Organic Electroluminescent Materials and Devices (1996)

Numerous organic materials are capable of producing electroluminescence (EL). In recent years, researchers have directed considerable effort at developing thin-film organic EL devices that are more power-efficient1-3 and can be driven by a low voltage – typically less than 10 V. We have seen substantial improvements in efficiency, color, and stability. These improvements were made possible, in large part, by the development of a multilayer device structure prepared by vapor deposition and by the discovery of several important classes of organic EL materials with excellent transport and luminescent properties. These advances stimulated commercial interest in developing a new display technology that offers significant advantages.4 Recombination electroluminescence in a molecular solid has a maximum quantum yield of approximately 25%, i.e., 0.25 photons per electron.5 With other losses, such as that due to optical coupling, an overall yield of 5% should be achievable from organic EL devices throughout the visible region. Assuming this 5% quantum yield, a 5-V drive, and a set of blue, green, and red emission bands with an arbitrary Gaussian shape, the luminous characteristics can be derived (Table 1). These values indicate that broadband organic EL emitters with a bandwidth of 75–100 nm can produce a color gamut similar to that of a CRT. Investigators have also discovered that extremely high luminous efficiencies can be realized from these emitters if they can be driven with a sufficiently low voltage. Efficient organic EL devices generally have a basic two-layer structure consisting of a hole-transport layer (HTL) and an electron-transport layer (ETL) sandwiched between injecting electrodes [Fig. 1(a)]. The mismatched energy levels in the two organic layers creates potential barriers at the interface where both electrons and holes can be localized and where the probability of their recombination can be maximized. In effect, the mismatched levels reduce the pathway for non-radiative carrier annihilations at the injecting contacts. Also, the channel for radiative recombinations can be systematically optimized by introducing suitable fluorescent centers near the interfacial region. Efficient organic EL devices generally have a basic two-layer structure consisting of a hole-transport layer (HTL) and an electron-transport layer (ETL) sandwiched between injecting electrodes. The energy-level diagram is shown in (a). (b) The addition of a thin emitting layer (EML) helps in tuning the EL colors. So, the two-layer design can be extended to include a thin emitting layer (EML) between the HTL and ETL [Fig. 1(b)]. This three-layer structure is particularly useful for tuning the EL colors. The organic layers in the EL device can be thought of as low-mobility solids, and the conduction current through these layers can be space-charge or injection limited. The space-charge limitation can be avoided by using ultra-thin organic films on the order of a thousand angstroms (Å), while low-barrier contacts are usually designed by matching the organic layers with electrode materials having the proper work functions. Common electrodes are ITO for the anode and a low-work-function alloy for the cathode. The ITO contact is relatively inert and can be further modified to provide a more stable hole injection into the HTL.6 Mg:Ag (Ref. 2) and Al:Li (Ref. 7) are examples of useful cathodes prepared by co-evaporation. The low-work-function components are responsible for the electron-injection efficiency, but they are highly susceptible to corrosion. The alloyed compositions have produced some degree of stabilization.8 A number of organic EL materials have been found to produce high-performance devices (Fig. 2). Most HTL materials are based on aromatic amines known for their high hole mobility9 relative to other organic materials. Amines generally have a low ionization potential and small electron affinity associated with a large energy gap. Thus, the amine-based HTL also functions effectively as an electron-blocking layer in the multilayer EL device. Within the amine class, the benzidines10 – TPD and NPB, for example – are the most important because of their superior electrochemical stability. A number of organic EL materials have been found to produce high-performance devices. The representative materials in the figure have been classified according to their functions in the device structure. Representative spectra of a set of organic EL devices indicate a green that is close to a CRT green, but red and blue spectra that are too broad. The luminous efficiencies are relatively high. Alq is one of the most frequently used ETL materials. It is a complex of 8-hydroxyquino-line with a tris ligand configuration about the central Al ion. The high molecular symmetry provides the Alq film with an unusually good thermal and morphological stability.11 The electron transport in the Alq film has been found to be trap-limited.12 The electron mobility is low – on the order of 10° cm2/V-s – and is highly field-dependent. Nevertheless, the EL response time of the Alq-based device at high field is quite fast (less than 1 μs) because of the thin layer. Alq ETL layers are usually kept below 1000 Å in order to minimize the drive voltage. The most efficient EL devices use an EML layer consisting of a host material doped with a small number of fluorescent molecules.13 This dopant-host scheme offers a simple means of producing a fluorescent medium with a quantum yield near one. In addition, a common host material can be used for a range of colors. For instance, efficient green and orange-red ELs have been obtained from an Alq host with dopants such as Comarin 6, QA, and DCJT. BAlq, which belongs to the Alq class, is another host material. A blue EL emission can be obtained from BAlq with perylene as the dopant.14 Other host materials include distyryiarylene derivatives, such as DPVBi,15 which are excellent blue emitters by themselves. The EL spectra of a set of organic EL devices emitting in blue, green, and red are shown in Fig. 3, and their luminance characteristics are summarized in Table 2. These EL devices have demonstrated relatively high luminous efficiencies and are based on doped Alq as the EML layer in a multilayer structure as described in Fig. 1(b), with NPB as the HTL and Alq as the ETL. The chromaticity of the green EL is closest to the CRT equivalent; the blue and red colors are less pure due to the broad width of the dopant emission bands that extend into the green region. Further improvement in narrowing the bandwidths of the blue and red emissions will be needed to give more saturated hues. The EL colors can also be adjusted by changing the dopant concentration in the host. Besides Alq-based materials, several classes of organic materials have been reported as having excellent efficiencies. In particular, a luminance efficiency of 15 lm/W has been claimed for a green EL using a QA-doped BeQ2.16 The same dopant in Alq has shown an efficiency17 of about 8 lm/W. These remarkably high efficiencies may be attributed to the high fluorescence yield of the dopant QA in the BeQ or Alq host and to the use of a more highly injecting cathode that lowers the drive voltage. For blue EL, the DPVBi class of materials has been found to be one of the most efficient. A luminous efficiency of 1.5 lm/W was reported for a doped DPVBi device.18 Remarkably stable EL devices have been demonstrated, particularly with emitters based on doped Alq (Fig. 4). The figure compares the stability of three green Alq devices driven by a constant current of 20 mA/cm2, using one undoped and two doped emitters. For the undoped Alq, the half-life, T1/2, is about 4000 hours, operating from an initial luminance of 510 cd/m2. For the quinacridone QA-doped Alq, the initial luminance (1600 cd/m2) is about triple that of the undoped device because of the enhanced luminance efficiency. The stability is, however, rather poor, providing a T1/2 of only about 300–400 hours. With a modified quinacridone MQA-doped Alq, an EL device with superior stability and high luminance has been achieved: the T1/2 is more than 6000 hours with an initial luminance of 1400 cd/m2. These examples show that EL stability can be profoundly affected by a particular choice of dopant, among other factors. The T1/2 value is dependent on the luminance level produced by the EL device and a longer T1/2 is obtainable for operation at a lower luminance. Thus, a half-life in excess of 50,000 hours can be reasonably projected for the MQA-doped Alq device operating at an initial luminance of about 100 cd/m2. Several other EL materials have also shown excellent operational stability. Using rubrene as the emitting dopant in the HTL,19 yellow EL with a T1/2 of about 3500 hours has been obtained with a initial luminance of 500 cd/m2. Blue EL20 with a T1/2 of about 1500 hours and an initial luminance of 300 cd/m2 has been reported for perylene-doped BAlq. Considerable progress has been made in organic thin-film EL devices based on a multilayer structure. Color EL devices are readily obtainable with luminance efficiencies much greater than 1 lm/W throughout the visible spectrum, and long operational life has been demonstrated. Depending on the luminance, a half-life of a few thousand hours to a few tens of thousands of hours can be obtained from organic EL emitters. With this level of performance, organic EL has emerged as a new emissive display technology with the potential for a wide range of applications. Remarkably stable EL devices have been demonstrated, particularly with emitters based on doped Alq. These three green Alq devices were driven by a constant current of 20 mA/cm2.