On morphological, structural and electrical properties of vacuum deposited pentacene thin films

Vacuum 83 (2009) 1159–1163

Contents lists available at ScienceDirect

Vacuum journal homepage: www.elsevier.com/locate/vacuum

On morphological, structural and electrical properties of vacuum deposited pentacene thin films M. Girtan a, *, S. Dabos-Seignon a, A. Stanculescu b a b

Angers University, POMA Laboratory, FRE CNRS 2988, 2 Bd. Lavoisier, 49045 Angers, France National Institute of Materials Physics, Bucharest, Romania

a r t i c l e i n f o

a b s t r a c t

Article history: Received 16 July 2008 Received in revised form 15 January 2009 Accepted 1 March 2009

Films of different thickness (50, 100, 150 and 200 nm) were deposited by thermal evaporation in vacuum on two types of substrates glass and ITO. The deposition was performed under a pressure of 106 mB with a rate of 0.25 nm/s. Films surface investigations showed morphological and structural changes in function of films thickness and the nature of the substrate. Films optical transmission was analysed in the 280–1600 nm spectral range and the electrical measurements were done in low vacuum (101:102 mB) and in dark. Ó 2009 Elsevier Ltd. All rights reserved.

Keywords: Vacuum thermal evaporation Organic molecules

1. Introduction

2. Experimental

The employment of organic molecules in microelectronics offers the possibility for creating low weight, mechanical flexibility and low cost electronic devices. In recent years many molecules have been studied. Among these, in the past decade, we remark a growing interest for pentacene (C22H14) films in the preparation of organic electronic devices, especially for field effect transistors [1–15]. The pentacene organic molecule is constituted by a linear chain of five planar benzenic rings. The good charges mobility and small optical bad gap of this material make it suitable also for photovoltaic applications. However, at present, only few researches concerning use of pentacene for solar cells, were done [16–20]. In order to understand and to improve the properties of organic devices more investigations on thin films physical properties are necessary. The aim of our paper is to study the structural, morphological, electrical and optical properties of pentacene films of different thickness, deposited by thermal vacuum evaporation on two types of substrates namely glass and Indium Tin Oxide (ITO), in order to put in evidence which are the parameters that could influence the physical properties of films and therefore the performances of the organic devices.

Pentacene thin films of different thickness (50, 100, 150 and 200 nm) were deposited by thermal evaporation in vacuum onto unheated glass and ITO substrates, in an Edwards evaporation plant. The source material was Aldrich pentacene powder with 99.99% purity. The deposition was performed under a pressure of 106 mB and a rate of 0.25 nm/s. Soda lime glass plates from Knittel Gla¨ser and commercially Indium Tin Oxide (ITO) films with a thickness of 850 nm and a mean roughness of 10 nm, from Thin Films Devices, were used as substrates. The deposition rates of pentacene and films thickness were controlled using an Edwards FTM 7 quartz oscillator. The distance between evaporation source and substrates holder was about 15 cm. The structural characterisation was made by Cu-Ka (l ¼ 1.5406 Å) X-ray diffractometry (XRD) and the films surface morphology was determined by Atomic Force Microscopy (AFM). The transmittance spectra were recorded in the wavelength range 280–1600 nm with unpolarized light, at room temperature, using a Lambda 19 UV–vis spectrophotometer. The electrical measurements were done in dark and at low vacuum (101:102 mB) using surface type cells. 3. Results and discussion

* Corresponding author. Tel.: þ33 241 735359; fax: þ33 241 735216. E-mail address: [email protected] (M. Girtan). 0042-207X/$ – see front matter Ó 2009 Elsevier Ltd. All rights reserved. doi:10.1016/j.vacuum.2009.03.001

The XRD patterns for films having various thicknesses, deposited on glass, are illustrated in Fig. 1. As one can see, films present a layered structure with a (001) orientation parallel to substrate.

1160

M. Girtan et al. / Vacuum 83 (2009) 1159–1163 Table 1 The 2q values corresponding to Figs. 1 and 2 peaks and dhkl are the calculated distances for l ¼ 1.5406 Å; TFP indicate the thin film phase and BP indicate the bulk phase. Sample Pentacene films on glass (a) – 50 nm TFP (b) – 100 nm TFP BP (c) – 200 nm

TFP

BP Pentacene films on ITO (a) – 50 nm TFP (b) – 100 nm TFP (c) – 200 nm

TFP

Observed 2q from XRD ( )

dhkl (Å)

h

k

l

5.90 5.90 11.61 6.29 5.90 11.61 17.36 6.29 12.40

14.97 14.97 7.62 14.04 14.97 7.62 5.10 14.04 7.13

0 0 0 0 0 0 0 0 0

0 0 0 0 0 0 0 0 0

1 1 2 1 1 2 3 1 2

5.79 5.79 11.52 5.79 11.52

15.25 15.25 7.68 15.25 7.68

0 0 0 0 0

0 0 0 0 0

1 1 2 1 2

Fig. 1. XRD spectra of various thickness pentacene films, grown in high vacuum onto glass substrates.

Reported studies on pentacene films structure revealed the existence of at least four polymorphs [21] but more frequently were observed two polymorphs [22]: one is attributed to a thermodynamically stable phase called ‘‘single-crystal phase’’ or ‘‘bulk phase’’ (BP) and the other one to a kinetically favoured, metastable phase called ‘‘thin film phase’’ (TFP). For our samples, the first order diffraction peak, attributed to ‘‘film phase’’, appears at 2q ¼ 5.91, which corresponds to a vertical periodicity of 14.94 Å. As might be expected the intensity of peaks increases gradually with films thickness. For thicker films the

results show the coexistence of both phases: (TFP and BP), the second diffraction peak appears at 6.29 and is attributed to ‘‘bulk phase’’. On the contrary, the polymorphism for thicker films deposited, in the same conditions, on ITO wasn’t evidenced (Fig. 2). These confirm the observations of other authors indicating that pentacene films polymorphism is depending on substrate nature and on films thicknesses [21,23]. The inset in Fig. 2, depicts the XRD pattern of ITO substrate and Table 1 resumes the diffraction data for pentacene films deposited on glass and respectively ITO substrates. The crystallites mean size (for TFP) was calculated using Debye-Scherrer formula [24] and the obtained values are tabulated in Table 2. It results that the

Fig. 2. XRD spectra of various thickness pentacene films onto ITO film substrates within inset of the XRD spectra of ITO film substrate.

M. Girtan et al. / Vacuum 83 (2009) 1159–1163

1161

Table 2 Average crystallite size, D, for thin film phase of pentacene films having various thicknesses, deposited on glass and ITO respectively.

Table 3 Roughness parameters of pentacene films, having various thicknesses, deposited on glass and ITO respectively.

Sample

Sample

Film thickness (nm) 50

100

200

58 60

78 60

50

D (nm) Pentacene film on glass Pentacene film on ITO

35 58

Film thickness (nm) 100

200

19.1 12.7

24.2 12.6

RMS (nm) Pentacene film on glass Pentacene film on ITO

12.4 11.0

Fig. 3. AFM images of pentacene films of different thickness deposited onto ITO film and glass substrates: pentacene film on glass substrate a) 50 nm, b) 100 nm c) 200 nm; pentacene film on ITO film substrate d) 50 nm, e) 100 nm, f) 200 nm. Here ‘‘P’’ label means pentacene film on glass and ‘‘P/ITO’’ means pentacene film on ITO.

1162

M. Girtan et al. / Vacuum 83 (2009) 1159–1163

ON ITO ON GLASS

THICKNESS 200 nm 100

Z, nm

80 60 40 20 0 0,0

0,5

1,0

1,5

2,0

X,µm Fig. 4. Surface profile of 200 nm films deposited on glass and ITO respectively.

crystallites sizes increase with thickness, for films deposited on glass and remains quite unchanged for films deposited on ITO. AFM images were done for films having various thicknesses, deposited on glass and also on ITO. Fig. 3, shows the atomic force microscopy films topography. These pictures reveal that pentacene morphology is strongly influenced by the substrate nature. The grains are much larger for films deposited on glass and their sizes increase with the thickness of the films, while for films deposited on ITO, grains are smaller than of those deposited on glass and no change in grain size is observed with the increase of film thickness. The Root Mean Square (RMS) values of roughness obtained by the analysis of AFM data are given in Table 3. Fig. 4 evidenced the surface profiles in the case of 200 nm films deposited on glass and ITO respectively. The profile peaks of

pentacene films deposited on glass reveal an enlargement, which could be attributed to the coexistence of the two phases mentioned above. All these results indicate a good agreement between AFM observations and current XRD analysis. The optical band gap of pentacene films was calculated by using the spectral data. Fig. 5 presents the absorption spectra of films having different thickness (deposited on glass). A series of electronic transitions are observed with the absorption onset at 1.85 eV. Here a is the absorption coefficient and hn is the photon energy. The optical gaps were obtained by plotting (ahn)2 vs. hn (see the inset in Fig. 5). Considering the band gap model and assuming direct optical transitions, this plots yield a band gap HOMO-LUMO of 1.80 eV, which confirms the values given in literature for films deposited on other type of substrates [25]. This value of band gap makes this material a good candidate for organic photovoltaic applications. Another important result is that the HOMO-LUMO pentacene gap does not show any variation with films thickness and neither with substrate nature (the same values were obtained for pentacene films deposited on ITO). Jurchescu et al. [26] showed that pentacene films are generally sensitive to the atmosphere exposure and at illumination. They observed that water present in ambient air could form trapping sites which conduct to a decrease of conductivity. In contrast, oxygen absorption by diffusing of air into pentacene films, increases the conductivity. This phenomenon is reversible and the air could be completely removed by re-evacuation. To avoid such effects and keep the same studying conditions, we have done our measurements in dark and under vacuum. The films resistivity was measured during heating, using electrical fields of different intensities. The temperature was varied from 290 K to 400 K; this range corresponds in general, to the range of materials heating at exposure to Sun. Electrical investigations required many repetitive measurements. The films were deposited on ITO patterned electrodes with a distance between electrodes of

Fig. 5. The absorption spectra a vs. hn (photons energy in eV) of pentacene films with various thickness (50 nm, 100 nm, 150 nm) deposited on glass. Inset (ahn)2 vs. hn plots of different thickness pentacene films.

M. Girtan et al. / Vacuum 83 (2009) 1159–1163

1163

influenced by the substrate nature and neither by the film thickness, despite of the fact that AFM studies revealed big differences on the surface morphology in function of substrate nature and film thickness. The electrical resistivity shows a more pronunciation variation with temperature if measurements are done using high electric fields (more than 8  103 V/m), at low electric fields no significant changes in resistivity values with temperature were noticed for the studied domain.

Acknowledgments Authors are grateful to Prof. J.M. Nunzi from Queen’s-University, Kingston for providing the necessary vacuum system set-up, to Prof. N. Mercier from CIMA – Laboratory, Angers for providing the necessary facilities for XRD studies and ANR-OxTiMIBPhotobatterie for the financial support.

References Fig. 6. Pentacene film electrical resistivity vs. temperature, for various intensities of electrical field.

about 1 mm. Fig. 6 illustrates the results for a 150 nm thick pentacene film, but similar behaviours were putted in evidence for films of various thickness. For the same temperature, the resistivity increases constantly with the applied electric field. Equally, should be noticed that, within the studied domain of temperatures, the charge transport is thermally activated only for higher electric fields (1.6  104 V/m). These results are of great importance for the potentially future applications of pentacene films in solar cells. Knowing that during the exposure to solar radiation, cells temperature increase, this analyze put in evidence that we must be taking into account supplementary transport phenomena in function of the order of magnitude of electric fields present in films. Generally films thickness being very small, the electric field into films becomes very important. As an example: by applying a transversal bias of 1 V (currently value used to determine the current–voltage characteristics) to a film of 100 nm thickness the magnitude of the electric field inside the films is of 107 V/m. 4. Conclusions Crystalline pentacene films were obtained by vacuum thermal evaporation. The obtained films show a (001) orientation parallel to the substrate. Between structural and morphological properties a good correlation was found. The optical band gap is not

[1] Dimitrakopoulos CD, Malenfant PRL. Adv Mat 2002;14(2):99. [2] Lin YY, Gundlach DJ, Nelson SF, Jackson TN. IEEE, Electron Devices Lett 1997;18(12):606. [3] Lin YY, Gundlach DJ, Nelson SF, Jackson TN. IEEE, Trans Electron Dev 1997;44(8):1325. [4] Nelson SF, Lin YY, Gundlach DJ, Jackson TN. Appl Phys Lett 1998;72(15):1854. [5] Klauk H, Halik M, Zschieschang U, Schmid G, Radlik W, Weber W. J Appl Phys 2002;92(9):5259. [6] Knipp D, Street RA, Volkel A, Ho J. J Appl Phys 2003;93(1):347. [7] Schon JH, Berg S, Kloc Ch, Batlogg B. Science 2000;287(5455):1022. [8] Newman CR, Frisbie CD, Da Silva Filho DA, Bredas JL, Ewbank PC, Mann KR. Chem Mater 2004;16(23):4436. [9] Afzali A, Dimitrakopoulos CD, Breen TL. J Am Chem Soc 2002;124(30):8812. [10] Rost C, Karg S, Reiss W, Loi MA, Murgia M, Muccini M. Synth Met 2004;146(3):237. [11] Majewski JA, Schroeder R, Grell M. Appl Phys Lett 2004;85(16):3620. [12] Pannemann Ch, Diekmann T, Hilleringmann U. Microel Eng 2003;67–68:845. [13] Cui T, Liang G. Appl Phys Lett 2005;86(6):1. [14] Kumaki D, Yahiro M, Inoue Y, Tokito S. J Photopol Sci Technol 2006;19(1):41. [15] Lee S, Koo B, Park J-G, Moon H, Hahn J, Kim JM. MRS Bull 2006;31(6):455. [16] Yoo S, Domercq B, Kippelen B. Appl Phys Lett 2004;85:22. [17] Pandey AK, J-Nunzi M. Appl Phys Lett 2006;89:213506. [18] Yang J, Nguyen TQ. Organic Electronics 2007;8(5):566. [19] Sullivan P, Jones TS. Organic Electronics 2008;9(5):656. [20] Palilis LC, Lane PA, Kushto GP, Purushothaman B, Anthony JE, Kafafi ZH. Organic Electronics 2008;9(5):747. [21] Mattheus CC, Dros AB, Baas J, Oostergetel GT, Meetsms A, Boer JL, et al. Synth Met 2003;138:475. [22] Dimitrakopoulous CD, Brown AR, Pomp A. J Appl Phys 1996;80(4):2501. [23] Bouchoms IPM, Schoonveld WA, Vrijmoeth J, Klapwijk TM. Synth Met 1999;104(3):175. [24] Cullity BD, Stock RS. Elements of X-ray diffraction. 3rd ed. Prentice Hall; 2001. [25] Lee Kim J, Kim SS, Kim Kibum, Hoon Jae, Im Seongil. Appl Phys Lett 2004;84(10):1701. [26] Jurchescu OD, Baas J, Palstra TTM. Appl Phys Lett 2005;87(5). art no. 052102.