N-doped Electron Transport Layer Research Articles

Improved Efficiency Roll-Off and Operational Lifetime of Organic Light-Emitting Diodes with a Tetradentate Platinum(II) Complex by Using an n-Doped Electron-Transporting Layer.

The efficiency roll-off and operational lifetime of organic light-emitting diodes (OLEDs) with a tetradentate Pt(II) emitter is improved by engaging an n-doped electron-transporting layer (ETL). Compared to those devices with non-doped ETL, the driving voltage is lowered, the charged carrier is balanced, and the exciton density in the emissive layer (EML) is decreased in the device with n-doped ETL with 8-hydroxyquinolinolatolithium (Liq). High luminance of almost 70,000 cd m−2 and high current efficiency of 40.5 cd A−1 at high luminance of 10,000 cd m−2 is achieved in the device with 50 wt%-Liq-doped ETL. More importantly, the extended operational lifetime of 1945 h is recorded at the initial luminance of 1000 cd m−2 in the 50 wt%-Liq-doped device, which is longer than that of the device with non-doped ETL by almost 10 times. This result manifests the potential application of tetradentate Pt(II) complexes in the OLED industry.

Investigating the effect of the conductivity of the electron transportation layer on the organic light-emitting diodes

In this paper, we choose three types of n-doped electron transport layer (ETL) materials with different chemical structures to fabricate only-electron carrier device for studying the conductivity of them, namely, 100 nm ETL1, 40 nm ETL2, and 23 nm ETL3. It is also analyzed that the effect of the potential barrier height on the carrier injection efficiency of the single-carrier device when they directly contact with the electrode having the different work function ITO (indium tin oxide film) electrode. When the thickness of the organic electron transport layer is thin, the current density is mainly limited by the injection barrier in the interface, However, When the thickness of the organic electron transport layer is thick, the organic electron transport layer with very low carrier mobility is a dominant factor in limiting the current through the device. If ETL1 is in contact with a lower work function aluminum electrode relative to the ITO electrode, the effect of the injection limit on the current is weaker. Considering Poole-Frenkel effect and space charge limited current effect, the field strength-dependent carrier mobility of ETL1 material is estimated by linear-fitting the current-electric field characteristic curve of only-electron device. Organic red light-emitting diode is fabricated with ETL1 thicknesses of 10 nm and 100 nm, respectively. For the organic light-emitting device with 100 nm ETL1, it reaches 14.9 cd/A at a current density of 30 mA/cm2 , and the color coordinates is (0.658, 0.341).

Solution-processed conductive interconnecting layer for highly-efficient and long-term stable monolithic perovskite tandem solar cells

Although stacking of multiple subcells with complementary bandgaps via an interconnecting layer (ICL) to form multi-junction tandem devices appears to be one of the most promising strategies to increase the power conversion efficiency (PCE) of organic-inorganic hybrid perovskite solar cells (PeSCs), the realization of high efficiency and long-term stable tandem devices based on solution-processed ICL remains highly challenging. In this study, a promising solution-processed ICL consisting of cross-linked p-doped hole transport layer (HTL) and n-doped fullerene electron transport layer for efficient and stable monolithic perovskite tandem solar cells is demonstrated. For the first time, high-performance photo-cross-linkable p-doped HTL material based on poly-[bis(4-phenyl)(2,4,6-trimethylphenyl)amine] (PTAA) is developed by incorporating 1,2-bis[4-(azido-methyl)phenyl]-1,2-diphenylethene (TPE-MN3) and molybdenum tris-[1-(trifluoroethanoyl)-2-(trifluoromethyl)ethane-1,2-dithiolene] (Mo(tfdCOCF3)3) as the cross-linking agent and p-type dopant, respectively. This HTL exerts multiple positive impact on the device characteristics, including good solvent resistance, appropriate energy level matching, good electrical conductivity, and high compatibility with different perovskite systems. With these desirable characteristics, the series-connected tandem solar cells afford a PCE up to 18.69%, which represents the highest efficiency reported to date for monolithic all-perovskite tandem cells. Additionally, owing to high conductivity of the interfacial layers, a promising PCE up to 16.20% is attained even when the active area is extended to 1.2 cm2. More encouragingly, the encapsulated tandem cells exhibit remarkable long-term stability, maintaining ≈ 91% of its initial PCE after 9300 h (≈387 days) of air exposure. This work represents an important step forward toward the realization of large-area solution-processed perovskite tandem solar cells with high efficiency and long-term stability.

Improvement on the performance of electrophosphorescent organic light-emitting device employing n-doped bis-4,6-(3,5-di-3-pyridylphenyl)-2-methylpyrimi-dine as electron-transporting layer

Abstract The effects of using Cesium azide as an n-dopant in Bis-4,6-(3,5-di-3-pyridylphenyl)-2-methylpyrimi-dine on the performances of green phosphorescent organic light-emitting device had been studied in detail for the first time. By optimizing the device structure, phosphorescent organic light-emitting device having a maximum current efficiency of 76.4 cd/A and a maximum external quantum efficiency of 21.7% was realized. In particular, the maximum power efficiency and the power efficiency at 100 cd/m2 were as high as 99.9lm/W and 99.4lm/W, respectively, which were better than the parameter on power efficiency of most reported Ir (ppy)3-OLEDs without out-coupling enhancement. Origins of the improvement on device characteristics in the n-doped device were systematically investigated by experimental and theoretical methods and the role of Cesium azide in enhancing the electron conductivity of the n-doped electron-transporting layer was discovered by X-ray photoelectron spectroscopy measurement.

N-Type Doping of Fullerenes for Planar Perovskite Solar Cells

Modification of TiO2 by using fullerenes is an efficient strategy to further improve the device efficiency and enhance the cell stability of perovskite solar cells (PSCs). However, the intrinsic issues of low electron mobility and electrical conductivity of fullerene materials may restrict their potential application in PSCs. Here, we demonstrated an n-doped electron-transporting layer by doping bis(1-[3-(methoxycarbonyl)propyl]-1-phenyl)-[6,6]C62 (bis-PCBM) with decamethylcobaltocene (DMC) to fabricate n–i–p structure PSCs. We successfully realized an n-type doping of bis-PCBM via a solution-processed doping process. DMC doping played a series of roles in adjusting the energy levels, improving the electron mobility and enhancing the film conductivity of bis-PCBM. A bis-PCBM:DMC composite film could act as an underlay for the deposition and growth of a subsequent perovskite layer owing to its smooth morphology. In addition, the bis-PCBM:DMC composite film also presented good solvent resistance, which coul...

Enhanced electron current in inverted organic light emitting diodes with an n-doped electron transport layer adopting high and low doping levels

Inverted organic light emitting diodes (IOLEDs) have been fabricated using Li2CO3 n-doped bathocuproine (BCP:Li2CO3) to boost electron injection and transport. It is found that the 8:1 BCP:Li2CO3 in mass ratio is more conductive than the 4:1 BCP:Li2CO3, mostly because the higher-doping level Li2CO3 scatters electrons more severely than the lower-doping level one. Nevertheless, the electron injection from cathode to 4:1 BCP:Li2CO3 is confirmed more efficient than that from cathode to 8:1 BCP:Li2CO3, since the higher doping level creates narrower depletion zone at the cathode interface and thereby makes electron tunneling more efficient, compared to the lower doping level. In addition, the transport barrier from 4:1 BCP:Li2CO3 to emissive layer is found smaller than that from 8:1 BCP:Li2CO3 to emissive layer. The IOLED using the cathode/10 nm 4:1 BCP:Li2CO3/10 nm 8:1 BCP:Li2CO3/10 nm 4:1 BCP:Li2CO3 structure shows increased performance than its counterparts using the cathode/30 nm 4:1 BCP:Li2CO3 and cathode/30 nm 8:1 BCP:Li2CO3 structures, as a result that the former structure offers an optimized trade-off between reducing the injection loss, conduction loss, and electrical loss due to transport barrier from BCP:Li2CO3 to emissive layer, relative to the latter two. We provide a simple and effective method to improve the performance of organic electronic devices based on managing the doping levels of n-doped electron transport layers.

Lowering the driving voltage and improving the luminance of blue fluorescent organic light-emitting devices by thermal annealing a hole injection layer of pentacene**Project supported by the National Natural Science Foundation of China (Grant No. 60906022), the Natural Science Foundation of Tianjin, China (Grant No. 10JCYBJC01100), the Key Science and Technology Support Program of Tianjin, China (Grant No. 14ZCZDGX00006), and the

We chose pentacene as a hole injection layer (HIL) to fabricate the high performance blue fluorescent organic lightemitting devices (OLEDs). We found that the carrier mobility of the pentacene thin films could be efficiently improved after a critical annealing at temperature 120 °C. Then we performed the tests of scanning electron microscopy, atomic force microscopy, and Kelvin probe to explore the effect of annealing on the pentacene films. The pentacene film exhibited a more crystalline form with better continuities and smoothness after annealing. The optimal device with 120 °C annealed pentacene film and n-doped electron transport layer (ETL) presents a low turn-on voltage of 2.6 V and a highest luminance of 134800 cd/m2 at 12 V, which are reduced by 26% and improved by 50% compared with those of the control device.

Inkjet deposition of a hole-transporting small molecule to realize a hybrid solution-evaporation green top-emitting OLED

The QUPD molecule has been deposited by inkjet printing as a hole-transport layer in top-emitting green OLEDs. A systematic study of the QUPD-based ink formulation has been done and different solvent mixtures have been investigated, in order to find the best composition (QUPD in toluene/IPA/anisole, 8/1/1 v/v/v) leading to the best film forming properties. Spin-coated PEDOT-PSS has been used as hole injecting layer. Subsequent layers have been deposited by vacuum sublimation. The resulting hybrid, solution-sublimation, OLEDs have been encapsulated by atomic layer deposition using Al2O3 material. In order to overcome the issue related to the thickness control of the organic layers deposited from solution, second order cavity length OLEDs have been fabricated by modifying the n-doped electron transport layer thickness. In that case, the relative OLED efficiency variation (10.5%) due to the thickness variation is far less compared to first order cavity length (34%) allowing a better reproducibility of the OLED fabrication. In the end, high efficiency (18 lm/W) green OLEDs of two different sizes, 0.44 cm2 and 4 cm2, have been fabricated, using an inkjet printed QUPD layer as hole transporting layer.

Multilayered phosphorescent polymer light-emitting diodes using a solution-processed n-doped electron transport layer

Efficient multilayered green phosphorescent polymer light-emitting devices (PhPLEDs) were successfully fabricated using a solution-processed n-doped small molecular electron transporting layer (ETL) composed of 1,3,5-tris(N-phenyl-benzimidazol-2-yl)-benzene (TPBi) and CsF. We found that the electroluminescence properties of the devices with n-doped ETLs are significantly improved. The maximum luminance efficiency of the device with 7.5wt% CsF doped TPBi ETL reached 26.9cd/A, which is 1.5 times as large as that of the undoped device. The impedance spectra of the devices and electron transport properties of the CsF doped ETLs demonstrate that doping dramatically decreases the impedance and enhances the electrical conductivity. Similarly, enhanced performance of PhPLED is also observed by use of CsF-doped 4,7-diphenyl-1,10 -phenanthroline (BPhen) ETL. These results demonstrate that CsF can be used as an effective n-dopant in solution-processed devices.

ITO-free large-area top-emission organic light-emitting diode by blade coating

Large-area top-emitting organic light-emitting diodes (TEOLEDs) with multi-layer structure are successfully demonstrated using the solution-processable blade coating on ITO-free substrate. The semitransparent cathode of TEOLED is composed of lithium fluoride (LiF), aluminum (Al) and silver (Ag). The composition of 3nm Al and 10nm Ag has a transmittance of 56% and a sheet resistance of 11Ω/□. It is applied to the green phosphorescence device with an emissive area of 2cm by 2.5cm. The maximum current efficiency is 25.2cd/A with high light-emission uniformity within 10% variation. The large-area TEOLEDs show comparable current efficiency as the small-area devices with an emissive area of 2mm by 2mm (having the same device structure) and better efficiency than traditional large-area bottom-emitting devices. Cesium fluoride (CsF) and n-doped electron transport layer are applied to improve electron injection. At 6V, the luminance is raised from 141cd/m2 to 502cd/m2 and 304cd/m2, respectively. In n-doped device, a simple Al/Ag cathode is used without LiF.

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 10years. 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 10years. 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.

Highly efficient tandem organic light-emitting devices utilizing the connecting structure based on n-doped electron-transport layer/HATCN/hole-transport layer.

In this work, we conducted studies of tandem organic light-emitting devices (OLEDs) based on the connecting structure consisting of n-doped electron-transport layer (n-ETL)/1,4,5,8,9,11-hexaazatriphenylene hexacarbonitrile (HATCN)/hole-transport layer. We investigated effects of different n-ETL materials and different HATCN thicknesses on characteristics of tandem OLEDs. Results show that the tandem OLEDs with n-BPhen and a 20 nm layer of HATCN in the connecting structure exhibited the best performance. With these, highly efficient and bright green phosphorescent two-emitting-unit tandem OLEDs, with drive voltages significantly lower than twice that of the single-unit benchmark device and current efficiencies higher than twice that of the single-unit benchmark device, were demonstrated.

White light emission from alternating current organic light-emitting devices using high frequency color-mixing

We report on a novel approach to obtain white emission from organic light-emitting devices, which require no charge injection from external electrodes and instead use a capacitively coupled alternating current driving signal for device operation. In our approach, both half-cycles of a sinusoidal driving signal are used to independently address a phosphorescent green and a triplet-harvesting blue/red unit. Both emission units are spatially separated and connected by sharing a common n-doped electron transport layer. Upon application of a sinusoidal driving voltage, the device alternately emits green and purple light pulses with durations of 25% of the signal period. For high frequencies, the human eye cannot resolve this modulation and the emitted light is perceived as white. This approach yields devices with warm-white color coordinates (0.43, 0.44), a CRI of 76.5, and high brightness values of 1000 cd m−2. We show evidence proving that the devices operate on a capacitive-coupling based driving scheme and that the emission is solely due to charge carriers provided by the doped transport layers. High dielectric constant insulating layers between electrodes and organic layers efficiently suppress charge carrier injection and improve the Zener tunneling mechanism that is required to replenish the depleted transport layers after each emission cycle.

High-efficiency green phosphorescent organic light-emitting diodes with double-emission layer and thick N-doped electron transport layer

We have developed green phosphorescent organic light-emitting diodes (OLEDs) with high external quantum efficiency of 59.7% and power efficiency of 243lm/W at 2.73V at 0.053mA/cm2. A double emission layer and a thick n-doped electron transport layer were adopted to improve the exciton recombination factor. A high refractive index hemispherical lens was attached to a high refractive index substrate for extracting light trapped inside the substrate and the multiple-layers of OLEDs to air. Additionally, we analyzed an energy loss mechanism to clarify room for the improvement of our OLEDs including the charge balance factor.

Organic Light‐Emitting Diodes: The Mechanism of Charge Generation in Charge‐Generation Units Composed of p‐Doped Hole‐Transporting Layer/HATCN/n‐Doped Electron‐Transporting Layers (Adv. Funct. Mater. 4/2012)

A tandem white organic light-emitting diode (OLED) composed of blue and orange emitting units interconnected by a charge-generation unit (CGU) in which both electrons and holes are efficiently generated is presented. On page 855, Jang-Joo Kim and co-workers analyze the charge generation mechanism of the CGU composed of p-doped hole-transporting layer (HTL)/1,4,5,8,9,11-hexaazatriphenylene hexacarbonitrile (HATCN)/n-doped electron-transporting layer (ETL). The energy level alignment shows that the electron injection at the HATCN/n-ETL junction limits the charge generation in the CGU.

Role of n-dopant based electron injection layer in n-doped organic light-emitting diodes and its simple alternative

We investigate the enhancement mechanism of the electroluminescence (EL) of alkali metal based n-doped organic light-emitting diodes (OLEDs). The dual role of the n-dopant (carrier transport and lowering of the injection barrier) induces a trade-off. When the electron transport layer (ETL) is optimally doped by the n-dopant for the highest conductivity, the amount of n-dopant at the ETL/cathode interface is insufficient to form enough chemical bonds with the cathode for efficient carrier injection. This insufficient amount of n-dopant limits the carrier injection properties. To solve this problem, we demonstrated that the addition of an electron injection layer (EIL) comprised of the n-dopant could increase its presence at the interface and, thereby, improve the carrier injection properties and, consequently, the EL efficiency. Moreover, simply using an alkali-metal alloy (rather than co-deposition) on the n-doped ETL as a cathode, instead of using the additional EIL, greatly improves the EL efficiency of the OLEDs. The alkali-metal alloy cathode increased the interfaced states at the ETL/cathode. The proposed model was confirmed by x-ray photoemission spectroscopy experiments on the alkali-metal n-dopant/electrode interface.

Electron Injection and Transport Mechanisms of an Electron Transport Layer in OLEDs

Electron injection and transport are key issues in the performance of organic light-emitting diodes (OLEDs). In this paper, an efficient n-doped electron transport layer (ETL) composed of 4,7-diphenyl-1,10-phenanthroline (Bphen) and lithium acetate (CH3COOLi) is demonstrated. The results reveal that the Fermi level moves toward the lowest unoccupied molecular orbital (LUMO), which consequently reduces the electron injection barrier and further enhances the efficiency of electron injection from the aluminum cathode. The mobility of electrons in the Bphen layer increased with CH3COOLi doping. In addition, a sky-blue fluorescent OLED was fabricated using the CH3COOLi-doped Bphen ETL, exhibiting a high luminance and efficiency. We also proved that as an n-type dopant, LiAC is more capable than LiF.

Organic light-emitting diodes based on new n-doped electron transport layer

Organic light-emitting diodes with 8-hydroxy-quinolinato lithium doped 4′7-diphyenyl-1, 10-phenanthroline as electron transport layer (ETL), and etrafluro-tetracyano-quinodimethane doped 4,4′,4″-tris(3-methylphenylphenylamono) triphenylamine as hole transport layer (HTL) are demonstrated. The conductivity of carrier transport layers with different doping concentration is examined by hole-only and electron-only devices. Compared with the referenced device (without doping), the current efficiency and power efficiency of the p–i–n device are enhanced by approximately 51% and 89%, respectively. This improvement is attributed to the improved conductivity of the transport layers and the efficient charge balance in the emission zone.

Electrical characterization of organic light-emitting diodes using dipotassium phthalate as n-type dopant

An efficient n-doped electron transport layer composed of 4,7-diphenyl-1,10-phenanthroline (BPhen) and dipotassium phthalate (PAK2) has been developed. By temperature-dependent admittance spectroscopy, the incorporation of PAK2 into BPhen is found to raise the Fermi level from 1.7eV to only around 0.5eV below BPhen’s lowest unoccupied molecular orbital, which further enhances the efficiency of electron injection from an Al cathode. When this n-doped layer is adopted in an organic light-emitting diode device, the green fluorescent 10-(2-benzothiazolyl)-1,1,7,7-tetramethyl-2,3,6,7-tetrahydro-1H,5H,11H-benzo[l]-pyrano[6,7,8-ij]quinolizin-11-one do-ped device can achieve a current efficiency of 16cd∕A and a power efficiency of 10.9lm∕W at 1000cd∕m2.

Passivated p-type silicon: Hole injection tunable anode material for organic light emission

We find that hole injection can be enhanced simply by selecting a lower-resistivity p-Si anode to match an electron injection enhancement for organic light emitting diodes with ultrathin-SiO2-layer-passivated p-Si anode (Si-OLED). For a Si-OLED with ordinary AlQ electron transport layer, the optimized resistivity of the p-Si anode is 40Ωcm; for that with n-doped Bphen electron transport layer, it decreases to 5Ωcm. Correspondingly, the maximum power efficiency increases from 0.3to1.9lm∕W, even higher than that of an indium tin oxide control device (1.4lm∕W). This passivated p-type silicon is a hole injection tunable anode material for OLED.