Valence Band States of Conducting Polymer Films

WDS'5 Proceedings of Contributed Papers, Part III, 569 573, 25. ISBN 8-86732-59-2 MATFYZPRESS Valence Band States of Conducting Polymer Films A.T. Wroble, S. Tepavcevic, A. Zachary, and L. Hanley Department of Chemistry (mc 111), University of Illinois at Chicago, Chicago, Illinois, USA 667-761. Abstract. Surface polymerization by ion-assisted deposition (SPIAD) has been used to deposit polythiophene-like films (3T SPIAD) on AlxOy, TiO2, and Au substrates for potential use as an electron donor material in organic solar cells. Ultraviolet photoelectron spectroscopy (UPS) has been used to determine the energy level alignment at the interfacial region between 3T SPIAD films and their corresponding substrates. No changes were observed in the UP spectrum for either the barrier to hole injection (EFVB) or the Fermi level of the substrate-vacuum level offset (EF) as the work function of the substrate was changed from 3.5 to 4.8 ev or as the thiophene ion current was varied from 5 to 25 na with constant terthiophene flux. The constant values for EFVB and EF can be explained by the presence of gap states at the interface, resulting in an interface dipole and the offset of the vacuum levels of the 3T SPIAD film and substrate. The vacuum level offset allows the Fermi levels of the substrate and 3T SPIAD film to align without the movement of the 3T SPIAD Fermi level from its initial position. Introduction The development of technology for the efficient and cost-effective conversion of solar energy to electricity has been an important research goal over the past few decades. The non-polluting, renewable nature of solar energy makes it a viable alternative to the use of traditional energy sources such as fossil fuels. Conventional silicon-based solar cells have offered solar energy-to-electricity conversion efficiencies of greater than 24% in laboratory settings [1]. However, organic-based solar cells composed of conjugated semi-conducting polymers as electron donors and C 6 derivatives as electron acceptors have the potential advantage of facile processing and cheaper cost than conventional silicon-based solar cells. The major drawback of organic-based solar cells is their seemingly low efficiency, reaching only 5.7% for a tandem configuration [2]. The energy level alignment at the interfaces of electrode/electron donor, electrode/electron acceptor, and electron donor/acceptor in organic-based solar cell systems appears to be one key to understanding factors that will enhance the efficiency of solar energy-to-electricity conversion. Ultraviolet photoelectron spectroscopy (UPS) has been extensively used to characterize the energy level alignment at these types of interfaces for this purpose [3, 4]. The use of surface polymerization by ion-assisted deposition (SPIAD) has been previously reported as a method for growing polythiophene films (3T SPIAD) [5 7]. In this paper, we will present the UPS analysis of substrate/3t SPIAD interface to determine the effect of the variation of substrate work function and deposition parameters on the energy level alignment at this interface. Experimental Preparation of Substrates Silicon wafers (Atomergic Chemetals Corp., Si (1) p-type, boron doped) were coated with 1 nm aluminum or gold films (Evaporated Coatings, Inc.). Aluminum with native oxide and gold substrates were ultrasonicated in acetone and isopropanol prior to being introduced into the vacuum chamber. Further, gold substrates were sputtered with 1 kev He + for 1 hour. Titanium dioxide films were prepared by doctor-blading a slurry of nanophase TiO 2 powder (Nanophase Technologies Corp.) in distilled water onto a clean indium tin oxide coated glass substrate. The TiO 2 films were dried in air and heated to 45 o C for 1 hour to produce anatase TiO 2 films. All substrates were analyzed by UPS prior to deposition of polythiophene film to confirm the substrate work function. 569

Deposition Method Surface polymerization by ion-assisted deposition (SPIAD) was accomplished by simultaneous deposition of thermally evaporated terthiophene neutrals (3T) and 2 ev thiophene ions (T + ) to form polythiophene films (3T SPIAD). The ions were formed by 8 ev electron impact ionization of thiophene vapor, accelerated to 1 kev, mass-selected by a Wien filter, decelerated to 2 ev, refocused, and guided onto the substrate. The most efficient polymerization was previously reported [6] at an ion-to-neutral ratio of 1/15 and was used in all depositions except where noted otherwise. All films were grown for two hours. The morphology and thickness of these films has been investigated previously [8]. Briefly, scanning electron microscopy images of 3T SPIAD films showed the presence of large rounded features from 1 to 9 µm in diameter surrounded by a featureless region that is composed of sulfur-containing graphitic carbon. The island layer was estimated to be 45-85 nm thick whereas the thickness of the featureless layer was estimated as 7-2 nm. Ultraviolet and X-Ray Photoelectron Spectroscopies (UPS and XPS) A helium discharge UV source (Model UV 1, Thermo VG Scientific) operating in He (II) mode (hν = 4.8 ev) was used to obtain UPS valence band spectra. XPS was performed using a monochromatic Al K α x-ray source (15 kev, 25 ma emission current, model VSW MX1 with 7 mm Rowland circle monochromator, VSW Ltd., Macclesfield, UK) and a concentric hemispherical analyzer (15 mm, model Class 15, VSW Ltd.) with multichannel detector operating in constant energy analyzer mode. The spectrometer was calibrated by the position of the Au(4f 7/2 ) peak at 83.66 ev and the gold Fermi level at ev binding energy of a He + sputter-cleaned polycrystalline gold foil. During analysis of the substrates and 3T SPIAD films, the samples were positioned such that they were in electrical contact with the gold foil to align their Fermi levels. The location of the deposited film on the substrate was determined by the position on the sample that gave the highest sulfur signal as seen in XPS. Energy level alignment parameters of the substrate/3t SPIAD interface were determined from UPS as has been described in the literature [3, 4] and is detailed below. Figure 1 shows a generalized scheme of the energy level alignment between the substrate and the 3T SPIAD film. The work function (Φ) of the substrate was determined with UPS by subtracting the position of the secondary electron cutoff (E C ) from the excitation energy (hν=4.8 ev): hν E C = Φ (1) The energy offset between the Fermi level (E F ) of the substrate and the vacuum level (VL) of the 3T SPIAD film was determined by subtracting the excitation energy from E C to define the position of the vacuum level of the 3T SPIAD film: E C hν = VL (2) and then subtracting VL from E F to obtain the Fermi level-vacuum level offset, E F : E F VL = E F (3) The energetic difference between E F of the substrate and the valence band onset (E VB ) of the 3T SPIAD film was determined by identifying the position of E VB, defined as the intercept between the tangent to leading edge of the lowest binding energy feature and the zero-intensity background line [9], and subtracting E F from E VB : E VB E F = E F VB (4) E F VB is often termed the barrier to hole injection. Since E F has been arbitrarily set to ev by adjusting the spectrometer settings, E F is numerically equal to the absolute value of VL, and E F VB is numerically equal to E VB. The ionization potential (IP) of the 3T SPIAD film is an inherent property of the film and should remain constant regardless of the substrate. Furthermore, the ionization potential is not expected to vary with deposition conditions. The IP was determined by the sum of E F and E F VB : E F + E F VB = IP (5) A combination of UPS and near edge absorption fine structure spectroscopy has previously been used [1] to compare the electronic structure of 3T SPIAD and 3T evaporated films by concentrating on changes in valence band features, band gap, and E F VB. 57

Figure 1. General scheme of energy level alignment at interface between substrate and 3T SPIAD film. Vacuum levels of substrate and 3T SPIAD film may or may not be aligned depending on substrate. Results Variation of Substrate Work Function 3T SPIAD films were deposited onto Al x O y, TiO 2, or Au substrates and were subsequently analyzed by XPS and UPS. UPS determination of the work functions of aluminum, gold, and titanium dioxide substrates prior to deposition shows agreement of our data (see Table 1) with values reported in the literature for similar substrates: Φ(Al x O y ) = 3.9 ev [3], Φ(TiO 2 ) =4.1 ev [11], Φ(Au) = 4.7-5.4 ev [3, 4]. Figure 2a shows the UP spectra for 3T SPIAD films deposited on each of these substrates. These spectra contain a continuous emission band that begins at 1.2-1.4 ev, corresponding to the π * anti- bonding orbitals of 3T SPIAD films, which is consistent with previously reported spectra for polythiophene and longer chain thiophene oligomers [12 14]. A peak at ~4 ev is assigned to the n non-bonding orbitals, and a group of peaks from ~6-1 ev represents the π bonding orbitals. Figure 2b focuses on the π* anti-bonding region of each spectrum and shows the method for determination of the valence band onset position. The position of E VB is shown to be near 1.4±.2 ev for all 3T SPIAD films, which suggests that E VB F does not shift with respect to the substrate work function for 3T SPIAD films. Table 1 summarizes the parameters E VB F, E F, IP, and Φ for 3T SPIAD films grown on various substrates. 6 5 3T SPIAD on Au 3T SPIAD on AlxOy 3T SPIAD on TiO2 π 25 2 3T SPIAD on Au 3T SPIAD on AlxOy 3T SPIAD on TiO2 4 3 2 n 15 π* 2 4 6 8 1 5 EVB~1.4eV for all substrates..5 1. 1.5 2. 2.5 3. 3.5 4. 4.5 5. (a) Figure 2. a) UP spectra of 3T SPIAD films on Al x O y, Au, and TiO 2 substrates. b) π* anti-bonding region of UP spectra showing determination of E VB. (b) 571

Table 1. E VB F, E F, IP(3T SPIAD), and Φ(substrate) as calculated from UP spectra of substrates and 3T SPIAD films. Substrate Φ (ev) E VB F (ev) E F (ev) IP (ev) Al x O y 3.5±.3 1.3±.2 4.±.2 5.3±.3 TiO 2 4.±.3 1.4±.2 4.±.2 5.4±.3 Au 4.8±.3 1.3±.3 4.1±.2 5.4±.3 Variation of Ion Current 3T SPIAD films were deposited onto Al x O y substrates with ion currents of 5, 15, and 25 na and constant 3T flux and were subsequently analyzed by XPS and UPS. Figure 3a shows the UP spectra of 3T SPIAD films grown at various ion currents. All characteristic emissions of 3T SPIAD films are present in the spectrum for each ion current as discussed above. However, the peak due to emission from π* anti-bonding orbitals is not as pronounced for the film grown at 5 na ion current as compared to the emission seen for films grown at 15 and 25 na. The decreased emission can be more clearly seen in Figure 3b. E F VB, E F, IP, and Φ for 3T SPIAD films grown at various ion currents are summarized in Table 2. E F VB is within the range of 1.3-1.4 ev for all 3T SPIAD films grown on Al x O y regardless of the deposition conditions. 25 2 3T SPIAD on AlxOy, 15 na 3T SPIAD on AlxOy, 25 na 3T SPIAD on AlxOy, 5 na 8 3T SPIAD on AlxOy, 15 na 3T SPIAD on AlxOy, 25 na 3T SPIAD on AlxOy, 5 na 15 6 4 5 2 2 4 6 8 1..5 1. 1.5 2. 2.5 3. 3.5 4. 4.5 5. (a) Figure 3. a) UP spectra of 3T SPIAD films on Al x O y substrate grown at 5, 15, and 25 na ion current. b) π* anti-bonding region of UP spectra showing determination of E VB. (b) Table 2. E F VB, E F, IP(3T SPIAD), and Φ(substrate) as calculated from UP spectra of Al x O y substrate and 3T SPIAD films. Φ(Al x O y ) = 3.6±.3 ev. Ion Current (na) E VB F (ev) E F (ev) IP (ev) 5 1.4±.2 4.±.2 5.4±.3 15 1.4±.2 4.1±.2 5.5±.3 25 1.3±.3 4.±.2 5.3±.3 Discussion Surface polymerization by ion-assisted deposition has been used to grow polythiophene-like films onto Al x O y, TiO 2, and Au substrates to observe the effect of substrate work function and deposition conditions on the interfacial energy level alignment. UPS analysis has shown that E F VB and E F remain the same for films grown on substrates with work functions in the range 3.5-4.8 ev and for films that have been grown with various thiophene ion currents and constant terthiophene flux. Different degrees of variation of E F VB and E F were reported in the literature for semiconducting polymers such as 4,4 -N,N'-dicarbazolyl biphenyl, tris (8-hydroxyquinoline) aluminum, (N,N - diphenyl- N,N -bis (1-naphthyl)- 1,1 -biphenyl-4,4 -diamine, 3,4,9,1 perylenetetracarboxylic 572

dianhydride [9, 15], and poly(9,9-dioctylfluorene) [3] with respect to substrate work function. Direct correspondence between changes in substrate work function and energy level alignment was seen for poly(9,9-dioctylfluorene) [3], whereas no shift was observed for 3,4,9,1 perylenetetracarboxylic dianhydride [9, 15]. This phenomenon appears to be a property of the film, and an explanation for the constant value of E F VB with respect to substrate was given by Hill and coworkers [9]. The ability of the Fermi level to move within the band gap of the semiconductor the 3T SPIAD film in this case is reduced due to the presence of states within the gap. As the Fermi level of the semiconductor attempts to align with the Fermi level of the substrate, the gap states are more or less populated depending on the direction the Fermi level attempts to move. An interface dipole is formed as a result of the net space charge at the substrate/semiconductor interface, which causes the vacuum levels of the substrate and semiconductor to offset. The vacuum level offset makes it possible for the Fermi levels of the substrate and semiconductor to align without the semiconductor Fermi level moving from its initial position. Acknowledgements. This work is funded by the National Science Foundation under grant no. CHE- 241425. ATW s visit to Charles University, Prague, Czech Republic was funded by Kontakt 1PO4ME754. References [1] S. E. Shaheen, D. S. Ginley, G. E. Jabbour, MRS Bulletin, 25, 3, 1. [2] J. Xue, S. Uchida, B. P. Rand, S. R. Forrest, Appl. Phys. Lett., 25, 86, 5757. [3] W. R. Salaneck, M. Logdlund, M. Fahlman, G. Greczynski, Th. Kugler, Mater. Sci. Engin. R. 21, 34, 121. [4] H. Ishii, K. Sugiyama, E. Ito, K. Seki, Adv. Mater. 1999, 11, 65. [5] S. Tepavcevic, Y. Choi, L. Hanley, J. Amer. Chem. Soc., 23, 125, 2396. [6] S. Tepavcevic, Y. Choi, C. Wu, L. Hanley, Lang., 24, 2, 8754. [7] Y. Choi, S. Tepavcevic, Z. Xu, L. Hanley, Chem. Mater., 24, 16, 1924. [8] S. Tepavcevic, A. M. Zachary, A. T. Wroble, Y. Choi, L. Hanley, submitted. [9] I. G. Hill, D. Milliron, J. Schwartz, A. Kahn, Appl. Surf. Sci. 2, 166, 354. [1] S. Tepavcevic, A. T. Wroble, M. Bissen, D. J. Wallace, Y. Choi, L. Hanley, J. Phys. Chem. B, 25, 19, 7134. [11] G. Liu, W. Jaegermann, J. He, V. Sundstrom, L. Sun, J. Phys. Chem. B 22, 16, 5814. [12] H. Fujimoto, U. Nagashima, H. Inokuchi, K. Seki, Y. Cao, H. Nakahara, J. Nakayama, M. Hoshion, K. Fukuda, J. Chem. Phys. 199, 92, 477. [13] P. Dannetun, M. Boman, S. Stafstrom, W. R. Salaneck, R. Lazzaroni, C. Fredriksson, J. L. Bredas, R. Zamboni, C. Taliani, J. Chem. Phys., 1993, 99, 664. [14] H. Ahn, J. E. Whitten, J. Macromol. Sci., 23, A4, 1357. [15] I. G. Hill, A. Rajagopal, A. Kahn, Y. Hu, Appl. Phys. Lett. 1998, 73, 662. 573