Opto-electronic properties of Epoxy/Silicone blend based thin films

: Recently, the rise of two dimensional amorphous nanostructured thin films have ignited a big interest because of their intriguingly isotropic structural and physical properties leading to potential applications in the nano-optoelectronics. However, according to literature, most of optoelectronic properties are investigated on chalcogenides related heterostructures. This has motivated the present work aiming to provide a new platform for the fabrication, examination of the properties and the applications of 2D nanostructured thin films based on epoxy/silicone blend. Thin films of Epoxy/Silicone loaded with nitrogen doped carbon nanotubes (N-CNTs) were prepared by sol-gel method and deposited on Indium Tin Oxide (ITO) glass substrates at room temperature. Further examination of optical properties aimed the investigation of optical pseudo-gap and Urbach energy and enabled the determination of processed films thickness based on Manifacier and Swanepol method. The results indicated that the unloaded thin films have a direct optical transition with a value of 3.61 eV followed by noticeable shift towards narrowing gaps depending on the loading rate. Urbach's energy is 0.19 eV for the unloaded thin films, and varies from 0.43 to 1.33 eV for the loaded thin films with increasing the rate of N-CNTs. It is inversely variable with the optical pseudo-gap. Finally, Epoxy/Silicone loaded with N-CNTs nanocomposites films can be developed as active layers with specific optical characteristics, giving the possibility to be used in electro-optical applications.


INTRODUCTION
To some extent, the performance of electronic devices in photovoltaic applications is tightly depending on the energetic alignment of the valence and conduction band edges at interfaces.
The electrostatic and well as van der Waals stacking interactions are the key parameters controlling interfacial electronic effects and resulting in a better dispersion of the fillers within the composites. Photovoltaic solar energy is an electrical energy produced from solar radiation by photovoltaic solar cells [1]. This kind of energy is the smartest way to produce electricity, and it has many advantages, such as: direct generation of electricity from sunlight [2][3]. It is a renewable source and clean [4][5][6][7][8][9][10][11], as well as friendly to the environment [5,8,[11][12][13][14]. The Photovoltaic energy is useful in different applications and devices [15][16][17][18][19][20][21].
The prices of the solar cells based on (Si) have declined so speedily that panel expenses now make up < 30 % of the costs of a fully installed "solar-electricity-system" [22]. Since of their fragility, Si thin sheets cannot be treated on their own, but they must be mechanically supported.
The researchers proposed to scale thick substrates by adding different materials such as: Aluminum, silver, nickel and epoxy [23][24][25][26]. They are generally composed of a mixture of inorganic particles embedded in a polymer matrix.
In the recent years, the research interest in the development of a new material of polymerinorganic nanocomposites with improved properties has been very high [27][28][29][30][31][32][33], and most research has been directed towards the use of materials in the form of thin films. The nanocomposites allow to improve mechanical, electrical, optical, optoelectronic and magnetic properties. For this reason, many studies have shown that hybrid nanocomposites are used in optoelectronic or optical applications requiring high visible transparency and shielding against ultra-visible [34][35][36][37][38][39][40].
Today, energetic deposition means are widely used for the manufacture of thin film optical components [41]. These processes allow the fabrication of thin film materials with excellent repeatability, whose optical properties are very close to those of the solid material, thus opening doors to higher performance treatments that are insensitive to the constraints of the external environment. This was only possible with the technology developments in thin films deposition using several physical deposition techniques such as reactive sputtering [42], electron beam evaporation [43] and arc deposition [44][45]. In particular, the sol-gel method has emerged as one of the most promising processes, as it is economical and efficient in the production of thin films [46] as well as transparent and homogeneous films which are suitable for a variety of substrates. These different properties make the sol-gel process a method of choice for the production of either amorphous or crystalline materials.
In this paper, we report a simple and economical method for elaborating thin films of epoxy/ silicone blend loaded by Nitrogen-doped carbon nanotubes (N-CNTs). The nanocomposites are thin films having interesting optical properties. These films may offer potential new opportunities for photovoltaic applications caused by their specific chemical and electrical properties performant [47][48][49][50].

EXPERIMENTAL WORK  2.1Material and Methods
The elaborated matrix contains Epoxy which is colorless viscous liquid of 99.9% purity, supplied along with the hardner from Toronto Research Chemicals and silicone gel from Keol having high purity level (>99%).
The fillers are the nitrogen-doped carbon nanotubes (N-CNTs) which were prepared using physical vapor deposition according to explained protocol in our previous paper [51].
Substrate cleaning is a very important step that takes place in a clean room, as this step determines the adhesion and homogeneity of the deposited layers. The substrates must be free of grease, dust and scratches. The substrates chosen for our study are blades Indium Tin Oxide (ITO). he procedure for cleaning the substrates is as follows: 1) Brushing with detergent, rinsing with de-ionized water, 2) Ultrasonic cleaning for ten minutes in a beaker filled with detergent, 3) Rinse with de-ionized water, 4) Ultrasonic cleaning again, but this time in a beaker filled with water deionized, for seven minutes, First, epoxy and Silicone were mixed in a 50ml beaker with the weight percentage of epoxy is always kept higher than that of Silicone and after a strong stirring the hardener is added to avoid anisotropy and in order to keep homogeneity . The neat matrix contains 75 wt% of epoxy, 10 wt% of Silicone and 15 wt% of hardener. Afterward, the N-CNTs were added with weight percent (0.00; 0.07; 0.1 and 0.2 % of N-CNTs) in order to obtain homogeneous nanocomposites with consideration that they are formulated using the same process.
The resulting mixture was deposited on the ITO glass substrate at room temperature. The prepared films were thermally cured at 103 for 1hour then at 115°C for 30min in the oven to obtain the Epoxy/silicone N-CNT films.

 2.2.Technical Characterizations
After preparing the thin films of Epoxy/Silicone N-CNT, microstructural and optical characterization were carried out, using, respectively , the scanning electron microscopy and UV-Visible technique which is based on the property of material and its ability to absorb certain wavelengths of the UV-visible domain. This method determines the transmission T (%) of a material for a given wavelength λ (nm) that has been judiciously chosen. The optical transmission spectrum for the elaborated thin films were registered using UV-Visible spectrophotometer (Jasco V-530) over the wavelength range of 300-800 nm. The microstructure of the processed composite thin films were examined through SEM micrographs which were picked up for epoxy/silicone blend loaded with 0.2 wt % N-CNTs using FEI Quanta FEG 450 scanning electronic microscope (SEM).

RESULTS AND DISCUSSION
With 1 and 2 are the refractive indices at two adjacent maxima (or minima) at λ1 andλ2.
The average values of thickness d of the studied thin films determined by this equation is about 700 nm.
The refractive index in the spectral region of the high, low and medium absorption zones can be calculated, it follows that the refractive index is given by the expression: In addition, the Swanepoel coefficient (N) in the transparent spectral region can be calculated by the following expression: Where S is the refractive index of the glass, T and T represent the maximum and minimum values for the transmission curve.
The absorption α of the of the Epoxy/Silicone N-CNT nanocomposite is linked to transmittance through Bouguer-Lambert-Beer relation [54]: If transmittance T is expressed in (%), the absorption coefficient is shown by: We can from the transmittance spectra (T) calculate the optical gap value of semiconductors from the Tauc formula (Eg (eV)) defined by the following equation using [55][56][57][58][59]: The relation can be rewritten in a logarithmic form such as: Where α is the absorption coefficient, ν is the absorption frequency, B is constant, h is Planck's constant and n is dependent on the type of optical transition, The constant n depends on the nature of the optical gap, it is for a direct optical gap and 2 for an indirect optical gap.
Note that, the interband transitions are accompanied by a change of electronic dynamics.
Because the laws of energy and momentum conservation must be satisfied, the indirect electronic band-to-band transitions are phonon-assisted, phonons must be involved in the electronic interband transition to provide the necessary momentum. Their energetic contribution is negligible if for instance the exponent takes the value of n=2 [60].
To determine whether the electronic transition that has occurred in the samples studied is direct or indirect, the optical gap Eg must first be determined using equation 6 to plot αhν versus hν, the value of the optical gap is obtained by extrapolating the linear part of the curve, at the intersection of this line with the x-axis given in Table 1, then the photon energy lnh  Eg is plotted versus lnh) which has been fitted with equation 7 in the linear region of the curve, using the average-squares method, where Eg, n and B are fitted parameters, this process can show that the type of optical transition of the pure epoxy/silicone mixture is direct with the power factor = 0.51 and the same result was obtained for the Epoxy/Silicone N-CNT w% loaded. To verify the validity of the method used, we plotted of (αhν) 2 versus hν as shown in Figure 2 according to the Tauc model [61], gives the value of the direct optical gap. The extrapolation of the linear part of the absorption edge (αhν) 2 gives the band gap energy . The use of the power factor n obtained gives a good agreement between the optical gap bands and the Tauc slopes, almost the same optical gap Eg is found (Table 1), so the correlation between experience and theory is compatible.  Figure. 2: Plot of ( ℎ ) versus photon energy ℎ , for "Epoxy/silicone N − CNT @ wt %".
Where (@ = 0.00; 0.07; 0.1 0.2) Obviously, the optical pseudo-gap obtained is high for Neat Epoxy (is of 3,6 eV), because it is optically transparent and this means that no absorption is possible in the visible, they cannot be excited without being loaded with another element to decrease Eg, so we load with N-CNT and we see that there is a diminution in Eg in 3,6 to 3,1 eV (Table 1) (Table 1 and Fig.3).
So the system is progressively becoming more conductor with addition of N-CNT, this seems to change slightly the structure of the electronic bands of the Neat Epoxy. In addition, the variation of optical pseudo-gap with increasing N-CNT loaded concentration can also be correlated with surface roughness and film density.
When variations in interatomic distance, length or angle of bonding are produced in the material, a so-called "disorder" occurs, in this case, the strip edges described in the case of crystalline networks and delimited by valence energy Ev and conduction energy Ec can disappear (Fig.4), so-called localized states formed in band tails at the borders of the optical pseudo-gap in the valence band and the conduction band are observed.    (Table 1 and Fig.6).  The cured composites exhibited a very dense and relatively smooth surface. The SEM photographs revealed that N-CNTs particles were found to be uniformly dispersed throughout the epoxy/silicone blend matrix. This result revealed that there is a good miscibility between the phases.in a good agreement with elsewhere findings [67].

CONCLUSION
The optical properties were studied from UV-Visible spectroscopy to examine the transmission spectrum, Optical pseudo-gap, Tauc verified pseudo-gap and Urbach energy, based on the envelope method proposed by Swanepoel.
The study shows that the films obtained show a high transmittance for the unloaded thin films of Neat Epoxy/Silicone blend about 88% and an average transmittance for the loaded thin film of Epoxy/Silicone N-CNT about 42 to 67% in the visible range and opaque in the UV range.
The results indicate that the film has a direct optical transition with an optical pseudo-gap of 3.61 eV for unloaded thin films, and 3.55 to 3.19 eV for loaded thin films depending on the loading rate. The optical pseudo-gap was appropriately adapted to the direct transition model proposed by Tauc, its value was 3.6 eV for unloaded thin films, and from 3.38 to 3.1 eV for loaded thin films, then determining Urbach's energy which is inversely tended with Eg which varies from 0.19 eV for unloaded thin films, and from 0.43 to 1.33 eV for loaded thin films.
The obtained results show the success of the method Sol-Gel to elaborate Epoxy/Silicone loaded with N-CNT films with properties adapted to the physical applications. These results also show that it was possible to modify the loaded Epoxy films by inserting a loading. In the near future, this gives hope for applications such like waveguides, electrochemistry, optical fibers, and solar cells.