Electronic Structure and Electrical Properties of Interfaces between Metals and -Conjugated Molecular Films

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1 Electronic Structure and Electrical Properties of Interfaces between Metals and -Conjugated Molecular Films ANTOINE KAHN, 1 NORBERT KOCH, 2 WEIYING GAO 1 1 Department of Electrical Engineering, Princeton University, Princeton, New Jersey Physik von Makromolekülen, Institut für Physik, Humboldt-Universität zu Berlin, Newtonstrasse 15, D Berlin, Germany Received 5 April 2003; revised 13 May 2003; accepted 21 May 2003 ABSTRACT: 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 -conjugated molecular films and metals. It introduces several mechanisms currently believed to affect the formation of metal organic interface barriers Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 41: , 2003 Keywords: interfaces; metal-organic; injection INTRODUCTION The development of organic-based devices applicable to light emission, light-energy conversion, microelectronics, and macroelectronics has received considerable attention over the past few years and is leading to commercial products that should appear on the market in the near future. These organic technologies are based on organic thin-film structures grown by vacuum deposition or spun from the liquid phase. The structures often comprise multiple layers of organic materials and contacts for electron or hole injection. Metal organic (MO) and organic organic (OO) interfaces are, therefore, ubiquitous in small-molecule and polymer devices alike. They are the Correspondence to: A. Kahn ( kahn@ee.princeton. edu) Journal of Polymer Science: Part B: Polymer Physics, Vol. 41, (2003) 2003 Wiley Periodicals, Inc. keys, but quite often also the bottlenecks, for the injection of charge carriers into the films, their transport through the device, or eventually their containment for efficient radiative recombination in a designated part of the device. Given their obvious importance, these MO and OO semiconductor interfaces have been the subject of multiple fundamental and phenomenological investigations aimed at understanding their electronic and chemical structure, at controlling interface energy barriers, and, in the case of MO interfaces, at optimizing the injection of charge carriers. 1,2 At the beginning of the intensive period of work on organic devices, which followed the groundbreaking demonstration of the organic light-emitting diode (OLED) by Tang and Slyke, 3 it was generally assumed that the electronic structure of MO interfaces followed the simple rule of vacuum level alignment, known as the Schottky Mott limit in the community working on metal inorganic semiconductor interfaces. According to this 2529

2 2530 KAHN, KOCH, AND GAO Figure 1. Energy diagram of an MO semiconductor interface (a) without and (b) with a dipole barrier ( ). e and h are the electron and hole barriers, respectively, and E vac (O) and E vac (M) are the organic and metal vacuum levels, respectively. rule, the prediction of electron-and hole-injection barriers at a metal semiconductor interface is simple once the properties of the separated constituents of the interface are known: the holeinjection barrier ( h ) is the difference between the ionization energy (IE) of the organic material and the work function of the metal ( M ), and the electron-injection barrier ( e ) is the difference between M and the electron affinity (EA) of the organic film [Fig. 1(a)]. Despite widespread evidence disproving the validity of the Schottky Mott limit at inorganic semiconductor interfaces, vacuum level alignment was originally accepted for organic films because of the assumption that their surfaces, which consist of closed-shell molecular entities, would not undergo significant interactions with the metal surface. However, the work done on a wide variety of MO interfaces since the mid 1990s reveals a far more complex situation, which can be broadly characterized by a breakdown of the vacuum level alignment rule [Fig. 1(b)]. 2,4 This situation reminds us of the inorganic semiconductor interfaces investigated throughout the 1970s and 1980s and of the difficulties of sifting through the relevant interface mechanisms of formation of Schottky barriers, such as chemistry-induced defects and metal-induced gap states. Yet, in several respects, organic interfaces appear more complex and difficult to model than their inorganic counterparts. First, the complexity of standard inorganic semiconductor/metal interfaces is compounded at MO interfaces by the complex physics of charge injection and transport in molecular solids, which makes it difficult to identify various mechanisms. The weak van der Waals bonding and small intermolecular overlap of wave functions lead to transport by thermally activated intermolecular hopping (we are not considering here the highmobility, high-field, low-temperature transport achieved in molecular crystals such as anthracene, which belongs to the regime of band transport 5 ). The physics of charge transport in amorphous or microcrystalline molecular thin films is dominated by charge localization resulting from polarization of the medium and relaxation of molecular ions. These energies are much larger than transfer integrals or the temperature. Depending on the charge-injection conditions and applied fields, charge transport in these low-dielectricconstant and nearly charge-depleted materials is described by space-charge-limited, trap-limited, or injection-limited models. It is, therefore, difficult to extract accurate information on interface electronic structure and electron and hole barriers directly from transport data such as current voltage (I V) measurements. Trends on barrier heights can be, and have been, obtained from transport measurements, but the accurate determination of barrier heights and the understanding of the electronic structure of the interface presume that injection can be separated from transport and that the latter is well understood. That is not always the case at present.

3 ELECTRICAL PROPERTIES OF INTERFACES 2531 Second, the morphology and chemistry affect the behavior of MO interfaces. The morphology of interfaces can vary greatly with processing. The softness of organic materials is likely to lead to morphologies and interface metallurgical widths that are significantly different for metals deposited on organic films (generally vacuum-evaporated from a hot source) than for organic films deposited on metals. The diffusion of metal atoms and clusters through the loose organic matrix can be significant in the former case, leading to charge-transfer reactions (often called doping) and the formation of organometallic complexes deep in the organic film. The comparison of electrical behaviors of metal-on-organic interfaces versus organic-on-metal interfaces is a closely related issue and is very important for devices that include multiple organic layers and contacts. This is discussed later in this review. Metal-molecule chemistry can introduce interface electronic states, which, like their inorganic counterparts, affect the energetics of interfaces. Direct investigations and measurements of interface chemistry, morphology, and energetics on organic interfaces are, therefore, of paramount importance for understanding, controlling, and optimizing organic devices. Our strategy for uncovering the basic mechanisms and properties of MO interfaces has been to use film processing based on ultrahigh vacuum (UHV) in conjunction with ultraviolet photoemission spectroscopy (UPS) and X-ray photoemission spectroscopy (XPS) to investigate valence states and core levels and to use inverse photoemission spectroscopy (IPES) to investigate empty electronic states during the formation of interfaces. UHV conditions are generally regarded as remote from the conditions under which organic thin films are being processed and will be processed for commercial applications. Yet, UHV processing permits investigations of fundamental interface properties designed to extract information related to the intrinsic nature of the materials under investigation, rather than to extrinsic factors linked to the contamination of organic films and metal electrodes. The UHV-based work described in this review focuses exclusively on films of -conjugated molecules that can be sublimated and condensed on a variety of substrates. The surface/interface analysis techniques mentioned previously are used in conjunction with in situ I V measurements on simple devices. The contact potential difference (CPD) is used to eliminate artifacts due to photon-or electron-related probes, which occasionally induce nonequilibrium situations in wide-band-gap organic materials. This powerful combination of electronic, chemical, and electrical probes allows us to correlate information on the interface electronic structure and chemistry with the actual charge carrier injection through the interface under bias and to begin the process of separating injection from transport. Following this introduction is a rapid description of the experimental techniques and the type of information they provide. The review proceeds with examples that demonstrate the breakdown of the vacuum level alignment rule at MO interfaces. The bulk of the review focuses on the various mechanisms of molecular level alignment and the formation of interface dipoles and discusses issues of electrical symmetry at MO interfaces versus organic metal (OM) interfaces, with an emphasis on morphological issues. The review closes with a section on interface modification via electrical doping. EXPERIMENTAL The results described herein have been obtained from experiments performed in situ on molecular films deposited in UHV by sublimation from solid sources. The organic material is purified ex situ and then thoroughly degassed in vacuo. Highpurity films are obtained in that fashion. The valence states of the organic films, particularly the molecular states close to the highest occupied molecular orbital (HOMO), are investigated by UPS performed with synchrotron radiation or radiation from a helium discharge lamp [fixed photon energies: ev (HeI) and 40.8 ev (HeII)]. A typical valence spectrum of an organic thin film, measured here by synchrotron radiation UPS from N,N -diphenyl-n,n -bis(1-naphthyl)-1,1 -biphenyl-4,4 -diamine ( -NPD) deposited on Au, is shown in Figure 2(a). The valence states close to the HOMO are shown on the right panel. The bottom spectrum corresponds to the clean Au substrate and includes the metal Fermi edge at a binding energy (BE) reference of 0 ev and the Au 4f levels at BE 4 6 ev. The -NPD features progressively appear upon incremental deposition of the molecular film. The -NPD spectrum consists of several well-defined features related to the density of states of the system and orbitals of the molecular solid. The highest kinetic energy (lowest BE) feature corresponds to the HOMO (BE 2 ev). The 1.4-eV energy dif-

4 2532 KAHN, KOCH, AND GAO Figure 2. -NPD on Au: (a) valence UPS spectra, showing the position of the leading edge of the HOMO with respect to the Au Fermi level; (b) the shift of the photoemission onset, indicating the presence of an interface dipole; (c) a comparison between UPS and CPD measurements of the change in the work function ( ), relative to the clean substrate surface, induced by the deposition of the organic film; and (d) a schematic interface energy diagram deduced from the photoemission measurements. The spectra in part b were taken with the sample biased at 3 V to clear the detector work function. ference between the leading edge of the HOMO and the Fermi level constitutes the hole-injection barrier, and it is displayed on the schematic of the interface electronic structure [Fig. 2(d)]. Figure 2(b) shows the low-energy secondary-electron distribution with the sharp photoemission cutoff (at

5 ELECTRICAL PROPERTIES OF INTERFACES ev on the Au substrate). This cutoff, also called the photoemission onset, is related to the vacuum level (E vac ), as no electron with less energy can escape from the solid. Translated by the photon energy, this onset gives the position of E vac with respect to the other photoemission features (e.g., the HOMO). The IE of the molecular film is defined as the energy difference between E vac and the leading edge of the HOMO peak. A summary of IEs measured for several molecular films is given in Figure 3. An important and recurrent aspect of the results reviewed in this article is the observation of an abrupt shift of E vac between the metal and organic upon deposition of the first molecular layer of the organic films on the metal. The example in Figure 2 gives a shift of 1.2 ev. The shift is generally the manifestation of the formation of an interface dipole barrier. It is central to the electronic structure of MO interfaces and is discussed at length in this review. The chemistry of MO interfaces is probed with XPS. We use the K line from an aluminum anode (1486 ev) and the M line from a zirconium anode (151 ev). The low kinetic energy of electrons photoexcited by the latter provides for a very surface-sensitive chemical probe. Both photon lines produce spectra with an energy resolution of approximately 0.5 ev. A central aspect of our interface work is the study by IPES of empty states in the energy region of the lowest unoccupied molecular orbital (LUMO) of the organic materials. In IPES, an electron with energy above E vac of the sample is directed toward the surface of the film, penetrates the solid, and decays into empty states. The transition releases a photon, which is collected to map the density of empty states of the material. In our laboratory, IPES is done in the isochromat mode; that is, the electron energy is varied while the energy of the detected photons remains fixed. The system has a resolution of 0.5 ev. 6 Experiments on organic films are performed with incident electron current densities in the range of approximately 10 7 to 10 4 A/cm 2 to prevent degradation of the molecular material. The combination of UPS and IPES spectra gives a complete picture of the valence states, energy gap, and empty states of the materials, as shown for zinc phthalocyanine (ZnPc) in Figure 4. CPD measurements are performed with a vibrating UHV Kelvin probe. This method provides an independent verification, performed in the dark, of the vacuum level and work function changes upon the formation of MO interfaces. Finally, the relationship between the electronic structure, the chemistry, and the charge-carrierinjection characteristics at organic interfaces under investigation is established via I V measurements on devices grown and measured in UHV. The complete in situ process eliminates the effects of the electrode oxidation and deterioration of the organic layer in air on the transport characteristics. ELECTRONIC STRUCTURE AND ENERGY BARRIERS AT MO INTERFACES Organic-on-Metal Interfaces A good starting point for evaluating the electronic structure of a metal semiconductor interface is to compare the metal work function ( ) and semiconductor EA (for electron injection) and IE (for hole injection). A comparison of these quantities for standard metals and organic materials used in our studies is shown in Figure 5. The range of organic IEs and EAs is extensive in comparison with standard inorganic semiconductors. for Au is very close to IE for -NPD and larger than IE for copper phthalocyanine (CuPc), ZnPc, pentacene, or -sexithiophene ( -6T), and this leads to the prediction that Au forms a good hole-injection contact for these materials. However, as mentioned earlier in this review, a central aspect of the electronic structure of MO interfaces is the breakdown of the Schottky Mott, or vacuum alignment, rule [Fig. 1(b)]. As an example, the type of vacuum level shift measured in Figure 2(b) for -NPD/Au signals the formation of an interface dipole ( ) that offsets the electronic structure of the two materials. Thus, instead of the eV hole barrier suggested by Figure 5 for -NPD/Au, measurements show that the barrier is approximately 1.4 ev [Fig. 2(d)]. This state of affairs is more the rule than the exception at MO interfaces. The electronic structures of a number of interfaces are summarized in Figure 6. Nearly all these interfaces are formed by the deposition of the organic film on the metal. The reverse deposition sequence, that is, metal on organic, is discussed in a later section. In each of the eight panels, the data points represent the position of the Fermi level measured by UPS between the HOMO (bottom horizontal bar; at the same time serving as a metal work function scale)

6 2534 KAHN, KOCH, AND GAO Figure 3. Summary of IE values measured by UPS of films of organic materials. IE is defined as the energy difference between the leading edge of the HOMO and the vacuum level obtained from the photoemission cutoff. The molecules displayed include pentacene, ZnPc, CuPc, Alq 3, 4,4 -N,N -dicarbazolyl-biphenyl (CBP), bathocuproine (BCP), -6T, -NPD, PTCBI, hexadecafluoro copper phthalocyanine (F 16 -CuPc), PTCDA, tetracyanoquinodimethane (TCNQ), and F 4 -TCNQ. and LUMO (top horizontal bar) as a function of the metal work function. Each data point represents an experiment of the type presented in Figure 2(a,b). The HOMO is measured by UPS at the interface, and the position of the LUMO is obtained by the addition of the transport, or singleparticle, gap. 7 The additional polarization due to the proximity of the metal, which narrows the

7 ELECTRICAL PROPERTIES OF INTERFACES 2535 Figure 4. Combined UPS IPES spectra of a 100-Åthick film of ZnPc deposited on Au. The inset shows the schematic electronic structure deduced from UPS and IPES. interface single-particle gap, is neglected in this discussion. 8 The dashed line in each panel represents the position that the Fermi level would assume as a function of the metal work function if the Schottky Mott rule was in effect. The vertical segments are the dipole barriers ( ) measured at these interfaces. When the data point is above the Schottky Mott prediction (the majority of the cases), represents a step down of E vac from the metal to the organic film. In some cases involving low work function metals, such as Mg, and high- EA materials, such as 3,4,9,10-perylenetetracarboxylicdianhydride (PTCDA), is in the opposite direction. Figure 6 leads to the conclusion that interface dipoles are ubiquitous at MO interfaces prepared in UHV. We emphasize here in UHV as it is increasingly apparent that substrates such as Au exposed to air before organic deposition lead to smaller dipoles and smaller hole-injection barriers. 9 Substrate contamination is discussed more in a later section. Interestingly, the range of the Fermi level (E F ) position as a function of the metal work function in Figure 6, that is, the S parameter, varies substantially from organic to organic and generally increases with the singleparticle gap in a way that is reminiscent of inorganic semiconductor/metal interfaces. 10 Dipoles with both signs can be found, depending on the relative values of the organic EA and the metal work function ( ). These dipoles have different Figure 5. Comparison between metal work functions and IE and EA, or HOMO and LUMO positions, as determined by UPS and IPES, of various organic materials. The zero of the energy scale represents a common vacuum level.

8 2536 KAHN, KOCH, AND GAO Figure 6. Measured interface position of E F with respect to HOMO and LUMO as a function of the metal work function for eight different molecular materials. In each panel, the thick horizontal bottom and top bars represent the HOMO (with a work function scale) and LUMO, respectively. Dashed LUMO bars mean that the LUMO position is not precisely known. The data points were obtained via UPS for organic-onmetal interfaces. The dashed oblique lines correspond to the Schottky Mott limit of the Fermi level position, and the vertical lines give the magnitude of the measured interface dipole barriers. origins, which we place into three different categories: (1) charge transfer based on relative values of and the organic EA and IE, (2) a chemical reaction leading to the formation of gap states and pinning of E F at the interface, and (3) molecule-induced modification of the metal. Another class of mechanisms based on metal-induced gap states and the charge neutrality level of the semiconductor is only beginning to be investigated for organic films. 11 Although clearly important, this theoretical approach is still in its infancy and has not yet produced a meaningful set of interface data. Interface Charge Transfer The first category addresses interfaces such as 3,4,9,10-perylenetetracarboxylic bisbenzimidazole (PTCBI)/Ag 12 (Fig. 6). In such cases, the metal before the formation of the interface is smaller or equal to the organic EA, leading to an electron transfer to the LUMO of the interface molecules. The result is a dipole barrier equal to an upward step of E vac from the metal to the organic film. The raising of the organic electronic structure with respect to the metal Fermi level stops the net electron transfer. The partially occupied LUMO level relaxes into the gap, forming interface gap states visible by UPS above the HOMO and below E F [Fig. 7(a)]. The charges transferred from the metal to the organic film remain localized at the interface, as shown by the rapid attenuation of the gap state photoemission signal upon further PTCBI deposition. A general feature of these interfaces is the absence of mo-

9 ELECTRICAL PROPERTIES OF INTERFACES 2537 Figure 7. (a) UPS spectra of the Fermi level, gap, and HOMO energy region as a function of PTCBI deposition on Ag. 12 The two short vertical bars mark the position of the filled gap states. (b) Schematic energy diagram showing the interface dipole and molecular levels at the interface and in the film lecular level shift (or band bending in the conventional semiconductor jargon) in the organic semiconductor away from the interface [Fig. 7(b)]. The molecular level alignment with respect to the metal Fermi level is set at the interface, and no significant band bending shift is detected in the organic film, at least on the thickness scale relevant to thin-film devices ( Å) and in nominally undoped layers. This is entirely consistent with the fact that very few free charges are available in these relatively wide-gap, purified, and undoped materials However, similar investigations of interfaces of electrically doped molecular layers deposited on metal surfaces do show the formation of band bending and depletion regions, and this is consistent with the presence of charges in the film (discussed later). Other interfaces such as F 16 CuPc-on-Mg, F 16 CuPc-on- Al, PTCBI-on-Mg, PTCDA-on-Mg, and PTCDAon-In also exhibit a dipole barrier going up from the metal to the organic film. Although the type of charge transfer mentioned previously plays a definite role, chemistry is also a significant factor at these interfaces. 16,17 Formation of Covalent Bonds at Interfaces The second category corresponds to reactive interfaces in which the energy level alignment is controlled by chemistry-induced electronic states. The number of such chemical reactions is too large to review within the boundaries of this article. Our purpose is to give here a sense of the mechanism through the example of the extensively studied interface between tris(8-hydroxyquinoline)aluminum (Alq 3 ) and Mg. Mg:Alq 3 is an important interface for its technological relevance to OLEDs, as Alq 3 is one of the most widely used electron-transport and green-emitter materials and the Mg:Ag alloy is a good electron injector for Alq 3. Figure 5 shows that the Mg work function falls in the Alq 3 gap, and this is a situation that gives no a priori reason for charge transfer and an interface dipole. Experimental observations show a dipole barrier (pointing down from the metal into the organic layer) of approximately 0.5 ev. Measurements 18,19 also show that the interface electronic structure includes interface states resulting from the formation of a new Mg:Alq 3 organometallic complex. A detailed investigation of

10 2538 KAHN, KOCH, AND GAO Figure 8. (a) UPS spectra of the Fermi level, gap, and HOMO energy region as a function of Alq 3 deposition on Mg. 19 The short full vertical line gives the position of the chemistry-induced gap states. (b) Structural representation of the organometallic complex product of the Mg Alq 3 reaction. 20 core levels at the Alq 3 /Mg interface, 19 coupled to first-principle dynamic calculations of Mg interacting with a three-dimensional unit cell of Alq 3, 20 reveals the formation of a complex involving a Mg atom and two molecules, the formation of covalent MgOC bonds, and MgOO coordination [Fig. 8(b)]. The interaction goes, therefore, beyond the formation of a cation anion pair with the transfer of a (partial) electron from Mg to the molecule, as seen with other species such as alkali metal atoms The new interface complex gives rise to a density of filled states detected with UPS with energy in the former gap of pristine Alq 3 [Fig. 8(a)]. The appearance of these filled states in the gap leads to pinning of the Fermi level in the upper part of the organic gap at the Mg:Alq 3 interface, as shown schematically in Figure 9. The alignment of the Fermi level across the interface requires an electron charge transfer from the organic to the metal. The charge comes from the interface states, and we calculate that approximately 1 out of 20 interface organometallic molecules needs to be ionized to set up the observed 0.5-eV dipole. Interfaces between Alq 3 and two other reactive metals, Al and Sm, have been investigated and found to exhibit electronic structures quite similar to that of Mg:Alq 3, with gap states pinning E F in the upper part of the gap. The Sm work function being only 2.7 ev, however, a considerably smaller dipole of about 0.15 ev appears at this interface. The I V characteristics of these interfaces are, of course, most important in terms of device performance. They are reviewed in a later section in the context of a comparison between organic-onmetal and metal-on-organic interfaces. Pillow Effect on Molecular Level Alignment Recent experimental and theoretical studies suggest that a significant fraction of the interface dipole barrier at interfaces such as p-sexiphenyl, pentacene, N,N -diphenyl-n,n -(3-methylphenyl)- 1,1 -biphenyl-4,4 -diamine (TPD), or -NPD on Au 25 corresponds to a lowering of the metal work function ( M ) by the adsorbed molecules. 4,26,27 The typical evolution of M for such a system (e.g., hydrocarbons on noble metal surfaces) is depicted in Figure 2(c). M drops by a substantial amount with the first monolayer of the organic material, giving the appearance of an interface

11 ELECTRICAL PROPERTIES OF INTERFACES 2539 Figure 9. Mechanism of the formation of the dipole at the Alq 3 /Mg interface: separate materials (left), chemical reaction and formation of gap states that pin the Fermi level (middle), and electron transfer from a organometallic complex to the metal to establish thermodynamic equilibrium (right) dipole barrier, and remains essentially constant for higher coverages. This experimental observation has been reported for many physisorptive OM systems. Note that the measurements presented in Figure 2(c) are performed by UPS and Kelvin probe CPD measurements. The excellent agreement between the two sets of data demonstrates unambiguously the reality of the work function drop. The work function of a metal is the sum of bulk and surface contributions, 28,29 that is, the bulk chemical potential ( ) and surface dipole (SD). The latter depends sensitively on the surface structure and is in part set by the tail of electrons spilling out from the metal surface into the vacuum. This SD contribution is modified by the presence of an adsorbate. With large adsorbates such as conjugated organic molecules, the repulsion between the molecule electrons and the metal surface electrons compresses the electron tail (the pillow effect) and lowers M. This, in turn, causes the abrupt downward shift of the vacuum level from the metal to the organic film at the interface. The result of this lowering of M is a shift of the molecular energy levels toward lower energy and a corresponding increase in the energy difference between the metal E F and the HOMO of the organic film, that is, the hole-injection barrier. The barrier is, therefore, systematically increased with respect to a vacuum level alignment situation, with the unfortunate result of a significant reduction in hole-injection performance. The large SD contribution to M (Au) and that of other large work function metals leads to the large interface dipole barrier observed at organic interfaces with these metals. Conversely, when organic molecules are physisorbed on metals with small SD contribution to the work function, the change in M (i.e., the shift of the vacuum level) is accordingly small. This can be seen, for example, at the interfaces between p-sexiphenyl and pentacene and Sm ( M 2.7 ev). The interaction at these OM interfaces is physisorptive, 30,31 and the interface dipole is smaller than 0.2 ev. Naturally, the pillow effect occurs also at OM interfaces with stronger (chemical) interactions, but it is more difficult to differentiate between the various contributions to the interface dipoles (e.g., covalent bonding with ionic character). The measured interface dipole is the sum of all contributing dipoles, and various components can be identified only through computations. 26 Metal versus Conducting Polymer Electrodes: Avoiding the SD The pillow effect appears to play against the intuitive approach of using a high work function metallic material to achieve low hole-injection barriers. We should emphasize again that rela-

12 2540 KAHN, KOCH, AND GAO Figure 10. Energy diagrams of interfaces formed between -NPD, pentacene, and p-sexiphenyl and Au and PEDOT:PSS. tively good contacts to hole-transport materials have been obtained with Au exposed to air. 9 The reasons for the substantial difference between these and the UHV-processed interfaces remain to be investigated. Nevertheless, the idea previously developed finds an interesting confirmation in the successful use of the conducting polymer mixture poly(3,4-ethylene dioxythiophene)/poly- (styrene sulfonate) (PEDOT:PSS) to enhance hole injection. Indeed, one possibility for circumventing the problem of substrate M reduction by adsorbed molecules is to design a material with large and small SD contributions to M.An example of this kind of material is PEDOT:PSS. It exhibits good optical, electrical, and processing characteristics and has a work function of approximately 5 ev, equivalent to that of Au. However, the work function of PEDOT:PSS is mainly controlled by the energy levels created by the charge transfer between the sulfonate and the ethylene dioxythiophene moieties. 32 This charge transfer does lead to dipoles within the polymer, but they have a random orientation and cancel each other macroscopically. The surface electron dipole layer contribution to the work function is, therefore, minimal, as the conducting organic polymer is made of closed-shell molecules and has far fewer free electrons than a metal such as Au. Thus, its work function does not have a significant surface electron tail contribution, and the adsorption of molecules should only slightly modify the work function of the polymer. This, in turn, should enable the formation of smaller hole-injection barriers at a contact with a conjugated organic material. The hole-injection barriers ( h ) and interface dipoles ( ) measured by UPS for three conjugated organic materials ( -NPD, pentacene, and p-sexiphenyl) deposited onto Au and PEDOT:PSS confirm this suggestion. 25 Summary energy diagrams of the six interfaces are shown in Figure 10. The values are consistently much larger for Au (1.15 ev for -NPD, 1.05 ev for pentacene, and 0.8 ev for p-sexiphenyl (6P)) than for PEDOT:PSS

13 ELECTRICAL PROPERTIES OF INTERFACES 2541 indium tin oxide (ITO) bottom contact covered with a spin-cast film of PEDOT:PSS, a 150-nm film of -NPD, and 45 nm of Au. In type II devices, both the top and bottom contacts are made of Au. The two structures result in similar -NPD morphologies, as confirmed by atomic force microscopy measurements: the root-mean-square roughness of -NPD is 1.8 nm on Au versus 1.1 nm on PEDOT:PSS, which is small in comparison with the nominal thickness of the film. The I V characteristics [Fig. 11(b)] correspond to hole injection from the bottom PEDOT:PSS or top Au electrode in type I devices, or from the bottom Au or top Au electrode in type II devices. The key result is the difference of several orders of magnitude in the hole current density injected from PEDOT:PSS versus Au, which confirms the 1-eV lowering of the hole barrier at the interface with the former. The I V characteristics for the type II device show identical hole injection from top and bottom Au contacts, despite the difference in the deposition sequences of vastly dissimilar materials. The next section addresses this specific issue, which is an important one in the context of devices built vertically with a number of different organic and inorganic (e.g., metal) layers. Figure 11. (a) Structures of type I and type II devices and (b) I V characteristics for ITO/(PEDOT:PSS)/ - NPD(150 nm)/au(45 nm) (type 1) devices and ITO/ (PEDOT:PSS)/Au(80 nm)/ -NPD(150 nm)/au(45 nm) (type 2) devices. (0.3 ev for -NPD, 0.1 ev for pentacene, and 0.35 ev for 6P). The minus sign for 6P on PEDOT:PSS indicates a dipole barrier of opposite direction; that is, the vacuum level rises from the polymer electrode to the molecular film. No evidence for a chemical reaction or the formation of interface electronic states in the gap of the organic film is obtained from UPS for any of the six interfaces investigated. However, we cannot rule out the occurrence of charge-transfer reactions at these OO interfaces below the detection limit of UPS. The consistently lower hole-injection barrier obtained with PEDOT:PSS is reflected in I V characteristics of simple unipolar devices [Fig. 11(a)]. We emphasize that all devices are built and tested in UHV to eliminate all possibilities of ambient contamination. Type I devices include an Metal-on-Organic Interfaces versus Organic-on- Metal Interfaces: Are They Electrically Symmetric? In a vacuum deposition environment, one intuitively expects a sharper interface for a soft organic material deposited on an already formed metal surface than for the converse deposition sequence (Fig. 12, top). The latter is likely to produce more in-diffusion of thermally excited metal atoms impinging on the organic surface and lead to a broader interface. By and large, this picture is confirmed by experimentation, with many examples of in-diffusion of deposited metal species. A couple of examples are reviewed in this section. The question we develop here, however, is whether the morphological difference between organic-on-metal and metal-on-organic interfaces is reflected in the electrical characteristics of the contact. This is a more complex issue, the assessment of which requires a good understanding of the interface chemistry and electronic structure. We begin with the Mg-on-Alq 3 and Alq 3 -on-mg interfaces and their Al and Sm counterparts. Early investigations of these two systems by Bulovic et al. 33 concluded that the top Mg electrode, that is, Mg evaporated on Alq 3, formed a substantially better electron-injection contact than the

14 2542 KAHN, KOCH, AND GAO Figure 12. (a) Abrupt interface obtained by the deposition of organic molecules on a metal surface (top) and the Mg1s core level as a function of Alq 3 deposition on Mg (bottom). Note that the scale of the upper two spectra are multiplied by 2 and 100, respectively. (b) Diffused interface obtained by the evaporation of metal atoms on an organic film (top) and the C1s core level as a function of Mg deposition on Alq 3 (bottom). bottom Mg electrode, giving a current density larger by three orders of magnitude at equal bias. Shen et al. 34 demonstrated that this large difference could be traced to contamination (i.e., oxidation) of the reactive bottom Mg electrode during diode fabrication in a standard 10 6 to 10 7 Torr vacuum and was, therefore, of extrinsic origin. We briefly review the results of an investigation of these interfaces and device fabrication in UHV, in which all extrinsic factors related to the contamination of highly reactive metals are eliminated. The bottom part of Figure 12 compares the evolution of the C1s core level of Alq 3 as a function of deposition of Mg on the organic film with that of the Mg1s core level of a Mg film substrate as a function of Alq 3 deposition. The attenuation rates versus the nominal thicknesses of deposited

15 ELECTRICAL PROPERTIES OF INTERFACES 2543 Figure 13. I V characteristics corresponding to electron injection from top and bottom electrodes in (a) Mg/Alq 3 /Mg and (b) Sm/Alq 3 /Sm devices (the thickness of the Alq 3 film is 120 nm). (c) The position of the Fermi level and electron-injection barriers measured at Alq 3 interfaces with Mg, Sm, and Al. material show that the morphologies of the two interfaces are quite different: the Alq 3 -on-mg interface is abrupt, whereas Mg evaporated on Alq 3 penetrates the organic matrix (the slow attenuation of the C1s component could be explained by Mg clustering on the organic surface, but the absence of a metallic Mg component in the Mg core level actually argues against this possibility). From the point of view of chemistry, gap states, and electrical barriers, the top and bottom interfaces are very similar, giving nearly identical XPS and UPS signatures of core-level chemical shifts, states in the gap of the material, and HOMO versus the E F position. 19 Furthermore, despite strong morphological differences, the Mg-on-Alq 3 and Alq 3 -on-mg contacts produce identical electron injection over five orders of magnitude [Fig. 13(a)]. The same is true for Mg:Ag/Alq 3 /Mg:Ag, Sm/Alq 3 /Sm [Fig. 13(b)] and Al/Alq 3 /Al devices. 19 We believe that the dominating and equalizing factor in these top and bottom interfaces is the chemistry-induced density of gap states, which pins the Fermi level at the same position, regardless of the deposition sequence. These results also dispel the notion that a chemical reaction coupled with diffusion enhances charge injection by creating a (physically and electronically) broad density of interface states that could play the role of a

16 2544 KAHN, KOCH, AND GAO Figure 14. (a) Schematic of the metal/alq 3 /metal structure. The shaded area represents the thin oxide layer introduced by the exposure of the Mg:Ag bottom contact layer to ambient pressure. (b) Energy alignment of the Mg/Alq 3 /Mg structure with the bottom electrode oxidized. (c) I V characteristics for the Mg:Ag/Alq 3 /Mg:Ag structure built on a oxidized bottom contact. ladder of electronic states between the metal Fermi level and the transport levels of the organic film. These reactive UHV-fabricated and UHVtested interfaces behave in a very symmetric fashion. The issue of environmental impact on the fabrication of interfaces is an important one. Commercial devices will be fabricated in an environment in which reactive materials such as Mg do get somewhat contaminated. Figure 14 shows the I V characteristics of a Mg:Ag/Alq 3 /Mg:Ag structure in which the organic film is deposited on a bottom Mg electrode contaminated by an exposure to oxygen approximately equivalent to that corresponding to a normal fabrication time in a low vacuum (ca Torr). 34 The electron current injected from the bottom contact is one to two orders of magnitude smaller than that injected from the top contact. However, as already demonstrated for UHV-processed devices, this difference is not related to the contact morphology. The F 16 CuPc/Au system, however, offers an example in which the difference in morphology, due to different deposition sequences, does matter. 35 The interaction of molecules deposited on the metal surface (bottom electrode) is nonreactive

17 ELECTRICAL PROPERTIES OF INTERFACES 2545 atoms act as p-dopants in the organic film and alter the balance of charges and the position of the Fermi level. 35 Unlike in the previous case, the electronic structure of these interfaces is not dominated by a chemical reaction. Figure 15. Electron current injected from top and bottom Au electrodes in the Au/F 16 CuPc/Au structure. The inset is a device structure showing preferential Au diffusion at the top interface. 35 and does not induce gap states in the organic material. However, the in-diffusion of isolated metal species (top electrode) evaporated on the organic film leads to doping or a reaction inside the organic film. A UPS XPS investigation of interfaces between these two materials has shown that (1) Au atoms evaporated on F 16 CuPc diffuse in the organic film 35 and (2) the position of the Fermi level is higher by ev with respect to the HOMO at the F 16 CuPc-on-Au interface than at the Au-on-F 16 CuPc interface. Consequently, the electron-injection barrier is larger at the top Au contact, as reflected in the I V characteristics of the Au/F 16 CuPc/Au structure (Fig. 15). The difference in the E F positions reflects a fundamental difference between the MO interactions that take place at these two interfaces. The F 16 CuPc-on-Au interface is the result of a nonreactive adsorption of molecules on a fully formed metallic surface and is abrupt. The mechanisms that control the molecular level alignment depend on the relative values of the work functions of the two solids and on the effect of the adsorbed molecules on the metal surface electronic structure, as described previously. In this case, however, EA of F 16 CuPc is 4.8 ev, and the work function of Au is ev, leading to a position of E F close to the top of the transport gap [the pillow effect is minimized here by the fact that M (Au) is so close to EA(F 16 CuPc)]. The other interface is formed by the deposition of metal atoms that diffuse into the film and alter the electronic properties of the organic material in the interface region. The Au Modification of MO Interfaces by Electrical Doping We shall conclude this brief and clearly nonexhaustive review of MO interfaces with a short look at electrical doping as an efficient way of enhancing carrier injection. Doping introduces charges in the organic film and leads to the formation of a depletion region. Heavy doping at or near the interface narrows the depletion region to the point at which carrier tunneling occurs and considerably enhances injection. This technique is widely used to form ohmic contacts on inorganic semiconductors. The doping of molecular films, with both inorganic and organic dopants, has been investigated by several groups 22,36 40 and has already led to significant improvements in the efficiency and drive voltages of some OLEDs Our work focuses on the use of organic molecules as dopants, and we briefly review here the mechanism and impact of p-doping of hole-transport materials such as ZnPc and -NPD with highly electronegative tetrafluorotetracyanoquinodimethane (F 4 - TCNQ; Fig. 3). P-doping requires the transfer of an electron from the host molecule to the dopant, leading to the formation of a hole. From the molecular level point of view, this requirement imposes that EA of the dopant be comparable to the IE of the host molecule. Combinations of UPS and IPES measurements, similar to those presented in Figure 4, show that this is the case for ZnPc: F 4 -TCNQ 46,47 and -NPD:F 4 -TCNQ (Fig. 16). The excellent match between the key molecular levels leads to efficient electron transfer and doping. Furthermore, the structural properties of the molecular films allow the introduction of a large concentration of the dopants without a major impact on the electronic structure of the host material. Evidence of doping comes from UPS measurements of the molecular level position upon the formation of an undoped or doped organic film on a metal surface [Fig. 17(a,b)]. Although the undoped film shows no shift of the molecular level away from the interface (as discussed previously), the doped film exhibits a depletion region with a 0.48-eV upward band bending between the inter-

18 2546 KAHN, KOCH, AND GAO Figure 16. Electronic structure (IE and EA) of ZnPc, -NPD, and F 4 -TCNQ determined by a combination of UPS and IPES (as in Fig. 4). face molecular level position imposed by the metal-molecule interaction and the bulk molecular level position imposed by doping. The width of the depletion region for a doping level of 0.3% is approximately 150 Å. Note that the interface barrier of the doped film is nearly identical to that of the undoped film. The anchoring of the molecular level by the mechanisms discussed in the previous sections is strong. However, the narrow width of the depletion region, obtained by the doping of the organic film near the interface only, is sufficient to produce considerable tunneling through the top of the barrier, leading to an increase in the hole injection of four orders of magnitude at the -NPD/Au interface [Fig. 17(c)]. 48 It is easy to see how this type of interface engineering can be of great benefit to organic devices with performance limited by difficult carrier injection over large interface barriers. For that reason, there is considerable incentive for further developing efficient and stable p-and n-type molecular dopants that can be easily inserted and controlled inside the host matrix. CONCLUSIONS In this article, we provided a rapid overview of the basic energetics of interfaces formed between metals and vacuum-evaporated films of -conjugated molecules. These interfaces play a central role in the performance of thin-film organic devices such as OLEDs, thin film transistors Figure 17. Energy of the molecular levels near the interface between Au and (a) undoped -NPD and (b) -NPD:0.5% F 4 -TCNQ (the measured width of the depletion region is shown in part b; the interface dipole, work function of Au, and -NPD IE are indicated in each case). (c) I V characteristics measured in situ for Au/ -NPD/Au structures with ( ) undoped -NPD and ( ) -NPD doped with 0.5% F 4 -TCNQ in an 80-Å region next to the interface.

19 ELECTRICAL PROPERTIES OF INTERFACES 2547 (TFTs), and photovoltaic cells and must be well understood and controlled for manufacturing reliable and efficient devices. We reviewed some of the techniques and methodologies used to unravel the mechanisms that control interface energetics and showed how photoemission and inverse photoemission spectroscopies play such an important role in determining interface energy barriers. The principal, and by now well-known, discovery of these interface investigations is that energetics of MO vacuum-processed interfaces do not simply derive from vacuum level alignment. The rich physics and chemistry of MO interface formation make it very difficult to predict interface properties and performance solely on the basis of separately determined material parameters such as the work function and EA. Several mechanisms ranging from simple charge exchange to Fermi level pinning by chemistry-induced states and metal surface modification by adsorbed molecules lead to substantial deviations from the ideal Schottky Mott picture, marked by the formation of interface dipole barriers. We showed examples of the impact of processing, such as the deposition sequence, and environment on molecular level alignment and charge injection across the interfaces. The importance of the interface morphology in particular, the diffusion of reactive metal atoms versus nonreactive metal atoms or clusters in the organic matrix was discussed in terms of the asymmetry of the interface barrier and charge injection at organic-on-metal interfaces versus metal-on-organic interfaces. Finally, electrical doping with molecular dopants was introduced as an efficient method for considerably enhancing charge injection at interfaces. Support for the work described in this review by the National Science Foundation (DMR ), the New Jersey Center for Organic Optoelectronics, and the New Energy and Industrial Technology Development Organization of Japan is gratefully acknowledged. The authors are grateful to J. Schwartz for providing key insights into the chemistry of interfaces and for a longstanding collaboration on a number of projects summarized in this review. REFERENCES AND NOTES 1. Salaneck, W. R.; Stafström, S.; Brédas, J.-L. Conjugated Polymer Surfaces and Interfaces: Electronic and Chemical Structure of Interfaces for Polymer Light Emitting Devices; Cambridge University Press: Cambridge, England, Conjugated Polymer and Molecular Interfaces: Science and Technology for Photonic and Optoelectronic Applications; Salaneck, W. R.; Seki, K.; Kahn, A.; Pireaux, J.-J., Eds.; Marcel Dekker: New York, Tang, C. W.; Slyke, S. A. Appl Phys Lett 1987, 51, Ishii, H.; Sugiyama, K.; Ito, E.; Seki, K. Adv Mater 1999, 11, Karl, N.; Kraft, K.-H.; Marktanner, J.; Münch, M.; Schatz, F.; Stehle, R.; Uhde, H.-M. J Vac Sci Technol A 1999, 17, Wu, C. I.; Hirose, Y.; Sirringhaus, H.; Kahn, A. Chem Phys Lett 1997, 272, Hill, I. G.; Kahn, A.; Soos, Z. G.; Pascal, R. A. Chem Phys Lett 2000, 327, Tsiper, E. V.; Soos, Z. G.; Gao, W.; Kahn, A. Chem Phys Lett 2002, 360, Malliaras, G. G. Cornell University. Private communication (2003). 10. Sze, S. Z. Physics of Semiconductor Devices, 2nd ed.; Wiley: New York, Vazquez, H.; Oszwaldowski, R.; Pou, P.; Ortega, J.; Perez, R.; Flores, F.; Kahn, A. Euro Phys Lett, submitted. 12. Hill, I. G.; Schwartz, J.; Kahn, A. Org Electron 2000, 1, Hayashi, N.; Ishii, H.; Ouchi, Y.; Seki, K. J Appl Phys 2002, 92, List, E. J. W.; Kim, C. H.; Shinar, J.; Pogantsch, A.; Petritsch, K.; Leising, G.; Graupner, W. Synth Met 2001, 116, Möller, S.; Weiser, G. Chem Phys 1999, 246, Shen, C.; Kahn, A.; Schwartz, J. J Appl Phys 2001, 90, Hirose, Y.; Kahn, A.; Aristov, V.; Soukiassian, P.; Bulovic, V.; Forrest, S. R. Phys Rev B 1996, 54, Shen, C.; Hill, I. G.; Kahn, A.; Schwartz, J. J Am Chem Soc 2000, 122, Shen, C.; Kahn, A.; Schwartz, J. J Appl Phys 2001, 89, Meloni, S.; Palma, A.; Schwartz, J.; Kahn, A.; Car, R. J Am Chem Soc, in press. 21. Ramsey, M. G.; Steinmuller, D.; Netzer, F. P. Phys Rev B 1990, 42, Greczynski, G.; Fahlman, M.; Salaneck, W. R.; Johansson, N.; dos Santos, D. A.; Bredas, J. L. Thin Solid Films 2000, 363, Koch, N.; Leising, G.; Yu, L. M.; Rajagopal, A.; Pireaux, J. J.; Johnson, R. L. J Vac Sci Technol A 2000, 18, Mason, M. G.; Tang, C. W.; Hung, L. S.; Raychaudhuri, P.; Madathil, J.; Giesen, D. J.; Yan, L.; Le, Q. T.; Gao, Y.; Lee, S. T.; Liao, L. S.; Cheng, L. F.; Salaneck, W. R.; dos Santos, D. A.; Bredas, J. L. J Appl Phys 2001, 89, 2756.