Tandem Organic Light-emitting Diodes Research Articles

Emission Spectral Stability Modification of Tandem Organic Light-Emitting Diodes through Controlling Charge-Carrier Migration and Outcoupling Efficiency at Intermediate/Emitting Unit Interface.

An intermediate unit (IMU) of tandem organic light-emitting diode (TOLED) can affect the emission spectral stability because of the different charge-carrier migration and outcoupling efficiency at intermediate/emitting unit interface. In this work, an IMU consisting of carbon 60 and copper(II) phthalocyanine, inserted between red-emitting unit and blue-emitting unit, were constructed and fabricated. We focus on the effect of the relative location of the red-emitting unit and blue-emitting unit in the device configuration on the emission spectral stability of TOLED. In the color-tunable TOLED, a maximum current efficiency of 20.4 cd/A, along with light-emitting spectra tuning from red emission at (0.63, 0.31) according to Commission Internationale de L’Eclairage coordinates at a bias of 10 V to white emission at (0.34, 0.27) at 22 V, is realized. Meanwhile, in the color-stable TOLED, a maximum current efficiency of 15.5 cd/A and a stable white emission at (0.31, 0.27) in a wide range of biases from 12 to 22 V are also obtained. This work might inspire a promising approach for the fabrication of color-tunable and color-stable OLEDs for both information display and solid-state lighting.

High performance hybrid tandem white organic light-emitting diodes by using a novel intermediate connector

Using Liq/Ca/HAT-CN as an intermediate connector, high-performance hybrid tandem WOLEDs with EQE of 39.57% and CRI of 93 were developed.

Diphenanthroline Electron Transport Materials for the Efficient Charge Generation Unit in Tandem Organic Light-Emitting Diodes

In this paper, we report two new phenanthroline-based compounds, 1,4-bis(2-phenyl-1,10-phenanthrolin-4-yl)benzene (p-bPPhenB) and 1,3-bis(2-phenyl-1,10-phenanthrolin-4-yl)benzene (m-bPPhenB), for the charge generation unit of tandem organic light-emitting diodes (OLEDs). These two compounds exhibited high electron mobility of (5.8–4.4) × 10–3 cm2/(V s), a very small injection barrier at the p–n junction interface, a high glass transition temperature of 123.9–182.1 °C, and exceptionally good operational stability. Because of such excellent characteristics, a single-stack red phosphorescent OLED (PhOLED) with p-bPPhenB showed a low driving voltage (2.7 V) and significantly improved maximum power efficiency (56.8 lm/W), external quantum efficiency (30.8%), and device lifetime (LT95, 130 h) compared to those of the control device using bathophenanthroline (Bphen) (3.7 V, 39 lm/W, 27.1%, and 13 h). Furthermore, a two-stack (tandem) red PhOLED using p-bPPhenB in the charge generation unit exhibited superior cha...

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

Abstract An intermediate connector of Cs 2 CO 3 /Al/MoO 3 used in tandem organic light-emitting diodes (OLEDs) was investigated in this work. Here, an ultrathin Cs 2 CO 3 /Al was used as an electron-injection layer (EIL) from MoO 3 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 Cs 2 CO 3 /Al/MoO 3 connector is also estimated. The electrical properties and EL spectra of tandem OLEDs show that the Cs 2 CO 3 /Al/MoO 3 can function well for charge generation and transport, and the current density-voltage curves of Cs 2 CO 3 /Al/MoO 3 based special multilayer device shows the photovoltaic effect as a photovoltaic cell.

Progress on material, structure and function for tandem organic light-emitting diodes

Abstract Tandem organic light-emitting diodes (TOLEDs) generally consist of two or more electroluminescent (EL) units connected electrically in series via an intermediate layer. It has been found that the current efficiency of TOLEDs is potentially several folds higher than that of single EL unit device. Based on the latest great research efforts, herein we aim to present an overview of primary principles and designation for both efficient EL unit and intermediate layer in the aspects of functional materials, energy level, device architecture and optical properties. Moreover, the design of monochromatic, white and color tunable TOLEDs, is also presented to guide the precise regulation of charge carriers in the intermediate layer and exciton distribution in distinct EL units. Finally, we end this work by presenting the challenges of TOLEDs to achieve the appealing commercial interest for information display and solid-lighting source. It can be anticipated that novel functional materials and structural construction can be devised upon suitable device engineering to achieve high performance TOLEDs.

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

Buffer-modified C60/pentacene as charge generation layer (CGL) is investigated to achieve effective performance of charge generation. Undoped green electroluminescent tandem organic light-emitting diodes (OLEDs) with multiple identical emissive units and using buffer-modified C60/pentacene organic semiconductor heterojunction (OHJ) as CGL are demonstrated to exhibit better current density and brightness, compared with conventional single-unit devices. The current density and brightness both can be significantly improved with increasing the thickness of Al. However, excessive thickness of Al seriously decreases the transmittance of films and damages the interface. As a result, the maximum current efficiency of 1.43 cd·A-1 at 30 mA·cm-2 can be achieved for tandem OLEDs with optimal thickness of Al. These results clearly demonstrate that Cs2CO3/Al is an effective buffer for C60/pentacene-based tandem OLEDs.

Doping-free tandem white organic light-emitting diodes

Tandem white organic light-emitting diodes (WOLEDs) are of great research interest since they can greatly boost the performance compared with the single-unit counterparts. However, their structures are more complicated than those of single-unit OLEDs. Besides, to achieve high performance, the doping technology is required to tandem OLEDs, particularly for tandem WOLEDs, further complicating the structures. Herein, doping-free tandem WOLEDs, for the first time, have been demonstrated. By managing an effective doping-free charge generation layer to interconnect doping-free emitting layers/charge transport layers, high-performance doping-free tandem WOLEDs have been developed. The blue-yellow device accomplishes the simplified structure/short fabrication time/reduced cost/high efficiency/low efficiency roll-off/low voltage/high luminance trade-off, which cannot be achieved by previous tandem WOLEDs. Remarkably, the efficiency (81.2cdA−1) is ∼2-fold higher than the highest efficiency of previous doping-free WOLEDs and even higher than those of some best doping tandem WOLEDs. The maximum luminance is 44,886cdm−2, which is the highest for doping-free WOLEDs. Besides, the blue-red device can exhibit a color rendering index (CRI) of 67, which is even higher than that of some representative three-color tandem WOLEDs. Such findings may not only represent a significant step for doping-free WOLEDs, but unlock a novel avenue that doping-free tandem WOLEDs are promising to achieve the simplicity and high performance trade-off.

Role of MoO3-modified organic photovoltaic-type charge generation layer in tandem organic light-emitting diodes

The role of each part in MoO3-modified organic photovoltaic (OPV)-type charge generation layer (CGL) was investigated by incorporation into hole-only devices. The OPV heterojunction (HJ) consists of fullerene (C60) and copper phthalocyanine (CuPc). According to the electrical characteristics of hole-only devices, it was found that charge generation mainly took place at the interface between MoO3 and hole transport layer rather than the interface of C60–CuPc HJ under an electric field. The C60–CuPc HJ exhibited good charge generation in the presence of photons due to photovoltaic effect, despite its poorer charge generation upon an application of electric field. In addition, charges can be also generated at CuPc/MoO3 interface upon a relatively high electric field since the absorption band of charge transfer complex formed by CuPc and MoO3 is within visible region. These results, which had been rarely reported in the previous studies on OPV-type CGL, can well explain the experimental results of tandem organic light-emitting diodes (OLEDs) with MoO3 modified OPV-type CGLs.

Enhanced charge generation in doped organic/organic p–n heterojunction for improving the performance of tandem organic light emitting diode

AbstractThe doped organic/organic p–n heterojunctions have been applied as charge generation structure (CGS) in tandem organic light emitting diodes (TOLEDs). It is found that the field‐induced charge generation takes place more efficiently at the interface between Li2CO3 n‐doped bathocuproine (BCP:Li2CO3) and MoO3 p‐doped 4,4′‐N,N′‐dicarbazole‐biphenyl (CBP:MoO3) than at the interface between BCP:Li2CO3 and MoO3 p‐doped 4,4‐bis[N‐1‐naphthyl‐N‐phenylamino]biphenyl (NPB:MoO3). It is because the process of electrons tunneling through the depletion zone from the highest occupied molecular orbit (HOMO) of CBP:MoO3 to the lowest unoccupied molecular orbit (LUMO) of BCP:Li2CO3 is more efficient than that from the HOMO of NPB:MoO3 to the LUMO of BCP:Li2CO3. Compared to the TOLED using the conventional CGS of 10‐nm BCP:Li2CO3/20‐nm NPB:MoO3, the one using the CGS of 10‐nm BCP:Li2CO3/10‐nm CBP:MoO3/10‐nm NPB:MoO3 shows increased device performance. In addition, the interconnecting property of CGS of 10‐nm BCP:Li2CO3/x nm CBP:MoO3/20 − x nm NPB:MoO3 shows a strong dependence on the thickness of CBP:MoO3. We provide a new insight on optimizing Ohmic loss in the CGSs, useful for improving the performance of TOLEDs.

Buffer-modified n/p-type and p/n-type planar organic heterojunctions as charge generation layers for high performance tandem organic light-emitting diodes

High performance tandem organic light-emitting diodes (TOLEDs) were demonstrated for the first time using both buffer-modified n/p-type and p/n-type planar organic heterojunctions (OHJs) as charge generation layers (CGLs), and the optimized configurations of these two CGLs were “LiF/4,7-diphenyl-1,10-phenanthroline (Bphen)/tris(phenylpyrazole)iridium (Ir(ppz)3)/Ir(ppz)3:molybdenum oxide (MoOx)/MoOx” and “LiF/Ir(ppz)3/Bphen/MoOx”, respectively. Compared to the single-unit OLED, both n/p-type and p/n-type CGLs based TOLEDs exhibited about two folds enhancement in current efficiency. The influence of both MoOx and LiF as carrier extraction layers on the charge generation process of CGLs was conscientiously investigated. The working mechanism of two CGLs was discussed in details. It was also found that the novel design concept, a buffer-modified p/n-type planar OHJ could generate enough charges under a forward applied voltage and the carriers extraction was a tunneling process. Our results provide the great practicality of implementing planar OHJs as CGLs in high performance TOLEDs.

Dynamic processes of charges generation in intermediate connectors for tandem organic light emitting diodes

The role of transition metal oxides (TMOs) based intermediate connectors in tandem organic light emitting diodes (OLEDs) has been studied via capacitance-voltage and current-voltage characteristics, in order to elucidate the dynamic processes of charges generation and transport within externally applied voltages. The TMO-based intermediate connectors are composed of molybdenum trioxide (MoO3) and cesium azide (CsN3)-doped-4, 7–diphenyl-1, 10-phenanthroline (BPhen) layers, where MoO3 and CsN3 are used due to low deposition temperatures. From the obtained results of capacitance and current density, charges generation in CsN3:BPhen/MoO3/NPB is proposed to the defect states in thermally evaporated MoO3, which offers a minimal energy offset for charges generation. Moreover, our results clearly indicate that charges generation efficiency is not only relying on the MoO3-NPB interface, but also influenced by CsN3:BPhen-MoO3 interface. CsN3 doped BPhen layer further improves charges separation efficiency, which finally results in favorable charges transport into the adjacent layers and ensure to function efficiently for tandem OLEDs.

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 MoO3-x. Although a-ZSO has a very small work function of 3.4 eV, it forms Ohmic contact with MoO3-x with a high work function of 6.6 eV. It is discovered that the interface state appears between a-ZSO and MoO3-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.

Highly efficient tandem OLED based on C60/rubrene: MoO3 as charge generation layer and LiF/Al as electron injection layer

Tandem organic light-emitting diodes (OLEDs) have received much attention in solid-state lighting due to their high current efficiency, long lifetime and excellent stability. The highly efficient blue fluorescent tandem OLEDs based on the charge generation layer (CGL) of C60/rubrene: MoO3 and the electron injection layer (EIL) of LiF/Al were fabricated. The ultra-thin Al layer in EIL was introduced to further increase electron injection from CGL to the emission unit. We found that the maximal current efficiency and power efficiency of optimal tandem device can reach to 43.1cd/A and 15.1lm/W, respectively, which are approximately 2.8 and 1.9 times compared with those of single-emissive-unit device. Moreover, compared with the traditional tandem device, the driving voltage of the optimal device is reduced by 6V, and the turn-on voltage is reduced by 2.4V. We analyzed the mechanism and characterization of these tandem devices. The effective charge separation and transport of C60/rubrene: MoO3, and excellent electron injection ability of ultra-thin Al layer are the main factors for the remarkable enhancement in both current efficiency and power efficiency of tandem OLEDs.

C70/C70:pentacene/pentacene organic heterojunction as the connecting layer for high performance tandem organic light-emitting diodes: Mechanism investigation of electron injection and transport

A high performance tandem organic light-emitting diode (OLED) is realized by employing a C70/C70:pentacene/pentacene organic heterojunction as the efficient charge generation layer (CGL). Not only more than two time enhancement of external quantum efficiency but also significant improvement in both power efficiency and lifetime are well achieved. The mechanism investigations find that the electron injection from the CGL to the adjacent electron transport layer (ETL) in tandem devices is injection rate-limited due to the high interface energy barrier between the CGL and the ETL. By the capacitance-frequency (C-F) and low temperature current density-voltage (J-V) characteristic analysis, we confirm that the electron transport is a space-charge-limited current process with exponential trap distribution. These traps are localized states below the lowest unoccupied molecular orbital edge inside the gap and would be filled with the upward shift of the Fermi level during the n-doping process. Furthermore, both the trap density (Ht) and the activation energy (Ea) could be carefully worked out through low temperature J-V measurements, which is very important for developing high performance tandem OLEDs.

High-performance tandem organic light-emitting diodes based on a buffer-modified p/n-type planar organic heterojunction as charge generation layer

High-performance tandem organic light-emitting diodes (TOLEDs) were realized using a buffer-modified p/n-type planar organic heterojunction (OHJ) as charge generation layer (CGL) consisting of common organic materials, and the configuration of this p/n-type CGL was “LiF/N,N′-diphenyl-N,N′-bis(1-napthyl)-1,1′-biphenyl-4,4′-diamine (NPB)/4,7-diphenyl-1,10-phenanthroline (Bphen)/molybdenum oxide (MoOx)”. The optimized TOLED exhibited a maximum current efficiency of 77.6 cd/A without any out-coupling techniques, and the efficiency roll-off was greatly improved compared to the single-unit OLED. The working mechanism of the p/n-type CGL was discussed in detail. It is found that the NPB/Bphen heterojunction generated enough charges under a forward applied voltage and the carrier extraction was a tunneling process. These results could provide a new method to fabricate high-performance TOLEDs.

Stacking multiple connecting functional materials in tandem organic light-emitting diodes

Tandem device is an important architecture in fabricating high performance organic light-emitting diodes and organic photovoltaic cells. The key element in making a high performance tandem device is the connecting materials stack, which plays an important role in electric field distribution, charge generation and charge injection. For a tandem organic light-emitting diode (OLED) with a simple Liq/Al/MoO3 stack, we discovered that there is a significant current lateral spreading causing light emission over an extremely large area outside the OLED pixel when the Al thickness exceeds 2 nm. This spread light emission, caused by an inductive electric field over one of the device unit, limits one’s ability to fabricate high performance tandem devices. To resolve this issue, a new connecting materials stack with a C60 fullerene buffer layer is reported. This new structure permits optimization of the Al metal layer in the connecting stack and thus enables us to fabricate an efficient tandem OLED having a high 155.6 cd/A current efficiency and a low roll-off (or droop) in current efficiency.

Enhancement of tandem organic light-emitting diode performance by inserting an ultra-thin Ag layer in charge generation layer

White organic light-emitting diodes (WOLEDs) have attracted both scientific and industrial interest in the solidstate lighting and display applications due to their exceptional merits,such as high luminances,low power consumptions, high efficiencies,fast response times,wide-viewing angles,flexibilities and simple fabrications.The power efficiency of WOLED has been step-by-step improved in the last 20 years,however,the lifetime of WOLED is still unsatisfactory, which greatly restricts the further development of WOLED.In general,the tandem structure can be used to obtain high-efficiency and long-lifetime WOLED.One of the most important features of this kind of structure is that the different-colors emitting units can be connected by the charge generation layer.Therefore,the key to achieving a highperformance tandem device is how to design the charge generation layer.In this paper,we first develop a tandem green OLED by using an effective charge generation layer with an ultra-thin Ag layer between 4,7-diphenyl-1,10-phenanthroline:CsCO3 and hexaazatriphenylenehexacabonitrile,achieving high luminance,low voltage,high efficiency and long lifetime.The green tandem device with ultra-thin Ag layer (device C) obtains a highest luminance of 290000 cd/m2,which is 1.4 and 1.9 times higher than those of the tandem devices without ultra-thin Ag (device B) and singleunit device (device A),respectively.The driving voltage of device C is 7.2 V at 1000 cd/m2,1.4 V lower than that of device B.Besides,the maximum current efficiency of device C is 60.4 cd/A,which is 2.4% and 220% higher than those of device B (59 cd/A) and device A (18.7 cd/A),respectively.The power efficiency of device C is 26 lm/W,which is 21% higher than that of device B (21.5 lm/W).Moreover,the lifetime (T80) of device C reaches 250 h at an initial luminance of 10000 cd/m2,which is nearly 100 times higher than that of device B (2.7 h).Finally,we fabricate a white tandem device with the optimized charge generation layer,achieving a current efficiency and power efficiency of 75.9 cd/A and 36.1 lm/W at 1000 cd/m2,respectively.In addition,the lifetime (T80) is 77 h at an initial luminance of 10000 cd/m2.All the excellent performances are ascribed to the introduction of the ultra-thin Ag layer into the charge generation layer, which can effectively block the charge generation layer from diffusing.This exciting discovery can provide an effective way to design efficient and stable WOLED,which is beneficial to the solid-state lighting and display markets.

Tandem Organic Light-Emitting Diodes.

A tandem organic light-emitting diode (OLED) is an organic optoelectronic device that has two or more electroluminescence (EL) units connected electrically in series with unique intermediate connectors within the device. Researchers have studied this new OLED architecture with growing interest and have found that the current efficiency of a tandem OLED containing N EL units (N > 1) should be N times that of a conventional OLED containing only a single EL unit. Therefore, this new architecture is potentially useful for constructing high-efficiency, high-luminance, and long-lifetime OLED displays and organic solid-state lighting sources. In a tandem OLED, the intermediate connector plays a crucial role in determining the effectiveness of the stacked EL units. The interfaces in the connector control the inner charge generation and charge injection into the adjacent EL units. Meanwhile, the transparency and the thickness of the connector affect the light output of the device. Therefore, the intermediate connector should be made to meet both the electrical and optical requirements for achieving optimal performance. Here, recent advances in the research of the tandem OLEDs is discussed, with the main focus on material selection and interface studies in the intermediate connectors, as well as the optical design of the tandem OLEDs.

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.

Effects of Aluminum Interlayer in Charge Generation Unit for Tandem Organic Light-Emitting Diodes

Effects of Aluminum Interlayer in Charge Generation Unit for Tandem Organic Light-Emitting Diodes