This article provides a review about electroluminescence from organic materials and deals in detail with organic light-emitting diodes (OLEDs), light-emitting electro-chemical cells (LECs) and electrogenerated chemilumi-nescence (ECL) reflecting different electrooptical appli-cations of conjugated materials. It is written from an organic chemist's point of view and pays particular attention to the development of organic materials involved in corresponding devices. In recent years a substantial amount of both academic and industrial research has been directed to organic electroluminescence in an effort to improve the processability and tunability of organic materials and the longevity of OLEDs and LECs. On the eve of the commercialization of organic electrolumi-nescence this review provides an overview of lifetimes and efficiencies attained and reflects materials and device concepts developed over the last decade. In this context electrogenerated chemiluminescence is discussed with respect to its importance as a versatile tool to simulate the fundamental electrochemical processes in OLEDs.

The electroluminescence of organic materials

Organic transistors and circuits show great promise for the realization of futuristic roll-up displays, adaptive sensors for humanoid robots and ubiquitous radio-frequency identification tags. But today's organic circuits require operating voltages of 15 to 30 volts (10 to 20 batteries' worth), and they draw enough power to drain those batteries in a day. To overcome this major hurdle, Hagen Klauk et al. have developed a method of fabricating organic circuits that run on a single 1.5-volt battery for several years. The key to the method is the use of a layer of an insulating organic material just one molecule thick; although the layer is very thin, it leaks only a small amount of current, while it provides for a large capacitance. Two different types of organic semiconductors are used to fabricate transistors, logic gates and ring oscillators. A report of the development of organic electronic circuits, which require only a single 1.5V battery and last for several years. The main ingredient is the use of a single layer of an insulating organic material. Although the layer is very thin, it leaks only small amount of current, while providing for a large capacitance. The prospect of using low-temperature processable organic semiconductors to implement transistors, circuits, displays and sensors on arbitrary substrates, such as glass or plastics, offers enormous potential for a wide range of electronic products1. Of particular interest are portable devices that can be powered by small batteries or by near-field radio-frequency coupling. The main problem with existing approaches is the large power consumption of conventional organic circuits, which makes battery-powered applications problematic, if not impossible. Here we demonstrate an organic circuit with very low power consumption that uses a self-assembled monolayer gate dielectric and two different air-stable molecular semiconductors (pentacene and hexadecafluorocopperphthalocyanine, F16CuPc). The monolayer dielectric is grown on patterned metal gates at room temperature and is optimized to provide a large gate capacitance and low gate leakage currents. By combining low-voltage p-channel and n-channel organic thin-film transistors in a complementary circuit design, the static currents are reduced to below 100 pA per logic gate. We have fabricated complementary inverters, NAND gates, and ring oscillators that operate with supply voltages between 1.5 and 3 V and have a static power consumption of less than 1 nW per logic gate. These organic circuits are thus well suited for battery-powered systems such as portable display devices2 and large-surface sensor networks3 as well as for radio-frequency identification tags with extended operating range4.

Ultralow-power organic complementary circuits

Electroluminescent devices based on organic materials are of considerable interest owing to their attractive characteristics and potential applications to flat panel displays. After a brief overview of the device construction and operating principles, a review is presented on recent progress in organic electroluminescent materials and devices. Small molecular materials are described with emphasis on their material issues pertaining to charge transport, color, and luminance efficiencies. The chemical nature of electrode/organic interfaces and its impact on device performance are then discussed. Particular attention is paid to recent advances in interface engineering that is of paramount importance to modify the chemical and electronic structure of the interface. The topics in this report also include recent development on the enhancement of electron transport capability in organic materials by doping and the increase in luminance efficiency by utilizing electrophosphorescent materials. Of particular interest for the subject of this review are device reliability and its relationship with material characteristics and interface structures. Important issues relating to display fabrication and the status of display development are briefly addressed as well.

https://ir.nctu.edu.tw:443/bitstream/11536/28337/1/000179449700001.pdf

Recent progress of molecular organic electroluminescent materials and devices

Polymers for flexible displays: From material selection to device applications

The field of organic thin films and devices is progressing at an extremely rapid pace. Organic–metal and organic–organic interfaces play crucial roles in charge injection into, and transport through, these devices. Their electronic structure, chemical properties, and electrical behavior must be fully characterized and understood if the engineering and control of organic devices are to reach the levels obtained for inorganic semiconductor devices. This article provides an extensive, although admittedly nonexhaustive, review of experimental work done in our group on the electronic structure and electrical properties of interfaces between films of π-conjugated molecular films and metals. It introduces several mechanisms currently believed to affect the formation of metal–organic interface barriers. © 2003 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 41: 2529–2548, 2003

Electronic structure and electrical properties of interfaces between metals and π-conjugated molecular films

Oxygen plasma treatment of indium tin oxide (ITO) results in a change in work function and electron affinity by ∼0.5 eV. This change correlates with the measured increase in injected current in simple “hole-only” organic devices with O-plasma treated ITO electrodes. Neither addition nor removal of surface hydroxyl functionality accounts for the observed work function and electron affinity changes. X-ray and ultraviolet photoelectron spectroscopies show a new type of oxygen species is formed. Oxidation of surface Sn-OH to surface Sn-O• units is proposed to account for the observed changes in O-plasma treated ITO; this proposal can explain a wide variety of previously described ITO surface activation results.

/pdf/surface-oxidation-activates-indium-tin-oxide-for-hole-4ql2x5ope2.pdf

Surface oxidation activates indium tin oxide for hole injection

The chemistry, electronic structure, and electron injection characteristics at interfaces formed between tris(8-hydroxy quinoline) aluminum (Alq3) and magnesium (Mg) and aluminum (Al) are studied via x-ray photoemission spectroscopy, ultraviolet photoemission spectroscopy, and current–voltage (I–V) measurements. Both metal-on-Alq3 and Alq3-on-metal interfaces are investigated. All interfaces are fabricated and tested in ultrahigh vacuum in order to eliminate extrinsic effects related to interface contamination. The propensity for Mg and Al to form covalent metal–carbon bonds leads to broad and heavily reacted interfaces when the metal is deposited on the organic film. For this deposition sequence, we propose the formation of an organometallic structure where a single metal atom attaches to the pyridyl side of the quinolate ligand of the molecule and coordinates with an oxygen atom of another ligand or of a neighboring molecule. The other deposition sequence leads to significantly more abrupt interfaces wh...

Chemical and electrical properties of interfaces between magnesium and aluminum and tris-(8-hydroxy quinoline) aluminum

Interfaces formed by evaporating Au on copper hexadecafluorophthalocyanine (F16CuPc) and F16CuPc on Au are studied via ultraviolet photoemission spectroscopy and x-ray photoemission spectroscopy. The energy position of the molecular levels is found to depend on the deposition sequence. The Au-on-F16CuPc interface exhibits a lower hole injection barrier than the F16CuPc-on-Au interface. This behavior is attributed to the diffusion of Au into the organic matrix at the Au-on-top interface, in which they behave as acceptors. Band bending consistent with p doping, i.e., upward away from the metal interface, is observed in Au-doped F16CuPc films. Current–voltage measurements on Au/F16CuPc/Au sandwich structures show a higher electron injection barrier at the top Au electrode contact than at the bottom electrode contact, also consistent with p-type doping F16CuPc by Au.

Electronic structure, diffusion, and p-doping at the Au/F16CuPc interface

The role of interface states in controlling the electronic structure of Alq3/reactive metal contacts

Modern organic light emitting diode (OLED) devices are typically prepared in part by vapor phase deposition of a low work function metal cathode onto an organic electron carrier/ photoemitter. Tris(8-hydroxyquinolino)aluminum (Alq 3, 1) is a commonly used electron carrier/photoemitter, and it is reasonable to propose that chemical reduction of Alq 3 (with concomitant formation of metallic cations) can occur as a result of metal atom deposition, by analogy with well-known reduction processes of aromatic compounds which can be accomplished by activated or dissolved alkali and alkaline earth metals. Since the products of Alq3 reduction might vary in structure according to the deposited metal, the choice of cathodic metal may affect the performance of an OLED not only obviously, because of work function considerations, but also subtly, because of details of chemistry at the Alq3/cathode interface. It is not surprising, then, that analysis of possible chemical reactions between Alq 3 and several atomic alkali and alkaline earth metals (Li, K, Mg, and Ca) has been of considerable interest recently, both from theoretical and experimental perspectives. 1-4 Two items are noteworthy because of their absence from these reports. First, while they focus strongly on the expected shift tolower core level binding energies for the nitrogen atoms of the reduced quinolinate ligands, these studies either ignore any core level binding energy shifts for the oxygens of the ligands or suggest that such shifts are minor (and no mention is made of binding energy shifts for the central Al atom, either). Most surprising is that, while Mg (as Mg/Ag) is the most commonly used OLED contact with Alq 3, no experimental studies correlating the chemistry of Mg atom deposition on Alq 3 with spectroscopic observations have been reported. Since chemical reduction of polynuclear aromatic hydrocarbons with activated Mg can take a course quite different from that of Li, K, and Ca, the operationally significant Mg contact may in fact be structurally unrelated to these recently described models based on these other metals. We have now studied the deposition of this operationally key metal (and also of Al) onto thin films of Alq 3, and we find, by XPS and UPS analysis, a strong shift to higher binding core level energies occurs both at the ligand oxygens and at the central Al atom of the Alq3, which can be explained using simple organometallic models based on “solution”-derived observations. All procedures were carried out in an ultrahigh vacuum system consisting of three inter-connected chambers, which were used for surface preparation, film growth, and analysis, respectively. A 100 Å thick film of Alq3 was deposited on a gold substrate by thermal evaporation from an Al 2O3 crucible. Magnesium was then deposited incrementally on the Alq 3 film. XPS and UPS measurements were performed following each successive deposition event; the base pressure was maintained between 10 -8 and 10-10 Torr, for deposition and analysis, respectively. XPS data were collected using achromatic Al KR (1486.6 eV) or Zr Mú (151.4 eV) irradiation produced by a VSW double-anode source. Valence state data (UPS) were obtained using the He(I) ultraviolet emission (21.2 eV) line. 5 Changes in Mg(2p), C(1s), N(1s), O(1s), and Al(2p) core level binding energies (BE) were measured as a function of exposure of Alq 3 to Mg vapor. Analysis following initial Mg deposition (4 Å) showed the Mg(2p) peak to consist primarily of a high binding energy component (BE ) 51.6 eV), corresponding to Mg(II), and a smaller component at BE ) 49.3 eV. This latter peak grew with increasing exposure, and is assigned to metallic Mg. Peaks for C(1s), N(1s) and O(1s) all shifted by 0.6( 0.1 eV to higher BE after initial exposure to Mg, which is attributed to a shift of the whole molecular level structure of the organic film initially aligned with the Au substrate (see Table 1 in Supporting Information). While the peak shape and half-width for the C(1s) and Al(2p) peaks remained nearly constant regardless of Mg exposure, the N(1s) and O(1s) peak shapes changed considerably. Consistent with reports of K and Li deposition onto Alq3, reaction with Mg gave rise to a new, lower binding energy N(1s) peak component (experimental BE ) 399.1 eV; corrected BE) 398.5 eV; Figure 1). But in contrast, a new O(1s) peak component also appeared, which was shifted by 1.4 eV (corrected for molecular level realignment) to higher binding energy vs Alq 3 (experimental BE) 533.7 eV; corrected BE ) 533.1 eV; Figure 2). The Al(2p) signal also shifted to higher binding energy (experimental BE ) 75.5 eV; corrected BE) 74.9 eV). In agreement with previous measurements, 6,7 UPS measurements show that, while deposition o f 2 Å of Mg onto Alq3 attenuates spectral features, several peaks remain identifiable and are shifted (by realignment of molecular levels) to higher binding energies by∼0.6 eV. A new state, at approximately 1.5 eV above the (corrected) HOMO of Alq 3, was also detected † Department of Electrical Engineering. ‡ Department of Chemistry. (1) Choong, V. E.; Mason, M. G.; Tang, C. W.; Gao, Y. Appl. Phys. Lett. 1998, 72, 2689. (2) Zhang, R. Q.; Hou, X. Y.; Lee, S. T. Appl. Phys. Lett. 1999, 74, 1612. (3) Johanson, N.; Osada, T.; Stafstro ̈m, S.; Salaneck, W. R.; Parente, V.; dos Santos, D. A.; Crispin, X.; Bre ́das, J. L.J. Chem. Phys. 1999, 111, 21572163. (4) Curioni, A.; Andreoni, W.J. Am. Chem. Soc. 1999, 121, 8216-8220. (5) XPS and UPS spectra were collected with a Perkin-Elmer double-pass cylindrical mirror analyzer. Resolution for the UPS and XPS measurements were 0.15 and 0.7 eV, respectively. Energy scales were calibrated using the Fermi edge of a clean gold surface and the Mg(2p) core level of a freshly deposited Mg layer. Al KR irradiation was used to collect O(1s), C(1s), and N(1s) data, and Zr Mú irradiation was used for Mg(2p) and Al(2p) measurements. Figure 1. N(1s) XPS spectra of Alq 3 with increasing Mg exposure. 5391 J. Am. Chem. Soc. 2000,122,5391-5392

Chongfei Shen

Papers

Surface oxidation activates indium tin oxide for hole injection

Chemical and electrical properties of interfaces between magnesium and aluminum and tris-(8-hydroxy quinoline) aluminum

Electronic structure, diffusion, and p-doping at the Au/F16CuPc interface

The role of interface states in controlling the electronic structure of Alq3/reactive metal contacts

Organometallic Chemistry at the Magnesium- Tris(8-hydroxyquinolino)aluminum Interface