Electrical and optical properties of ultrasonically sprayed Al-doped zinc oxide thin films

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Abstract

Aluminium-doped ZnO (AZO) films were deposited by ultrasonic spray pyrolysis (USP) technique to investigate its potential application as antireflection coating and top contact layer for copper indium gallium diselenide (CIGS) based photovoltaic cells. The solution used to prepare AZO thin films contained 0.2 M of zinc acetate and 0.2 M of aluminium pentanedionate solutions in the order of 2, 3 and 4 at.% of Al/Zn. AZO films were deposited onto glass substrates at different substrate temperatures starting from 450 °C to 500 °C. XRD and FESEM analysis revealed the structural properties of the films and almost all the films possessed crystalline structure with a preferred (0 0 2) orientation except for the 4 at.% of Al. Grain size of AZO films varied from 29.7 to 37 nm for different substrate temperatures and atomic percentage of aluminium. The average optical transmittance of all films with the variation of doping concentration and substrate temperature was 75–90% in the visible range of wavelength 600–700 nm. Optical direct band gap value of 2, 3 and 4 at.% Al-doped films sprayed at different temperatures varied from 3.32 to 3.46 eV. Hall studies were carried out to analyze resistivity, mobility and carrier concentration of the films. AZO films deposited at different substrate temperatures and at various Al/Zn ratios showed resistivity ranging from 0.12 to 1.0 × 10−2 Ω cm. Mobility value was ∼5 cm2/V s and carrier concentration value was ∼7.7 × 1019 cm−3. Minimum electrical resistivity was obtained for the 3 at.% Al-doped film sprayed at 475 °C and its value was 1.0 × 10−2 Ω cm with film thickness of 602 nm. The electrical conductivity of ZnO films was improved by aluminium doping.

Introduction

ZnO is a II–VI group semiconductor material with wide direct band gap and wurtzite structure. The high stability, melting point and excitation energy makes it a promising ultraviolet (UV) and blue optoelectronic material. In addition, ZnO thin films offer a variety of applications in solar cells, optoelectronic, and gas sensitive devices [1]. A transparent conducting electrode is a necessary component in all flat panel displays (FPDs). Commercially most important material for a transparent conducting film nowadays is Sn-doped In2O3 (ITO), owing to its unique characteristics of high visible transmittance (90%), low DC resistivity, high infra-red reflectance and absorbance in the microwave region. The high quality of ITO films deposited by sputtering of oxide targets has already been successfully achieved on a commercialized production scale [2]. On the other hand, ZnO films have attracted interest as a transparent conductive coating material, because of the materials characteristics such as (1) cheap and abundant element, (2) production of large-scale coatings, (3) allow tailing of ultraviolet absorption, (4) high stability in hydrogen plasma, (5) low growth temperature, (6) nontoxic and (7) easy to fabricate. Electrical resistivity of ZnO thin film is readily modified by the addition of impurity or post-deposition annealing [2], [3]. Aluminum-doped zinc oxide (AZO) thin films have been prepared by thermal evaporation [4], chemical vapour deposition (CVD) [5], sol–gel [6], [7], pulsed laser deposition (PLD) [8] magnetron sputtering [2], [9], and spray pyrolysis [3], [10], [11], [12], [13], [14], [15], [16], [17], etc.

Among these methods, spray pyrolysis is cheap and useful for large area applications. This method is simpler, fast, material efficient, easy to use, carried out in non-vacuum system and permits to obtain films with the required properties for optoelectronic applications. In the ultrasonic spray pyrolysis (USP) system, an alcoholic solution containing the precursor-salts is nebulized by an ultrasonic actuator and then transported to a heated substrate. The advantage of USP over conventional pneumatic spraying is low consumption of material, and better control of the spray flux with a soft carrier-gas flow, which allows the deposition of very thin layers with homogeneous thickness. Already reports are available on the In2S3 thin buffer layers deposited by USP over CIGS layers in solar cells, where the desired film thickness is in nano-scale [15], [16]. Since not many systematic reports are available on the USP deposition of AZO thin films, this study reports the effect of [Al]/[Zn] ratio in the starting solution and substrate temperature on the optical and electrical characteristics of these thin films which are explored for its application in photovoltaic devices as transparent conducting oxide contact.

Section snippets

Experimental details

Al-doped ZnO thin films were prepared by ultrasonic spray pyrolysis method. An ultrasonic spray pyrolytic apparatus (vertical configuration type) was used. A schematic of homemade experimental setup is shown in Fig. 1. 0.2 M starting solution was prepared from zinc acetate dehydrate (Zn(CH3COO)2·2H2O) (Merck) dissolved in methanol (Baker) and acetic acid (Merck). Aluminum pentanedionate (C15H21AlO6) (Alfa) previously diluted in deionized water and acetic acid (Merck) at 0.2 M was added to the Zn

Results and discussion

When aerosol droplets arrive close to the heated glass substrates, a pyrolytic process takes place and a highly adherent film of ZnO forms on the substrates. Possible reaction mechanism for the ZnO thin films deposited by spray pyrolysis has been reported by Paraguay et al. [18]. In the present work similar kind of reaction mechanism takes place except for the replacement of zinc atoms by aluminium atoms. Fig. 2 shows simulated pattern of the ZnO structure incorporated with aluminium. AZO

Conclusions

Al-doped ZnO (AZO) films were successfully prepared by a homemade ultrasonic spray pyrolysis (USP) technique. The effects of substrate temperature and aluminium concentration on the electrical and optical characteristics of AZO thin films were studied. XRD analysis shows that the sprayed AZO thin films are of polycrystalline texture with a hexagonal structure. FESEM analyses revealed polycrystalline morphology of the films. Optical transmittance at 600–700 nm ranged from 75 to 90%, depending on

Acknowledgements

The authors wish to thank for the technical assistance of Dr. Jaime Vega Perez, Miguel A. Avendaño, and M.A. Luna-Arias. B.J. Babu is thankful to CONACYT for the scholarship provided to pursue Doctoral program.

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