English

• 1. INTRODUCTION

  Photovoltaic power generation is one of the most promising sources of renewable clean energies, in which the SSCs account for more than ~90% of the photovoltaic market share. Developing high-effi-ciency SSCs is the future direction, and one of the key issues to improve the solar cell efficiency is optimizing Ag/Si contact interface during metalli-zation process^{[1, 2]}. Screen printing of silver paste and high-temperature sintering have been applied to achieve the metallization in the large-scale pro-duction of SSCs. Thus, the sintering mechanism^[3] and Ag/Si ohmic contact mechanism are research hotspots in the research field of SSCs. Some factors affecting electrochemical properties of the Ag-Si ohmic contact are listed as follows: sintering process^{[3, 4]}, interface morphology of Ag/Si contact interface^[5-15], glass composition of siver paste^[16-24] and cell structure ^[25](Fig. 1). Chung et al.^[3] have investigated the influence of O₂ on Ag ionization in molten lead borosilicate glass during sintering process, in which the Ag powder can dissolve into the molten glass as Ag⁺ ions via the interaction of silver with oxygen in ambient atmosphere, the increased P_O2 can increase the Ag⁺concentration in the molten glass, and plays a crucial role on the fire-through Ag/Si contact formation. Dieter K et al. have reviewed the ohmic contact of SSCs^[4], in which some basic principles for Ag-Si contanct are demonstrated, including the tunnelling mechanism, Fermi level pinning, field emission, thermal/field emission, as well as current transport of surface states. The Ag-Si contact formation process has also been reported, as well as the optimization of formulation and sintering process to achieve high-performance solar cell silver paste^{[5, 6]}. Cheng et al. have investigated the two-dimension distribution of Ag crystallites for SSCs sintering at different temperature, in which their microstructural observations give new insights into the tunneling mechanism of current flow of Ag/Si interface from under-fired to over-fired conditions^[8].

  Figure 1

  

  Figure 1.  Factors affecting Ag/Si contact interface: sintering process^[18], copy right of Elesver; interface morphology^[7], copy right of Elesver; glass composition^[16], copy right of Elsever; and cell structure^[25], copyright of IEEE

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  The interface morphology of Ag/Si contact inter-face has also been intensely reported. Usually, the nano-structure of Ag/Si interface contains a mixture of contact features such as ultra-thin interface glass, nano-scale Ag precipitating in the ultra-thin interface glass, and sub-micron Ag crystallites at the Si surface^{[9, 10]}. It is reported that a uniform ultra-thin interfacial glass containing nano-Ag colloids can be formed in the Ag/Si contact interface under opitimally-fired conditions with enhanced solar cell performance due to the reduced contact resistivity^[8]. Under over-fired condition, some Ag crystallites attach into the Si emitter surface and significantly disturb the poten-tial distribution, resulting in lowered solar cell performance. Thus, the size of large Ag crystallites at Si emitter surface shall be kept to a minimum^[12].

  The interface morphology of Ag/Si contact is vital for improving the photoelectric conversion efficiency of the SSCs. By tuning the glass composition, the Ag/Si contact interface can be optimized. For a typical silver paste, it usually consists of three cons-tituents: silver particles as the conductive phase, glass frit as inorganic binder, and organic compounds as dispersant. Despite the minor content (< 8 wt%) in silver paste, the glass frit plays a key role on forming fine contact between the silver and silicon wafer^{[10, 11]}. Some basic functions of glass frit include etching the reflective layer Si₃N₄, forming a Ag-Si ohmic contact with the silicon substrate, and so on. During sintering process, the glass melts can wet and dissolve the Ag powder at the sintering temperature by generating a liquid glassy-phase. Besides, this glassy-phase can flow downward to the surface of SSCs to etch through the SiNx anti-reflection layer (70~80 nm)^[4]. Various kinds of glass frits have been developed, such as TeO₂-based^[17], Pb-Te-O based^[18-22], abd Lead-free glass frits^[23] and so on. For example, a new type of lead-free silver paste using a TeO₂-based glass as the front electrode of SSCs has been reported by Cai et al., which shows a resistance of about 3.1~3.7 μΩ cm after sintering^[17]. Pi et al. have studied the effects of glass powder on the etching process of the Si₃N₄ antireflection film and silver resistivity, and reveal that tellurium glass can make the combination of silver and silicon better^[21].

  Finding and developing new etching materials for the metallization of SSCs, realizing low-temperature sintering, lead-free^[23] and studying the related func-tion performance and mechanism are an important topic for solar cell metallization. As a rarely reported compound, the research on properties of Ag₂TeO₃ as etching material in SSCs is rather lacking. The synthetic method of Ag₂TeO₃ has been reported by Sharma et al.^[25]. In this study, the Ag₂TeO₃-based glass frit is applied to further optimize the Ag/Si contact interface of SSCs. Besides, the corresponding optimization mechanism is also demonstrated.

  2. OPTIMIZATION MECHANISM OF Ag₂TeO₃-BASED GLASS FRIT

  Ag₂TeO₃ was directly synthesized by AgNO₃ (purity 99.9%) with TeO₂ (purity 99.99%, particle size D50 = 100 nm). The mixture is evenly mixed and annealed in a muffle furnace at 750 ℃ for 100 min. Then the obtained Ag₂TeO₃ is ball-milled to powder with an average particle size D₅₀ = 10 μm. X-ray diffraction (XRD, Bruker, D8) is applied to characterize the phase structure of the synthesized Ag₂TeO₃ (Fig. 2a), which is consistent with the PDF card of #83-1779, indicating the successful synthesis of Ag₂TeO₃. The Ag₂TeO₃ phase belongs to a P2₁/a(14) space group with monoclinic phase structure. Compared with the previous report^[24], the synthesis of Ag₂TeO₃ in this study is much simple, and the direct solid-reaction of AgNO₃ and TeO₂ can also avoid the contamination of other metal ions and improve the experimental quality.

  Figure 2

  

  Figure 2.  Optimizing Ag/Si contact interface with Ag₂TeO₃-based glass frit. (a) X-ray diffraction pattern of Ag₂TeO₃, and corresponding crystal structure (inset, grey sphere represents the Ag atom, yellow sphere the Te atoms, and red spheres the O atom); (b) DSC/TGA curves of Ag₂TeO₃ reacting with Si₃N₄ in oxygen-containing atmosphere; (c) Thermos-gravimetric curves of Ag₂TeO₃ and PbO reacting with Si₃N₄ in O₂ atmosphere, respectively; (d) TEM morphologies of the cross-section Ag/Si contact interface, and (e) Corresponding EDS-mapping images

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  The TGA and DSC curves of the mixture of Ag₂TeO₃ with Si₃N₄ (Fig. 2b) indicates that the redox reaction between Ag₂TeO₃ with Si₃N₄ starts at ~545 ℃, which is much lower than the melting temperature of Ag₂TeO₃ with Si₃N₄, indicating a solid reaction. Besides, from the DSC curves, we also reveal that the redox exothermic reaction mainly occurs around ~600 ℃, which is beneficial to increase the temperature of the system and accelerate the reaction. During the redox reaction process, N₂ is released, and the corresponding reactions are shown as equations (1) and (2):

  $ \begin{aligned} & 6 \mathrm{Ag}_2 \mathrm{TeO}_3+\mathrm{Si}_3 \mathrm{~N}_4=12 \mathrm{Ag}+ \\ & 3 \mathrm{SiO}_2+6 \mathrm{TeO}_2+2 \mathrm{~N}_2 \uparrow \end{aligned} $

  (1)

  We also observe that the TGA curve begins to increase at ~750 ℃, and it reaches the highest point at ~900 ℃, indicating that the reaction in this period is dominated by equation 2. Meanwhile, beyond ~800 ℃, the weight gains in TGA curve are obvious, while exothermic heat is weak, which may demonstrate that the endothermic reaction of the system absorbing oxygen and the exothermic reaction of the etching reaction may cancel out each other, so there is no obvious dynamic change of energy.

  $ \begin{aligned} & 2 \mathrm{Ag}_2 \mathrm{TeO}_3+\mathrm{Si}_3 \mathrm{~N}_4+2 \mathrm{O}_2=4 \mathrm{Ag}+ \\ & 3 \mathrm{SiO}_2+2 \mathrm{TeO}_2+2 \mathrm{~N}_2 \uparrow \end{aligned} $

  (2)

  Fig. 2c depicts the comparison of TGA curve of Ag₂TeO₃ and PbO with Si₃N₄, respectively, which shows a significant difference. For PbO reacting with Si₃N₄, the mass remains stable until ~700 ℃. Above this temperature, an obvious weight gain is observed, which is very different from that of Ag₂TeO₃ reacting with Si₃N₄. This result indicates that the O₂ in atmosphere mainly dominates the etching reaction for PbO. The corresponding reaction is as equations (3) and (4).

  $ 6 \mathrm{PbO}+\mathrm{Si}_3 \mathrm{~N}_4=6 \mathrm{~Pb}+3 \mathrm{SiO}_2+2 \mathrm{~N}_2 $

  (3)

  $ 2 \mathrm{~Pb}+\mathrm{O}_2=2 \mathrm{PbO} $

  (4)

  Besides, the etching reaction of Ag₂TeO₃ and Si₃N₄ starts at about ~540 ℃, which is ~160 ℃ lower than that of the traditional PbO. Thus, the advantage of Ag₂TeO₃ lies in the lower reaction temperature and more abundant oxygen source.

  TEM together with EDS is also applied to analyze Ag/Si contact interface after introducing Ag₂TeO₃-based glass frit. Fig. 2d indicates that the silver particle generates along the surface of silicon, with length of about 300~500 nm and thickness of 50 nm, which is the product of etching reaction of Ag₂TeO₃ and Si₃N₄ in the glass. The micro-region of the silver-silicon contact is also presented, in which the charges can transport or tunnelling from the silicon emitter surface to the silver grid to achieve the collection and transport of charges^[8]. Fig. 2e shows the EDS-mapping results of the silicon-silver contact micro-region. It can be seen that the Ag and O in the silicon-silver contact interface is distributed separately, so the interface etching product is pure silver particles.

  To confirm the optimizing performance of Ag₂TeO₃ based glass frit, the solar cells are assembled and tested. The maximum current density of the solar cell reaches 38.37 mA/cm², the open voltage is about 0.638 V, and the photoelectric conversion efficiency is up to 18.41%, with an average value larger than ~18%, proving that the silver grid of solar cells using Ag₂TeO₃ as etching material presents good charge collection ability.

  The distribution position and morphology of silver particles have a great influence on the solar cell performance. L. Liang et al. have studied the key features on the interface micro-regions of the solar cell. In addition to some residual SiN_x, the major components are a thin glass layer with nano-Ag colloids, and the size of Ag particles on the surface of the silicon emitter is about 10~100 nm^[10]. It can also be seen apparently that the silver particles distribute discretely on the interface with the average distances more than ~50 nm, which is disadvantage for the charge collection. C. Ballif et al. have studied the resistance of the direct contact between silver and silicon, and find that the resistivity in some regions was four orders of magnitude lower than the typical value^[5], indicating that the contact between silver and silicon could significantly reduce the contact resistance of the solar cell and collect the charge better. Besides, C. Ballif et al. have also revealed that the silver-silicon contact interface is composed of Ag crystals with size of about 200~500 nm, and with an average penetration of ~130 nm into silicon. They consider that the silver is grown from glass melts, and similarly, the silver particles also exhibit discrete distribution^[13], which is also harmful for the charge collection. In this study, the silver nanoparticles are shown as strips in the silicon-silver contact interface, indicating that a higher concentration of silver ions is provided in the process of growth due to the addition of Ag₂TeO₃ in the glass (Fig. 3), that's to say, Ag₂TeO₃ can produce more silver in the silicon-silver contact interface after etching Si₃N₄, thus enabling the continuous distribution of silver particles, which can directly contact with silicon to collect charge, and output through a silver grid and then improve the solar cell efficiency.

  Figure 3

  

  Figure 3.  Optimization mechanism of Ag₂TeO₃-based glass frit on the Ag/Si contact of silicon solar cells

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  From the above results, it can be seen that the addition of Ag₂TeO₃ to the glass of silver paste, on the one hand, can realize the low temperature sintering of silver grid, and on the other hand, the product of the reaction between Ag₂TeO₃ and Si₃N₄ is conductive metal silver, which presents much higher conductivity advantage than other materials, and the effect of improving the efficiency of silicon solar cells is particularly prominent.

  3. SUMMARY & PERSPECTIVE

  In summary, in this study, the Ag₂TeO₃-based glass frit is applied to further optimize the Ag/Si contact interface of SSCs, and thus effectively improve the solar cell effeciency. The starting tempe-rature of Ag₂TeO₃ etching Si₃N₄ is ~545 ℃, which is about ~160 ℃ lower than that of the traditional PbO etching agent. By applying the Ag₂TeO₃-based glass frit, the solar cells obtain a high current density of ~38.37 mA/cm² for Ag₂TeO₃-containing solar cells, an open voltage about ~0.638 V, as well as an average efficiency of more than 18%. This outstan-ding solar cell performance is mainly due to the optimized Ag/Si contact interface. The silver-silicon interface was observed by TEM/EDS, which indi-cates that silver nanoparticles are observed con-tinuously distributing on the silicon-silver contact interface, with a length of about 300~500 nm, and a thickness of ~50 nm. This kind of silver can collect and transmit charge better, and thus can reduce contact resistance and improve solar cell efficiency. This study opens up a new direction for the metalli-zation of silicon solar cells from the perspective of material development, and also promotes the development of low-temperature sintered silver paste with less lead or lead-free.

  
  
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• 

  Figure 1  Factors affecting Ag/Si contact interface: sintering process^[18], copy right of Elesver; interface morphology^[7], copy right of Elesver; glass composition^[16], copy right of Elsever; and cell structure^[25], copyright of IEEE

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  Figure 2  Optimizing Ag/Si contact interface with Ag₂TeO₃-based glass frit. (a) X-ray diffraction pattern of Ag₂TeO₃, and corresponding crystal structure (inset, grey sphere represents the Ag atom, yellow sphere the Te atoms, and red spheres the O atom); (b) DSC/TGA curves of Ag₂TeO₃ reacting with Si₃N₄ in oxygen-containing atmosphere; (c) Thermos-gravimetric curves of Ag₂TeO₃ and PbO reacting with Si₃N₄ in O₂ atmosphere, respectively; (d) TEM morphologies of the cross-section Ag/Si contact interface, and (e) Corresponding EDS-mapping images

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  Figure 3  Optimization mechanism of Ag₂TeO₃-based glass frit on the Ag/Si contact of silicon solar cells

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