Hsing-Lin Wang

High-efficiency solution-processed perovskite solar cells with millimeter-scale grains

Large-crystal perovskite films The performance of organic-inorganic hybrid perovskite planar solar cells has steadily improved. One outstanding issue is that grain boundaries and defects in polycrystalline films degrade their output. Now, two studies report the growth of millimeter-scale single crystals. Nie et al. grew continuous, pinhole-free, thin iodochloride films with a hot-casting technique and report device efficiencies of 18%. Shi et al. used antisolvent vapor-assisted crystallization to grow millimeter-scale bromide and iodide cubic crystals with charge-carrier diffusion lengths exceeding 10 mm. Science, this issue p. 522, p. 519 Solution processing techniques enable the growth of high-quality, large-area perovskite crystals for solar cells. State-of-the-art photovoltaics use high-purity, large-area, wafer-scale single-crystalline semiconductors grown by sophisticated, high-temperature crystal growth processes. We demonstrate a solution-based hot-casting technique to grow continuous, pinhole-free thin films of organometallic perovskites with millimeter-scale crystalline grains. We fabricated planar solar cells with efficiencies approaching 18%, with little cell-to-cell variability. The devices show hysteresis-free photovoltaic response, which had been a fundamental bottleneck for the stable operation of perovskite devices. Characterization and modeling attribute the improved performance to reduced bulk defects and improved charge carrier mobility in large-grain devices. We anticipate that this technique will lead the field toward synthesis of wafer-scale crystalline perovskites, necessary for the fabrication of high-efficiency solar cells, and will be applicable to several other material systems plagued by polydispersity, defects, and grain boundary recombination in solution-processed thin films.

Strongly enhanced current densities in superconducting coated conductors of YBa2Cu3O7-x + BaZrO3.

There are numerous potential applications for superconducting tapes based on YBa2Cu3O7–x (YBCO) films coated onto metallic substrates1. A long-established goal of more than 15 years has been to understand the magnetic-flux pinning mechanisms that allow films to maintain high current densities out to high magnetic fields2. In fact, films carry one to two orders of magnitude higher current densities than any other form of the material3. For this reason, the idea of further improving pinning has received little attention. Now that commercialization of YBCO-tape conductors is much closer, an important goal for both better performance and lower fabrication costs is to achieve enhanced pinning in a practical way. In this work, we demonstrate a simple and industrially scaleable route that yields a 1.5–5-fold improvement in the in-magnetic-field current densities of conductors that are already of high quality.

Graphene/graphene-tube nanocomposites templated from cage-containing metal-organic frameworks for oxygen reduction in Li-O2 batteries

Nitrogen-doped graphene/graphene-tube nanocomposites are prepared by a hightemperature approach using a newly designed cage-containing metal-organic framework (MOF) to template nitrogen/carbon (dicyandiamide) and iron precursors. The resulting N-Fe-MOF catalysts universally exhibit high oxygen-reduction activity in acidic, alkaline, and non-aqueous electrolytes and superior cathode performance in Li-O2 batteries.

Alternating‐current light‐emitting devices based on conjugated polymers

Most polymer electroluminescent devices to date are represented as tunnel diodes and operate under direct‐current (dc) driving field. Here we report the fabrication of symmetrically configured alternating‐current (ac) light‐emitting (SCALE) devices based on conjugated polymers. The new devices consist of an emissive polymer layer sandwiched between two redox polymer layers. This configuration enables the SCALE devices to work under both forward and reverse dc bias as well as in ac modes. The nearly ohmic electrode/redox polymer contacts improve the charge injection efficiency significantly and make the SCALE device operation insensitive to electrode work functions. Symmetric operation supports the key role of redox polymer/emissive polymer interface states.

Application of polyaniline (emeraldine base, EB) in polymer light-emitting devices

Abstract We report the fabrication of several multilayer light-emitting-diode (LED) devices based on a novel conjugated polymer, poly(2,5-dihexadecanoxy phenylene vinylene pyridyl vinylene) (PPV-PPy V), involving the use of polyaniline (emeraldine base, EB) as an insulating layer between the emissive polymer layer and the electrodes. In all the above devices with various configurations (‘3-layers’, ‘4-layers’-1’, ‘4-layers-2’ and ‘5-layers’), only the symmetrically configured a.c. light-emitting (SCALE) (‘5-layers’) device shows the capability of operating in both forward and reverse bias modes and in an a.c. mode. The SCALE devices have a typical turn-on voltage of about 4–6 V and work well under both forward and reverse bias modes. It is important to note that the total resistance ( R= V/I +) of the four devices at any given applied potential decreases as the number of insulating polymer layers increases, suggesting that the nature of the electrode/polymer interface plays a critical role in determining the characteristics of the devices.

A carbon-nanotube-supported graphene-rich non-precious metal oxygen reduction catalyst with enhanced performance durability

A non-precious metal catalyst for oxygen reduction in acid media, enriched in graphene sheets/bubbles during a high-temperature synthesis step, has been developed from an Fe precursor and in situ polymerized polyaniline, supported on multi-walled carbon nanotubes. The catalyst showed no performance loss for 500 hours in a hydrogen/air fuel cell. The improved durability is correlated with the graphene formation, apparently enhanced in the presence of carbon nanotubes.

Mechanistic understanding of surface plasmon assisted catalysis on a single particle: cyclic redox of 4-aminothiophenol

Surface plasmon assisted catalysis (SPAC) reactions of 4-aminothiophenol (4ATP) to and back from 4,4′-dimercaptoazobenzene (DMAB) have been investigated by single particle surface enhanced Raman spectroscopy, using a self-designed gas flow cell to control the reductive/oxidative environment over the reactions. Conversion of 4ATP into DMAB is induced by energy transfer (plasmonic heating) from surface plasmon resonance to 4ATP, where O2 (as an electron acceptor) is essential and H2O (as a base) can accelerate the reaction. In contrast, hot electron (from surface plasmon decay) induction drives the reverse reaction of DMAB to 4ATP, where H2O (or H2) acts as the hydrogen source. More interestingly, the cyclic redox between 4ATP and DMAB by SPAC approach has been demonstrated. This SPAC methodology presents a unique platform for studying chemical reactions that are not possible under standard synthetic conditions.

Angular-dependent vortex pinning mechanisms in YBa2Cu3O7 coated conductors and thin films

We compare the angular-dependent critical current density (Jc) in YBa2Cu3O7 films deposited on MgO templates grown by ion-beam-assisted deposition (IBAD), and on single-crystal substrates. We identify three angular regimes in which pinning is dominated by different types of correlated and uncorrelated defects. Those regimes are present in all cases, but their extension and characteristics are sample dependent, reflecting differences in texture and defect density. The more defective nature of the films on IBAD turns into an advantage as it results in higher Jc, demonstrating that the performance of the films on single crystals is not an upper limit for the IBAD coated conductors.

Reversible fluorescence quenching in carbon nanotubes for biomolecular sensing

Biosensing applications of single-walled carbon nanotubes have been demonstrated in solid-state device structures. Bioanalyte sensing schemes based on coupling of reversible nanotube fluorescence quenching to redox reactions paired to enzymatic peroxide generation have also been pursued. Here we show a new approach to highly sensitive nanotube-based optical sensing. Single-walled carbon nanotubes interacting with dye-ligand conjugates--a redox-active dye molecule that is covalently bound to a biological receptor ligand (such as biotin in this case)--showed fluorescence quenching. Further interaction between the receptor ligand on the conjugates and target analytes (avidin in this case) induced the recovery of the quenched fluorescence, forming the basis of the sensing scheme. Nanomolar sensitivity was attained with high specificity for the target analyte. This is a versatile approach because a wide range of conjugation possibilities exists between the potential receptors and redox quenchers.

Blue electroluminescent devices based on soluble poly(p‐pyridine)

We have fabricated unilayer electroluminescent devices from soluble poly(p‐pyridine) (PPy). The solubility of PPy in weak acids allows direct spin casting of the polymer films. The electroluminescence spectrum peaks at 2.5 eV (497 nm) corresponding to white light weighted towards the blue end of the spectrum. The photoluminescence spectrum peaks at 2.35 eV (530 nm). The operating voltages of the devices ranged from 4 to 12 V with current densities of 6 to 8 mA/mm2. We compare our devices with similar blue emitting devices based on poly(p‐phenylene).