RTP_2002

Solar Energy Materials Silicon; Solar cell; Screen-printing 1. Introduction Low cost and high efficiency are the keys to large-scale acceptability of photovoltaic (PV) systems. The cost break down of current Si PV modules reveals that wafer, cell processing, and module assembly account for approximately 45%, 25% and 30% of the module cost, respectively [1]. The cost of silicon wafer can be *Corresponding author. E-mail address: ebong@crd.ge.com (A. Ebong). 0927-0248/02/$-see front matter r 2002 Elsevier Science B.V. All rights reserved. PII: S0927-0248(02)00047-8 reduced by low-cost solar grade polysilicon feedstock material, increased wafer size, reduced kerf losses during slicing, and thinner substrates. However, the single crystalline silicon grown with CZ method and cast multi-crystalline silicon accounts for more than 75% of the PV cells fabricated today. The lower efficiency realized from CZ substrates compared to FZ wafers has been partly attributed to defects and light-induced degradation due to the presence of B i and O i [2]. Glunz et al. [3] have shown that cell processing involving prolonged heat treatments for CZ substrates can reduce the lifetime degradation and produce efficiency improvement of around 1% absolute. However, conventional furnace processing (CFP) can take more than 1h at 850–9001C for phosphorus diffusion alone. This could limit the throughput of a manufacturing line. The purpose of this study is to develop and demonstrate rapid technologies for emitters without sacrificing cell efficiency on single crystal silicon materials, including FZ, CZ, and magnetic CZ silicon. 2. Device fabrication Our approach towards rapid thermal technologies for high-efficiency cells involves: (1) rapid emitter formation by diffusion under tungsten halogen lamps by belt furnace processing (BFP) and rapid thermal processing (RTP), instead of conventional infrared furnace processing, and (2) use of screen-printed (SP) aluminum followed by 2min RTP for simultaneous back surface field (BSF) and in situ oxide formation. The belt emitter was formed at 9251C in 6min, RTP emitter was formed at 8801C in 3min in a single wafer RTP system and a conventional furnace emitter was formed at 8651C in about 1h. After the emitter formation, Al was SP on the back and formed in an RTP system at 8501C for 2min in oxygen ambient. This resulted in simultaneous formation of excellent Al BSF and front oxide passivation. Cells were fabricated with SP as well as photolithography (PL) contacts. The PL cells had only one masking step involving lift-off. No surface texturing, point contacts or selective emitter to keep the cell design simple. In SP cells Ag contacts were fired through a single layer PECVD SiN AR coating. 3. Results and discussion The emitter profiles for BFP and the RTP for 80–90O/ 2.4mA lower J sc ; and 0.028 reduction in FF. Detailed modeling [4] has shown that the 2% efficiency loss for SP cells can be attributed to poor metal conductivity, high contact resistance 1.E+14 1.E+15 1.E+16 1.E+17 1.E+18 1.E+19 1.E+20 1.E+21 0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 Junction depth (micron) Doping concentration (cm -3 ) BLP RTP Fig. 1. Emitter profiles for 80–90O/& belt line and rapid thermal processing. Table 1 Electrical output parameters Of CFP, RTP and BFP solar cells Cell ID V oc (mV) J sc (mA/cm 2 )FF(%)Z (%) BFP-PL-FZ 636 37.3 80.2 19.0 RTP-PL-FZ 641 36.2 81.9 19.0 CFP-PL-FZ 634 37.1 80.5 18.9 BFP-SP-FZ 629 34.9 77.4 17.0 BFP-PL B-doped CZ 610 36.1 79.5 17.5 RTP-PL B-doped CZ 608 35.1 79.0 16.9 BFP-MCZ-PL 636 35.6 81.3 18.4 A. Ebong et al. / Solar Energy Materials & Solar Cells 74 (2002) 51–55 53 (0.2%), high surface recombination (0.4%), sheet loss (0.1%), emitter doping (0.3%), AR coating and absorption in SiN (0.3%) and higher grid shading (0.5%). Table 1 also shows that PV grade CZ material gave efficiencies ofB17% for both BFP and RTP cells with PL contacts. This 2% lower efficiency of CZ cells compared to FZ cell is attributed to lower bulk lifetime. In order to overcome this deficiency, crucible-grown magnetic CZ was used. This material gave 18.4% efficient cells with PL contacts or a 1.4% higher efficiency compared to CZ silicon. A combination of rapid technologies and magnetic CZ silicon can produce lower cost, high throughput and high-efficiency cells. 4. Conclusions This paper shows that rapidly formed 80O/&emitters ino6min in the hot zone of a conveyor belt furnace or in 3min in an RTP system can produce 19% efficient cells with no surface texturing, point contacts, or selective emitter. We also achieved 19% efficient cells on FZ by emitter and SP Al-BSF formation in o10min in the RTP system. Finally, SP manufacturable cells with 45O/& emitter and SP Al-BSF formed in the conveyor belt furnace gave 17% efficient cells on FZ silicon. The SP cell gaveB2% lower efficiency along with a decrease in J sc and fill factor (FF). This is attributed to the poor blue response, higher series resistance and higher contact shading in the SP devices. 0 10 20 30 40 50 60 70 80 90 100 350 550 750 950 1150 Wavelength (nm) IQE (%) SP-17% BFP-PL-19% RTP-PL-19% IQE Reflectance Fig. 2. IQE and hemispherical reflectance of 19% efficient BFP-PL and RTP-PL and 17% SP cells. A. Ebong et al. / Solar Energy Materials & Solar Cells 74 (2002) 51–5554 References [1] A. Rohatgi, P. Doshi, T. Krygowski, Pushing the frontiers of silicon PV technologies: novel approaches to high-efficiency, manufacturable silicon cells, AIP Proceedings of NREL/SNL Photovoltaics Program Review, 1997, pp. 109–115. [2] R.L. Crab, Photon induced degradation of electron irradiated silicon solar cells, Proceedings of the Ninth IEEE Photovoltaic Specialists Conference, Silver Springs, 1972, pp. 329–330. [3] S.W. Glunz, S. Rein, W. Warta, J. Knobloch, W. Wettling, Comparison of lifetime degradation in boron, gallium doped p-type CZ-Si. Technical Digest, 11th PVSEC, Saporo, Hokkaido, Japan, 1999, pp. 549–552. [4] P. Doshi, J. Mejia, K. Tate, A. Rohatgi, Modeling and characterization of high-efficiency silicon solar cells fabricated by rapid thermal processing, screen printing, and plasma-enhanced chemical vapor deposition, IEEE Trans. Electron Devices 44 (9) (1997) 1417–1424. A. Ebong et al. / Solar Energy Materials & Solar Cells 74 (2002) 51–55 55