Comparative studies of structural and electrical ...

Comparative studies of structural and electrical properties of co-doped ZnO thin films prepared by direct current sputtering as a front contact for copper indium gallium di-selenide solar cell Chandan Ashis Gupta, S. Mangal, and U. P. Singh Citation: J. Renewable Sustainable Energy 5, 031609 (2013); doi: 10.1063/1.4807617 View online: http://dx.doi.org/10.1063/1.4807617 View Table of Contents: http://jrse.aip.org/resource/1/JRSEBH/v5/i3 Published by the American Institute of Physics.

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JOURNAL OF RENEWABLE AND SUSTAINABLE ENERGY 5, 031609 (2013)

Comparative studies of structural and electrical properties of co-doped ZnO thin films prepared by direct current sputtering as a front contact for copper indium gallium di-selenide solar cell Chandan Ashis Gupta,1 S. Mangal,2 and U. P. Singh1,a) 1 2

School of Electronics Engineering, KIIT University, Bhubaneswar, Odisha 751024, India School of Applied Sciences, KIIT University, Bhubaneswar, Odisha 751024, India

(Received 29 December 2012; accepted 25 April 2013; published online 5 June 2013)

Doped and co-doped ZnO thin films are currently under intense investigation and development for optoelectronic and solar cell applications. Here in this study Aluminum and Boron (ZAB), Gallium and Boron (ZGB), and Gallium and Aluminum (ZGA) co-doped ZnO thin films were deposited on glass substrate using DC magnetron sputtering at room temperature. A comparative study of the above co-doped ZnO thin films was done on the basis of its structural and electrical properties for solar cell application. All thin films have shown excellent optical properties with more than 80% transmission in the visible range of the light. From the X-ray diffraction patterns, it is found that the films were polycrystalline in nature and the ZAB thin film is more crystalline than the other co-doped ZnO thin films. The surface morphology showed different growth structure of the films. For ZAB and ZGA thin films, the rounded grains were observed and for ZGB thin film some rounded as well as corn type grains were observed. The electrical properties of all the thin films were measured using Hall measurement system at room temperature. For ZGB and ZGA thin films, the resistivity was obtained in the order of 103 ohm cm and for ZAB thin film the lowest resistivity of the order of 104 ohm cm was obtained which is ideal for transparent conducting oxide thin films to be used as window cum-front contact in multi-junction solar cell such as CIGS C 2013 AIP Publishing LLC. [http://dx.doi.org/10.1063/1.4807617] solar cell. V

I. INTRODUCTION

Zinc Oxide (ZnO) is an attractive material for large variety of applications. ZnO thin films are used in particular for transparent contacts in photovoltaic solar cells. It has emerged as one of the most promising materials, due to its optical, structural and electrical properties associated with the high chemical and mechanical stability. It is a wide band gap oxide semiconductor with a direct energy gap of about 3.37 eV. The doping of ZnO, transparent conductive oxide (TCO) films with Al, Ga and In is being considered for manufacturing transparent electrodes due to their high luminous transmittance, good electrical conductivity, good adhesion to the substrate and the fact that they are chemically inert.1–6 Study of these properties is important as it helps in optimizing film parameters for better device applications. Doped and co-doped ZnO thin films are currently under intense investigation and development for optoelectronic and energy conversion applications.7 TCO films can be used as transparent electrodes in solar cells and flat-panel displays, as well as for infrared-reflecting and low-emissivity windows.8 For solar cell application, the properties of TCO films should include a large band gap (nearly 3.2 eV), low resistivity (103 to 104 ohm cm), and a very good optical transmittance (80% to 90%) in

a)

Author to whom correspondence should be addressed. Electronic mail: [email protected]. Tel.: þ91 674 2378251. Fax: þ91 674 2725481.

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C 2013 AIP Publishing LLC V

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J. Renewable Sustainable Energy 5, 031609 (2013)

the visible range. The conductivity of the ZnO films can be changed by several orders of magnitude by doping with Al, Ga, etc. by creating oxygen vacancies. In the co-doping process, Al and Ga atoms become attached to the ZnO, by substitution and function as donors thus contributing to n-type conductivity. In this study we are focusing on the comparative studies of optical, electrical and structural properties of Aluminum and Boron (ZAB), Gallium and Boron (ZGB), and Gallium and Aluminum (ZGA) co-doped ZnO thin films for solar cell application. II. EXPERIMENTAL DETAILS

Co-doped ZnO thin films were prepared using DC sputtering on glass substrate. The glass substrates were first cleaned in boiling soap solution followed by ultrasonic cleaning in propanol and deionized (DI) water and finally dried in blowing N2. ZAB, ZGA, and ZGB thin films were prepared on glass substrate by using DC magnetron sputtering at room temperature. Different targets, such as ZnO:Al2O3 (2 wt. %), ZnO:B2O3 (2 wt. %) and ZnO:Ga2O3 (2 wt. %) having diameter 2 in., thickness 3 mm and purity 99.99% were used to deposit the ZnO film. Before that chamber pressure was maintained 7 106 mbar where the deposition was carried out under a working pressure of 8 103 mbar. During deposition of ZnO film, the substrate was rotated at a speed of 20 rpm and 100 W sputtering power was maintained throughout the ZnO layer growth. The crystallographic properties of films were analyzed by X-ray Diffraction (XRD) using the Cu Ka (with k ¼ 1.5405 A) radiation (XRD-Bruker D8 advance, D-Ray at 40 kV and 40 mA, increment is 0.5 , scan speed is 1 s/step, M-ray incidence angle is 2 , and scan range was 20 –80 ). The surface morphology of the films was measured by scanning electron microscopy (SEM-Carl Zeiss Gmbh, Cemini column with 0.1 keV to 30 keV with a resolution of 1.1 nm). The electrical properties (resistivity, Hall mobility, conductivity, and the carrier concentration) were obtained from Hall-effect measurement (Ecopia HMS 3000) at room temperature. The optical transmission spectra were obtained using UV-visible spectrophotometer (Shimadzu 2450). The thickness of the films was measured by surface profilometer system (Ambios Technology XP-200). The thickness of all the films which was measured by surface ˚. profile meter is found to be in the range of 2500–3000 A III. RESULT AND DISCUSSION

Fig. 1 shows the optical transmission spectra of ZAB, ZGB, and ZGA samples prepared at room temperature using DC sputtering technique in the wavelength range of 300–800 nm. From Fig. 1, it was found that the transmittance of all the samples shows more than 80% in the

FIG. 1. Transmittance and band gap plots of ZAB, ZGB, and ZGA films.

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TABLE I. Particle size and band gap values of ZAB, ZGB, and ZGA films. FWHM ( )

Crystallite size (nm)

Band gap (eV)

ZAB

0.6

13.48

3.15

ZGB ZGA

0.7 0.68

11.53 11.89

2.96 3.1

Sample name

visible region of light which clearly indicates that all the films are transparent in nature. In case of ZAB thin film, the highest transmittance (above 90%) was observed and in case of ZGA film lowest transmittance value was obtained. The presence of Al layer on the top of the ZGA film surface is responsible for slight deterioration of transmittance.9 The variation of transmittance in all the films is thought to be caused by the increase of free carrier absorption.10 The relation between the absorption coefficient a and the bandgap of a semiconductor with the incident photon energy h is given by ðahÞn ¼Bðh Eg Þ; where B is a constant, Eg is the band gap energy, and n ¼ 2 for direct band gap transition. The direct band gap value of the films was determined by a plot of (ah)2 as a function of photon energy h (Ref. 11) which is shown in Fig. 1. The band gap values are given in Table I. The band gap is lowest in case of ZGB sample which is due to low particle size compared to other co-doped thin films. In general, the changes in band gap energy of the respective thin films have been related to variations in the mean crystalline size, the internal stress, the free carrier concentration, and the defects caused by a decreased level of crystallinity and due to co-doped ZnO thin films.12–17 In Fig. 2, XRD patterns of the co-doped ZnO thin films prepared by DC sputtering were plotted as a function of diffraction angle (2h). The peaks (2h) for ZAB, ZGB, and ZGA are observed in between 33.24 and 34.2 . All the samples grown at room temperature show a (002) preferred orientation which indicates the hexagonal ZnO wurtzite phase. Apart from this strong peak some other weak peaks are also observed at (100), (101), (102), (110), (202), (112), (004), (201) and (103), which indicate that the films are polycrystalline in nature. These weak peaks can be indexed to wurtzite hexagonal structure of ZnO.18–22 The intensity of these

FIG. 2. XRD plot of ZAB, ZGB, and ZGA films.

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weak peaks decreases evidently showing the crystal reorientation effect.23 The XRD data are useful for calculation of particle size of films. The particle size of (002) phase of the respective co-doped ZnO thin films has been calculated using Scherrer’s formula which can be expressed as D ¼ 0:9 k=b cos h; where D is the crystallite size, k is the wavelength of the X-ray radiation used, b is the full width at half maximum (FWHM) of the diffraction peak, and h is the Bragg diffraction angle of the XRD peak.24,25 The FWHM and the particle size of the respective films are given in Table I. FWHM values can be used to evaluate crystallinity, i.e., the smaller the FWHM value, the greater the crystallinity. From Table I, it was found that the FWHM value obtained for ZAB film is lower than for the other doped ZnO films, indicating that the particle size in case of ZAB film is maximum compared to other films. Figures 3–5 show the SEM micrographs (magnification 100 KX) of ZAB, ZGB, and ZGA thin films. A noticeable surface morphological variation on the film surface was found in these SEM micrographs with large grain size on film surface. It indicates that during deposition of films by sputtering onto substrates the growth has taken place by nucleation and coalescence process. Randomly distributed h002i oriented nuclei may have first formed and these nuclei then have grown to form observable islands. As islands increase their size by further deposition and come closer to each other, the larger ones appeared to grow by coalescence of smaller one.26–28 The films show different morphology of surface grains, which are dependent on the deposition parameters and addition of dopants.26 In case of ZGA samples, the grain size is rounded. There are both small and large rounded shaped grains present on the surface of the film. In case of ZAB, the film exhibits petals like surface grains along with the presence of some rounded grains in the film. However, in case of ZGB corn shaped grains are present along with tiny rounded grains. From SEM data, we observed that the particle size is maximum in case of ZAB sample which is also supported from XRD result. The electrical properties of the co-doped ZnO thin films are determined from the Hall effect measurement. The Hall measurement data have been shown in Table II. The carrier concentration value in case of ZAB sample is slightly more than that of ZGA and ZGB samples. This is due to the increase of lattice distortion in case of ZAB thin film.7 Besides this a small

FIG. 3. SEM image of ZAB film.

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FIG. 4. SEM image of ZGB film.

variation of resistivity is noticed. From the literature review, it was found that the variation of the resistivity depends on the film deposition methods and the deposition process parameters.29,30 The resistivity of ZAB thin film is less than that of other two co-doped samples. From Table II, a decreasing trend in resistivity was noted which indicates the decrease in the grain boundary related barriers and the surface states.31 The electrical conductivity of the respective co-doped thin films is contributed by Al or Ga or B ions which are located on substitutional sites of Zn ions and Al or Ga or B interstitial atoms as well as from oxygen vacancies and interstitial Zn atoms which play a crucial role on the surface state as well as conductivity or resistivity of this film.32 From Table II, we observe a little bit variation of mobility in the

FIG. 5. SEM image of ZGA film.

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TABLE II. Hall effect measurement values of ZAB, ZGB, and ZGA films. Carrier concentration (cm3)

Mobility (cm2/V s)

Resistivity (ohm cm)

ZAB

9.1 1020

12.33

5.56 104

ZGB ZGA

20

7.38 4.13

1.02 103 2.16 103

Sample name

8.29 10 6.99 1020

respective co-doped ZnO thin films. This is due to the variation in the layer thickness and the mean crystallite size of the films.7,29 IV. CONCLUSION

In this study, the co-doped ZnO thin films (ZAB, ZGB, and ZGA) were deposited on glass substrates by DC magnetron sputtering. The main focus is to use these co-doped ZnO thin films as front contact in CIGS solar cell. More than 80% transmissions for all the samples were observed and for ZAB thin film more than 90% transmission was obtained which is ideal for any TCO to be used as front contact in CIGS solar cell. From XRD study, it was observed that the crystallinity of ZAB sample is improved compared to ZGB and ZGA samples. Also from SEM result, the better crystallinity of the all the films was observed. The electrical properties such as mobility, carrier concentration and resistivity were measured using Hall measurement system. We found that the carrier concentration and mobility of ZAB thin film are more than those of the other two co-doped ZnO thin films. Also for ZAB thin film lowest resistivity in the order of 104 ohm cm was observed which was ideal for any doped or co-doped ZnO thin film to be used as front contact in CIGS solar cell. Here, a comparative study of different co-doped ZnO thin films was done on the basis of its optical, structural and electrical properties and all the results showed that ZAB thin film prepared by DC sputtering has better optical, structural and electrical properties than those of the other two co-doped ZnO thin films for solar cell application. ACKNOWLEDGMENTS

This work was supported by research funds of Ministry of New and Renewable Energy (MNRE) and Defence Research and Development Organization (DRDO), New Delhi, India. 1

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