Contents lists available at ScienceDirect Solar Energy journal homepage: www.elsevier.com/locate/solener Development of nanoparticle copper screen printing pastes for silicon heterojunction solar cells Boon Heng Teo a,b, ⁎ , Ankit Khanna b , Vinodh Shanmugam b , Ma Luisa Ortega Aguilar b , Maryknol Estrada Delos Santos b , Darius Jin Wen Chua c , Wei-Chen Chang d , Thomas Mueller b a Graduate School for Integrative Sciences and Engineering, National University of Singapore, University Hall, Tan Chin Tuan Wing, #04-02, 21 Lower Kent Ridge, Singapore 119077, Singapore b Solar Energy Research Institute of Singapore (SERIS), Block E3A, #06-01, 7 Engineering Drive 1, Singapore 117574, Singapore c Materials Science and Engineering Department, University of California Los Angeles, 410 Westwood Plaza, CA 90095-1595, United States d Geckos Technology Corp, 6F-11, No 38, Taiyuan St, Zhubei City, Hsinchu County 30265, Taiwan ARTICLE INFO Keywords: Low temperature curing Screen printing Cu nanoparticles Silicon heterojunction Solar cells ABSTRACT This paper reports the development of copper screen printing pastes for silicon heterojunction solar cells. Nanoparticle copper paste formulations with a varying amount of copper (percentage by weight) were evaluated in terms of printability, line resistance, and contact formation to Indium-Tin Oxide (ITO) transparent conductive oxide layers. The screen-printed Cu samples were cured under vacuum conditions (< 300 ppm O 2 ) at tem- peratures between 200 °C and 400 °C for 30 min. Scanning electron microscopy was used to investigate Cu na- noparticle sintering at the microstructural level and determine optimal curing conditions for the pastes. The optimized Cu paste formulation yielded consistent finger widths between 53 and 60 μm and finger heights above 20 μm. The average specific contact resistivity of the Cu-ITO contact for the best-performing paste formulation under optimal curing conditions was 0.4 mΩ·cm 2 . The resistivity of printed Cu lines after curing at 400 °C for 30 min was 27 μΩ·cm. In terms of printability and contact resistance to ITO, the paste formulations developed in this study are suitable for application to silicon heterojunction cells. Steps to further improve the resistivity of the printed Cu lines are discussed. Insights from this study revealed the critical influence of Cu paste compo- sition, rheology, screen printing parameters, and curing conditions on the properties of printed electrodes. 1. Introduction Heterojunction silicon wafer solar cells (abbreviated ‘HJ’ in this study) comprise of a front-side hydrogenated amorphous silicon in- trinsic thin layer, a-Si:H(i), and a boron-doped emitter, a-Si:H(p), and rear-side a-Si:H(i) and a phosphorus-doped back-surface field, a-Si:H (n). The doped a-Si layers may be amorphous, nano-crystalline, or micro-crystalline in their morphology. Full-area transparent conductive oxide (TCO) layers, usually Indium Tin Oxide (ITO), cap the doped a- Si:H layers on both sides. Finally, Ag or Ag alloyed metal pastes are screen printed on both sides. The Ag contacts, which are chemically inert and low in resistivity, are designed in an “H” grid fashion, opti- mized by taking into consideration the trade-off between metal shading-related optical losses and resistive losses. Fig. 1 shows a con- ventional bifacial HJ structure. The excellent surface passivation of c-Si wafers achieved by a-Si:H(i) layers results in high open-circuit voltages (> 735 mV) and high photoconversion efficiencies (> 24%) for heterojunction cells (Yoshikawa et al., 2017; Adachi et al., 2015; Taguchi et al., 2014; Masuko et al., 2014). The best reported bifacial HJ cell photoconversion efficiency is 25.1% (Adachi et al., 2015). The current world record ef- ficiency for a single-junction c-Si solar cell under one-Sun illumination is 26.7% and this was achieved by an interdigitated back-contact het- erojunction (IBC- HJ) structure (Yoshikawa et al., 2017). High-efficiency HJ solar cells have matured from R&D to industry (Haschke et al., 2018). It is predicted that HJ cells will obtain a pho- tovoltaic (PV) market share of 10% in 2025 (Fischer, 2017). Despite the high efficiencies achieved by HJ cells, standard industrial Passivated Emitter and Rear Cells (PERC) dominate the PV market share. This is primarily due to the high production costs of HJ cells (Louwen et al., 2016). A potential solution is to explore alternatives to Ag contacts, which accounts for at least 20% of the total cost of a HJ cell (Chang et al., 2016). Investigations into Cu contacts as an alternative to Ag contacts for https://doi.org/10.1016/j.solener.2019.07.055 Received 10 April 2019; Received in revised form 14 June 2019; Accepted 19 July 2019 ⁎ Corresponding author. E-mail address: boonheng.teo@u.nus.edu (B.H. Teo). Solar Energy 189 (2019) 179–185 0038-092X/ © 2019 International Solar Energy Society. Published by Elsevier Ltd. All rights reserved. T