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>> Transparent Electrodes for Top-Emitting Devices of Organic Light Emitting Diodes (OLEDs) 4th,June,2018
Organic Light-Emitting Diodes (OLEDs) have become popular because of their advantages such as fast response time, low operating voltage, high contrast, and large size and flexible panels.[1-4] Especially in recent years, OLEDs have been widely used in display panels for mobile phones (small screens) and televisions (big screens). In 2016, the number of mobile phone displays on the Chinese market has reached 99 million using OLEDs, and 77-inch large-screen OLEDs have been used. TV has also been on the market, indicating that the era of OLED display is truly coming.
The initial OLEDs were bottom-emission devices. The structure of the devices from top to bottom was: opaque metal cathode/organic functional layer/transparent anode. Light was emitted from the anode and was therefore called bottom emission, as shown in Figure 1(a). Show.
Figure 1
(Color online) Bottom (a) and top (b) emission OLED
Figure 2
Electrical model (a) and optical model (b) for DMD electrode
In the active display, the OLED light emitting device is controlled by a thin film transistor (TFT). Therefore, if the device emits light in the form of a bottom emission, light passes through the substrate and is blocked by TFTs and metal lines on the substrate, thereby affecting the actual Luminous area. If the light is emitted from the top of the device, the circuit design of the substrate will not affect the light emitting area of the device. Under the same brightness, the operating voltage of the OLED is lower, and a longer service life can be obtained. Therefore, the top emission device is the first choice for active display of small screens such as mobile phones. The structure of the top emission device is: Transparent or translucent cathode/organic functional layer/reflective anode [5], as shown in Figure 1 (b). In the top emitting device, the choice of transparent electrode is the most important, and a suitable transparent electrode will greatly improve the performance of the device.
Translucency and conductivity are two important parameters for evaluating transparent electrodes. Transmittance is determined by the transmittance of the film T, which can be measured by the spectrophotometer. Conductivity is often characterized by square resistance Rs, which can be measured by the four-point resistance test method. For transparent electrodes, good light transmission performance and excellent conductivity are often not satisfied at the same time, and it needs to be considered comprehensively. The parameter for characterizing photoelectric comprehensive performance is ΦH=T10/Rs[6], where Rs is the square resistance of the film, usually Need to reach the order of 10–2 to meet application requirements. In the following, the status quo of the development of top-emission transparent electrodes in OLEDs is mainly introduced in terms of light transmission and conductivity of various types of electrodes.
1 transparent conductive oxide (TCO) electrode
1.1 Indium Tin Oxide (ITO)
Conductive metal oxide, the most commonly used is ITO, its work function is about 4.5 ~ 4.8eV [7], is generally used as the anode conductive material, is a very stable, conductive and transparent material, Its resistivity is about 1×10–3 to 7×10–5Ωcm, and the transmittance in the visible light range is close to 90%. Therefore, the cathode of the first top-emission OLED device is ITO [8].
In general, ITO is deposited on a glass substrate by magnetron sputtering. During the film formation process, high-energy ions continuously hit the glass substrate, eventually forming a dense and uniform crystalline transparent conductive film[9]. However, when the organic functional layer film is pre-deposited on the substrate, bombardment of high-energy particles can seriously damage the organic layer, causing irreversible deterioration of the device performance. In order to solve this problem, a buffer layer is introduced between the organic layer/ITO. Buffer media layer can be divided into inorganic and organic layers.
(i) Inorganic barriers. In 1996, Gu et al. [8] first used 10 nm of Mg:Ag (30:1) plus 40 nm of ITO as the cathode for the top emission, and the transmittance was about 70% in the visible range, in 8-hydroxyquinoline aluminum. The emission peak at 530 nm of (Alq3) is 63%. The device structure is: ITO/TPD(20nm)/Alq3(40nm)/Mg:Ag(10nm)/ITO(40nm) (TPD is N,N'-Bis(3-methylphenyl)-N,N'-bis (phenyl)benzidine), because it is a penetrating device, it can emit light up and down, the light intensity on each side is about 500 cd/m2 (10V working voltage), and the external quantum efficiency is 0.1%, which is lower than the same The structure of the traditional bottom emission device is about 0.25%. Mg and Ag are co-evaporated on top of the organic layer. The thickness is less than the skin depth of the light and is used to enhance electron injection while protecting the underlying organic layer. In order to avoid the damage of the organic layer and the short circuit of the electrode caused by sputtered ITO, the sputtering power used is only 5W, and the deposition rate is only 0.05/s, so the sputtering of 40nm ITO is more than 2h, even at low power. Sputtering, the device also has a large leakage current, during the sputtering process, Mg will oxidize, so that the resistance of the Mg:Ag/ITO interface increases, the starting voltage is higher than the traditional bottom-emitting OLED devices by 3V.
In addition to the Mg:Ag-ITO transparent cathode, Burrows et al. [10] also studied a series of metal-ITO transparent cathodes, such as Ca-ITO, LiF/Al-ITO. When the thickness of the metal layer is 10 nm, the transmittance of the Mg:Ag electrode and the Mg:Ag-ITO electrode is only about 50%, while the transmittance of the LiF/Al-ITO electrode is less than 20%. If it is a Ca-ITO electrode, The maximum transmission rate exceeds 80%. In addition, the use of Ar plasma during sputtering can reduce damage to the organic layer [11]. When the sputtered atoms pass through Ar plasma, the high-energy atoms undergo multiple scattering to reduce the energy. Therefore, increasing the Ar pressure (p) or the distance (L) between the sputtering target and the substrate will reduce the Destruction of the organic layer. The inorganic metal layer can provide good ohmic contact at the interface while providing protection to the organic layer, which is beneficial to the injection of carriers from the electrode to the organic transport layer. However, the metal thin layer will greatly limit the light transmission of the electrode. When the Mg:Ag alloy has a thickness of 8 nm, the transmittance of the electrode may not even reach 50%. This is a disadvantage of increasing the metal barrier layer.
Some of the transition metal oxides (TMO) can also be vapor deposited to form a TMO-ITO electrode [12]. In 2008, Meyer et al. [12] studied the protective effect of WO3. Compared with the aforementioned metal barriers, oxides have the advantage of higher light transmittance, can effectively reduce the microcavity effect, and at the same time, TMO has improved electrodes and Carrier injection at the organic layer interface. In fact, the device reported by Meyer et al. is an ITO cathode/organic active layer/WO3-ITO anode inverted organic light emitting diode (IOLED). By varying the thickness of the WO3 layer (~60 nm), the device ITO/Bphen:Li(40 nm)/TPBi(5 nm)/TPBi:Ir(ppy)3(15 nm)/TCTA(40 nm)/WO3(60 nm) )/ITO(60nm) (Bphen is bathophenanthroline, TPBi is 1,3,5-tris(1-phenyl-1H-benzimidazol-2-yl)benzene, and Ir(ppy)3 is tris(2-phenylpyridine)iridium, TCTA The leakage current of 4,4′,4′′-tris(carbazol-9-yl)-triphenylamine is very low (10–4 mA/cm2), the transmittance of the transmissive OLED exceeds 75%, and the power efficiency reaches 30lm/W, current efficiency is 38?cd/A.
(ii) Organic barriers. In 1998, Forrest et al. [13] used organic materials instead of inorganic metal as the barrier layer to increase the transmittance in the visible light region. There were three kinds of materials used, copper phthalocyanine (CuPc), phthalocyanine Zinc (zinc phthalocyanine, ZnPc), lanthanum compounds (3,4,9,10-perlyenetetracarboxylic dianhydride, PTCDA), found that the effect of ZnPc and CuPc is similar, and the energy barrier between ZnPc and CuPc and ITO is relatively large, and therefore decreases. With the injection efficiency, the starting voltage of the device rises from 4.2V (Mg:Ag as the top emission device of the cathode) to 5.2V. Switching to PTCDA as a barrier layer results in poorer results, with a brighter starting voltage of 20 V and a quantum efficiency of only 1% of that of ITO/CuPc as a cathode device.
The reason why CuPc has relatively good injection efficiency is that Cu-O bonds are formed during the sputtering of ITO, thereby introducing many intermediate energy bands and surface states, making it easier to inject electrons; at the same time, CuPc also acts as a protective organic layer. The effect is that if the thickness of CuPc is reduced from 6?nm to 3nm, the device leakage current increases. In addition, the introduction of very thin Li (0.2nm) at the interface between the electrode and the organic layer helps to increase electron injection by comparing ITO/CuPc/NPB/Alq3/CuPc/Li/ITO (NPB is N, N'-Bis -(1-naphthalenyl)-N,N'-bis-phenyl-(1,1'-biphenyl)-4,4'-diamine) and bottom emitting ITO/CuPc/NPB/Alq3/Mg:Ag devices [ 14], found that their current-voltage curves are very similar, and the voltage of the former is higher at current densities above 10 mA/cm2. 2,9-dimethyl-4,7-diphenyl-1,10-phenanthroline (BCP) can also be used as an organic protective layer [15]. In the device of the above structure, using BCP instead of CuPc, the external quantum efficiency increases by 40%, and the electron injection and electron transport capability of BCP is better than that of Alq3 and CuPc, and the transmittance is in the visible region with BCP/Li/ITO as an electrode. Close to 90%, ηext=1.0%.
The disadvantage of the organic-ITO electrode is that the heat generated during the sputtering of ITO will crystallize the organics, which will cause changes in the geometry of the surface, making the contact between the ITO electrode and the organic layer worse and after the organic barrier is introduced. It will bring a new potential barrier to carriers, making the exciton recombination zone move to the cathode side, reducing the luminous efficiency.
In general, as a buffer layer, it is desirable to satisfy: (1) adequate light transmission; (2) certain electrical conductivity; (3) ohmic contact formation; (4) no organic layer destruction during film formation; stability. Whether inorganic metal or organic material acts as a barrier for blocking high-energy particles, it can achieve a good effect and reduce the leakage current of the device. However, they solve the old problems and introduce new problems: The metal layer has insufficient light transmittance, and organic substances. The introduction of this will bring new potential barriers to carrier transport.
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