Antireflective Coating Nanocrystalline α-Fe2O3 Layer for Solar Cell Applications

A new antireflective nano-coating of α-Fe2O3 for glass surfaces using dip-coating technique working in the visible and near infrared optical spectrum was developed and characterized. α-Fe2O3 nanoparticles were synthesized by hydrothermal process. Thermal transformation strategy is designed to transfer β-FeOOH into αFe2O3. The structure and morphology of the iron oxide nanoparticles were analyzed by XRD, FTIR, and SEM. Optical properties of antireflective coatings (ARC) deposited from ethanolic solution of α-Fe2O3 have been characterized. 93.4 % optical transmittance using optimal dip coating conditions (dipping rate – 3 cm/10 sec, coating time – 5min, and temperature – 25 oC) for annealed powder at 300 °C for 1 hr.


Introduction
Antireflective coatings (ARC) are widely used for reflectance minimization in numerous applications, such as flat panel displays [1,2] , organic lightemitting diodes (OLEDs) [3] , thin-film solar cells [4,5] , lasers, and all kinds of lenses [6]. In the cases of flat panel displays and OLEDs, the AR films are deposited on the glass substrates to suppress Fresnel reflection at the air-glass interface [7].
A key issue in preparing the AR coating is the formation of large area, and uniform layer. Different self-assembly processes can be in principle used to achieve this goal, such as spin coating [8,9] , dip coating [10,11] , layer-by-layer assembly [12,13] , soft lithography [14,15] , and evaporation-induced selfassembly [16] .
Weakly-ferromagnetic α-Fe2O3 (hematite) is of particular interest as a cheap, environmentally friendly and thermodynamically stable iron oxide [17]. Synthetic hematite (α-Fe2O3) has been extensively used in the field of pigments [18] , catalysts [19] , magnetic materials [20] , anticorrosive agents [21] and sensors [22] , owning to its low cost, environmental friendliness, and high resistance to corrosion [23] . The synthesis of iron oxides nanocrystals of different size and shape has attracted considerable interesting in recent years [24] .
In the present study, we demonstrated the preparation of nanocrystaline α-Fe2O3 as AR coating on lime glass substrates by dip-coating technique and showed the dependence of variation of the AR behavior on different deposition process parameters and heat treatment.

Materials and methods 2.1. Materials
Iron(III) chloride (FeCl3 anhydrous, 98%) was obtained from Acros organics company (Geel, Belgium). hydrochloric acid (HCl) and ethanol absolute (C2H5OH) were purchased from Fisher chemical company. All chemical reagents utilized as received without further purification.

Method
The α-Fe2O3 hydrosol was synthesized by using hydrolysis method as the raw material and its thin films were deposited on glass substrates by dip coating technique. Predetermined amounts of stock solutions of FeCl3 and HCl were mixed in a flask at the ratio of 1:3 (v/v), and the deionized water was added till the final concentration of Fe +3 is 0.01 M. This mixture was preserved in water bath at 90 • C for 24 hrs, and then cooled to room temperature. The resulting orange-red solution was centrifuged (6000 rpm for 15 mins) and dried at 60˚C overnight on vacuum oven.
The as-prepared powder was annealed at 300 • C and 500 • C for 1h. After annealing, red powder was produced. Thin films of S1, S2, and S3 samples were deposited on soda-lime glass substrates using dip-coating technique (As mentioned in Table 1). The dip coating solutions were prepared by dissolve the produced powders in 50 mL absolute ethanol. Table 1: powder and thin films samples Powder sample Dip coating sample (thin film on two side of glass substrate) S1 As-prepared S1L 0.025 g/50 ml ethanol S2 Annealed at 300 ˚C for 1 hr S2L S2LA 0.025 g/50 ml ethanol S2LB 0.05 g/50 ml ethanol

Characterizations
X-ray powder diffraction (XRD) patterns were obtained using X-Ray diffractometer (Schimadzu 7000, Japan), operating with Cu Kα radiation (λ=0.154060 nm). Fourier transform infrared (FT-IR) spectra were recorded on a Shimadzu FTIR-8400 S (Japan) transform infrared spectrometer in the region of 400-4000 cm -1 using KBr pellets. Scanning electron microscopy (SEM) measurements were performed used a JEOL, JSM-6360 LA to examine the morphology and the homogeneity of the surface.
Optical transmittance was measured covering the spectral regions from 190 nm to 1100 nm with a standard UV-Vis double beam PC scanning spectrophotometer (LABOMED, INC). Fig. 1 showed the X-ray diffraction (XRD) patterns of the β-FeOOH prepared by hydrothermal process and the α-Fe2O3 product obtained after annealing process [27] . For S1 sample, it cauld be seen that, the peaks at 2θ of 26 The crystallite size can be determined from the broadening of corresponding X-ray spectral peaks by Scherrer formula (Equation 4) [26] . L = K λ / (β cos θ) (4) Where L is the crystallite size, λ is the wavelength of the X-ray radiation (Cu Kaα =0.15418 nm), K is usually taken as 0.89, and β is the line width at halfmaximum height, after subtraction of equipment broadening. The calculated crystallite size of S2 and S3 had the value of 30.1 nm and 61.94 nm, respectively. This result could be explained according to the growth rate Equation 5 which given by [27] : u=a0υ0[exp(-Q/KT)][1-exp(-∆Fυ/KT)] (5) where a0 is the particle diameter, υ0 is the atomic jump frequency, Q is the activation energy for an atom to leave the matrix and attach itself to the growing phase, and ∆Fυ the molar free energy difference between the two phases. For non-crystallization, ∆Fυ >> KT, so Equation (5) can be reduced to [28] :

Structural analysis
The crystallite size increases with increasing the heat treatment temperature. For S2 sample, it was found that, the β-FeOOH precursor was converted into a pure rhombohedral structure of α-Fe2O3 (according to card number 00-033-0664) after annealing process at 300 ˚C for 1 hr. Diffraction peaks of α-Fe2O3 were observed but with low intensity. No other diffraction peaks were observed, indicating that no impurity existed and the β-FeOOH completely transformed into the α-Fe2O3.    2 presented FTIR spectra obtained from S1, S2, and S3.
It was clear that the signals from S3 were relatively strong compared to S1 and S2. The FTIR spectrum of S1 showed characteristic vibrations of H2O and Fe-OH bands. A broad bands 3340 and 1626 cm -1 were assigned to O-H and H-O-H bending vibrations of β-FeOOH and H2O. Also, the bands at 644 and 862 cm -1 were assigned to Fe-O vibrational mode of β-FeOOH. The experimental results demonstrated that β-FeOOH was obtained as a result of the hydrothermal interaction between FeCl3 with HCl. After annealing process at 300 ˚C for 1 hr (S2), FTIR broad bands of S1 were weakened. Also, two new weak bands at 578, 447 cm −1 were appeared which characteristic of α-Fe2O3 spheres [29] .
Increasing the annealing temperature into 500 ˚C (S3), these two peaks at 3420 and 1640 cm -1 were completely disappeared. The sharp bands at 575 and 485 cm -1 were observed. The FTIR results indicate the formation of α-Fe2O3 in the present experiment after annealing process at 300 ˚C and 500 ˚C for 1 hr, which are consistent with the XRD result [30] . Fig. 2. FTIR spectra of S1, S2, and S3 samples. Fig. 3 illustrated the SEM images of the as prepared nano-powder and annealed samples (S1), (S2) and (S3). For S1, very small spindle or elongated particles were produced with size range of 55-65 nm, demonstrating the increase of the overall dimension relative to β-FeOOH [31] . When the as-prepared powder was annealed at 300 ˚C for 1h (S2), irregular spherical grains of α-Fe2O3 nano-particles with diameter of 100-150 nm was appeared. When the annealing temperature was increased to 500 ˚C for 1hr (S3), the particles were aggregated with each other and form spherical grains with large a diameter 500 nm. Table 2 showed the direct optical transmittance of S2LA, and S2LB samples without and with heat treatment at 200 ˚C for 15 min. Also the effect of immersing time of glass substrate on the dip-coating solution was demonstrated. It was found that, the heat treated sample with immersing time of 5 mins was given higher transmittance than untreated samples. It means that, using of low concentration of α-Fe2O3 ethanolic solution increase the transparency of the film and the heat treatment remove any impurities and enhance the adhesion of the material on the substrate. Fig. 3. SEM of S1, S2, and S3 samples. Table 2. Effect of solution concentration on the optical transmission of samples without and with heat treatment at 200 C for 15 min Table 3 showed the effect of number of layers deposited on glass substrate. The direct transmittance was enhanced in the whole spectrum for AR film deposited with one layer and dipping for 5 min with heat treatment sample compared to that of the bare glass as shown in Fig. 4.   Fig. 4. Optical transmittance spectra of S2LA thin films with different number of layers deposited on glass substrates with immersing interval 3 min (left) and 5 min (right). The optical transmittance of films deposited on glass substrates using powder samples S1, S2, and S3 were presented in Table 4 and shown in Fig. 5. It was found that, the morphology of the samples has a great effect on the transmittance of the samples. For elongated particles of β-FeOOH in S1L and the large grains of α-Fe2O3 in S3L caused reflection or scattering for the light which incident on the samples which lead to reduce in the whole transmittance value of the films [32] .  5. Optical transmittance spectra of thin films of S1L, S2LA, and S3L samples; without (left) and with HT (right).

Conclusions
Nanocrystalline iron oxide (III) has been prepared by hydrothermal method. orthorhombic β-FeOOH phase was produced after hydrothermal process. After annealing at high temperature (300 ˚C), rhombohedral α-Fe2O3 was obtained. Increasing the annealing temperature into 500˚C lead to increase the crystallinity of the produced α-Fe2O3 nanoparticles with crystallite size of 30 nm. For antireflective coating of nanocrystalline α-Fe2O3 layer on soda lime glass substrates deposited by dip-coating technique (dipping rate -3 cm/10 sec, coating time -5min, and temperature -25 ºC), the maximum optical transmittance was 93.4 % for annealed powder at 300 °C for 1 hr.