Tandem organic light-emitting diodes with buffer- modified C60/ZnPc as charge generation layer

In this paper, a significant enhancement in current efficiency of the green tandem organic light-emitting diodes (TOLEDs) is demonstrated, which is based on a buffer-modified charge generation layer (CGL) of fullerene carbon (C60)/zinc-phthalocyanine (ZnPc). Al and MoO3 were used as the buffer-modified layers on both sides of the bilayer C60/ZnPc, respectively. Experimental results show that the inserted Al and MoO3 layers can effectively increase the electron extraction of the CGL for obtaining the device performance enhancement. Compared with that of the green TOLEDs without buffer-modified layers in CGL (37.3 cd?A-1), the current efficiency of the green TOLEDs is increased to 54.1 cd?A-1. Further study results find that the performance can also be improved by optimizing the thickness of Al in the CGL. The maximum current efficiency and maximum luminance of the green TOLEDs achieve 63.5 cd?A-1 and 17 873 cd?m-2, respectively, when the multilayer structure of the CGL is Al (3 nm)/C60 (5 nm)/ZnPc (5 nm)/MoO3 (3 nm).     

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.     

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.     

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.     

Simplified phosphorescent organic light-emitting diodes with a WO3-doped wide bandgap organic charge transport layer

The effects of p-type doping of wide bandgap ambipolar 4,4′-N,N′-dicarbazolebiphenyl (CBP) with WO₃ were investigated through detailed electrical device characterization. It was found that, to achieve effective doping for improved hole injection and transport, the doping level should be greater than 20 mol% and the doped layer should be at least 10 nm thick. A large downward shift of the Fermi level in WO₃-doped CBP causes band bending and depletion at the doped/undoped CBP interface, resulting in an additional energy barrier which hampers hole transport. Simplified green phosphorescent organic light-emitting diodes (PhOLEDs) with CBP as the hole transport and host material were fabricated. With a WO₃-doped hole transport layer, the PhOLEDs attained brightness of 11,163 cd/m² at 20 mA/cm², and exhibited an improved reliability under constant-current stressing as compared to undoped PhOLEDs.     

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.     

Ionic liquid modified zinc oxide injection layer for inverted organic light-emitting diodes

We have demonstrated a novel approach for fabricating efficient hybrid organic–inorganic light emitting diodes (HyLEDs) by introducing dopants into solutions processable metal oxides as an interfacial layer. The doped ZnO is prepared by adding ionic liquid (IL) to a precursor solution for the ZnO. In this way a heavily doped ZnO:ILs cathode was obtained that enhances the electron injection properties and assures a good wetting of the organic active materials.     

Fabrication of flexible organic light-emitting diodes with Alq3 as emitting layer

Fabrication of flexible organic light-emitting diodes（FOLEDs） with ITO/PVK ：TPD/Alq3/Al configuration prepared on PET substrates is reported. Alq3 is used as the light-emitting material. The curves of the current density vs. voltage,optical current vs. voltage and quantum efficiency vs. current density of the devices are investigated. Compared the devices with the ones that have the same configuration and are fabricated under the same conditions but on glass substrates,the characteristics of the two kinds of devices are very similar except that the threshold voltage of the flexible FOLEDs is a little higher. Under the driving voltage of 20V,the corresponding brightness and the external quantum efficiency are 1000 cd/m^2 and 0. 27%, respectively. In addition, the anti-bend ability of the devices is tested and the reasons of failure of the devices are analyzed.     

Efficient single layer RGB phosphorescent organic light-emitting diodes

We report efficient single layer red, green, and blue (RGB) phosphorescent organic light-emitting diodes (OLEDs) using a “direct hole injection into and transport on triplet dopant” strategy. In particular, red dopant tris(1-phenylisoquinoline)iridium [Ir(piq)₃], green dopant tris(2-phenylpyridine)iridium [Ir(ppy)₃], and blue dopant bis(3,5-difluoro-2-(2-pyridyl)phenyl-(2-carboxypyridyl)iridium [FIrpic] were doped into an electron transporting 1,3,5-tris(N-phenylbenzimidazole-2-yl)benzene (TPBi) host, respectively, to fabricate RGB single layer devices with indium tin oxide (ITO) anode and LiF/Al cathode. It is found that the maximum current efficiencies of the devices are 3.7, 34.5, and 6.8 cd/A, respectively. Moreover, by inserting a pure dopant buffer layer between the ITO anode and the emission layer, the efficiencies are improved to 4.9, 43.3, and 9.8 cd/A, respectively. It is worth noting that the current efficiency of the green simplified device was as high as 34.6 cd/A, even when the luminance was increased to 1000 cd/m² at an extremely low applied voltage of only 4.3 V. A simple accelerated aging test on the green device also shows the lifetime decay of the simplified device is better than that of a traditional multilayered one.     

Tandem white organic light-emitting device using non-modified Ag layer as cathode and interconnecting layer

Tandem white organic light-emitting device (WOLED) using non-modified Ag film as cathode and interconnecting layer is demonstrated. Effective electron injection is achieved when Ag is deposited on 4,7-diphenyl-1,10-phenanthroline electron transporting layer without any modified layer. Single OLED with Ag cathode shows comparable performance to that of device with Mg:Ag cathode. Such tandem WOLED exhibits low driving voltage, high power efficiency (15.1 lm/W at 1000 cd/m²) and low efficiency roll-off. The working mechanisms of single and tandem devices were discussed in detail. These results could provide a simple method to fabricate high performance tandem white OLED.     

Solution-processed hexaazatriphenylene hexacarbonitrile as a universal hole-injection layer for organic light-emitting diodes

The use of 1,4,5,8,9,11-hexaazatriphenylene hexacarbonitrile (HAT-CN) thin layers, particularly the solution-processed type, as an efficient hole-injection layer (HIL) for organic optoelectronic devices is demonstrated herein. Among the solvents commonly used for solution processing, 2-propanone was found to selectively dissolve HAT-CN, allowing the fabrication of a rigid film. The alignment of the electronic energy levels of the solution-processed HAT-CN and thermally polymerized 2,7-disubstituted fluorene-based triaryldiamine (VB-FNPD) species was evaluated using ultraviolet photoelectron spectroscopy. The results revealed that the lowest unoccupied molecular orbital of HAT-CN and the highest occupied molecular orbital of VB-FNPD were very close to the Fermi level, which facilitated charge transfer at the interface and improved hole injection. The utilization of HAT-CN as HIL resulted in a dramatic enhancement of the performance of solution-processed red, green, and blue organic light-emitting diodes. The external quantum efficiency, current efficiency, and power efficiency of the HAT-CN-based devices were higher than or almost similar to those of optimized poly(3,4-ethylenedioxythiophene):poly(styrenesulfonate) (PEDOT:PSS)-based devices. Because of the efficient carrier-injection capability and the capacity to prevent interfacial mixing and erosion during fabrication, solution-processed HAT-CN is promising as a novel alternative to conventional PEDOT:PSS HILs.     

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.     

Interfacial charging originated from the conductivity decrease of C60 layer in IZO/pentacene/C60/Al organic double-layer solar cells

The origin of interfacial charging process in double-layer organic solar cells (OSCs) was studied by using the normal structure of Indium–Zinc–Oxide/pentacene/C₆₀/Al and its inverted double-layer system. Optical electric-field-induced second-harmonic generation (EFISHG) measurement was employed and results suggested that interfacial charging in these two kinds of OSCs led to charge accumulation with opposite charge polarity, owing to the conductivity decrease of C₆₀ layer. Applying the EFISHG measurements to the inverted OSCs also showed that the significant charge accumulation on donor–acceptor interface is responsible for the low I–V performance of the inverted OSCs. Thus, Maxwell–Wagner type interfacial charging, which is governed by the conductivity of C₆₀, can cause the degradation of the I–V performance of OSCs. The protection of C₆₀ layer from the conductivity decrease is a way to improve OSCs performance.     

High efficiency p-i-n organic light-emitting diodes with a novel n-doping layer

This study demonstrated p-i-n organic light-emitting diodes (OLEDs) incorporating a novel n-doping transport layer which is comprised of cesium iodide (CsI) doped into tris-(8-hydroxyquinoline) aluminum (Alq₃) as n-doping electron transport layer (n-ETL) and a p-doping hole transport layer (p-HTL) which includes molybdenum oxide (MoO₃) doped into 4,4′,4″-tris[2-naphthyl(phenyl)amino] triphenylamine (2-TNATA). The device with a 15 wt.% CsI-doped Alq₃ layer shows a turn on voltage of 2.4 V and achieves a maximum power efficiency of to 4.67 lm/W as well, which is significantly improved compared to these (3.6 V and 3.21 lm/W, respectively) obtained from the device with un-doped Alq₃. This improvement is attributed to an increase in the number of electron carriers in the transportation layer leading to an efficient charge balance in the emission zone. A possible mechanism behind the improvement is discussed based on X-ray photoelectron spectroscopy (XPS).     

Phosphorescent dye-doped hole transporting layer for organic light-emitting diodes

Hole transport materials are critical to the performance of organic light-emitting diodes (OLEDs). While 1,1-bis(di-4-tolylaminophenyl)cyclohexane (TAPC) with a high triplet energy is widely used for high efficiency phosphorescent OLEDs, devices using TAPC as a hole transport layer (HTL) have a short operating lifetime due to the build-up of trapped charges at the TAPC/emitting layer (EML) interface during device operation. In this work, to solve the operating stability problem, instead of using conventional HTLs, we use a(fac-tris(2-phenylpyridine)iridium (III))(Ir(ppy)₃) doped layer as an HTL to replace the conventional HTLs. Because of the hole injecting and transporting abilities of the phosphorescent dye, holes can be directly injected into the emitting layer without an injection barrier. OLEDs based on a phosphorescent dye-doped HTL show significant improvement in operational stability without loss of efficiency.     

基于LiF/Al/F4-TCNQ/NPB电荷产生层的叠层有机电致发光器件的特性研究

采用结构为LiF/Al/F4-TCNQ/NPB的电荷产生层,制备出了双发光单元叠层有机电致发光器件(OLED:Organic Light Emitting Device)。通过对比实验发现当F4-TCNQ层的厚度为8nm、Al层的厚度为5nm时,电荷产生层产生电荷的能力较强且具有良好的透光率。基于此,本文制备了发光层为CBP:6%Ir(ppy)3的叠层OLED,通过与单发光单元OLED的性能比较发现:采用LiF/Al/F4-TCNQ/NPB作为电荷产生层制备的叠层OLED的最大电流效率与功率效率分别为51.6cd/A、28.4lm/W,为单发光单元OLED的2.16倍、1.8倍,此外采用这种结构的电荷产生层有效解决了叠层OLED由于工作电压高而导致功率效率并未得到提升的问题;另一方面,采用有机材料F4-TCNQ代替传统无机金属氧化物作为电荷产生层中的电荷产生部分,能够避免无机金属氧化物高温升华对Al层薄膜的破坏,提升了器件的效率并且降低了器件的roll-off现象。     

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.     

Vertical InGaN light-emitting diodes with Ag paste as bonding layer

Vertical InGaN-based light-emitting diodes (LEDs) were fabricated with a Si substrate using Ag paste as bonding layer. Vertical LEDs with Ag paste bonding layer were bonded with Si substrate at a low temperature of 140 °C. In addition to the low-temperature bonding process, the soft property of Ag paste could better alleviate thermal stress compared with conventional eutectic metal bonding layer such as Au–Sn. Under the same test conditions, these two LEDs showed similar optical and electrical properties and reliability. However, LEDs with Ag-paste bonding layer were fabricated through a low-temperature bonding process. The characteristic of soft solder enables a relatively wider process window, such as bonding pressure and temperature, and a higher yield as compared with the vertical LEDs with Au–Sn eutectic bonding layer.     

Color stable phosphorescent white organic light-emitting diodes with double emissive layer structure

In this paper, we report color stable phosphorescent white organic light-emitting diodes (OLEDs) based on a double emissive layer (EML) structure composed of blue and red/green phosphorescent units. Deep hole trapping situation of red and green dopants at the red/green EML could induce less voltage dependent white spectral characteristics by restricting the change of exciton generation zone. A wide band-gap host material, 2,6-bis(3-(carbazol-9-yl)phenyl)pyridine (26DCzPPy), was used for achieving such deep-trap generation. Fabricated phosphorescent white OLED shows a slight color coordinate change of (?0.002, +0.002) from 1000 cd/m² to 5000 cd/m² with power efficiency of 38.7 lm/W and current efficiency of 46.4 cd/A at 1000 cd/m². In addition, negligible color changes were observed by delaying red dopant saturation time using optimum red dopant concentration.