Many investigations have been done for over from decades on transparent conducting oxide (TCO) thin layers for various applications in the field of science and technology
In the literature there is an extensive study on MZO films using various transition metal ions such as Vanadium (V), Nickel (Ni), Niobium (Nb), Iron (Fe), Chromium (Cr), Gold (Au), Copper (Cu), Cobalt (Co), Zirconium (Zr), Molybdenum (Mo), Manganese (Mn), Titanium (Ti), and Halogens such as Chlorine (Cl) and Fluorine (F) and also some metalloids, metals such as Germanium (Ge), Boron (B), Bismuth (Bi), Aluminium (Al), Tin (Sn), Indium (In), and Gallium (Ga). However, the investigations reported on MZO thin films are meager, particularly in relation to deposition parameters. The valency difference between Mo6+ and Zn2+ ions is (+4), which will become very beneficial for doping ZnO with Mo. Each Molybdenum atom will provide four free electrons to the parent zinc lattice so that a small amount of Mo-doping can contribute more free electrons that could severely alter the electrical conductivity. Many thin film deposition techniques were used to grow MZO layers such as spray pyrolysis, and sputtering. To compare all these deposition techniques, spray pyrolysis is an easily adoptable technique for growing ZnO thin films over large areas and can be commercialized simple and low cost. Further, it is a method that no needs to create vacuum. In the present investigation zinc chloride was used as a precursor for zinc to deposit MZO layers instead of zinc acetate
A detailed investigation of structural, morphological, optical, and electrical properties has been done in relation to Zn-precursor molarity. Rietveld refinement analysis and Haze analysis, which were previously unreported, were performed on Mo: ZnO layers. CTS/MZO heterojunction solar cells have not yet been reported. The observed results are comparable to improving the efficiency of solar cells through the use of environmentally friendly materials.
MZO films were prepared using the spray pyrolysis technique on Corning 7059 glass substrates taking zinc chloride (Aldrich 98 %) as the zinc source and MoCl2 (Aldrich 95 %) as the Mo source with methanol as solvent at a constant substrate temperature, 400 °C by changing Zn molarity in the range, 0.01 M – 0.20 M while the doping concentration of Mo was maintained constant 2 at. % (see
For the evaluation of chemical composition of grown films and to know the valency state of Mo ion in ZnO lattice, X-ray photoelectron spectroscopy (XPS) spectra were recorded in the binding energy range 0-1350 eV.
The XRD patterns of grown MZO layers using different precursor molar concentrations with Mo-doping of 2 at. % was shown in
The analysis of Rietveld refinement was also done to know the presence of Mo in ZnO lattice, for MZO films prepared using a Zn- molar concentration of 0.15 M. In addition, R weighted profile (Rwp), R profile (Rp), R structure factor, R Bragg factor (RBragg), goodness of fit (GOF) such structural parameters were also calculated. EXPO software was used to calculate the unit cell parameters from the Rietveld refinement data.
The calculated Rietveld refinement parameters such as Rp, Rwp, GOF, R structure factor, RBragg, and unit cell parameters were listed in
|
|
a, (Å) |
4.69 |
b, (Å) |
5.73 |
c, (Å) |
4.89 |
α ,(°) |
90 |
β, (°) |
90.31 |
γ, (°) |
90 |
Cell volume (Å)3 |
131.27 |
Volume per atom (Å)3 |
10.94 |
Density (g/cm3) |
5.70 |
Crystal system & Space group number |
Monoclinic & (P1c1) |
Laue group & Point group symbols |
2/m &2/ m |
|
|
Atoms |
28 |
Bonds |
24 |
Polyhedra |
06 |
|
|
Rp |
7.651 |
Rwp |
12.065 |
Goodness of fit |
1.57 |
R-Structure factor |
47.884 |
R-Bragg factor |
64.834 |
|
|
|
|
|
Mo1 |
0.0000 |
0.8119 |
0.2500 |
1.000 |
Zn1 |
0.5000 |
0.6918 |
0.7500 |
1.000 |
O1 |
0.2538 |
0.6236 |
0.4014 |
1.000 |
O2 |
-0.2165 |
0.8950 |
0.5603 |
1.000 |
The lattice parameters were evaluated by following expressions (1) to (3) mentioned below
The lattice parameters were measured and it can be seen that the value of the lattice constant and the interplanar distance (d) are decreased by increasing the molar concentration of the precursor present in the growth film.
From the data obtained, it is found that MZO films with a starting solution molarity of 0.10 M showed minor change in the lattice parameters with a = 0.326 nm, c = 0.563 nm and d = 0.282 nm. The Debye-Scherrer formula
where β represents full width at half maximum (FWHM) in radians, θ represents angle of diffraction, λ represents X-rays wavelength and ‘n’ represents the correction factor (n = 0.9), The evaluated values are tabulated (see
The evaluated (δ) values are tabulated in
where θ represents angle of diffraction and β represents full width at half maximum (FWHM) in radians. The films with precursor molar concentration 0.10 M exhibited low value of ε, indicates less defects between the grown MZO films and substrate. Further the lattice strain increases for higher precursor molar concentrations. This might be due to variation of nucleation mechanism in the films. Polycrystalline films preferential orientation can be obtained by evaluating texture coeffcient Tc, which is evaluated using the below expression (7)
where I(hkl) is the relative intensity measured on the preferred orientation plane (hkl) and N is the number of reflections observed on the XRD graph. I0(hkl) represents standard diffraction pattern (JCPDS: 750576) intensity. It is found from the values obtained that MZO films grown with 0.10 M of precursor solution exhibited a high Tc and less lattice defects than other MZO films, which shows that for precursor solution of 0.10 M more number of crystallites of MZO layers are oriented along the (002) plane. The Lorentz factor greatly influences peak intensity, which depends on the integrated intensity. Lorentz factor can be evaluated using the following formula.
where θ represents the angle of diffraction. The evaluated values of L are tabulated (see
where θ represents the angle of diffraction and β represents the FWHM. The evaluated Stacking faults values are tabulated (see
|
|
|
|
|
|
|
|
0.01 |
(002) |
9.26 |
11.66 |
0.0036 |
5.31 |
3.02 |
0.0092 |
0.05 |
(002) |
55.20 |
0.32 |
0.0008 |
5.67 |
3.25 |
0.0024 |
0.10 |
(002) |
84.47 |
0.14 |
0.0004 |
6.51 |
3.55 |
0.0008 |
0.15 |
(002) |
52.35 |
0.36 |
0.0007 |
2.96 |
2.80 |
0.0019 |
0.20 |
(002) |
8.46 |
13.97 |
0.0040 |
2.60 |
2.07 |
0.0076 |
Here, t represents the thickness of the film. The thickness of the film was taken as 500 nm. The energy band gap (Eg) of the grown layers was evaluated by the following expression,
where A is a constant, hυ represents the energy of photon and n=1/2 shows a direct optical transition in these deposited films.
The ratio of intensity of diffused light to the total intensity of transmitted and reflected light on textured interface is called Haze. It predicts the degree of scattering of light. The Haze value can be calculated from following relation,(12)
The change of Haze parameter with wavelength in air medium is shown in
The scattering phenomena always influenced by the incident medium refractive index, magnitude of light and interface morphology.
The Hall measurements clearly showed n-type conductivity for all MZO films with resistivity varying from 15×10-2 Ωcm to 1.9×10-2 Ωcm with carrier concentration varying in the range, 2.28- 7.8×1018 cm-3 and carrier mobility changing between 15cm2/V-s and 42 cm2/V-s (See
|
|
|
|
0.01 |
15.3 x10-2 |
3.88 x1018 |
15 |
0.05 |
5.4 x10-2 |
5.7 x1018 |
20.32 |
0.10 |
1.9 x10-2 |
7.8x1018 |
42 |
0.15 |
4.1 x10-2 |
4.4 x1018 |
34.46 |
0.20 |
9.3 x10-2 |
2.28 x1018 |
29.52 |
In order to test the potentiality of the optimized MZO window layer, a hetero-junction solar cell was fabricated in substrate configuration with the device structure, SLG/Mo/Cu2SnS3/CdS/Mo:ZnO/Au/Cu. The schematic diagram of the junction cross-section is shown in
The fabricated solar cells exhibited an open circuit voltage (Voc) of 0.14 V, short circuit current density (Jsc) of 6.46 mA cm-2, a fill factor (FF) of 0.27, and a conversion efficiency of 0.25 %. In general, the quality of the absorber decides performance of the device. The collection of more number of charge carriers is mainly depends on the uniform and homogeneous surface of the absorber layer in which no voids
MZO layers were successfully deposited by chemical spray pyrolysis technique by varying the zinc precursor molar concentration in the range, 0.01 – 0.20 M maintaining a constant substrate temperature of 400 °C and dopant Mo content of 2 at. % constant. The comprehensive structural, optical and electrical properties of the films prepared at different Zn precursor concentrations were studied. The as deposited MZO films were polycrystalline in nature and exhibiting hexagonal wurtzite structure with (002) preferred orientation. The energy band gap values of the films varied in the range, 2.70 to 3.65 eV, respectively with the change of Zn- molar concentration. Investigation of electrical properties showed that the films grown at 0.10 M precursor concentration exhibited a low resistivity of 1.9x10-2 Ωcm, high mobility 42 cm2/V-S and carrier concentration 7.8x1018 cm-3 Compared to other films. Hence, MZO films grown at 0.10 M precursor concentration with a Mo content of 2 at. % exhibited better opto-electrical properties that could be used as window layers in hetero-junction photovoltaic cells. Hetero-junction solar cell was fabricated with MZO film as a window layer that showed photovoltaic response.
Sumalatha Chevva, Sreenivasulu Reddy Tirumalareddygari and Phaneendra Reddy Guddeti have equally contributed to this work
Sumalatha Chevva, Sreenivasulu Reddy Tirumalareddygari and Phaneendra Reddy Guddeti have equally contributed to this work.