2836 IEEE TRANSACTIONS ON ELECTRON DEVICES, VOL. 48, NO. 12, DECEMBER 2001 Enhanced Silicon Solar Cell Performance by Rapid Thermal Firing of Screen-Printed Metals Ji-Weon Jeong, Student Member, IEEE, Ajeet Rohatgi, Fellow, IEEE, Vijay Yelundur, Abasifreke Ebong, Senior Member, IEEE, Mark D. Rosenblum, and Juris P. Kalejs Abstract—Rapid thermal processing (RTP) of screen-printed (SP) Al on the back and silver (Ag) grid on the front produced significant improvement in back surface field (BSF) of n -p-p float-zone (FZ) Si solar cells. Two-step firing was found to form more effective BSF than co-firing, resulting in 0.6–1.0% increase in absolute cell efficiency. In addition, RTP was found to be more effective than the beltline processing (BLP), resulting in 0.5–1.0% increase in absolute cell efficiency. Although the Al-BSF formed by the BLP was inferior to the RTP, the difference between the two is virtually eliminated during the subsequent RTP contact firing. Internal quantum efficiency (IQE) analysis of the solar cells gave effective back surface recombination velocities of 5000 cm/s and 1500 cm/s for co-firing in the BLP and the RTP, respectively. Two-step firing produced of 1500 cm/s and 700 cm/s in the BLP and the RTP, respectively. However, for the two-step firing, involving BLP BSF formation followed by RTP contact firing, was found to be 700 cm/s, which indicates that RTP contact firing with a faster ramp-up (100 C/s) restores the poor-quality BLP BSF. On the other hand, BLP contact firing with a slow ramp-up 10 C/s degrades the high-quality RTP BSF, increasing from 700 cm/s to 1500 cm/s. Index Terms—Back surface field (BSF), photovoltaic cells, rapid thermal processing (RTP), screen-printing (SP), silicon. I. INTRODUCTION F OR photovoltaics (PVs) to be competitive with conven- tional energy sources, the cost of silicon (Si) PV modules must decrease by at least a factor of two for large-scale ap- plications. Since about 70% of the current Si PV module cost is associated with the substrate material and cell fabrication, low-cost Si substrates in conjunction with rapid and cost effec- tive fabrication techniques should be developed, without sacri- ficing cell efficiency, in order to achieve the cost goal of $1/W for PV modules. Rapid thermal processing (RTP) has the poten- tial of reducing the fabrication time and processing steps, while increasing the cell performance and throughput of a manufac- turing line. RTP has become an active area of investigation for Si solar cell fabrication [1]–[4]. It has been used for many Si solar cell Manuscript received May 1, 2001; revised July 31, 2001. The review of this paper was arranged by Editor P. N. Panayotatos. J.-W. Jeong, A. Rohatgi, and V. Yelundur are with the University Center of Excellence for Photovoltaics Research and Education, School of Electrical and Computer Engineering, Georgia Institute of Technology, Atlanta, GA 30332 USA (e-mail: gt0652c@prism.gatech.edu). A. Ebong was with the Georgia Institute of Technology, Atlanta, GA 30332 USA. He is now with GE Corporate Research and Development, Schenectady, NY 12301 USA. M. D. Rosenblum and J. P. Kalejs are with ASE Americas, Inc., Billerica, MA 01821 USA. Publisher Item Identifier S 0018-9383(01)10124-3. fabrication steps including shallow n emitter diffusion [5], high-quality oxide growth for surface passivation [6], p alu- minum (Al) back surface field (BSF) formation [7], and screen- printed (SP) contact firing [8]. RTP is currently being studied at the laboratory level because a continuous high-throughput RTP machine is not available today. As a result, a high-throughput conveyor beltline processing (BLP) is preferred by the PV in- dustry. However, RTP could provide a promising technology for low-cost and high-efficiency Si solar cells in the near future be- cause of fast processing, accurate temperature control, and ben- eficial optical effects of high-energy photons in the visible and ultraviolet (UV) range [9]. Screen-printing of Al paste on the backside for the BSF and silver (Ag) paste for the front grid, in conjunction with BLP, is widely used for the commercial Si solar cells today. In this paper, we have conducted a detailed and systematic investiga- tion of BLP and RTP firing of SP Al and Ag contacts, indi- vidually and in combination, to demonstrate and explain why the implementation of RTP improves the Al-BSF, contacts, and the cell efficiency; and how to maximize its beneficial effect. SP solar cells were fabricated on high-quality single crystalline float-zone (FZ) Si substrate using various BSF and contact firing schemes in a beltline furnace and an RTP system. Detailed de- vice characterization and analysis were used to quantify the per- formance enhancement due to RTP over BLP. II. DEVICE FABRICATION AND CHARACTERIZATION Simple n -p-p solar cells were fabricated with SP contacts and a plasma-enhanced chemical vapor deposited silicon nitride (PECVD SiN) single-layer antireflection (SLAR) coating. The area of all the solar cells fabricated in this study is cm . We fabricated nine cm cells on 4-in diameter wafers. Alloying of SP Al on the backside and firing of SP Ag grid con- tacts on the front were performed in a beltline furnace as well as a single-wafer rapid thermal processor. In our beltline furnace, samples are heated by tungsten–halogen lamps rather than the resistive elements. During BLP, samples move along a belt and are heated by the lamps; furthermore, unlike RTP, the samples are directly placed on the belt and are exposed to the air flowing through the beltline furnace. While the temperature profile can be accurately controlled in the RTP system; it is determined by the length of the belt, belt moving speed, and temperature set- ting in the beltline furnace (the length of the belt is 30 in long in the beltline furnace used in this study). In the RTP system, sam- ples are heated only radiatively by the tungsten–halogen lamps that emit photons in the wavelength range of ultraviolet (UV) to infrared (IR). 0018–9383/01$10.00 © 2001 IEEE