The electronic properties of the interface between

JOURNAL OF APPLIED PHYSICS 103, 033710 共2008兲

The electronic properties of the interface between nickel phthalocyanine and a PEDOT:PSS film F. Petraki,1 S. Kennou,1,a兲 and S. Nespurek2 1

Department of Chemical Engineering, University of Patras and FORTH/ICE-HT, Gr-26504 Patras, Greece Institute of Macromolecular Chemistry, Academy of Sciences of the Czech Republic, 162 06 Prague 6, Czech Republic

2

共Received 23 July 2007; accepted 4 December 2007; published online 13 February 2008兲 Thin films of nickel phthalocyanine 共NiPc兲 are deposited on an as-received and on a mildly sputtered PEDOT:PSS film, spin coated on fluorine tin oxide coated glass. The electronic properties of the PEDOT:PSS surface, both as loaded and upon thermal treatment and sputtering, as well as of the interfaces between NiPc and PEDOT:PSS are studied by x-ray and UV photoelectron spectroscopies in order to investigate both the electronic and the chemical properties of the materials. Surface analysis of the PEDOT:PSS films showed that upon sputtering the insulating PSS film is removed leading to lower work function, as well as to an increase of the density of occupied states close to the Fermi level. The investigation of the interfaces between NiPc and PEDOT:PSS revealed charge transfer and a pinning of the Fermi level across the interface. The hole injection barrier was found significantly lower compared with that for the NiPc/Au interface, indicating that the presence of the PEDOT:PSS layer facilitates the carrier injection between the electrode and the organic semiconductor. © 2008 American Institute of Physics. 关DOI: 10.1063/1.2840135兴 I. INTRODUCTION

Conjugated polymers are currently studied for two reasons: 共i兲 In the semiconducting 共undoped兲 state, they are used for fabrication of many electronic devices, such as photodetectors, solar cells, light-emitting devices, and active elements in polymer lasers. 共ii兲 In the doped state, they exhibit a very high conductivity and can be used for the fabrication of thin film transistors, antistatic coatings, electromagnetic interference 共EMI兲 shielding, etc. The common feature of many conjugated polymers is the alternating pattern of double and single bonds between adjacent carbons. Poly关3,4共ethylenedioxy兲thiophene兴 共PEDOT兲 is the derivative of polythiophene with dioxyethylene and it was first introduced in 1990 as an antistatic coating in photographic films and in printed circuit boards and microactuators. PEDOT is presently used as a hole-injecting buffer layer in organic lightemitting diodes, in polymer-based displays, electrochromic windows, rechargeable polymer batteries, and antistatic coatings, as it exhibits relatively high conductivity, good transparency, and electrochemical stability. Indium tin oxide 共ITO兲 anodes covered by PEDOT exhibit longer lifetime and higher charge injection efficiency. PEDOT also offers many advantages over other conducting polymers because of its simple processing ability, film-forming properties, optical transparency, good mechanical strength, and atmospheric stability. The solubility problem was circumvented by using a water-soluble polyelectrolyte, sulfonated polystyrene 共PSS兲, as the charge-balancing dopant in polymerization to yield the PEDOT:PSS system. The electrical conductivity was reported at about 10 S cm−1.1 Author to whom correspondence should be addressed. Tel.: ⫹30-2610996324. FAX: ⫹30-2610-993255. Electronic mail: kennou@ chemeng.upatras.gr.

a兲

0021-8979/2008/103共3兲/033710/6/$23.00

The interfaces between PEDOT:PSS and different conjugated organic materials 共␣-NPD, 6P, pentacene兲 have been studied previously and are compared with the corresponding interfaces with Au, whereby it is found that the hole injection barrier and the interfacial dipole on PEDOT:PSS is drastically smaller than on Au, although the substrates have similar work functions.2 The same behavior for the hole injection barrier is also observed for the CuPC/PEDOT:PSS interface.3 Phthalocyanines 共Pc’s兲 are p-type semiconductors with low mobility and low carrier concentration.4 Their unique properties such as low temperature and low-cost preparation processes, mechanical flexibility, light weight, thermal, chemical, and photochemical stabilities, and their beneficial optical and electronic properties make them promising materials for applications in gas sensor technology and in electronic devices.5 The electrical properties of metal phthalocyanines and H2Pc have been widely investigated in the form of sandwich structures with inorganic electrodes 共Au, Al, ITO兲 and have shown that they provide a considerable electric field with relatively low voltages.5,6 CuPc is one of the most studied metal phthalocyanines 共MPc’s兲, which forms abrupt interfaces with gold, while it interacts with ITO through charge transfer.7–9 In the case of the NiPc/metals 共Au, Ag兲 interfaces studied by photoelectron spectroscopies, the highest occupied molecular orbital 共HOMO兲 cutoff was determined at 1.0 eV and the ionization potential 共IP兲 at 5.0 eV, while the barrier for hole injection was calculated ⬃0.9 eV. The magnitude of the interfacial dipole was found 1.0 eV in the case of the NiPc/Au interface.10 In this work, the changes upon thermal and sputtering treatment of a PEDOT:PSS film grown on fluorine tin oxide coated glass were studied and the electronic properties of the interface between nickel phthalocyanine and PEDOT:PSS, both as loaded and after mild argon-ion sputtering, were in-

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© 2008 American Institute of Physics

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J. Appl. Phys. 103, 033710 共2008兲

vestigated by core level x-ray photoemission spectroscopy 共XPS兲 and valence band ultraviolet photoemission spectroscopy 共UPS兲. The aim of this study is to compare the properties of the NiPc/PEDOT:PSS interface with those of the previously studied NiPc/Au interface,10 in order to investigate the role of a PEDOT:PSS film interceded between the organic active layer and the electrode in the device performance, by exhibiting or not an efficient charge injection. Fluorine tin oxide 共FTO兲 is used instead of ITO as the holeinjecting electrode, because it is less expensive to produce and because indium is reported to diffuse into the organic active layer under device operation. Furthermore, FTO is presently used in solar cells, as it exhibits high optical transmission 共⬃90% 兲 independent of the fluorine doping.11 II. EXPERIMENTAL SECTION

The experiments were carried out in an ultrahigh vacuum system, which allows the preparation of interfaces with high purity and controlled thickness. The system, described previously,12 is equipped with a hemispherical electron energy analyzer 共SPECS LH-10兲 and a twin anode x-ray gun. Commercial PEDOT:PSS 共Baytron P VP AI4083兲 was centrifuged at 1200 rpm for 15 min and then deposited on a FTO substrate by spin coating at 5700 rpm for 15 s. The thickness of the prepared film was 35 nm. The surface of PEDOT:PSS was studied as prepared, after annealing at 110 ° C for 1 h and after in situ mild argon sputtering for 5 min with an argon pressure of 4 ⫻ 10−6 mbar, an accelerating voltage of 0.2 keV, and an incidence angle of 45°. Commercial NiPc powder from Alfa Aesar was thermally evaporated in a stepwise manner in the preparation chamber, from a home-made deposition source kept at 425 ° C, after purification in UHV by heating at 150 ° C for about 15 h. NiPc thin films with total thickness of ⬃7.0 nm were deposited at room temperature on both as-received and sputtered PEDOT:PSS surfaces in a vacuum of about 5 ⫻ 10−8 mbar. After each deposition step the interfaces were characterized by XPS and UPS. In order to investigate the effects of annealing and sputtering on the PEDOT:PSS film, XPS measurements were carried out with a nonmonochromatic Mg K␣ excitation energy 共1253.6 eV兲 and an analyzer pass energy of 25 eV. For the study of the NiPc/ PEDOT:PSS interface, Al K␣ radiation at 1486.6 eV and an analyzer pass energy of 97 eV were used. All UPS measurements were taken by using the He I 共21.22 eV兲 excitation line. The spectrometer was calibrated by the Au4f 7/2 core level 共84.00⫾ 0.05 eV兲 from a clean Au foil. All XP spectra were fitted with mixed Gaussian-Lorentzian peaks after Shirley background subtraction. A negative bias of 12.30 V was applied to the sample during UPS measurements in order to separate sample and analyzer high binding energy cutoffs and estimate the absolute work function from the UV photoemission spectra. The analyzer resolution as determined from the width of the Au Fermi edge was 0.16 eV. III. RESULTS AND DISCUSSION

The XPS O1s peak from the as-loaded PEDOT:PSS surface exhibits a binding energy of 531.6 eV. Two chemical

FIG. 1. XP 共Mg K␣兲 spectra of the S2p peak for the as-received, annealed, and sputtered PEDOT:PSS film.

species are expected to contribute to the O1s signal, one attributed to the O v S bonds in the PSS polymer chain and one attributed to the C–O–C bonds from PEDOT.13 The XPS C1s peak for the as-loaded PEDOT:PSS film exhibits a binding energy of 284.5 eV and consists of contributions from carbon atoms with different chemical bonds in the polymer chain and of residual contamination from the exposure of the sample to the atmosphere.14 Upon heat treatment and sputtering, there is no significant change in the characteristics of the XPS O1s and C1s peaks 共not shown兲. Figure 1 presents the changes in the XPS S2p peak of the PEDOT:PSS film upon annealing and sputtering. The S2p spectrum of the as-received surface appears in two components. The component at lower binding energies is attributed to the contribution of PEDOT and, in particular, to sulfur atoms linked to carbon in the chemical structure of PEDOT and the peak at higher binding energies 共BE’s兲 corresponds to the sulfur atoms in the PSS polymer. The PEDOT component is fitted in all cases with a single spin-orbit doublet at 163.8⫾ 0.1 and 165.0⫾ 0.1 eV binding energies and a 2p3 / 2 to 2p1 / 2 ratio of 2:1, the splitting of 1.2 eV being that expected for S2p. The contribution of PSS to the S2p peak is due to the doping ions PSS−H+ 共spin-orbit doublet at 168.4 and 169.6 eV binding energies兲 and to the PSS−Na+ salt 共Refs. 13 and 15兲 共spin-orbit doublet at 168.0

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Petraki, Kennou, and Nespurek

FIG. 2. He I UP spectra of the as-received, annealed, and sputtered PEDOT: PSS film. In the inset, the density of states close to Fermi level is shown magnified.

and 169.2 eV binding energies兲. The original ratio of PSS–Na to the PSS–H in the PSS layer is found equal to 10:1. Upon thermal treatment, this ratio is reduced to about half of that value, while upon gentle argon-ion sputtering, it decreases farther to 4.5:1 which means that sodium initially segregates at the surface of the film and is partly removed by annealing and sputtering. The PEDOT-to-PSS ratio in the surface region can be calculated from the total area under the S2p core level spectra and it was found 1:5 for the as-received substrate. According to the supplier, the nominal PEDOT-to-PSS monomer ratio is 1:6. After annealing for 1 h up to 110 ° C, this ratio becomes 1:4 and increases more to 1: 1.5 after mild argon sputtering. Upon further sputtering, this ratio remains constant, indicating that the excess of the PSS layer has been removed from the surface and there remains the bulk PEDOT:PSS material. This implies that there exists phase segregation with an excess of PSS on the surface of the asreceived samples, as reported previously.15 In order to estimate the thickness d of the outermost PSS layer, we consider PSS and PEDOT as two separated polymers growing in the form of a “layer over layer” structure ending in a pure PSS layer and compare the intensities from the contribution to the S2p peak of each polymer. The thickness is given by the expression d = ␭ ln关共IPSS/I0PSS兲/共IPEDOT/I0PEDOT兲 + 1兴, where ␭ = 27⫾ 3 Å 共Ref. 15兲, is the electron inelastic mean free path, and Ix, I0x are the measured and expected signal intensities from species X, respectively.13 The above equation yields an outermost PSS layer thickness of 4.32 nm for the as-received and 2.16 nm for the argon sputtered sample, verifying that the top PSS layer is gradually removed from the surface during sputtering. Figure 2 shows the changes of the He I UP spectra for PEDOT:PSS upon annealing and sputtering. The valence

J. Appl. Phys. 103, 033710 共2008兲

band structure of the as-loaded PEDOT:PSS film is composed of three features at about 8.4, 5.7, and 3.3 eV of binding energies associated with ␲ bands, whereas the cutoff is located at ⬃2.0 eV in accordance with previous studies.16 The annealing treatment does not affect significantly the valence band structure, while the density of states close to the Fermi level slightly increases 共inset of Fig. 2兲. After mild argon-ion sputtering for 5 min, the characteristic peaks from the PEDOT:PSS valence band are not distinguishable and the density of states close to the Fermi level increases. This indicates that the surface is covered by an insulating PSS layer which is removed, revealing the PEDOT:PSS blend underneath, which exhibits occupied valence band states close to the Fermi level, as expected for a p-doped PEDOT layer. Furthermore, the work function of the surface decreases from 4.90 eV for the as-received PEDOT:PSS film to 4.70 eV after gentle argon sputtering. Upon NiPc deposition on PEDOT:PSS, the characteristic XPS S2p and O1s peaks related to the substrate are gradually attenuated. The C1s peak is not suitable for monitoring the deposited organic film thickness and the possible energetic shifts, as there is a contribution from the carbon of the substrate. The film thickness was estimated from the attenuation of the S2p peak upon deposition 共increasing the intensity of the N1s peak兲, assuming a layer-by-layer growth for the organic overlayer at small coverages. The experimental results showed similar behavior of the NiPc characteristic XPS core level peaks during deposition of the organic material on both the as-loaded and the sputtered PEDOT:PSS substrates. The figures shown in the present work refer to the deposition of NiPc thin films on the sputtered PEDOT:PSS surface. Figures 3共a兲 and 3共b兲, shows the change of the N1s and Ni2p3/2 XPS peaks during interface formation. For comparison, the binding energy of the same peaks in the case of the previously studied NiPc/Au interface10 is also presented at the NiPc/PEDOT:PSS interface, the position of both the N1s and Ni2p3/2 XPS peaks remains stable up to 3.2 nm of organic film thickness, while for thicker films up to 6.5 nm, the peaks exhibit an upward shift of 0.3 eV. The observed stability of the binding energy up to 3.2 nm in the NiPc related XPS peaks indicates a pinning of the Fermi level position in the gap of the phthalocyanine. For thicker films up to 6.5 nm the observed shift toward higher binding energies is attributed to band bending at the organic energy levels. Therefore, the total band bending at the NiPc overlayer, according to the XPS data, is Vb = 0.30⫾ 0.05 eV. In the case of Au, the N1s and Ni2p3/2 XPS peaks exhibited a continuous upward change, as the organic film thickness increased up to ⬃1.5 nm 共total shift of 0.3 eV兲. The final energy position of both peaks is different from that in the case of NiPc/ PEDOT:PSS interfaces, which indicates a different position of the Fermi level implying a p doping for the NiPc layer on PEDOT:PSS. The UP spectra, as shown in Fig. 4, gradually change during deposition. The characteristic valence band peaks of the PEDOT:PSS film decrease as the organic film becomes thicker and new features attributed to NiPc appear. The valence band structure of NiPc is fully developed at a film

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Petraki, Kennou, and Nespurek

J. Appl. Phys. 103, 033710 共2008兲

FIG. 5. Valence band structure of an ⬃3.0 nm thick NiPc film deposited on a polycrystalline Au foil and on a sputtered PEDOT:PSS substrate.

FIG. 3. XPS binding energy variation of the N1s 共a兲 and Ni2p3/2 共b兲 peaks vs thickness for the NiPc/PEDOT:PSS interface.

FIG. 4. UP spectra of the NiPc/PEDOT:PSS interface. In the inset, the broadening of the HOMO peak in going to thinner organic films is shown magnified.

thickness of ⬃1.5 nm and consists of four peaks in agreement with previous studies of NiPc/metal interfaces.10 No other features in the energy range of the UP spectra are observed, which would point toward a chemical reaction at the interface. The valence band features exhibit similar shifts in the NiPc related XPS peaks. The HOMO cutoff position is located at 0.50⫾ 0.05 eV and remains almost constant up to 3.2 nm of organic film thickness. As the NiPc film thickness increases, there is a continuous shift of the HOMO cutoff position toward higher BE, reaching 0.80 eV for 6.5 nm of NiPc. The expected position for the bulk is near the midgap, as for the NiPc/Au interface 共1.00 eV兲.10 The above shift of the HOMO position implies a pinning of the Fermi level in the gap at the interface. In the present study, this position is not reached even up to ⬃7.0 nm of NiPc, indicating that there is a charge transfer at the interface, which corresponds to a p-type doping for the NiPc layer in agreement with the XPS results. Figure 5 shows the valence band structure for an ⬃3.0 nm thick NiPc film deposited on Au and on PEDOT:PSS. The HOMO cutoff position in the case of PEDOT:PSS is closer to the Fermi level, in agreement with the XPS results. According to the inset of Fig. 4, the width of the HOMO peak for a 6.5 nm thick organic film is 0.47 eV and becomes broader 共0.52 eV兲 for a 1.5 nm NiPc film. The broadening of the HOMO peak at the early steps of deposition is an additional indication of the presence of a NiPcsubstrate interaction through charge transfer. The above results were also obtained in the study of the NiPc/ PEDOT:PSS 共as-loaded兲 interface, indicating that the

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Petraki, Kennou, and Nespurek

J. Appl. Phys. 103, 033710 共2008兲

different work function of the substrate does not affect the behavior of the core levels and the characteristic valence band features of NiPc. Similar behavior was also reported previously for the CuPc/PEDOT:PSS interface studied by XPS and UPS.3,17 The hole injection barrier 共⌽bh兲, which is equal to the energy difference between EF and the HOMO cutoff position measured for the NiPc共3.2 nm兲/PEDOT:PSS interface, is ⌽bh = HOMOcutoff = 0.50 ⫾ 0.05 eV, in both the as-loaded and the sputtered NiPc/PEDOT:PSS interfaces and is independent of the work function of the substrate. It is important to note that the ⌽bh measured for the NiPc/PEDOT:PSS interface 共0.50 eV兲 is significantly lower than that in the case of NiPc/Au interface 共0.90 eV兲 for the same NiPc coverage,10 in spite of the quite similar work functions, suggesting that the presence of the PEDOT:PSS layer leads to a much more favorable energy level alignment for hole injection between the substrate and the upper organic layer. The high binding energy cutoff region in the UP spectra, which defines the work function of the surface, is also shifted upon NiPc deposition. For a 3.2 nm NiPc film, the work function is equal to 4.30⫾ 0.05 eV resulting in an interface dipole 共eD兲: eD = ⌬⌽ = 共4.70 − 4.30兲 eV = 0.40 ⫾ 0.10 eV,

FIG. 6. Schematic energy level diagram of the NiPc共3.2 nm兲/PEDOT:PSS interface for the 共a兲 as-loaded and 共b兲 mildly sputtered PEDOT:PSS surfaces.

for the NiPc/PEDOT:PSS共as-loaded兲 interface and eD = ⌬⌽ = 共4.90 − 4.30兲 eV = 0.60 ⫾ 0.10 eV, for the NiPc/PEDOT:PSS 共sputtered兲 interface. The magnitude of the eD in the case of the NiPc/PEDOT:PSS interface is compatible with those generally obtained at organic/ organic interfaces, but it is significantly lower than that observed for the NiPc/Au interface 共⬃0.90 eV兲.10,18 The direction of the dipole in both NiPc/PEDOT:PSS and NiPc/Au interfaces indicates that the vacuum level decreases from the substrate to the organic film. Figure 6 shows the energy level diagrams at the NiPc/ PEDOT:PSS interfaces for the as-loaded and the sputtered PEDOT:PSS surface, as derived from the combination of the photoemission results for a NiPc film thickness of about 3.2 nm. The hole injection barrier 共⌽bh兲 is found 0.50 eV in both interfaces, which means that the adjustment of equilibrium conditions in bulk NiPc is independent of the work function of the underlying conduction polymer substrate. Also, the PEDOT:PSS layer between the FTO substrate and the NiPc layer enables the formation of lower hole injection barriers as compared to the NiPc/Au contact 共⌽bh = 0.90 eV兲, indicating that the presence of a PEDOT:PSS layer can improve the performance of the organic electronic devices. The properties of the NiPc/PEDOT:PSS interface are quite different from those observed on polycrystalline Au, although the substrates have similar work functions. In the case of NiPc deposition on gold, an abrupt interface was formed and a continuous shift of the molecular core level peaks by ⬃0.3 eV was observed from submonolayer to several monolayers. This was attributed to a change in the po-

larization due to a decrease in the screening of the photohole by the metal electrons as the distance between the photoexcited molecule and the metal substrate increases with film thickness. No similar change in the polarization is observed for the polymer substrate, which does not possess the free electron density available in the metal. Therefore, in the NiPc/PEDOT:PSS interface the organic core levels remain stable up to an ⬃3.2 nm thick organic film indicating a pinning of the Fermi level, in contrast to the continuous change observed in the organic core levels in the case of the NiPc/Au interface. The large interface dipole and the higher hole injection barrier observed at the NiPc/Au interface compared with the case of PEDOT:PSS can be explained by the fact that upon NiPC deposition on the metal surface, the contribution of the metal surface dipole to the work function changes. The work function of a metal consists of bulk and surface contributions, such as the chemical potential 共␮兲 and the surface dipole 共SD兲.18 The SD depends on the structure of the surface and is determined by the tail of the electron cloud from the metal surface toward the vacuum. The presence of organic molecules changes the SD of the metal, because the repulsions between the electrons from the metal surface and the organic molecules compress the electron tail, resulting in the lowering of the work function of the surface and of the vacuum level. As a result, the energy difference between the Fermi level of the metal 共Au兲 and the HOMO position of the organic 共NiPc兲 film, which determines the barrier for hole injection, increases. In the case of PEDOT:PSS which has similar work function to Au, the work function is controlled by the energy levels created by the charge transfer between

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J. Appl. Phys. 103, 033710 共2008兲

Petraki, Kennou, and Nespurek

the sulfonate and the ethylene dioxythiophene moieties. The dipoles that are formed in the polymer have a random orientation and cancel each other macroscopically and as a result the contribution of the surface dipole is minimal. In the absence of surface electron tail in the PEDOT:PSS, which is a conducting organic polymer made of closed-shell molecules with much fewer free electrons than a metal such as Au, the NiPc molecules will only slightly modify the work function of the polymer, which would lead to the formation of smaller interface dipoles and to much more favorable energy level alignment for hole injection at the contact with the NiPc film.18 The interface dipole is formed by the mixing of the organic materials which takes place at the interface and it is associated with charge transfer across the interface and an associated Fermi level pinning. The direction of the electron transfer between the organic layers can be also expressed in terms of the electrochemical potential ␮e. A comparison of ␮e for PEDOT:PSS and Pc according to previous studies predicts an oxidation for the Pc’s molecules.3 This charge transfer leads to a pinning of the Fermi level in the gap of the NiPc corresponding to a p-type doping for the NiPc layer. As a result, the presence of a PEDOT:PSS layer between the FTO electrode and the NiPc active organic layer can significantly reduce the barrier for the injection of holes from FTO to the NiPc layer and positively affect the transport properties of the organic device. IV. CONCLUSIONS

The surface electronic structure of the as-loaded PEDOT:PSS film and the effects of annealing and sputtering were investigated by XPS and UPS. Mild argon-ion sputtering removes the excess surface layer of PSS increasing the density of occupied states close to the Fermi level. The deposition of NiPc on the as-loaded and the sputtered PEDOT:PSS surface and the investigation of the formed interfaces by photoelectron spectroscopies showed a charge transfer and a pinning of the Fermi level across the interface as observed by the position and the broadening of the HOMO

peak at the early stages of NiPc deposition. The hole injection barrier is found significantly lower compared with that in the case of the NiPc/Au interface, indicating that the presence of the PEDOT:PSS layer between the electrode and the organic semiconductor decreases the potential barrier which can result in an efficient charge carrier injection. ACKNOWLEDGMENTS

This work was supported by Grant No. KAN 401770651 from the Grant Agency of the Academy of Sciences of the Czech Republic and No. 1041/2006-32 from the Ministry of the Education, Youth and Sports of the Czech Republic. 1

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