Miniproject for FMI 040

Polymers and Molecules for Organic Electronics

By

Jingjing Chen

Mohammad Kamruzzarnan Chowdhurry

Bo Zhang

Teng Wang

Supervisor:

Thorvald Andersson

Date: 2006/05/16

Physics and chemistry of organic electronics 1. Basic concept Polymers have traditionally been considered as insulating materials; however the serendipitous discovery of highly conducting polyacetylene marked the birth of a new field: organic (molecular) electronics [1]. Organic electronics are defined as organic materials in electronics [2], which can be classified as organic semiconductor, organic optical conductor, organic electric conductor and supper conductor, conducting polymer, organic devices, and etc. For the past forty years organic materials have been essential to the unparalleled performance increase in semiconductors, storage and displayers at consistently lower costs. However, the majority of these organic materials is either used as sacrificial stencils (photoresists) or passive insulators, and takes no active role in the electronic functions of a device. They do not conduct current to act as switches or wires, and they do not emit light [3]. To meet the requirements for the high density and high speed in electronics, which the current inorganic materials become incapable, nontraditional materials such as conjugated organic molecules, short-chain oligomers, longer-chain polymers, and organic-inorganic composites are being developed that that emit light, conduct current, and act as semiconductors. 2. Electrical transport in organic materials The mechanism of electrical conducting of organic materials is different from inorganic materials. The free electrons, ions, and vacancies in inorganics are replaced by charges (electron polarons and hole polarons, which are referred to simply as electrons and holes). Like inorganic semiconductors, organic semiconductors (OS) can be doped to produce conduction of positive charges (holes) or negative charges (electrons). A hole can most easily be thought of as an absence of an electron in the valence band of an atom. Electrons from neighboring atoms jump from atom to atom, effectively moving the hole. The organic compounds have “conjugated backbones” that are able to conduct the charges, that is, the molecular forms double and single-electron bonds, which allow charges to move along the backbone and through the crystal, much like charge moves through silicon. 2.1 Electronic structure of organics It has been pointed that the ability of organic materials to transport charges (holes and electrons) due to the π-orbital overlap of neighboring molecules provides their semiconducting and conducting properties. The recombination of the charge carriers under an applied field can lead to the formation of an exciton that decays radiatively to produce light emission [3].

2.1.1 π-Bonding [4] Molecular orbitals are obtained by combining the atomic orbitals on the atoms in the molecule. An example for a molecular building block is the Benzene molecule. The Benzene consists of six Carbon atoms (Fig. 1). Each Carbon atom is bond to two other C atoms and to a Hydrogen atom. The valence atomic orbitals of Carbon can be combined to form hybrid atomic orbitals, which are all energetically equivalent. In the case of Benzene only two π-orbitals hybridize with the s-orbital, which is called sp2−Hybridisation: three identical hybrid orbitals lying on a plane with an angle of 120 between them (Fig. 2). In Benzene the three hybrid orbitals form a covalent bond to the neighboring C atoms and to the Hydrogen atom. The molecule is therefore planar. The fourth electron sits on the remaining non hybridized π-orbital, perpendicular to the molecular plane. Each of these π-orbitals overlaps with the neighboring ones. This overlap results in a so called π-bonding. The electrons involved in the π-bonds are not strongly tight to a specific atom and can be considered delocalized and in this way a negative cloud above and below the molecule is formed (Fig. 2). The same type of delocalization is found on chains formed by interconnected benzene rings. These chains are called oligomers because they are synthesized by joining one molecular unit (for example benzene) to itself a few times. And they are called conjugated because the lowest energy π-orbital extends uniformly over the whole molecule. Such orbitals can provide channels that permit the transport of additional electrons from one side of the molecule to the other when a bias is applied. Such conductive structures are called Tour wires, from the name of the chemist that first synthesized them. The possibility to conduct currents remain also when other groups are inserted between the benzene rings, as long as the conjugation among the π-bonded components is maintained.

Fig. 1. Benzene molecular

Fig. 2. Each sp2 hybridized C in the ring has an unhybridized π-orbital perpendicular to the ring which overlaps around the ring [5].

2.1.2 LUMO and HOMO In solid state physics, the energy sub bands (bonding and antibonding) are achieved by adding atoms to one another in all three dimensions forming for example a crystal. Here, in the case of oligomers, it is very similar to solids. Every time an atom, for example a carbon atom, is added to the chain, the number of discrete energy sub bands is increased. The resulting energy gap is the energy between the so called HOMO (Highest Occupied Molecular Orbital) and the LUMO (Lowest Unoccupied Molecular Orbital) energy levels (about 3eV, Polyphenylene chain. According to this paper the Polyphenylene chain acts like a semiconductor. In comparison to semiconductor physics the conduction band is called LUMO and the valence band is called HOMO. 2.2 Electrical transport [6] Charge transport in organic materials is known to substantially differ from that in conventional inorganic semiconductors. In organic materials, molecules are held together by weak van der Waals forces; accordingly, valence and conduction bands are narrow, and transport in delocalized levels unlikely. It has been pointed out that transport in organic semiconductor occurs via small polaron hopping [7] the small polarons move via thermally activated hopping to neighboring sites, resulting in a simple activated dependence of the mobility with temperature. [8]. At very low temperature, the transport mechanisms in well-ordered materials can be described in terms of band-like motion. The current flows under the form of either polarons delocalized over several chains or free carriers. The total valence and conduction bandwidths built from th e interaction of the HOMO and LOMO levels are then the key parameters governing the hole and electron mobilities. At room temperature, the charge carriers are expected to be localized over a single unit (due to the strong electron-phonon coupling underlined above); this leads to geometry relaxation under the form of localized positive or negative polarons. The transport mechanism can then be described in term of sequential jumps of the relaxed charges between adjacent chains (also referred to as thermally activated polaronic hopping process). 3. Function mechanisms of organic semiconductors 3.1 Organic light emitting diode (OLED) 3.1.1 OLED structure

Organic materials have many excellent properties such as high photoconductivity, electroluminescence (EL) and nonlinear optical effects, and so on. EL devices fabricated using organic thin film has gained increasing attention since Tang and van Slyke [9]. An organic light-emitting diode (OLED) is a thin-film light-emitting diode (LED), in which the emission layer is an organic compound. OLED technology is intended primarily as picture elements in practical display devices (Fig. 3).

Fig. 3. Schematic cross section (not to scale) of a typical, single heterostructure organic light-emitting device (OLED) [2]. To a first approximation the working principle, as shown as Fig. 4, can be described by electrons and holes which are transported within the molecular film (electrons in the LUMO and holes in the HOMO level) where they recombine radiatively, this effect is called electroluminescence. In order to inject electrons from one side and holes from the other, different metals (i.e. with different work function) are used for the two contacts. Injection of the electrons and holes occurs due to emission over the barrier or tunnelling across it. Because holes are much faster than electrons in organic materials, and the electrons also have to surmount a higher barrier, there is an imbalance in the current contribution in favour of the holes. This translates in poor efficiency. To overcome this problem one uses a metal for the electron injection contact with low work function and different organic layers for electrons and holes. By using two different molecular films one creates a barrier for holes due to the lower HOMO level of the electron transport layer. In the same way a barrier for the electrons is got. These barriers create an accumulation of electrons and holes at the heterostructure interface which results in a higher efficiency [4].

Fig. 4. Working principle of OLED [10].

3.1.2 OLED materials The organic materials for OLED can be conjugated polymers and small molecules, as shown in Table 1.

Table 1. Organic materials for OLED [11]

Conjugated polymers Small molecules

PPV poly(p-phenylene vinylene)

Alq or Alq3 tris-(8-oxyquinolato) aluminum

MEH-PPV poly[2-methoxy-5-(2’-ethyl-hexoxy)-1,4-phenylene vinylene]

Bebq2 bis-(10-oxybenzo[h]quinolato)beryllium

P3AT poly(3-alkylthiopphene)

Eu(TTFA)3Phen tris-(4,4,4-trifluoro-1-(2-thienyl)-1,3-butanediono)-1,10-phenanthroline europium (III)

PPy poly(p-pyridline)

DPVBi (1,4´-bis-(2,2-diphenylvinyl)biphenyl

CN-PPV (R1=R2=C6H13) Poly[2,2-bis(hexyloxy)-1,4-phenylene-(1-cyanovinylene)

PBD 2-(4-biphenyl)-5-(4-t-butylphenyl)-1,3,4-oxadiazole

DPOP-PPV (R=phenyl) poly(1,4-phenylene-1,2-diphenoxypheny vinylene)

TPD or TAD N,N´-diphenyl-N,N´-bis-(3-methylphenyl)-(1,1´)-biphenyl-4,4´-diamine

PDPA poly(diphenyl acetylene)

NPB N,N´-diphenyl-N,N´-dinaphthyl-(1,1´)-biphenyl-4

,4´-diamine

DHO-PPE (R1=R2=hexyl) poly[1,4-(2,5-dihexoxy)-phenylene ethynylene)

MTDATA

PPP poly(para-phenylene)

spiro-2P 9,9´-spirobifluorene

LPPP “laddered poly(para-phenylene)”

perylene

PDAF poly(9,9-dialkylfluorene)

quinacridne

PPCP pentaphenylcyclopentadiene

DCM 4-(dicyanomethylene)-2-methyl-6(p-dimethylaminostyryl)-4H-pyran

rubrene

3.2 Organic thin-film transistor (OTFT) [12-16] Another application newly developing is the organic thin film transistor (Fig. 5). The doping of organic semiconductors is different from their inorganic counterparts in that organic compounds are oxidized to produce a scarcity of electrons (p-type) or reduced to an excess of electrons (n-type). There has been difficulty with developing n-type organic semiconductors that are air stable, because the oxidizing nature of the atmosphere tends to de-dope the electron rich n-type semiconductor, that is, atmospheric oxygen combines with the electrons in the semiconductor and reduces the doping level. However, there has been some recent success in creating air stable n-type semiconducting materials. Both types of semiconductors used together will allow for creation of CMOS-type circuits, with all the low power advantages of that technology.

Fig. 5. A 256-transistor array produced by Lucent using a rubber stamp printing process [12]

An OTFT is analogous to its inorganic counterpart in basic design and function. It is a three-terminal device, in which a voltage applied to a gate electrode controls current flow between a source and drain electrode under an imposed bias. A basic schematic is shown in Fig. 6, where Vg and Vds are the applied gate and source-drain voltages, respectively. A charge (usually positive) is injected into the semiconducting layer from the source electrode and charges generated, termed polarons, are swept to the drain electrode under the influence of a voltage applied at a third electrode, the gate. In such a configuration the device is capable of current amplification, i.e., small changes in the applied gate voltage can produce large increases in current passing between the source and grain. The effectiveness of the device is limited by the ability of the semiconducting layer to transport charge, a parameter termed the field effect mobility.

Fig. 6. Basic schematic of a field-effect transistor [13].

The mobility, µ, describes how easily charge carriers can move within the active layer under the influence of an electric field and is, therefore, directly related to the switching speed of the device. This parameter can be extracted from current-voltage measurements, and would ideally be as large as possible. Typical values range from 0.1-1 cm2/V.s for amorphous-Si (a-Si) devices, with the best organic materials achieving mobilities of 1-10 cm2/V. The on/off ratio, defined as the ratio of the current in the “on” and “off” states, is indicative of the switching performance of OTFTs. A low off current is desired to eliminate leakage while in the inactive state. Ratios as high as 106-suitable for most applications-can be reached by current-generation OTFT. 3.3 Materials for thin film transistor The materials used for OTFT are controlled by their charge mobility. Table 2 lists the highest field-effect mobility (m) values measured from OTFTs as reported in the literature, annually from 1986 through 2000 and separately for each of the most promising organic semiconductors. Table 2. Highest field-effect mobility (m) values measured from OTFTs as reported in the literature annually from 1986 through 2000 [16].

*Values for Ion/Ioff correspond to different gate voltage ranges and thus are not readily comparable to one another. The reader is encouraged to read the details of the experiments in the cited references. †This result has not yet been reproduced. Deposition Methods 1. Integrated Thin Film Deposition System [17] The present interest in use of organic thin films in optoelectronics stems from many technological benefits intrinsic to these materials. Organic thin film are simple to grow over both small and large areas, and easy to integrate with both conventional technologies and less conventional materials such as flexible, self assembled, or conformable substrates. Although functional use of organics has been demonstrated in the form of light emitters, photodetectors, optical elements, and active electronic logic components, many basic electronic and optical

properties of these solids are still not well understood. Much research is needed. Similarities with conventional inorganic semiconductors provide a physical framework for further investigations, but a large number of phenomena in organic materials have no analogy and require development of novel physical concepts. The completed growth system will integrate the method for physical and vapour phase deposition of hybrid organic/inorganic thin-films with a low-pressure RF/DC sputtering chamber, an evaporative growth chamber, and a chemical vapour deposition chamber. The completed vacuum system will be capable of depositing molecular organics, polymers, metals, metal oxides, inorganic nanodots, and colloids in a controlled layer-by-layer fashion. An in-situ shadow masking system will enable fabrication of complex patterned structures inside a vacuum environment, while the integrated N2-filled, dry glove box will facilitate handling, measuring, and packaging of organic thin film samples that are susceptible to reactions with atmospheric oxygen and water vapour.

Fig. 7. In-situ growth and testing of multilayer structures and devices

Fig. 8. In-situ shadow masking stage

Fig. 9. A simple deposition chamber 2. Vacuum-grown π-architectures Sublimation in a vacuum environment has been used to produce highly ordered films by varying different experimental conditions. The temperature of the substrate during the process of sublimation and post-treatments such as thermal annealing turned out to be crucial steps for the improvement of the degree of molecular order on the micrometer scale for oligo-thiophene films grown by sublimation in high vacuum and studied by AFM. Making use of this vacuum sublimation procedure, and keeping low deposition rates, layers consisting of monocrystalline domains with lateral sizes on the hundreds of nanometres scale were produced. Sub-monolayer quantities of conjugated species have been deposited in UHV on metallic surfaces and the tip of a STM has been employed to manipulate single molecules in situ, inducing conformational molecular switches (Fig. 10). This represents a step forward towards the fabrication of molecular tunnel-wired nano-robots.

Fig. 10. A single-molecule rotor operating within a supramolecular bearing at room temperature. (A), (B) STM images of hexa-tert-butyl decacyclene molecules at just below monolayer coverage on Cu (100). (A) In a nanoscopic void, the central molecule is imaged as a six-lobed structure (see the circle) and is immobilized at a high-symmetry site defined by the surrounding molecules. In (B), a lateral translation by one Cu lattice spacing (0.26 nm) shifts the molecule to a lower symmetry site where it is imaged as a torus, indicating rotation (image area: 5.75 nm by 5.75 nm). (C), (D) Snapshots of the molecular mechanical simulations based on the molecular coordinates taken from the STM data. The central rotor was rotated to compute the rotational barriers in the fixed and rotating states, which were found to be above and below room temperature, respectively. 3. Self-assembly behaviour of oligo-thiophenes [18] It is intriguing to compare the self-assembly behaviour of oligo-thiophenes bearing different side substituents. While the alkyl side-groups induce the formation of lamellae characterized by a staggered arrangement of the single molecules (Fig. 11(a)), the addition of hydrogen-bond-forming groups in the α- and ω-positions of the main chains can be used to design a precise stacking of the functionalised oligo-thiophenes enabling a notable increase of the intermolecular overlap of the π-orbitals (Fig. 11(b), (c)).

Fig. 11. STM images at the graphite–solution interface of (a) 1 in trichlorobenzene (the inset shows an enlargement) and (b) 2 in 1-octanol. The red arrow in (b) indicates the location of the thiophene rings within a lamella. The yellow arrows indicate the location of the urea groups. (c) Shows the packing model for (b). 4. Work-function modulation [19] When a gas molecule enters the semiconductor it can partially transfer electronic charge in a process that is analogous to the formation of charge-transfer complexes between electron donor and acceptor molecules. The amount and distribution of shared charge is governed by the ‘donacity’ of the molecule40 and by the affinity of the CP for electrons, which is given by its work function. The work function consists of two terms, the work needed to bring an electron from the bulk of the material up to the surface dipole layer (chemical potential), and the energy of extraction of an electron through the surface dipole layer up to the vacuum level.

Fig. 12. Comparison between the work-functionresponses of conducting polymer layers todifferent vapours. Initial work function (versusAu reference) was adjusted electrochemicallyto different initial values44.

Fig. 13. Conducting polymers in field-effect transistors. a, Thin film transistor; b, insulated gate field-effect transistor. Mobility [20] Fig. 13(a) and (b) respectively show the dependence of field-effect mobility m on the charge

per unit area on the semiconductor side of the insulator QS, and the gate field E. The solid circles correspond to a pentacene-based device with a 0.12-mm-thick SiO2 gate insulator thermally grown on the surface of a heavily doped n-type Si wafer that acted as the gate electrode. The open circles correspond to a similar device with a 0.5-mm-thick SiO2 gate insulator. The mobility for the SiO2-based devices is calculated in the saturation regime using a gate sweep, and is then plotted versus the maximum VG used in each gate sweep. The mobility increases linearly with increasing QS and E and eventually saturates.

Fig. 14. Dependence of field-effect mobility on (a) the charge per unit area on the semiconductor side of the insulator QS, and (b) the gate field E for pentacene OTFTs with different gate insulators. By growing amorphous films of pentacene, which is achieved by keeping the substrate temperature low during deposition, we make a film that is practically insulating. When the substrate temperature is kept at room temperature during deposition, a very well-ordered film is deposited, and the mobility measured at room temperature is very high for an organic semiconductor. When a mixture of the thin-film phase and the single-crystal phase is grown, the mobility is very low, possibly because of the high defect concentration resulting from the coexistence of the two phases. Pentacene transistor drain–source contacts can be made in one of two configurations top contact and bottom contact. The performance of pentacene devices with the bottom-contact configuration is inferior to that of devices with the top-contact configuration. Consequently, most high-performance pentacene TFTs reported in the literature have the top contact configuration, and shadow masking is generally used to pattern the source and drain contacts on top of the pentacene. This is a process that cannot be used in manufacturing.

Energy level 1. The Electronic Structure of an Organic Solid [21] We start the examination of the basic concepts with the electronic structure of a hydrogen atom (Fig. 15(A)). The ordinate is the electron energy. The potential well is the Coulombic potential by the atomic nucleus. Various atomic orbits (AOs) are formed in this well, and an electron occupies the lowest 1s orbital. The horizontal part of the potential well is the vacuum level (VL), above which the electron can escape from the atom. Fig. 15(B) shows the electronic structure of a polyatomic molecule or a single polymer chain. The effective potential well of an electron is formed by the atomic nuclei and other electrons. The wells of the nuclei are merged in the upper part to form a broad well. Deep AOs are still localized in the atomic potential well (core levels), but the upper AOs interact to form delocalized molecular orbits (MOs). The outermost horizontal part of the potential well is again the VL. The energy separations from the highest occupied MO (HOMO) or lowest unoccupied MO (LUMO) to the VL are the gas phase ionization energy (Ig) or the electron affinity (Ag) of the molecule, respectively. When molecules or polymer chains come together to form an organic solid, the electronic structure becomes like Fig. 15(C). Since the molecules interact only by the weak van der

Waals interaction, the top part of the occupied valence states (or valence band) and the lower unoccupied states (conduction band) are usually localized in each molecule, with narrow intermolecular band widths of < 0.1 eV. Thus the electronic structure of an organic solid largely preserves that of a molecule or a single chain, and the validity of usual band theory (which assumes itinerant electrons) is often limited. The top of the occupied state and the bottom of the unoccupied state are often noted as HOMO and LUMO, reflecting the correspondence with the molecular state. The situation in Fig. 15(C) is often simplified to those in Fig. 15(D) and Fig. 15(E). Although the VL in Fig. 15(E) is shown as if it is inside the solid, it is actually on the outside. In Fig. 15(C)-(E), the Fermi level is also indicated (EF). Since the electrons fill the energy levels following the Fermi statistics, the concept of Fermi level is always valid.

(A) (B)

(D)

Simplified

(C)

(E)

Fig. 15. (A) Hydrogen atom. (B) Polyatomic molecule. (C) Organic solid. (D) (E) Simplification of (C).

Doping 1. Background [23] The discovery that polyacetylene can be doped after synthesis, at room temperature, and with a variety of dopants, opened the field of conducting polymers. Subsequently, the doping methodology was generalized and applied to a large number of conjugated systems. The ability of these π-electron polymers to be reversibly doped at room temperature, and after synthesis, makes them fundamentally different from conventional covalent semiconductors. The doping of conductive polymers implies (1) charge transfer (by oxidation, p type, or reduction, n type), (2) the associated insertion of a counter ion (for overall charge neutrality), and (3) the simultaneous control of the Fermi level or chemical potential. Through doping the electronic and optical properties of conducting polymers can be controlled over the full range from insulator to metal. 2. Why doping--Application of doping [22] Doping plays an important role in semiconductor device applications, and the possibility of doping in organic electronics has been extensively explored recently. For example, doping of conjugated polymers can increase the conductivity by many orders of magnitude. Furthermore, doping has also been widely used in organic light emitting diodes (OLEDs), and proven to be one of the most effective methods to improve the performance of the devices. There have been numerous examples of using dopants to control the luminescence of the emitter and to enhance the stability. In addition, doping can be used to modify the properties of the metal–organic interface by adjusting the energy level alignment. Despite these exciting observations, the fundamental relationship between the guest and host energy levels is not yet clearly understood. 3. Doping methods [23] The solution doping method is convenient. For p-type doping, the following reaction may be used:

xyy

gasxy PFCHONCHPFNO ])()[()()()(2 6222/6−+−+ +→+ .

In this (or similar) reaction, the is the oxidizing agent, and the counter ion enters

the structure to assure charge neutrality; the reaction is carried out in a suitable solvent. For n-type doping, the following reaction may be used:

+NO )( 6−PF

xy

yneutralx CHNaNaphthCHNaphthNay ])([()()())(( −+−+ +→+ .

The act as the reducing agent and the as the counter ion. )( −Naphth +Na

In the case of electrochemical doping, the reaction is carried out in an electrochemical cell with the polymer as one electrode. The polymer is reduced or oxidized (i.e., electrons are added or removed) electrochemically through the external circuits. Electrical neutrality is provided by an ion from the electrolyte. For example, polyacetylene can be used as one electrode and Li metal as the counter electrode, with Li+ClO4

- as the electrolyte (dissolved in a suitable solvent). The electrochemical method is the most elegant of the doping methods, for it is reversible, it allows control at the resolution of parts per million, and it makes possible a novel experimental methodology based on in situ experiments. Moreover, the doping is accomplished by directly controlling the electrochemical potential µ of the polymer (the cell voltage is precisely µ measured with respect to the reference electrode). Since the doping involves first-order phase transitions, both electronic and structural, it is useful to control µ rather than the dopant concentration whenever possible. Organic/Metal and Organic/Organic Interfaces The interfaces between organic/metal and organic/organic materials can be found in all organic semiconducting devices, such as organic thin-film transistor (OTFT, shown in Fig. 16.) and organic light emitting diode (OLED, shown in Fig. 17.), and they determine the functions of the devices in many cases. Therefore, these interfaces have been intensively investigated. In this section, the basic knowledge of organic/metal and organic/organic interfaces is generally introduced and discussed.

Fig. 16. Schematic of organic semiconducting p-type thin-film transistor with top contacts. [24]

Fig. 17. Schematic of typical organic light-emitting diode. [24]

The review by K. Seki et al. [21] provides an understanding of the basic properties of organic/metal and organic/organic interfaces. To study the interfacial electronic structure, the following important factors have been considered. The first one is the energy level alignment at the interface. Fig. 18. (a) illustrates the electronic structure of a contact of a metal and thin organic solid layer assuming common vacuum levels (VLs) at the interface. Actually, a dipole layer may be formed at the interface due to charge transfer across the interface, redistribution of electron cloud, interfacial chemical reaction, and so on. As a result, there is a shift of virtual VL Δat the interface. Band bending should also be considered if the organic layer is relatively thick within the interface. In order to achieve the electrical equilibrium with the alignment of the Fermi levels of the two sides, the charges in the organic layer are distributed, and thus make the energy levels bent. This band bending produces the built-in potential Vbi with in a diffusion layer of thickness W. The interfacial energy diagram with band bending is shown in Fig. 19. (a) and (b), corresponding to Fig18. (a) and (b) respectively. Besides these conceptual aspects, there are also some practical factors affecting the interfacial electronic structure, including the possible chemical reaction and diffusion at the interface, and the atmosphere of the experiments. The discussion above are based on a organic/metal interface, but can be easily extended to organic/organic interfaces.

(a) (b)

Fig. 18. a) Schematic representation of electronic structure of a contact of a metal and thin organic solid layer assuming common vacuum levels (VLs) at the interface. b) Interfacial energy diagram with a shift of VL Δ at the interface due to the dipole layer formation. [21]

(a) (b)

Fig. 19. Interfacial energy diagram with band bending. [21] Ultraviolet photoemission spectroscopy (UPS), in which a sample is irradiated with high energy monochromatic light and the energy distribution of emitted electrons (UPS spectrum) is measured, is a common and powerful technique for studying the electronic structure of material and can give a direct picture about the alignment of the electronic structure of an interface. We will not introduce the details of UPS in this article, but focus on the results measured by UPS method. The interfacial energy diagram of Alq3/Al interface is shown in Fig. 20 (a) and (b) is the traditional way of estimation. The VL lowering is in the range of 1.0 eV and the value of εv

F is in the range of 2.7 eV. The energy diagrams of the model interfaces for the Al/Alq3/TPD/ITO device obtained by UPS are illustrated in Fig. 21. More results about various interfaces are presented in Fig. 22 by I.G. Hill et al. [25].

(a) (b)

Fig. 20. The interfacial energy diagrams of Alq3/Al interface obtained from a) UPS and b) the traditional way of estimation. [21]

Fig. 21. Energy diagrams of the model interfaces for the Al/Alq3/TPD/ITO device obtained by UPS. [21]

Fig. 22. The alignment of molecular levels at 12 organic/organic heterointerfaces. [25]

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