chemical species at polymer/ito interfaces: consequences for the band alignment in light-emitting...

C. R. Acad. Sci. Paris, t. 1, Série IV, p. 409–423, 2000Surface, interfaces, films/Surfaces, interfaces, films(Physicochimie/Physical chemistry)

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Chemical species at polymer/ITO interfaces:consequences for the band alignmentin light-emitting devicesThomas KUGLER a, William R. SALANECK b

a ACRED AB, Bredgatan 34, S-60221 Norrköping, Swedenb Department of Physics, IFM, Linköping University, S-561 83 Linköping, Sweden

(Reçu le 28 janvier 2000, accepté le 3 mars 2000)

Abstract. The influence of chemical species present at the interface between the electroluminescentpolymer and the ITO electrode in light-emitting devices on the band edge energies ofoverlayers of semiconducting conjugated polymers has been studied using photoelectronspectroscopy. The formation of InCl3 during the conversion of precursor-PPV on ITOwas directly monitored with XPS. Ultrathin films of poly(bis-(2-dimethyloctylsilyl)-1,4-phenylenevinylene) were studied directly on ITO, as well as with an intermediate layerof an electrically conducting polymer using UPS. The initial work function of the ITOwas varied chemically from 4.4 eV to 4.8 eV. In addition, the work function of ITO waschangedin situ, within a given sample, by exposure to X-rays. For the polymer spin-coateddirectly on ITO, the vacuum levels are aligned. With the electrically conducting polymerblend, poly(3,4-ethylenedioxythiophene) doped with poly(4-styrene sulfonate) spin-coatedon ITO, the Fermi levels are aligned, as expected. Therefore, with a conducting polymerblend intermediate layer between the polymer and the ITO, the polymer bands align to thevacuum level of the conducting polymer blend on ITO, and the barrier to hole injection intothe polymer is determined by the work function of the conducting polymer blend insteadof the work function of the ITO. 2000 Académie des sciences/Éditions scientifiques etmédicales Elsevier SAS

XPS / UPS / precursor-PPV / bis-DMOS-PPV / PEDOT-PSS

Espèces chimiques aux interfaces polymère/ITO :conséquences sur l’alignement des niveaux énergétiquesdans les diodes électroluminescentes

Résumé. Nous avons étudié par spectroscopie de photoélectrons l’influence des espèces chimiquesprésentes dans les diodes électroluminescentes à l’interface polymère électroluminescent-ITO sur les énergies de bord de bande des polymères semi-conducteurs conjugués.La formation de InCI3 durant la conversion du précurseur du PPV sur l’ITO a étédirectement suivie par XPS. Des films ultra minces de poly(bis-(2-diméthyloctylsilyl)-1,4-phénylènevinylène) ont été étudiés par UPS après dépôt direct sur ITO, et égalementaprès dépôt sur une couche intermédiaire de polymère conducteur. Le travail de sortiede FITO a été modifié par voie chimique de 4,4 eV à 4,8 eV. De plus, le travail de sortiede FITO pour un échantillon donné, a été modifié in situ par irradiation aux rayons X.Les niveaux de vide sont alignés pour le polymère directement déposé par spin-coatingsur l’ITO. Comme attendu, les niveaux de Fermi sont alignés avec le polymère conducteur,

Note présentée par Guy LAVAL .

S1296-2147(00)00140-2/FLA 2000 Académie des sciences/Éditions scientifiques et médicales Elsevier SAS. Tous droits réservés. 409

T. Kugler, W.R. Salaneck ORGANIC ELECTROLUMINESCENCE

poly(3,4-éthylènedioxythiophène) dopé poly(4-styrène sulfonate), déposé par spin-coatingsur FITO. Par conséquent, avec une couche intermédiaire de polymère conducteur entrele polymère EL et FITO, les niveaux énergétiques s’alignent avec le niveau de vide dupolymère conducteur et la barrière à l’injection de trous dans le polymère EL est déterminéepar le travail de sortie du polymère conducteur et non par le travail de sortie de FITO. 2000 Académie des sciences/Éditions scientifiques et médicales Elsevier SAS

XPS / UPS / précurseur du PPV / bis-DMOS-PPV / PEDOT-PSS

1. Introduction

Poly(p-phenylene vinylene), or ‘PPV’, and its derivatives are some of the most promising and commonmaterials as the active luminescent component in organic light-emitting diodes (LEDs) [1–5]. Typically,a single layer polymer-based LED is fabricated by spin- or blade-coating a thin polymer layer on top of anoptically transparent, high work function, hole-injecting electrode, like indium-tin-oxide (ITO), followedby the deposition of a low work function, electron-injecting metal electrode, e.g., calcium (see figure 1).Such devices, encapsulated in glass and epoxy, are now reliably yielding operation times of over 7000 h(continuously driven in dc current drive, in air at room temperature) [6], and Idemitsu Kosan demonstrateda QVGA TV screen atAsia Display ’97with evaporated molecules.

High quality thin films of PPV are usually prepared by a precursor route involving a solution processablesulphonium salt polyelectrolyte (unsubstituted PPV is insoluble) [7,8]. In the fabrication of single layerlight emitting devices, typically, a solution of the precursor is spin-coated onto the ITO substrate and thenconverted to PPV at elevated temperatures (near 200◦C) in inert atmosphere or vacuum [7,8]. During theconversion process, tetrahydrothiophene and HCl are eliminated from the precursor polymer, yielding fullyconjugated PPV, as illustrated infigure 2. These byproducts will then diffuse out of the PPV film andbe pumped away. However, the strong acidity of HCl may result in chemical interactions with the ITO-substrate [9], leading to the formation of indium chloride (InCl3), which was reported to act as ap-typedopant [10]. The presence of such dopant species not only influences the electrical characteristics of thedevice, but may also cause considerable photoluminescence quenching [11].

Previous publications on the issue include an investigation using Rutherford back scattering, RBS, anddepth profiling with X-ray photoelectron spectroscopy (XPS) to demonstrate the correlation between thepresence of indium and the degradation of the electroluminescence spectra in polymer-based light-emittingdiodes [12]. Furthermore, capacitance-voltage measurements have indicated the doping of PPV films byspecies, which are formed by reactions of the conversion products of precursor-PPV (tetrahydrothiopheneand/or HCl) with the ITO substrate [11,13,14]. In addition, thermally stimulated current (TSC) techniqueshave been used to establish the existence of trap distributions in PPV light-emitting devices with ITO

Figure 1. Single layer polymer-based lightemitting diode: a thin layer of an

electroluminescent polymer is sandwichedbetween the hole-injecting electrode (ITO onglass) and the electron-injecting electron (e.g.,

calcium).

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ORGANIC ELECTROLUMINESCENCE Chemical species at polymer/ITO interfaces

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Figure 2. Reaction scheme for the thermal conversion of precursor-PPV to PPV.

as the hole-injecting electrode [13]. Finally, depth profiling of ITO/PPV/Al devices with secondary ionmass spectroscopy (SIMS) has shown enhanced chlorine and indium count rates throughout the entire PPVlayer [15].

Recent improvements in device performance often involve the insertion of an ultrathin, conductingpolymer layer between the hole-injecting ITO electrode and the electroactive (i.e., PPV) layer in polymer-based light emitting devices. It is therefore important to understand the nature of the chemical and electronicinteractions at the polymer/ITO interface, which, to a great extent, determine many of the device properties.

In general, however, the nature of the electronic band edge alignment at interfaces of organic materialson conducting substrates is still a subject of controversy [16,17]. The discussion focuses on two classes oforganic materials: so-called ‘small molecules’, typified bytris(8-hydroxyquinoline) aluminum, or ‘Alq3’[16,18–27]; and conjugated polymers, typified by poly(p-phenylenevinylene), or ‘PPV’ [28,29], and itssoluble substituted derivatives.

The interfaces between ‘small molecules’ and metallic or metal-oxide substrates, studied using ultravioletphotoelectron spectroscopy (UPS), have been discussed at length in the recent literature, where particularattention has been paid to the relationship of the energy barrier to hole injection as a function of thework function of the metal substrate [16,21,23,24,26,27]. In almost all cases, studies have been carriedout in UHV, with molecules deposited from the vapor phase on ultra-clean metal surfaces, which had beenprepared in UHV.

On the other hand, the interfaces of conjugated polymers on metals are less well understood, especiallysince ‘ideal’ interfaces are essentially impossible to prepare in the same way as for molecules on cleanmetal surfaces in UHV.

This paper reviews the main results of our extensive X-ray photoelectron spectroscopy (XPS) andultraviolet photoelectron spectroscopy (UPS) studies on a major issue associated with fabricating directPPV-on-ITO interfaces: the formation of chemical dipoles and their influence on the band alignment atthe interface. In addition, a carefully chosen set of different materials has been used to illustrate themain behavior types encountered in general polymer-on-ITO interfaces: the semiconducting conjugatedpolymer poly(bis-(2-dimethyloctylsilyl)-1,4-phenylenevinylene), orbis-DMOS-PPV (see figure 3), waschosen because of the ease of preparation of high-quality ultra-thin films from toluene solutions (seefigure 4), and the availability of ultra-pure material [30].

Interfaces ofbis-DMOS-PPV with carefully controlled surfaces of ITO, exhibiting a range of workfunctions, were investigated. In particular, by exposure to X-rays, the work function of ITO was changedin a reproducible wayin situ, in order to follow the influence of changes in work function of the substrateon the band edge parameters of the semiconductor in a single sample. Moreover, the work function ofthe ITO substrate was changed using any one of several chemical cleaning procedures. Also, the effect

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Figure 3. The chemical structure ofpoly(bis-(2-dimethyloctylsilyl)-1,4-phenylenevinylene), orbis-DMOS-PPV.

Figure 4. 5× 5 µm tapping-mode AFM height pictures ofpoly(bis-(2-dimethyloctylsilyl)-1,4-phenylenevinylene), orbis-DMOS-PPV, on an evaporated gold substrate: the films were spin-coated from a chloroform (a), and a toluene (b)solution, respectively (solutions ca. 0.6 wt%). The insets in the upper left corners display the phase-contrast pictures.At the bottom, a cross-section displays the surface roughness of the respective picture (note the difference in height

scale).

Figure 5. The chemical structures ofpoly(3,4-ethylenedioxythiophene), or PEDOT, andpoly(styrene

sulfonic acid), or PSSH.

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of InCl3 — a byproduct of the precursor-PPV conversion on ITO — and of ultrathin interfacial layers,consisting of eitherpoly(4-styrenesulfonate), denoted PSSH, or of an electrically conducting polymer blend,namelypoly(3,4-ethylenedioxythiophene) doped withpoly(4-styrenesulfonate) [31] denoted PEDOT-PSS(see figure 5) have been studied.

2. Experimental

The ITO substrates used in these studies were obtained from Hörnell Elektrooptik, Sweden, andmanufactured by Applied Films, USA. ITO surfaces with different work functions were prepared usingseveral chemical techniques: (a) washing with acetone, followed by isopropanol, 5 min each, in anultrasonic cleaner; (b) treatment with a dilutedaqua regiasolution after (a); or (c) treatment with a solutionof 30 wt% H2O2, 25 wt% NH4OH, and deionized water at 80◦C for 20 min, followed by rinsing withdeionized water [32] after (a). The work functions of these types of surface were (a) 4.4, (b) 4.6 and(c) 4.8 eV (±0.1 eV).

Precursor-PPV, in a water/methanol solution, was obtained from Cambridge Display Technologies(CDT), England, and stored atT < −14 ◦C. Samples for study were made by spin-coating thin films(20 nm) of precursor-PPV on either ITO (which had first been cleaned by a UV-ozone treatment), or thenaturally oxidized surface of optically-flat Si(100) wafer substrates (for reference). The coating procedurewas calibrated to yield films of about 10 nm (occasionally, 20 nm) thickness. Since thermal conversionresults in a loss of about 50% of the molecular weight of the precursor polymer, the thickness of theconverted PPV films was in the range of 10 nm.

Bis-DMOS-PPV, was applied to freshly prepared ITO, in air, by spin-coating from a 0.6 wt% solution intoluene. The resulting film thicknesses, as estimated from XPS, were roughly 10 nm.

Thin films of PEDOT-PSS were spin-coated from an aqueous dispersion, in atmosphere, and insertedimmediately into UHV, as for thebis-DMOS-PPV samples, above. Resultant film thicknesses were also inthe neighborhood of 10 nm, as estimated by XPS.

Films ofbis-DMOS-PPV on PEDOT-PSS-on-ITO, andbis-DMOS-PPV on PSSH-on-ITO were preparedsuccessively, as described above.

Samples were inserted immediately into a vacuum inter-lock on the spectrometer, and pumped down top < 1 · 10−6 Torr using a turbo-pump. The time from coating (in air) to pump-down was less than 10 min.Subsequently, samples were moved into the UHV portion of the spectrometer, with a base pressure (beforeinsertion)p < 10−10 mbar, where the first spectra could be recorded within about 20 min. Although, inthe case of precursor-PPV, some interfacial chemistry had already taken place at the time of the first dataacquisition, the full conversion to PPV required heating to at least 200◦C in UHV over night (indicatingthe time scale of the conversion process).

XPS and ultraviolet photoelectron spectroscopy (UPS) were carried out on a Scienta ESCA 200spectrometer [33], using monochromatized Al(Kα) radiation for XPS, or a doubly differentially pumpedHe-resonance lamp for UPS. The electronic structural effects reported here were studied by determining theposition of the vacuum level from the secondary electron cut-off in He I (21.22 eV) radiation UPS spectra,while the position of the valence band edges were determined using He II radiation (40.8 eV). Data analysiswas performed using the curve-fitting routines included in the Scienta ESCA 200 software.

3. Results and discussion

3.1. Characterization of the morphological, electronic and chemical structure of ITO surfaces

The ITO substrates, obtained from Hörnell Elektrooptik, Gagnef, Sweden, are particularly flat, asdetermined by Atomic Force Microscopy (see figure 6), enabling careful studies of the interface withouteffects from rough surface morphology, as discussed previously [32].

Figure 7displays photoelectron spectra of the valence band region of pristine ITO using photon energiesof 1486.6 eV (AlKα) and 40.813 eV (He II). The binding energy ranges accessible with the different

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Figure 6. Tapping-mode AFM height pictures of the ITO substrates: (a)5× 5 µm; and (b)1× 1 µm. The white spotscorrespond to isolated protrusions with a maximum height of about 25 nm. Zooming into the flat areas in between the

protrusions, tiny droplets can be seen with az-range of approx. 1 nm.

Figure 7. Photoelectron spectra of the valenceband region of pristine ITO using photon

energies of 1486.6 eV (AlKα) and 40.813 eV(He II); normalization to equal intensity of the

In(4d) derived feature at 18.1 eV bindingenergy. A clear step in the density-of-states at

the Fermi energy appears in the blown-up(×20 and×200) AlKα spectrum.

excitation sources depend on their respective photon energies. Whereas AlKα radiation allows mappingof the whole valence band region including an oxygen-2s-derived feature at 26.4 eV binding energy, He IIradiation is restricted to the energy range between the Fermi energy and the onset of the He I spectrumat around 20 eV binding energy. Both the AlKα and the He II spectrum display a pronounced In(4d)-derived feature at about 18.1 eV binding energy. Using this peak for normalising the intensities of the twospectra, a large increase of the intensity in the binding energy range from 12 to 2 eV is observed in theHe II spectrum as compared to the AlKα spectrum. This effect illustrates an important advantage of UPS

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over XPS for valence band photoelectron spectroscopy: the relative increase of the photo-ionisation crosssections for atomic subshells in the low binding energy valence region for low photon energies [34]. Anotherimportant consequence of the difference in photon energy is the much larger information depth (ca. 10 nm)in the case of AlKα radiation as compared to He II radiation. Thus, while valence band photoelectronspectroscopy using He II radiation is restricted to the outermost surface layers of a sample, the advantagewith using (monochromatized) AlKα radiation for valence band spectroscopy is that the higher kineticenergy of the photoelectrons (larger information depth) allows probing of the electronic structure of buriedinterfaces.Figure 7 illustrates this point: whereas no finite density-of-states (DOS) is observed right at theFermi energy in the He II spectra of pristine ITO, a clear-cut, though low-intensity, step appears in the(blown-up) AlKα spectrum. These findings point to the presence of an insulating layer at the uppermostsurface of ITO.

The difference in electronic structure at the ITO surface as compared to the bulk is paralleled bycorresponding differences in the chemical composition.Figure 8 displays the oxygen, O(1s), core levelspectrum of ITO for both normal emission and an electron take-off angle of 10 degrees (80 degrees relativelyto the surface normal). In both cases, the experimental peak was fitted with three peak components atbinding energies of 532.4, 531.2 and 530.2 eV. The low take-off angle (high angle relatively to the surfacenormal) corresponds to an increased surface sensitivity as compared to normal emission. Therefore, therelative increase of the high binding energy peak components at 532.4 eV and 531.2 eV and the resultingincrease of the overall oxygen-to-indium atomic ratio from 1.3 to 1.6 indicates the presence of an oxygen-rich phase at the surface. A corresponding change occurs in the In(3d5/2) spectrum displayed infigure 9.For the lower electron take-off angle, the peak component at 445.1 eV increases relative to the lower bindingenergy component at 444.5 eV. We therefore conclude that the uppermost ITO surface is composed of

Figure 8. O(1s) photoelectron spectra of pristine ITOat normal emission (filled dots) and at an electron

take-off angle of 10 degrees (open dots); normalizationto equal area of the corresponding In(3d)

photoelectron spectra. The experimental peaks werefitted with three peak components at binding energies

of 532.4, 531.2 and 530.2 eV.

Figure 9. In(3d) photoelectron spectra of pristineITO at normal emission (filled dots) and at an

electron take-off angle of 10 degrees (open dots);normalization to equal total area. The experimental

peaks were fitted with two peak components atbinding energies of 445.1 and 444.5 eV.

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a non-conducting, oxygen-rich phase, possibly amorphous indium oxide and/or indium hydroxide, whichwe assume to be most likely to be attacked by the HCl formed during the conversion of precursor-PPV.

3.2. Chemical species at the ITO/precursor-PPV interface

The chemical species involved in the thermal conversion of precursor-PPV may be identified from theXPS core-electron spectra of precursor-PPV, as shown for a thin film on an oxidized Si(100) substrate:figure 10shows the first three scans over the binding energy region appropriate for studying the chlorinecontent. It displays the Cl(2p) components over a narrow binding energy range, where the experimentalspectrum is fitted with four peak components. Note that the notation ‘Cl(2p)’ corresponds to the spin-splitCl(2p1/2,3/2) doublet of well-known energy splitting and intensity ratio (1.6 eV and1 : 2, respectively).The first scan, at the bottom, was taken about 20 min after spin-coating of the precursor-film and insertioninto UHV, and took about 20 min to accumulate. The remaining two spectra were taken at further intervalsof approximately 20 min.

Keeping in mind that the Cl(2p) features correspond to spin-split doublets, the peaks in the spectramy be identified (from left to right) as follows: the high binding energy components of the Cl(2p) spectracorrespond to eliminated HCl, and the low binding energy components to the precursor sulphonium chloridesalt. It is clear that the content of the chloride anion (Cl−) decreases over the time scale of the experiment,while the content of hydrochloric acid (HCl) increases. In other words, already after less than 1 h at roomtemperature, a partial elimination of HCl from the precursor sulphonium chloride salt indicates the onsetof the conversion process. Corresponding changes occur in the S(2p) photoelectron spectrum (not shownhere): 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 film.

Obviously, the four-peak spectrum may be fitted well with only two spin-split components of givenenergy splitting and intensity ration.

For precursor-PPV on ITO substrates, however, the situation is more complex. As shown previously, theuppermost 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 infigure 11, arequite different from those for a silicon substrate (figure 10): Clearly, the intensities of the features maynot be fitted with only two spin-split components. A third set of spin-split components, open circles in thefigure, is necessary in order to account for the observed intensity distribution. Reference measurementswere performed on InCl3-coated ITO in order to determine and identify the Cl(2p)-binding energy in InCl3on ITO. By comparison with our own reference spectra, the third and lowest intensity double is assigned tothe chlorine in InCl3. Already within 1 h at room temperature, InCl3 is present at the polymer-ITO interface,as a result of the chemical reaction of the HCl released in the thermal elimination reaction with the ITOsubstrate.

By studying the various spectra offigures 10and 11 more closely, it may be noted that the rate ofconversion is faster in the case of precursor-PPV on ITO: already in the first scan in each figure, a significantdifference may be seen. Differences persist even at later stages of conversion. It appears that the indiumchloride generated at the polymer-ITO interface acts as a catalyst for the elimination reaction. Moreover,from the angular dependence of the Cl(2p) spectrum, it is clear that the InCl3 lies closer to the ITO substratethan the polymer surface; indicating that the InCl3 is actually formed at the ITO-polymer interface, fromwhere it diffuses into the polymer film. This agrees with temperature dependent impedance spectroscopymeasurements and theoretical modeling performed by Scherbel et al. [35], who found that the bulk of PPVconverted on ITO is composed of a highly doped region at the ITO interface and a region with lower dopingat higher distances from the interface. Moreover, the boundary between these two regions is not sharp butthere is a gradual change in dopant concentration.

The occurrence ofp-doping is in agreement with our observation, that the Cl(2p) binding energies fora given chemical species is approx. 0.5 eV lower in the precursor-PPV film on ITO as compared to the

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Figure 10.As prepared films of precursor-PPV onsilicon: narrow scan XPS spectra of the Cl(2p) bindingenergy range. The experimental data were fitted withfour peak components: the two peaks at low bindingenergy (spin-split components) are attributed to the

chloride ions of the precursor sulphonium salt, while thetwo peaks at high binding energy are attributed to the

eliminated HCl. The formation of HCl on the time-scaleof three successive scans (20 min per scan) indicates apartial conversion of precursor-PPV in UHV already at

room temperature.

Figure 11.Cl(2p) photoelectron emission of asprepared films of precursor-PPV on ITO: again, the

formation of HCl on the time-scale of three successivescans (20 min per scan) indicates the partial conversionof precursor-PPV in UHV already at room temperature.

Moreover, in the case of ITO as the substrate, theexperimental spectra had to be fitted with six

(spin-split) peak components, corresponding to threedistinct chemical species: the two extra peak

components, open circles in the figure, indicate theformation of InCl3.

Table. XPS binding energies for narrow scan spectra in the Cl(2p) binding energy range for asprepared films of precursor-PPV on silicon and ITO

Species XPS binding energies (eV)

Cl− InCl3 HCl

Cl(2p1/2)/Cl(2p3/2) Cl(2p1/2)/Cl(2p3/2) Cl(2p1/2)/Cl(2p3/2)

Prec.-PPV on SiOx 199.0/197.4 −/− 202.1/200.5

Prec.-PPV on ITO 198.5/196.9 199.8/198.2 201.6/200.0

film on SiOx (seethe table). However, the observed shift in the binding energies might also result fromthe formation of interfacial dipoles due to the protonation of the ITO surface by the HCl formed during theconversion process. In the following, we will focus on UPS investigations of how the band-alignment atpolymer–ITO interfaces is influenced by chemical species relevant in the context of polymer light-emittingdevices.

3.3. Band alignment at ITO/polymer interfaces

Figure 12 shows the energy level scheme and the corresponding experimental UPS intensity curvesfor bis-DMOS-PPV on a gold substrate, in order to establish the parameters to be discussed below. The

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nomenclature and structure of the diagrams are now standard, and follow that of Seki and coworkers, andKahn and coworkers [20–23,25,26].

Note, in particular, the following:(i) the difference in kinetic energy between the fastest photo-electrons from the gold substrate and from

the polymer over-layer corresponds to the offset between the valence band edge in the polymer over-layer and the Fermi energy in the conducting gold substrate,εF

VB. It determines the barrier for holeinjection from the substrate into the valence band of the polymer;

(ii) the energy difference between the vacuum level of the polymer over-layer and the Fermi energy in theconducting substrate,εF

VAC, is εFVAC = hv − (Emax

k (BDMOS-PPV) + εFVB). When the width of the

band bending region at the substrate/polymer interface is well below the polymer film thickness,εFVAC,

as probed by UPS, is equal to the work function of the polymer (which is a material constant). This,however, requires a high concentration of free charge carriers, which is only the case in electricallyconducting polymers (due to doping);

Figure 12.The band parameters which may be derivedfrom the UPS spectra, as presented in previously

reported studies [16,26], is diagrammed. The UPSspectra shown are for a clean gold substrate (left) and

bis-DMOS-PPV on an identical substrate (right).

Figure 13.The band parametersεFVAC, ∆, εF

VB andIP(see text) ofbis-DMOS-PPV are shown as a functionof the substrate work function. The substrates are: fullcircles= ITO type (a); full triangles= ITO type (b);

full squares= ITO type (c); open circles=PEDOT-PSS on ITO; open triangles= PSSH on ITO;

and open diamonds= InCl3 on ITO. Two decimalplaces are given, since the data are averaged over a

large number of measurements.

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(iii) the offset between the vacuum levels of the polymer coated and the uncoated gold substrate isΛ = εF

VAC −ΦAu (where, infigure 12, ΦAu is the work function of the gold substrate);(iv) the ionization potentialIP for the polymer, as probed by UPS, isIP = εF

VB +εFVAC [29]. It is a specific

property of the polymer and therefore independent of the energy level alignment at the interface.In UPS measurements, it is important to clearly define the significance of the term ‘vacuum level’: the

vacuum level of a metal substrate as measured by UPS (denoted asEVAC(Au) in figure 12) corresponds tothe energy of an electron at restjust outsidethe sample [17].

The main findings of our studies are discussed below.Figure 13shows a series of values ofεFVAC, ∆,

εFVB, andIP . The ITO work function ranges from 4.4 eV (method (a)), to 4.8 eV (method (c)) [32]. Even

higher values were obtained for ultra-thin layers of PEDOT-PSS, PSSH, and InCl3 on ITO: 5.0, 5.2 and5.7 eV.

For spin-coatedbis-DMOS-PPV, the position of the vacuum level,εFVAC, follows the work function of

the substrate, ranging from 4.4 eV (for ITO (a)) to 5.3 eV (for InCl3-coated ITO). The offsets,∆, forbis-DMOS-PPV are very small (−0.2 eV in the average) andindependent of the substrate work function.

An important consequence of this vacuum level alignment is that an increase of the substrate workfunction results in a corresponding decrease of the difference between the valence band edge in thebis-DMOS-PPV layer and the Fermi level of the ITO substrate:εF

VB ranges from 1.6 eV forbis-DMOS-PPVon ITO-(a) to 0.7 eV forbis-DMOS-PPV on InCl3 on ITO. This result is highly relevant in the contextof polymer LED’s sinceεF

VB determines the barrier for hole injection(ΦpB) from the ITO-electrode into

the electroluminescentbis-DMOS-PPV layer, an increase of the ITO-work function will reduce the barrierfor hole injection, thereby following the Schottky–Mott rule for the case of a zero offset∆ [36]. Theionization potentialIP , as probed by UPS, corresponds to the sum of the distances of the valence bandedge,εF

VB, and of the vacuum level,εFVAC, from the Fermi level of the ITO substrate. AsεF

VB andεFVAC

evolve strictly anti-parallel with changing substrate work function, their sum is constant (approximately5.9 eV) and independent of the energy level alignment at the interface (see figure 13).

By exposure to non-monochromatized X-rays (MgKα), the work function of ITO can be changedinsitu: figure 14displays the binding energy of the In(3d5/2) emission of a bare ITO substrate as a functionof X-ray exposure time. The change is exponential in time and resulted in a lowering of the binding energyby 0.3 eV within approximately 180 seconds.

The effect can be used in order to follow the influence of changes in work function of the substrate on theband edge parameters of the semiconductor in a single sample: infigure 15are shown several examples ofhow the the position of the vacuum level,εF

VAC, of the semiconductor over-layer (bis-DMOS-PPV) followsthe changes in the work function of the ITO substrate, from exposure to X-rays,in situ. The changes in thework function, in each case, were determined by exposing two samples to the X-rays: one sample with theover-layer under study, and one control sample without any over-layer. There are three different situations:(i) For bis-DMOS-PPV on ITO, andbis-DMOS-PPV on PSSH on ITO, the slope dεF

VAC/dΦ isapproximately 1.0 and the data points lie close to the diagonal in theεF

VAC vs. Φ diagram. Thisindicates an almost perfect vacuum level alignment (∆ ca.−0.2 eV). The (weak) interfacial dipolelayer is independent of the change of the substrate work function induced by irradiation with X-rays.

(ii) For PSSH on ITO, the slope dεFVAC/dΦ is still close to 1.0. However, there is a large offset∆

(+0.4 eV) from the diagonal in theεFVAC vs.Φ diagram. This indicates a modification of the respective

vacuum level alignment by the presence of a substantial interfacial dipole layer. A likely source forthe formation of this dipole layer is the protonation of the ITO surface (PSSH is a strong acid). Theabrupt change of the potential across the dipole layer [37] leads to an increase of the vacuum level ofPSSH on ITO relative to the vacuum level of the bare ITO substrates (positive sign of∆ in figure 15).The presence of an interfacial dipole layer modifies the barrier for hole-injection from the ITOsubstrate into the electroluminescentbis-DMOS-PPV layer,Φp

B (see figure 12):

ΦpB(∆) = IP − (ΦITO + ∆) = Φp

B −∆.

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Figure 14.The evolution of the In(3d5/2) binding energyas a function of the X-ray irradiation time as the work

functions of the ITO substrate is changed by irradiationwith non-monochromatized X-rays (MgKα). The change

is clearly exponential in time and occurs with a timeconstant of 34 s (for the X-ray intensity in our

experimental set-up).

Figure 15.The changes inεFVAC of bis-DMOS-PPV as

the work functions of the different ITO substrates arechanged by irradiation with X-rays. The samples arecoded as in the caption offigure 3. The gray diagonalline of unit slope corresponds to an ideal vacuum level

alignment (Schottky–Mott limit).

(iii) For PEDOT-PSS on ITO, andbis-DMOS-PPV on PEDOT-PSS on ITO, a completely differentbehavior is observed. In both cases, the vacuum levelεF

VAC remains constant (within experimentalaccuracy) upon changing the work functionΦ of the ITO substrate by X-ray irradiation. Asa consequence, the offset∆ for PEDOT-PSS on ITO changes from∼0.0 to +0.5 eV, as seen infigure 15. In the case of PSSH on ITO, the offset∆ remains unchanged upon irradiation with X-rays.Given the fact that PEDOT-PSS films are made up of nanometer-sized grains covered with surfacesegregated PSSH [38], a modification of∆ due to X-ray induced changes of the chemical structure atthe PEDOT-PSS/ITO interface can be excluded. In contrast to PSSH, PEDOT-PSS is an electricallyconducting polymer with a high concentration of free charge carriers. Therefore, the charge in thePEDOT-PSS layer arising from the Fermi level alignement with the ITO substrate is contained withina distance from the ITO interface much smaller than the thickness of the PEDOT-PSS film (flat bandsituation), thus resulting in the constancy of the vacuum levelεF

VAC observed by UPS.To illustrate this point further, considerbis-DMOS-PPV spin-coated onto ITO substrates with strongly

different work functions, both with and without a thin, interfacial PEDOT-PSS layer: ITOs (a) and (c). Therespective substrate work functions are 4.4, and 4.8 eV (±0.1 eV). The results are displayed infigure 16.Without interfacial PEDOT-PSS layer,εF

VB for the bis-DMOS-PPV layer is strongly dependent on thesubstrate work function. The values are 1.6 and 1.2 eV, respectively. With an interfacial PEDOT-PSSlayer, however,εF

VB is 1.0 eV forbis-DMOS-PPV on both ITOs with a PEDOT-PSS interfacial layer. Thisconfirms the establishment of the flat band situation within a distance from the ITO interface smaller thanthe PEDOT-PSS film thickness.

Note that in a recent study of device performance as a function of the nature of the ITO substrate, Kimet al., observed a dependence of injection onset parameters on the treatment conditions of the ITO substrate,despite the presence of a PEDOT-PSS intermediate layer [39], in apparent contradiction with the resultsreported above. The ITO substrates by Kim et al., however, had different chemical compositions, different

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Figure 16.The resultant band diagrams are shown. Onthe left side is the case ofbis-DMOS-PPV on differentsubstrates: ITO of types (a) and (c). On the right is the

case ofbis-DMOS-PPV on the same substrates, but withan interfacial layer of PEDOT-PSS.

surface morphologies, different resistance, and different work functions. It has already been shown [40]that ITOs with similar work functions but different morphologies lead to notable differences in LEDperformance (e.g., turn-on voltages, etc.). The present results are confined to identical starting ITO samples,with a fixed surface morphology, while the work functions were varied independently. The work functionchanges reported here do not appear to lead to any major changes in morphology or surface chemistry.

4. Conclusions

The uppermost layer of ITO is composed of an insulating, oxygen-rich phase, presumably In(OH)3.The formation of indium chloride (InCl3) can be observed at the polymer-ITO interface during the

conversion process of precursor-PPV films on ITO: the HCl released in the conversion process interactswith the surface of the ITO-substrate, leading to the formation of indium chloride, which diffuses into thepolymer. PPV films on ITO substrates, under conditions found published previously, are dopedp-type byresidual InCl3.

Both the cleaning procedure of ITO and acidic byproducts of the conversion of precursor-PPV like HCland InCl3 (by formation of chemical dipoles at the interface through protonation of the ITO) influence theworkfunction of the ITO electrode.

Keeping the ITO substrate otherwise constant, changes in work function of the ITO substrate lead tochanges in the energy offset of the valence band edge of a spin-coated semiconductor overlayer. Thesechanges in offset are equivalent to changes in expected barrier heights for hole injection at the ITO contactto the light emitting semiconductor in polymer based LED’s.

Upon insertion of an electrically conducting polymer interfacial layer between the ITO contact and thesemiconducting polymer, the energy barrier to hole injection is determined only by the work function of theconducting polymer film, and appears to be independent of the work function of the ITO substrate, whenother substrate (ITO) parameters are kept constant.

The results show unequivocally, that interfacial interactions are of major importance in understanding,and ultimately improving, the performance of polymer-based electro-luminescent devices.

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Acknowledgements.The H-PSS and PEDOT-PSS was supplied byCambridge Display Technology, CDT(Cam-bridge, UK). The authors acknowledge Annika Andersson for performing the XPS-measurements on the conversionof precursor-PPV on ITO and silicon substrates. Research on condensed molecular solids and polymers in Linköpingis supported in general by grants from the Swedish Natural Science Research Council (NFR), the Swedish ResearchCouncil for Engineering Sciences (TFR), two European Commission TMR networks (project number 1354 SELOA,and project number 0261 SISITOMAS), and a Brite/EuRam contract (project number 4438 OSCA).

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