Investigation of Ag-bulk/glassy-phase/Si heterostructures of printed Ag contacts on crystalline Si solar cells

doi:10.1016/j.solmat.2008.02.032

Solar Energy Materials and Solar Cells

Volume 92, Issue 9, September 2008, Pages 1011-1015

https://doi.org/10.1016/j.solmat.2008.02.032 Get rights and content

Abstract

The interface structure of screen-printed silver contacts on a crystalline silicon solar cell has been studied by transmission electron microscopy (TEM). TEM results confirmed that the glassy-phase plays an important role in contact properties. There are at least three different microstructures present in optimal fired contacts. The location where silver-bulk directly contacts silicon is observed through SEM, and this is actually a very thin glass layer in between. In addition, high-density silver embryos on silicon were found for samples fired optimally. The results presented in this study suggest that Ag-bulk/thin-glass-layer/Si contact is the most decisive path for current transportation.

Introduction

The screen-printed Ag thick-film is the most widely used front-side contact in industrial crystalline silicon solar cells. The front contacts have the roles of efficiently making contacting with the Si and transporting the photogenerated current without adversely affecting the cell properties and without damaging the p–n junction. Although it is rapid, has low cost and is simplicity, high-quality screen-printed Ag contact is not easy to make due to the complicated composition in the Ag paste. Commercially available Ag pastes generally consist of Ag powders, lead-glass frit powders and an organic vehicle system. The organic constituents of the Ag paste are burned out at temperatures below 500 °C. Ag particles, which are ∼60–80 wt% and can be different in shape and size distribution, show good conductivity and minor corrosive characteristics. The concentration of glass frit is usually less than 5 wt%; however, the glass frit in the Ag paste plays a critical role for achieving good-quality contacts to high-doping emitters [1], [2], [3], [4]. The optimization of the glass frit constitution can help achieve adequate photovoltaic properties.

The melting characteristics of the glass frit and also of the dissolved Ag have significant influence on contact resistance and fill factors (FFs) [5]. Glass frit advances sintering of the Ag particles, wets and merges the antireflection coating. Moreover, glass frit forms a glass layer between Si and Ag-bulk, and can further react with Si-bulk and forms pin-holes on the Si surface upon high-temperature firing. The contact microstructures should have a high impact on current transport across the contact interface. Therefore, more work is needed to understand the structure and electrical behaviors of the Ag-bulk/Si contact structures.

The objective of this work was to improve the understanding of front-side contact formation by analyzing the individual contact types and their role in the Ag-bulk/Si contact. Transmission electron microscopy (TEM) was used to study the microstructures and features at the contact interface. TEM of contact cross-section revealed high-density Ag-embryos on Si-bulk in samples fired optimally. The density of Ag-embryo, counted via TEM, is more than 2×10¹⁶ cm⁻² in some locations. This study also confirms that the spacing between Ag-embryos and Ag-bulk can be less than 5 nm. It is suggested that Ag-bulk/thin-glass-layer/Si contact is the most decisive path for current transportation. Possible conductance mechanisms of electrons across the contact interface are also discussed.

Section snippets

Experiment

This study is based on industrial single-crystalline silicon solar cells with an SiN_x antireflection coating, screen-printed Ag thick-film front contacts and a screen-printed Al-BSF. The contact pattern was screen printed using a commercial PV145 Ag paste from DuPont on top of the SiN_{_x} antireflective coating (ARC) and fired rapidly in a belt furnace. The exact Ag paste compositions are not disclosed by the paste manufacturers. The glass frit contents are estimated from the results found in this

Results and discussion

The microstructural properties of the screen-printed Ag-bulk/Si contacts were examined by TEM at 200 kV. TEM results confirmed that the glassy-phase plays an important role in contact properties. The typical Ag-bulk/Si microstructure, which includes localized large glassy-phase region, is shown in Fig. 1(a). The area where Ag-bulk directly contact with Si through SEM observation is actually with a very thin glass layer (<5 nm) in between as shown in Fig. 1(b). This possibly can be attributed to

Conclusion

The structural properties of the screen-printed Ag-bulk/Si contacts were investigated in this study. It was found that in optimally fired contacts there are at least three different microstructures. TEM results confirmed that the glassy-phase plays an important role in contact properties. The location where Ag-bulk directly contacts Si, through SEM observation, is actually a very thin glass layer (<5 nm) in between. In addition, high-density Ag-embryos on Si-bulk were found for samples fired

Acknowledgments

The authors would like to thank the support of Bureau of Energy (MOEA) under the Project no. 95-D0132. We are grateful to Cher-Zer Yang, Chia-Pin Chuang, Wen-Chuan Hou and Chin-Rong Chang for preparing solar cell samples used for this work. We also thank Chih-Jen Lin for his TEM operation.

References (12)

G. Schubert et al.
Solar Energy Mater. Solar Cells
(2006)
V. Kumar et al.
Solid State Electron.
(1977)
C. Ballif et al.
Appl. Phys. Lett.
(2003)
M.M. Hilali et al.
J. Electrochem. Soc.
(2006)
L. Weber
Metall. Mater. Trans. A
(2002)
M. M Hilali et al.
J. Electrochem. Soc.
(2005)

There are more references available in the full text version of this article.

Cited by (87)

Innovative recycling of high purity silver from silicon solar cells by acid leaching and ultrasonication
2024, Solar Energy Materials and Solar Cells
Precious and scarce silver (Ag) is used as a front electrical contact in silicon solar panels. With massive amounts of solar panel waste coming to end-of-life, it is imperative to recover all the Ag from these modules. In this paper, we propose a novel method to easily reclaim Ag from end-of-life silicon solar cells using low concentration sulfuric acid (H₂SO₄) leaching followed by ultrasonication. Our process simplifies the Ag recycling procedure by directly recovering the Ag contacts from solar cells, eliminating the need for secondary precipitation/electrodeposition. First, scanning electron microscopy (SEM) with energy dispersive X-ray spectroscopy (EDX) and X-ray photoelectron spectroscopy (XPS) was used to study the leaching and ultrasonication process on rejected solar wafers. Next, a solar panel from landfill was heated in a furnace to burn off the polymer encapsulant. The silicon wafers were then collected from the burning process and leached in 2 M H₂SO₄ at two different shaking speeds for 48 h at room temperature. Finally, the end-of-life silicon wafer pieces were collected, sonicated in water, and then the sonication water was centrifuged to show a proof-of-concept to recover the Ag contacts. Inductively coupled plasma optical emission spectroscopy (ICPOES) is used to monitor dissolved elements in the leachate as a function of time and shaking speed. XPS is used to evaluate the composition of the silicon cell wafer surface before and after H₂SO₄ leaching. SEM is used to image the recovered Ag contacts morphology and EDX confirms the recovered particles contain high amounts of Ag. A proposed schematic illustrates the authors’ hypothesis for the peeling mechanism.
Effect of MoO<inf>3</inf>/WO<inf>3</inf> modulation on high-temperature wettability and crystallization behavior of glass and its application in solar cell
2024, Ceramics International
The role of glass in the metallization contact of crystalline silicon solar cells is crucial. Numerous studies have been conducted to investigate the reaction between the main components in glass and SiN_x antireflection layer during sintering. Additionally, the low content component in glass plays an indispensable role in etching and wetting silicon base. The present study investigated the impact of ionic radius on glass properties through manipulation of the elemental composition of transition metals Mo and W. The results indicated a significant decrease in the characteristic temperature point of glass and an increase in wettability after the addition of MoO₃ and WO₃. Furthermore, the thermal expansion of glass was observed to increase with higher WO₃ content. Screen printing for the preparation of silver electrodes revealed that glass-silver pastes had optimal printing properties. When the quality ratio of MoO₃ and WO₃ was 7:3, it effectively enhanced the precipitation of Bi₂WO₆ and Pb_7.90Mo_0.10O_12.15 crystals in the glass, leaded to a reduction in the band gap of the glass. This resulted in an increased imprint of silver crystals within the glass layer, with more nano-silver crystals were deposited near the pyramid. Consequently, an optimal metallization contact was established between the silver grid and emitter. The average aspect ratio of silver grid improved from 0.415 for AG-1 to 0.469 for AG-3, which enhanced the photoelectric conversion efficiency of crystalline silicon solar cells.
Firing behavior of lead-containing and lead-free metallization silver paste for monocrystalline silicon solar cells
2023, Solar Energy Materials and Solar Cells
In this work, PbO-TeO₂-Bi₂O₃-SiO₂ lead-containing glass and TeO₂-Bi₂O₃-ZnO-Li₂O lead-free glass were synthesized in order to investigate the difference between them and find out the reason why lead-free paste can't be used on monocrystalline wafer commercially. Their flowability and reactivity with SiN_x were characterized. Lead-free glass presents lower flow temperature, better wettability and stronger etching ability to SiN_x than lead-containing glass. Button test was carried out and large white particles was identified on the interface for lead-free glass. These results indicates the excessive flowability and etching to SiN_x for lead-free glass. The morphology of grid line and interface between silver electrode and silicon wafer were investigated on dependence of temperature. The lead-containing and lead-free glasses went through similar evolution of silver sintering and interface microstructure with lead-free glass better facilitating silver sintering through liquid phase sintering due to its lower flow temperature. The thicker glass layer and more Ag crystallites with large size can be observed from cross-sectional micrographs for lead-free glass. Therefore, lead-free paste exhibits lower V_oc and FF than lead-containing glass owing to its ecxessive corrosion of SiN_x, thicker glass layer and large Ag crystallites.
Differently shaped Ag crystallites and four current transport paths at sintered Ag/Si interface of crystalline silicon solar cells
2023, Solar Energy Materials and Solar Cells
The comprehensive understanding of the Ohmic contact mechanism on the front metalization is highly desirable for further improvement of crystalline silicon solar cells performance. However, there are still controversial views about current transport paths between silver electrode and silicon emitter. To clarify this significant issue, we applied the selective acid etching (AE) process and the method of mechanical stripping process to prepare proper samples for direct observation of the Ag/Si contact interface. It is revealed that four kinds of Ag crystallites are grown with variant shapes and sizes on the pyramid tips, {111} planes, edges and tiny bumps at valleys of the textured Si surface resulting from anisotropic etching of Si emitter by the flowing molten glass frit during firing and the consequent epitaxially growth of Ag on the etching pits during cooling. All these Ag crystallites are in direct contact with Si emitter, serving as the most important contact points on the Si side. Meanwhile, three kinds of spherical Ag particles are formed in the resolidified glass frit during cooling which are crucial tunneling paths, conducting between glass-covered Ag crystallites on silicon and Ag bulk above. As a whole, four current transport paths are suggested mainly via Ag crystallites, Ag particles or both. In addition, the four electron transport paths are further confirmed by the Kelvin probe force microscopy characterization. The clarification and detailed understanding of all current transport paths are key points to design better silver paste and sintering process, and hence to improve the solar cell performance.
Improvement of etching ability of lead-free glass frits by compositing low content PbO nanoparticles and its effect on metallization contact of solar cells
2023, Materials Chemistry and Physics
Numerous research studies have been conducted on lead-free glass frits for crystalline silicon solar cells due to the high toxicity of Pb. However, lead-free glass frits have poor metallization performance. The excellent performance of PbO in glass frits cannot be completely replaced by other compounds. In this study, in order to reduce the PbO content in glass frits while keeping its excellent properties, a method for the preparation of low-content lead oxide composited glass frits was established. High temperature microscopy revealed that the temperature coefficients of the glass frit decreased as the lead content increased. FT-IR and Raman spectroscopy results indicated that the main structure of the glass remain intact under compositing PbO. The specific surface area of the glass frit increased after compositing PbO. The etching ability of lead-free glass frit to silicon wafer was improved. After the glass frit was prepared into silver paste, it found that the silver electrode formed excellent metallized contact with silicon and more silver microcrystals were deposited on the silicon surface to increase the conductive path.
Effect of TeO<inf>2</inf>-based lead-free glass on contact formation of front side silver metallization for monocrystalline silicon solar cells
2022, Solar Energy Materials and Solar Cells
Citation Excerpt :
These Ag particles formed through precipitating of Ag ions in the glass at the cooling stage due to declining of its solubility in the glass. They also contribute to the flowing of electrons through the multiple tunnel effect [25,26]. However, the amount of Ag particles is not adequate in this case.
TeO₂–Bi₂O₃–ZnO–Li₂O–MgO lead-free glass was synthesized based on the consideration of environmental protection and the demand for further conversion efficiency improvement. Reactivity of Bi₂O₃ powder with SiN_x and Si as well as lead-free glass with SiN_x and Si was investigated using differential thermal analysis. DSC curves show that Bi₂O₃ doesn't react with SiN_x or Si under 800 °C, whereas lead-free glass can react with SiN_x and Si at approximately 600 °C indicating early softening and melting of glass facilitates the reaction between Bi₂O₃ and SiN_x or Si. The flowing behavior of lead-free glass at varied temperatures was studied and the morphology of the interface between the glass and the silicon wafer was examined. The white particles formed above silicon were identified as Bi–Te alloy or BiTeO_x through stoichiometrically calculating. However, their formation was suppressed by the addition of silver powder. Ag crystallites and Ag particles start to generate on the interface, as the content of silver powder was increased to 8 wt%. These Ag particles and Ag crystallites were observed on the interface in the finished solar cell.

View all citing articles on Scopus

View full text

Investigation of Ag-bulk/glassy-phase/Si heterostructures of printed Ag contacts on crystalline Si solar cells

Abstract

Introduction

Section snippets

Experiment

Results and discussion

Conclusion

Acknowledgments

Solar Energy Mater. Solar Cells

Solid State Electron.

Appl. Phys. Lett.

J. Electrochem. Soc.

Metall. Mater. Trans. A

J. Electrochem. Soc.