electrode interfaces in light emitting devices

Current Applied Physics 1 (2001) 98±106

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Photoelectron spectroscopy study of the energy level alignment at polymer/electrode interfaces in light emitting devices Grzegorz Greczynski a, Thomas Kugler b,*, William R. Salaneck a a

Department of Physics, Link oping University, S-561 83 Link oping, Sweden b ACREO AB, Bredgatan 34, S-602 21 Norrk oping, Sweden Received 28 August 2000; accepted 8 September 2000

Abstract The band alignment at the interface between electroluminescent polymers and the electrodes in polymer-LEDs was studied using photoelectron spectroscopy. Chemical factors like the formation of InCl3 during conversion of precursor-PPV on ITO could be directly monitored with XPS. Films of electroluminescent polymers were studied on a range of ITO and metal electrodes with di€erent work functions, as well as with an intermediate, electrically conducting polymer layer, using UPS. Furthermore, the study of the band alignment at polymer electrode interfaces was extended to three-layer structures: the results con®rm the common assumption that the potential drop over the polymer layer in polymer LEDs is equal to the di€erence between the electrode work functions. Ó 2001 Elsevier Science B.V. All rights reserved. PACS: 73.20; 79.60; 71.20.R; 79.60.F; 73.61.P; 78.60.F; 71.20 Keywords: Interface; Photoelectron spectroscopy; ITO; Electrode; Electroluminescence; Polymer; Band alignment; LED

1. Introduction In electronic devices based on organic materials, and for light emitting devices (LEDs) in particular, the characteristics of charge carrier injection from the metal contacts into the electroluminescent polymer layer is a crucial issue for the overall performance. However, the details of the energy level alignment at interfaces between organic layers and metal electrodes is still a subject of controversy [1]. The discussion focuses on two classes of organic materials: so-called ``small molecules'', typi®ed by tris(8-hydroxyquinoline) aluminum, or ``Alq3 '' [2]; and conjugated polymers, typi®ed by poly(p-phenylenevinylene), or ``PPV'' [3,4], and its soluble substituted derivatives. The interfaces between small molecules and metallic or metal-oxide substrates, studied using ultraviolet photoelectron spectroscopy (UPS), have been discussed at length in the recent literature, where particular attention has been paid to the relationship of the energy barrier for hole injection as a function of the work function of the metal substrate [5±7]. Work in this area

*

Corresponding author. E-mail address: [email protected] (T. Kugler).

has mostly focused on interfaces prepared under ultra high vacuum conditions (UHV), by means of the vapor deposition of an organic over-layer onto ultra-clean metallic substrates, thereby minimizing possible contamination. It has been shown that these ``ideal'' interfaces do not necessarily follow the Schottky±Mott rule, which means that there is no perfect alignment between the vacuum levels of the organic ®lm and the metal substrate. This emphasizes the importance of factors giving rise to the formation of interfacial dipole layers, such as charge transfer through the interface, interfacial electronic states, chemical interactions, and image forces, among others. In the case of conjugated polymers, the spin coating deposition process (implying the presence of a solvent and air) results in an unavoidable passivation of the polymer±substrate interface. Although this makes the situation rather complex from the point of view of fundamental studies, the information obtained on such real-life interfaces is directly applicable to the analysis of the performance of actual devices. PPV and its derivatives are some of the most promising and common materials as the active luminescent component in organic light-emitting diodes (LEDs). High-quality thin ®lms of PPV are usually prepared by a precursor route involving a solution processable

1567-1739/01/$ - see front matter Ó 2001 Elsevier Science B.V. All rights reserved. PII: S 1 5 6 7 - 1 7 3 9 ( 0 0 ) 0 0 0 1 9 - 5

G. Greczynski et al. / Current Applied Physics 1 (2001) 98±106

sulphonium salt polyelectrolyte (unsubstituted PPV is insoluble) [8]. A solution of the precursor is spin-coated onto the ITO electrode and converted to PPV at elevated temperatures (near 200°C) in inert atmosphere or vacuum. During the conversion process, tetrahydrothiophene and HCl are eliminated from the precursor polymer, yielding fully conjugated PPV. Due to the strong acidity of HCl, this may result in chemical interactions with the ITO-substrate [9], leading to the formation of indium chloride (InCl3 ), which was reported to act as a p-type dopant [10]. Recent improvements in device performance often involve the insertion of an ultra-thin, conducting polymer layer between the hole-injecting ITO electrode and the electroactive (i.e., PPV) layer in polymer-based light emitting devices [11]. It is, therefore, important to understand the nature of the chemical and electronic interactions at the polymer/ITO interface, which, to a great extent, determine many of the device properties. This paper reviews the main results of our extensive X-ray photoelectron spectroscopy (XPS) and ultraviolet photoelectron spectroscopy (UPS) studies on a major issue associated with fabricating direct PPV-on-ITO interfaces: the formation of chemical dipoles and their in¯uence on the band alignment at the interface was studied for InCl3 ± a byproduct of the precursor-PPV conversion on ITO ± and for ultra-thin interfacial layers consisting of poly(4-styrenesulfonate), denoted PSSH. Furthermore, a carefully chosen set of di€erent polymers and electrode materials has been used to illustrate the basic alignment types encountered at polymer-onmetal interfaces: the examples that have been studied in this context are semiconducting poly(bis-(2-dimethyloctylsilyl)-1,4-phenylenevinylene), or bis-DMOS±PPV [12] on indium tin oxide (ITO) substrates, semiconducting poly(9,9-dioctyl¯uorene) (PFO) on a range of metal substrates [13] (see Fig. 1), and electrically conducting poly(3,4-ethylenedioxythiophene) doped with poly(4styrenesulfonate) [14], denoted PEDOT±PSS (see Fig. 1). In particular, by exposure to X-radiation, the work function of ITO was changed in a reproducible way in situ, in order to follow the in¯uence of changes in work function of the substrate on the band edge parameters of the semiconductor in a single sample. Finally, the photoelectron spectroscopy study of the energy band edge alignment was extended to three-layer metal/semiconductor/metal structures, representing the full LED-type architecture for single-emissive-layer devices, using carefully chosen, multi-layered structures, namely PEDOT±PSS/PFO/Au and PEDOT±PSS/PFO/Al. 2. Experimental The experimental procedures have been detailed in previous publications [12,13].

99

Fig. 1. Chemical structures of poly bis-(2-dimethyloctylsilyl)-1,4phenylenevinylene), denoted bis-DMOS±PPV, poly(9,9-dioctyl¯uorene), denoted PFO, poly(3,4-ethylenedioxythiophene), denoted PEDOT, and poly(styrene sulfonic acid), denoted PSSH.

3. Results and discussion 3.1. Chemical species at the ITO/precursor-PPV interface The chemical species involved in the thermal conversion of precursor-PPV may be identi®ed from the XPS core-electron spectra of precursor-PPV, as shown for a thin ®lm on an oxidized Si(1 0 0) substrate: Fig. 2 shows the ®rst three scans over the binding energy region appropriate for studying the chlorine content. It displays the Cl(2p) components over a narrow binding energy range, where the experimental spectrum is ®tted with four peak components. The ®rst scan, at the bottom, was taken about 20 min after spin-coating of the precursor-®lm and insertion into UHV, and took about 20 min to accumulate. The remaining two spectra were taken at further intervals of approximately 20 min. Keeping in mind that the Cl(2p) features correspond to spin-split doublets, the peaks in the spectra may be identi®ed (from left to right) as follows: the high binding energy components of the Cl(2p) spectra correspond to eliminated HCl, and the low binding energy components to the precursor sulphonium chloride salt. It is clear that the content of the chloride anion (Clÿ ) decreases over the time scale of the experiment, while the content of hydrochloric acid (HCl) increases. In other words,

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Fig. 2. As-prepared ®lms of precursor-PPV on silicon: Narrow XPS scans of the Cl(2p) binding energy range.

Fig. 3. As-prepared ®lms of precursor-PPV on ITO: Narrow XPS scans of the Cl(2p) binding energy range.

already after less than 1 h at room temperature, a partial elimination of HCl from the precursor sulphonium chloride salt indicates the onset of the conversion process. Corresponding changes occur in the S(2p) photoelectron spectrum [9] (not shown here): a partial elimination of tetrahydrothiophene (low binding energy components in the spin-split S(2p) spectrum) from the precursor sulphonium chloride salt (high binding energy spin-split S(2p) components) can be observed already at room temperature in an as-prepared ®lm. Obviously, the four-peak spectrum may be ®tted well with two spinsplit components of given energy splitting and intensity ration. For precursor-PPV on ITO substrates, however, the situation is more complex. As shown previously [9], the uppermost ITO surface is covered with an insulating, oxygen-rich layer, presumably indium hydroxide. The detailed features in the Cl(2p) spectra for precursor-PPV spin-coated on ITO, shown in Fig. 3, are quite di€erent from those for a silicon substrate (Fig. 2): clearly, the intensities of the features may not be ®tted with only two spin-split components. A third set of spin-split components, open circles in the ®gure, is necessary in order to account for the observed intensity distribution. By comparison with reference spectra, the third and lowest intensity double is assigned to the chlorine in InCl3 . From the angular dependence of the Cl(2p) spectrum, it is clear that the InCl3 lies closer to the ITO substrate than the polymer surface; indicating that the InCl3 is actually formed at the ITO-polymer interface, from where it di€uses into the polymer ®lm. This agrees with

temperature-dependent impedance spectroscopy measurements and theoretical modeling performed by Scherbel et al. [10], who found that the bulk of PPV converted on ITO is composed of a highly doped region at the ITO interface and a region with lower doping at higher distances from the interface. Moreover, the boundary between these two regions is not sharp but there is a gradual change in dopant concentration. The occurrence of p-doping is in agreement with the observation that the Cl(2p) binding energies for a given chemical species is approximately 0.5 eV lower in the precursor-PPV ®lm on ITO as compared to the ®lm on SiOx . However, such a shift in the binding energies might also result from the formation of interfacial dipoles due to the protonation of the ITO surface by the HCl formed during the conversion process. 3.2. Band alignment at ITO/polymer interfaces In the following, we will focus on UPS investigations of how the band-alignment at polymer±ITO interfaces is in¯uenced by chemical species relevant in the context of polymer light emitting devices. Fig. 4 shows the energy level scheme and the corresponding experimental UPS intensity curves for bis-DMOS±PPV on a gold substrate, in order to establish the parameters to be discussed below. Note, in particular, the following: (a) The di€erence in kinetic energy between the fastest photoelectrons from the gold substrate and from the polymer over-layer corresponds to the o€set between the valence band edge in the polymer over-layer and

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In UPS measurements, it is important to clearly de®ne the signi®cance of the term ``vacuum level'': the vacuum level of a metal substrate as measured by UPS F (denoted as EVAC (Au) in Fig. 4) corresponds to the energy of an electron at rest just outside the sample [1]. The main ®ndings of our studies are discussed in the following: Fig. 5 shows a series of values of EFVAC , D, EFVB , and IP. The ITO work function ranges from 4.4 eV (method (a)), to 4.8 eV (method (c)). Even higher values were obtained for ultra thin layers of PEDOT±PSS, PSSH, and InCl3 on ITO: 5.0, 5.2, and 5.7 eV. For spin-coated bis-DMOS±PPV, the position of the vacuum level, EFVAC , follows the work function of the substrate, ranging from 4.4 eV (for ITO (a)) to 5.3 eV (for InCl3 -coated ITO). The o€sets, D; for bis-DMOS± PPV are very small ()0.2 eV in the average) and independent of the substrate work function. An important consequence of this vacuum level alignment is that an increase of the substrate work function results in a corresponding decrease of the difference between the valence band edge in the bisDMOS±PPV layer and the Fermi level of the ITO substrate: EFVB ranges from 1.6 eV for bis-DMOS±PPV on ITO (a) to 0.7 eV for bis-DMOS±PPV on InCl3 on ITO. This result is highly relevant in the context of polymer LEDs: since EFVB determines the barrier for hole injection (UpB ) from the ITO-electrode into the electroluminescent bis-DMOS±PPV layer, an increase of the ITO work function will reduce the barrier for hole injection, thereby following the Schottky±Mott rule for the case of Fig. 4. The band parameters which may be derived from UPS HeI spectra. The spectra shown are for a clean gold substrate (left) and bisDMOS±PPV on an identical gold substrate (right).

the Fermi energy in the conducting gold substrate, EFVB . It determines the barrier for hole injection from the substrate into the valence band of the polymer. (b) The energy di€erence between the vacuum level of the polymer over-layer and the Fermi energy in the conducting substrate, EFVAC , is EFVAC ˆ hm ÿ …Ekmax …BDMOS±PPV† ‡ EFVB †:

…1†

(c) The o€set D between the vacuum levels of the polymer coated and the uncoated gold substrate is de®ned as D ˆ EFVAC ÿ UAu ;

…2†

where in Fig. 4, UAu is the work function of the gold substrate. (d) The ionization potential IP for the polymer, as probed by UPS, is IP ˆ

EFVB

‡

EFVAC :

…3†

It is a speci®c property of the polymer and therefore independent of the energy level alignment at the interface.

Fig. 5. The band parameters EFVAC ; EFVB and IP (see text) of bisDMOS±PPV are shown as a function of the substrate work function. The substrates are: full circles ˆ ITO type (a); full triangles ˆ ITO type (b); full squares ˆ ITO type (c); open circles ˆ PEDOT±PSS on ITO; open triangles ˆ PSSH on ITO; and open diamonds ˆ InCl3 on ITO.

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a zero o€set ``D'' [15]. The ionization potential IP, as probed by UPS, corresponds to the sum of the distances of the valence band edge, EFVB , and of the vacuum level, EFVAC , from the Fermi level of the ITO substrate. As EFVB and EFVAC evolve strictly anti-parallel with changing substrate work function, their sum is constant (approximately 5.9 eV) and independent of the energy level alignment at the interface (see Fig. 5). By exposure to non-monochromatized X-rays (MgKa), the work function of ITO can be changed in situ. The e€ect can be used to follow the in¯uence of changing the work function of the substrate on the band edge parameters of the semiconductor in a single sample: Fig. 6 shows several examples of how the the position of the vacuum level, EFVAC , of the semiconductor over-layer (bis-DMOS±PPV) follows the changes in the work function of the ITO substrate, from exposure to Xrays, in situ. The changes in the work function, in each case, were determined by exposing two samples to the X-rays: one sample with the over-layer under study, and one control sample without any over-layer. There are three di€erent situations: (a) For bis-DMOS±PPV on ITO, and bis-DMOS± PPV on PSSH on ITO, the slope dEFVAC =dU is approximately 1.0 and the data points lie close to the diagonal in the EFVAC vs. U diagram. This indicates an almost perfect vacuum level alignment (D ca. )0.2 eV). The (weak) interfacial dipole layer is independent of the change of the substrate work function induced by irradiation with X-rays.

(b) For PSSH on ITO, the slope dEFVAC =dU is still close to 1.0. However, there is a large o€set D (+0.4 eV) from the diagonal in the EFVAC vs. U diagram. This indicates a modi®cation of the respective vacuum level alignment by the presence of a substantial interfacial dipole layer. A likely source for the formation of this dipole layer is the protonation of the ITO surface (PSSH is a strong acid). The abrupt change of the potential across the dipole layer [16] leads to an increase of the vacuum level of PSSH on ITO relative to the vacuum level of the bare ITO substrates (positive sign of D in Fig. 6). The presence of an interfacial dipole layer modi®es the barrier for hole-injection from the ITO substrate into the electroluminescent bis-DMOS±PPV layer, UPB …D† (see Fig. 4) UpB …D† ˆ IP ÿ …UITO ‡ D† ˆ UpB ÿ D:

…4†

(c) For PEDOT±PSS on ITO, and bis-DMOS±PPV on PEDOT±PSS on ITO, a completely di€erent behavior is observed. In both cases, the vacuum level EFVAC remains constant (within experimental accuracy) upon changing the work function U of the ITO substrate by X-ray irradiation. As a consequence, the o€set, D, for PEDOT±PSS on ITO changes from 0:0 to +0.5 eV, as seen in Fig. 6. In the case of PSSH on ITO, the o€set, D, remains unchanged upon irradiation with X-rays. Given the fact that PEDOT±PSS ®lms are made up of nanometer-sized grains covered with surface segregated PSSH [17], a modi®cation of D due to X-ray-induced changes of the chemical structure at the PEDOT±PSS/ ITO interface can be excluded. In contrast to PSSH, PEDOT±PSS is an electrically conducting polymer with a high concentration of free charge carriers. Therefore, the charge in the PEDOT±PSS layer arising from the Fermi level alignment with the ITO substrate is contained within a distance from the ITO interface much smaller than the thickness of the PEDOT±PSS ®lm (¯at band situation), thus resulting in the constancy of the vacuum level EFVAC observed by UPS. 3.3. Band alignment at metal/polymer interfaces

Fig. 6. The changes in EFVAC of bis-DMOS±PPV as the work functions of the di€erent ITO substrates are changed by irradiation with X-rays. The samples are coded as in the caption of Fig. 5. The gray diagonal line of unit slope corresponds to an ideal vacuum level alignment (Schottky±Mott limit).

Recently, we have demonstrated that UPS spectra  in may be taken on spin-coated PFO ®lms up to 1600 A thickness without any surface charging e€ects [13]. This means that photoelectron spectroscopy may be carried out on ®lms displaying the same thickness range as used in real devices [3]. Fig. 7 displays EFVAC , derived from UPS, for thick ®lms of PFO on four di€erent substrates, PFO/Al, PFO/ Si, PFO/PEDOT±PSS, and PFO/Au. We observed the same behavior as for the ultra-thin bis-DMOS±PPV ®lms on ITO: Increasing the substrate work function (from 3.9 eV for Al to 5.4 eV for Au) results in a corresponding increase of EFVAC for the PFO-coated substrates. The vacuum levels of the PFO ®lms and the

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F Fig. 7. EVAC versus substrate work function for PFO (black circles) and PEDOT±PSS (open circles). The long-dash diagonal represents the Schottky±Mott limit, whereas the horizontal line corresponds to a metallic-like behavior of the overlayer. The substrates used are: Al (3.9 eV), SiO2 (4.4 eV), PEDOT±PSS (4.7 eV), and Au (5.4 eV).

metallic substrates are aligned. In other words, PFO follows the Schottky±Mott rule, marked in the ®gure by the long-dash diagonal line. In this respect, the photoelectron spectroscopy measurements are in agreement with internal photoemission experiments, where the tracking of the metal work function by the Schottky barriers at the polymer/metal interface has been observed [18]. Again, in agreement with the previous experiments on ITO (see above), PEDOT±PSS ®lms on Au, Al, or Si substrates behave di€erently: EFVAC of PEDOT±PSS, is independent of the work function of the substrate. The results are displayed in the energy band diagram in Fig. 8: the vacuum levels of the substrate and the spincoated PFO ®lms are aligned (D  0) [12,13]. For metal substrates with work functions from 3.9 eV (for Al) up to 5.4 eV (for the present type of Au), the Fermi level of the metal substrate always falls within the forbidden energy gap of PFO (Eg ˆ 2.9 eV; Ip ˆ 5.9 eV). This fact has important consequences for charge transfer through the metal±polymer interface. First, with in the band gap of the PFO, there is a very low density of the extrinsic charge carriers, typically on the order of 1016 cmÿ3 [13,19]. Thus, charge transfer from the polymer to metal, if any, is very small, even if the Fermi energy of the metal should be situated below the impurity levels of PFO. Furthermore, electronic charges are stored in PFO (as in other conjugated polymers) in the form of bipolarons [20]. The negative bipolaron formation energy per particle estimated from the alkali metal doping of PFO is about 2.4 eV, relative to the valence band edge [21]. This is however only a lower limit in the present case,

103

Fig. 8. The energy level positions before establishment of electrochemical equilibrium. The positions of the Fermi levels of various metallic substrates used in this work are indicated to the left of the scheme. The energy band edges of PFO: conduction band (CB) edge and valence band (VB) edge are shown in the center. The diagram for PEDOT±PSS is repeated once more for clarity on the right-hand side of the ®gure.

since the presence of counter-ions (to supply electrons in doping) shifts the bipolaron energy levels deeper into the band gap. For both PFO/Au and PFO/Al, the Fermi level of the metal is situated below the negative bipolaron formation energy per particle (or above the positive bipolaron formation energy per particle), the value needed in order to ``®ll bipolaron states''. This implies that a charge transfer from the metal to polymer cannot occur [22]. Consequently, there is no electrostatic potential drop in a contact region, and vacuum level alignment results. In the absence of signi®cant charge transfer, the electrochemical potential in the polymer layer is determined by the work function of the substrate. It can be tuned within the limits given by the positive and negative bipolaron formation energy per particle (for PFO the latter is estimated to be roughly 2.4 eV from the VB edge). Beyond this limit charge transfer from metal to polymer is allowed, and the Fermi energy is pinned at a value corresponding to the bipolaron formation energy [22]. 3.4. Band alignment in three-layer PEDOT±PSS/PFO/ metal structures The experimental sequence for the photoelectron spectroscopy investigation of PEDOT±PSS/PFO/metal three-layer structures was as follows: First, XPS spectra of as-prepared thin ®lms of PFO-on-metal were taken. Next, UPS spectra of the PFO over-layers were recorded to deduce the postition of the vacuum level, EFVAC , and the valence band edge, EFVB , relative to the Fermi energy in the metal substrate. Finally, after depostition of the PEDOT±PSS layer, UPS as well as XPS (for precise

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evaluation of the core level binding energies of oxygen O(1s) and sulfur S(2p), the two unique elements present in PEDOT±PSS) were performed on the resultant PEDOT±PSS/PFO/metal three-layer structures. According to Fig. 8, the work function values for two of the substrates, Al and Au, span a wide range from 3.9 to 5.4 eV. This allowed us to study two distinct situations: (1) For vacuum level aligned PFO/Au, the high work function of the substrate forces the position of the electrochemical potential in the PFO layer to be close to the valence band (VB) edge (only 0.7 eV away). This means that when the PEDOT±PSS layer is applied, before equilibrium of the electrochemical potential has been established, the electrochemical potential of the PFO (on Au), at 5.2 eV, lies energetically below the Fermi level of PEDOT±PSS (4.7 eV). (2) For the PFO/ Al structure, the low-work function of the Al substrate brings the electrochemical potential of PFO higher up in the energy gap, as far as 2.0 eV away from the VB edge. As a consequence the electrochemical potential of the PFO/Al is now above the Fermi level of PEDOT±PSS. Thus the two extremes may be studied; two cases of the initial relative alignment of the parameters de®ning the equilibrium in both layers that are to be brought into contact. The experimental results for the Au substrates are displayed in Fig. 9 (the corresponding data for the Al substrates are in complete analogy, therefore they are discussed in the text without showing them explicitly in the ®gures). The sequence of the UPS He I spectra shows, from top to the bottom, the VB regions of the following samples: (A) PEDOT±PSS/Au, (B) PFO/Au and (C) PEDOT±PSS/PFO/Au, respectively. The ®rst two serve as references for the interpretation of the third. As shown in the inset in panel A, the Fermi edge of PEDOT±PSS is clearly resolved. It appears as a wellde®ned cut-o€ of the density of states exactly at zero binding energy (spectra recorded relative to Fermi level of the substrate). Panel B shows the VB region of PFO. Again, the inset displays the lowest binding energy part of the spectrum, the valence band edge. No ®nite density of states is observed near the Fermi energy of the substrate (0 eV in the ®gure), as expected for a semiconductor. Note that the thickness of the layers under  or more consideration were on the order of 1200 A which is far above the limit for the observation of any signal originating from the gold substrate. The UPS spectrum for PEDOT±PSS/PFO/Au is shown in panel C. The most important points are as follows: (i) The Fermi edge of PEDOT±PSS appears exactly at the same position as the Fermi energy of the substrate. This is a direct con®rmation of the fact that the electrochemical potential is constant across the whole three-layer structure; thus, the entire layered system is in equilibrium. (ii) The overall shape of the VB spectrum of PEDOT±PSS/PFO/Au is precisely the same as for PE-

Fig. 9. UPS spectra of the valence band region: (A) PEDOT±PSS/Au; (B) PFO/Au; (C) PEDOT±PSS/PFO/Au. Insets in the upper-right corners show the magni®cation of the binding energy region close to the Fermi level. Note especially the Fermi edge of PEDOT±PSS visible at zero binding nergy (panels A and C). All the spectra are referred to the Fermi level of the metal substrate.

DOT±PSS/Au. (iii) The vacuum level of PEDOT±PSS/ PFO/Al and PEDOT±PSS/PFO/Au is the same as for PEDOT±PSS/Au alone. In agreement with the UPS data, the XPS binding energies of sulfur S(2p) and oxygen O(1s) core levels of PEDOT±PSS are exactly the same for all structures: PEDOT±PSS/PFO/Au (PEDOT±PSS/ PFO/Al) and PEDOT±PSS/Au (PEDOT±PSS/Al), independent of the energy level alignment prior to the contact between PEDOT±PSS and PFO/Au (PFO/Al). The energy level diagrams for the three-layer structures as determined by photoelectron spectroscopy are

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Fig. 10. The energy level diagrams deduced from the photoemission measurements for PEDOT±PSS/PFO/Au (on the left) and PEDOT±PSS/PFO/Al (on the right).

displayed in Fig. 10: independent of the initial (before contact) position of the electrochemical potential in the polymer layer relative to the Fermi level of PEDOT± PSS, the alignment occurs over the three-layer structure. Consequently the potential drop across the polymer layer equals the di€erence between the work functions of the metal and the PEDOT±PSS contacts.

4. Conclusions Indium chloride (InCl3 ) can be spectroscopically observed at the polymer±ITO interface during the conversion process of precursor-PPV ®lms on ITO: HCl released in the conversion process interacts with the surface of the ITO-substrate, leading to the formation of indium chloride, which di€uses into the polymer. Two essentially di€erent types of behavior were observed for thin polymer layers on metal and ITO substrates, prepared by spin coating under ambient conditions: (1) The electronic structures of semi-conducting polymers and metallic substrates are essentially vacuum level aligned, the interface is almost in the Schottky±Mott limit. The electrochemical potential in the semi-conducting polymer over-layer is determined by the work function of the substrate. High ITO work functions result in small energy o€sets between the valence band edge of the semiconducting polymer layer and the Fermi energy in the ITO, thus reducing the barrier heights for hole injection into the semi-conducting polymer in polymer-based LEDs. (2) The second case is that of an electrically conducting polymer on a metallic substrate. PEDOT±PSS shows typically metallic behavior, in that the Fermi level of the conducting

polymer over-layer is aligned with that of metallic substrate by means of charge transfer through the interface. This charge transfer results in the formation of a contact potential, i.e., the electronic structure of the conducting polymer is not vacuum level aligned to the metallic substrate. The work function of the (metallic) PEDOT± PSS ®lm is independent of the underlying substrate, since it is an intrinsic property of the ``metal''. Thus, when an interfacial layer of a conducting polymer is inserted between the ITO substrate and a semiconducting polymer, the energy barrier to hole injection into the semiconducting polymer is determined by the work function of the conducting polymer. It is independent of the work function of the ITO substrate, provided that other substrate (ITO) parameters are kept constant. The three-layer structures PEDOT±PSS/PFO/Au(Al) allowed to investigate the energy level alignment in device structures composed of a semiconducting polymer sandwiched between metallic contacts with di€erent work function: in both cases, the electrochemical potential was aligned across the entire three-layer structure.

Acknowledgements The authors gratefully acknowledge Annika Andersson for performing the XPS-measurements on the conversion of precursor-PPV on ITO and silicon substrates. Cooperation with Cambridge Display Technology, especially for supply of the polymer materials, is gratefully acknowledged. This work was carried out under a Brite/ EuRam contract BRPR-CT97-0469, OSCA (project number 4438). In general, research on condensed molecular solids and polymers in Link oping is

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