Densification and resistance of Ta 2 O 5 and ZnO co-doped SnO 2 ceramic targets for low-cost TCO films of solar cells

doi:10.21203/rs.3.rs-4868636/v1

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Densification and resistance of Ta 2 O 5 and ZnO co-doped SnO 2 ceramic targets for low-cost TCO films of solar cells

https://doi.org/10.21203/rs.3.rs-4868636/v1

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Low cost transparent and conducting oxide (TCO) films are important for solar cells, and sintering densification of SnO₂-based ceramic targets for depositing TCO film are difficult. Ta₂O₅ and ZnO were used as dopants and harmless sintering aid, respectively. The targets with different ZnO contents show single rutile structure. The sintering densification was improved by ZnO, and the dense microstructure has fine grains with an average size of 0.2-3 µm. The decomposition and volatilization of targets sintered at higher temperature decrease density. The 0.85 wt% ZnO doped target at the optimal sintering temperature of 1500°C obtain the highest relative density (> 99%) and low resistance (< 50 Ω). The enhanced density of SnO₂-based targets by ZnO can effectively improve the density, and further inhibit the formation of nodules on the target surface during magnetron sputtering.

SnO2

Ta2O5

ZnO

Densification

Resistance

Transparent conductive oxide (TCO) films with high transmittance in visible light region and low resistivity are widely used in flat panel displays, solar cells, and touchscreens [1–2]. Currently, the widely used TCO films are tin-doped indium oxide (ITO) films. However, ITO films used high-cost indium and have biotoxicity. Therefore, low-cost films based ZnO, SnO₂ were proposed. SnO₂-based TCO films, such as SnO₂:F (FTO), SnO₂:Sb (ATO), have received more attention [3–6]. High-mobility Ta-doped SnO₂ (TTO) films were grown on glass by pulsed laser deposition using a TiO₂ seed-layer, which shows high mobility of 83 cm²/V·s, low resistivity of 2.8×10⁴Ω·cm [7]. However, the investigation of the TTO ceramic targets for magnetron sputtering should be further strengthened.

Nano oxide powder is key issue for sintering dense ceramic, and the synthesis methods of SnO₂ fine powders include hydrothermal, vapor phase, electrolysis and sol-gel. The precursor prepared by SnCl₄ and ammonia was heated hydrothermally at 160°C for 12 h to obtain SnO₂ nano powders [8]. Two-step vapor phase oxidation process oxidizes Sn to SnO in low-oxygen environment, and further oxidizes SnO to SnO₂ in oxygen-enriched environment [9]. SnO₂ nanoparticles with the highest volume at 83.11 nm in a range of 25–150 nm was electrolytically synthesized at the potential of 60 V and 0.06 M HCl [10]. Nanocrystalline precursors were prepared by sol-gel, and then calcined at different temperatures to obtain SnO₂ nano powders with different particle sizes [11]. These reports provide a basis for the industrial synthesis of SnO₂ nano powders.

Pure insulating SnO₂ ceramics are difficult to sintering densification, which is independent of sintering temperature, so the dopants are key issue for densification and conductivity [12]. The multiple sintering processes of ITO targets were widely investigated [13], which is also suitable for TTO targets. The ATO powders sintered by the spark plasma sintering (SPS) at 1000°C obtained high density (99.2% relative density) ATO targets [14]. Nearly fully dense FTO targets with 98.5% relative density were prepared by the SPS at temperatures ranging from 850°C to 900°C [15]. However, the ATO targets sintered at 1250°C and air atmosphere shown low density (95%) [16]. The ZnO-SnO₂ binary ceramic targets sintered at 1600°C have high density (99.78%), fine grains and homogeneous structure [17]. The SnO₂ ceramics co-doped with Sb and Zn sintered at 1300°C exhibited enhanced electrical conductivity and high densities [18]. The Zn²⁺, Ba³⁺, Cu²⁺, Mn²⁺, Si⁴⁺ were used to improve the conductivity of SnO₂-based ceramics [19–23]. These findings are promising for the further improvement of SnO₂-based ceramics. However, the study of the dense SnO₂-based targets doped with other elements and its sintering densification is insufficient.

In this work, the sintering behavior of 3 wt% Ta₂O₅ doped SnO₂ target were improved by adding ZnO with the contents of 0.7 wt%, 0.85 wt%, and 1.0 wt%. The phase structure, microstructure, density and resistance of the targets were investigated.

High purity SnO₂ (99.99%), Ta₂O₅ (99.95%) and ZnO (99.95%) powders were used as raw materials. The 3 wt% Ta₂O₅ doped SnO₂ (TTO) targets with ZnO contents of 0.7 wt%, 0.85 wt%, and 1.0 wt% were designed. The powders were dispersed with anhydrous ethanol with a solids content of 50% in a ball milling tank. The slurry was mixed and milled at conditions of 410 rpm and 24 h, and then dried at 80°C for 24 h. The powders were first sieved through a 150-mesh sieve, and then added an aqueous solution with 3 wt% polyvinyl alcohol (PVA) and 2 wt% polyethylene glycol (PEG) as binder. The mixture of powders and binder was granulated by 80-mesh sieve. The granulated powders were pressed into discs with a diameter of 13 mm and a thickness of 2 mm. The discs with a relative density of about 55% were dewaxed at 600°C for 3 h, and then sintered in a muffle furnace at 1350°C, 1400°C, 1450°C, 1500°C and 1550°C for 4 h.

X-ray diffraction (XRD, D8 Advance, Bruker) was used to study the phase structure of targets. Scanning electron microscopy (FE-SEM, Tecnai-450, FEI) was used to observe microstructure and the elemental distribution. The average grain size of targets was statistically analyzed using Nano measure software. The density of targets was tested using the Archimedes method, and the relative density used in this work is 6.988 g/cm³. The resistance of targets was measured by a multimeter (VC9205, Beicheng).

The morphologies of SnO₂ and Ta₂O₅ and ZnO powders in Fig. 1 shows that the SnO₂ powders shows nanoscale size, and the Ta₂O₅ and ZnO powders have submicron size. The 0.85wt% ZnO doped mixed powders are well dispersed, which is conducive to homogeneous mixing. Figure 2 shows images of the TTO targets without ZnO sintered at different temperatures. It can be seen that the TTO targets illustrate almost no shrinkage in the temperature range of 1350–1500°C. Therefore, Ta₂O₅ has no sintering aid effect for SnO₂ ceramic, which is similar with pure SnO₂ ceramics [24].

Figure 3 shows the XRD pattern of the 0.7 wt% ZnO doped TTO targets sintered at 1350–1550°C. It can be seen that the TTO targets show three strong diffraction peaks of the (110), (101) and (211), and the peaks originating from the impurity phases is invisible. Therefore, the rutile structure indicates that the Zn and Ta enters into the lattice of SnO₂ matrix [25–26]. As shown in Fig. S1 and Fig. S2, when the ZnO doping content is increased to 0.85 wt% and 1.0 wt%, the TTO targets also show the rutile structure without impurity phase.

Figure 4 and Fig. 5 shows the surface and cross-sectional morphologies of the 0.7 wt% ZnO doped TTO targets sintered at different temperatures, respectively. It can be seen from Fig. 4 that the TTO targets have dense microstructure, tightly packed grains, clear grain boundaries. As shown in Fig. 5, the size and number of pores in targets gradually decrease with increasing sintering temperature, indicating the increasing density. In addition, the fracture along grain boundaries can be observed. The size of the grains shown in targets also gradually increases with increasing sintering temperature [27].

Figure 6 shows the grain size distribution of the 0.7 wt% ZnO doped TTO targets sintered at different temperatures. It can be seen that the TTO targets have fine and uniform grains, and the grain size increases from 0.63 µm to 1.45 µm with increasing sintering temperature from at 1350°C to 1550°C.

The surface (Fig. S3, Fig. S6) and cross-sectional (Fig. S4, Fig. S7) morphologies of the 0.85 wt% and 1.0 wt% ZnO doped TTO targets sintered at different temperatures confirm that the TTO targets also have dense structure, tightly packed grains, clear grain boundaries and intergranular fracture. With increase sintering temperature, the grain size gradually increases. Moreover, the pores are almost invisible in the 1.0 wt% ZnO doped TTO targets sintered at higher temperature. It is well known that pure SnO₂ ceramics are difficult to sinter densification, so ZnO can promote sintering densification [28]. Fig. S5 and Fig. S8 show the distribution and average size of grains. It can be seen that the TTO targets illustrate uniform grain distribution. With increasing sintering temperature, the average grain size of the 0.85 wt% ZnO doped TTO targets increases from 0.63 µm to 1.45 µm. The 1.0 wt% ZnO doped TTO targets show a slight increase in size from 1.08 µm to 1.28 µm.

Figure S9 shows the cross-sectional morphologies of three types of TTO target sintered at 1600°C. It can be seen that be a number of pores exist in targets, indicating that the sintering temperature of 1600 ℃ exceeds the optimal temperature. The 1600 ℃ results in the decomposition and volatilization of SnO₂ and ZnO [29]. Fig. S10 shows the elemental distribution of the 0.85 wt% ZnO doped TTO target. It can be seen that the Sn, Ta and Zn elements illustrate uniform distribution in target.

Figure 10 shows the density of the ZnO doped TTO targets sintered at different temperatures. It can be seen that the density of three types of TTO targets increases with increasing ZnO content and sintering temperature. Further increasing sintering temperature results in decrease in density because of decomposition and volatilization. The 0.85 wt% ZnO doped TTO targets sintered at 1500°C obtains the optimal density of 99.9%, so ZnO can promote the sintering densification of SnO₂-based ceramic targets [30].

Figure 11 shows resistance of the ZnO doped TTO targets sintered at different temperatures. It can be seen that the resistance gradually decreases with increasing sintering temperature, and the optimal resistance can be obtained at 1500 ℃. However, the effect of the ZnO doping content on resistance is not significant. At low sintering temperature of 1350 ℃, the TTO targets show high resistance because of low density. However, the decomposition and volatilization at high sintering temperature also results in low density[31]. The optimum sintering temperature in this work is 1500 ℃. Therefore, the optimal resistance of the TTO targets obtained at 0.85 wt% ZnO doping content and 1500 ℃ is 36 Ω.

In this work, the SnO₂-based ceramic targets for low-cost TCO films of solar cells was modified by 3 wt% Ta₂O₅ and ZnO with 0.7 wt%, 0.85 wt% and 0.85 wt%. All the TTO targets show single rutile structure without impurity phases. The sintering densification has been significantly improved by ZnO. The dense microstructure with fine grains illustrates an average grain size in range of 0.2-3 µm. Higher sintering of above 1500°C is not conducive to increasing density because of the decomposition and volatilization of SnO₂ and ZnO. The resistance gradually decreases with increasing sintering temperature, and is insensitive to ZnO doping content. Therefore, The TTO targets with the highest relative density (> 99%) and low resistance (< 50 Ω) were achieved at the optimal sintering temperature of 1500°C and 0.85 wt% ZnO content. The TTO ceramic targets with an enhanced density can effectively inhibit an abnormal discharging during sputtering, and meet the conductivity demand of direct current magnetron sputtering.

Author Contribution

Jane was responsible for writing the manuscript, while Mr. Hsu, Mr. Zhu, and Mr. Shang Xu were responsible for checking and improving it.

Acknowledgements

This work was financially supported by the Joint Fund of NSFC-Guangxi (U21A2065), Guangxi Natural Science Foundation (2021GXNSFAA220020), the National Natural Science Foundation of China (62364007), Science and Technology Major Project of Guangxi (AA21077018).

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Densification and resistance of Ta 2 O 5 and ZnO co-doped SnO 2 ceramic targets for low-cost TCO films of solar cells

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1. Introduction

2. Experimental procedures

3. Results and discussion

4.Summary

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Author Contribution

Acknowledgements

References

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