Advanced Materials

The cover shows a surface acoustic-wave driven light shutter formed from a polymer-dispersed liquid crystal (PDLC) film. In regions without surface acoustic waves (SAWs) the LC molecules in the droplets show a radial alignment, while within the SAW working region, the LC molecules show an alignment perpendicular to the substrate. With further optimization, this PDLC-based light shutter, reported on p. 1656 by Tony Jun Huang and co-workers, could possess advantages such as low power consumption, high throughput, and excellent optical properties.

Single molecules provide ideal systems to investigate charge transport on the molecular scale, a subject of intense current interest for both practical applications and achieving a fundamental understanding of novel physical phenomena that take place in such mechanisms. The review article by Takhee Lee and co-workers on p. 1583 focuses on experimental aspects of electronic devices made with single molecules.

Single molecules provide ideal systems to investigate charge transport on the molecular scale, which is a subject of intense current interest for both practical applications and achieving a fundamental understanding of novel physical phenomena that take place in molecular-scale charge transport. This review article focuses on experimental aspects of electronic devices made with single molecules.

Carbon-based spin valves, composed of a C₆₀ layer sandwiched between two magnetic materials show room temperature spin transport for a fullerene thickness up to 30 nm. This result, interpreted by a multistep tunnelling transport model, represents an advance in the field of carbon spintronics.

Chirality-triggered wettability switching is an important challenge for responsive surfaces and a good example of mimicking nature. Through hydrogen bonding interactions among chiral recognition units, mediating units, and functional switching units, smart polymer films can reversibly change their surface properties, including wettability and morphology, according to the chirality of biomolecules in solution, as reported by Taolei Sun and Guangyan Qing on p. 1615.

Chiral recognition of monosaccharide enantiomers is translated into large-extent wettability switching and other macroscopic surface properties, e.g., volume, on a smart copolymer film containing dipeptide units by a cooperative hydrogen-bonding interaction. This work points to a promising direction for developing novel chirality-responsive materials, which find broad applications in controllable chiral separation, chiral medicine, smart biodevices, etc.

The Nature of polarization fatigue in BiFeO₃ was revealed using monodomain epitaxial thin films with different orientations. We have found that the fatigue strongly depends on switching path. Fatigue-free behavior is demonstrated in ferroelastic switching (71° and 109°). In contrast, a significant polarization fatigue occurs in ferroelectric switching (180°) which involves multiple switching paths and formation of charged domain walls that effectively pin domain walls and hence reduce the total switched volume.

Patternable solution-crystallized organic transistors are developed with very high carrier mobility that exceeds 10 cm² V⁻¹ s⁻¹. The devices feature a newly synthesized air-stable compound 2,9-didecyldi-naphtho[2,3-b:2’,3’-f]thieno[3,2-b]thiophene (C₁₀-DNTT) and are formed from hot solution. A method of oriented growth is introduced to obtain the single-crystalline films of C₁₀-DNTT, regulating the crystallizing direction and the positions in a single process.

Silk fibroin, one of the silk proteins produced by the silk work, is shown to be an excellent gate dielectric material for pentacene organic thin-film transistors (OTFTs). The pentacene OTFT described exhibits a very high field-effect mobility value of 23.2 cm² V⁻¹ s⁻¹ and a low operating voltage of –3 V. This, combined with its advantages of low cost and light weight, confers on it great potential for high-speed flexible electronics applications.

A mutual ferromagnetic and ferroelectric coupling (multiferroic behavior) in Cu-doped ZnO is demonstrated via deterministic control of Cu doping and defect engineering. The coexistence of multivalence Cu ions and oxygen vacancies is important to multiferroic behaviors in ZnO:Cu. The samples show clear ferroelectric and ferromagnetic domain patterns. These domain structures may be written reversibly via electric and magnetic bias.

A solid-state polymerizable conductive monomer with good conductivity and penetration to the photoelectrode is introduced to iodine-free, solid-state dye-sensitized solar cells (DSSCs) using an easily accessible and widely applicable fabrication method. A conducting polymer as a hole transporting material effectively penetrates into the TiO₂ pores and polymerizes with heating to mild temperatures. The fabricated DSSCs exhibited the highest energy conversion efficiency of 5.4% in N719 dye.

High purity crystalline ZnSe compound semiconductor waveguides are fabricated inside optical fibers via high-pressure chemical vapor deposition. These fiber waveguides exhibit very low loss (e.g., <1 dB cm⁻¹ at 1550 nm wavelength). The superior optical and electronic properties of crystalline compound semiconductors can now be exploited in a fiber geometry.

Ratiometric encoding with two types of photon-upconverting nanoparticles is demonstrated. Different concentrations of filter dyes on the nanoparticle’s surface are used to tune selectively one of the dual emission bands (I_code). The second emission band (I_ref), by contrast, serves as an internal reference to obtain ratiometric codes. Combining UCNPs with distinct emission bands expands the coding capacity exponentially.

A surface acoustic wave (SAW)-driven light shutter is demonstrated using poly­mer-dispersed liquid crystals (PDLCs). Our experiments show that a PDLC film switches from a non-transparent state to a transparent state when subjected to a SAW. The working mechanism is analyzed theoretically and the acousto-optic properties of the PDLC film are characterized. This swit­ching behavior results from the acoustic streaming-induced realignment of liquid crystal (LC) molecules as well as SAW-induced thermal diffusion. Such a device shows an excellent performance in terms of imaging quality and optical contrast, which is important for display applications and smart windows.

Highly efficient polymer solar cells based on novel photocrosslinkable donor–acceptor conjugated polymers are fabricated and their long-term thermal stability is reported. After 72 h of thermal annealing at 150 °C, a stable power conversion efficiency as high as 4.7% is maintained. The control of active layer morphology and device performance through annealing is correlated with the synthetic design of the photocrosslinkable polymer.

Conjugated zwitterionic polyelectrolytes with free mobile ions, good alcohol solubility, and excellent electron injection ability are developed. PLEDs based on a Al cathode with a conjugated zwitterionic polyelectrolytes electron injection layer showed more than 70 times higher efficiency than that of bare Al cathode devices.

We present a new experimental technique that affords direct quantification of the fraction of charge carriers lost in poly(3-hexylthiophene):fullerene solar cells by bimolecular recombination. Depending on annealing conditions up to 17% of carriers recombine bimolecularly under solar illumination. We explain our findings with a closed analytical expression for the photocurrent generated by an organic solar cell.

Studies of the healing of artificial defects in the surface of highly oriented pyrolitic graphite (HOPG) by chemical vapor deposition (CVD) of acetylene show that only single-layer depth defects could be healed completely during the CVD treatment. A promising method is introduced for defect control in surfaces of graphite, graphene, and graphene-based materials, with important implications for their application.

Highly efficient and air-stable inverted polymer solar cells are fabricated with a poly(2,7-carbazole) derivative (PCDTBT):fullerene derivative (PC₇₀BM) active layer. Low-temperature-annealed sol-gel-derived zinc oxide (ZnO) and molybdenum oxide films are selected as the electron and hole transporting layers, respectively. Power conversion efficiency (PCE) of 6.3% is demonstrated, which is the highest reported to date for inverted polymer solar cells.