that these magnetic oxide nanowires obtained by the bottom-up synthesis are promising as nanoscale building blocks in spintronic devices

Effects of High Temperature and Thermal Cycling on the Performance of Perovskite Solar Cells: Acceleration of Charge Recombination and Deterioration of Charge Extraction

In this work, we investigated the effects of high operating temperature and thermal cycling on the photovoltaic (PV) performance of perovskite solar cells (PSCs) with a typical mesostructured (m)-TiO₂–CH₃NH₃PbI_3–<i>x</i>Cl_<i>x</i>–spiro-OMeTAD architecture. After temperature-dependent grazing-incidence wide-angle X-ray scattering, in situ X-ray diffraction, and optical absorption experiments were carried out, the thermal durability of PSCs was tested by subjecting the devices to repetitive heating to 70 °C and cooling to room temperature (20 °C). An unexpected regenerative effect was observed after the first thermal cycle; the average power conversion efficiency (PCE) increased by approximately 10% in reference to the as-prepared device. This increase of PCE was attributed to the heating-induced improvement of the crystallinity and p doping in the hole transporter, spiro-OMeTAD, which promotes the efficient extraction of photogenerated carriers. However, further thermal cycles produced a detrimental effect on the PV performance of PSCs, with the short-circuit current and fill factor degrading faster than the open-circuit voltage. Similarly, the PV performance of PSCs degraded at high operation temperatures; both the short-circuit current and open-circuit voltage decreased with increasing temperature, but the temperature-dependent trend of the fill factor was the opposite. Our impedance spectroscopy analysis revealed a monotonous increase of the charge-transfer resistance and a concurrent decrease of the charge-recombination resistance with increasing temperature, indicating a high recombination of charge carriers. Our results revealed that both thermal cycling and high temperatures produce irreversible detrimental effects on the PSC performance because of the deteriorated interfacial photocarrier extraction. The present findings suggest that the development of robust charge transporters and proper interface engineering are critical for the deployment of perovskite PVs in harsh thermal environments