Enhancing power conversion efficiencies and operational stability of organic light-emitting diodes by increasing carrier injection efficiencies at anode/organic and organic/organic heterojunction interfaces (original) (raw)
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Journal of Applied Physics, 2008
We show that the performance of organic light-emitting diodes (OLEDs) is markedly improved by optimizing the thickness of a hole-injection layer (HIL) of molybdenum oxide (MoO3) inserted between indium tin oxide and N,N′-diphenyl-N,N′-bis(1-naphthyl)-1,1′-biphenyl-4,4′-diamine (α-NPD). From results of the electroluminescence (EL) characteristics of OLEDs with various thicknesses of a MoO3 HIL, we found that the OLED with a 0.75-nm-thick MoO3 HIL had the lowest driving voltage and the highest power conversion efficiency among the OLEDs. Moreover, the operational lifetime of the OLED was improved by about a factor of 6 by using the 0.75-nm-thick MoO3 HIL. These enhanced EL characteristics are attributable to the formation of an Ohmic contact at the interfaces composed of ITO/MoO3/α-NPD.
Current efficiency in organic light-emitting diodes with a hole-injection layer
2008
We have systematically investigated the effect of layer structures on the current efficiency of prototypical hole-injection layer ͑HIL͒/hole-transport layer ͑HTL͒/electron-transport layer ͑ETL͒ organic light-emitting diodes based on 4 , 4Ј ,4Љ-tris͓N-͑3-methylphenyl͒-N-phenylamino͔triphenylamine ͑MTDATA͒ as the HIL, 4 , 4Ј-bis͓N-͑1-naphthyl͒-N-phenylamino͔biphenyl ͑NPB͒ as the HTL, and tris͑8-quinolinolato͒aluminum ͑Alq͒ as the ETL. With bilayer devices, the current efficiency is limited by exciplex emissions in the case of MTDATA/Alq and quenching of Alq emissions by NPB + radical cations in NPB/Alq. The improved current efficiency in trilayer MTDATA/NPB/Alq devices can be attributed to a reduction in NPB + radical cations at the NPB/Alq interface and a strong electric field in the NPB layer.
Enhanced device efficiency in organic light-emitting diodes by dual oxide buffer layer
We have demonstrated an improvement in device performance of fluorescent organic light-emitting diodes (OLEDs) by inserting a dual anode buffer layer composed of tungsten oxide (WO 3) and molybdenum oxide (MoO 3). The advantage of adding dual anode buffer layers with different deposition sequences over individual and composite oxide buffer layers has been systematically analyzed based on their electronic and optical properties. The incorporation of single and composite buffer layers has been revealed to induce a very low injection barrier for holes in tri-layer fluorescent OLEDs which results in a charge imbalance in the emission layer. In contrast, a proper sequence of buffer layers (WO 3 /MoO 3) exhibiting higher contact angle (lower surface energy) and higher surface roughness, together with a step-wise increment of potential barrier leads to a better overall charge balance in the active emission layer. Therefore, an enhanced current efficiency and power efficiency of ∼5.8 cd/A and ∼5.2 lm/W respectively were recorded for the WO 3 /MoO 3 buffer unit, which was better than the insertion of individual and composite layers.
Organic/Organic Heterointerface Engineering to Boost Carrier Injection in OLEDs
Scientific Reports, 2017
We investigate dynamic formation of nanosheet charge accumulations by heterointerface engineering in double injection layer (DIL) based organic light emitting diodes (OLEDs). Our experimental results show that the device performance is considerably improved for the DIL device as the result of heterointerface injection layer (HIIL) formation, in comparison to reference devices, namely, the current density is doubled and even quadrupled and the turn-on voltage is favorably halved, to 3.7 V, which is promising for simple small-molecule OLEDs. The simulation reveals the (i) formation of dynamic p-type doping (DPD) region which treats the quasi Fermi level at the organic/electrode interface, and (ii) formation of dynamic dipole layer (DDL) and the associated electric field at the organic/organic interface which accelerates the ejection of the carriers and their transference to the successive layer. HIIL formation proposes alternate scenarios for device design. For instance, no prerequisite for plasma treatment of transparent anode electrode, our freedom in varying the thicknesses of the organic layers between 10 nm and 60 nm for the first layer and between 6 nm and 24 nm for the second layer. The implications of the present work give insight into the dynamic phenomena in OLEDs and facilitates the development of their inexpensive fabrication for lighting applications.
Improving the performance of organic light-emitting diodes
SPIE Newsroom, 2008
Organic light-emitting diodes (OLEDs) are promising candidates for large-area full-color flat panel displays due to their ease of fabrication and convenience for many applications. 1 OLEDs work through the passage of an electric current across a fluorescent or phosphorescent organic layer resulting in an excitation/emission profile of the material used. With OLEDs, the injection efficiency of electrons is a critical parameter and depends to a great extent on the work function (the minimum energy needed to move an electron out of a substance) of the electrode. A thin hole-injection layer (HIL) or an anode buffer layer (ABL) between the indium tin oxide (ITO) anode and the organic emitting layer are usually adopted to enhance the performance of the hole-injection process. 2-6 Thus, current electroluminescent devices typically have the following layered configuration: ITO anode/HIL or ABL/organic emitting layer/tris(8-hydroxyquinoline) aluminum (Alq 3 )/lithium fluoride (LiF)/aluminum cathode. Our recent work suggests that either an HIL composed of metal phthalocyanine (MPc) or an ABL of Li-doped zinc oxide (LZO) should improve the holeinjection efficiency. The organic, inorganic, and Al layers of our test device were successively deposited using vacuum vapor evaporation at room temperature. The LZO powders with a doped concentration of 5% Li were prepared by sintering a mixture of ZnO and Li 2 CO 3 powders in air. Various MPc layers were tested for their effect on injection efficiency (see ). The turn-on voltage of the devices decreases from 5.3V to 4.3V when CoPc or CuPc layers are inserted: see (b). Compared to the non-MPc device, higher emission efficiency was observed in all MPc devices. The CuPc device achieved the highest efficiency as shown in . For the same emission intensity, the higher efficiency suggests that a much lower current density is required. Figure 1.
Highly power efficient organic light-emitting diodes with a p-doping layer
Applied Physics Letters, 2006
In this letter, the authors demonstrate p-in organic light-emitting diodes ͑OLEDs͒ incorporating a p-doped transport layer which comprises tungsten oxide ͑WO 3 ͒ and 4 , 4Ј ,4Љ-tris͑N-͑2-naphthyl͒-N-phenyl-amino͒triphenylamine ͑2-TNATA͒ to replace the volatile tetrafluro-tetracyanoquinodimethane. The authors propose the 2-TNATA: WO 3 composition functions as a p-doping layer which significantly improves hole injection and conductivity of the device that leads to the fabrication of tris͑8-quinolinolato͒aluminum based p-in OLEDs with long lifetime, low driving voltage ͑3.1 V͒, and high power efficiency ͑3.5 lm/ W͒ at 100 cd/ m 2 .
Journal of The Electrochemical Society, 2007
We report on the advantages of an anode buffer layer of Li-doped ZnO ͑LZO͒ on the electro-optical properties of organic light-emitting diodes ͑OLEDs͒. LZO layers with different thicknesses were prepared by thermally evaporating the LZO powders and then treating them with ultraviolet ͑UV͒ ozone exposure. The turn-on voltage of OLEDs decreased from 4 V ͑4.2 cd/m 2 ͒ to 3 V ͑5.1 cd/m 2 ͒, the maximum luminance value increased from 16780 to 28150 cd/m 2 and the power efficiency increased from 2.74 to 5.63 lm/W when a 1 nm thick LZO layer was inserted between indium-tin oxide ͑ITO͒ anodes and -naphthylphenylbiphenyl diamine hole-transporting layers. X-ray and ultraviolet photoelectron spectroscopy were performed to show that the formation of the LZO layer and the work function increased by 0.64 eV when the LZO/ITO layer was treated by UV-ozone for 20 min. The surface of the LZO/ITO film became smoother after the UV-ozone treatment. Thus, the hole-injection energy barrier was lowered by inserting an LZO buffer layer, resulting in the decrease of the turn-on voltage and the increase of the power efficiency in OLEDs.
Bulletin of Materials Science
In this study, high-performance of organic light-emitting diodes (OLEDs) with a buffer layer of MoO 3 is demonstrated. With an optimal thickness of MoO 3 (12 nm), the luminance efficiency is found to be increased compared to the single layer anode OLED. To study the influence of MoO 3 buffer layer on OLED performance, we deposited MoO 3 films with different thicknesses on the fluorine-doped tin oxide (FTO) surface and studied J-V and L-V characteristics of the OLED devices. Also, further analysis was carried out by measuring sheet resistance, optical transmittance and surface morphology with the FESEM images. Here, we found that MoO 3 (12 nm) buffer layer is a good choice to increase the efficiency of FTO-based OLED devices within the tunnelling region. Here, the maximum value of current efficiency is 6.15 cd A −1 .
Journal of Alloys and Compounds, 2009
The advantages of using an anode buffer layer of ZnO on the electro-optical properties of organic light emitting devices (OLEDs) are reported. ZnO powders were thermal-evaporated and then treated with ultra-violet (UV) ozone exposure to make the ZnO layers. The turn-on voltage of OLEDs decreased from 4 V (4.2 cd/m 2 ) to 3 V (3.4 cd/m 2 ) and the power efficiency increased from 2.7 lm/W to 4.7 lm/W when a 1nm-thick ZnO layer was inserted between indium tin oxide (ITO) anodes and ␣-naphthylphenylbiphenyl diamine (NPB) hole-transporting layers. X-ray and ultra-violet photoelectron spectroscopy (XPS and UPS) results revealed the formation of the ZnO layer and showed that the work function increased by 0.59 eV when the ZnO/ITO layer was treated by UV-ozone for 20 min. The surface of the ZnO/ITO film became smoother than that of bare ITO film after the UV-ozone treatment. Thus, the hole-injection energy barrier was lowered by inserting an ZnO buffer layer, resulting in a decrease of the turn-on voltage and an increase of the power efficiency of OLEDs.