Analyzing nanostructures in mesogenic host–guest systems for polarized phosphorescence

Feasibility of polarized phosphorescent organic light-emitting devices (OLEDs) had been previously demonstrated by combining a discotic Pt(II) complex with a glassy-nematic oligofluorene host to form a mesogenic host–guest phosphorescent emitting system. Previous photophysical studies suggested that in the host–guest film, the Pt(II) complex tended to aggregate into columnar stacks, exhibiting metal–metal-to-ligand charge transfer (MMLCT) emission. Both host molecules and guest aggregates in the host–guest films could be oriented by a conductive alignment layer, giving rise to polarized phosphorescence from the Pt(II) aggregates. Nevertheless, film morphologies and nanostructures of the mesogenic host–guest systems have remained to be elucidated. In this work, grazing incidence X-ray scattering (GIXS) was carried out to analyze nanostructures in both neat films of the discotic Pt(II) complex and mesogenic host–guest mixture films. In addition, confocal laser scanning microscopy (CLSM) was also utilized for visualization of the morphologies of mesogenic host–guest systems. The columnar axes of nanostructured Pt(II) stacks lying on the alignment-treated surfaces were found to be preferentially oriented perpendicular to the rubbing direction, which is responsible for the observed linearly polarized phosphorescence.     

Enhanced green electrophosphorescence by using polyfluorene host via interfacial energy transfer from polyvinylcarbazole

For green-emitting Ir complex-doped polymer devices, high efficiencies have been achieved only when non-conjugated poly(N-vinylcarbazole) (PVK) was used as the host polymer. It is commonly believed that conjugated polyfluorenes (PFO) are not suitable for the use as host of green phosphorescent complexes due to the presence of the low-lying triplet state. In this paper we reported that despite very inefficient PL phosphorescent emission of Ir(Bu-PPy)₃ in the PFO films, strong green electroluminescence (EL) and high device efficiencies have been observed for devices from such blend films when multilayer device with PVK was used as anode buffer. We analyze the PL spectra in steady state and transient response and found that the energy transfer via PVK interlayer plays important role in the efficient EL performance. This fact clearly indicates the efficient electrophosphorescence might be achieved even if the triplet energy of phosphorescence dye is higher than that of the host. These findings could significantly broaden our selection in polymer hosts for phosphorescent devices.     

Photo‐ and Rubbing‐Alignable Brush Polyimides Bearing Three Different Chromophores

Three new photoreactive brush polyimides (PSPIs), each bearing a different type of chromophore (cinnamoyl (CA), 3‐(2‐furyl)acryloyl (FA), and methacryloyl (MA)) in their bristles (i.e., side groups), are successfully synthesized, and are found to produce good‐quality films with smooth surfaces through conventional spin‐casting and drying processes. These PSPI polymers are thermally stable up to 320 °C. This is the first quantitative investigation of the photoaligning and rubbing‐aligning processabilities of PSPI polymer films, and of the abilities of the resultant films to control the orientation and anchoring of liquid‐crystal (LC) molecules. The chromophores of both poly(1‐cinnamoyloxy‐2,4‐phenylene hexafluoroisopropylidenediphthalimide) (6F‐DAP‐CA) and poly(1‐3‐(2‐furyl)acryloyloxy‐2,4‐phenylene hexafluoroisopropylidenediphthalimide) (6F‐DAP‐FA) PSPIs are found to undergo photodimerization in thin films and, to a lesser extent, photoisomerization, resulting in insoluble, crosslinked films. The MA chromophores of 6F‐DAP‐MA PSPI are found to undergo photopolymerization in thin films, which might include photodimerization to a lesser extent, resulting in insoluble, crosslinked films. Thin films of the PSPI polymer chains are found to have excellent unidirectional orientation ability as a result of either photoexposure with linearly polarized UV light (LPUVL) or rubbing. Both the photoaligned and the rubbing‐aligned polymer chains in the PSPI films are demonstrated to effectively induce the alignment of nematic LCs along their orientation directors by anisotropic interactions between the preferentially oriented polymer chain segments and the LCs. The contribution to LC alignment of the microgrooves developed in the rubbed films is found to be very low. The anchoring energies of the LCs on the photoaligned film surfaces are comparable to those on the rubbing‐aligned film surfaces; the anchoring energies are found to be in the range 0.45–2.25 × 10^–5 J m^–2, and to depend on which film treatment process is used and which chromophore bristle is present. In summary, the new PSPIs reported in this paper are promising LC alignment‐layer candidates with rubbing‐free processing for the production of advanced LC‐display (LCD) devices, including LCD televisions with large display areas.     

A Series of Energy‐Transfer Copolymers Derived from Fluorene and 4,7‐Dithienylbenzotriazole for High Efficiency Yellow,Orange, and White Light‐Emitting Diodes

Eight random and alternating copolymers PF‐DTBTA derived from 2,7‐fluorene and 4,7‐dithienylbenzotriazole (DTBTA) were synthesized. Thin solid films of the energy‐transfer copolymers possess high absolute photoluminescence (PL) quantum yields (Φ_PL) between 60?72%. Inserting PVK layer between anode and emissive layer could show higher electroluminescence (EL) performances due to PVK‐enhanced hole injection. Random copolymers PF‐DTBTA1?15, with DTBTA molar contents from 1% to 15%, displayed yellow EL spectra with high external quantum efficiency (EQE_max) up to 5.78%. PF‐DTBTA50, the alternating copolymer, showed an orange EL with EQE_max of 3.3%. The good Φ_PL and EQE_max of the PF‐DTBTA50 with very high DTBTA content indicate that DTBTA is a high efficiency chromophore with very low concentration quenching effects in the solid state PL and EL processes. PF‐DTBTA0.03?0.1 could emit white EL due to partial energy transfer from fluorene segments to DTBTA units. Moreover, white EL devices, with forward‐viewing maximum luminous efficiency up to 11 cd/A and stable white EL spectra (CIE coordinates of (0.33, 0.43)) in high current range from 5 mA to 60 mA, could be realized from the non‐doped polymer with simple binary structure. Our results suggest that DTBTA has big potential to construct high performanced EL polymers or oligomers.     

有机白光LED

有机白光LED主要有电致发光与光致发光两类。电致发光的主要机理有:量子阱发光、激基复合物发光和能量转移。人们对有机电致发光白光LED研究较多,不久将有产品问世;而对光致发光研究较少。与电致发光相比,光致发光造价更小,其荧光量子效率或许比电致发光更高。有机白光LED制备工艺简单、成本低功耗小,具有巨大的应用价值及潜在的市场。对发光显示技术,有机白光LED代表了一条“便宜”经济的路径。综述了有机白光LED的机理及发展现状。     

2D Versus 3D Crystalline Order in Thin Films of Regioregular Poly(3‐hexylthiophene) Oriented by Mechanical Rubbing and Epitaxy

Highly oriented films of regioregular poly(3‐hexylthiophene) (P3HT) are prepared by two methods: mechanical rubbing and directional epitaxial crystallization. The structure, nanomorphology, and optical and charge‐transport properties of the oriented films are investigated by electron diffraction, high resolution transmission electron microscopy (HR‐TEM), absorption spectroscopy, and transistor field‐effect measurements. In rubbed films, P3HT chains align parallel to the rubbing direction and the crystalline domains orientation changes from preferential edge‐on to flat‐on orientation. The maximum in‐plane orientation probed by absorption spectroscopy is a function of the polymer molecular weight M_w; the lower the M_w, the higher the in‐plane orientation induced by rubbing. The anisotropy of field‐effect mobility measured parallel and perpendicular to the rubbing shows the same trend as the absorption. The M_w‐dependence of the orientation is explained in terms of chain folding and entanglement that prevent the reorientation and reorganization of the π‐stacked chains, especially when M_w ≥ 50 kDa. For comparison, P3HT films are oriented by directional epitaxial crystallization using a zone‐melting technique. Electron diffraction and HR‐TEM show that epitaxial and rubbed films differ in terms of intralamellar order within layers of π‐stacked chains. Comparison of UV‐vis absorption spectra for the different samples suggests that the vibronic structure is sensitive to intralamellar disorder.     

A Versatile Method to Fabricate Highly In‐Plane Aligned Conducting Polymer Films with Anisotropic Charge Transport and Thermoelectric Properties: The Key Role of Alkyl Side Chain Layers on the Doping Mechanism

A general method is proposed to produce oriented and highly crystalline conducting polymer layers. It combines the controlled orientation/crystallization of polymer films by high‐temperature rubbing with a soft‐doping method based on spin‐coating a solution of dopants in an orthogonal solvent. Doping rubbed films of regioregular poly(3‐alkylthiophene)s and poly(2,5‐bis(3‐dodecylthiophen‐2‐yl)thieno[3,2‐b ]thiophene) with 2,3,5,6‐tetrafluoro‐7,7,8,8‐tetracyanoquinodimethane (F₄TCNQ) yields highly oriented conducting polymer films that display polarized UV–visible–near‐infrared (NIR) absorption, anisotropy in charge transport, and thermoelectric properties. Transmission electron microscopy and polarized UV–vis–NIR spectroscopy help understand and clarify the structure of the films and the doping mechanism. F₄TCNQ^? anions are incorporated into the layers of side chains and orient with their long molecular axis perpendicular to the polymer chains. The ordering of dopant molecules depends closely on the length and packing of the alkyl side chains. Increasing the dopant concentration results in a continuous variation of unit cell parameters of the doped phase. The high orientation results in anisotropic charge conductivity (σ) and thermoelectric properties that are both enhanced in the direction of the polymer chains (σ = 22 ± 5 S cm^?1 and S = 60 ± 2 µV K^?1). The method of fabrication of such highly oriented conducting polymer films is versatile and is applicable to a large palette of semiconducting polymers.     

InGaN-AlInGaN multiquantum-well LEDs

InGaN-GaN and InGaN-AlInGaN multiquantum-well (MQW) light-emitting diodes (LEDs) were both fabricated and their optical properties were evaluated by photoluminescence (PL) as well as electroluminescence (EL). We found that the PL peak position of the InGaN-AlInGaN MQW occurs at a much lower wavelength than that of the InGaN-GaN MQW. The PL intensity of the InGaN-AlInGaN MQW was also found to be larger. The EL intensity of the InGaN-AlInGaN MQW LED was also found to be larger than that of the InGaN-GaN MQW LED under the same amount of injection current. Furthermore, it was found that EL spectrum of the InGaN-AlInGaN MQW LED is less sensitive to the injection current. These observations all suggest that we can improve the properties of nitride-based LEDs by using AlInGaN as the barrier layer     

咔唑在Sol-Gel制备的SiO_2薄膜中的发光机理

利用溶胶-凝胶(sol-gel)工艺制备了能够发射红光的含咔唑的SiO2薄膜。测量了薄膜样品的发射谱,发现当激发波长从610nm连续减小到400nm时,样品的发射波长从760nm连续蓝移到了550nm左右。将含咔唑的SiO2薄膜溶解在乙醇中,经逐步稀释后所得溶液的发光可以从绿光变到紫光。基于它们的发射谱、激发谱和吸收谱,探讨了咔唑在SiO2薄膜中发红光的机理。结果表明,咔唑分子被紧缩在SiO2薄膜内部的微孔中,因受到挤压而使其共轭尺寸变大,从而导致咔唑的发光特性发生改变。     

各向异性PDLC散射膜的特性研究

采用摩擦表面取向层的方法制备了有光学各向异性结构的ＰＤＬＣ膜。研究了液晶盒的厚度、摩擦强度,以及在紫外光诱导相分离过程中的光照强度对ＰＤＬＣ膜的光学各向异性程度和电光特性的影响。对ＰＤＬＣ的光学各向异性进行了研究,并给出了要获得低阈值电压、高各向异性的聚合物分散液晶各向异性膜的盒厚、摩擦强度以及光照度等条件。被取向后的ＰＤＬＣ各向异性膜可被用来作为具有很好光电特性的电可调散射式偏振片。     

Structural and Luminescent Properties of SiO<Subscript><Emphasis Type="Italic">x</Emphasis></Subscript>N<Subscript><Emphasis Type="Italic">y</Emphasis></Subscript>(Si) Nanocomposite Films

With a view to creating Si LEDs, the structural and luminescent properties of SiO _x N _y films containing Si nanocrystals in the SiO _x N _y matrix are studied experimentally. It is found that the film structure (nanocrystal size and concentration, the presence of an amorphous phase, etc.) and the spectrum and intensity of photoluminescence (PL) and electroluminescence (EL) are strongly dependent on the Si stoichiometric excess δ and annealing conditions. At δ≈ 10%, unannealed films are amorphous and contain Si clusters of size < 2 nm, as deduced from the TEM and microdiffraction data obtained. Annealing at 800–1000°C for 10–60 min produces Si crystals 3–5 nm in size with a concentration of ≈10¹⁸ cm^?3. The annealed films exhibit room-temperature PL and EL over the wavelength range 400–850 nm with intensity peaks located at 50–60 and 60–70 nm, respectively. The PL and EL spectra are found to be qualitatively similar. This suggests that both the PL and the EL should be associated with the formation of luminescent centers at nanocrystal–matrix interfaces and in boundary regions. However, the two phenomena should differ in the mechanism by which the centers are excited. With the EL, excitation should occur by impact processes due to carrier heating in high electric fields. It is found that as δ increases, so does the proportion of large amorphous Si clusters with a high density of dangling bonds. This enhances nonradiative recombination and suppresses luminescence.     

Photoluminescence and Electroluminescence Imaging of Carrot Defect in 4H-SiC Epitaxy

Electroluminescence (EL) and photoluminescence (PL) imaging techniques were successfully used to reveal defect features in 34 carrots in 4H-SiC epilayers. Because EL and PL techniques are nondestructive and require minimal sample preparations, many carrots can be examined over a reasonable time. Our findings showed that some carrots had the basic components consistent with the model proposed by Benamara et al., but also contained additional features beyond the basic components. Eight carrots contained multiple bright line features, and 25 out of 50 bright line features exhibited multiple line characteristics. Eleven carrots were discovered to contain basal plane faults (B’s) and the corresponding small stacking faults (S’s) near the carrot heads. Not all the carrots lie along the off-cut direction; a couple were oriented 5° from the off-cut. Of the seven carrots with shape lengths -200 μm, one had a corresponding bright line length of 370 μm. Based on this line length, the calculated depth of origin was about midway in the epilayer, while all of the other carrots originated from the substrate. In summary, both EL and PL techniques were consistent in showing that carrots exhibit variable defect structures on a microscopic scale.     

Polarizing polymer solar cells based on the self-organization of a liquid crystalline polymer

We manufactured polarizing polymer solar cells (PSCs) utilizing a liquid crystalline polymer (i.e., poly(2,5-bis(3-dodecylthiophen-2-yl)thieno[3,2-b]thiophene) (PBTTT)) as an electron donor material and a material that selectively absorbs polarized light. The oriented PBTTT films prepared using a self-organization process exhibited a high dichroic ratio of ca. 6.35 at the absorption peak. The polarizing PSCs based on oriented PBTTT–PC₇₁BM photoactive layers exhibit an anisotropic photovoltaic effect under polarized illumination along the two orthogonal axes. The polarizing PSCs have a larger power conversion efficiency under parallel-polarized illumination than that of isotropic PV devices under unpolarized illumination. Based on picosecond fluorescent spectra, the parallel excitation produces a slower ground state recovery and a longer exciton lifetime than perpendicular excitation for PBTTT molecules in a uniaxially oriented arrangement.     

富Si的SiN薄膜光致发光及电致发光研究

采用等离子体化学气相沉积法(PECVD)制备了三种不同化学计量比的富Si的SiN薄膜,并对其光致发光及电致发光性能进行了研究.研究发现,随着Si含量的增加,薄膜的光致发光峰位从460 nm红移到610 nm,且半高宽不断增加;而薄膜的电致发光峰位并没有随着Si含量的改变而变化,始终处于600 nm附近,这主要是由于富Si的SiN薄膜的光致发光与电致发光的机理是不同的.光致发光来源于SiN薄膜缺陷态能级之间辐射复合,而电致发光则是由注入的裁流子先迟豫到较低的缺陷态能级,然后经过辐射复合而得到.     

利用P-MBE在Si(111)衬底上生长氧化锌薄膜及其光学性质的研究

在Si(111)衬底上利用等离子体辅助分子束外延(P-MBE)生长氧化锌(ZnO)薄膜,研究了在不同衬底生长温度下(350～750℃)制备的ZnO薄膜的结构和光学性质．随着衬底温度的升高,样品的X射线及光致发光的半高宽度都是先变小后变大,衬底温度为550℃样品的结构及光学性质都比较好,这表明550℃为在Si(111)衬底上生长ZnO薄膜的最佳衬底温度;同时,我们还通过550℃样品的变温光致发光谱(81～300K)研究了ZnO薄膜室温紫外发光峰的来源,证明其来源于自由激子发射．     

Studies of deep centers in dilute GaAsN and InGaAsN films grown by molecular beam epitaxy

Deep levels spectra DLTS, 77 K photoluminescence (PL) spectra and photosensitivity were measured for GaAsN and InGaAsN films with low N and In concentration grown by molecular beam epitaxy and in GaAs films grown on GaAsN buffer. It is shown that the bandedge luminescence intensity is greatly decreased in GaAsN, GaAs/GaAsN and particularly in InGaAsN structures compared to the homoepitaxial GaAs. Comparison of the DLTS and PL spectra strongly suggests that the main recombination center in such films is the EL3-like electron trap whose concentration greatly increases upon In and N incorporation into the solid solution. Based on published results the trap is associated with substitutional oxygen on As site and the results are discussed in view of such possible assignment.     

Orientation management of α-sexithiophene layer for the application in organic photovoltaic devices

Management of α-sexithiophene (6T) molecular orientation is a key issue to improve the performance of organic photovoltaic (OPV) devices based on 6T thin films, which act as p-type semiconductor layers; however, it is difficult to produce 6T layers with the molecules lying on smooth surfaces, including oxide covered substrates. We have succeeded in orienting the 6T molecules on oriented conductive polythiophene (PT) films, parallel to the oriented PT molecular axis; i.e. parallel to the substrate, by evaporating 6T in vacuum. Here, we reported planar heterojunction (PHJ) OPV devices consisting of 6T films with molecular orientation parallel to the substrate. The orientation of 6T led to improvement of the power conversion efficiency (PCE) in these PHJ OPVs. The average PCE of PHJ OPVs with oriented PT was 2.8 times higher than that without PT. The PCE and the maximum value of the short-circuit photocurrent action spectra of the OPV with the PT was larger under irradiation of polarized light parallel to the 6T molecular axis than that orthogonal to the axis. On the other hand, the PCE and the action spectra of the OPV without the PT did not show the response to polarized light.     

The contact characteristics of SiCN films for opto-electrical devices applications

This paper reports the study of ohmic and Schottky contacts to rapid-thermal chemical vapor deposition (RTCVD) deposited SiCN films in detail. Two prototypical blue-violet light-emitting devices with SiCN material based on the study have been developed. Both photoluminescence (PL) measurement and electroluminescence (EL) photography are used to evaluate the devices’ optical performances. The experimental results show the potential of SiCN for advanced blue and ultraviolet (UV) optoelectronic applications.     

有机小分子掺杂的聚合物发光二极管电致发光及其发射机制

在新型空穴传输聚合物聚TPD(PTPD)中掺杂电子传输有机小分子荧光染料Rubrene制成薄膜器件.考察了影响聚合物掺杂小分子薄膜器件发光性能的因素.实验表明,通过在器件中掺杂,可以控制器件所发光的颜色.研究了PTPD掺杂Rubrene分子薄膜的电致发光光谱和光致发光光谱.由实验可知.在光致发光中存在从PTPD向Rubrene的能量传递和电荷转移,而电致发光则存在从PTPD向Rubrene的能量传递和Rubrene分子对载流子的俘获.即掺杂器件的发射机制为载流子陷阱和Forster能量转换过程的共同作用.     

有机电致发光显示板

本文介绍高性能多层有机电致发光显示器件。该器件可分为三类。多层器件的发光亮度超过１００ｃｄ／ｍ’，驱动电压低于２０Ｖ。文章也介绍了高亮度器件的发光机理和器件的稳定性。