A novel series of soluble conjugated random and alternating copolymers (PFO-TST) derived from 9,9-dioctylfluorene (FO) and 1,1-dimethyl-3,4-diphenyl-2,5-bis(2‘-thienyl)silole (TST) were synthesized by palladium(0)-catalyzed Suzuki coupling reactions. The feed ratios of FO to TST were 99:1, 95:5, 90:10, 80:20, and 50:50. Chemical structures and optoelectronic properties of the copolymers were characterized by elemental analysis, NMR, UV absorption, cyclic voltammetry, photoluminescence (PL), electroluminescence (EL), a photovoltaic cell, and a field effect transistor. The elemental analyses of the copolymers indicated that FO and TST contents in the copolymers were very close to the feed compositions. The random copolymers exhibited a PFO-segment-dominated UV absorption peak at ∼385 nm and a narrow band gap TST absorption at ∼490 nm. For the alternating copolymer, only a broad absorption band was found, demonstrating a mixed and TST-dominated electronic configuration. Compared with the solution PL, complete PL excitation energy transfer from the PFO segment to the TST unit could be achieved by film PL at lower TST content. It was found that the EL spectra of the copolymers with a device configuration of indium−tin oxide/poly(3,4-ethylenedioxythiophene)/poly(vinylcarbazole) (PVK)/copolymer/Ba/Al were red shifted and had better red light CIE coordinates with improved external quantum efficiency, compared with corresponding device performances without the PVK layer. With the alternating copolymer as the electron donor and methanofullerene [6,6]-phenyl C61-butyric acid methyl ester as the electron acceptor, an energy conversion efficiency of 2.01% was achieved under an AM1.5 simulated solar light at 100 mW/cm2, which is among the highest values so far reported for bulk-heterojunction photovoltaic cells. The field effect hole mobility of the alternating copolymer is 4.5 × 10-5 cm2/(V s) using polyacrylonitrile as an organic insulator on a gate electrode.

In recent years, white polymer light-emitting diodes (WPLEDs) have received great attention because of their potential application in full-color flat-panel displays and solid-state lighting. A variety of approaches have been proposed for the realization of white emission in PLEDs.[1] One of the successful approaches in small-molecule organic light-emitting diodes (OLEDs) fabricated by thermal deposition is to use a multilayer device system consisting of two or more active layers, where each layer emits a primary color. [2,3] The highest device performance in multilayer phosphorescent OLEDs reached external quantum efficiency (QE) and power efficiency of 18.7 % and 37.6 lm W -1 , respectively, at a luminance of 500 cd m -2 with Commission Internationale de L'Éclairage (CIE) coordinates of (0.40,0.41), as reported by the Forrest group.[4]However, it is very difficult to fabricate multilayer PLEDs by solution processing because of the intermixing of different layers as a result of dissolution of the previously deposited layer. The most widely used approach for the manufacturing of PLEDs is to use the single-layer polymer blend system, [5][6][7][8][9] where the emitting layer consists of green and red emitters (small molecule or polymer) blended into a wide-gap bluelight-emitting polymer host and spin-coated onto an indium tin oxide/poly-(3,4-ethylenedioxythiophene) (ITO/PEDOT) substrate. Like most blended devices, the phase behavior of the guest and host is very sensitive to the driving voltage and the operating and shelf life; as a result, the color coordinates are not very stable. [5][6][7] Gong et al. [10] reported the first polymer multilayer white-light-emitting devices with a triplet phosphore doped into a blue-green polyfluorene host with water-soluble polyelectrolytes as the hole-transport layer (HTL) and electron-transfer layer (ETL). Recently, efforts have been made to prepare a single-component white polymeric emitter based on insufficient energy transfer, because phase segregation of chromophores can be significantly reduced by incorporating RGB (red-green-blue) chromophores into a single polymer chain. Lee et al. first reported a single fluorene-based copolymer composed of blue-, green-, and red-light-emitting units (although the RGB chromophores were not in full conjugation in the main chain) with a maximum brightness of 820 cd m -2 at 11 V with CIE coordinates of (0.33,0.35).[11] At almost the same time, Wang and co-workers adopted a slightly different synthetic strategy by which a green-emitting component was attached to the pendant chain and a red-emitting component was incorporated into the blue-emitting polyfluorene backbone.[12] The electroluminescent device exhibited a luminance efficiency of 1.59 cd A -1and CIE coordinates of (0.31,0.34). A similar strategy with two chromophores for producing white-light-emitting polymers has been reported, with a luminous efficiency (LE) of 3.8 cd A -1 and CIE coordinates of (0.32,0.36), [13] and a luminous efficiency of 7.3 cd A -1 and CIE coordinates of (0.3...

The invisibility cloak, a long-standing fantastic dream for humans, has become more tangible with the development of metamaterials. Recently, metasurface-based invisibility cloaks have been proposed and realized with significantly reduced thickness and complexity of the cloaking shell. However, the previous scheme is based on reflection-type metasurfaces and is thus limited to reflection geometry. In this work, by integrating the wavefront tailoring functionality of transparent metasurfaces and the wave tunneling functionality of zero-index materials, we have realized a unique type of hybrid invisibility cloak that functions in transmission geometry. The principle is general and applicable to arbitrary shapes. For experimental demonstration, we constructed a rhombic double-layer cloaking shell composed of a highly transparent metasurface and a double-zero medium consisting of dielectric photonic crystals with Dirac cone dispersions. The cloaking effect is verified by both full-wave simulations and microwave experimental results. The principle also reveals exciting possibilities for realizing skin-thick ultrathin cloaking shells in transmission geometry, which can eliminate the need for spatially varying extreme parameters. Our work paves a path for novel optical and electromagnetic devices based on the integration of metasurfaces and metamaterials.

We study metamaterials with an anisotropic effective permittivity tensor in which one component is near zero. We find that such an anisotropic metamaterial can be used to control wave propagation and construct almost perfect bending waveguides with a high transmission rate (>95%). This interesting effect originates in the power flow redistribution by the surface waves on the input and output interfaces, which smoothly matches with the propagating modes inside the metamaterial waveguide. We also find that waves in such anisotropic epsilon-near-zero materials can be reflected by small-sized perfect magnetic conductor defects. Numerical calculations have been performed to confirm the above effects.The electromagnetic properties exhibited by metamaterials are remarkable. Indeed, they may provide almost arbitrary effective permeabilities and/or permittivities [1][2][3][4][5][6][7] . In previous studies on metamaterials, various interesting phenomena have been discovered, such as negative refraction 1 , perfect lens 3 , invisibility cloaks [4][5][6][7] , etc. Metamaterials with near zero parameters are also an important and intriguing class. Recently, epsilon-near-zero (ENZ) metamaterials with permittivity near zero, mu-near-zero (MNZ) metamaterials with permeability near zero, and index-near-zero (INZ) metamaterials with both permittivity and permeability near zero have been extensively studied and various applications have been proposed, such as directive emission devices [8][9][10][11][12] , creating subwavelength channels and bends [13][14][15][16][17][18][19] , tailoring the wave front 20,21 , realizing total transmissions and reflections in a channel by engineering defects [22][23][24][25] , bends designed through the principles of transformation optics [31][32][33][34] , and zero-index materials 13,[17][18][19] , etc. Silveirinha et al. 13 first proposed using isotropic epsilon-near-zero (IENZ) metamaterials to create subwavelength channels and bends, which was later experimentally realized by Liu et al. 17 and Edwards et al. 18,19 . However, in order to ensure high transmission, such isotropic channel is required to be very narrow in width.In this letter, we investigate the properties of anisotropic epsilon-near-zero (AENZ) metamaterials in which one component of the permittivity tensor is near zero. We find such a material can achieve almost perfect bending waveguides, which does not have the requirement of narrow channel width for high transmission. More interestingly, we can embed a small-sized perfect magnetic conductor (PMC) defect in the AENZ metamaterials to confine the wave propagation. It should be noted that with only one component near zero, such AENZ metamaterials are much easier to achieve than IENZ metamaterials in practice. Possible realizations of such AENZ metamaterials include metal-dielectric multilayered structures 35,36 , metal wire arrays 26,37 , etc. Before we study the bending waveguide of AENZ metamaterials, we first investigate the case of a straight waveguide. As illustrated in Fig....

Absorption of microwaves by metallic conductors is typically inefficient, albeit naturally broadband, due to the huge impedance mismatch between metal and free space. Reducing metal to ultrathin profile may improve absorption efficiency, but a maximal 50% absorption limit induced by the field continuity exists. Here, we experimentally show that broadband, perfect (100%) absorption of microwaves can be realized in a single layer of ultrathin conductive film when illuminated coherently by two oppositely directed incident beams. Our experiments keep the field continuity and simultaneously break the 50% limit. Inheriting the intrinsic broadband feature of metals, complete absorption is observed to be frequency independent in microwave experiments from 6 to 18 GHz. Remarkably, this occurs in films with thicknesses that are at the extreme subwavelength scales, ∼λ/10 000 or less. Our work proposes a way to achieve total electromagnetic wave absorption in an ultrawide spectrum of radio waves and microwaves with a simple conductive film.