Ellipsometric study of an organic template effect: H 2 Pc/PTCDA

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1 Organic Electronics 7 (2006) Ellipsometric study of an organic template effect: H 2 Pc/PTCDA O.D. Gordan a, *, T. Sakurai b, M. Friedrich a, K. Akimoto b, D.R.T. Zahn a a Institut für Physik, Technische Universität Chemnitz, Reichenhainer Str. 70/457, D Chemnitz, Germany b Institute of Applied Physics, University of Tsukuba, Tennodai, Tsukuba, Ibaraki , Japan Received 22 March 2006; received in revised form 24 July 2006; accepted 24 July 2006 Available online 18 August 2006 Abstract A key prerequisite for fabricating opto-electronic devices with organic molecules is to control the molecular growth mode. In this work we present an ellipsometric study of the templating effect of 3,4,9,10-perylene tetracarboxylic dianhydride (PTCDA) on the growth mode of metal free phthalocyanine (H 2 Pc). The difference in the anisotropic dielectric function between the H 2 Pc films grown on bare glass or silicon substrates compared to the anisotropic dielectric function of H 2 Pc films grown on PTCDA indicates a drastic change in molecular orientation. From the strong in-plane/out-of-plane anisotropy the average molecular tilt angle of H 2 Pc was found to be around 52 for the films grown on bare substrates and around 25 for H 2 Pc films grown on PTCDA. A splitting in the H 2 Pc Q-band was observed for the H 2 Pc films grown on PTCDA which indicates a deviation from the perfect H 2 Pc a-phase crystal. Ó 2006 Elsevier B.V. All rights reserved. PACS: Ef; Jk Keywords: Ellipsometry; Anisotropy; Dieletric function 1. Introduction The development of organic electronics progressed rapidly in this decade and organic photovoltaic cells are regarded as one of the future high potential molecular electronic applications with the advantage of low cost production of printable and flexible devices [1]. Among novel research subjects for improving photovoltaic cell efficiencies, such as the development of low band-gap materials * Corresponding author. Tel.: ; fax: address: ovidiu.gordan@physik.tu-chemnitz.de (O.D. Gordan). and device structures, especially controlling the molecular orientation has a great potential for improving the electrical and optical properties of organic films because the anisotropic molecular orbitals contribute to carrier transport and visible light absorption [2]. Nevertheless, only few attempts to control the molecular orientation have been made for the application of photovoltaic cells because most organic semiconductors are difficult to be controlled in their molecular orientation on substrate surfaces of glass and metal oxides due to a weak adsorbate-substrate interaction [3]. It was recently reported using X-ray studies [4] and a combination of X-ray and infrared (IR) [5] that 3,4,9,10-perylene tetracarboxylic dianhydride /$ - see front matter Ó 2006 Elsevier B.V. All rights reserved. doi: /j.orgel

2 522 O.D. Gordan et al. / Organic Electronics 7 (2006) (PTCDA) template layers can control the stacking direction of H 2 Pc and phthalocyanine derivative films parallel to the surface of a glass substrate. Such a molecular templating technique is one of the easiest methods for fabricating device structures. Moreover, the orientation of the H 2 Pc molecular plane formed on PTCDA is beneficial for photovoltaic cell applications because the stacking direction of the p orbitals predominantly contributes to carrier transport and optical absorption [2]. Especially for this specific device application it is very important to know the optical properties (or the dielectric function) of the active organic layers. In this work these properties were determined using spectroscopic ellipsometry, which is a non-destructive and very surface sensitive technique. The structural as well as the orientational changes within the H 2 Pc layers can be detected from the magnitude of the optical anisotropy. 2. Experiment H 2 Pc and PTCDA layers were deposited on glass (Corning 7059) and SiO 2 /Si (001) substrates by organic molecular beam deposition (OMBD) at a base pressure of mbar. The organic source materials were purified three times by gradient sublimation and were charged into Knudsen cells. The glass substrates were cleaned in ultrasonic baths of acetone and ethanol and then dried in nitrogen flow before being loaded into the OMBD chamber. The SiO 2 /Si structure was fabricated depositing SiO 2 on Si(0 0 1) wafers by electron beam evaporation. After a PTCDA template layer was grown, thick H 2 Pc layers were deposited on it. The sample holder was held at room temperature during all deposition processes. The deposition rate was measured using a quartz oscillator close to the substrate holder and was maintained below 0.1 Å/s. Ellipsometric measurements were performed using a variable angle spectrometric ellipsometer (VASE, J.A. Woollam Co. Inc.). In order to determine the thickness and optical properties of the samples, the measurements were carried out in a large spectral range, from 0.73 to 5 ev, with a 0.02 ev energy step. Measuring the samples at incidence angles around the Brewster angle of the substrate increases the sensitivity of this technique. The samples on glass substrates were measured at several angles of incidence, between 55 and 70 with a step of 5. The measurements for the SiO 2 /Si substrates were performed between 65 and 75 with the same step. In addition, the transmittance of the organic layers on glass substrates was also measured. In ellipsometry the change in the polarisation state of light is measured after reflection on a sample. The relation between the amplitudes of the reflected E r and incident E i electric field can be written in the Jones matrix formalism as E rp E rs ¼ r pp r sp Eip r ps r ss E is The diagonal elements of the Jones matrix represent the change of amplitude and phase of the p- and s-components, while the off-diagonal elements describe the transfer of energy from the p-component to the s-component and vice versa. For isotropic samples or anisotropic uniaxial samples with in-plane isotropy the off-diagonal elements of the Jones matrix are zero. In this case the change in the polarization state can be described in terms of the ellipsometric parameters W and D. The ellipsometric parameters are related to the ratio of Fresnel reflection coefficients r pp and r ss by q ¼ r pp ¼ tan W expðidþ: r ss For the evaluation of the ellipsometry spectra a model which describes the interaction of light with a specific sample structure has to be taken into consideration. A detailed description of the ellipsometry principles and theory can be found in Ref. [6]. 3. Results 3.1. Single layers In order to assess the in-plane anisotropy, all samples were measured at different azimuthal angles. The measurement revealed that the ellipsometric spectra are independent of the azimuthal angle. This indicates that all samples have in-plane isotropy. However, due to their intrinsic molecular anisotropy and their crystalline growth it is known that H 2 Pc [7,8] and PTCDA form anisotropic layers [9,10]. Therefore, an out-of-plane optical anisotropy was considered when modeling the optical response of the organic layers. In this case the in-plane contributions are equal, i.e. e x = e y = e k, while the outof-plane one, perpendicular to the substrate surface, e z = e? 5 e k is different. As the transmission measurements were performed at normal incidence, these data can be used to determine e k for the films deposited on transparent substrates.

3 O.D. Gordan et al. / Organic Electronics 7 (2006) The first step in data evaluation is thickness determination. For an accurate determination of the thickness the optical properties of the substrates are taken into account. The optical response of the silicon reference substrate was simulated using the existent dielectric function in the data base for silicon [11] and for the silicon oxide [12]. The thickness of the silicon oxide was found to be 31 (±0.02) nm. The dielectric function of the glass substrate was determined from a reference ellipsometric measurement. During the fit the data from both ellipsometry and transmission measurements are considered. Especially for anisotropic samples this procedure is particularly useful in lowering the correlation factor between the floating parameters used to simulate the in-plane and out-of-plane contributions. The film thicknesses of the organic layers were determined in the transparent range of the material where the optical properties are described by the Cauchy dispersion relation: nðkþ ¼A þ B k 2 þ C k 4 ; where n is the film refractive index and k is the wavelength. In the numerical fitting procedure A, B, C and the film thickness are floating parameters. Usually A and B are enough to describe the optical dispersion in the transparent range and C is set to zero. For uniaxial anisotropic films two sets of Cauchy relations are used to describe the optical properties in a direction parallel with the film surface and perpendicular to it, respectively. The thicknesses obtained from the ellipsometric data evaluation are presented for the layers on glass in the transmittance plots and in Table 1 for the layers on SiO 2 /Si. After the thickness determination the transmission spectra can be used to determine e k. Considering an isotropic model the simulation of the ellipsometric spectra using the extinction coefficient determined from the absorption together with the Kramers Kronig consistent refractive index does not provide satisfactory agreement with the experimental data. Therefore an uniaxial model has to be considered for the ellipsometric data. In this model two sets of dielectric functions are used to model the sample properties in the directions parallel and perpendicular to the sample surface. In order to have a Kramers Kronig (KK) consistent solution, the imaginary part of the dielectric function is simulated using Gaussian oscillators while the real part is solved employing the KK relation. In order to reduce the correlation between parameters during the fitting procedure, three samples with different thicknesses are considered in the model together with their transmission data. The validity of this evaluation holds as long as the organic films have the same optical properties regardless of the film thickness. In Fig. 1 the simulated and experimental transmittance data of the H 2 Pc films on glass substrates are plotted. The reference glass transmittance is also shown. The anisotropic dielectric function of H 2 Pc is presented in Fig. 2. As all transmission measurements were performed at normal incidence, the simulation of the transmittance spectra is done using the in-plane components. The shape of the absorption bands is in agreement with previous reports [9,13] and indicates that the films consist of a-phase H 2 Pc with its well known herringbone structure [7,8]. Taking into account that the absorption features in the presented spectral range are induced by p p * transitions, which are polarized in the Table 1 The thicknesses of the single layers determined from ellipsometry in the transparent range of the H 2 Pc and PTCDA films on SiO 2 / Si substrates Substrate Material Thickness/nm SiO 2 /Si H 2 Pc 36.7 ± ± ± 1.1 SiO 2 /Si PTCDA 10 ± ± 0.4 Fig. 1. Transmittance spectra of H 2 Pc layers on glass substrates. For comparison the transmittance of the glass substrate is also plotted. The continuous lines represent the model simulation of the transmittance using the in-plane component of the dielectric function.

4 524 O.D. Gordan et al. / Organic Electronics 7 (2006) molecular plane, an average molecular orientation can be deduced from the differences in the extinction coefficient between the xy (in-plane) and the z (perpendicular) contribution [9]. Considering in a first approximation that the H 2 Pc is planar and the overall absorption intensity is the result of a scalar product between the electric field vector and the transition dipole, the average orientation of the molecules with respect to the substrate was found to be 52. This indicates that the growth mode of H 2 Pc is a-phase with the molecular stacking axis oriented parallel with the substrate surface. Due to the reduced ellipsometric sensitivity for the outof-plane component [14] we estimate a possible error in the tilt angle determination of ±4. While we consider that we detect the right anisotropy magnitude, the error is estimated from the slightly different solutions that can be obtained for the out-of-plane component. A similar analysis procedure was applied for the H 2 Pc films grown on the silicon oxide/silicon substrates. The in-plane dielectric function was found to be identical with the one presented in Fig. 2. The out-of-plane component was slightly different leading to a tilt angle of 53. As the in-plane component of the dielectric function is identical for the H 2 Pc films grown on glass with the one of H 2 Pc films grown on silicon substrates and the difference in the out-of-plane component is within the error limits we consider that these films exhibit similar growth modes. These findings are in agreement with recent studies of H 2 Pc orientation studied by infra-red reflection absorption spectroscopy (IRRAS) [5], X-ray diffraction [15] and consistent with previous results derived from ellipsometry for CuPc [10]. A similar analysis was performed for PTCDA molecules grown on glass and silicon oxide/silicon substrates. Fig. 3 shows the anisotropic dielectric function of PTCDA. In Fig. 4 the corresponding experimental transmittance spectra of PTCDA on glass and the corresponding model simulation are presented. The difference in the imaginary part of the dielectric function between the in-plane component and the out-of-plane one indicates that the molecules lie flat on these substrates [5]. The shape of the in-plane dielectric function is in agreement with previous reports [16,17]. Fig. 2. The real and imaginary parts of the dielectric function of the H 2 Pc films deposited on glass and SiO 2 /Si Heterostructures In the ellipsometric data evaluation of the heterostructures H 2 Pc/PTCDA/substrate, the previously determined dielectric function was used for the PTCDA layer. The thicknesses of the PTCDA layers were estimated from transmission measurements, in the range where PTCDA has its strong absorption feature and H 2 Pc has a very low absorption. With the PTCDA thickness fixed, the previously described ellipsometric procedure was applied to find the H 2 Pc thickness. Compared to H 2 Pc single layers a good data simulation was obtained only if surface roughness was considered in the model. Again the thicknesses obtained for the layers on the glass substrates are presented in Fig. 3. The real and imaginary parts of the dielectric function of the PTCDA films deposited on glass and SiO 2 /Si.

5 O.D. Gordan et al. / Organic Electronics 7 (2006) Fig. 4. Transmittance spectra of PTCDA layers on glass substrates. For comparison the transmittance of the glass substrate is also plotted. The continuous lines represent the model simulation of the transmittance using the in-plane component of the dielectric function. Fig. 5. The real and imaginary parts of the dielectric function of the H 2 Pc films deposited on PTCDA. the transmittance figures, while the thicknesses for the layers on SiO 2 /Si are summarized in Table 2. Fig. 5 shows the anisotropic dielectric function of the H 2 Pc layer deposited on PTCDA. As can be seen the in-plane component now is stronger than the out-of-plane one suggesting that the H 2 Pc molecules have a considerably lower tilt angle with respect to the substrate. This is most likely due to the strong interaction of the PTCDA p orbitals with the p orbitals of the first monolayer of H 2 Pc, which leads to a templating effect [4,5,15]. Applying the procedure described above, a 25 average tilt angle was calculated for the molecular orientation. This value is very close to the 27 value deduced from IRRAS [5] and in agreement with a crystalline growth of H 2 Pc with the stacking axis perpendicular with the substrate surface, since the angle between the normal to the molecular plane and b-axis is Fig. 6 shows the transmittance data of the H 2 Pc films on PTCDA layers thicker than 6 nm and the model simulation. The total thickness of the H 2 Pc layers is indicated. In ellipsometry the surface roughness is modelled using an effective medium approximation with 50% voids. The total thickness thus is calculated by adding the surface roughness divided by 2. As can be seen in Fig. 6 the model Table 2 The thicknesses of the organic layers in the heterostructures: H 2 Pc/PCTDA/Substrate determined from the simulation of the ellipsometry spectra using the dielectric functions from figs. 3 (PTCDA) and 5 (H 2 Pc) Substrate PTCDA Thickness/nm H 2 Pc Thickness/nm SiO 2 /Si SiO 2 /Si Surface roughness/nm Fig. 6. Transmittance spectra of H 2 Pc layers on PTCDA/glass substrates with PTCDA thickness higher than 7 nm. For comparison the transmittance of the glass substrate is also plotted. The continuous lines represent the model simulation of the transmittance using the in-plane component of the dielectric function.

6 526 O.D. Gordan et al. / Organic Electronics 7 (2006) simulation matches perfectly for the thick H 2 Pc layers, but a small deviation in the transmission data can be noticed for the 7 nm H 2 Pc layer. This deviation was also observed in the ellipsometric spectra. This is an indication that the thin H 2 Pc layer has a small deviation in the optical constants, when compared to thicker layers. The difference is probably due to a small difference in the molecular packing [15], as the structure of the first H 2 Pc monolayers have a deviation from the a-phase H 2 Pc due to the templating effect of the PTCDA. The same model was applied successfully to H 2 Pc layers grown on PTCDA layers with thicknesses lower than 1.4 nm. This proves that even a very small thickness of PTCDA is sufficient to induce the template effect. Fig. 7 shows the transmittance data for these layers grown on the glass substrate. Also in this case a small deviation from the experimental data was noticed for the 34 nm H 2 Pc layer. As this layer is the thickest, it seems to support a small molecular reorientation with the increasing thickness. However, for this layer a higher uncertainty in calculating the PTCDA layer thickness was noticed due to the small value, probably less than 0.3 nm. The data evaluation for the SiO 2 /Si substrate was more difficult because it was hard to estimate the thickness of the PTCDA layer beneath H 2 Pc. The previous determined dielectric functions of the H 2 Pc on PTCDA and of PTCDA on glass were used to determine the thickness. This evaluation gave a Fig. 8. Comparison between the in-plane imaginary dielectric function of H 2 Pc on glass and SiO 2 /Si and the in-plane imaginary dielectric function of H 2 Pc on PTCDA. very good simulation of the ellipsometric spectra in the measured spectral range, indicating that the films on SiO 2 /Si substrates have the same optical properties like the ones on glass substrates. Fig. 8 shows the imaginary part of the in-plane dielectric function for the H 2 Pc layers grown on glass and SiO 2 /Si substrates, compared with the one of the films grown on PTCDA. Besides the higher values for H 2 Pc grown on PTCDA due to the lower tilt angle with respect to the substrate, which demonstrate the template effect induced by PTCDA, a splitting of the Q-band can be observed. The splitting is also visible in the transmission measurements. While the difference can be attributed to the observation of the optical properties of the a- phase H 2 Pc in different crystal direction, the splitting can also be a sign of a small deviation from the perfect H 2 Pc a-phase crystal. As the degree of overlapping of the H 2 Pc p orbitals changes the shape of the Q-band thus making it possible to distinguish between the different crystalline forms of H 2 Pc [7,8], the possibility that PTCDA induces a deviation from the perfect a-phase crystal has to be taken into consideration [15,18]. 4. Summary Fig. 7. Transmittance spectra of H 2 Pc layers on PTCDA/glass substrates with PTCDA thickness less than 2 nm. For comparison the transmittance of the glass substrate is also plotted. The continuous lines represent the model simulation of the transmittance using the in-plane component of the dielectric function. The optical response of H 2 Pc layers grown on SiO 2 /Si and glass substrates and on PTCDA were determined using spectroscopic ellipsometry. Due to the intrinsic anisotropy of the H 2 Pc and PTCDA, the optical anisotropy was considered when modeling the optical response of organic layers. For a

7 O.D. Gordan et al. / Organic Electronics 7 (2006) H 2 Pc layer grown on a glass substrate, the tilt angle of H 2 Pc molecule with respect to the substrate surface was found to be 52 while its angle for H 2 Pc deposited on PTCDA layer was estimated to be approximately 25. Such variation of the H 2 Pc molecular orientation is consistent with IR-RAS and XRD results reported previously [5] and is caused by the van der Waals intermolecular interaction between PTCDA and H 2 Pc at the heterointerface. The simulated results of transmittance spectra, which were calculated from the in-plane components of the dielectric functions, match perfectly with the experimental results. While the lower orientation angle of the H 2 Pc on PTCDA increases the light absorption efficiency, the overlapping of the p orbitals in a direction perpendicular to the film surface should also decrease the electrical resistance in this direction. This effect could thus be useful for the organic solar cells construction. The splitting of the Q-band that was observed in both transmission and the imaginary part of the inplane dielectric function for H 2 Pc/PTCDA heterostructure might be caused by a small deviation from the perfect a-h 2 Pc crystal. Therefore, the reliable optical parameters of H 2 Pc films grown on glass and PTCDA can be determined from the simulation of ellipsometric spectra proving that this technique can be a useful tool for improving the structural and optical properties of the organic films. Acknowledgement We gratefully acknowledge the financial support of the Deutsche Forschungsgemeinschaft, Graduiertenkolleg 829 Akkumulation von einzelnen Molekülen zu Nanostrukturen. References [1] C.J. Brabec, N.S. Sariciftci, J.C. Hummelen, Adv. Funct. Mater. 11 (2001) 15. [2] M. Pope, C. Swenberg, Electronic Processes in Organic Crystals and Polymers, second ed., Oxford University Press, New York, [3] S. Tokito, J. Sakata, Y. Taga, Thin Solid Films 256 (1995) 182. [4] S. Heutz, R. Cloots, T.S. Jones, Appl. Phys. Lett. 77 (2000) [5] T. Sakurai, S. Kawai, J. Shibata, R. Fukasawa, K. Akimoto, Jpn. J. Appl. Phys. 44 (2005) [6] R.M.A. Azzam, N.M. Bashara, Ellipsometry and Polarized Light, Elsevier, Amsterdam, [7] N.B. McKeown, Phthalocyanine Materials, Cambrige University Press, [8] C.C. Leznoff, A.B.P. Lever, Phthalocyanines, Properties and Applications, VCH, New York, [9] O. Gordan, M. Friedrich, D.R.T. Zahn, Thin Solid Films (2004) 551. [10] O. Gordan, M. Friedrich, D.R.T. Zahn, Org. Electron. 5 (2004) 291. [11] G.E. Jellison, Opt. Mater. 1 (1992) 41. [12] E. Palik (Ed.), Handbook of Optical Constants of Solids, vol. 1. Academic Press, p [13] H. Arwin, D.E. Aspnes, Thin Solid Films 138 (1986) 195. [14] D.E. Aspnes, J. Opt. Soc. Am. 70 (10) (1980) [15] T. Sakurai, R. Fukasawa, K. Akimoto, Jpn. J. Appl. Phys. 45 (2006) 255. [16] O. Gordan, S. Hermann, M. Friedrich, D.R.T. Zahn, J. Appl. Phys. 97 (2005) [17] M. Friedrich, Th. Wagner, G. Salvan, S. Park, T.U. Kampen, D.R.T. Zahn, Appl. Phys. A 75 (2002) 501. [18] S. Heutz, G. Salvan, S.D. Silaghi, T.S. Jones, D.R.T. Zahn, J. Phys. Chem. B 107 (2003) 3782.