Printer Friendly

Effect of tin as dopant on zinc oxide thinfilms for solar cell applications.

INTRODUCTION

To achieve higher conductivity in ZnO thinfilms, various tetravalent metal dopants such as Tin ([Sn.sup.4+]), Silicon ([Si.sup.4+]), Geranium ([Ge.sup.4+]) and Titanium ([Ti.sup.4+]) are added to it. The use of higher valance dopants provides more free electrons which leads to lower the amount of impurity doping and enhance the quality and optical properties of thinfilms [1]. Among the tetravalent metal dopants, Sn doped ZnO is transparent conducting oxide (TCO) material due to its high conductivity and good optical transmittance. [2] Young et al reported that the optical band gap in free standing transparent ZnO films can be reduced with dopant Sn using the Sol-gel method. This study elobarates the study on the preparation and characterization of Tin doped Zinc Oxide (TZO) thinfilms. Fabrication of ZnO thinfilms doped with Sn at varying concentrations under different coating condition has been carried out. The effect of Sn incorporation on structural and electro-optical properties of ZnO films has been investigated. This research work suggests the optimized SZO film with enhanced properties suitable for solar cell applications.

Experimental Details:

ZnO thinfilms have been fabricated with tin as dopant at different molar concentrations. The Sol solution is prepared by dissolving Zinc acetate in the solvent mixture ethanol and mono -ethanolamine. The optimized concentration of Zinc acetate to prepare the precursor solute is 0.36 mol/L. Tin chloride (Sn[Cl.sub.4] 5[H.sub.2]O) is used as the source material for tin. The solution is refluxed at 60 [degrees]C for 1 hour and then allowed for aging in the ambient. The coating is made on the third day of aging which is the optimized critical time for coating. The Sol solutions are spin coated on glass substrate maintaining the coating parameters under different conditions. The Sn doped ZnO thinfilms are fabricated with various concentrations of Tin chloride viz., 0.004, 0.006, 0.008, 0.01 mol %.

The coating parameters such as number of coating layers, spin rate and heat treatment temperature have been varied as given in the Table 1 Each set of thinfilms fabricated under fixed coating parameters are prepared up to 12 number of coating layers to obtain the required thickness of ZnO thinfilms. All prepared films are subsequently annealed at 450[degrees]C in vacuum for one hour. The Sn doped ZnO thinfilms prepared with various combinations of the coating parameters with different concentrations of Sn[Cl.sub.4] are detailed in the Table.1.

Structural Properties Of Sn Doped Zno Thin Films:

The XRD pattern of tin doped ZnO films at 8, 10 and 12 layers of coating are shown in Figure 1. These thinfilms have been fabricated using precursor solution prepared with dopant concentration 0.01 mol %, coated at spin rate 3500 rpm and heat treated at 350 [degrees]C. No phase corresponding to tin or other tin compound is observed in the XRD patterns.

Characteristic peaks corresponding to (100), (002) and (101) orientations [3] denoting Wurtizite structure have been observed in all the coated thinfilms with number of coating layers such as 8, 10 and 12. It could be seen that the peaks are more pronounced for the thinfilm at 10 coating layers than the other films with 8 and 12 layers. However the intensity of diffraction is found to be low for all the films coated at this spin rate. It indicates the degree of crystallinity of ZnO particles is very low for all the films when coated at spin rate 3500 rpm. On the other hand the crystallinity is well acquired by the thinfilms coated at other spin rate 2500 and 4000 rpm as observed in the Figure 2 which gives the diffraction pattern for Sn doped ZnO thinfilms coated at different spin rate. The intensity of the ZnO orientations along (002) plane found to be dominant. Whereas diffractive intensities of (100) and (101) planes are very low. It depicts the well formed crystalline nature of Caxis preferred orientation of ZnO particles. The low intensity values of ZnO peaks as observed at 3500 rpm in the Figure 2 follows the similar result as observed earlier.

The average particle size of Sn doped ZnO thinfilms gets increased at higher spin rate, may be due to clustering of crystallites. Value of Lattice parameters for these films increases as spin rate increases denotes the increase in the volume of the unit cells due to the introduction of Sn2+ ions. Figure 3 gives the diffraction pattern for Sn doped ZnO thinfilms at different heat treatment temperature. Prominent ZnO orientations are observed at temperatures 350 and 400 [degrees]C with increasing peak intensities towards higher temperature. Whereas the diffractive peaks observed with feeble intensities for the film heat treated at 300 [degrees]C. Figure 4 shows the diffraction pattern for Sn doped ZnO thinfilms at different concentrations of the dopant such as 0.005, 0.006 and 0.1 mol %. It is observed that the film with 0.005 mol % dopant concentration is found to have high intensed peaks corresponding to ZnO planes. The XRD pattern also showed that these diffraction peaks found to have minimal reduction in intensity for the films with the lower dopant concentration. This infers the crystalinity of ZnO particles might be distorted by the introduction of dopant and whose ionic radii are lesser than that of Zn. The value of texture coefficients are below 1 for (101) and (100) orientations refers feeble growth of ZnO crystallites along these planes. Whereas TC for (002) orientations are found to be more than 2 infers well grown crystallites along c axis indicating the preferred c-axis oriented crystallites. The extent of micro strain occurred at different (hkl) lattice orientation found to be low at higher spin rates. This may be due to as the higher spreading of particles that would create more interspace distance between particles. This in turn measures low dislocation density at higher spin rates.

Fig. 3&4: XRD Pattern of SZO Thinfilms Spin Coated with Different Heat Treament Temperature and Dopant concentration

The average particle size found to be decreased towards higher temperatures. This may be due to agglomerization of ZnO particles XRD pattern of SZO thinfilms spin coated at different heat treatment temperature dopant concentration takes place at higher temperature. The lattice parameters 'a' and 'c' evaluated also shows a gradual decrement in its value with the increase in temperature. The value of texture coefficient is found to be more than 1 along (002) orientation and a similar observation is expected for all the coated films. The calculated values of micro strain and dislocation density are low at higher heat treatment temperature. This may be due to the formation of uniform crystallites on the substrate, free from defects.

The particle size increases initially with dopant concentration and then found to decrease with dopant concentration. This is due to the lesser ionic radius of Sn+4 (0.071) which substitutes [Zn.sup.2+] (0.074), thereby decreasing the crystallite size [4]..The variation of crystallite size does not change systematically which may due to lattice disorder produced in the films at higher dopant concentrations due to the difference in their ionic radii [4]. The value of lattice parameters are found to be reduced at dopant concentration of 0.008 mol % compared to 0.005 and 0.01 mol %. The values of micro strain and dislocation density are found to be low at 0.008 mol %. The value of texture co-efficient is found to be above 1 for all the C-axis orientated planes. The values of FWHM, d spacing and diffraction angle are also given in Table 5.4. The narrower FWHM means larger the crystallite size as observed from determined values of crystallite size. The values of d-spacing and diffraction angle obtained in the XRD data matches well with JCPDS data corresponding to ZnO.

Optical Properties Of Sn Doped Zno Thin Films:

UV-Visible spectroscopy:

The optical transmittance of Sn doped ZnO thinfilms prepared with different coating conditions are studied and their optical properties are analysed and reported. For the ZnO film the percentage of transmittance in the visible region (390-800 nm) is about 80-87 % with a gradual fall near the fundamental absorption at 380 nm denotes the absorption edge occurs in the UV region. Addition of tin as dopant improved the transmittance in the visible region as detailed below. All the thinfilms are transparent in the visible region above the wavelength 400 nm as shown in Figure 5. It is observed thinfilm with 10 number of coating layers is found to have its maximum transmittance percentage about 95 % at wavelength 850 nm[5].This is comparatively higher than other coating layers. Sharp absorption edges in the UV region are observed for all the doped films that hint at a direct type transition. These absorption edges slightly shifted towards shorter wavelengths as the number of Sn atoms increased with the number of coatings. This indicates that, this blue shift is been observed when Sn dopant has been added within ZnO thinfilms[3].

The band gaps of Sn doped thinfilm have been calculated using the transmittance spectra at different no of coatings. Plots of [alpha]2 Vs E in the absorption region near the fundamental absorption edge is shown in the Figure 6. The extrapolation of the linear region of the graph to the photon energy axis measures the optical band gap .Its value has been determined as 3.1 eV for the thinfilms with 8,10 and 12 numbers of coating layers. Figure 7 and 8 shows the transmission and bandgap profile respectively for the Sn doped ZnO thinfilms, coated at different spin rate. High transmittance percentage of 95 % as well as large band gap value of 3.1 eV is observed for thinfilm coated at 4000 rpm. It is also observed the increase of band gap from 3 to 3.1 eV, with spin rate varying from 2500 to 4000 rpm. Figure 9 and 10 show the transmittance and band gap of films coated with different heat treatment temperature. Films heat treated at 350 [degrees]C is found to have maximum transmittance of 83 % and its band gap value is found to be 3.1 eV. As the temperature increases from 300 to 400 [degrees]C, the band gap initially increases from 3 eV to 3.1 eV and then decreases to 3.05 eV.

Figure 11 and 12 show the transmittance spectra and bandgap plot of the ZnO thinfilms spin coated with different dopant (Sn) concentrations. High transmittance of 88 % is observed for the film doped with 0.008 mol % of dopant and the band gap is found to be 3.1 eV. As the dopant concentration increases from 0.005 to 0.01 mol %, the band gap increases[4]R.K.nath from 3.05 eV to 3.01 eV. The change in band gap can be attributed due to Burstein-Moss band gap widening and band gap narrowing due to electron -electron impurity scattering [6].

Photoluminescence Spectroscopy Studies:

The effects of Tin doping on the luminance emission of the SZO films are studied by Photoluminescence measurements. The Photoluminescence emission spectra of ZnO thinfilms doped with 0.008 mol % of dopant concentration coated with spin rate 3500 rpm at heat treatment temperature 400 [degrees]C is shown in the Figure 13. The spectra showed a weak UV emission band and a broad visible emission band. The UV emission band occurs at 485 nm which is associated with recombination of electrons with holes in the conduction band as reported by [7] Lin et al &[8] Deng et al. The broad visible emission bands can be divided into three emission bands. Two of them are orange emission bands centered at 557 nm and 563nm, the third one lies due to orange emission bands at 611 nm. The red emission band occurred at 663nm. The orange luminance is associated with the amount of oxygen in the films. [9] had reported, the red emission band is considered to have resulted from excess oxygen.

FTIR Spectroscopy studies:

Figure 14 shows the FTIR spectra of ZnO thinfilms doped with 0.008 mol % of dopant concentration, coated with spin rate 3500 rpm at heat treatment temperature 400 [degrees]C. The observed absorption band near 3500 cm-1 represents the presence of hydroxyl groups (O-H mode) on the surface of the samples and the absorption peak observed at 2900 cm-1 is due to C-H stretching frequencies. The absorption peak appearing at 2600 cm-1 is due to absorption of atmospheric CO2 by metallic cat ion. The characteristic peak observed at 482 cm-1 is attributed to the ZnO stretching vibrations.

Electrical Properties Of Sn Doped Zno Thin Films:

The sheet resistance of ZnO films found to be decreased with the addition of Sn as dopant. This is attributed to increase of electron contribution which is caused by additional two free electrons generated for each Sn 4+ ion occupied Zn2+ ion site. Because the radius of the Sn4+ ion (0.069nm) is smaller than that of the Zn2+ (0.074), the grain size of the SZO films is smaller than that of ZnO films. The sheet resistance of the film is found to be reduced with a small amount of Sn doping. This could be due to the replacement of Zn2+ by Sn4+ ions which increases electron concentration, thereby decreasing the resistivity[9]. With increase in dopant concentration the resistivity is found to be increasing significantly. This could be explained as at higher concentration, the disorder produced in the lattice due to the difference in ionic radii of Zn2+ and Sn4+ increases the efficiency of scattering mechanism such as phonon scattering and ionized impurity scattering which in turn causes increase in resistivity [4].

The values of sheet resistance have been measured for the SZO thinfilms fabricated under different coating conditions. The lowest value of sheet resistance has been observed as 150 M[OMEGA]/sq-cm. Figure 15 (a) to (d) shows the variation of sheet resistance with number of coating layers, spin rate, heat treatment temperature and dopant concentration respectively. The various coating parameters which offer lower sheet resistance are (i) number of coating layers : 10, (ii) spin rate : 3500 rpm, (iii) heat treatment temperature : 350 [degrees]C and (iv) dopant concentration : 0.008 mol %.

Sem And Edax Analysis:

The surface features of Sn doped ZnO thinfilms are studied by SEM analysis. SEM images of undoped and Sn doped ZnO films are shown in Figure 16 Where (a), (b) and (c) corresponds to undoped ZnO thinfilms with increasing magnifications 5kx, 40 kx, and 80 kx respectively, (e), (f) and (g) corresponds to ZnO: Sn films with increasing magnifications 3 kx, 10 kx and 24 kx respectively. These micrographs show that the surface morphologies on the films are influence by the introduction of dopant. The streaks or wrinkle like structure on the surface of the thinfilms is seen in the images of undoped films. Whereas the similar pattern but the wrinkles [ 3] formed in a closely packed manner, appears in the images of Sn doped films[10]. Thus enhancement of uniformity and hence reduction of surface roughness of the film can be observed from these images. The average crystallite size found to be reduced on Sn (0.008 mol %) doping when compared with ZnO films as observed from the images shown in the Figure 16. (c) and (f) which agrees with the results obtained from the XRD measurements. The EDAX spectra as shown in Figure 17 illustrate the presence of the elemental peak corresponding to Tin.

Conclusions:

The Sn doped ZnO thinfilms were deposited with various concentrations of Tin chloride viz., 0.004, 0.006,

0. 008.and 0.01 mol %. From the XRD data, the change in the values of lattice constants, crystallite size and micro strain due to Sn doping were investigated. These structural properties showed the influence of [Sn.sup.4+] incorporation in the [Zn.sup.2+] ion. The ZnO thinfilms prepared with 0.008 mol % of Sn doping into ZnO, exhibits a lower value of sheet resistance 150 MQ/sq-cm and these results are in good agreement with values reported in earlier literature [4].

High transmittance of 88 % was observed for the SZO film doped with 0.008 mol % of dopant and its band gap was found to be 3.1 eV. The near band edge absorption was observed to occur around 380 nm for all the spin coated SZO films. The emission spectra of the SZO films obtained from the photoluminescence spectra at the excitation wavelength 350 nm reveals the characteristic nature of ZnO. The FTIR analysis shows the absorption peaks which relate the characteristic bonds of ZnO. The SEM micrographs of Sn doped ZnO thinfilms demonstrate that the surface morphologies of the films are influenced by the introduction of dopant. Enhancement of uniformity and hence reduction in surface roughness of the film can be observed from the SEM micrographs. The present studies indicate that Tin doped Zinc Oxide thinfilms with enhanced properties could be achieved using the following optimized coating parameters viz., spin rate at 4000 rpm, number of coating layers at 10, heat treatment temperature at 350 [degrees]C and dopant concentration of Sn with 0.008 mol %.

ACKNOWLEDGMENTS

Authors acknowledge the support rendered by the College management, principal and HOD of the department of Physics of Thiagarajar College of Engineering, Madurai.

REFERENCES

[1.] Ataev, B.M., et al., 1999. Thermally stable, highly conductive, and transparent ZnO layers prepared in situ by chemical vapor deposition.Materials Science and Engineering B: Solid-State Materials for Advanced Technology, 65: 159-163.

[2.] Karl, W. Boer, 2013. Handbook of the Physics of Thin-Film Solar Cells, Springer publications, XL882: 451.

[3.] Chien-Yie Tsay, Hua-Chi Cheng, Yen-Ting Tung, Wei-Hsing Tuan & Chung-Kwei Lin, 2008. Effect of Sn-doped on microstructural and optical properties of ZnO thinfilms deposited by sol-gel method.Thin Solid Films, 517: 1032-1036.

[4.] Rajarshi Krishna nath & Siddhartha Sankar nath, 2009. Sn-doped Zinc Oxide thinfilms for Methanol. Sensors & Transducers Journal, 108(9): 68-179.

[5.] Jin-Hong Lee, Kyung-Hee Ko & Byung-Ok Park, 2003. Electrical and optical properties of ZnO transparent conducting films by the sol-gel method. Journal of Crystal Growth, 247: 119-125.

[6.] Benny Joseph et al., 2005. A study on the chemical spray deposition of zinc oxide thin films and their structural and of ZnO films by atmospheric pressure chemical-vapor deposition using zinc acetylacetonate and ozone.Thin Solid Films, 343.

[7.] Kuan Jen Chen, Fei Yi Hung, Yen Ting Chen, Shoou Jinn Chang Zhan Shuo Hu, 2010. Surface characteristics. Optical and electrical properties on Sol-Gel Synthesized Sn -Doped ZnO Thinfilm, The Japan Institute of Metals Materials Transactions, 51(7): 1340-1345.

[8.] Farid Jamali Sheini, Dilip S. Joag & A. Mahendra, 2010. Electrochemical synthesis ofSn doped ZnO nanowires on zinc foil and their field emission studies.Thin Solid Films, 519: 184-189.

(1) M. Senthamizh selvi, (2) N. Sankarasubramanian, (3) R. Vasuki, (4) S. Rajathi

(1,2,3,4) Department of Physics, Thiagarajar College of Engineering, Madurai, 625015, India.

Received 28 February 2017; Accepted 22 May 2017; Available online 6 June 2017

Address For Correspondence: M. Senthamizh Selvi, Department of Physics, Thiagarajar College of Engineering, Madurai, Tamilnadu, India. E-mail: mssphy@tce.edu.

Caption: Fig. 1&2: XRD Pattern of SZO Thinfilms Spin Coated with Different Number of Coating Layer and Spin Rate

Caption: Fig. 5 & 6: Transmission & Band gap energy of SZO thinfilms spin coated with number of coating layers

Caption: Fig. 7&8: Transmission spectra & Band gap energy of SZO thinfilms spin coated with spin rates

Caption: Fig. 9&10: Transmission Spectra and Band Gap Energy SZO Thinfilms Deposited at Different Heat Treatment Temperature

Caption: Fig. 11&12: Transmittance Spectra and Band Gap Energy of Sn Doped ZnO Thinfilms Spin Coated with Different Sn Concentration

Caption: Fig. 13: Photoluminescence spectra of 0.008 mol % Sn doped ZnO thinfilm

Caption: Fig. 14: FTIR Spectra of 0.008 mol % Sn Doped ZnO thinfilm

Caption: Fig. 15: Variation of sheet resistance of SZO films with (a) Number of coating layers, (b) Spin rate, (c) Heat treatment temperature and (d) Dopant concentration

Caption: Fig. 16(a-c) SEM micrograph of undoped ZnO thinfilm magnified with (a) 5kx(b)40 kx and (c) 80 kx;

Caption: Fig. 16: (d)-(f) SEM micrograph of TZO thinfilms magnified with (d) 3 kx, (e) 10 kx and (f) 24 kx keV

Captiion: Fig. 17: EDAX spectra of 0.008 mol % Sn doped ZnO thinfilm
Table. 1 Different coating parameters used in the preparation
of SZO thinfilms

Variable Coating   Varied as   Fixed coating        Remark
parameters                     parameters

Number of          8           Tin chloride         Zinc acetate
                                 concentration        Concentration
                                 -0.01 mol%           is fixed at
coating layers     10          Spin Rate-3500 rpm     0.36 mol %
                   12          Heat treatment
                                 temperature
                                 -350[degrees]C
Spin rate          3000        Tin chloride
                                 concentration
                                 -0.008 mol %
(rpm)              3500        Heat treatment
                                 temperature
                                 -350[degrees]C
                   4000        No of coating
                                 layers-10
Heat treatment     300         Tin chloride
                                 concentration
                                 -0.008 mol %]
temperature        350         Spin Rate-3500 rpm
C [degrees]c)      400         No of coating
                                 layers-10
Tin chloride       0.006       Spin Rate-3500 rpm
concentration      0.008       Heat treatment
                                 temperature
                                 -350[degrees]C
{mol%}             0.01        No of coating
                                 layers-10

Table 2 Structural Properties of Sn Doped Thinfilms Coated at
Different Solute Concentrations

Sn            FWHM      d-             (hkl)     [theta]   D (nm)
concentrati   [beta]    spacin g
on mol. %               ([Angstrom])

0.005         1         2.81           (100)     16        8.62
              0.4       2.60           (002)     17.2      21.70
              1         2.47           (101)     IS        S.72
0.008         1         2.81           (100)     16        S.62
              0.5       2.60           (002)     17.2      17.36
              0.4       2.47           (101)     IS        21.81
0.01          1         2.81           (100)     15        S.62
              0.39      2.60           (002)     17        22.25
              1         2.48           (101)     IS        S.72

Sn            a              c              TC      [delta]x
concentrati   ([Angstrom])   ([Angstrom])           [lO.sup.15]
on mol. %                                           lin/[m.sup.2]

0.005         3.250          5.206          0.907   13.43
                                            1.430   2.12
                                            0.661   13.14
0.008         3.247          5.204          0.95S   13.43
                                            1.215   3.31
                                            0.826   2.10
0.01          3.249          5.219          0.135   13.44
                                            2.722   2.01
                                            0.141   13.13

Sn            [epsilon]x     [sigma]
concentrati   [10.sup.-4]
on mol. %     [lin.sup.-2]
              [m.sup.-4]

0.005         41.94          S.50
              440.57
              952.36
0.008         41.94          8.68
              723.8
              1064.4
0.01          41.96          43.5
              67.33
              1292.5
COPYRIGHT 2017 American-Eurasian Network for Scientific Information
No portion of this article can be reproduced without the express written permission from the copyright holder.
Copyright 2017 Gale, Cengage Learning. All rights reserved.

Article Details
Printer friendly Cite/link Email Feedback
Author:Selvi, M. Senthamizh; Sankarasubramanian, N.; Vasuki, R.; Rajathi, S.
Publication:Advances in Natural and Applied Sciences
Geographic Code:1USA
Date:Jun 1, 2017
Words:3732
Previous Article:Preparation and characterization of zinc doped CdS thin films for photovoltaic applications.
Next Article:Experimental investigation on influence of process parameters in electric discharge machining of P20 mould steel.
Topics:

Terms of use | Privacy policy | Copyright © 2021 Farlex, Inc. | Feedback | For webmasters