168 IEEE JOURNAL OF PHOTOVOLTAICS, VOL. 4, NO. 1, JANUARY 2014 Electrical and Microstructural Analysis of Contact Formation on Lightly Doped Phosphorus Emitters Using Thick-Film Ag Screen Printing Pastes Vinodh Shanmugam, Jessen Cunnusamy, Ankit Khanna, Prabir Kanti Basu, Yi Zhang, Chilong Chen, Arno F. Stassen, Matthew B. Boreland, Thomas Mueller, Bram Hoex, and Armin G. Aberle Abstract—Screen printing of the metallization of phosphorus diffused emitters is a well-established process for industrial sili- con wafer-based solar cells. Previously, screen printed silver pastes typically required a very high phosphorus surface doping concen- tration to ensure a low-resistance ohmic contact. Recently, paste manufacturers have focused on the development of silver pastes ca- pable of contacting phosphorus emitters with progressively lower surface concentrations, to minimize surface recombination losses and enable higher cell conversion efficiencies. In this paper, we report on the progress of contacting inline-diffused phosphorus emitters, of which the surface concentrations have been reduced by an etch-back process, using two different pastes. Solar cells with emitter surface concentrations ranging from 4.0 × 10 20 to 1.7 × 10 20 phosphorus atoms/cm 3 were made using two differ- ent silver pastes. We present a microstructural analysis of the contact formation, which indicates the possible dominant current transport mechanisms for the two pastes. A high density of sil- ver crystallites formed with a very narrow interfacial glass layer makes the Sol 9600 paste suitable for contacting lowly doped phos- phorus emitters. Efficiency gains of 0.2%–0.3% (absolute) were achieved, reaching a maximum efficiency of 18.6% on 156 mm × 156 mm p-type pseudo-square Cz mono-crystalline silicon solar cells. Index Terms—Crystalline silicon solar cells, inline diffused emitter, metallization, photovoltaics, screen printing, silver pastes. I. INTRODUCTION S CREEN printing technology is the dominant metallization process for industrial silicon wafer solar cells and is a well- established process for contacting phosphorus diffused (n + ) emitters on p-type silicon (Si) wafers. Screen printed silver Manuscript received August 27, 2013; revised October 11, 2013; accepted November 6, 2013. Date of publication December 2, 2013; date of current ver- sion December 16, 2013. SERIS is sponsored by the National University of Singapore and Singapore’s National Research Foundation through the Singa- pore Economic Development Board. V. Shanmugam, J. Cunnusamy, A. Khanna, P. K. Basu, T. Mueller, B. Hoex, and A. G. Aberle are with the Solar Energy Research Institute of Singapore, 117574, Singapore (e-mail: vinodh.shan@nus.edu.sg; jessen.cunnusamy@nus. edu.sg; ankit.khanna@nus.edu.sg; prabir.basu@nus.edu.sg; tomte.mueller@ gmail.com; Bram.Hoex@nus.edu.sg; armin.aberle@nus.edu.sg). M. B. Boreland was with the Solar Energy Research Institute of Singapore. He resides in N.S.W., Australia (e-mail: mattboreland@yahoo.com.au). Y. Zhang, C. Chen, and A. F. Stassen are with Heraeus Materials, Singapore (e-mail: yi.zhang@heraeus.com; chilong.chen@heraeus.com; arno.stassen@ heraeus.com). 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.2013.2291313 (Ag) pastes typically require a high phosphorus surface doping concentration to ensure a low-resistance ohmic contact. Indus- trial silicon wafer solar cells generally use phosphorus emitters with surface doping concentrations above 5 × 10 20 atoms/cm 3 to enable screen printed Ag metallization contacts with a low specific contact resistance. However, recombination losses in the emitter region increase with increasing phosphorus surface concentration [1]. A high phosphorus surface concentration re- sults in a poor response to short wavelengths (blue response) due to an increased Auger recombination in the emitter [2], [3]. Innovative Ag pastes have been developed to enable contacting phosphorus emitters with a lower surface doping concentration. This improves both the voltage (due to reduced heavy doping effects) and the current (due to improved blue response) of the solar cells. In Si wafer solar cells, forming a good contact between the emitter and the screen-printed Ag paste is essential to achieving high PV efficiency [4]. The Ag paste consists of Ag powder, glass frits, and organic materials [5]. To establish a good con- tact with the diffused emitter, the screen printed Ag paste must etch through the insulating antireflection coating (ARC), typi- cally silicon nitride (SiN x ), with minimum damage to the p-n junction [6]. The glass frits in the Ag paste melt during the fir- ing process and then etch through the SiN x layer to facilitate the formation of the electrical contact [7]. The organic mate- rials in the paste act as a carrier of the Ag powder and are responsible for the adhesion of the paste to the Si wafer during printing [6], [8]. Current transport through the screen-printed Ag/Si structure is relatively complex and several models have been proposed regarding its mechanism [9], [10]. The main reason for the complexity regarding the current transport mechanisms of the screen-printed contacts is the lateral nonuniformity of the con- tact, which is characterized by complex interfacial regions con- sisting of a resistive glassy layer, crystallites, colloids, and pin- holes. The high series resistance is often a problem with screen- printed contacts on solar cells, which is mainly due to melted glass frits that flow preferentially toward the Ag–Si interface during the high-temperature (>800 ◦ C) firing process. This cre- ates an interfacial glass layer between the Ag contact and the Si, which increases the contact resistance between the Ag and the Si, resulting in an increased series resistance of the solar cell. This interfacial glass layer is highly resistive (in the order of 10 9 Ωcm), and it prevents any electrical conduction through the 2156-3381 © 2013 IEEE