Enhancing the electron injection in polymer light-emitting diodes using a sodium stearate/aluminum bilayer cathode

In this contribution the molecule sodium stearate (NaSt) is used for the first time as electron injection layer in combination with the fluorescent polymer phenylene substituted poly (para-phenylenevinylene) (Ph-PPV) in organic light-emitting diodes (OLEDs). The fabricated devices show current efficiencies up to 8.4 cd/A, indicating that the employed NaSt/aluminum (Al) bilayer cathode has adequate electron injection capabilities in conjunction with Ph-PPV and, therefore, NaSt has the potential to become a non-toxic alternative to the widely-used alkali halide lithium fluoride (LiF).Numerical simulations of the device structure are performed which are in good agreement with the experiments. Additionally, it is shown that the NaSt/Al cathode of the presented device cannot be simply modeled by using a low work function contact, as it is commonly done for the LiF/Al cathode in simulations of multilayer devices. Instead, an alternative approach is introduced in which an insulator in combination with the Fowler–Nordheim tunneling and the direct tunneling model is chosen to describe the charge carrier injection through the NaSt layer.     

Lithium phenolate complexes for an electron injection layer in organic light-emitting diodes

We synthesized π-conjugated lithium phenolate complexes, lithium 2-(2-pyridyl)phenolate (LiPP), lithium 2-(2′, 2′′-bipyridine-6′-yl)phenolate (LiBPP), and lithium 2-(isoquinoline-1′-yl)phenolate (LiIQP). These complexes showed lower sublimation temperatures of 305–332 °C compared to 717 °C of LiF. The organic light-emitting devices (OLEDs) using these complexes as an electron injection layer exhibited high efficiencies which are comparable to that of the device using LiF. Especially, a 40-nm thick film of LiBPP or LiPP was effective as an electron injection material, providing low driving voltages, while such a thick film of LiF serves as a complete insulator, resulting in high driving voltages.     

Dual enhancing properties of LiF with varying positions inside organic light-emitting devices

A multilayer organic light-emitting device (OLED) has been fabricated with a thin (0.3 nm) lithium fluoride (LiF) layer inserted inside an electron transport layer (ETL), aluminum tris(8-hydroxyquinoline) (Alq₃). The LiF electron injection layer (EIL) has not been used at an Al/Alq₃ interface in the device on purpose to observe properties of LiF. The electron injection-limited OLED with the LiF layer inside 50 nm Alq₃ at a one forth, a half or a three forth position assures two different enhancing properties of LiF. When the LiF layer is positioned closer to the Al cathode, the injection-limited OLED shows enhanced injection by Al interdiffusion. The Al interdiffusion at least up to 12.5 nm inside Alq₃ rules out the possible insulating buffer model in a small molecule bottom-emission (BE) OLED with a thin, less than one nanometer, electron injection layer (EIL). If the position is further away from the Al cathode, the Al diffusion reaches the LiF layer no longer and the device shows the electroluminescence (EL) enhancement without an enhanced injection. The suggested mechanism of LiF EL efficiency enhancer is that the thin LiF layer induces carrier trap sites and the trapped charges alters the distribution of the field inside the OLED and, consequently, gives a better recombination of the device. By substituting the Alq₃ ETL region with copper phthalocyanine (CuPc), all of the electron injection from the cathode of Al/CuPc interface, the induced recombination at the Alq₃ emitting layer (EML) by the LiF EL efficiency enhancer, and the operating voltage reduction from high conductive CuPc can be achieved. The enhanced property reaches 100 mA/cm² of current density and 1000 cd/m² of luminance at 5 V with its turn-on slightly larger than 2 V. The enhanced device is as good as our previously reported non-injection limited LiF EIL device [Yeonjin Yi, Seong Jun Kang, Kwanghee Cho, Jong Mo Koo, Kyul Han, Kyongjin Park, Myungkeun Noh, Chung Nam Whang, Kwangho Jeong, Appl. Phys. Lett. 86 (2005) 213502].     

Improved out-coupling efficiency of organic light emitting diodes by manipulation of optical cavity length

The relationship between thickness of electron transport layer (ETL) and device performance of organic light-emitting diodes (OLEDs) was investigated. Especially, we prepared various OLEDs by varying the thickness of ETL to investigate the difference of device performance. Very interestingly, the device efficiency of green phosphorescent organic light emitting diodes (PHOLEDs) was significantly improved when the thickness of ETL was optimized even though we did not change any materials for such devices except that we applied highly conductive Li doped ETL. This means that the only one factor which is associated with an improvement of device efficiency could be originated from the constructive optical interference. As a result, the simple modification of PHOLEDs only by changing the optical thickness condition causes a dramatic improvement of current efficiency (up to 82.4 cd/A) as well as external quantum efficiency (EQE, up to 23.8%), respectively. Those values correspond to the much more improved ones (by ∼34.4%) compared to those obtained from the normal devices with thin ETL as a reference.     

Improving the performance of the organic thin-film transistors with thin insulating lithium fluoride buffer layer

The pentacene-based organic thin-film transistors (OTFTs) with a thin insulating lithium fluoride (LiF) buffer layer between the pentacene and source/drain electrodes were fabricated. Compared with conventional OTFTs, the introduction of the buffer layer (1 nm) leads to field-effect mobility increases from 0.16 to 0.5 cm²/Vs, and threshold voltage downshifts from −19 to −8 V for the linear region. The on/off current ratio is improved to a level of 10⁵ for the off-state current decreasing. These improvements are attributed to (i) tunneling injection through the LiF layer and (ii) interface dipole energy barrier decreasing and contact resistance reduction between pentacene and Au. The results demonstrate that it is an effective method to improve the device characteristics by using a buffer layer.     

Role of LiF in polymer light-emitting diodes with LiF-modified cathodes

We report on high-efficiency polymer light-emitting diodes (PLEDs) based on poly [2-methoxy-5-(3′,7′-dimethyloctyloxyl)]-1,4-phenylene vinylene (OC1C10) with LiF-modified cathodes. Devices with different cathodes are made and characterized by the electroabsorption technique to measure their built-in voltage. Devices with a LiF/Al bilayer cathode or a LiF:Al composite cathode, all show significantly improved performance as compared to those with bare Al cathodes. The improvement is correlated with enhanced electron injection due to a decrease of the electron injection barrier, which is also indicated by the electroabsorption measurements. The same effect is also observed with LiF(0.6 nm)/Mg cathodes. However, inserting the same LiF thin film between Ag and OC1C10 does not improve the device performance. Cathodes composed of ultra-thin films of LiF(0.6 nm)/Al(1 nm) or LiF:Al(2 nm) covered by Ag (100 nm) show the same performance as LiF(0.6 nm)/Al bilayer cathode or a LiF:Al composite cathode, indicating that the enhancement is specific to LiF and Al. Our experiments can be explained by assuming that Li-ions can dissociate from LiF and diffuse into the OC1C10 layer, leading to an n-type zone close to the polymer/cathode interface. This n-doped layer at the interface facilitates electron injection at the cathode/polymer interface and eventually leads to the formation of an Ohmic contact.     

LiF作电子注入缓冲层对有机电致发光器件电容的影响

已经有多种方法分析了LiF作为电子注入缓冲层对有机电致发光器件的影响,用LiF/Al双层阴极和发光层Alq3制成的有机电致发光器件(OLED),可以降低器件的开启电压,提高器件的发光效率、发光亮度。文章主要对OLEDs(A):Al/Alq3/ITO和(B):Al/LiF(1nm)/Alq3/ITO的C-V特性进行了研究,当在阴极和发光层Alq3之间加上1nm厚的LiF层作为电子注入缓冲层以后,器件的电容由不加LiF时的72500pF减小到12500pF,由于电容的减小,有效地降低了器件的功耗,进而提高了器件的寿命,节约了能源,进一步改善了器件的性能。     

Influence of cathode roughness on the performance of F8BT based organic–inorganic light emitting diodes

Hybrid light emitting diodes (HyLED) with a structure of FTO/ZnO/F8BT/MoO₃/Au/Ag is fabricated and the influence of surface roughness of cathode (FTO/ZnO) is investigated. The roughness of FTO could be decreased from 9.2 nm to 2.2 nm using a mild polishing process. The ZnO film, deposited by spray pyrolysis, functions as an electron injection layer. The roughness of the FTO/ZnO surface is found also highly dependent on the ZnO thickness. For thin ZnO films (20 nm), polishing results in better efficacy and power efficiency of LED devices, with nearly a two times improvement. For thick ZnO films (210 nm), the overall FTO/ZnO roughness is almost independent of the FTO roughness, hence both polished and unpolished substrates exhibit identical performance. Increasing ZnO thickness generally improves the electron injection condition, leading to lower turn on voltage and higher current and power efficiencies. However, for too large ZnO thickness (210 nm) the ohmic loss across the film dominates and deteriorates the performance. While the polished substrates show less device sensitivity to ZnO thickness and better performance at thin ZnO layer, best performance is obtained for unpolished substrates with 110 nm ZnO thickness. Larger interface area of ZnO/F8BT and enhanced electric filed at sharp peaks/valleys could be the reason for better performance of devices with unpolished substrates.     

Enhancement of electroluminescence efficiency and stability in phosphorescent organic light-emitting diodes with double exciton-blocking layers

A high efficiency phosphorescent organic light-emitting diode (OLED) has been fabricated by introducing a double exciton-blocking layer (d-EBL) between the hole-transporting layer and the light-emitting layer in the device. The device exhibits a yellow emission with a maximum current efficiency of 58.5 cd/A at 117 cd/m², corresponding to the power efficiency of 50.9 lm/W, which is two times improved compared with that of devices having only one traditional single exciton-blocking layer (s-EBL). The efficiency improvement has been investigated through the electroluminescence (EL) spectral analyses in the phosphorescent guest-doped and the non-doped OLEDs. The results demonstrate that the electrons are blocked and the excitons are confined more effectively in the d-EBL-based devices than that in the s-EBL-based devices. In addition, over two times improvement in the lifetime is also achieved in the devices with the d-EBL compared with the devices having a traditional s-EBL.     

利用LiF空穴阻挡层提高有机发光二极管效率

通过引入LiF,明显提高了基于八羟基喹啉铝双层有机发光二极管的发光效率.2 nm 厚的 LiF 空穴阻挡层可将器件的发光效率从 2.6 cd/A 提高到 6.3 cd/A,研究结果表明,LiF 空穴阻挡层可以有效调节空穴的注入与传输,平衡器件中的空穴与电子,提高有机发光二极管的发光效率.     

p-Doped p-phenylenediamine-substituted fluorenes for organic electroluminescent devices

Two novel p-phenylenediamine-substituted fluorenes have been designed and synthesized. Their applications as hole injection materials in organic electroluminescent devices were investigated. These materials show a high glass transition temperature and a good hole-transporting ability. It has been demonstrated that the 2,3,5,6-tetrafluoro-7,7,8,8-tetracyanoquinodimethane (F4-TCNQ) doped p-phenylene-diamine-substituted fluorenes, in which F4-TCNQ acts as p-type dopant, are highly conducting with a good hole-transporting property. The organic light emitting devices (OLEDs) utilizing these F4-TCNQ-doped materials as a hole injection layer were fabricated and investigated. The pure Alq₃-based OLED device shows a current efficiency of 5.2 cd/A at the current density of 20 mA/cm² and the operation lifetime is 1500 h with driving voltage increasing only about 0.7 mV/h. The device performance and stability of this hole injection material meet the benchmarks for the commercial requirements for OLED materials.     

Efficient inverted organic light-emitting devices using a charge-generation unit as electron-injection layers

Efficient electron injection from cathode to electron transport layer is generally required to realize high-performance inverted organic light-emitting devices (IOLEDs). In this work, highly efficient IOLEDs are developed by employing a non-doped charge-generation unit of 1,4,5,8,9,11-hexaazatriphenylene-hexacarbonitrile (HAT-CN)/Al/LiF as electron-injection layers (EILs). The ultraviolet photoelectron spectroscopy (UPS) shows that the insertion of HAT-CN layer reduces the work function of EILs by 0.21 eV. Combined with the current-voltage characteristics of electron-only devices, the role of the HAT-CN layer in promoting electron injection is confirmed. What's more, the double EILs device with Rubrene as a probe verifies the strong hole blocking ability of HAT-CN/Al/LiF. Based on the EILs, the inverted blue phosphorescent OLEDs with a maximum current efficiency of 29.4 cd/A are realized. The excellent performance proves the superiority of HAT-CN/Al/LiF as EILs.     

具有新型空穴缓冲层材料TiO2的有机薄膜发光器件

以溶胶凝胶法制备的TiO2作为空穴缓冲层．在结构为ITO／TiO2／NPB／Alq／LiF／A1的器件中,改善了器件的发光效率。研究了TiO2厚度对器件发光特性的影响。在电流密度为100mA／cm^3时,有缓冲层的器件发光效率为5cd／A,而没有缓冲层的器件发光效率为3．45cd／A,有TiO2缓冲层的器件发光效率有了明显提高。     

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.     

Highly efficient green phosphorescent organic light emitting diodes with simple structure

A series of simple structures is investigated for realization of the highly efficient green phosphorescent organic light emitting diodes with relatively low voltage operation. All the devices were fabricated with mixed host system by using 1,1-bis[(di-4-tolylamino)phenyl]cyclohexane (TAPC) and 1,3,5-tri(p-pyrid-3-yl-phenyl)benzene (TpPyPB) which were known to be hole and electron type host materials due to their great hole and electron mobilities [μ_h(TAPC): 1 × 10^?2 cm²/V s and μ_e(TpPyPB): 7.9 × 10^?3 cm²/V s] [1]. The optimized device with thin TAPC (5–10 nm) as an anode buffer layer showed relatively high current and power efficiency with low roll-off characteristic up to 10,000 cd/m². The performances of the devices; with buffer layer were compared to those of simple devices with single layer and three layers. Very interestingly, the double layer device with TAPC buffer layer showed better current and power efficiency behavior compared to that of three layer device with both hole and electron buffer layers (TAPC, TpPyPB, respectively).     

Enhancement of electrical properties in pentacene-based thin-film transistors using a lithium fluoride modification layer

We investigated the electrical performances of pentacene-based thin-film transistors with a thin LiF film as a modification layer between the source/drain electrodes and the active layer. Au, Pt, and Pd were employed as the source/drain (S/D) electrodes of the organic thin-film transistors (OTFTs). The thickness of the LiF layer was varied to optimize the electrical performances of the OTFTs. We found that the electrical performances of the devices with LiF layer are better than those without inserting the LiF layer, regardless of which one metal used for the S/D electrodes. The electrical performances of the devices in terms of drain current, on resistance, threshold voltage, on/off current ratio and mobility have been optimized when the thickness of the LiF buffer layer was at 1.0 nm. These improvements are attributed to work function of S/D contact materials were modified by the dipole of the inserted LiF layer and an ultrathin LiF layer provides the tunneling condition to enhance the carrier injection efficiently. Additionally, LiF layer with optimal thickness (1.0 nm) can modify surface roughness of pentacene film and partly enhance the carrier injection.     

用ZnS薄膜作为空穴缓冲层的高效率有机发光二极管

用磁控溅射方法制备的ZnS薄膜作为有机发光器件(OLEDs)的空穴缓冲层,使典型结构的 OLEDs(ITO/TPD/Alq/LiF/Al) 的发光性能得到改善.ZnS 缓冲层厚度对器件性能影响的实验结果表明,当ZnS缓冲层厚度为 5 nm 时,器件的亮度增加了2倍多;当ZnS缓冲层厚度为5、10 nm时,器件的发光电流效率增加40%.研究结果表明 ZnS 薄膜是一种好的缓冲层材料,它能够提高器件的发光效率,改善器件的稳定性.     

Efficient electron transport in 4,4′-bis[N-(1-napthyl)-N-phenyl-amino] biphenyl and the applications in white organic light emitting devices

The electron transport capability of 4,4′-bis[N-(1-napthyl)-N-phenyl-amino] biphenyl (α-NPD) was investigated by fundamental physical measurements named as current–voltage (I–V) electrical property evaluation and displacement current measurement (DCM). In electron-dominated devices, the I–V characteristics of α-NPD were similar as that of (8-hydroxyquinolino) aluminum (Alq₃) owing to their same order of electron mobilities. The interface of Al/LiF and α-NPD was proven to be an Ohmic contact through the evaluation of I–V characteristics at low bias regime (<3 V). And an electron injection barrier, 0.21 eV, at Al/LiF/α-NPD was obtained by extrapolating the temperature dependent I–V curves. The electron transport behavior in α-NPD film was further confirmed by DCM evaluations. Furthermore, an efficient white organic light emission device was successfully fabricated by using α-NPD as hole transport layer and electron transport layer, respectively.     

Thermally cross-linkable host materials for enabling solution-processed multilayer stacks in organic light-emitting devices

A thermally cross-linkable host material, i.e., two vinylbenzyl ether groups containing a carbazole derivative (DV-CBP), was developed for solution-processed multilayer organic light-emitting devices (OLEDs). DV-CBP was thermally cross-linked at styrene end-groups through curing at approximately 180 °C in the absence of a polymerization initiator. This cross-linking reaction rendered the emissive layer insoluble and enabled the subsequent solution deposition of an upper electron-transporting layer. Furthermore, photoluminescence quantum efficiencies of the emissive layer were maintained at greater than 75% throughout the cross-linking reaction. A solution-processed small-molecule electron-transporting layer on top of the cross-linked emissive layer led to lower driving voltages and higher efficiencies in the OLEDs compared to those of a device with a vacuum-deposited Ca electrode on the emissive layer.     

Doping-free orange and white phosphorescent organic light-emitting diodes with ultra-simply structure and excellent color stability

We demonstrate simplified doping-free orange phosphorescent organic light-emitting diodes (OLEDs) based on ultrathin emission layer. The optimized orange device has the maximum current efficiency of 52.1 cd/A and power efficiency of 36.3 lm/W, respectively. Efficient simplified doping-free white OLEDs employing blue and orange ultrathin emission layers have excellent color stability, which is attributed to the avoidance of the movement of charges recombination zone and no differential color aging. One white device exhibits high efficiency of 33.6 cd/A (30.1 lm/W). Moreover, the emission mechanism of doping-free orange and white OLEDs is also discussed.