returned to the bell jar until no more iodine was taken up [14]. Sometimes, in the case of post-doped films, the samples were doped by introducing them, with a ...
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Polymer Degradation and Stability 66 (1999) 257-262
Degradation and Stability
Influence of the iodine doping process on the properties of organic and inorganic polymer thin films K. Napo "-", G. Safoula b, J.C. Bernède b, K. D'Almeida b, S. Touirhi C,
K. Alimi ct, A. Barreau e "Laboratoire sur l'Energie Solaire, Faculté des Sciences, Université du Bénin, BP 1515, Lome, Togo "Equipe Couches Minces et Matériaux Nouveaux, EPS£, 2, Rue de la Houssinière, BP 92208, 44322 Nantes Cedex 3, France e Département de Physique, Faculté des Sciences de Sfax, 3038 Sfax, Tunisia dFaculté des Sciences de Monastir, Tunisia "Centre Commun de Microscopie â Balayage de l'Université de Nantes, 2 Rue de la Houssinière, BP 92208, 44322 Nantes Cedex 3, France Received 21 May I 999; accepted 28 May I 999
Abstract Iodine has been used to dope semi-conductive polymers in thin films form; one organic, the poly(N-vinylcarbazole), one inorganic, the selenium. When iodine doped powders are used to obtain the a-films (ante-doped films), the heating necessary to evaporate the powders induces some partial polymers degradation with unstable compound formation. After deposition, these compounds re-evaporate from the substrates forming pinholes and empty paths, which destroy the homogeneity of the films and damage their properties. These annoyances can be avoided when the films are deposited from pure powders and post-doped. It is shown that when the films are iodine doped after decomposition more iodine is present in the films; these are more homogenous, which allows optimum conductivity properties. © 1999 Elsevier Science Ltd. All rights reserved.
l. Introduction Doping of polymers with the aim of electronic device fabrication is an important research area since the 1980s. Many experiments have been done using halogen dopants (Cl, Br, I) because polymer doping with these elements can be very easily achieved by simple chemical process. However it has been shown that if the high electron affinity of Cl and Br allows to obtain stable charge transfer complex (CT-complex) between these halogens and polymers, in return there is often a progressive attack of the polymer backbone by the halogen [1,2]. Therefore it appears that iodine doping, even if it is less stable, is more promising because it does not induce any polymer degradation. More precisely we have shown in the case of poly(Nvinylcarbazole) (PVK) that, while chlorine and even bromine [3,4] doping induce partial degradation of the polymer, there is only CT-complex formation in the case of iodine doping. In return there is a slow degradation * Corresponding author. Tel.: 125528.
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with time of the sample properties. However we have shown that the stability of the complex can be improved by using a mild annealing treatment [5-8]. Similarly we have studied the effect of chlorine and iodine doping on the properties of polycrystalline selenium. The hexagonal selenium is composed of parallel chains and therefore it can be considered as an inorganic polymer. Here also it has been shown that after doping there is some selenium degradation by the chlorine as shown by the presence of SeCl4 while there is no compound with iodine [9-11]. Usually polymer thin films are obtained by spin coating, however the use of solvent becomes difficult in the case of multilayered structures and moreover purity of the films is not optimum [12]. Therefore an increasing interest is geared to vacuum deposition. We have shown that PVK thin films can be obtained by vacuum evaporation even if there is some decrease of chain length during the process [13]. In the present paper we discuss the properties (stability, degradation) of PVK thin films obtained by evaporation of pure powder followed by iodine post-deposition doping and iodine pre-doped powder. The same study on pre- and post-doped selenium thin films is also reported.
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In the light of these results some information on the effects of the iodine doping process on polymers ( organic or not) is discussed.
2. Experiments
All films have been deposited by evaporation in vacuum (better than 10-4 Pa). In order to compare the properties of the pre- and post-doped samples they have been characterized by X-ray diffraction (XRD), ultraviolet absorption, thermal gravimetric analysis, X-ray electron spectroscopy (XPS), scanning electron microscopy (SEM), microprobe analysis and conductivity measurements. In the present paper we display only the characterization techniques whose results depend on the doping process.
Pyrex tube. Then the films were annealed very carefully because they peeled off from the substrates very easily. The annealing cycle was: increase of the temperature at a constant speed of l K min-1 to achieve the heating temperature (363 K) of the sample. This temperature was kept for 6 h, then the power supply to the oven was stopped and the sample slowly cooled down. Even by using such a process more than half of the samples were destroyed during the annealing when polished substrates were used, therefore unpolished glass substrates were also used, which reduces the loss. In the case of the films obtained from pre-doped selenium powder they were crystallized by in situ annealing, under 105 Pa argon pressure in order to try to avoid selenium sublimation. The annealing was carried out at 363 K for an hour. 2.2. Characterization techniques
2.1. Preparation of the films The substrates which were glass, silica slides, mica sheets, CaF2, NaCl single crystals and metal plates, were heated at 400 K for I h prior to deposition. Then they were cooled down to 300 K, the substrate temperature being controlled by a copper-constantan thermocouple attached by silver paste to the surface of the substrate. The PVK films were obtained by thermal evaporation of pure or iodine-doped powder from a heated molybdenum boat. The evaporation temperature, which was measured by a chrome! alumel thermocouple stuck on the boat, was kept below 590 Kin order to avoid strong polymer degradation. The evaporation rate (1 nm s-1) and the film thickness (100 nm-5 µm) were measured in situ using the vibrating quartz method. The iodine-doped powder and/or thin films were obtained by introducing the sample in a bell jar. Simultaneously some iodine in a separate glass cup was placed in the apparatus. The bell jar was placed under vacuum at room temperature. Periodically the samples were removed, weighed and returned to the bell jar until no more iodine was taken up [14]. Sometimes, in the case of post-doped films, the samples were doped by introducing them, with a small amount of iodine, in a vacuum-sealed Pyrex tube for 72 h. Some of them were then annealed for 24 hat T = 370 K. The selenium films were thermally evaporated from pure or iodine-doped powder using a tantalum boat. The substrates were polished and unpolished slides of glass and silica; they were heated at 400 K during l h prior to deposition. The substrates were cooled down to 300 K before deposition. The selenium evaporation rate was 4 nm s-1 while the thickness of the films was 500 nm to 10 µm. The post-doped films were obtained by introducing a film with a small amount of iodine in a vacuum-sealed
Only the techniques whose results will be discussed below are described. Observation of the morphology of the films was performed using a JEOL 6400F field-effect scanning electron microscope (SEM) to compare the surface topography of the pre-doped and post-doped samples. Microprobe analysis was performed using a JEOL 5600 LV scanning electron microscope equipped with a PGT X-ray microanalysis system, in which X-rays were detected by a germanium crystal. X-ray photoelectron spectroscopy (XPS) 1 measurements were performed using a magnesium cathode (1253.6 eV) as X-ray source. lt was operated at 10 kV and 10 mA; the energy resolution was I eV at a pass energy of 50 eV. High resolution scans with good signal-to-noise ratio were obtained in the Cis, Nls, Ols, Se3d and 13d regions of the spectrum. The quantitative analyses were based on the determination of the Cis, Nls, Ols, Se3d and l3d5/2 peak areas with 0.2, 0.36, 0.61, 0.57 and 6.4, respectively, as the sensitivity factors (the sensitivity factors of the spectrometer were given by the manufacturer). All spectra were recorded under identical conditions. The iodine depth profile in the samples was studied by recording successive XPS spectra after ion etching for short periods. Using an ion gun, sputtering was accomplished at pressures of less than 5.1 10-4 Pa with a l O mA emission current and 3 kV ion beam energy. In such conditions all the surface of the sample was sputtered. The optical measurements,2 in the visible and ultraviolet domain, were carried out at room temperature 1 XPS analyses were performed with a Leybold spectrometer at the University of Nantes, CNRS. 2 The optical measurements were carried out in the Laboratory of Crystalline Physics, Institute of Materials, Nantes.
K. Napo et al./ Polymer Degra dation and Stability 66 ( 1999) 257-262
using a Cary spectrometer. The optical density (O.D) was measured at wavelength 2-0.3 µm. For electrical measurements metal electrodes were evaporated before sample deposition. The conductivity measurements were carried out with planar M/thin film/ M samples. The distance between the electrodes was 1 mm, and the electrodes were 1 cm wide. Ohmic I-V characteristics were obtained using gold electrodes in the case of PVK. In the case of selenium, we have shown earlier [10] that chromium thin films prevent peeling of the selenium film, give good ohmic contact and do not diffuse into the films even after annealing, therefore Cr/Se/Cr samples have been used for electrical measurements. The film de conductance was measured by conventional methods with an electrometer.
259
(a)
a-PVK!ilm (b)
3. Results In order to facilitate the description and then the discussion of the results, the films obtained from iodine doped powder (pre-doped films) will be called a-films while those doped after deposition will be called p-films from the Latin prefixes ante (before) and post (after). Some measurements (IR absorption, Raman scattering, ... ) giving the same results for a-doped and p-doped films will not be described here. p-PVK!ilm
3. I. Morphological studies The scanning electron study is summarized for both, PVK and selenium in Figs. 1 and 2. lt can be seen that, while continuous films are obtained in the case of pfilms (Figs. 1 b and 2b ), strongly disturbed samples are obtained in the case of a-films (Figs. la and 2a). Even after optimization of the experimental conditions to obtain clean samples, i.e. even when silica substrates are used, the same discrepancy is obtained. Very smooth surfaces are obtained in the case of p-PVK films (Fig. 1 b) the samples being nearly amorphous as checked by XRD (not shown). For a-PVK films if the samples are still nearly amorphous they exhibit large roughness with pinholes and empty paths (Fig. la). In the case of selenium, if the results exhibit a similar discrepancy, it is more spectacular. After the annealing process, the XRD measurements (not shown) demonstrate that all the films are crystallized in the hexagonal structure. In the case of p-Se films, if some cracks are present there are no pinholes in the films. Some spherulites are visible (Fig. 2b); such features are well known to be present in crystallized selenium films [15,16]. The a-Se films appear to be discontinuous. If the crystallite size is about 1 to 2 µm, a high density of empty and broad grain boundaries are visible. In fact empty paths are present all over the sample, the channel
Fig. I. Scanning electron micrographs of: (a) a-PVK films (film thickness 2 µm); (b) p-PVK films (film thickness 2 µm).
gap being about 0.1 nm large (Fig. 2a). The microprobe analysis has only been used in the case of Se because the elements constituting PVK are too light for the experimental technique. In the case of a-Se films the iodine concentration is too small to be quantitatively estimated and the repartition of iodine in the films cannot be studied. However it has been shown earlier by secondary ion mass spectroscopy (SIMS) that iodine accumulates at the surface while the curve exhibits a stabilization of the concentration in the bulk, moreover, there is some iodine content difference from one point to another in the same sample [11]. For p-Se films, the average iodine percentage is about 1 to 2 at%. Usually the spherulites are too small to measure the iodine concentration in one of them only. 3.2. XPS analysis XPS surface analysis, before any sputtering, has been largely discussed earlier, mainly in the case of PVK
K. Napo et al./ Polymer Degradation and Stability 66 ( /999) 257-262
260
[6, 13,17]. In the present work we will discuss mainly the iodine depth profile. The surface analysis has shown that, as usual in the case of polymers [18], the surface contamination of the PVK is quite small. There is only 5-10 at% of contaminating oxygen, 4-5 at% of nitrogen and the remainder is carbon. Therefore, in the experimental uncertainty range of the XPS technique, there is a good agreement between the theoretical and experimental N, C atomic ratio. This is true before and after doping. The iodine will be discussed below. In the case of selenium the presence of oxygen and carbon at the surface of the films was detected, but after more than 5 nm of etching the 02 would not be quantifiable. The iodine depth profiles in PVK samples are reported in Fig. 3. lt can be seen that after a small decrease of iodine concentration during the first minutes of etching there is stabilization of the concentration in the bulk whatever the sample. However it can be seen that, if there is about 1-2 at% of iodine in p-PVK films, there is only 0.3-0.5 at% in the a-PVK films, i.e. the iodine
concentration is one order of magnitude smaller than in the initial powder. The iodine depth profiles in selenium samples are reported in Fig. 4. There is a striking similarly with the iodine profiles in PVK: • The same iodine surface accumulation • The same stabilization of the iodine concentration in bulk • The same variation in iodine concentration from one type of sample to another one, i.e. 3 at% in the doped powder, 0.4 at% in p-Se films and only 0.2 at% in the a-Se films.
at. %1 3
2
o
o
o
O
(a)
+
+
+
+
(e)
o
o
o
o (b)
1
2
3
4 min
1
o
Fig. 3. XPS iodine depth profile in PVK sample: (a) iodine doped PVK powder (3 at¾ of iodine); (b) a-PVK film; (c) p-PVK film.
at.% I 3
o
o
o
o
o
(a)
a-Se (b)
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p-Se
Fig. 2. Scanning electron micrographs of: (a) a-Se film (film thickness 5 µm); (b) p-Se film (film thickness 5 µm).
o
X
X
X
X
X
X
X
(e)
o
o
o
o
o
o
0
(b)
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Fig. 4. XPS iodine depth profile in selenium sample: (a) iodine doped selenium powder (6 at¾ of iodine); (b) a-Se film; (e) p-Se film.
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K. Napo et al./ Polymer Degradation and Stability 66 ( 1999) 257-262
3.3. Electrical and optical measurements Room temperature conductivity measurements are summarized in Table 1. lt can be seen that, if there is a systematic increase of the conductivity of the doped films, the p-films have a conductivity two orders of magnitude higher than that of a-films whatever the sample is: PVK or Se, organic or inorganic polymer. The optical density has been measured for each kind of sample; the most striking result, shown in Fig. 5, has been obtained with selenium. lt can be seen that the strong increase of the optical density, obtained in pure polycrystalline selenium film, when the wavelength }, ~ 0.8 µm is systematically far smaller in the case of aSe films. Moreover the optical density of a-Se films, when the wavelength (is larger than 0.8 µm, is stronger than that of the other samples. The same tendency is obtained in the case of PVK films.
4. Discussion
0.8 µm the roughness of the films induces diffusion of the light (false absorption) [20] which increases the optical density. In the case of post deposition, the doping takes place at lower temperature and no degradation reaction takes place with the films. There is only electron charge transfer from the material to the iodine with complex salt formation. The doping conditions are nearly similar to those of the powders and the iodine concentration introduced in the films is nearly similar. The stability of the complex is good enough to avoid iodine degradation
Î%
OD
0-1
3
2
10
Sample Pure PVK film a-PVK film p-PVK film Pure Se film a-Se film p-Se film
10-11 10-8 10-6 10-1 10-s 10-3
100 ......_..____.___.___,___,___.._____.___,___,
0.4
1.2
2
Q ). (}lm)
Fig. 5. Optical density of selenium's films: (A) pure polycrystalline film; (B) a-Se film; (C) p-Se film.
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and neither hole nor empty paths appear in the films. The films are homogeneous, their conductivities are the highest and the optical densities correspond to the expected values.
5. Conclusion
Iodine is often used to dope conductive polymers because it allows one to obtain a CT-complex without degradation of the polymers. Here it has been used to dope two semi-conductive polymers in thin films form; one organic, PVK, one inorganic, selenium. We have shown that, when iodine doped powders are used to obtain the a-films, the heating process necessary to evaporate the powders induces some partial polymer degradation with unstable compound formation. After deposition, these compounds re-evaporate from the substrates forming pinholes and empty paths, which destroy the homogeneity of the films and damage their properties. These annoyances can be avoided when the films are deposited from pure powders and post-doped. These p-filrns have properties similar to doped powder and they are homogeneous in such way that they can be used in different devices in the visualization domain (videcon, xerography ... ) References [I] Sandman DJ, Elman BS, Homll GR, Hefter S, Velazquez CS. ACS Symp. Series 337, 1987. p. I 18.
[2] Block H, Cowd MA, Walter SM. Polymer 1977;18:781. [3] Safoula G, Touihri S, Postic M, Bernède JC, Molinié Ph. J Chim Phys I 997;94: 1602. [4] Safoula G, Touihri S, Bernède JC, Jamali M, Ra biller C, Molinié Ph, Napo K. Polymer I 999;40:531-9. [SJ Bernède JC, Alimi K, Safoula G. Polym Degrad Stab l 994;46:269. [6] Alimi K, Safoula G, Bernède JC, Rabiller C. J Polym Sci: Part B Polym Phys I 996;34:845. [7] Safoula G, Bernède JC, Alimi K, Molinièe Ph, Touihri S. J Appl Polym Sci I 996;60: 1733. [8] Safoula G, Bernède JC, Touihri S, Alimi K. Eur Polym J 1998;34: 1871. [9] Popov A, Geller I, Karalonets A, Patoya N. J Non-Cryst Solids 1980;36:87 l. [IO] Burgaud P, Brenède JC, Safoula G, Ameziane A. Phys Stat Sol (a) I 986;95:721. [I I) Safoula G, Bernède JC, Latef A, Rzepka E, Spiesser M. Mater Chem Phys l988;20:571. [12) Burrows PE, Bulovic V, Gu G, Kozlov V, Forrest SR, Thompson ME. Thin Solid Films 1998;331:101. [13] Touihri S, Safoula G, Bernède JC, Le Ny R, Alimi K. Thin Solid Films 1997;304: 16. [14) Gutierrez M, Ford WT, Pohl HA. J Polym Sci Polym Chem Edn l 984;22:3739. [I SJ Audière JP, Mazières C, Carballes JC. J Non-Cryst Solids 1978;27:41 l. [16] Audière JP, Mazières C, Carballes JC. J Non-Cryst Solids 1979;34:37. [I 7] Touihri S, Safoula G, Bernède JC. Polym Degrad Stab I 998;60:481. [18) Briggs D, Seah MP. Practical surface analysis, vol. I, Auger and X-ray photoelectron spectroscopy. 2nd ed. Wiley, 1990. p. 3591. [I 9] Chizhevskaya SN, Abrikosov NKh, Azizoua BB. Inorg Mater 1973;9: 198. [20) Messoussi R, Bernède JC, Safoula G, Rzepka E, Spiesser M. Phys Stat Sol (a) 199 I; I 23: 175.
