Microstructure and resistivity of ITO targets sintered by cold and microwave sintering for solar cell applications

Abstract

High mobility of ITO films for solar cells is enhanced by decreasing SnO₂ content in ITO gargets. However, the sintering densification of ITO targets becomes difficult. The density of ITO targets with low SnO₂ content is enhanced by TiO₂, SiO₂ and cold sintering. The green bodies of ITO are first compacted by cold sintering and then further fully densified by microwave sintering. Three types of ITO targets exhibit single cubic bixbyite structure, dense microstructure and transgranular fracture. ITO targets sintered at a microwave temperature of 1450 °C have a high relative density of over 99% and low resistivity of below 4.87 × 10⁻⁴ Ω·cm. The fine grains have a size of less than 2.0 μm. Therefore, the synergistic effect of additives and the sintering process can resolve the difficulty of sintering densification of ITO targets with low doping content.

Keywords

ITO cold sintering microwave sintering density resistivity

Introduction

Compared to traditional fossil fuels, the electrical energy generated by solar cells is more sustainable and cleaner. Silicon solar cells based on the passivated emitter and rear contact (PERC) technology have reached multigigawatt levels.¹ For enhancing conversion efficiency, silicon heterojunction (SHJ) technology is emerging as a highly promising alternative.² Transparent and conductive oxide (TCO) films are crucial transparent electrodes for the SHJ solar cells. TCO films with low resistivity (<10⁻³ Ω·cm) and high transmittance (>85%) in the visible light region are usually used for colour filters, thin-film transistors, solar cells and touch panels.^3–6 The TCO films based on In₂O₃:Sn (ITO), ZnO:Al (AZO), ZnO:Ga (GZO), SnO₂:Sb (ATO) and SnO₂:F (FTO) were widely investigated. Among them, the traditional 10 wt-% SnO₂ doped ITO films deposited by magnetron sputtering are widely used in photoelectric devices.⁷ However, the traditional ITO films with high carrier concentration have high absorption in the near-infrared region, which limits the performance enhancement of the SHJ solar cells.^8,9

Solar energy in the infrared band should be fully utilised to improve the conversion efficiency. The transmittance of TCO films in the infrared band is improved by reducing carrier concentration. However, low carrier concentrations result in an increase of the resistivity of TCO film, which decreases fill factor.¹⁰ The high mobility of TCO films can compensate for low carrier concentration, maintaining the excellent conductivity of TCO film.¹¹ Decreasing doping content in TCO films can obtain low carrier concentration and high mobility. Therefore, the mobility of ITO films can be enhanced by reducing SnO₂ content.¹² ITO films doped with 5 wt-% and 3 wt-% SnO₂ show high mobility and are used for solar cells.¹³ Therefore, further decreasing SnO₂ content, such as 1 wt-%, is very important for achieving high mobility ITO films.

ITO targets with high density and low resistivity are important for depositing ITO films by magnetron sputtering.¹⁴ As is known to all, ITO targets with low density (a relative density of 98%) are prone to abnormal discharge during sputtering, resulting in nodule formation on the surface of the target. Nodules further cause the formation of defects in films during large-scale production.¹⁵ It is difficult to manufacture high-density ITO targets, especially those with low doping contents.¹⁶

The ITO targets with 10 wt-% SnO₂ achieved ultra-high density (≥99.7%) by ITO nanopowders and oxygen atmosphere sintering,¹⁷ which replaces high-pressure oxygen atmosphere and hot isostatic pressing sintering technology.^18,19 However, the sintering densification of ITO targets with low SnO₂ content is more difficult than that of the 10 wt-% SnO₂ doped ITO targets because a large amount of SnO₂ dopants can act as sintering aids. Therefore, the traditional fabrication technology cannot resolve the sintering densification of the low SnO₂ content doped ITO targets.

For In₂O₃-based TCO films, there are many dopants used to improve mobility, such as TiO₂, ZrO₂, MoO₃, WO₃, CeO₂, Ta₂O₅ and Hf₂O₅.^20–23 Therefore, the TiO₂ is a good candidate for dopant and sintering aid. The TCO film without etching patterns can use the SiO₂ as sintering aids.²⁴ Recently, novel cold sintering (CS) was proposed, which promotes sintering densification of ceramics at low temperatures (≤400 °C).²⁵ If the density of ITO green bodies is enhanced by CS, the sintering temperature will be decreased, and the sintering densification will be promoted. In our previous work, the TiO₂ and SiO₂ have indeed enhanced the density of ITO targets,²⁶ but the density and grain size should be further optimised.

In this work, the low SnO₂ contents (3 wt-%, 2 wt-% and 1 wt-%) doped ITO targets with TiO₂ and SiO₂ additives were used to investigate sintering densification, and the green bodies of ITO were compacted tightly by cold sintering. The effects of microwave sintering on the phase structure, microstructure, density and resistivity were studied.

Experimental procedures

The high purity (99.99%) nano powders of In₂O₃ (BET = 8.2 m²/g), SnO₂ (BET = 7.8 m²/g), TiO₂ (BET = 55 m²/g) and SiO₂ (BET = 300 m²/g) were used as raw materials. ITO targets with 3 wt-%, 2 wt-% and 1 wt-% SnO₂ contents were defined as the 973 ITO, 982 ITO and 991 ITO, respectively. The 0.25 wt-% TiO₂ and 0.02 wt-% SiO₂ used as additives were added to ITO powders. The raw powders were weighted according to the quality ratio and milled in a tank. Anhydrous ethanol was used as the dispersant, and the weight ratio of zirconia balls to powders in the slurry was 5:1. The slurry was milled by a planetary ball mill at 360 rpm for 6 h. After drying the slurry, the mixed fine powders were obtained by grinding and sieving. ITO powders (2 g) were shaped by dry pressing method using metal mould with 100 MPa pressing pressure, and the pressed green bodies have a diameter of 13 mm and a thickness of about 2 mm. ITO powders and acetic acid solution (2 mol/L) were mixed to form the uniform slurry for cold sintering, and then the slurry (3.5 g) was filled into the metal mold of cold sintering. The green bodies of ITO were compacted by cold sintering at 400 °C and 200 MPa for 2 h. The dense green bodies with a diameter of 13 mm and a thickness of 2.5 mm were further sintered by microwave sintering in an oxygen atmosphere with a flow rate of 5 L/min. The sintering temperatures were 1400 °C, 1450 °C, 1500 °C and 1550 °C for 2 h.

X-ray diffraction (XRD, D8 Advance, Bruker) investigated phase structure. Scanning electron microscopy (FE-SEM, Tecnai-450, FEI) observed the microstructure and the elemental distribution. The average grain size of ITO targets was statistically analyzed by Nano measure software. The density of ITO targets was tested by the Archimedes method, and then the relative density was calculated by comparing the tested density with the theoretical density. The resistivity of ITO targets was measured by four-point probe method (MCP-T700, Mitsubishi).

Results and discussion

The In₂O₃ and SnO₂ raw powders and its mixed powders have good dispersity analyzed in our previous work.²⁶ Figure 1 shows the microstructure of ITO green bodies prepared by dry pressing and cold sintering. It can be seen that ITO green bodies prepared by dry pressing and cold sintering illustrate tightly packed particles. The relative density of the green bodies obtained by cold sintering can reach about 70%, which is higher than the 58% of the green bodies by dry pressing. However, a density increase of 12% is difficult to distinguish by SEM images. Therefore, the tightly packed fine particles contribute to the diffusion reaction between particles during the sintering densification procedure.

Figure 2 shows the phase structure of ITO green bodies prepared by dry pressing and cold sintering. By comparing the diffraction peaks of ITO green bodies with those of the In₂O₃ from the JCPD standards (bottom of Figure 2), it can be confirmed that there was no solid solution reaction among the In₂O₃, SnO₂, TiO₂, SiO₂ raw powders during cold sintering.²⁷ The diffraction peaks originating from the additives are not obvious because the content of the additives is relatively low. According to the report,²⁸ the acetic acid solution added to the powders is expelled from the targets during the cold sintering process.

The XRD patterns of the three types of ITO targets sintered by microwave sintering at different temperatures are shown in Figures 3 and 4. It can be seen from Figure 3 that the diffraction peaks of the 973 ITO targets can match the diffraction peaks of the In₂O₃ cubic bixbyite structure (PDF# 06-0416). However, the intensity of the diffraction peaks increases with increasing sintering temperature. Therefore, the crystallinity of the 973 ITO targets is enhanced, which can improve its electrical properties.²⁹ Moreover, the diffraction peaks from the impurity phases are invisible because the SnO₂ content in these ITO targets is lower than the solid solubility of 6–8 wt-% SnO₂.³⁰ The 982 and 991 ITO targets sintered at 1400–1550 °C in Figure 4 also show single cubic bixbyite structure without other impurity phases, which is similar to the results of the 973 ITO target because the 2 wt-% and 1 wt-% SnO₂ in the 982 and 991 ITO targets are less than 3 wt-% in the 973 ITO target.

Figure 5 shows the surface and cross-sectional morphologies of the 991 ITO targets sintered by microwave sintering at different temperatures. It can be seen from Figure 5(a) that the grains of the 991 ITO target are clear and tightly stacked together. The grain size increases gradually with increasing sintering temperature. The cross-sectional morphologies in Figure 5(b) exhibit a dense microstructure. The 991 ITO targets illustrate dense microstructure with a few pores at a low temperature of 1400 °C. At higher sintering temperatures, only a small number of tiny closed pores exist. The pores appear inside the grains at 1550 °C, which is caused by the rapid growth of the grains and the inability of the timely releasing pores at high temperatures. High density of the 991 ITO targets obtained at lower sintering temperatures is contributed to the dense green bodies enhanced by cold sintering and the rapid sintering of microwave energy. The higher the density of the green bodies, the easier the sintering densification of the targets. It is well known that the grain size of the target affects not only the mechanical properties of the target but also the uniformity of the film.³¹ The distribution of grain size in Figure 6 illustrates that the grain size is more centralised, indicating a uniform distribution of ITO grains. In addition, the average grain size of 0.98 μm at 1400 °C increases to 1.56 μm at 1550 °C. The grain size (0.98–1.56 μm) of the 991 ITO in this work is lower than the size (about 50 μm) of the 3 wt-% SnO₂-doped In₂O₃ targets sintered by normal pressure oxygen atmosphere in our previous work.³² In addition, the 991 ITO target sintered by normal pressure oxygen atmosphere as shown in Figure 5(c) illustrates large grain size. Therefore, the fine grains are attributed to the synergistic effect of the dense green body and the rapid microwave sintering.

Figures 7(a) and 9(a) show the surface morphologies of the 982 and 973 ITO targets sintered at different temperatures. These targets also illustrate clear and tightly stacked grains, and the grain size gradually increases with increasing sintering temperature, which is similar to the results of the 991 ITO targets. The cross-sectional morphologies in Figures 7(b) and 9(b) show the transgranular fracture, suggesting that the ITO targets have excellent mechanical properties.³² The dense microstructure with a small number of fine pores for the ITO target sintered at 1400 °C is observed because the large pores are difficult to close at low sintering temperature. Sintering in the temperature range of 1450–1550 °C results in a dense and pore-free microstructure.

Figures 8 and 10 show the grain size distribution of the 982 and 973 ITO targets. It can be seen that the centralised distribution of grain size is observed, indicating that the grains grow uniformly without abnormal growth. The average grain size of the 982 ITO target increases from 0.99 μm at 1400 °C to 1.58 μm at 1550 °C, and the 973 ITO targets also illustrate a slight increase in size from 1.10 μm at 1400 °C to 1.66 μm at 1550 °C. Therefore, the 982 and 973 ITO targets show fine grains like the 991 ITO targets.

The uniform distribution of elements in the ITO target has a significant effect on the microstructure, conductivity and nodule formation.³³ The elemental distribution in targets analyzed by the 973 ITO target is shown in Figure 11. It can be seen that the added Sn, Ti and Si elements are uniformly distributed in the In₂O₃ matrix. Combining the elemental distribution and crystal phases in Figures 3 and 4, it can therefore confirm that the dopants are dissolved in the lattice of the In₂O₃.

Figure 12 shows the relative density of the 973, 982 and 991 ITO targets sintered at different temperatures. It can be seen that the density of these ITO targets firstly increases and then slightly decreases with increasing temperature. Moreover, the more the content of SnO₂, the higher the density of the ITO targets. The most difficult of sintering densification is the 991 ITO target, but its density increases from 98.25% to 99.01% and then decreases to 98.06%. The best density for these ITO targets is obtained at 1450 °C, and the densities of the 991, 982 and 973 ITO targets are 99.01%, 99.22% and 99.34%, respectively. Even at 1400 °C, the density of the 991 ITO target can reach 98%, which is higher than that of the 10 wt-% SnO₂ doped ITO target prepared by dry pressing and oxygen sintering.³⁴ As is well known, higher sintering temperature and longer sintering time were used to enhance sintering density, resulting in an increase of grain size and abnormal grain growth.³⁵ However, an abnormal grain growth is invisible, and the density has slightly decreased. The rapid microwave sintering inhibits grain growth, and results in a decrease in density because the closed pores are difficult to eliminate during rapid microwave sintering at high temperatures.³⁶ Therefore, the low SnO₂ content doped ITO targets achieves high density and fine grains by cold and microwave sintering, which also contributes to decreasing resistivity.

Figure 13 shows the resistivity of the 973, 982 and 991 ITO targets sintered at different temperatures. It can be seen that the resistivity of these ITO targets firstly decreases and then increases with increasing sintering temperature, which is similar with the evolution of the relative density. The optimal resistivity of the 991, 982 and 973 ITO targets sintered at 1450 °C are 4.87 × 10⁻⁴ Ω·cm, 4.26 × 10⁻⁴ Ω·cm and 3.73 × 10⁻⁴ Ω·cm, respectively. Moreover, the higher the content of SnO₂, the lower the resistivity of the ITO targets. The carrier concentration depends on the SnO₂ doping content, so the heavily doped ITO targets have low resistivity. Therefore, the excellent conductivity of these ITO targets should contribute to depositing ITO films with low resistivity.

In order to highlight the advantages of combining cold sintering with microwave sintering (defined as B method), the samples prepared by dry pressing and microwave sintering (defined as A method) were used for comparative analysis. The 991 ITO target is the most difficult to achieve sintering densification, so it is used for comparative analysis. The comparison of the density, resistivity and grain size of the 991 ITO targets prepared by two methods is provided in Figure 14. It can be seen that the B method can achieve sintering densification at low sintering temperature, which is better than that of the A method. The densely packed particles in green bodies pressed by cold sintering are helpful for atomic migration and diffusion, resulting in high density.³⁷ Moreover, the B method tends to choose a lower sintering temperature. The 991 ITO targets prepared by the two methods have a small difference in resistivity, and the moderate sintering temperature for the B method helps to achieve low resistivity. However, the difference in grain size between the two methods is significant, and the B method can obtain fine grains within a wide sintering temperature range because the dense green bodies contribute to enhancing densification and inhabiting grain grow.³⁸

Conclusions

In summary, in order to obtain dense ITO targets with low SnO₂ content of 1 wt-%, 2 wt-% and 3 wt-%, the synergistic effect of the additives and cold sintering was proposed. The TiO₂ and SiO₂ act as sintering aids. Cold sintering obtains the dense green bodies with about 70% relative density. The tightly packed particles promote the sintering densification during microwave sintering procedure. The three types of ITO targets show the cubic bixbyite structure, dense microstructure and fine grains. The average grain sizes of all the ITO targets were less than 2.0 μm. The optimal sintering temperature has been significantly decreased. All the ITO targets sintered at 1450 °C can achieve a relative density of over 99% and a resistivity of less than 4.87 × 10⁻⁴ Ω·cm. Therefore, the difficult of sintering densification for low SnO₂ content doped ITO targets can be resolved by additives, cold and microwave sintering, promoting the development of ITO targets for solar cell applications.

Footnotes

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).

Declaration of conflicting interests

The authors declared no potential conflicts of interest with respect to the research, authorship, and/or publication of this article.

Funding

The authors disclosed receipt of the following financial support for the research, authorship, and/or publication of this article: This work was supported by the the National Natural Science Foundation of China, the Joint Fund of NSFC-Guangxi, Science and Technology Major Project of Guangxi, Natural Science Foundation of Guangxi Zhuang Autonomous Region (grant numbers 62364007, U21A2065, AA21077018, and 2021GXNSFAA220020).

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