1. Introduction

Zinc oxide (ZnO), a II-IV semiconductor, has a wide direct gap of 3.37 eV at room temperature and large exciton binding energy of 60 meV^[1], which has attracted much attention because of its wide prospects in optoelectronic devices, such as solar cells, light emitting diodes (LED), laser diodes and acoustic-optical devices^{[2, 3, 4, 5]}. In solar cells, ZnO thin films are used as an anti-reflective coating (ARC) and transparent conductive oxide (TCO) due to their high optical transmittance in the visible light region, high band gap energy ($E_{\rm g}$ $\approx$ 3.3 eV), optimum refractive index ($n$ $\sim $ 2.0), and natural n-type electrical conductivity^{[6, 7]}. ZnO can be used as a heat mirrors, piezoelectric devices^[8], thin films^[9], and for chemical and gas sensing^[10].

ZnO thin films have been prepared using various methods, such as molecular beam epitaxy (MBE)^[11], chemical vapor deposition^[12], electrochemical deposition^[13], pulsed laser deposition (PLD)^[14], sol-gel process^[15], reactive evaporation^[16], magnetron sputtering technique^[17], and spray pyrolysis^[18], which have been reported to prepare thin films of ZnO. The spray pyrolysis technique is one of these techniques to prepare large-scale production for technological applications. It is possible to alter the mechanical, electrical, optical, and magnetic properties of ZnO nanostructures.

In the present study, nanostructure ZnO based thin films can be deposited by spray pyrolysis technique on glass substrate at two substrate temperatures of 300 C and 350 C. The thin films were prepared with 30 mL of deposition rate, the aim of this work is to study the effect of substrate temperature on crystalline structure, optical gap energy, and electrical resistivity.

2. Experimental

In this study, the organic solar cells were fabricated through this process. We fabricated the organic solar cells, which consisted of a mirror layer inside (ITO glass) and a substrate holder. We established the substrate layer to maintain the substrate temperature using the mirror layer. The organic solar cells use solar energy to heat a substrate glass with deposition of ZnO solution on substrate (see Figure 1). The reflective layer is designed to reflect the maximum amount of solar energy incident upon it back through the glass substrate. The structure of the spray deposition system and fabricated organic solar cells are shown in Figure 1. The substrate was heated by absorber plate and we maintained a temperature of 300 C and 350 C when the active solution was sprayed onto the substrate glass.

ZnO solution were prepared by dissolving 0.1~M (Zn(CH$_{3}$COO)$_{2}$, 2H$_{2}$O) in asolvent containing equal volume absolute methanol solution (CH$_{3}$OH) (99.995 %) purity. We then added a few drops of concentrated HCl solution as a stabilizer, and the mixture solution was stirred at 60 C for 120~min to yield a clear and transparent solution. The reaction can be represented as:

$$\begin{split} {}& {\rm Zn(CH}_{3}{\rm COO})_{2}\cdot 2{\rm H}_{2}{\rm O}+2{\rm R}-{\rm OH} \to \\[2mm]& {\rm Zn (CH}_{3}{\rm COO})_{2} + 2{\rm R}-{\rm OH} + 2{\rm H}_{2}{\rm O}.\end{split}$$

(1)

The chemical reaction may take place on the heated substrate to produce ZnO thin film as follows: when a droplet of the solution reaches the heated substrate, a chemical reaction of the zinc acetate with water solution takes place under the stimulated temperature, which provides the formation of ZnO thin films^[2].

$$\begin{split} {\rm Zn(CH}_{3}{\rm COO})_{2}\cdot 2{\rm H}_{2}{\rm O}{}& \frac{300\; \du {\rm or} 350\; \du}{\rm deposited as}\to \\& {\rm ZnO}+{\rm CO}_{2}+{\rm CH}_{4} + {\rm steam}.\end{split}$$

(2)

The resulting solutions were sprayed on the heated glass substrates by a spray pyrolysis technique, the substrates were heated by using the solar cells method, and this was prepared in our laboratory. The thin films were deposited at two substrate temperatures of 300 C and 350 C, onto glass whose size was 30 $\times $ 17.5 $\times $ 1 mm$^3$, which transforms the liquid to a stream formed with uniform and fine droplets of 35 $\mu $m average diameters.

The structures of the thin films were determined by X-ray diffraction (XRD, Bruker AXS-8D) with CuK$\alpha $ radiation ($\lambda$ $=$ 0.15406 nm), the scanning range of (2$\theta$) was between 20$^\circ$ and 60$^\circ$. The optical transmittance of the films was measured in the range of 300-900 nm by using an ultraviolet-visible spectrophotometer (SHUMATZU 1800) and the electrical resistivity was measured in a coplanar structure obtained with evaporation of four golden stripes on the deposited film surface using the Keithley Model 2400 low voltage source meter instrument.

3. Results and discussion

The XRD spectrum of the ZnO thin films grown at 300 and 350 C are shown in Figure 2. The obtained XRD spectra matched well with the space group P63mc (186) (No. 36-1451)^[19]. As can be seen, the only diffraction peak was observed at $2\theta $ $=$ 35.29$^\circ$ and 34.5$^\circ$ for sprayed films at 300 and 350 C, respectively, which is related to the plan of (002). Zaier et al.^[20] found that for sprayed ZnO thin films at a substrate temperature ranging between 250 C and 450 C, a single (002) diffraction peak of ZnO films is detected in all films. The spectra shows that all of the films have a preferential growth along $c$-axis or (002) plane but with different peak intensity. The film deposited at 350 C has a higher and sharper diffraction peak, indicating an improvement in (002) peak intensity compared to the other films.The crystalline quality of thin films was enhanced at 350 C. Similar observations have been found by other researchers^{[21, 22]}.

The lattice constant $c$ and diffraction peak angles of ZnO thin films (see Table 1) are calculated using the following equation^[21]:

$d_{hkl} =\left(\frac{4}{3}\frac{h^2+hk+k^2}{a^2}+\frac{l^2}{c^2}\right)^{-\frac{1}{2}}, $

(3)

where $a$, $c$ are the lattice parameters, ($h, k, l)$ is the Miller indices of the planes, and $d_{hkl}$ is the interplanar spacing.

The strain $\varepsilon$ values in our films were estimated from the observed shift in the (002) diffraction peak between their positions in the XRD spectra via the formula^[21]:

$\varepsilon =\frac{c-c_0 }{c_0} \times 100 %, $

(4)

where $\varepsilon$ is the mean strain in ZnO thin films (Table 1), $c$ the lattice constant of ZnO thin films, and $c_{0}$ the lattice constant of bulk (standard $c_{0}$ $=$ 0.5206 nm).

The crystallite size of ZnO thin films are estimated using the well-known Debye-Scherrer formula^[23]:

$G=\frac{0.9\lambda }{\beta \cos \theta }, $

(5)

where $G$ is the crystallite size, $\lambda$ is the wavelength of X-ray ($\lambda$ $=$ 1.5406 Å), $\beta$ is the full width at half-maximum (FWHM), and $\theta$ is the half diffraction angle of the centroid of the peak. Table 1 presents the structure properties of ZnO films whichmeasured the (002) diffraction peak at 300 and 350 C. It can be seen from Table 1 that good crystallinity isachieved in the films sprayed at 350 C, which wasalso obtained by Reference~^[22].

Figure 3 shows the optical transmission spectra of ZnO thin films with substrate temperature. As can be seen, there is a height transparent spectra $T(\lambda)$ of the thin films in the visible region, and the average transmission is over 90 %. The region of the absorption edge in the all layers due to the transition between the valence band and the conduction band is located between 360-390 nm. In this region, the transmission decreased because of the onset fundamental absorption.

The optical band gap energy $E_{\rm g}$ was measured from the transmission spectra using the following relations^[24]:

$(Ah\nu)^2=C(h\nu -E_{\rm g}), $

(6)

where $A$ is the absorbance, $d$ is the film thickness; $T$ is the transmission spectra of thin films; $\alpha $ is the absorption coefficient values; $C$ is a constant, $h\nu $ is the photon energy $(h\nu =\frac{1240}{\lambda ({\rm nm})}$ (eV)); and, $E_{\rm g}$ is the band gap energy of the semiconductor. As was shown in (Figure 3), a typical variation of $(Ah\nu )^2$ as a function of photon energy ($h\nu )$ used for deducing optical band gap $E_{\rm g}$ is determined by extrapolation of the straight line portion to zero absorption ($A=0$)^[11], the values of $E_{\rm g}$ are listed in Table 2. In addition, we have used the Urbach tail energy ($E_{\rm u})$, which is related to the disorder in the film network, which is expressed as^{[21, 23]}:

$A=A_0 \exp \frac{h\nu }{E_{\rm u}}, $

(7)

where $A_0$ is a constant, $h\nu $ is the photon energy, and $E_{\rm u}$ is the Urbach energy. The Urbach tail energy $E_{\rm u}$ is calculated from the slope of $h\nu $ versus photon energy ($h\nu )$ plot and the obtained values are shown in Table 2.

As is clearly seen in Figure 4, the optical gap energy of ZnO thin films arechanged from 3.265 to 3.294 eV with the increase of substrate temperature from 300 and 350 C, respectively. This indicates a narrowing of the optical band gap. The decrease in optical energy gap might be interpreted as a Moss-Burstein shift, which was ascribed to an increase of free carrier concentration, this leads to a downward shift of the Fermi level to below the band edge^[25]. The same phenomena are carried out by Ali et al.^[26], who reported similar results in ZnO thin films prepared by the radio frequency technique. However, in Figure 5 we obtained the inverse result for the Urbach energy: the Urbach energy $E_{\rm u}$ decreases from 89.8 to 83.4~meV as substrate temperature increases from 300 and 350~C, indicating the reduction of structural disorder and defects in the ZnO thin films. This is related to an improvement in the crystalline structure, as indicated by XRD spectra.

Table 2 shows that the electrical resistivities of our films are in qualitative agreement with the band gap energy (see Table 2). The increase in the electrical resistivity can be explained by the increase ofthe potential barriers because the introduced atoms are segregated into the grain boundaries, this interpretation is consistent with the authors^{[18, 19, 21, 22, 23, 24]}. This can be explained by decreasing of the crystallite size (see Table~1). One can note that the substrate temperature effect is clearly observed in the layer quality.

4. Conclusions

In conclusion, highly transparent conductive ZnO thin films were deposited on glass substrate by the spray pyrolysis technique. The ZnO thin films were deposited at two different temperatures (300 and 350 C), the substrates were heated using the solar cell method. The influence of deposition temperature on structural, optical and electrical properties was investigated. The films exhibit a hexagonal wurtzite structure with a strong (002) preferred orientation. The average transmittance of obtain films is about 90 %-95 %, which was measured by a UV-vis analyzer. The band gap energy varies from 3.265 to 3.294 eV for the deposited ZnO thin film at 300 and 350 C, respectively. The electrical resistivity measured of our films is in the order of 0.36 $\Omega $$\cdot $cm.

Acknowledgments

This work was supported in part by the National Project Research (PNR) and VTRS laboratory of El-Oued University, X-ray diffraction data in this work were acquired with an instrument supported by the University of Biskra. We thank Prof. A. Chala and B. Gasmi (Biskra University) for the assistance in XRD data acquisition.