Orientation Control of Linear‐Shaped Molecules in Vacuum‐Deposited Organic Amorphous Films and Its Effect on Carrier Mobilities

The molecular orientation of linear‐shaped molecules in organic amorphous films is demonstrated to be controllable by the substrate temperature. It is also shown that the molecular orientation affects the charge‐transport characteristics of the films. Although linear‐shaped 4,4′‐bis[(N‐carbazole)styryl]biphenyl molecules deposited on substrates at room temperature are horizontally oriented in amorphous films, their orientation when deposited on heated substrates with smooth surfaces becomes more random as the substrate temperature increases, even at temperatures under the glass transition temperature. Another factor dominating the orientation of the molecules deposited on heated substrates is the surface roughness of the substrate. Lower carrier mobilities are observed in films composed of randomly oriented molecules, demonstrating the significant effect of a horizontal molecular orientation on the charge‐transport characteristics of organic amorphous films.     

Different orientation of the transition dipole moments of two similar Pt(II) complexes and their potential for high efficiency organic light-emitting diodes

The efficiency of organic light-emitting diodes (OLEDs) is especially limited by their low light outcoupling efficiency. An approach for its enhancement is the use of horizontally oriented emitter molecules with respect to the substrate. In this study we quantitatively determine the orientation of the optical transition dipole moments in doped films of two similar phosphorescent Pt(II) complexes having a linear molecular structure. These emitters are employed in OLED devices and their efficiency is analyzed by optical simulations. For an OLED with slightly more horizontally oriented emitter molecules an external quantum efficiency (η_EQE) of 15.8% at low current-density is realized, indicating a relative improvement of outcoupling efficiency of 5.3% compared to the isotropic case. However, a very similar complex adopting isotropic molecular orientation yields η_EQE of only 11.5% implying an imperfect charge carrier balance in the OLED device and a shift of the recombination zone. Furthermore, we highlight the enormous potential of horizontal molecular orientation of emitting molecules in OLEDs.     

Fabrication of tunable [Al2O3:Alucone] thin-film encapsulations for top-emitting organic light-emitting diodes with high performance optical and barrier properties

The optical and barrier properties of thin-film encapsulations (TFEs) for top-emitting organic light-emitting diodes (TEOLEDs) were investigated using TFEs fabricated by stacking multiple sets of inorganic–organic layers. The inorganic moisture barrier layers were prepared by atomic layer deposition (ALD) of Al₂O₃ using trimethylaluminum (TMA) and O₃ as precursors and are shown to be efficient barriers against gases and vapors. The organic alucone layers were produced by molecular layer deposition (MLD) using TMA and ethylene glycol as precursors. The [Al₂O₃:Alucone] ALD/MLD films were used because their adjustable inorganic–organic nanolaminate composition allows for the tuning of the optical properties, thereby enhancing their application potential for the design and fabrication of high performance light out-coupling structures for TEOLEDs. By carefully adjusting the relative thickness ratio of the inorganic–organic encapsulation materials, optimized light extraction was achieved and the films not only maintained their high moisture barrier strength but also showed excellent optical performance.     

Molecular packing correlated fluorescence in TIPS-pentacene films

The molecular packing and optical properties of exposed and buried layers (i.e. the layers at the top surface and near the substrate, respectively) were systematically studied in 6,13-Bis(triisopropylsilylethynyl)pentacene (TIPS-pentacene) films coated by spin-coated (SC) and droplet-pinned-crystallization (DPC) methods. Buried layers in both films exhibit intense photoluminescence (PL) resembling the behaviors of the molecules in dilute solution ascribing to weak π-π stacking, while the exposed layers show extremely weak PL due to strong crystallinity. Polarized excitation PL spectra demonstrate that molecular orientation of the buried layers is quasi-ordered in the film coated by DPC method and completely disordered in the film coated by SC method. Besides, the strong crystallinity of the exposed TIPS-pentacene is verified by grazing incident wide-angle X-ray scattering measurement. The distinct differences in optical and structural properties between the exposed and buried layers indicate that TIPS-pentacene films are inhomogeneous in vertical direction due to interfacial effect, which affects the performance of photodiode fabricated with both films. The understanding of the molecular packing correlated fluorescence in TIPS-pentacene films is vital for optimizing the film structure to achieve high performance organic electronic devices.     

Boron-doped silicon film as a recombination layer in the tunnel junction of a tandem solar cell

Boron-doped hydrogenated silicon films with different gaseous doping ratios (B2H6/SiH4) were deposited in a plasma-enhanced chemical vapor deposition (PECVD) system. The microstructure of the films was investigated by atomic force microscopy (AFM) and Raman scattering spectroscopy. The electrical properties of the films were characterized by their room temperature electrical conductivity (σ) and the activation energy (Ea). The results show that with an increasing gaseous doping ratio, the silicon films transfer from a microcrystalline to an amorphous phase, and corresponding changes in the electrical properties were observed. The thin boron-doped silicon layers were fabricated as recombination layers in tunnel junctions. The measurements of the Ⅰ-Ⅴ characteristics and the transparency spectra of the junctions indicate that the best gaseous doping ratio of the recombination layer is 0.04, and the film deposited under that condition is amorphous silicon with a small amount of crystallites embedded in it. The junction with such a recombination layer has a small resistance, a nearly ohmic contact, and a negligible optical absorption.     

The Root Causes of the Limited Stability of Solution‐Coated Small‐Molecule Organic Light‐Emitting Devices: Faster Host Aggregation by Exciton–Polaron Interactions

The degradation mechanism is compared in organic light‐emitting devices (OLEDs) fabricated by solution‐coating to that in vacuum‐deposited OLEDs. Devices comprising various host materials made by vacuum‐deposition or solution‐coating are investigated. Changes in devices electroluminescence (EL) spectra during prolonged electrical driving are compared and analyzed. Hole‐only devices are also utilized, and employed to study the effects of charges and excitons, separately and combined. The results reveal that the faster degradation of solution‐processed devices relative to their vacuum‐deposited counterparts under electrical stress is due to a faster aggregation of the host materials. Interactions between excitons and polarons in the emitting layers of the devices induce this aggregation phenomenon. Although this phenomenon affects both vacuum‐deposited and solution‐coated emitting layers, it is found to occur much faster in the later. The findings shed light on the root causes of the limited stability of solution‐processed OLEDs.     

Highly Crystalline Rubrene Light-Emitting Diodes with Epitaxial Growth

Conventional organic optoelectronic devices suffer from low carrier mobility limited by the static and dynamic disorder. Organic crystals with long-range order can circumvent the effects of disorder and significantly improve the charge transport. While highly ordered organic crystals offer the desirable electronic coupling strength and charge transport, their integration into large-area optoelectronic devices remains a challenge. Here, monolithic integrated triclinic crystal rubrene light-emitting diodes (LEDs) are presented using epitaxial growth with functional additives being engineered into the films. Superior charge transport, excellent operational and long-term stability in these light-emitting devices are demonstrated. By comparing two rubrene-based LEDs, one made from amorphous and one from crystalline rubrene layers, their exciton dynamics are estimated using comprehensive transient electroluminescence simulation. The crystalline LEDs show high triplet-triplet annihilation (TTA) rate constant similar to TTA rate constant of triclinic single crystals determined by optical spectroscopy. At the same time, the crystalline phase enhances drastically the singlet-fission and bimolecular annihilation rates, which reduces the overall performance of the LED compared to its amorphous counterpart. Finally, an outlook on the potential applications of rubrene and/or its derivatives crystalline films are provided for enhancing the performance of organic and hybrid optoelectronic devices.     

Fabrication of flexible conductive graphene/Ag/Al-doped zinc oxide multilayer films for application in flexible organic light-emitting diodes

Graphene/Ag/Al-doped zinc oxide (AZO) multilayer films were fabricated by using chemical vapor deposition and magnetron sputtering methods. The electrical and optical properties of the transparent conductive graphene/Ag/AZO films were investigated. The graphene/Ag/AZO film can maintain high conductivity and transmittance without obvious degradation during bending test. A green flexible organic light emitting diode with a structure of graphene/Ag/AZO/N,N-diphenyl-N,N-bis(1-napthyl)-1,1-biphenyl-4,4-diamine/tris(8-hydroxyquinoline) aluminum(III)/lithium fluoride/Al exhibited a stable green emission and light-emitting efficiency during the cycle bending test. The multilayer films hold promise for application in flexible optoelectronic devices.     

光诱导化学汽相淀积非晶硅薄膜及其在太阳电池中的应用

本文较系统地评述了光诱导化学汽相淀积(LCVD)技术淀积非晶硅薄膜的开发现状,主要介绍了LCVD淀积非晶硅薄膜 的机理.评价了LCVD淀积非晶硅薄膜的电学和光学特性,最后介绍了这种薄在制备晶硅太阳电池方面的应用.     

Bright Blue Light—emitting Diodes Based on a Novel Fluorescent Dye Containing Heterocyclic Groups

Novel molecular material ,1-benzothiazoly-3-pheny1-pyrazoline (BTPP) was found to function as bright blue light emitting dye in organic electroluminescent device, and its optical and electric characteristics were investigated. This heterovyclic compound exhibited good characteristics of blue photoluminescence and electroluminescence,which had the emission peak at 450nm .The single layer light-emitting devices using BTPP as light -emitting material dispersed in poly(N-vinylcarbazole)(PVK) and double layer ones using PBD as hole block layer above the light-emitting layer were fabricated using conventional spin-casting and vaccum vapour deposition methods. The introduction of PBD has enhanced electron injection and luminance efficiency, compared with the single layer LEDs.     

Fabication of Organic／Polymeric Superlattice Structure and Its Use for Electroluminescent Device^①②

Superlattices consisting of alternating layers of organic/polymeric materials have been fabricated from tris(8-hydroxyquinoline) aluminum(Alq3) and poly(N-vinylcarbazole)(PVK) by a multisource-type high-vacuum organic molecular deposition.The characteristics of superlattice structures are determined by the smallangle X-ray diffraction,optical adsorption and photoluminescence.The electroluminescent devices with the superlattice structure have also been fabricated and the emission characteristics are discussed.     

Current Enhancement in Organic Films through Gap Compression by Cold and Hot Isostatic Pressing

The spatial gaps in organic films are compressed using cold and hot isostatic pressing (CIP and HIP, respectively) with the aim of enhancing their electrical characteristics. The microscopic gaps formed in amorphous organic films by inefficient molecular packing are difficult to compress using CIP and HIP; however, the macroscopic gaps formed between grains and other grains or substrates in polycrystalline organic films can be compressed using CIP and HIP. The gap compression by CIP and HIP in polycrystalline films enhances their electrical characteristics. Conversely, the electrical characteristics of amorphous films remain unchanged after CIP and HIP. HIP gives almost the same results as CIP in terms of gap compression and current enhancement, probably because the expected activation of molecular motion at high temperature is suppressed under high applied pressure. CIP markedly improves the performance of organic light‐emitting diodes, organic solar cells, and organic field‐effect transistors containing polycrystalline films. These findings are important for understanding the carrier injection and transport mechanisms of organic films containing gaps as well as enhancing the performance of future organic devices, especially those with polycrystalline films.     

Sunlight-like white organic light-emitting diodes with inorganic/organic nanolaminate distributed Bragg reflector (DBR) anode microcavity by using atomic layer deposition

White organic light-emitting diodes (WOLED) with inorganic/organic/hybrid nanolaminate distributed Bragg reflector (DBR) prepared by atomic layer deposition (ALD) has been firstly developed, and the improved structural, optical, and electrical properties have been observed. The functional layer consists of periodically alternating layers of ZrO₂/Zircone (zirconium alkoxides with carbon-containing backbones) films fabricated using ALD. By well adjusting the thickness and emissive dyes of the emission layer, three white light diodes were realized based on three cavity modes with different resonance wavelength and intensity. The narrowed lectroluminescent (EL) spectra from the microcavity organic light emitting diodes (MOLEDs) are of high-purity, and the off-resonant optical mode is highly suppressed due to the excellent optical properties of the DBR layers with high reflectivity in a wide stop-band range. The CIE chromaticity coordinates of devices are tuned from (0.33, 0.34), (0.43, 0.40) to (0.57, 0.40), which are all on the planckian locus, and the color temperature is adjusted from 5783 K, 3842 K to 2245 K. This work exhibits a simple and novel approach to achieve a sunlight-like WOLED by constructing the inorganic/organic nanolaminate DBR using ALD, which will be very important for healthy display and lighting.     

Molecular orientation and thermal stability of thin-film organic semiconductors

Molecular orientation in organic semiconductors plays a critical role in maximizing external quantum efficiencies of organic light-emitting diodes. It was generally believed that the molecular packing of organic semiconductors is either amorphous or liquid-crystal-like with a preferred molecular orientation distributed uniformly throughout the film. In this paper, however, we report that the orientation of organic molecules in physical-vapor deposited films varies drastically depending on thickness. The thermal stability of the molecular network, measured by its characteristic glass transition temperature, also varies as a function of the film thickness. Based on a two-layered film-structure model, we propose a simple function to quantify the molecular dipole orientation S parameter as a function of film thickness. This function describes well experimental data. In addition to contributing to external quantum efficiency, the molecular orientation parameter S is found to have a strong impact on disruptive change in material density after thermal anneal and glass transition.     

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.     

有机电致发光器件中有机薄膜的制备方法

有机电致发光器件中有机薄膜的制备方法非常重要,不同方法制备的薄膜质量不同,这直接影响着器件的效率;制备方法直接影响到产业化中的器件制备成本。根据材料的不同,有机小分子常用真空蒸镀的方法,而高分子材料常用旋涂的方法制备薄膜。随着有机电致发光器件制备工艺的发展,相继出现了其他的制备工艺,如:有机蒸汽喷印(organic vapor jet printing)、有机气相沉积(organic vapor phase deposition)、丝网印刷(screen printing)和喷墨打印(ink jet printing)技术等,这对有机电致发光显示器产业化发展具有巨大的推动作用。文章综述了这些制备方法,比较了它们的优缺点,以及这些工艺对产业化的影响。     

Two-dimensional-growth small molecular hole-transporting layer by ultrasonic spray coating for organic light-emitting devices

Two-dimensional-growth small molecular organic thin film with high quality is fabricated by ultrasonic spray coating technology (USC) from the toluene solution of 4,4′,4″-tris (carbazol-9-yl) triphenylamine (TCTA). In comparison to the vacuum thermal evaporation (VTE) TCTA film, the USC-TCTA film obtained from liquid-phase solution possesses more uniform surface topography. The differences between the USC-TCTA and the VTE-TCTA in optical property, electrical property and formation mechanism are also studied in detail. Besides, to evaluate the hole transport and electron blocking ability of USC-TCTA film, the organic light-emitting devices (OLEDs) employing USC-TCTA film as hole transport layer are fabricated successfully. Additionally, the green OLEDs based on USC-TCTA film perform as well as the ones with VTE-TCTA film in current density, luminance, efficiency and color stability, and show even better tolerance to high bias voltage.     

有机层界面对有机电致发光器件性能的影响

采用真空蒸镀的方法,制备了以ITO/2T-NATA(15 nm)/NPB(25 nm)/Alq3(30nm)/LiF(1 nm)/Al(100 nm)为基本结构的绿光器件,实验中在NPB(25 nm)与Alq3(30 nm)有机层界面处加入周期性不同的NPB(10 nm)/Alq3(10 nm)结构的有机层.通过实验测得的数据,研究了周期性的空穴传输层与发光层结合这种特殊结构对绿光器件发光性能的影响.根据实验结果,发现在有机层界面处,加入周期不同的NPB(10 nm)/Alq3(10 nm)层虽然会提高器件的起亮电压,但会改善器件的发光效率,而对器件的发光波长与发光区域以及发光亮度影响不大.     

Optoelectrical characteristics of organic light-emitting devices fabricated with different cathodes

Organic light-emitting devices (OLEDs) with various cathode structures were prepared on indium tin oxide (ITO) substrates by vacuum sublimation technique, and the effects of the device cathodes on the electroluminescence (EL) characteristics of OLEDs were studied in terms of the luminance, efficiency, driving voltage and threshold voltage. The results demonstrate that the optical and electrical performance of OLEDs depend on the properties of the devices' cathodes and the characteristics of the cathode–organic interface and the organic–organic interface. The optoelectrical performance of a device with composite cathodes is better than that of the devices with metal alloy and pure metal cathodes. The improvement in the device performance can be attributed to a more efficient electron injection at the cathode–organic interface, a better balanced hole and electron recombination in the light-emitting layer and fewer accumulated charges near the organic–organic interface.     

Flexible organic light-emitting diodes on a polyestersulfone (PES) substrate using Al-doped ZnO anode grown by dual-plasma-enhanced metalorganic deposition system

This study proposes flexible organic light-emitting diodes (OLEDs) grown on polyestersulfone (PES) using Al-doped zinc oxide (AZO) as the anode, fabricated by the dual-plasma-enhanced chemical vapor deposition (DPEMOCVD) system. The experimental results including crystalline structure, optical, and electrical characteristics indicate that the quality of AZO films grown on PES depends on the deposition temperature and Al content. The optimal deposition temperature and Al content for AZO film are 185  C and 2.88 at%, respectively. Further increasing or decreasing the deposition temperature and Al content degrades the quality of AZO films. The optimal AZO film deposited on the PES substrate was used as the anode for flexible OLED. It shows a similar performance compared to OLEDs using commercial indium–tin-oxide (ITO) as the anode on glass, and represents enhanced characteristics to that of the commercial ITO anode on a flexible polyethylene naphthalate (PEN) substrate. This indicates that the DPEMOCVD-deposited AZO film on the PES substrate can be the anode for flexible OLEDs.