新型蓝光OLED材料和器件

基于有机材料的电致发光技术可以应用于超薄平面显示和有机固体激光器等方面,在近20年来取得了飞速的发展,基本实现了红、绿、蓝三基色发光.绿色材料发展最快,基本达到了商业化实用阶段,而红色和蓝色材料的问题较多,特别是稳定、高效率的蓝光更具有挑战性.综述了近期我们对蓝光材料的分子设计与合成,以及蓝光器件的一些研究.主要包括金属络合物2,3-二甲基-8-羟基喹啉锂和聚芴衍生物树枝状的聚芴化合物的蓝光材料,以及利用基激复合态实现蓝光的电致发光器件.通过光致发光、电致发光光谱以及电压-电流-发光亮度特征曲线等研究了这些发光材料和器件的性质.现在蓝光器件的色纯度和工作寿命还有待提高.     

Direct measurement of the triplet exciton diffusion length in organic semiconductors

We present a new method to measure the triplet exciton diffusion length in organic semiconductors. N,N'-di-[(1-naphthyl)-N,N'-diphenyl]-1,1'-biphenyl)-4,4'-diamine (NPD) has been used as a model system. Triplet excitons are injected into a thin film of NPD by a phosphorescent thin film, which is optically excited and forms a sharp interface with the NPD layer. The penetration profile of the triplet excitons density is recorded by measuring the emission intensity of another phosphorescent material (detector), which is doped into the NPD film at variable distances from the injecting interface. From the obtained triplet penetration profile we extracted a triplet exciton diffusion length of 87±2.7 nm. For excitation power densities >1 mW/mm(2) triplet-triplet annihilation processes can significantly limit the triplet penetration depth into organic semiconductor. The proposed sample structure can be further used to study excitonic spin degree of freedom.     

有机电致发光白光器件的研究进展

在十多年的时间里,有机电致发光二极管(Organic Lightemitting Diodes,OLEDs)的研究和应用取得了长足的进展。有机电致发光器件具有许多优点,例如:自发光、视角宽、响应快、发光效率高、温度适应性好、生产工艺简单、驱动电压低、能耗低、成本低等,因此有机电致发光器件极有可能成为下一代的平板显示终端。有机电致发光白光器件因为可以用于全彩色显示和照明,已成为OLED研究中的热点。介绍了有机电致发光白光器件的研究进展,按发光的性质将白光器件分为荧光器件和磷光器件两类,按发光层数将白光器件分为单层和多层器件,对相关材料、器件结构、发光机理等方面进行了讨论。     

利用银纳米立方增强效率的多层溶液加工白光有机发光二极管

金属纳米粒子的局域表面等离子体共振效应常被用于增强有机发光二极管中激子辐射强度,其增强效果与金属纳米粒子的共振波长、共振强度及其与激子之间的耦合密切相关.本文将具有较强局域表面等离子体共振效应的银纳米立方引入多层溶液加工白光有机发光二极管中提升器件性能.在传统的溶液加工有机发光二极管中,发光层主体一般具有较强的空穴传输性,因此激子主要在发光层/电子传输层界面附近复合.本文将银纳米立方掺入电子传输层中,使银纳米立方与激子之间产生充分的耦合作用,提高激子发光强度.对银纳米立方包裹二氧化硅外壳,一方面优化纳米立方与激子之间的距离,另一方面减小其对器件中电荷传输的影响.通过优化银纳米立方的浓度,多层溶液加工白光有机发光二极管的电流效率达到30.0 cd/A,是基础器件效率的2倍.另外,由于银纳米立方的等离子体共振光谱较宽,同时增强了白光中蓝光和黄光的强度,因此引入银纳米立方基本没有影响白光的色度.研究结果表明引入金属纳米粒子是提升多层溶液加工发光二极管性能的有效方法.     

Analysis of undulated micro-cavity in organic light-emitting diodes

This paper investigates the internal physical mechanisms of undulated micro-cavity that controls the emission wavelength in organic light-emitting diodes (OLEDs). Finite-difference-time-domain method is applied to a previously manufactured OLED design featuring optical structure on a wavy over-coat layer. The emission spectrum of dipole oscillation is calculated with various amplitudes of undulation and emission angles. In addition, the guided mode of an undulated micro-cavity is analyzed compared with that of a flat micro-cavity.     

Interface and bulk controlled perovskite nanocrystal growth for high brightness light-emitting diodes [Invited]

Halide perovskites have attracted great attention due to their high color purity, high luminance yield, low non-radiative recombination rate, and solution processability. Although the external quantum efficiency of perovskite light-emitting diodes(Pe LEDs) is comparable with that of the organic light-emitting diodes(OLEDs) and quantum-dots light-emitting diodes(QLEDs), the brightness is still low compared with the traditional OLEDs and QLEDs. Herein, we demonstrate high brightness and high-efficiency Cs Pb Br3-based Pe LEDs using interface and bulk controlled nanocrystal growth of the perovskite emission layer. The interface engineering by ethanolamine and bulk engineering by polyethylene glycol led to highly crystallized and cubic-shaped perovskite nanocrystals with smooth and compact morphology. As a result, Pe LEDs with a high brightness of 64756 cd/m~2 and an external quantum efficiency of 13.4% have been achieved.     

Phosphorescent white organic light-emitting diodes with stable white color depending on luminance

We fabricated simple and color-stable phosphorescent white organic light-emitting diodes (OLEDs) without an interlayer using a single host of 1,3-bis(9-carbazolyl)benzene with iridium(III) bis[(4,6-difluorophenyl) pyridinato-N,C2’]picolinate and bis(1-phenylisoquinoline)(acetylacetonate) iridium(III) as blue and red phosphorescent emitters, respectively. The CIE 1931 color coordinate difference of the white OLEDs is (0.008, 0.007) when the luminance of the device is increased from approximately 265 cd/m² to 9156 cd/m², which is regarded as visually indistinguishable in practice. In addition, we also measured the decay time of excitons to investigate the emission mechanism in this device using transient photoluminescence and electroluminescence techniques.     

Excitons in highly optically excited gallium phosphide

A study has been made of luminescence in weakly (10¹⁵-10¹⁶ cm^-3) and heavily (10¹⁸-10¹⁹ cm^-3) N-doped GaP crystals induced by 1.78, 2.34 and 3.56 eV photons from Q-switched ruby or neodymium lasers with a KDP crystal for second harmonic generation. The results which were obtained at excitations up to 10²⁰ cm^-3 electron-hole pairs are interpreted as the transitions: single bound excitons, bound excitonic molecules, free excitons in weakly-doped GaP, as well as Auger processes and the formation of a new excitonic state similar to a solid metal with high density of single excitons and excitonic molecules bound to isoelectronic traps in heavily-doped GaP.     

Multi-layer ambipolar light-emitting organic field-effect transistors

Mutli-layer light-emitting organic field-effect transistors (OLETs) are shown to have high internal quantum efficiencies approaching 5%, a value much higher than the conventional organic light-emitting diodes (OLEDs). This work re-examines some data reported in the literature on OLETs and put forward a model that explains the charge transport and light emission process. Our analyses suggest that the reported improvements on the internal quantum efficiency of OLETs are directly linked to charge recombination and light emission and is independent of the drain-source current as well as the gate-induced charge density in the accumulation layer. Such independence allows the internal quantum efficiency to increase as the drain-source current decreases. The process differs from the charge transport in OLEDs where recombination and light emission are directly tied to the injected space charge densities thereby preventing the internal quantum efficiency of OLEDs to increase even when the device current is lowered.     

Exciplex elimination in an organic light-emitting diode based on a fluorene derivative by inserting 4,4’-N,N’-dicarbazole-biphenyl into donor/acceptor interface

Organic light-emitting diodes (OLEDs) composed of a novel fluorene derivative of 2,3-bis(9,9-dihexyl-9H-fluoren-2-yl)-6,7-difluoroquinoxaline (F2Py) were fabricated, and exciplex emission was observed in the device. To depress the exciplex in an OLED for pure colour light emission, 4, 4'-N,N'-dicarbazole-biphenyl (CBP) was inserted as a separator at the donor/acceptor interface. It was found that the device without the CBP layer emitted a green light peaking at 542~nm from the exciplex and a shoulder peak about 430~nm from F2Py. In contrast, the OLED with CBP layer emitted only a blue light peak at about 432~nm from F2Py. Device efficiencies were calculated by a simulative mode in an injection controlled type mechanism, and the results showed that exciplexes yield much lower quantum efficiency than excitons. The device with CBP has a higher power efficiency as no exciplex was present.     

发光区插入超薄LiF层对有机电致发光器件性能的影响

着重对比了在以DCM掺杂Alq₃为发光层的红光器件的发光区插入超薄LiF层后器件性能的改善。插入超薄LiF层后,器件的最大工作电流密度为487 mA/cm²,相应的最大电致发光亮度为76 740 cd/m²,最大外量子效率为5.9%。器件内量子效率为40%,超过了基于有机荧光小分子发光材料的有机电致发光器件的内量子效率的理论极限值25%。对器件内单线态及三线态激子的形成过程进行了分析,并推测:超薄LiF层的插入提高了器件内单线态电荷转移态/三线态电荷转移态的形成比例,进而提高了器件内单线态激子在激子总数中的比例,最终提高了器件的内量子效率。同时,超薄LiF层的插入改变了发光层内局域的内部电场,使器件的外量子效率不仅没有随电流密度的增加而降低,反而非线性增加。     

Picosecond decay kinetics of CdS luminescence studied by a streak camera

Using a picosecond laser and a streak camera we have observed the time dependence of the luminescence intensity of free excitons, bound excitons, and excitonic molecules in CdS. The observed kinetics show that the P band is due to bimolecular emission from free excitons and that bound excitons are generated from free excitons through monomelecular process and excitonic molecule through bimolecular process.     

Silver‐Based Nanoparticles for Surface Plasmon Resonance in Organic Optoelectronics

Organic optoelectronic devices including organic light‐emitting diodes (OLEDs) and polymer solar cells (PSCs) have many advantages, including low‐cost, mechanical flexibility, and amenability to large‐area fabrication based on printing techniques, and have therefore attracted attention as next‐generation flexible optoelectronic devices. Although almost 100% internal quantum efficiency of OLEDs has been achieved by using phosphorescent emitters and optimizing device structures, the external quantum efficiency (EQE) of OLEDs is still limited due to poor light extraction. Also, although intensive efforts to develop new conjugated polymers and device architectures have improved power conversion efficiency (PCE) up to 8%–9%, device efficiency must be improved to >10% for commercialization of PSCs. The surface plasmon resonance (SPR) effect of metal nanoparticles (NPs) can be an effective way to improve the extraction of light produced by decay of excitons in the emission layer and by absorption of incident light energy within the active layer. Silver (Ag) NPs are promising plasmonic materials due to a strong SPR peak and light‐scattering effect. In this review, different SPR properties of Ag NPs are introduced as a function of size, shape, and surrounding matrix, and review recent progress on application of the SPR effect of AgNPs to OLEDs and PSCs.     

Enhanced luminance of organic light-emitting diodes with metal nanoparticle electron injection layer

Improvement of the performance of organic light-emitting diodes (OLEDs) was achieved by implementing Magnesium-Nickel nanoparticles at the cathode–organic interface using pulsed laser deposition technique. The small geometry of Mg-Ni nanoparticles acts to enhance the localized electric field around them, thus increasing electron injection through tunneling, from the cathode to the organic layer. Improved current and luminance characteristics were demonstrated for both small molecule and polymer-based OLEDs when the nanoparticle layer was incorporated.     

Dynamical processes related to the Bose condensation of excitonic molecules in CdSe

The time dependences in the picosecond region of various dynamical processes of single excitons and excitonic molecules in CdSe are studied experimentally. The results are discussed in relation to the realization of the Bose condensation of excitonic molecules by the use of picosecond pulse excitation.     

纳米ZnO薄膜的激子光致发光特性

报道了纳米ZnO薄膜激子光致发光(PL)与温度的关系。首先利用低压金属有机化学气相沉积(LPMOCVD)技术生长ZnS薄膜,然后将ZnS薄膜在氧气中于800℃下热氧化2h获得纳米ZnO薄膜。X射线衍射(XRD)结果表明,纳米ZnO薄膜具有六角纤锌矿多晶结构且具有择优(002)取向。室温下观察到一束强的紫外(326eV)光致发光(PL)和很弱的深能级(DL)发射。根据激子峰的半高宽(FWHM)与温度的关系,确定了激子纵向光学声子(LO)的耦合强度(ГLO)。     

Enhancement of light extraction efficiency of organic light emitting diodes using nanostructured indium tin oxide

Improved outcoupling efficiency of organic light emitting diodes (OLEDs) is demonstrated by incorporating a nanostructured indium tin oxide (NSITO) film between a conducting anode and a glass substrate. NSITO film was fabricated using rf-sputtering at oblique angle (85°). Significant reduction in refractive index and improved transmission of NSITO film was observed. OLEDs were then fabricated onto NSITO film to extract the ITO-glass waveguided modes. Extraction efficiency was enhanced by 80% without introducing any detrimental effects to operating voltage, current density, and angular invariance of emission spectra of OLEDs.     

Optically detected magnetic resonance studies of luminescence‐quenching processes in π‐conjugated materials and organic light‐emitting devices

It is widely recognized that nonradiative quenching of excitons by other excitons and polarons become the dominant decay mechanism of these excitons at high excitation densities. These quenching processes cause the roll‐off in the efficiency of organic light‐emitting devices (OLEDs) and prevent lasing at high injection current densities. This review presents the optically‐detected magnetic resonance (ODMR) evidence for these photoluminescence‐ and electroluminescence‐quenching processes. And while it provides such evidence for quenching of singlet excitons by polarons and triplet excitons, it reveals the central role of the strongly spin‐dependent annihilation of triplet excitons by polarons, since under normal excitation conditions the steady‐state polaron and triplet exciton populations are 100–10⁴ times the singlet exciton population. In addition, it also suggests that quenching of singlet excitons by bipolarons, likely stabilized by a counterpolaron or countercharge at specific sites, may also be a significant quenching mechanism that also affects the charge transport properties.     

Novel Field Emission Organic Light Emitting Diodes with Dynode

This work presents novel field emission organic light emitting diodes(FEOLEDs) with dynode,in which an organic EL light-emitting layer is used instead of an inorganic phosphor thin film in the field emission display(FED).The proposed FEOLEDs introduce field emission electrons into organic light emitting diodes(OLEDs),which exhibit a higher luminous efficiency than conventional OLED.The field emission electrons emitted from the carbon nanotubes(CNTs) cathode and to be amplified by impact the dynode in vacuum.These field emission electrons are injected into the multi-layer organic materials of OLED to increase the electron density.Additionally,the proposed FEOLED increase the luminance of OLED from 10 820 cd/m2 to 24 782 cd/m2 by raising the current density of OLED from an external electron source.The role of FEOLED is to add the quantity of electrons-holes pairs in OLED,which increase the exciton and further increase the luminous efficiency of OLED.Under the same operating current density,the FEOLED exhibits a higher luminous efficiency than that of OLED.