Determination of the Anisotropic Optical Properties for Perfluorinated Vanadyl Phthalocyanine Thin Films

Determination of the anisotropic optical properties for perfluorinated vanadyl phthalocyanine thin films

O.D. Gordana) and M. Friedrich Semiconductor Physics, University of Technology Chemnitz, D-09107 Chemnitz, Germany W. Michaelis Physical Chemistry 1, Institute of Pure and Applied Chemistry, Faculty of Science, University of Oldenburg, D-26111 Oldenburg, Germany R. Kröger Institute of Solid State Physics, University of Bremen, D-28334 Bremen, Germany T. Kampen Physics Institute, University of Technology Chemnitz, D-09107 Chemnitz, Germany D. Schlettwein Physical Chemistry 1, Institute of Pure and Applied Chemistry, Faculty of Science, University of Oldenburg, D-26111 Oldenburg, Germany D.R.T. Zahn Semiconductor Physics, University of Technology Chemnitz, D-09107 Chemnitz, Germany

(Received 5 November 2003; accepted 22 January 2004)

Thin films of perfluorinated vanadyl phthalocyanine F16PcVO were prepared by physical vapor deposition in high vacuum on KBr and fused silica substrates. The absorption spectra in the visible region show that the films on different substrates have

different structure. The optical constants for F16PcVO films were obtained in the spectral range of 0.7–4.5 eV from the simulation of ellipsometry spectra with an anisotropic uniaxial model. From the difference between the in-plane and out-of-plane

components of the extinction coefficient the average tilt angle of the F16PcVO molecular planes with respect to the substrate plane was found to be 56° for fused silica substrates and between 0° and 3° for KBr substrates.

I. INTRODUCTION For any optoelectronic device application, the knowl- The growing interest in obtaining new electronic de- edge of the optical constants is vitally important. Unlike vices implies the application of new materials. In the last most inorganic semiconductors organic semiconductors decade, research has also focused on organic materials as show a pronounced optical anisotropy as a result of their potential candidates for active layers in devices. Amongst anisotropic molecular structure. Therefore the aim of this these materials which possess a high thermal and chemi- work is to determine the anisotropic optical properties cal stability, as well as high optical absorption in the using F16PcVO as a representative molecule of the Pc visible range are, e.g., phthalocyanines (Pc’s).1–7 This class of materials. Studies regarding the anisotropic op- class of organic materials has been extensively used in tical properties of organic semiconductors employing the past as dyes and more recently they have proven their spectroscopic ellipsometry are still rare. The implemen- tation of new mathematical algorithms15 based on4×4 applicability in devices such as organic photovoltaic 16 cells,3,8 organic field effect transistors (OFETs),9 organic transfer matrix formalism developed by Berreman is a 10 11 promising approach to tackle the problem. As reported light emitting diode (OLEDs), and gas sensors. The 17 phthalocyanine properties can be tuned by the choice of previously for metal-free phthalocyanine, the determi- the central metal ion and by chemical substitutions at nation of the anisotropic dielectric function from spec- the ligand. This was demonstrated for unsubstituted troscopic ellipsometry does not only yield physically re- phthalocyanines1,3,12 and in particular for perfluorinated liable values, but also allows the orientation of the mol- phthalocynanies.13,14 ecules with respect to the substrate to be determined.

II. EXPERIMENTAL a)Address all correspondence to this author. e-mail: [email protected] Organic thin films of perfluorinated vanadyl phthalo- DOI: 10.1557/JMR.2004.0264 cyanine (F16PcVO) were prepared by physical vapor

2008 J. Mater. Res., Vol. 19, No. 7, Jul 2004 © 2004 Materials Research Society O.D. Gordan et al.: Determination of the anisotropic optical properties for perfluorinated vanadyl phthalocyanine thin films deposition (PVD) under high vacuum conditions. The For the evaluation of the ellipsometry spectra a model

F16PcVO was synthesized as described in Ref. 13 and which describes the interaction of light with a specific prior to deposition purified by sublimation in a three material has to be taken into consideration. Afterwards a zone temperature gradient oven (Lindberg). The KBr(100) complex numerical fitting procedure is used to simulate substrates were cleaved by means of a steel knife from a the experimental spectra. The mean-square error (MSE) single crystal purchased from Dr. W. Schrader (Braun- which gives the differences between the model and the schweig, Germany). Immediately after cleavage the sub- experimental points is defined as follows strates were transferred into a high vacuum chamber MSE = (5 × 10−8 mbar) and annealed for 16 h at 500–600 K to desorb water and other adsorbates. The fused silica sub- 1 N ⌿mod − ⌿exp 2 ⌬mod − ⌬exp 2 ͱ ͫͩ i i ͪ + ͩ i i ͪͬ strates were bought from Menzel Gläser, Braunschweig, Α exp exp , 2N − M = ␴ ␴ Germany, and washed in an ultrasonic bath subsequently i 1 ⌿,i ⌬,i with acetone, ethanol, and de-ionized water. where N is the number of the experimental points, M is During the deposition the substrate temperature was the number of fit parameters, and ␴ is the standard de- kept constant at 42 °C for KBr crystals, and at 85 °C for viation for each point. The standard deviation is used to fused silica. The thickness of the organic material was weight the contributions of each data point to the mean- monitored by a 6 MHz resonance quartz crystal micro- squared error during the fitting process, such that very balance (QCM, Conrad Electronik, Braunschwelg, Ger- noisy data points are effectively excluded from the fit.18 many). The deposition rate was kept constant. Absorption spectra in the ultraviolet-visible (UV-vis) region were recorded using a double beam spectrometer III. RESULTS Specord M 40 (Carlzeiss, Jena, Germany). Afterward, A. UV-vis absorption ellipsometric measurements were carried out with a vari- The phthalocyanines are usually characterized by an able angle spectrometric ellipsometer (VASE, J.A. intense absorption band in the visible range (Q-band) and Woollam Co. Inc., Lincoln, NE). To determine the film another broad absorption band in the near ultraviolet thicknesses and the wavelength dependence of the opti- range (B-band)1 Both bands correspond to ␲−␲* transi- cal constants ellipsometric spectra were recorded at dif- tions. In Fig. 1 the absorption spectra of the F16PcVO ferent angles of incidence (50°, 60°, and 70°) in the range films on fused silica and KBr substrates are presented. of 0.73–4.5 eV with a 0.02 eV step for each sample. Prior Additionally the fused silica glass absorption spectrum is to the ellipsometric measurement all substrates were plotted. The shape of the B-band for the F16PcVO films back roughened to avoid back side reflections. Bare sub- on fused silica cannot be very well distinguished as the strates were also measured and their optical constants were determined in the same spectral range. In ellipsometry, the changes in the polarization state of the light are measured after the reflection on the sample.15,16,18,19 In the Jones matrix formalism the rela- tion between the amplitudes of the reflected Er and inci- dent Ei electric field is written as E r r E ͩ rpͪ = ͩ pp spͪͩ ipͪ . Ers rps rss Eis The diagonal elements of the Jones matrix represent the change in 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 uni- axial films with an in-plane isotropy on isotropic sub- strates 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 ⌿ and ⌬. The ellipsometric parameters are related to the ratio of Fresnel reflection coefficients rpp and rss by FIG. 1. UV-vis absorption spectra of F PcVO on fused silica and on r 16 ␳ = pp = tan⌿exp͑i⌬͒ . KBr substrates. Continuous line represents absorption spectrum of rss fused silica. The KBr crystal is absorption free in this range.

J. Mater. Res., Vol. 19, No. 7, Jul 2004 2009 O.D. Gordan et al.: Determination of the anisotropic optical properties for perfluorinated vanadyl phthalocyanine thin films fused silica glass starts to absorb at 450 nm. The KBr The surface roughness was taken into account using an crystal is absorption free in this range. In the case of an effective medium approximation layer. It consists of a isolated molecule the absorption band at lower energy mixture of 50% film material and 50% voids. The optical (Q-band) corresponds to highest occupied molecular refraction index in the absorption free range is simulated orbital-lowest unoccupied molecular orbital (HOMO- using a Cauchy relation for the isotropic model while two LUMO) transitions while the broad band of the film can different Cauchy dispersion relations are used for the be explained by strong excitonic interactions. Clear dif- uniaxial model to describe the optical refraction index for ferences of the F16PcVO spectra can be observed for in-plane and out-of-plane directions of the film. The different substrates. For the KBr substrate the spectrum is Cauchy relation is valid just for those spectral regions dominated by a strong absorption peak near 1.65 eV (750 where the material is transparent and the index of refrac- nm) while for the fused silica substrate the band centered tion n can be represented by a slowly varying function of at about 1.9 eV (650 nm) is the strongest. According to wavelength ␭ Schlettwein et al.13 the shape of the Q-band of the B C F16PcVO on KBr can be explained by a head-to-tail ar- ͑␭͒ = + + n A 2 4 . rangement of the chromophores, while the F16PcVO film ␭ ␭ on fused silica has a cofacial (face-to-face) arrangement of the molecules. During the fit the parameters are the film thickness, A Since the phthalocyanine’s ␲ orbitals are antisymmet- and B. Usually the A and B parameters describe the dis- ric with respect to the molecular plane, all allowed ␲−␲* persion well enough, so C is set to 0. The difference for transition are in-plane polarized. Considering in a first the film thickness between isotropic and anisotropic models for KBr substrates (Table II) is related to the approximation that the F16PcVO is planar and the overall absorption intensity is the result of a scalar product be- difference in the A value calculated for isotropic and tween the electric field vector and the transition dipole, anisotropic models. This is consistent with a strong in- an estimate of the average orientation of the molecules plane/out-of-plane anisotropy for F16PcVO on KBr sub- with respect to the substrate could be obtained from the strates, while the anisotropy is not so strong for films on differences in the absorption intensities for the two dif- fused silica substrates. 13 To determine the optical constants in a wider range, a ferent substrates. The F16PcVO molecules are thus ly- ing on the KBr substrates while for the fused silica substrate more complex model was used. For the isotropic one, a the molecules are predominantly vertical to the surface. point-by-point fit was performed keeping the film thick- ness and surface roughness fixed at the previously deter- B. Ellipsometry: Isotropic versus mined values. In addition, the wavelength dependence of anisotropic case optical constants was described by mathematical func- tions.18 The advantage is that the function can be made The first assessment of the ellipsometric spectra re- to be internally KK (Kramers–Kronig) consistent. The vealed that the off-diagonal terms of the Jones matrix rsp, ␧ shape of the imaginary part of the dielectric function 2 rps were 0. Rotating the samples in the azimuthal plane ␧ was simulated using a set of Gaussian functions while 1 indicated in-plane isotropy. Therefore isotropic and uni- is solved in KK consistency. The same procedure was axial anisotropic models were used to fit the experimen- applied for the uniaxial model using two sets of functions tal data. one set for the in-plane direction of the film and the other An uniaxial model and an isotropic model were used for the out-of-plane one. As starting point for the fit the using WVASE software. The film thicknesses for two optical constants determined from the isotropic model sets of samples were determined in the absorption free were used. Especially for the z component (normal to the range (below 1.1 eV) as presented in Table I for the fused surface) it is usually difficult to obtain reliable optical silica substrates and in Table II for the KBr substrates. constants for thin uniaxial films. To overcome this problem we used a special multiple-sample analysis TABLE I. Thickness and surface roughness for the F16PcVO films on fused silica substrates determined from ellipsometry using isotropic and uniaxial model. TABLE II. Thickness and surface roughness for the F16PcVO films on KBr substrates determined from ellipsometry using isotropic and Sample 1 Sample 2 uniaxial model. F16PcVO/fused silica F16PcVO/fused silica Sample 3 Sample 4 Film Surface Film Surface F PcVO/KBr F PcVO/KBr thickness roughness thickness roughness 16 16 Model MSE (nm) (nm) (nm) (nm) Model MSE Film thickness (nm) Film thickness (nm) Isotropic 1.4 81.9 ± 6.0 6.3 ± 1.0 24.1 ± 1.3 0 ± 0.3 Isotropic 1.1 62.6 ± 0.4 21.6 ± 0.1 Anisotropic 1.0 82.0 ± 3.0 19.4 ± 1.8 25.4 ± 0.9 11.6 ± 0.6 Anisotropic 1.0 76.5 ± 5.8 27.0 ± 0.1

2010 J. Mater. Res., Vol. 19, No. 7, Jul 2004 O.D. Gordan et al.: Determination of the anisotropic optical properties for perfluorinated vanadyl phthalocyanine thin films procedure,19,20 which assumes that films with different thicknesses but on the same substrate have the same op- tical constants.

C. F16PcVO films on fused silica substrates Figure 2 presents the ellipsometric spectra ⌿, ⌬ and .3.9 ס the fit applying the anisotropic model with a MSE The isotropic fit does not yield such a good result: Due to the fact that the anisotropic model .16.2 ס MSE has more floating parameters during the fit than the iso- tropic one, a lower MSE value for the first one is ex- pected. Therefore the value of the MSE itself cannot be used to judge which model is the more physically appro- priate. However, a comparison of the extinction coeffi- cients k obtained from the uniaxial model respectively from the isotropic model with that calculated from the FIG. 3. Extinction coefficient k for the F PcVO film on fused silica. absorption spectrum supports the appropriateness of the 16

FIG. 4. Anisotropic optical constants for F16PcVO film on fused FIG. 2. Ellipsometric ⌿ and ⌬ spectra at different angles of incidence silica: (a) extinction coefficient and (b) real port of the refractive for the 82 nm F16PcVO film on fused silica. Open symbols are the index. Continuous line represent in-plane components, dashed line experimental points and continuous lines the fits. out-of-plane components.

J. Mater. Res., Vol. 19, No. 7, Jul 2004 2011 O.D. Gordan et al.: Determination of the anisotropic optical properties for perfluorinated vanadyl phthalocyanine thin films

shoulder of the Q-band is not well resolved, even though the same number of oscillators was used as for the an- isotropic model.

The anisotropic optical constants for F16PcVO on fused silica are presented in Fig. 4. Since the F16PcVO molecule has an intrinsic optical anisotropy due to its structure, the anisotropy of the film can be explained with a preferential orientation of the molecules with re- spect to the substrate. Taking into account that the tran- sition dipole lies in the molecular plane an average tilt angle of the molecules of 56° with respect to the sub- strate was calculated from the differences between in- plane and out-of-plane absorption.

D. F16PcVO films on KBr substrates The films on KBr substrates were found to have opti- cal constants varying with thickness. This is most likely related to the growth mode which involves initial island growth. Apparently very thin films have a smaller aver- age density compared to thicker films. This also prohibits the application of the multi-sample analysis. The thinner film was fitted successfully with the isotropic model Our efforts were, however, concentrated on .(2 ס MSE) simulating the optical constants for the thicker film (see Table II). For this 76.5 nm thick film the isotropic fit yields a large MSE value of 22 while the MSE was lowered to 2 using an anisotropic fit. The ellipsometric spectra ⌿, ⌬ and the fit applying the anisotropic model are presented in Fig. 5. The in-plane values for the ex- tinction coefficient from this model together with the ones from the isotropic model and the one calculated from transmission are plotted in Fig. 6. The minor dif- ferences between the anisotropic model and transmission can be due to the presence of multiple cleavage planes in ⌿ ⌬ FIG. 5. Ellipsometric and spectra at different angles of incidence the KBr substrates. To model the ellipsometric data in a for the 63 nm F16PcVO film on KBr substrate. Open symbols are the experimental points and continuous lines the fits. anisotropic model, as can be seen in Fig. 3. The extinc- tion coefficient was calculated from transmission spectra of the film Tfilm and the bare substrate Tsubstrate as follows −ln͑T րT ͒ k = film substrate , UV−vis 2␲␯d where ␯ is the wavenumber, and d is the total film thick- ness. The total film thickness is the film thickness cal- culated from ellipsometry plus the surface roughness di- vided by 2. Since the absorption measurement was done in normal transmission the extinction coefficient calcu- lated can be compared only with the in-plane component of the uniaxial model. The extinction coefficient values from the anisotropic model is in very good agreement with that calculated from UV-vis for the Q-band, whereas the one calculated from the isotropic model is shifted considerably. Also the FIG. 6. Extinction coefficient k for the F16PcVO film on KBr substrates.

2012 J. Mater. Res., Vol. 19, No. 7, Jul 2004 O.D. Gordan et al.: Determination of the anisotropic optical properties for perfluorinated vanadyl phthalocyanine thin films three phase model (ambient-film-substrate) sharp inter- From the strong in-plane-/out-of-plane anisotropy the faces between these media have to be assumed. If the average orientation of the molecules can be calculated using substrate is rough or the interface between substrate and the procedure described above. Neglecting out-of-plane ab- film is otherwise degraded, then the three phase model is sorption the molecules lie perfectly flat on the KBr sub- inappropriate and more complex models have to be used. strate, while considering a very small absorption results in It is, however, obvious from the comparison in Fig. 6 that an average tilt angle of 3°. This is in very good agreement the isotropic approach leads to stronger deviations from with RHEED results obtained by Schlettwein et al.13 the extinction coefficient derived from absorption. The anisotropic optical constants for F16PcVO on KBr are IV. SUMMARY presented in Fig. 7 Since the out-of-plane absorption is Molecular thin films of F16PcVO were prepared by very small the optical properties can be modeled with PVD on KBr and fused silica substrates. The UV-vis both small absorption and without absorption in the en- spectra show that the films on fused silica and on KBr tire spectral range. It remains a future task to determine substrates have different structures. The optical constants the out-of-plane optical constants with higher accuracy. were determined from ellipsometry data. By comparing isotropic and anisotropic models the films are found to be uniaxial. The orientation of the molecules with respect to the substrate was deduced from the difference between the in-plane and the out-of-plane components of the op- tical constants.

ACKNOWLEDGMENT This work was financially supported by Deutsche Forschungsgemeinschaft, Graduiertenkolleg 829 “Akku- mulation von einzelnen Molekülen zu Nanostrukturen.”

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FIG. 7. Anisotropic optical constants for F16PcVO film on KBr sub- 18. J.A. Woollam: Guide to Use WVASE32. J.A. Woollam Co., Inc. strate: (a) extinction coefficient and (b) real port of the refractive 19. R.M.A. Azzam and N.M. Bashara: Ellipsometry and Polarized index. Continuous line represents in-plane components, dashed line Light (Elsevier, Amsterdam, The Netherlands, 1992). out-of-plane components. The out-of-plane real part, when no absorp- 20. W.A. Mcgahn, B. Johs, and J.A. Wollam: Thin Solid Films 234, tion is considered, is also plotted. 443 (1993).

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