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University Center of Excellence for Photovoltaics

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Lifetime Enhancement and Low-Cost Technology Development for High-Efficiency Manufacturable Silicon Solar Cells

(Georgia Institute of Technology, 2001-08) Rohatgi, Ajeet ; Yelundur, Vijay ; Jeong, Ji-Weon ; Georgia Institute of Technology. University Center of Excellence for Photovoltaic Research and Education

A low-cost, manufacturable defect gettering and passivation treatment, involving simultaneous anneal of a PECVD SiN(x) film and a screen-printed Al layer, is found to improve the lifetime in Si ribbon materials from 1-10 µs to over 20 µs. Our results indicate that the optimum anneal temperature for SiN(x)-induced hydrogenation is 700°C for EFG and increases to 825°C when Al is present on the back of the sample. This not only improves the degree of hydrogenation, but also forms an effective back surface field. Controlled rapid cooling was implemented after the hydrogenation anneal and contact firing to improve the retention of hydrogen at defect sites using RTP. RTP contact firing improved the performance of ribbon solar cells by 1.3-1.5% absolute when compared to slow, belt furnace contact firing. Enhanced hydrogenation and rapid heating and cooling resulted in screen-printed Si ribbon cell efficiencies approaching 15%. A combination of screen-printed Al and a two minute RTP anneal in an oxygen ambient produced simultaneously a high quality rapid thermal oxide (RTO) and an aluminum back surface field (Al-BSF) with a back surface recombination (BSRV) of 200 cm/s 2-3 Ohm-cm single and multicrystalline silicon solar cells. In addition, RTO/SiN(x) stack passivation was found to be superior to SiN(x) surface passivation. RTO/SiN(x) passivation reduces the BSRV to ~10 cm/s on 1-2 Ohm-cm p-type single crystal Si and also lowers the Joe of 40 and 90 Ohm/sq emitters by a factor of three and ten, respectively. Integration of RTP emitters, screen-printed RTP Al-BSF and RTO produced 19% and 17% efficient monocrystalline cells with photolithography and screen-printed contacts, respectively.
Investigation of High-Efficiency Screen-Printed Textured SI Solar Cells with High Sheet-Resistance Emitters

(Georgia Institute of Technology, 2005-01) Hilali, Mohamed M. ; Nakayashiki, Kenta ; Ebong, Abasifreke ; Rohatgi, Ajeet ; Georgia Institute of Technology. University Center of Excellence for Photovoltaic Research and Education

In this study it is found that the efficiency enhancement (Δη) resulting from the use of a 100 Ω/sq emitter instead of a conventional 45 Ω/sq emitter is substantially enhanced further by surface texturing. This enhancement is greater for textured cells by at least ~0.4% absolute over the enhancement for planar cells, and is mainly due to the greater difference in the front-surface recombination velocity (FSRV) between the high and low-sheet-resistance emitter textured cells. A FSRV of 60,000 cm/s resulted in a reasonably good V(oc) of ~642 mV for the 100 Ω/sq emitter textured cell. Our investigation of the Ag-Si contact interface shows a more regular distribution of Ag crystallite precipitation for the textured emitter (mainly at the peaks of the texture pyramids). The high contact-quality resulted in a series resistance of 0.79 Ω-cm, a junction leakage current of 18.5 nA/cm(2) yielding a FF of 0.784. This resulted in a record high-efficiency 4 cm(2) screen-printed cell of 18.8% (confirmed by NREL) on textured 0.6 Ω-cm FZ, with single-layer antireflection coating.
Comparison of Dielectric Surface Passivation of Monocrystalline and Multicrystalline Silicon

(Georgia Institute of Technology, 2002-05) Brody, Jed ; Rohatgi, Ajeet ; Georgia Institute of Technology. University Center of Excellence for Photovoltaic Research and Education

Reducing solar cell thickness is an attractive way to reduce material costs. However, model calculations in this paper show that if rear surface recombination velocity (S) is greater than about 1000 cm/s, a 100-μm-thick screen-printed cell on solar-grade material has a lower efficiency than a 300-μm-thick cell. The literature demonstrates that S < 1000 cm/s is readily achievable on monocrystalline materials. However, S on multicrystalline silicon (mc-Si) seems less thoroughly investigated. In this study, string ribbon mc-Si wafers of different resistivities are passivated with a thermal oxide, plasma-enhanced chemical vapor deposition (PECVD) nitride, and an oxide/nitride stack. For comparison, float zone (FZ) and Czochralski (Cz) monocrystalline wafers are passivated identically. By analyzing measured lifetimes under 500 nm and 1000 nm illumination, upper and lower limits on S are determined. For most of the monocrystalline wafers investigated in this study, the upper limit on S is less than 1000 cm/s, while for most of the multricrystalline wafers, 1000 cm/s falls within the error bars. Thus, thinning monocrystalline silicon should improve cell performance; however, it is difficult to conclude from this data that solar cell efficiency will improve when reducing thickness for the specified mc-Si materials and passivation technologies. In fact, results strongly suggest that S on string ribbon mc-Si is higher than S on identically passivated FZ.
Beneficial Impact of Low Frequency PECVD SiN(x):H-Induced Hydrogenation in High-Efficiency String Ribbon Silicon Solar Cells

(Georgia Institute of Technology, 2004-06) Yelundur, Vijay ; Rohatgi, Ajeet ; Hanoka, J. I. ; Reedy, R. ; Evergreen Solar, Inc. ; National Renewable Energy Laboratory (U.S.) ; Georgia Institute of Technology. University Center of Excellence for Photovoltaic Research and Education

PECVD SiN(x):H-induced hydrogenation of bulk defects in String Ribbon Si during RTP anneal is investigated in this study to enhance the carrier lifetime and understand the role of the plasma excitation frequency and an in-situ NH3 plasma pretreatment before SiN(x):H deposition. The results show that a low frequency SiN(x):H film with a NH3 plasma pretreatment annealed in RTP at 740°C for 60 seconds enhances the lifetime in String Ribbon Si from 5-6 μs to 90-100 μs. Secondary ion mass spectroscopy underneath SiN(x):H films deposited with deuterated ammonia (ND3) and silane shows greater deuterium incorporation in Si under the low frequency SiN(x):H film. Thus, hydrogen incorporated in Si during SiN(x):H deposition may act as an additional source that enhances hydrogen defect passivation during subsequent RTP treatments. In addition, the effect of the anneal time during RTA for hydrogenation is studied in an effort to reduce the hydrogenation time and improve the retention of hydrogen at defects in Si. The RTA time for hydrogenation is reduced to one second without loss of lifetime enhancement and leads to the fabrication of high-efficiency String Ribbon solar cells (17.9%) with photolithography-defined contacts. A rapid belt furnace contact co-firing scheme is developed based on the short RTA and produces screen-printed 4-cm2 String Ribbon solar cells with efficiencies as high as 15.9%.
Self-Doping Contacts and Associated Silicon Solar Cell Structures

(Georgia Institute of Technology, 1998-07) Meier, D. L. ; Davis, H. P. ; Shibata, A. ; Abe, T. ; Kinoshita, K. ; Bishop, C. ; Mahajan, S. ; Rohatgi, Ajeet ; Doshi, P. ; Finnegan, M. ; Georgia Institute of Technology. University Center of Excellence for Photovoltaic Research and Education ; EBARA Solar, Inc. ; EBARA RESEARCH Co. Ltd. Center for Electrophysics ; Carnegie-Mellon University. Dept. of Materials Science and Engineering

Contacts to <111> Si which are self-doping and self-aligning were investigated. Such contacts are applicable both to conventional cell structures as selective emitters and to more demanding structures such as interdigitated back contact cells. Emphasis was placed on alloyed contacts of AI for providing a self-doping p-type contact and of Ag-Sb for a self-doping n-type contact. Alloying at 900°C of 1.1% (wt.) Sb in Ag doped Si to a value of 2 x 10 (18) Sb/cm(3), suggesting a 5% (wt.) Sb is needed for ohmic contact. An AI alloy p-n junction was found to be suitable for a solar cell if placed at the back of the cell, with 13.2% efficiency and good IQE demonstrated for a fully screen-printed dendritic web cell. A prototype interdigitated back contact cell was fabricated by screen printing (AI and Ag) with tight alignment (100 11m lines and spaces) on a dendritic web substrate with an efficiency of 10.4%.
Understanding of the RTP-assisted Reduction of Hydrogen Dissociation from Defects in EFG Si

(Georgia Institute of Technology, 2004-01) Nakayashiki, Kenta ; Kim, Dong Seop ; Rohatgi, Ajeet ; Bathey, Bala R. ; RWE Schott Solar, Inc. ; Georgia Institute of Technology. University Center of Excellence for Photovoltaic Research and Education

This paper shows that very short, one second, firing of screen-printed Al on the back and SiN(x) anti-reflection coating on the front can significantly enhance the bulk lifetime in EFG Si through SiN(x)-induced hydrogenation of defects. This process improved average minority carrier lifetime from 3 μs to 93 μs, resulting in the open-circuit voltages as high as 613 mV. It is proposed that rapid firing at an appropriate temperature enhances the retention of hydrogen at defect sites by minimizing the hydrogen dissociation from defects. This is supported by a combination of simulations and experiments which reveal that the dissociation of hydrogen is extremely rapid at or below firing temperature of 700°C.
Comparison of Front and Back Surface Passivation Schemes for Silicon Solar Cells

(Georgia Institute of Technology, 1998-07) Moschner, J. D. ; Doshi, P. ; Ruby, D. S. ; Lauinger, T. ; Aberle, A. G. ; Rohatgi, Ajeet ; Institut für Solarenergieforschung Hameln ; Sandia National Laboratories ; Angewandte Solarenergie GmbH ; Georgia Institute of Technology. University Center of Excellence for Photovoltaic Research and Education

This work presents a comprehensive study on fast, low-cost methods for the electronic passivation of the phosphorus-diffused front surface and the non-diffused p-type rear surface of crystalline Si solar cells. Titanium dioxide is compared with rapidly-grown thermal oxide (RTO) and PECVD silicon nitrides from three different laboratories. Double layers of RTO and Ti02 or SiN are also investigated. We demonstrate that SiN and RTO single layers can provide very good passivation on both the front and back surface of solar cells. It is also shown that double layers consisting of a thin RTO film and silicon nitride can improve the passivation quality of most SiN layers, and enhance the stability under thermal treatment. With the proper choice of RTO, SiN, and thermal treatment, excellent surface recombination velocities on the back as well as very low emitter saturation currents can he reached using these fast, industrially feasible methods. All films used also provide or are compatible with a good antireflection coating of the cell surface.
String Ribbon Silicon Solar Cells with 17.8% Efficiency

(Georgia Institute of Technology, 2003-05) Kim, Dong Seop ; Gabor, A. M. ; Yelundur, Vijay ; Upadhyaya, A. D. ; Meemongkolkiat, Vichai ; Rohatgi, Ajeet ; Evergreen Solar, Inc. ; Georgia Institute of Technology. University Center of Excellence for Photovoltaic Research and Education

We have fabricated 4 cm(2) cells on String Ribbon Si wafers with efficiencies of 17.8% using a combination of laboratory and industrial processes. These are the most efficient String Ribbon devices made to date, demonstrating the high quality of the processed silicon and the future potential for industrial String Ribbon cells. Cofiring PECVD (Plasma Enhanced Chemical Vapor Deposition) silicon nitride (SiN(x)) and Al was used to boost the minority carrier lifetime of bulk Si. Photolithography front contacts were used to achieve low shading losses and low contact resistance with a good blue response. The firing temperature and time were studied with respect to the trade-off between hydrogen retention and aluminum back surface field (Al-BSF) formation. Bulk defect hydrogenation and deep Al-BSF formation took place in a very short time (~1 sec) at temperatures higher than 740 degrees C.
Comparison of Dielectrics and Iodine Solution for Monocrystalline and Multicrystalline Surface Passivation

(Georgia Institute of Technology, 2003-05) Brody, Jed ; Geiger, P. ; Hahn, G. ; Rohatgi, Ajeet ; Universität Konstanz. Fachbereichs Physik ; Georgia Institute of Technology. University Center of Excellence for Photovoltaic Research and Education

The effective lifetimes of monocrystalline and multicrystalline wafers were measured under dielectric and iodine-solution surface passivation using inductively coupled photoconductance. While all 18 spots measured on monocrystalline materials had significantly higher (>10%) lifetimes under iodine passivation than dielectric passivation, this condition was satisfied by only 12 of 18 spots measured on cast multicrystalline wafers and just 6 of 18 spots measured on string ribbon. Possible reasons for this behavior are discussed in this paper. Moreover, the differences in surface passivation effectiveness have also been investigated with lifetime maps in order to overcome measurement problems related with the inhomogeneity of ribbon silicon.
Study of Direct PECVD SiN(x)-Induced Surface Emitter and Bulk Defect Passivation in P-Type Silicon Solar Cells

(Georgia Institute of Technology, 2005-01) Upadhyaya, A. D. ; Sheoran, Manav ; Rohatgi, Ajeet ; Georgia Institute of Technology. University Center of Excellence for Photovoltaic Research and Education ; Georgia Institute of Technology. School of Physics

This paper shows that direct low-frequency (LF) deposition of SiN films at 425 °C by PECVD followed by a conventional screen-printed contact firing cycle is more effective than a high-frequency (HF) SiN film deposited at 300 °C in passivating both bulk defects and the emitter surface. The emitter saturation current density (Joe), was found to be higher for LF SiN compared to the HF SiN just after deposition. Joe values for LF SiN reduced dramatically after contact firing to 100-200 fA/cm(2), well below the Joe for HF SiN passivated emitters. Solar cells fabricated on float zone (FZ) Si and mc-Si grown by the Heat Exchanger Method (HEM) yielded efficiencies as high as 17.2% and 16.8%, respectively, when coated with LF SiN. The enhanced cell performance is corroborated by a higher short wavelength IQE response in FZ and HEM cells and a higher post hydrogenation lifetime in HEM mc-Si cells coated with LF SiN.