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1 Newly Developed Low Cost Metallization Paste for Si Hetero- Junction Solar Cell Kyotaro Nakamura1, Kazuo Muramatsu2, Aki Tanaka2 and Yoshio Ohshita1 1Toyota Technological Institute, 2NAMICS CORPORATION Japan 2 Table of Contents ✓ Introduction ✓ Basic concept of our new paste ✓ Cost reduction estimation ✓ Contact and conductor resistance ✓ SHJ solar cell fabrication ✓ Cross sectional SEM ✓ Cell fabrication results ✓ Conclusions 3 Introduction Cell technology Average stabilized efficiency values SHJ P-type mono PERC Market shares for different cell technology Si-heterojunction SHJ M. Despeisse, et al., 35th EUPVSEC, Brussels, 2018. Low temperature curing LTC Ag paste is commonly used for the electrode of SHJ solar cell at present. ITRPV 10th Edition 2019, March 2019. Si heterojunction SHJ solar cell is expected to increase the market share steadily, due to its several advantages, for example, high efficiency, excellent temperature coefficient and bifaciality. 4 Electroplated Cu electrode Kenji Yamamoto, et al., 31st EUPVSEC, Hamburg, 2015. Jay Chang,et al., PVSEC-23, Taipei, 2013. Laser microscope inspection of Electroplated Cu electrode Jian Yu, et al., Solar Energy, 146, 44–49, 2017. SHJ solar cells with electroplated fingers Introduction Eelectorde of SHJ ➢ LTC Ag paste is more expensive than HTF Ag paste. ➢ The conductor resistance of LTC Ag paste is much higher than that of HTF Ag paste. ➢ The consumption of Ag paste of SHJ cell is much larger than the conventional cell. Cost reduction of electrode is very serious issue for SHJ cell. Examples of investigations of electroplating electrodes for SHJ solar cells Electroplating process requires additional costs. The ratio of electrode on the fabrication cost of SHJ cell is very high. 5 Basic concept of our new paste LTC Ag paste Alternative base metal paste The market quotations of these base metals are much lower than Ag. We investigated Cu paste and Ni paste as candidates of the alternative paste. These pastes can drastically reduce the electrode cost. The alternative base metal paste is able to be used in the same way as LTC Ag paste. 6 Cu Ag Basic concept of our new paste Ag Ag Ag Ag Ag Ag Ag Ni Ag Ag Ag Ag Ag Ag Ag In the case of Cu paste, we coated Cu particle by Ag to prevent the contamination of Cu to substrate, and reduce the resistance. Because the resistivities of these base metals are higher than Ag, the conductor resistance of electrode made of 100 base metal paste should increase. In order to realize high performance using alternative base metal pastes, it will be the realistic approach to blend the base metal particles and Ag particles with appropriate blend ratio. 7 Cost reduction estimation The increase of Ag particle blend ratio decreases cost reduction rate. In the case of Ni paste, the Ni particles are cheaper than Ag coated Cu particles, so even if Ag particles are blended up to 58, 30 cost can be reduced.-80 -60 -40 -20 0 0 20 40 60 80 100E lec tro de co st red uc tio n ra te Ag particle blend ratio Ni Ag Ag Ag Ag Ag Ag Ag Ni Ag We calculated the cost reduction rate of Ag coated Cu paste and Ni paste as a function of Ag particle blend ratio. -50 -40 -30 -20 -10 0 0 20 40 60 80 100E lec tro de co st red uc tio n ra te Ag particle blend ratio Cu Ag Ag Ag Ag Ag Ag Ag Ag Ag coated Cu Ag In the case of Ag coated Cu paste, the blend ratio should be limited up to 35 to reduce 30 cost. 8 0.0 5.0 10.0 15.0 20.0 25.0 30.0 0 20 40 60 80 100 Co nta ct resi sta nc e m Ωcm 2 Ag particle blend ratio Contact resistance Ag blend ratio also impacts the performance of LTC paste, in particular, the contact and conductor resistance. We evaluated the contact and conductor resistance of Ag particle blended Ag coated Cu paste and Ni paste using TEG samples and TLM method. 0.0 5.0 10.0 15.0 20.0 25.0 30.0 0 20 40 60 80 100 Co nta ct resi sta nc e m Ωcm 2 Ag particle blend ratio Si TCO LTC paste electrode The contact resistance doesn’t largely depend on blend ratio. The value is lower than 8 mΩcm2, which is low enough for electrode of SHJ solar cell. Ni Ag Ag Ag Ag Ag Ag Ag Ni Ag Cu Ag Ag Ag Ag Ag Ag Ag Ag Ag coated Cu Ag In the case of Ni paste, similar result was obtained. 9 Conductor resistance In the case of Ni paste, the conductor resistance sharply increased under 80 of blend ratio. The conductor resistance heavily depends on blend ratio in the case under 40, and sharply increases with decrease of blend ratio. Up to 37, the conductor resistance keeps lower than 10μΩ⋅cm, which is low enough for electrode of SHJ solar cell. 0.0 10.0 20.0 30.0 40.0 50.0 0 20 40 60 80 100 Co nd ucto r r esi sta nce μΩ ・c m Ag particle blend ratio This result suggested that Ag coated Cu paste is better than Ni paste from the view point of cost- effectiveness.0.0 10.0 20.0 30.0 40.0 50.0 0 20 40 60 80 100 Co nd uc tor re sist an ce μΩ ・c m Ag particle blend ratio Ni Ag Ag Ag Ag Ag Ag Ag Ni Ag Ag coated Cu Ag Cu Ag Ag Ag Ag Ag Ag Ag Ag 10 SHJ solar cell fabrication N-type Cz Si PE-CVD RPD Printer Oven LTC paste electrodeTCO TCO p-a-SiH n-a-SiH i-a-SiH LTC paste electrode N-Cz Si Texturization Pre-deposition cleaning i/n a-SiH deposition on front side PE-CVD i/p a-SiH deposition on rear side PE-CVD ITO deposition on both sides RPD Screen printing of LTC paste on both sides Curing IV characteristics measurement Cross sectional SEM measurement Cu Ag Ag Ag Ag Ag Ag Ag Ag 11 Cross sectional SEM 1μm Ag Si 10μmSi 10μmSi 1μm Ag coated Cu Ag In the case of 37 Ag blended Ag coated Cu paste, small Ag particles are filled into the space among the large Ag coated Cu particles. And it helps the Ag coated Cu particles to contact to the substrate. Ag 100 Ag/Ag coated Cu37/63 In the case of 100 Ag paste, small Ag particles are connected each other and contacted to the substrate, too. Cu Ag Ag Ag Ag Ag Ag Ag Ag 12 Cell fabrication results Blend ratio IscA Voc V F.F. η Ag 100 8.869 0.726 0.785 20.96 Ag/Ag coated Cu37/63 8.999 0.723 0.773 20.89 The differences of Isc, Voc and F.F. between 100 Ag paste and Ag coated Cu paste are small, thus, the difference of cell efficiency is relatively just only 0.4. As a result, we were able to confirm that the newly developed LTC Ag coated Cu paste replaces LTC Ag paste, and it can reduce about 30 electrode cost of SHJ solar cell. 20.0 20.5 21.0 21.5 22.0 Ag 100 Ag/Ag coated Cu 37/63 η 8.70 8.80 8.90 9.00 9.10 9.20 Ag 100 Ag/Ag coated Cu 37/63 Isc A 0.700 0.705 0.710 0.715 0.720 0.725 0.730 0.735 0.740 Ag 100 Ag/Ag coated Cu 37/63 Voc V 0.700 0.720 0.740 0.760 0.780 0.800 0.820 Ag 100 Ag/Ag coated Cu 37/63 F.F . Cu Ag Ag Ag Ag Ag Ag Ag Ag 13 Conclusions We have developed a new low cost LTC paste which is suitable for SHJ cell. It is made of Cu particle coated with Ag, that is, Ag coated Cu paste. Because the resistivity of Cu is higher than Ag, it is the realistic approach to blend Ag coated Cu particles and Ag particles with appropriate blend ratio to ensure the enough performance of LTC paste. We estimated cost reduction rate due to Ag coated Cu paste, and investigated conductor and contact resistance of Ag coated Cu paste electrode, then, determined Ag particle blend ratio 37. Finally, we fabricated SHJ solar cells using the Ag coated Cu paste and compared with Ag paste. The difference of cell efficiency was relatively just only 0.4. In conclusion, we were able to confirm that the newly developed LTC Ag coated Cu paste replaces LTC Ag paste, and it can reduce about 30 of electrode cost of SHJ solar cell. 14 Acknowledgement E-mail kyotarotoyota-ti.ac.jp This work was supported by the New Energy and Industrial Technology Development Organization NEDO under the Ministry of Economy, Trade and Industry METI in Japan. Authors are grateful to Mr. M. Morimura of TTI for his support for the experiment.