Bimodally dispersed silver paste for the metallization of a crystalline silicon solar cell using electrohydrodynamic jet printing
Graphical abstract
Introduction
The global market of crystalline silicon solar cells has demanded higher cell efficiency at a lower manufacturing cost because of the intensive pressure related to the severe manufacturing capacity surplus of crystalline silicon solar cells worldwide [1]. To reduce manufacturing cost, the thickness of crystalline silicon solar cell wafers has been reduced from 370 μm to 180 μm since 1997; the thickness is predicted to be less than 150 μm in the near future [2]. To employ thinner crystalline silicon solar cell wafers, various non-contact printing techniques such as aerosol jet printing [3], [4], piezoelectric drop-on-demand (DOD) ink-jet printing [5], [6], [7], and dispensing printing [8], [9], [10] have been explored to replace the current screen-printing technique.
Aerosol jet printing utilizes a jet stream focused by a sheath gas from the concentric outer nozzle, and silver electrodes could be formed as fine as 14 μm in width [3]. However, the typical thickness of silver electrodes after the firing process does not exceed 2 μm [4], and a sophisticated additional process, i.e., light induced plating, is required to thicken the silver electrodes [11]. One typical problem at occurs as a result of the light induced plating process is that the electroplated silver electrodes are not only thickened but also widened. In particular, the increase in the width is approximately two times the increase in the thickness of the silver electrodes.
Direct front-side metallization of the silver electrodes was demonstrated using piezoelectric DOD ink-jet printing. However, the use of silver nanoparticles, which are capped with a polymeric dispersant, induces high volumetric shrinkage during the firing process because of the high volume ratio of the dispersant to the silver nanoparticles. Consequently, poor contact formation between the silver electrodes and the emitter layer of the crystalline silicon solar cell leads to a low cell efficiency of 12.1% [7].
Dispensing printing has been the most successful non-contact printing technique because it does not require a sophisticated additional process to thicken the silver electrodes. Moreover, the achieved cell efficiency of a polycrystalline silicon solar cell was as high as 16.77% [8], which is not only comparable to that of a screen-printed cell but also the highest among directly metallized polycrystalline silicon solar cells produced using non-contact printing techniques. However, the issue of nozzle clogging in dispensing printing would be is a persistent problem because the construction of fine silver electrodes is strongly dependent on the nozzle diameter.
Electrohydrodynamic (EHD) jet printing has recently attracted attention as an alternative to the aforementioned non-contact printing techniques. It has a very unique ability to produce ultra-fine patterns with either an ultra-fine nozzle [12], [13] or a large diameter nozzle [14]. Moreover, EHD jet printing can directly print fine conductive lines with an abnormally high aspect ratio [15]. Various sizes of silver particles can be employed unlike piezoelectric DOD ink-jet printing or dispensing printing because EHD jet printing can construct fine conductive lines with a large diameter nozzle. It can also employ moderately viscous silver paste, which is not possible with piezoelectric DOD ink-jet printing.
Despite all of these distinctive features of EHD jet printing, the cell efficiency of a polycrystalline silicon solar cell metallized with EHD jet printing remained as low as 13.7% when commercially available silver paste for screen-printing was used after dilution [15]. In contrast, the cell efficiency of a polycrystalline silicon solar cell metallized with screen-printing was 16.2% with the same silver paste. This discrepancy implies that the simple dilution of silver screen-printing paste is not effective for EHD jet printing in solar cell applications.
Therefore, this study primarily focuses on the development of silver paste for EHD jet printing in the front-side metallization of a crystalline silicon solar cell. With respect to the smooth flow of silver paste through the nozzle, the use of small silver particles is certainly beneficial, but it could also affect the sound contact formation between the silver electrodes and the emitter layer of a crystalline silicon solar cell. To evade the detrimental effects of small silver particles on the contact formation, we consider a bimodal dispersion of small and large silver particles, and the influence of the bimodal dispersion of silver particles on the electrical characteristics such as unit-line resistance and contact resistivity will be investigated. In addition, an effective means for controlling the viscosity of the bimodally dispersed silver paste for EHD jet printing without significantly altering the solid content of the paste will be introduced. Finally, the cell efficiency of a polycrystalline silicon solar cell metallized with EHD jet printing will be presented and compared with that of screen-printed cells.
Section snippets
Experimental
The in-house-developed bimodally dispersed silver paste for EHD jet printing was composed of (1) small (HP-0702, D50=0.13–0.35 μm, Heesung Metal Ltd., Republic of Korea) and large silver particles (HP-0710, D50=0.9–1.4 μm, Heesung Metal Ltd., Republic of Korea), as shown in Fig. 1(a) and (b); (Fig. 2) glass frit (V2172, D50≈4.52 μm, Ceradyne Inc., USA), as shown in Fig. 1(c); (Fig. 3) a dispersant (Zephrym PD 2246 SF, Croda International Plc., UK); (4) an organic binder (ethyl cellulose, CAS No.
Bimodal dispersion of silver particles
As an empirical rule of thumb, the particle size should be approximately one-hundredth of the nozzle diameter to evade nozzle clogging [20]. Though EHD jet printing potentially has fewer nozzle clogging problems than piezoelectric DOD ink-jet printing due to its unique capability for printing fine conductive lines with a relatively large diameter nozzle, the use of small silver particles might be deemed equally beneficial in terms of smooth nozzle flow. However, the use of extremely small
Conclusions
In this study, the electrical and rheological characteristics of the bimodally dispersed silver particles in silver paste were systematically investigated for EHD jet printing. With an increase in the weight ratio of small-to-large silver particles, the unit-line resistance consistently decreased. In contrast, the contact resistivity became minimal at the small-to-large silver particle weight ratio of 15 wt% but increased for higher weight ratios. This contact resistivity increase above 15 wt%
Acknowledgements
This work was supported by grants awarded under the New & Renewable Energy Technology Development Program of the Korea Institute of Energy Technology Evaluation and Planning funded by the Korean Ministry of Knowledge Economy (Grant No. 20113020010060), the Research and Development Program of the Korea Institute of Energy Research (Grant No. B4-2422), and the Basic Science Research Program of the National Research Foundation of Korea funded by the Ministry of Education, Science and Technology
References (30)
- et al.
Towards ink-jet printed fine line front side metallization of crystalline silicon solar cells
Sol. Energy Mater. Sol. Cells
(2011) - et al.
Investigations of thick-film-paste rheology for dispensing applications
Energy Procedia
(2011) - et al.
The fabrication of front electrodes of Si solar cell by dispensing printing
Mater. Sci. Eng. B
(2012) - et al.
Process development for a high-throughput fine line metallization approach based on dispensing technology
Energy Procedia
(2013) - et al.
Invisible Ag grid embedded with ITO nanoparticle layer as a transparent hybrid electrode
Sol. Energy Mater. Sol. Cells
(2014) - et al.
The effect of diameter ratio and volume ratio on the viscosity of bimodal suspensions of polymer lattices
J. Colloid Interface Sci.
(1997) - et al.
China’s solar photovoltaic industry development: the status quo, problems and approaches
Appl. Energy
(2014) Advances in crystalline silicon solar cell technology for industrial mass production
NPG Asia Mater
(2010)- et al.
Metal aerosol jet printing for solar cell metallization
Prog. Photovoltaics
(2007) - et al.
Silicon solar cells exceeding 20% efficiency
Prog. Photovoltaics
(2008)
Fabrication of an inkjet-printed seed pattern with silver nanoparticulate ink on a textured silicon solar cell wafer
J. Micromech. Microeng
Impact of effective volume ratio of a dispersant to silver nano-particles on silicon solar cell efficiency in direct ink-jet metallization
J. Micromech. Microeng
Analysis of series resistance of crystalline silicon solar cell with two-layer front metallization based on light-induced plating
Sol. Energy Mater. Sol. Cells
High-resolution electrohydrodynamic jet printing
Nat. Mater.
Organic transistors manufactured using inkjet technology with subfemtoliter accuracy
Proc. Natl. Acad. Sci. U.S.A
Cited by (35)
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 CellsA novel hybrid patterning technique for polymer PDMS micro and nanoscale nozzle by double casting
2022, Journal of Manufacturing ProcessesNano-inks for solar cells
2022, Smart Multifunctional Nano-inks: Fundamentals and Emerging ApplicationsParticle-less reactive inks
2022, Smart Multifunctional Nano-inks: Fundamentals and Emerging ApplicationsIn-depth analyses of p-type silicon solar cells: A comparison between commercial compact and laboratory LIBS systems
2021, OptikCitation Excerpt :In Fig. 10, we see a decrease in the intensity of silver as a function of depth. The presence of silver in the solar cell is not natural, it does not enter in the manufacture of metallurgical silicon, it is just a contamination by the screen-printed Ag fingers during metallization step on the front surface of the solar cell [41,46,47]. According to several studies, screen printing causes oxidation and rapid contamination of the Si wafer surface due to the diffusion of Ag in bulk Si [49–51].