This article has been accepted for inclusion in a future issue of this journal. Content is final as presented, with the exception of pagination. IEEE JOURNAL OF PHOTOVOLTAICS 1 266-nm ps Laser Ablation for Copper-Plated p-Type Selective Emitter PERC Silicon Solar Cells Pei-Chieh Hsiao , Ning Song , Xi Wang, Xiaowei Shen, Benjamin Phua, Jack Colwell, Udo R¨ omer, Benjamin Johnston, Sean Lim, Yuan Shengzhao, Pierre Verlinden, and Alison Lennon Abstract—Application of 266-nm picosecond (ps) laser ablation and copper (Cu)-plated metallization to p-type selective emitter (SE) passivated emitter and rear cells (PERC) is reported in this paper. Use of a 266-nm ps laser resulted in similar laser-induced periodic surface structures as observed for 355-nm ps laser ablation of a silicon (Si) nitride antireflection coating (ARC) on random- textured Si solar cell surfaces. In addition, it is shown that 266-nm ps laser ablation results in the formation of amorphous Si with an underlying distorted crystalline Si layer at the laser-ablated surfaces. The successful alignment of laser-ablated openings to the heavily doped SE regions resulted in a comparable cell efficiency of Cu-plated SE PERC cells to screen-printed controls, with a maximum cell efficiency of 20.6% being achieved for the Cu-plated cells. The plated cell performance was limited by the recombination losses, and in particular nonideal recombination caused by the use of a shallow emitter, which had been optimized for screen- printed metallization. Engineering of an SE with a junction depth of 0.52 μm in the heavily doped regions resulted in a 0.3% absolute increase in pseudo fill factor and demonstrated the importance of displacing the p-n junction from the laser-ablated Si surface. Although 355-nm ps laser ablation has been demonstrated to result in strong busbar adhesion in previous reports of Cu-plated cells, significant variability in the busbar adhesion of the fully plated SE PERC cells resulted by 266-nm ps laser ablation. The predicted increased sensitivity of 266-nm laser ablation to the ARC thickness and the possibility that surface oxides were not uniformly removed across wafers before plating may have affected the uniformity of silicide formation and hence the adhesion of the plated busbars. Index Terms—Copper (Cu) metallization, laser ablation, passi- vated emitter and rear cells (PERC), plating, selective emitter (SE), silicon (Si) solar cells. Manuscript received February 14, 2018; revised April 10, 2018; accepted May 3, 2018. This work was supported by the Australian Government through the Australian Renewable Energy Agency and Trina Solar through ARENA Grant 2014/RND003. (Corresponding author: Pei-Chieh Hsiao.) P.-C. Hsiao, N. Song, X. Wang, X. Shen, B. Phua, J. Colwel, U. R¨ omer, and A. Lennon are with the School of Photovoltaics and Renewable Energy Engineering, University of New South Wales, Sydney, NSW 2033, Australia (e-mail:, p.hsiao@unsw.edu.au; n.song@unsw.edu.au; wangxi@mtrc.ac.cn; xiaowei.shen@unsw.edu.au; b.phua@unsw.edu.au;jack.colwell@unsw.edu.au; u.romer@unsw.edu.au; a.lennon@unsw.edu.au). B. Johnston is with the Department of Physics, Macquarie University, Sydney, NSW 2109, Australia (e-mail:, benjamin.johnston@mq.edu.au). S. Lim is with the Mark Wainwright Analytical Centre, University of New South Wales, Sydney, NSW 2033, Australia (e-mail:, sean.lim@unsw.edu.au). Y. Shengzhao and P. Verlinden are with the State Key Laboratory of PV Science and Technology, Trina Solar Ltd., Changzhou 213031, China (e-mail:, yuanshengzhao@gclsi.com; PJVERLINDEN@IEEE.ORG). Color versions of one or more of the figures in this paper are available online at http://ieeexplore.ieee.org. Digital Object Identifier 10.1109/JPHOTOV.2018.2834629 I. INTRODUCTION R ECORD cell efficiencies for p-type silicon (Si) passivated emitter and rear cells (PERC) have rapidly increased to 23.45% [1] in recent years. However, challenges remain for the achievement of comparable energy conversion efficiencies in a cost-competitive manufacturing process. Copper (Cu) plating can provide advantages in terms of reduced metal cost and re- duced shading because of narrow fingers. However, it has failed so far to gain even small fractions of market share in manu- facturing, despite earlier concerns of poor adhesion of plated metal to Si having been addressed through the use of picosec- ond (ps) laser ablation to form the openings for the Cu-plated metal grids [2]–[6]. One of the key efficiency limiting features of the ps laser ablation and Cu plating process is the contact recombination introduced through surface laser damage [7], [8] and metal contacting lightly doped Si surfaces [9]. Contact recombination can be reduced through the use of a selective emitter (SE) and a number of manufacturers have introduced SE processes that can be used for both full-area alu- minum back surface field (Al-BSF) and PERC cells [1], [10], [11]. However, the use of an SE requires accurate alignment of the metal grid to the doping pattern, otherwise reductions in both open-circuit voltage (V OC ) and fill factor (FF) can result because of metal contacting lightly doped Si. Additionally for plated cells, if the junction is shallow, the FF can be further reduced through decreased pseudo FF (pFF), as metal can pen- etrate close to the junction resulting in nonideal recombination [9]. Alignment can be a manageable challenge for screen-printed SE PERC cells, if screen printing is used for both SE formation and metallization, because the same alignment method can be used (e.g., edge alignment with an identical wafer orientation). It has been demonstrated that aligned screen printing can be performed with an accuracy of ±10 μm for a print-on-print pro- cess [12], and similar alignment accuracy is achieved for align- ment of screen-printed metal to SE regions [13], [14]. However, alignment can be more complex when a laser is used to form the contact openings for SE cells where a printing method has been used to form the SE regions. First, few industrially available ps lasers have optical systems that enable alignment to markers (e.g., SE regions) on cells. Second, lasers and printers typically have different patterning aberrations (e.g., warping of screens, laser stage stability) and this can introduce misalignment be- tween the opened region and the heavily doped SE regions on the solar cell [15]. Special alignment algorithms or proce- dures are required for accurate laser contact opening to either 2156-3381 © 2018 IEEE. Personal use is permitted, but republication/redistribution requires IEEE permission. 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