Buffer-modified C60/pentacene as organic charge generation layer based on Al and MoO3

We demonstrate tandem organic light-emitting diodes (TOLEDs) with excellent performance using Al and MoO₃ buffer-modified C₆₀/pentacene as charge generation layer (CGL). Al and MoO₃ were used as the electron and hole injection layers of C60/pentacene CGL, respectively. Green phosphorescence TOLEDs with the structure of ITO/NPB/mCP:Ir(ppy)₃/TPBi/Al/C₆₀/pentacene/MoO₃/NPB/mCP:Ir(ppy)₃/TPBi/Cs₂CO₃/Al were fabricated. The results show that the inserted Al and MoO₃ can effectively increase the charge injection capacity of organic CGL, resulting the improvement of luminance and current efficiency of TOLEDs. The turn-on voltage of TOLEDs is much lower than that of single-unit device, and the current efficiency is more than 2 times larger than that of the single-unit device. TOLEDs can exhibit excellent photoelectric performance when the thicknesses of Al, C₆₀, pentacene and MoO₃ are 3 nm, 15 nm, 25 nm and 1 nm, respectively. The maximum luminance and current efficiency are 7 920.0 cd/m² and 16.4 cd/A, respectively. This work is significant to build new CGL structures for realizing high-performance TOLEDs.     

A red tandem organic light-emitting diode based on organic photovoltaic-type charge generation layer

In this paper, a significant enhancement in current efficiency of a red tandem organic light-emitting diode (OLED), which is based on an organic photovoltaic-type charge generation layer (CGL) of fullerene carbon 60/copper (Ⅱ) phthalocyanine, is introduced. The CGL can absorb a part of photons, radiated from emission zone, then form excitons, which are dissociated into free charges. It induces in lower driven voltage and better efficiency of tandem OLED. Compared with single emitter-unit OLED and tandem OLED with bulk heterojunction CGL, the luminous efficiency boosts remarkably with increasing current density and shows rather slower roll-off. Our results demonstrate that the organic photovoltaic heterojunction, consists of two matched n- and p-type organic semiconductors, is a promising CGL for tandem OLEDs with high efficiency.     

Efficiency enhancement in tandem organic light-emitting devices with a hybrid charge generation layer composed of BEDT-TTF-doped TPBi/mCP/HAT-CN

Tandem organic light-emitting devices (OLEDs) were fabricated with a hybrid organic charge generation layer (CGL) composed of bis(ethylenedithio)-tetrathiafulvalene (BEDT-TTF) doped 1,3,5-tris(N-phenylbenzimiazole-2-yl)benzene (TPBi), 1,3-bis(cabazol-9-yl)benzene (mCP), and 1,4,5,8,9,11-hexaazatriphenylene hexacarbonitrile (HAT-CN) in an attempt to enhance their current efficiency. While the operating voltage of the tandem OLEDs with a hybrid structure composed of BEDT-TTF-doped TPBi, mCP, and a HAT-CN CGL at 10 mA/cm² was 1 V lower than that of the tandem OLEDs with a typical CGL composed of BEDT-TTF-doped TPBi and a HAT-CN, the corresponding the current efficiency of the tandem OLEDs with a hybrid CGL at 10 mA/cm² was 2.9 cd/A higher than that of the tandem OLEDs with a typical CGL. The increase in the current efficiency and the decrease in the operating voltage of the tandem OLEDs with the hybrid CGL were attributed to enhanced electron injection due to the insertion of the mCP layer into the hybrid CGL.     

Organic semiconductor heterojunction as charge generation layer in tandem organic light-emitting diodes for high power efficiency

High-performance tandem organic light-emitting diodes (OLEDs) employing a buffer-modified C₆₀/pentacene organic semiconductor heterojunction (OHJ) as a charge generation layer (CGL) are demonstrated. The unique cooperation of charge generation, transport, and extraction processes occurred in the OHJ-based CGL remarkably reduces the operational voltage. As a result, an approximately twofold enhancement in power efficiency (21.9 lm W^?1 VS 10.1 lm W^?1) can be achieved that has previously been suggested to be difficult for tandem OLEDs. When the pentacene is replaced by zinc phthalocyanine (ZnPc), copper phthalocyanine (CuPc), or phthalocyanine (H₂Pc), a similar power efficiency improvement can be also achieved. The novel design concept of the buffer-modified OHJ-based CGL is superior to that of the conventional CGLs. The investigations on the operational mechanism are performed, from which it is found that the mobile charge carriers firstly are needed to be accumulated at both sides of the heterojunction interface and then transport along the two organic semiconductors in terms of their good carrier transport characteristics under an external electrical field, and finally inject into the corresponding electroluminescent (EL) units by the interfacial layers.     

高效叠层有机电致发光器件

采用Cs2CO3:Alq3/MoO3作电荷产生层,制备出高效双单元串联型叠层有机发光器件.双单元叠层有机发光器件发光性能受电荷产生层MoO3的厚度影响很大.当MoO3厚度为30 nm时,叠层器件表现出最好的器件性能,最大电流效率达到14.5 cd/A.在相当宽的低电流密度范围内,30 nm MoO3叠层器件的电流效率是...     

Efficient charge generation layer for tandem OLEDs: Bi-layered MoO3/ZnO-based oxide semiconductor

A novel oxide charge generation layer (CGL) with optical and electrical advantages for tandem organic light-emitting diodes (OLEDs) is proposed. The CGL comprises amorphous Zn-Si-O (a-ZSO) and MoO_3-x. Although a-ZSO has a very small work function of 3.4 eV, it forms Ohmic contact with MoO_3-x with a high work function of 6.6 eV. It is discovered that the interface state appears between a-ZSO and MoO_3-x, which contributes to the formation of quasi-Ohmic contact between the two oxides with dramatically different work functions. High performance of electron and hole injection/transport is achieved in tandem OLEDs with a very small voltage drop of 0.4 V at 100 mA/cm² with a twofold increase in current efficiency. This new CGL provides distinct advantages over conventional organic CGL materials with respect to processing simplicity, cost, and chemical stability.     

Effect of bulk and planar heterojunctions based charge generation layers on the performance of tandem organic light-emitting diodes

Tandem organic light-emitting diodes (OLEDs) were fabricated using organic planar and bulk heterojunctions based charge generation layers (CGLs), which were composed of cobalt phthalocyanine (CoPc) and fullerene (C₆₀). The electroluminescent (EL) characteristics of these two kinds of devices were systematically studied. The results showed that, compared to the corresponding devices with planar heterojunction (PHJ) based CGL, the tandem OLEDs with bulk heterojunction (BHJ) based CGL exhibited a dramatic improvement of performance. By investigating the electrical characteristics of CGLs, it was found that more hetero-interfaces introduced in the BHJ blend were beneficial for generating more interfacial dipoles and charge carriers, and the optimized charge transport pathways were favorable to promote both electron and hole mobilities. As a result, the improved charge carrier balance led to the efficiency enhancement of device performance. The results demonstrated the advantageous effect of BHJ blend film for the rational design of CGLs on the realization of high OLEDs performance.     

Highly stable and high power efficiency tandem organic light-emitting diodes with transition metal oxide-based charge generation layers

Tandem organic light-emitting diodes (OLEDs) have been studied to improve the long-term stability of OLEDs for 10 years. The key element in a tandem OLEDs is the charge generation layer (CGL), which provides electrons and holes to the adjacent sub-OLED units. Among different types of CGLs, n-doped electron transporting layer (ETL)/transition metal oxide (TMO)/hole transporting layer (HTL) has been intensively studied. Past studies indicate that this kind of CGL can achieve the desired efficiency enhancement, however, its long-term stability was reported not good and sometime even poor than a single OLED. This issue was not well addressed over the past 10 years. Here, for the first time, we found that this is caused by the unwanted diffusion of TMO into the underlying n-doped ETL layer and can be well resolved by introducing an additional diffusion suppressing layer (DSL) between them. Our finding will fully release the potential of TMO-based CGL in tandem OLEDs.     

LiF层对ITD/Alq~3/LiF∶Al器件性能的影响

分别制备了4种有机电致发光器件(OLEDs):ITO/Alq3/Al;ITO/Alq3/LiF(1.0nm):Al;ITO/Alq3/LiF(1.5nm)∶Al;ITO/Alq3/LiF∶(2.0nm)Al。研究了LiF的引入对金属电极与发光层界面的影响以及各种不同的界面态对器件发光性能的影响。研究结果表明:适当的LiF厚度的引入不仅可以改善器件的界面特性,而且可以提高器件的发光亮度及发光效率。     

Reverse breakdown behavior in organic pin-diodes comprising C60 and pentacene: Experiment and theory

Charge carrier transport under reverse voltage conditions is of major relevance in devices like organic photo-detectors, organic solar cells (tandem cells), organic light emitting diodes (generation contacts), and organic Zener diodes. We present organic pin-diodes comprising molecular doped layers of pentacene and C60 with an adjustable and reversible reverse breakdown behavior. We discuss the electric field and temperature dependence of the breakdown mechanism and propose a coherent charge transport scenario to describe the experimental findings. Within this model a field assisted tunneling of charge carriers over a rather large distance from valence to conductance states (and vice versa) governs the breakdown behavior. This is in accordance to experimental observations where charge carriers can overcome a layer thickness of 110 nm in the breakdown regime.     

Interfacial electronic structures of buffer-modified pentacene/C60-based charge generation layer

Interfacial electronic structures of MoO₃ and LiF-modified pentacene (PEN)/fullerene (C₆₀)-based charge generation layer (CGL) have been investigated with photoemission spectroscopy. Important characteristics controlling its functional effectiveness have been analyzed for charge transport properties in tandem organic lighting-emitting diodes (TOLEDs). It is found that a small energy offset at the PEN/C₆₀ heterojunction makes it easy to transfer electrons from PEN to C₆₀ even under a small applied bias, facilitating the occurrence of charge generation. The band bending observed in both PEN and C₆₀ is beneficial to exciton-dissociation and charge transport in opposite directions. At the MoO₃/PEN interface, the high work function (WF) of MoO₃ brings the highest occupied molecular orbital (HOMO) onset up to the Fermi level (E_F) not only for PEN but also for most hole transport layer (HTL) materials of the adjacent electroluminescent (EL) unit as this CGL is connected into TOLED. Therefore, holes can be efficiently injected from PEN into this EL unit. Similarly, at the C₆₀/LiF interface, the low WF of the LiF buffer layer makes the lowest unoccupied molecular orbital (LUMO) to pin close to the E_F not only for C₆₀ but also for most electron transport layer (ETL) materials of the other EL unit, which induces the electrons to inject easily from C₆₀ into that EL unit by tunneling through the thin LiF film. The favorable energy level alignment can effectively enhance charge generation, transport, and injection. The advantage of the MoO₃/PEN/C₆₀/LiF structure is that thus formed CGL can greatly reduce the voltage drop and thus enhance the power efficiency (PE) of the corresponding TOLED.     

The electro-optic performance and photovoltaic effect of organic devices based on cesium carbonate/Al/molybdenum trioxide intermediate connector

An intermediate connector of Cs₂CO₃/Al/MoO₃ used in tandem organic light-emitting diodes (OLEDs) was investigated in this work. Here, an ultrathin Cs₂CO₃/Al was used as an electron-injection layer (EIL) from MoO₃ to the adjacent electron transporting layer (ETL). To verify the function of this intermediate connector, the device performances were evaluated through current density-voltage-luminance characteristics, current density-efficiency curves, and EL spectra. Additionally, the effect of photon energy on carriers behavior in the Cs₂CO₃/Al/MoO₃ connector is also estimated. The electrical properties and EL spectra of tandem OLEDs show that the Cs₂CO₃/Al/MoO₃ can function well for charge generation and transport, and the current density-voltage curves of Cs₂CO₃/Al/MoO₃ based special multilayer device shows the photovoltaic effect as a photovoltaic cell.     

阴极蒸镀和隔离层对有机发光二极管性能的影响

制备了简单结构的有机发光二极管(OLED)ITO/NPB/Alq3/Al/Ag。实验结果表明,快速蒸镀法制备的Ag阴极越厚,器件性能越差,而慢速蒸镀200nmAg阴极时器件性能也较差。在Alq3与Al阴极之间插入BCP/C60/LiF隔离层后,即使快速蒸镀法制备的Ag厚达280nm,器件的最大电流密度、最大亮度和最大电流效率仍分别高达248.6mA/cm2、5380.7cd/m2和3.52cd/A。隔离层不仅保护NPB和Alq3基本不被玻璃化,还很好地与Alq3和Al阴极匹配,大大提高了器件性能。     

有机/聚合物异质结电致发光器件的光电性能

器件结构是影响有机发光器件(OLED)性能的重要因素之一.采用8-hydroxyquinoline-aluminum(AlQ)作为发光层(EML)和电子传输层(ETL),polyvinylcarbazole (PVK)作为空穴传输层(HTL),制备了具有有机小分子/聚合物异质结结构的OLED器件,通过其电压-电流-发光亮度(V-J-B)特性测试,研究了HTL的引入及其膜厚对器件性能的影响.实验结果表明,HTL的引入有效地改善了OLED的光电性能,同时HTL膜厚对器件性能具有显著影响,当HTL膜厚为20 nm时,所制备的OLED器件具有最小的驱动电压和启亮电压、最大的发光亮度和发光效率.

Abstract：

The device construction plays an important role in improving the optoelectronic performance of organic electroluminescence devices (OLEDs). Heterojunction OLEDs with a configuration of glass/ITO/PVK/AlQ/Mg/Al were fabricated by using 8-hydroxyquinoline-aluminum (AlQ) as the emission layer (EML) and electron transport layer (ETL) and polyvinylcarbazole (PVK) as the hole transport layer (HTL). The effect of the HTL thickness on the performance of OLEDs was investigated with respect to the driving voltage, turn-on voltage, electroluminescence brightness and efficiency of the devices. Experimental results demonstrate that the optical and electrical properies of OLEDs are closely related to the HTL thickness. The device fabricated with the HTL thickness of 20 nm possesses the best photoelectric properties such as the minimum driving voltage and turn-on voltage, and the maximum electroluminescence brightness and efficiency.     

Study of blocking effect of Cu-phthalocyanine layer in zinc oxide/pentacene/CuPc/C60/Al organic solar cells by electric field-induced optical second harmonic generation measurement

By employing electric-field-induced optical second-harmonic generation (EFISHG) measurement, we studied the blocking effect of sandwiched Cu-phthalocyanine (CuPc) layer on IZO/pentacene/Cu-phthalocyanine (CuPc)/C₆₀/Al organic solar cells (OSCs). Results evidently showed that the Maxwell–Wagner type interfacial charging appears at both pentacene/CuPc and CuPc/C₆₀ interfaces, depending on the CuPc layer thickness. When the thickness of CuPc layer was 8 nm, the charging at the two interfaces was significantly suppressed and the I–V characteristic was improved. CuPc layer is available as a blocking layer for the regulation of interfacial charging induced in IZO/pentacene/C₆₀/Al organic solar cells (OSCs). The blocking effect of the CuPc layer was discussed on the basis of Maxwell–Wagner model analysis.     

The Role of Transition Metal Oxides in Charge‐Generation Layers for Stacked Organic Light‐Emitting Diodes

The mechanism of charge generation in transition metal oxide (TMO)‐based charge‐generation layers (CGL) used in stacked organic light‐emitting diodes (OLEDs) is reported upon. An interconnecting unit between two vertically stacked OLEDs, consisting of an abrupt heterointerface between a Cs₂CO₃‐doped 4,7‐diphenyl‐1,10‐phenanthroline layer and a WO₃ film is investigated. Minimum thicknesses are determined for these layers to allow for simultaneous operation of both sub‐OLEDs in the stacked device. Luminance–current density–voltage measurements, angular dependent spectral emission characteristics, and optical device simulations lead to minimum thicknesses of the n‐type doped layer and the TMO layer of 5 and 2.5 nm, respectively. Using data on interface energetic determined by ultraviolet photoelectron and inverse photoemission spectroscopy, it is shown that the actual charge generation occurs between the WO₃ layer and its neighboring hole‐transport material, 4,4',4”‐tris(N‐carbazolyl)‐triphenyl amine. The role of the adjacent n‐type doped electron transport layer is only to facilitate electron injection from the TMO into the adjacent sub‐OLED.     

双层有机电致发光器件有机层厚度优化的数值研究

在陷阱电荷限制电流传导理论的基础上,提出了双层有机电致发光器件的数值模型,研究了结构为“阳极/空穴输运层( HTL) /发光层( EML) /阴极”的器件中电流密度和量子效率随有机层的特征陷阱能量、陷阱密度和载流子迁移率的依赖关系.研究发现,对于给定的HTL 和EML 的特征陷阱能量、陷阱密度和载流子迁移率,存在一个最优的HTL 和EML 之间的厚度比率,在此最优厚度比下,器件的电流密度和量子效率达到最大.通过有机层厚度的优化,器件的电流密度和量子效率可提高多达两个数量级.另外,还研究了最优厚度比随有机层特征陷阱能量、总陷阱密度和载流子迁移率之间的     

双层有机电致发光器件有机层厚度优化的数值研究

在陷阱电荷限制电流传导理论的基础上,提出了双层有机电致发光器件的数值模型,研究了结构为"阳极/空穴输运层(HTL)/发光层(EML)/阴极"的器件中电流密度和量子效率随有机层的特征陷阱能量、陷阱密度和载流子迁移率的依赖关系. 研究发现,对于给定的HTL和EML的特征陷阱能量、陷阱密度和载流子迁移率,存在一个最优的HTL和EML之间的厚度比率,在此最优厚度比下,器件的电流密度和量子效率达到最大.通过有机层厚度的优化,器件的电流密度和量子效率可提高多达两个数量级.另外,还研究了最优厚度比随有机层特征陷阱能量、总陷阱密度和载流子迁移率之间的定量关系.     

多层氧化物复合阴极透明OLED器件

制备了一种采用多层氧化物复合阴极的透明OLED,器件结构为:ITO/MoO3(10nm)/NPB(60nm)/Alq3(65nm)/Al(1nm)/MoO3(1nm)/Al(Xnm)/MoO3(30nm)。所采用的复合阴极结构为MoO3/Al/MoO3(MAM),同时在复合阴极(MAM)与电子传输层(Alq3)中间插入一层厚度为1nm的Al中间层,该薄Al层一方面提高了电极与有机层间界面的平整度,同时增强了电极的导电性;另一方面,在电子传输层与中间层Al薄膜之间形成了良好的欧姆接触,提高了电子的注入能力。改变MAM结构中Al的厚度,获得该透明OLEDs的最佳性能,在Al的厚度为18nm时器件亮度最高,为2 297cd/cm2。