Optimization of aluminum-doped zinc oxide films deposited at low temperature by radio-frequency sputtering on flexible substrates for solar cell applications

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Abstract

Aluminum-doped zinc oxide films were deposited at 100 °C on polyethylene terephthalate by radio-frequency magnetron sputtering. The sputtering parameters such as RF power and Argon working pressure were varied from 25 to 125 W and from 1.1 to 0.2 Pa, respectively. The structural properties of as-deposited films were analysed by X-ray diffraction, showing that all the deposited films were polycrystalline, with hexagonal structure and a strong preferred c-axis orientation (0 0 2). Full width at half maximum and grain sizes were around 0.27° and ranged from 24 to 32 nm, respectively. The strain state of the samples was also estimated from X-ray diffraction measurements, obtaining compressive stresses from 0.29 to 0.05 GPa. Resistivity as low as 1.1×10−3 Ω cm was achieved for the film deposited at 75 W and 0.2 Pa, sample that showed a low strain state of −0.06 GPa. High optical transmittance (∼80%) was exhibited when films were deposited at RF powers below 100 W. Band gap energies ranged from 3.36 to 3.39 eV and a refractive index of 1.80±0.05, constant in the visible region, was also obtained.

Introduction

Transparent conductive oxides (TCOs) have been extensively studied because of being one of the most important components for large area electronics devices such as solar cells, flat panel displays, optical sensors or touch screens [1], [2], [3], [4], [5]. For the choice of the proper TCO, it has to be taken into account that its optoelectronic properties have a big influence on device performance. In the field of photovoltaic applications, the main criteria that TCOs should fit are the following: first, to be highly transparent in the visible wavelength range where the solar cell is operating to minimize the photon absorption; second, to have high conductivity to reduce the resistive losses and finally, to have low carrier concentration to avoid absorption losses in the red and near-infrared (NIR) wavelength ranges [6]. On the other hand, an efficient light-trapping scheme is also essential to enhance the intrinsically low absorbance of the thin-film absorbers in the long wavelength range [7]. For this reason, surface morphology is also an important characteristic within this area of application, where a developed surface texture is required to provide light scattering and subsequent light trapping inside the solar cell structure [8], [9]. As a promising and suitable TCO material due to its potential properties, aluminum-doped zinc oxide (ZnO:Al) (AZO) has received great attention in the recent years [10], [11], [12], [13]. This is particularly interesting because of its low cost, a wide availability of its constituent raw materials and, besides, it is chemically stable in a hydrogen plasma, in comparison with other TCOs used such as indium–tin oxide (ITO) [14], [15]. Hence, the use of this material is a reliable and cost effective alternative for Si-based solar cells, thanks to the ease while etching with diluted acid, only to obtain a useful post-deposition texturing [16], [17].

There are several deposition techniques to grow AZO films such as chemical vapor deposition (CVD) [18], spray pyrolysis [19], [20], pulsed laser deposition [21], magnetron sputtering [2], [22] and sol–gel [23]. However, most of these techniques need moderate temperatures to achieve AZO showing low values of resistivity. On the other hand, low deposition temperatures are required to be compatible with the fabrication of the solar cells on several substrates, especially in thin-film cell technology on flexible substrates. Taking this into account, radio-frequency (RF) magnetron sputtering is considered as a favourable deposition technique. This technique allows the deposition at low temperatures, down to room temperature (RT), at high deposition rates and the films show good adhesion on substrates [24], [25]. Besides, the high energy of the particles enhances its mobility on the substrate surface during the deposition. This fact affects the crystallinity, the residual stress, the adhesion to the substrate and the microstructure of the film.

The deposition of TCOs on polymer substrates has many merits compared with the deposition on glass substrates, because considerably lower deposition temperatures are required. However, the use of plastic substrates permits obtaining devices that can be easily folded and are light and easy to carry. In particular, nowadays there is a great interest in replacing glass with polymer substrate in the thin-film solar cell technology. This fact would contribute to the cost reduction in the production process, being compatible with the use of roll-to-roll deposition systems [26]. Therefore, the flexibility of the polymeric substrate opens new application in the photovoltaic field that requires curved surfaces [27], [28].

In this work, the choice of the substrate, polyethylene terephthalate (PET), was based on the fact that this is a unique polyester that maintains its physical, mechanical, electrical and chemical properties for working temperatures up to 150 °C. Considering that for achieving AZO films with good electrical and optical properties high substrate temperatures are required, the deposition of this material at such a low temperature on this substrate becomes a big challenge [13]. Besides, PET is also transparent, with a transmission around 90% in the wavelength range 0.4–2.4 μm and with a refractive index of 1.65 [29], characteristics compatible with solar applications. Taking into account its cited properties and its low cost, recently there has been great interest in using this substrate as flexible substrate in the photovoltaic market [30].

The properties of AZO films are critically dependent on the deposition parameters, such as RF power, working pressure, target to substrate distance, substrate temperature and target specification (doping, density and purity). Several researchers have optimized these parameters in order to improve the performance of AZO films deposited on flexible substrates such as polyisocianate (PI), polypropylene adipate (PPA) [31], [32], [33], polyethylene naphthalene (PEN) [34], [35] and polycarbonate (PC) [36]. None of them shows at the same time similar characteristics as PET.

Taking this into account, in this work, the influence of sputtering parameters on the structural, electrical and optical properties of AZO films deposited on PET is investigated. Specifically, the tendency of the strain state of the thin films with the RF power and the sputtering working pressure is analysed. These studies have permitted establishing a relationship between the strain state and the film properties. The main purpose of this work is to demonstrate the feasibility of using these AZO films in photovoltaic devices either as front electrode or as back side reflector.

Section snippets

Experimental details

AZO thin films were deposited in a commercial MVSystem for RF magnetron sputtering on PET substrates of size 10×10 cm2 and with a standard thickness of 0.240 mm. This sputtering system has a vertically adjustable cathode. A commercial (plasma materials), 3-in-diameter ceramic target (ZnO:Al2O3, 98%:2% wt) with a purity of 99.99% and a density of 6.6 gr/cm3, was used for all the sputtering depositions. The control of temperature is performed by a type K reference thermocouple, which is mechanically

Results and discussion

AZO films presented in this work were physically stable and showed good adherence to the polymeric substrate. No break and peel-off on layers were observed after the deposition. The film thicknesses were estimated in the range 500–700 nm in all the analysed samples.

XRD measurements were performed to investigate the structural properties of the deposited films. Fig. 1a shows the θ/2θ XRD pattern of the samples deposited at 100 °C, 0.2 Pa and the RF powers of 25 and 100 W. As it can be observed, the

Conclusions

Al-doped ZnO thin films were successfully deposited on PET substrate at 100 °C using RF magnetron sputtering. The effect of sputtering parameters such as RF power and working pressure was investigated to achieve highly transparent and conductive thin films at very low substrate temperature. Analysing the structural properties, all the AZO thin films showed a strong c-axis orientation along the (0 0 2) plane. Other diffraction peaks such as the (1 0 1) and (1 0 0) reflections appeared in the XRD

Acknowledgements

Partial financial support was provided by the Spanish Ministry of Education under CLASICO (ENE2007-67742-004-01/ALT) and the Spanish National R&D Plan under PSE-MICROSIL. The author would like to thank Dr. Fernando Conde from C.A.I. Difracción Rayos X for his advice and guidance with the X-ray diffraction interpretation.

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