A Strained Donor-Acceptor Carbon Nanohoop: Synthesis, Photophysical and Charge Transport Properties.

Herein, we report the synthesis of a novel intramolecular donor-acceptor (D-A) system ([12]CPP-8TPAOMe) based on cycloparaphenylenes (CPPs) grafted with eight di(4-methoxyphenyl)amino groups (TPAOMe) as donors. Compared to [12]CPP, D-A nanohoop exhibited significant changes in physical properties, including a large redshift (>78 nm) in the fluorescence spectrum and novel positive solvatofluorochromic properties with a maximum peak ranging from 484 nm to 546 nm. The potential applications of [12]CPP-8TPAOMe in electron- and hole-transport devices were further investigated, and its bipolar behavior as a charge transport active layer was clearly observed.

Temperature-modulated crystal growth and performance for highly reproducible and efficient perovskite solar cells.

The annealing temperature (Ta) effect on CH3NH3PbI3 perovskite solar cells (PSCs) was studied. By utilizing a two-step technique, the Ta dependences of the optical absorption, grain size, and crystallinity of a CH3NH3PbI3 thin film have been revealed. It is found that the grain size of the CH3NH3PbI3 film increases monotonically with Ta. Meanwhile, the decomposed PbI2 emerges when Ta exceeds 120 °C and its content increases rapidly as Ta increases further. Consequently, the optical absorption of the CH3NH3PbI3 film and the efficiency of PSCs reach their maximum at Ta = 120 °C simultaneously. The highest and average device performances of PSCs achieved via this method are 17.61% and 16.40%, respectively. These results confirm the key role played by temperature and provide a route to the performance-optimization of PSCs.

Solution-processed solar cells based on inorganic bulk heterojunctions with evident hole contribution to photocurrent generation.

To develop solution-processed and novel device structures is of great importance for achieving advanced and low-cost solar cells. In this paper, we report the solution-processed solar cells based on inorganic bulk heterojunctions (BHJs) featuring a bulk crystalline Sb2S3 absorbing layer interdigitated with a TiO2 nanoarray as an electron transporter. A solution-processed amorphous-to-crystalline transformation strategy is used for the preparation of Sb2S3/TiO2-BHJs. Steady-state and dynamic results demonstrate that the crystalline structure in the Sb2S3 absorbing layer is crucial for efficient devices, and a better Sb2S3 crystallization favors a higher device performance by increasing the charge collection efficiency for a higher short-circuit current, due to reduced interfacial and bulk charge recombinations, and enhancing the open-circuit voltage and fill factor with the reduced defect states in the Sb2S3 layer as well. Moreover, an evident contribution to photocurrent generation from the photogenerated holes in the Sb2S3 layer is revealed by experimental and simulated dynamic data. These results imply a kind of potential non-excitonic BHJ for energy conversion.

Engineering the passivation routes of perovskite films towards high performance solar cells.

Passivation treatment is an effective method to suppress various defects in perovskite solar cells (PSCs), such as cation vacancies, under-coordinated Pb2+ or I-, and Pb-I antisite defects. A thorough understanding of the diversified impacts of different defect passivation methods (DPMs) on the device performance will be beneficial for making wise DPM choices. Herein, we choose a hydrophobic Lewis acid tris(pentafluorophenyl)borane (BCF), which can dissolve in both the perovskite precursor and anti-solvent, as the passivation additive. BCF treatment can immobilize organic cations via forming hydrogen bonds. Three kinds of DPMs based on BCF are applied to modify perovskite films in this work. It is found that the best DPM with BCF dissolved in anti-solvent can not only passivate multiple defects in perovskite, but also inhibit δ phase perovskite and improve the stability of devices. Meanwhile, DPM with BCF dissolved in both the perovskite precursor and anti-solvent can cause cracks and voids in perovskite films and deteriorate device performance, which should be avoided in practical applications. As a result, PSCs based on optimal DPMs of BCF present an increased efficiency of 22.86% with negligible hysteresis as well as improved overall stability. This work indicates that the selection and optimization of DPMs have an equally important influence on the photovoltaic performance of PSCs as the selection of passivation additives.

Coherent Phonon-Induced Gigahertz Optical Birefringence and Its Manipulation in SrTiO3.

Birefringence, which modulates the polarization of electromagnetic wave, has been commercially developed and widely used in modern photonics. Fostered by high-frequency signal processing and communications, feasible birefringence technologies operating in gigahertz (GHz) range are highly desired. Here, a coherent phonon-induced GHz optical birefringence and its manipulation in SrTiO3 (STO) crystals are demonsrated. With ultrafast laser pumping, the coherent acoustic phonons with low damping are created in the transducer/STO structures. A series of transducer layers are examined and the optimized one with relatively high photon-phonon conversion efficiency, i.e., semiconducting LaRhO3 film, is obtained. The most intriguing finding here is that, by virtue of high sensitivity to strain perturbation of STO, GHz optical birefringence can be induced by the coherent acoustic phonons and the birefringent amplitudes possess crystal orientation dependence. Optical manipulation of both coherent phonons and its induced GHz birefringence by double pump technique are also realized. These findings reveal an alternative mechanism of ultrafast optical birefringence control, and offer prospects for applications in high-frequency acoustic-optics devices.

Out-of-Plane Alignment of Conjugated Semiconducting Polymers by Horizontal Rotation in a High Magnetic Field.

The effective control of film morphology and molecular packing in the out-of-plane direction of semiconductor polymers plays a critical role in governing charge carrier transport in the direction perpendicular to the substrate. In this study, a highly out-of-plane alignment of the n-type polymer P(NDI2OD-T2) film has been successfully achieved by horizontal rotation in a high magnetic field (HR-HMF). The out-of-plane alignment of the P(NDI2OD-T2) film has showed a change from 72% face-on to 98.2% face-on lamellar texture as well as a 1.6-fold increase of the π-π stacking crystalline correlation length compared with that of as-cast polymer films without HR-HMF-induced alignment. Meanwhile, the film with near-perfect face-on molecular packing exhibited more than 18-fold enhancement of electron mobility compared to the unaligned film. The excellent electrical performance achieved with the HR-HMF process indicates its application potential for fabricating high-performance sandwich-type organic electronic devices, such as solar cells and light-emitting diodes.

Solvent Vapor-Assisted Magnetic Manipulation of Molecular Orientation and Carrier Transport of Semiconducting Polymers.

Effective control of the molecular orientation and the degree of ordering in organic semiconductors is important to achieve high-performance organic electronics. Herein, we have successfully achieved highly oriented films in centimeter scale for a naphthalenedicarboximide-based semiconducting polymer (P(NDI2OD-T2)) by solvent vapor annealing (SVA) of precast films under a high magnetic field (HMF). As revealed by the microstructural studies, the SVA-HMF films exhibit a remarkably higher degree of chain alignment and high morphological uniformity compared to the HMF-guided drop-cast films. Based on the structural evolution of the films with the SVA time, a mechanism is proposed to elucidate the alignment process, which emphasizes that the chain aggregates re-formed in the swollen films trigger magnetic alignment and determine the film order. Compared with the unaligned films, field-effect transistors of the magnetic aligned P(NDI2OD-T2) films have exhibited a 19-fold enhancement of electron mobility and an extraordinarily large mobility anisotropy of 125. Furthermore, a significantly reduced energetic barrier for activated transport is observed on the aligned devices from temperature-variable measurements. The improved performance achieved by the HMF-SVA process has indicated its potential for high-performance organic electronic applications.

Enhanced Spin Transport of Conjugated Polymer in the Semiconductor/Insulating Polymer Blend.

Conjugated polymers are of high potential in the development of spintronic devices. In this paper, we report systematic studies on spin transport properties of a semiconducting polymer PBDTTT-C-T in the permalloy/polymer/Pt trilayer using the spin pumping method. Pure spin current with long spin relaxation time is observed via the inverse spin Hall effect (ISHE) measurements. Furthermore, spin current is also found to propagate through the blend film consisting of a small amount of PBDTTT-C-T in an insulating matrix. The polymer blend exhibits a remarkably enhanced spin relaxation length (56 nm) and carrier mobility compared to pristine PBDTTT-C-T. From film microstructural characterizations, we propose that the enhanced spin/carrier transport properties are attributed to the formation of interlinked nanonetwork comprising of the PBDTTT-C-T chain bundles in the inert matrix to afford efficient intrachain charge conduction pathway. Temperature- dependent ISHE measurements support the spin-orbit coupling dominated spin relaxation mechanism.

Robust Ferroelectric Properties in (K,Na)NbO3-Based Lead-Free Films via a Self-Assembled Nanocomposite Approach.

(K,Na)NbO3-based lead-free ferroelectric materials are highly desired in modern electronic applications and have long been considered as a strong candidate for replacing (Pb,Zr)TiO3, but most of them are deficient in large remnant polarization and decent thermal stability. Here, a unique lead-free 0.95(K0.49Na0.49Li0.02)(Nb0.8Ta0.2)O3-0.05CaZrO3 with 2 wt % MnO2 addition (KNNLT-CZ-M) ferroelectric film with special nanocomposite structures grown on La0.7Sr0.3MnO3-coated SrTiO3(001) substrate is demonstrated. The KNNLT-CZ-M films display excellent ferroelectricity with a large twice remnant polarization of 64.91 µC/cm2, a superior thermal stability of ferroelectricity from -196 to 300 °C, and a high Curie temperature of 400 °C. These robust performances could be attributed to the densely arranged self-assembled nanocolumns (∼10 nm in diameter) in the films, which can vertically strain the matrix and enhance its b/a ratio. The formation of the nanocolumns critically depends on the CaZrO3 component. Our results may help the design of a new type of lead-free ferroelectric films and promote their potential applications in microelectronic devices.

Ambipolar transport in the smectic E phase of 2-propyl-5''-hexynylterthiophene derivative over a wide temperature range.

5-Hexyl-5''-hexynyl-2,2':5',2''-terthiophene exhibits the smectic E phase below 200 degrees C and does not crystallize when it is cooled to -100 degrees C. Between 200 and -100 degrees C, non-dispersive transport is observed for holes and electrons with time-of-flight spectroscopy. Over the entire temperature range, the electron mobility is approximately twice as high as that of the hole. The hole and electron transport characteristics in the smectic phase below 0 degrees C are explained by the Gaussian disorder model, which was proposed for amorphous organic semiconductors. The disorder parameters, sigma and Sigma, are almost the same for holes and electrons. However, the pre-exponential parameter mu(0) for the electron is twice as large as that for the hole, which can be attributed to the difference in the extension of the LUMO of the molecules. The energetic disorder sigma is primarily determined by the disorder in the orientation of the permanent dipoles of liquid crystal molecules.

Growth of Compact CH3NH3PbI3 Thin Films Governed by the Crystallization in PbI2 Matrix for Efficient Planar Perovskite Solar Cells.

As a convenient preparation technique, a two-step method, which is normally done by spin-coating CH3NH3I onto PbI2 film followed by a thermal annealing, is generally used to prepare solution-processed CH3NH3PbI3 films for planar perovskite solar cells. Here, we prepare the compact CH3NH3PbI3 thin films by the two-step method at a low temperature (<80 °C) and investigate the effects of PbI2 crystallization on the structure-property correlation in the CH3NH3PbI3 films. It is found that the importance of the crystallization in PbI2 matrix lies in governing the transition from the (001) plane of trigonal PbI2 to the (002) plane of tetragonal CH3NH3PbI3 in the rapid reaction process for atoms to coordinate into perovskite during spin-coating, which actually determines the morphology and the type of vacancy defects in resulting perovskite; a better crystallized PbI2 film has a much stronger ability to react with CH3NH3I solution and produces larger CH3NH3PbI3 grains with a higher crystallinity. The CH3NH3PbI3/TiO2 planar solar cell derived from a better crystallized PbI2 film exhibits significantly improved performance and stability as the result of the higher crystallinity inside the perovskite film. Moreover, it is demonstrated that the crystalline PbI2 film matrix subjected to the annealing after a slow heating process prior to contacting CH3NH3I solution is more effective for CH3NH3PbI3 formation than that with a direct annealing history. The results in this paper provide a guide for preparing high-quality CH3NH3PbI3 thin films for efficient perovskite solar cells and CH3NH3PbI3 interfacial films over the layers susceptible to temperature.

Room-temperature Magnetism in Carbon Dots and Enhanced Ferromagnetism in Carbon Dots-Polyaniline Nanocomposite.

Room temperature magnetic ordering is reported for very small carbon dots (CDs), mat-like polyaniline nanofibers (Mat-PANI) and a composite of CDs@Mat-PANI containing 0.315 wt% CDs. We have found saturation magnetization (M S ) of CDs, Mat-PANI and CDs@Mat-PANI at 5 (20/300) K equals to 0.0079 (0.0048/0.0019), 0.0116 (0.0065/0.0055) and 0.0349 (0.0085/0.0077) emu/g, respectively. The M S enhancement in CDs@Mat-PANI (200% and 40% at 5 K and 300 K, respectively) is attributed to electron transfer from Mat-PANI imine N-atoms to the encapsulated CDs. Changes in M S values reveal that 0.81 (0.08) electron/CD is transferred at 5 (300) K, which is supported by observation of CDs photoluminescence (PL) redshift while in CDs@Mat-PANI. Band-bending and bandgap-renormalization calculations are used to predict a redshift of 117 meV at 300 K as a result of the electron transfer, in excellent agreement with the PL data (110 meV). Raman, X-ray diffraction and X-ray photoelectron spectroscopy data are used to confirm the electron transfer process as well as the strong interaction of CDs with PANI within CDs@Mat-PANI, which increases the crystalline domain size of Mat-PANI from about 4.8 nm to 9.2 nm while reducing the tensile strain from about 6.2% to 1.8%.

Excellent spin transport in spin valves based on the conjugated polymer with high carrier mobility.

Organic semiconductors (OSCs) are characteristic of long spin-relaxation lifetime due to weak spin-orbit interaction and hyperfine interaction. However, short spin diffusion length and weak magnetoresistance (MR) effect at room temperature (RT) was commonly found on spin valves (SVs) using an organic spacer, which should be correlated with low carrier mobility of the OSCs. Here, N-type semiconducting polymer P(NDI2OD-T2) with high carrier mobility is employed as the spacer in the SV devices. Exceedingly high MR ratio of 90.0% at 4.2 K and of 6.8% at RT are achieved, respectively, via improving the interface structure between the polymer interlayer and top cobalt electrode as well as optimal annealing of manganite bottom electrode. Furthermore, we observe spin dependent transport through the polymeric interlayer and a large spin diffusion length with a weak temperature dependence. The results indicate that this polymer material can be used as a good medium for spintronic devices.

Synthesis of FeCo nanocrystals encapsulated in nitrogen-doped graphene layers for use as highly efficient catalysts for reduction reactions.

A facile strategy to fabricate FeCo nanocrystals with nitrogen-doped graphene shells has been designed, which involves one-step thermal decomposition of Prussian blue analogue (PBA) Fe3[Co(CN)6]2 spheres. The as-prepared product can be used as a non-precious-metal catalyst with a highly efficient catalytic activity and a magnetically separable capability in the reduction of 4-nitrophenol.

Preparation, optical and electrical properties of PTCDA nanostructures.

Film, nanorods (NRs), nanowires (NWs), and nanoparticles (NPs) of perylene-3,4,9,10-tetracarboxylic dianhydride (PTCDA) were prepared by organic molecular beam deposition (OMBD) on porous anodic alumina oxide (AAO) at different substrate temperatures (Ts). Scanning electron microscopy (SEM) study showed that the morphologies of the nanostructures (NS) formed on AAO strongly depend on the Ts. The absorption spectra of different PTCDA NS present strong absorbance in the wavelength range of 400-600 nm, and the photoluminescence (PL) spectra show a blue shift as Ts increases. The current versus voltage (I-V) characteristic illustrates that the electrical conductivity of the single-crystal NW is about 3 ± 0.1 S m(-1), which is much higher than the conductivity of PTCDA film reported previously.

CuO/Cu2O composite hollow polyhedrons fabricated from metal-organic framework templates for lithium-ion battery anodes with a long cycling life.

Novel CuO/Cu2O hollow polyhedrons with porous shells were fabricated by thermal decomposition of coordination compound [Cu3(btc)2]n (btc = benzene-1,3,5-tricarboxylate) polyhedrons at 350 °C. When tested as anode materials for lithium-ion batteries, these hollow polyhedrons exhibited a reversible lithium storage capacity as high as 740 mA h g(-1) at 100 mA g(-1) after 250 cycles even if the charge-discharge process is stopped for one week during the test time.