The Preparation and Properties of Al-Doped ZnO Thin Films as Transparent Electrodes for Solar Cell

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Physics Procedia 32 ( 2012 ) 789 – 794

18th International Vacuum Congress

The preparation and properties of Al-doped ZnO thin films as

transparent electrodes for solar cell

J.N.Ding

1

,

2

,

3

*, C.B.Tan

1

, N.Y.Yuan

1

,

2

,

3

*, X.W.Feng

4

, X.Y.Chang

4

, F.Ye

1

1_{Center for low-dimensional materials, micro-nano devices and system, Changzhou University, 1 Ge Hu Road, Changzhou 213164,China} 2_{School of Materials Science and Engineering, Changzhou University, 1 Ge Hu Road, Changzhou 213164, China}

3_{Key Laboratory of New Energy Source, Changzhou 213164, Jiangsu, China}

4_{Jiangsu Jinton andvanced optics and elec Display Tech.Co., Ltd, Changzhou 213164, Jiangsu, China}

*corresponding author: dingjn@cczu.edu.cn; nyyuan@cczu.edu.cn

Abstract

Transparent conductive oxides based on ZnO are promising materials for application in thin-film solar photovoltaic cells. Al-doped ZnO thin films with a large area of 1 m × 1.5 m were prepared by magnetic sputtering on glass substrate using a ceramic target (98 wt.% ZnO, 2 wt.% Al2O3) in different Ar+H2 ambient at different

substrate temperature. SiO2 layer with a thickness of 20 nm was deposited as a resistant layer. To investigate the

influence of H2-flow on the properties of AZO films, H2-flow rate was changed during the growth process with a

fixed Ar-flow rate. The effect of the substrate temperature and the H2-flow rate on the structure, electrical and

optical properties was studied. In order to enhance light scattering and absorption inside the cell, suitable surface texture is needed. The influence of wet chemical etching on surface roughness and haze of AZO were also investigated.

PACS: Type pacs here, separated by semicolons ;

Keywords: AZO, hydrogen and argon mixture gas, electrical and optical properties, wet chemical etching, surface roughness

Open access under CC BY-NC-ND license.

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1. Introduction

Transparent conducting oxides (TCOs) possess a wide range of applications in a variety of opto-electronic devices like flat panel displays [1] or thin film solar cells [2-3]. Most TCOs are based on SnO2, In2O3, ZnO and their

mixed compounds. In particular, the Al-doped ZnO (AZO) thin films have attracted the intense interest for its great potential applications in the optically transparent conducting layers as the electrode for thin-film solar cells due to their high electro-optical quality, high material availability and low material cost for large area applications [4-8]. Currently, many methods are being used to prepare AZO films, such as metal organic chemical vapor deposition, spray pyrolysis, sol-gel method, pulsed laser deposition, molecular beam epitaxy, reactive evaporation and magnetron sputtering [9-16]. Among the processes used to prepare AZO coatings, magnetron sputtering is considered to be a suitable technique due to its inherent characteristics such as low deposition temperature, high deposition rate, good adhesion between the film and the substrate, good surface uniformity, good controllability, scalability to large areas and simple equipment [17-18]. And sputtered AZO films are especially attractive since they promise lower cost than ITO films and higher conductivities and transparencies than SnO2 based TCO films. AZO is

used as standard front contact in CIS thin film solar cells and is investigated as an alternative TCO material for silicon thin film solar cells [19].

In this work, the AZO films were grown by intermediate magnetron sputtering system using the Al-doped ZnO ceramic target in an argon atmosphere. To investigate the influence of H2-flow on the properties of AZO films, H2

-flow rate was changed during the growth process with a fixed Ar--flow rate. The effect of the substrate temperature and the H2-flow rate on the crystalline structure, electrical, optical and surface roughness and haze properties were

studied.

2. Experimental details

Deposition of AZO thin films with a large area of 1.0 m × 1.5 m were performed by intermediate magnetron sputtering from an oxide ceramic target consisting of 98 wt% ZnO and 2 wt% Al2O3 in different Ar+H2 ambient at

different substrate temperature. The percentage of hydrogen (aH) was set as 0, 0.46, 0.84 and 1.17 during the growth

process. SiO2 layer with a thickness of 20 nm was deposited on glass as a resistant layer. Glass substrates were

placed in a vertical frame and moved in front of the target for AZO deposition at room temperature. The distance is 5 centimeters between the ceramic target and substrates. When the base pressure was better than 2.6×10-3 _{Pa, high}

purity Ar, Ar and H2 mixture gas were introduced through independent mass flow controllers after the vacuum

chamber was evacuated below 1.1× 10-1_{Pa. During the film deposition process the total pressure was kept at 7.0×}

10-1_{Pa and the sputtering power density employed was about 3.5 W/cm}2_{, which will not produce substantial}

substrate heating during preparation. The deposition rate was above 0.5 nm /s. And the substrate temperature ranging from 250 to 380°C was controlled using a feedback controlled heater and measured by a thermocouple gauge.

The thicknesses of the films were obtained using an Į-Step 500 surface-profile measurement system. The crystallographic structure was analyzed by x-ray diffraction (XRD). The square resistance of the different AZO coatings was determined with the four point probe system (Veeco FPP5000). The optical transmission spectra of films were recorded by the Uv-Vis spectrophotometer (Shimadzu UV-24SO) from 300 nm to 900 nm. For transmittance measurement, the beam was made to enter the film through the glass substrate and a blank glass slide was kept in the path of the reference beam for compensation. The surface morphologies were observed using atomic force microscope (AFM). Optical measurements of the surface roughness and haze were characterized by the light transmittance and fog degree tester (WGT-S). All measurements were performed at room temperature.

3. Results and discussion 3.1. Structure analysis

The AZO films prepared in this work were physically stable and had good adherence to the glass substrate.Fig. 1 shows the XRD patterns for AZO films deposited on SiO2/glass substrate. All of the films exhibit two diffraction

peaks at 2ș~34.4° and 2ș~72.54°, respectively. The two peaks are associated with the (002) and (103) plane of hexagonal phase ZnO, respectively. These experimental results are different from some previously reported data

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[20-22]. Neither metallic zinc or aluminum characteristic peaks nor aluminum oxide peak was observed from the XRD patterns, which implies that aluminum atoms replace zinc in the hexagonal lattice or aluminum segregate to the non-crystalline region in grain boundary [23]. As seen in Fig. 1(a), (b) and (d), with increased the substrate temperature, the intensity of the (002) peak drastically decreased and the intensity of the (103) peak rapidly increased, It is noteworthy that the intensity of (002) diffraction peak decreases when the substrate temperature was at 350°C. It might be tentatively presumed that the deposition induced lattice structure deformation. A transition of growth mode from (002) vertical growth to (103) lateral growth might have occurred due to the evolution of crystalline face energy during the process of deposition [24]. With increased the percentage of hydrogen, as shown in Fig. 1(c), (d), (e) and (f), the intensity of the (103) diffraction peak obviously declined and then increased, and it reached the minimum when aH was 0.46. The transition trend is just the opposite for the intensity of (002) peak.

Fig. 1 X-ray diffraction patterns as a function of growth temperatures and the various percentage of hydrogen for AZO films.

The AZO film surface morphology was measured by atomic force microscope (AFM) for the understanding of the film structural properties. Fig. 2 shows the surface morphology images of the as-deposited AZO films at different substrate temperatures and the various percentage of hydrogen. The surface roughness (Ra) of AZO film decreased with substrate temperatures increasing from 250°C to 380°C. At the same time, it can be seen from the above graphs, the film surface morphology has significant change with the percentage of hydrogen increased, but the effect was not linear.

Fig. 2 AFM images of the AZO films at different substrate temperatures and the various percentage of hydrogen: (a) 250°C, aH =0.46; (b) 350°C, aH =0.46; (c) 380°C, aH =0; (d) 380°C, aH =0.46; (e) 380°C, aH =0.84; and (f) 380°C, aH =1.17.

3.2. Electrical properties

As seen in Table 1, the film resistivity drastically decreased when the substrate temperature varied from 250 to 380°C and reduced slightly with the percentage of hydrogen increased. The film resistivity is, to some extent, related to the free carrier concentration and surface morphology of the film, rather than by the full width at half maximum or the integrated intensity ratio of the diffraction peaks [24]. According to the result of XRD, it is known that strong (002) preferential orientation did not ensure low resistivity at 250°C.

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3.3. Optical properties

The optical transmittance spectra of AZO films represent a weak dependence on the substrate temperature and the percentage of hydrogen as confirmed by Fig. 3 and Fig. 4. They are seen that the average transmittance of these samples was over 85% in the visible light region indicating a high optical transparency, regardless of the substrate temperature and the percentage of hydrogen.

Fig. 3 Optical transmittance of the AZO films at the same percentage of hydrogen: aH =0.46 and different substrate temperatures: (a) 250°C; (b) 350°C; and (c) 380°C.

Fig. 4 Optical transmittance of the AZO films at 380°C and the various percentage of hydrogen: (a) aH =0; (b) aH =0.46; (c) aH =0.84; and (d) aH =1.17.

The insert picture shows the red shifts of the optical absorption edge as the substrate temperature increased from 250 to 380°C as confirmed by Fig. 3 and the blue shifts of the optical absorption edge as the percentage of hydrogen increased as confirmed by Fig. 4. The observed results can be explained by the Burstein-Moss effect [25]. According to the Burstein-Moss theory, in heavily doped AZO films, donor electrons occupy states at the bottom of the conduction band. Since the Pauli principle prevents the state from being doubly occupied, the valence electrons require extra energy to be excited to higher energy states in the conduction band, which results in broadening of the optical band gap [24]. As the substrate temperature and the H2-flow increases, the average transmittance of the films

reduces slightly. However, the dependence of substrate temperature and the percentage of hydrogen on transmittance is not as sensitive as the resistivity's.

300 400 500 600 700 800 900 0 20 40 60 80 100 350 355 360 365 370 10 12 14 16 18 20 Tr an sm itta nc e( % ) Wavelength (nm) (a)(b)(c) (a) (b) (c) Tran sm it tan ce (%) Wavelength (nm) 400 500 600 700 800 900 0 20 40 60 80 100 350 355 360 365 370 10 12 14 16 18 20 Tr an sm itta nc e( % ) Wavelength (nm) (d)(c)(b)(a) (a) (b) (c) (d) T ransm it tance (%) Wavelength (nm)

The percentage of the H2 (%) 0 0.46 0.84 1.17

Temperature˄ć˅ Sheet resistance˄ȍ/Ƒ˅

250 42 26 25 24

350 24 24 22 20

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3.4 The etching effect on surface roughness and haze

The suitable surface roughness and haze can enhance light scattering and absorption inside the cell. The velvet structure can reduce directional reflection, increase internal reflection effects, promote the effective absorption of solar energy, and improve power efficiency. The as-deposited AZO films were etched in 1% hydrochloric acid solution. The time of corrosion was 10, 15 and 20s, respectively.

Fig. 5 shows the film haze obviously increases at 380°C when the percentage of hydrogen varies from 0 to 1.17. But it increases slightly when the aH varies from 0.64 to 1.17. It is noteworthy that the haze of the AZO film

deposited with the aH of 0.84 is much larger after 20 s etching in HCl. To investigate the change of film haze, the

AFM photographs of the etched AZO thin films deposited at 380°C with various percentages of hydrogen are shown in Fig. 6. Compared with the film deposited at Ar ambient, the surface roughness of the films increases obviously when deposited at Ar+H ambient. And the films deposited at Ar+H ambient exhibit uniformly textured surface with fine craters, while the surfaces of the film deposited at Ar ambient is regularly and smoothly. It could be seen that a large surface roughness will produce a larger haze, but the haze does not increase with the roughness linearly.

Fig. 5 The haze of the AZO films at 380°C and the various percentage of hydrogen by wet chemical etching: (a) aH =0; (b) aH =0.46; (c) aH =0.84; and (d) aH =1.17.

Fig. 6 AFM images of the AZO films at 380°C, 20s of the etching, and the various percentage of hydrogen: (a) aH =0; (b) aH =0.46; (c) aH =0.84; and (d) aH =1.17.

4. Conclusions

The effect of the substrate temperature and the percentage of hydrogen on the crystalline structure, electrical, optical and surface roughness of the as-deposited AZO films formed by a intermediate magnetron sputtering technique has been systematically investigated by X-ray diffractometry, atomic force microscope, four-point probe, spectrophotometry. The haze properties of the etched films were measured by light transmittance and fog degree tester. Several conclusions can be drawn as follows: (1) with the increase of the substrate temperature, the intensity of the (002) peak drastically decreases and the intensity of the (103) peak rapidly increases. (2) With the increased of percentage of hydrogen, the intensity of (103) peak obviously declines and then increases, and it reaches the minimum when aH is 0.46. But it is the opposite for the (002) peak. (3) The film resistivity decreases when the

substrate temperature and percentage of hydrogen increases. (4) The AZO films possess good transparency and the

10 15 20 5 10 15 20 (a) (b) (c) (d) 380 o C Haze t(sec)

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blue and red shifts of their optical absorption edges can be explained in terms of Burstein–Moss effect. (5) With the percentage of hydrogen increase, the surface roughness and haze of the etched AZO film increase.

Acknowledgments

This work was supported by New Century Excellent Talents (NCET-04-0515), Qing Lan Project (2008-04), Key Programs for Science and Technology Development of Jiangsu (BE2009028) and

the Priority Academic

Program Development of Jiangsu Higher Education Institutions

.

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