On the influence of silicon oxide nanoparticles on the optical and surface properties of hybrid (inorganic–organic) barrier materials

Thin Solid Films 517 (2009) 6275–6279

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Thin Solid Films j o u r n a l h o m e p a g e : w w w. e l s ev i e r. c o m / l o c a t e / t s f

On the influence of silicon oxide nanoparticles on the optical and surface properties of hybrid (inorganic–organic) barrier materials A. Laskarakis a,⁎, S. Logothetidis a, D. Georgiou a, S. Amberg-Schwab b, U. Weber b a b

Aristotle University of Thessaloniki, Department of Physics, Laboratory for Thin Films – Nanosystems and Nanometrology, GR-54124, Thessaloniki, Greece Fraunhofer-Institut für Silicatforschung, 97082 Würzburg, Germany

a r t i c l e

i n f o

Available online 25 February 2009 Keywords: Spectroscopic Ellipsometry Hybrid polymer Barrier materials Optical properties Flexible organic electronics

a b s t r a c t One of the major scientific and technological challenges for the production of flexible organic electronic devices is the device protection against atmospheric molecule permeation, which causes corrosion reducing its operation and lifetime. In this work, Spectroscopic Ellipsometry has been implemented to investigate the influence of silicon dioxide nanoparticles on the optical properties of hybrid polymers. The spectra analysis revealed valuable information about the electronic and vibrational response as well as the cross-linking mechanisms of these materials. The correlation of the optical properties with the synthesis parameters and the barrier response will contribute towards their optimization in order to be used as high barrier coatings for flexible organic electronics applications. © 2009 Elsevier B.V. All rights reserved.

1. Introduction The encapsulation of the active layers of organic electronic devices with flexible polymeric substrates represents one of the major challenges in order for the flexible electronic devices (FEDs) to reveal their full potential [1]. The protection of the device functional layers (such as electrodes, organic semiconductors and conductors) against the atmospheric gas molecules (H2O and O2) plays an important role for the functionality and stability of these devices (such as flexible displays as organic light emitting diodes—OLED, electrochromic displays, lighting, etc. and flexible photovoltaic cells—OPVs) [1,2]. The permeation of these molecules into the device structure leads to corrosion effects of the device active layers reducing the overall performance and lifetime of the device, prohibiting its market implementation [3–6]. Therefore, the use of permeation barrier films for highly sensitive materials is necessary. The requirements for gas protection depend on the specific application. For example, sensitive food products can be protected with a single polymer film coated with one inorganic layer, achieving permeation values of water vapor transmission rate (WVTR) of 10− 1 g/m2 d and oxygen transmission rate (OTR) of 10− 2 cm3/m2 dbar. Flexible electronic devices, such as flexible OLED displays and OPVs require a more complex layer stack as a permeation barrier in order to achieve ultra low permeation values of 10− 5 g/m2 d (WVTR) and 10− 5 cm3/m2 dbar (OTR) [1–3].

⁎ Corresponding author. Tel: +30 2310 998850, Fax: +30 2310 998390. E-mail address: [email protected] (A. Laskarakis). 0040-6090/$ – see front matter © 2009 Elsevier B.V. All rights reserved. doi:10.1016/j.tsf.2009.02.114

Concerning the inorganic barrier films (e.g. SiOx, AlOx), these can be deposited by physical vapor deposition methods [2,7]. These films show a preferential permeation of atmospheric gas molecules at defects sites, with different size (nano-, micro-and macro-defects) [8]. On the other side, hybrid (inorganic–organic nano-composite) barrier materials are synthesized via the sol–gel processes from organoalkoxysilanes, and they have strong covalent or ionic-covalent bonds between the inorganic and organic phases [6,9–11]. These materials combine the properties of their constituents, such as low processing temperatures, high optical transparency, chemical and thermal stability, and hardness [6,10,11]. If a hybrid barrier material is deposited and cured on a flexible polymer substrate (e.g. PolyEthylene Terephthalate—PET) it exhibits good barrier properties. But if it is combined with an inorganic barrier layer in a multilayer material structure, the barrier response of the hybrid/inorganic material system is significantly improved [9]. This has been reported to be due to the synergetic effect of the confinement of the molecule permeation to the defect zones of the inorganic layer, and to the formation of chemical bonds and crosslinking between the hybrid polymer and the inorganic layer [9,12]. The investigation of the optical properties of these hybrid materials and the study of their microstructure and bonding structure, will significantly contribute to the understanding of the bonding mechanisms and the processes that control this synergetic effect. In this work, we investigate the optical properties of several barrier layers deposited onto SiOx/PET substrates and the effect of the inclusion of SiO2 nanoparticles on their microstructure, optical properties and functionality. The optical properties were measured in an extended spectral region from the IR to the Vis-fUV spectral region in order to stimulate different light-matter mechanisms

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(bonding vibrations, interband transitions), that will provide significant insights on their properties. 2. Experimental details The hybrid nano-composite materials that are used as high barrier materials are inorganic–organic ORMOCER® polymers (Trademark of Fraunhofer-Gesellschaft zur Förderung der angewandten Forschung e.V. in Germany). These nano-composites have been synthesized via the sol–gel process at the Fraunhofer-Institut für Silicatforschung (ISC) and are characterized by strong covalent bonds between inorganic and organic moieties. [9] These molecular composites have the potential to combine certain structural properties of different classes of materials in ways not accessible by mixtures of macroscopic phases as this is the case in classical composites. The combination of these materials with inorganic layers is characterized by enhanced barrier response, capable for the encapsulation of flexible electronic devices since the alternative inorganic and hybrid materials provide a tortuous path that significantly reduces the permeation rate of O2 and H2O molecules. In order to increase the covalent bonds and to result to a more cohesive inorganic–organic network, which is expected to further increase the barrier properties, SiO2-np of 60 nm size have been included inside the hybrid polymers during their synthesis process. These SiO2-np of spherical size have been introduced in the hybrid layers during their synthesis, whereas the (hybrid + SiO2-np) materials have been lacquered by a spiral applicator onto the flexible polymeric substrates. Also, the thickness of the hybrid layers has been calculated approximately during the lacquering process. In this work, we have studied 7 samples with the same thickness (3–4 μm) but with different contents of SiO2-np, from 1% to 30% (6 samples) and a sample without nanoparticles for comparison. The optical properties of the materials have been investigated by the use of Spectroscopic Ellipsometry (SE) covering an extended spectral region from the Infra-Red (IR) to the Visible-far UltraViolet (Vis-fUV). The SE measurements in the IR spectral range (900–4000 cm− 1) were performed by a Fourier Transform IR Phase Modulated Spectroscopic Ellipsometry (FTIRSE), described in detail elsewhere [13]. The Vis-fUV SE measurements were performed by a Phase Modulated Spectroscopic Ellipsometer from Horiba Jobin-Yvon covering the energy range of 1.5– 6.5 eV with steps of 20 meV [14]. The combination of the complex pseudo-dielectric function bε(ω)N = bε1(ω)N +i b ε2(ω)N from SE mea-

surements in these two energy regions will provide insights on the thin film thickness, the band-to-band electronic transitions, and the bonding vibrations of the materials [15,16]. All the hybrid barrier materials were deposited by a spiral applicator onto inorganic SiOx layers that have been previously deposited onto the PET polymeric substrates (PET Melinex 401 of 50 μm thickness). These SiOx layers were deposited by electron beam evaporation and they have a thickness of 120 nm. It is important to note that the PET substrates are characterized by high optical anisotropy as a result of the stretching process that applied during their production [3,17]. Thus, all optical measurements were performed with the plane of incidence of light parallel to the stretching direction (or Machine Direction—MD) of the PET substrates, in order to obtain comparable results. Moreover, in order to avoid the contribution of the multiple reflections of light at the back surface of the polymer substrates on the measured spectra, the back side of the substrates has been roughened by sandpaper to scatter the refracted light inside the flexible polymer substrate [3,17]. Also, the permeation of oxygen and water vapor has been measured with standard testing devices from Mocon. Finally, the Contact Angle (CA) measurements were performed with an optical contact angle and surface tension meter CAM200 from KSV Instruments Ltd. The liquid which used was water and the volume of the drop was 5 μl. 3. Results and discussion Fig. 1 shows the measured pseudo-dielectric function bε(ω)N = bε1 (ω) N + i b ε2(ω)N of the hybrid barrier material deposited onto SiOx/ PET in the Vis-fUV energy region (1.5–6.5 eV). The thickness of the hybrid layer could not be controlled with precision during the lacquering process onto the polymer substrate, and initially it has been estimated to be in the range of a few microns. As it can be seen from Fig. 1, in the lower energy region, from 1.5–4 eV the bε(ω)N is dominated by interference fringes that it is attributed to the multiple light reflections at the ORMOCER®/SiOx/PET interface as the result of their optical transparency in this energy region. Above the energy of 4 eV the optical absorption takes place. For the detailed determination of the optical constants, the measured bε(ω)N has been analyzed by the use of three phase geometrical model that consists of a hybrid barrier with thickness d on top of a SiOx/PET material structure (air/SiOx/ORMOCER®/substrate).

Fig. 1. Measured pseudo-dielectric function bε(ω)N in the Vis-fUV spectral region of the hybrid barrier layer deposited onto PET.

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Fig. 4. Evolution of the calculated refractive index of the ORMOCER® + SiO2-np material deposited onto SiOx/PET as a function of the SiO2-np content. Fig. 2. Comparison of the SiO2-np content (%) in the ORMOCER® material as determined by the bε(ω)N analysis and as estimated during the ORMOCER® preparation onto the SiOx/PET substrates.

The optical response of the hybrid layer has approximated by the use of the Tauc-Lorenz (TL) oscillator model [18]. Also, in order to take into account the inorganic component and the SiO2-np embedded in the hybrid barrier layer and to determine the content of inorganic and organic components, we have implemented the Bruggemann effective medium approximation (BEMA) for the parameterization of the ORMOCER®'s optical response [19]. The analysis of the measured bε(ω)N in combination with the BEMA revealed the content of the SiO2-np in the ORMOCER® layer to be higher than the initially estimated (see Fig. 2). This can be attributed to the fact that the inorganic volume fraction includes both the SiO2-np and the inorganic component of the hybrid bonding network. Also, in Fig. 3 we show the dependence of the optical parameters deduced from the bε(ω)N analysis (fundamental gap Eg and Penn gap E0) as a function of the SiO2-np content in the ORMOCER® materials. From this figure it is clear that the increase of the content of SiO2-np from 0% to 30% leads to the reduction of both Eg and E0 values.

Fig. 4 shows the results from the oxygen transmission rate measurements of the studied hybrid materials. The increase of the SiO2-np content therefore is found to result to the increase of the OTR values leading to poorer barrier response. Moreover, as it can be seen from Figs. 3 and 4, the reduction of the Eg and E0 values are correlated with the increase of the OTR values. The stability of the calculated Eg and E0 values of the SiO2-np that is shown in the same figure justifies the validity of the performed analysis since it is expected for the SiO2-np to have stable Eg and E0 values, regardless their content in the ORMOCER® material. The analysis of the measured bε(ω)N revealed the refractive index of the ORMOCER® + SiO2-np material, which is also shown in Fig. 4. As it can be seen in this figure, the increase of the SiO2-np content is associated with a reduction in the refractive index values from n = 1.52 for the case of 0% content to n = 1.43 for the case of the higher content of 30% in the ORMOCER® material. This reduction in the refractive index can provide an information about the evolution of the density of the ORMOCER® materials, which seems to decrease with increasing SiO2-np content that can lead to poor barrier response due to the less cohesive inorganic–organic bonding network. Indeed, from representative barrier measurements, it has been found that the sample with the best barrier response in terms of the oxygen transmission rate (OTR) is the sample without SiO2-np (~0.004 cm3/m2 dbar at 23 °C–50%). For comparison the sample with 30% SiO2-np has a barrier response of 0.036 cm3/m2 dbar at 23 °C–50%). The above results are supported by the investigation of the bonding structure of the ORMOCER® materials by the use of FTIRSE. The vibrational properties of the films can be studied in the IR energy region, since the electric field of the IR beam causes the excitation of the bonding vibrations. These vibrations are modelled, to a first approximation, using a damped harmonic oscillator (Lorenz model). The effect of the several vibration modes on the complex dielectric function is described by the expression: e˜ðωÞ = e∞ +

X i

Fig. 3. Dependence of the determined optical parameters of the ORMOCER®, material, organic and SiO2-np part, as a function of the SiO2-np content.

2

ω20i

fi ω0i ; − ω2 + iCω

ð1Þ

where ω is the energy of light and ω0i is the absorption energy of the ith vibration mode. The constants fi and Γi denote the oscillator strength and the damping (broadening) of the specific vibration mode, respectively. The quantity ε∞ is the static dielectric constant and represents the contribution of the electronic transition that occurs at an energy ω0 in the NIR-Visible-UV energy region, on the dielectric function.

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Fig. 5. Measured imaginary part bε2(ω)N in the IR spectral region of the (ORMOCER® + SiO2-np) material of different SiO2-np contents deposited onto SiOx/PET. The characteristic absorption bands indicate the vibration modes of the various bonding configurations.

Fig. 5 shows the imaginary part bε2(ω)N of the measured bε(ω)N of the (ORMOCER® + SiO2-np)/SiOx/PET samples in the IR energy region. Above 1800 cm− 1, the samples are optically transparent and their FTIRSE spectra are dominated by Fabry-Pérot oscillations due to the multiple reflections of light at the PET interfaces. The characteristic peaks observed in the spectra indicate the bonding vibration modes of the IR-active chemical bonds of PET and the ORMOCER® layer. These include the glycol CH2 wagging peak at 1340 cm− 1 and the intense complex bands around 1240–1330 cm− 1 that arise mainly from ester group vibrations. Also, the stretching mode of the carbonyl CfO group of PET is shown at 1720 cm− 1. In the area of 1050–1100 cm− 1 the contribution of the Si–O bonding vibration can be seen. From Fig. 5 it is clear that the increase of the SiO2-np content in the ORMOCER® layer results to the increase of the Si–O vibration mode in the area of 1050 cm− 1. Also, some variations in the other vibration bands such as the CH2 and the CfO stretching vibrations, are shown at the hybrid layers with different contents of SiO2-np. This change is mainly attributed to the different thickness of the hybrid layers. We have to note that the large broadening of the Si–O vibration band can be justified by the large variations in bond lengths and bond angles (as it is the case in the inorganic part of the hybrid layers). The observed shift of the Si–O vibration band gives an indication that the “total” stoichiometry of the (hybrid + SiO2-np) material increases with increasing content of SiO2-np in the hybrid materials. For the parameterization of the Si–O stretching vibration, we have analyzed the measured bε(ω)N spectra by the use of a four phase model (air/(ORMOCER® + SiO2-np)/SiOx/PET substrate). The dielectric function and the thickness of the intermediate SiOx layer have been used as a reference since it has been modeled by using a plain SiOx/PET sample without the addition of ORMOCER® layer on top of it. For the analysis of the IR ellipsometry spectra we have used one oscillator at the range of 1050–1150 cm− 1 and the fit took place in a small energy region since we wanted to analyze the modification of the Si–O vibration mode (which includes the contribution of the SiO2np and the inorganic component of the hybrid layers) at the different contents of the SiO2-np in the hybrid layers. Thus, the only parameters that have been fitted are the thickness and the optical properties of the ORMOCER® layer, and the vibration band corresponding to the Si–O bonds. The calculated values of the Si–O

stretching vibration energy and their dependence with the content of the SiO2-np in the ORMOCER® material are shown in Fig. 6. From this figure it can be seen that the increase of the nanoparticle content is correlated with the increase of the Si–O vibration energy from 1058 cm− 1 (for 1% SiO2-np) to 1091 cm− 1 (for 30% SiO2-np). The increase of the Si–O vibration energy leads to the conclusion that the ORMOCER® materials with a higher content of SiO2-np are correlated to higher stoichiometry × values of the inorganic part [20]. Fig. 6 also shows the dependence of the measured contact angle values from the surface of the ORMOCER® materials. The error values of the contact angle are in the range of 0.02°–0.1°. It is clear from this figure that the increase of the SiO2-np content is associated with slightly higher contact angle values, leading to the conclusion that the higher amount of SiO2-np can be correlated to a more hydrophobic ORMOCER® material. However, this higher hydrophobic surface behavior of the ORMOCER® material with embedded 30% SiO2-np (in comparison with the sample without SiO2-np) cannot contribute much to the improvement of the overall barrier response of the

Fig. 6. Dependence of the calculated Si–O bonding vibration mode and the surface contact angle as a function of the SiO2-np content in the ORMOCER® material deposited onto SiOx/PET.

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multilayer system (as it has been discussed above). The increase of the Si–O vibration energy (and furthermore the material stoichiometry) is associated directly to the increase of the surface hydrophobicity of the samples, and moreover it is correlated to the reduction of the materials density (through the reduction of the refractive index values, shown in Fig. 4) and finally to the decrease of the barrier response.

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Acknowledgements The authors would like to thank the Fraunhofer-Institut für Verfahrenstechnik und Verpackung IVV for the barrier measurements. This work has been supported by EC under the STREP Project NMP3CT-2005-013883 “FLEXONICS”. References

4. Conclusions We have investigated the optical properties of several barrier layers deposited onto SiOx/flexible PET substrates and the effect of the inclusion of SiO2 nanoparticles on their microstructure, optical properties and functionality. The optical properties were measured by SE in an extended spectral region from the IR to the Vis-fUV spectral region in order to stimulate different light-matter mechanisms (bonding vibrations, interband transitions). SE provided important information on the evolution of the optical parameters (energy gap, absorption peaks) of the hybrid barrier films. As it has been found, the increase of the SiO2-np content leads to the reduction of the materials refractive index (lower density) and to the increase of the Si–O vibration energy (and furthermore the material stoichiometry). Also, the inclusion of SiO2-np in the ORMOCER® material, although it is originally intended to provide a more cohesive organic–inorganic network, is associated to less dense structures. In this way, the materials have poorer barrier response as it has been justified by representative barrier measurements. On the other side, the inclusion of SiO2-np leads to the increase of the surface hydrophobicity (higher contact angle values) and it is directly correlated to the Si–O bonding vibration energy. The above emphasizes the importance of optical characterization on the evaluation of barrier materials for the encapsulation to be used for the development of flexible electronic devices.

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