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University Center of Excellence for Photovoltaics

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  • Item
    Investigation of Modified Screen-Printing Al Pastes For Local Back Surface Field Formation
    (Georgia Institute of Technology, 2006-05) Meemongkolkiat, Vichai ; Nakayashiki, Kenta ; Kim, Dong Seop ; Kim, Steve ; Shaikh, Aziz ; Kuebelbeck, Armin ; Stockum, Werner ; Rohatgi, Ajeet ; School of Electrical and Computer Engineering ; College of Engineering ; University Center of Excellence for Photovoltaics ; Georgia Institute of Technology. University Center of Excellence for Photovoltaic Research and Education
    This paper reports on a low-cost screen-printing process to form a self-aligned local back surface field (LBSF) through dielectric rear surface passivation. The process involved formation of local openings through a dielectric (SiNx or stacked SiO(2)/SiN(x)) prior to full area Al screenprinting and a rapid firing. Conventional Al paste with glass frit degraded the SiN(x) surface passivation quality because of glass frit induced pinholes and etching of SiN(x) layer, and led to very thin LBSF regions. The same process with a fritless Al paste maintained the passivation quality of the SiN(x), but did not provide an acceptably thick and uniform LBSF. Al pastes containing appropriate additives gave better LBSF because of the formation of a thicker and more uniform Al-BSF region. However, they exhibited somewhat lower internal back surface reflectance (<90%) compared to conventional Al paste on SiN(x). More insight on these competing effects is provided by fabrication and analysis of complete solar cells.
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    Lifetime Enhancement in EFG Multicrystalline Silicon
    (Georgia Institute of Technology, 2000-09) Jeong, Ji-Weon ; Rohatgi, Ajeet ; School of Electrical and Computer Engineering ; College of Engineering ; University Center of Excellence for Photovoltaics
    P and AI gettering and SiN-induced hydrogenation of EFG Si have been investigated using manufacturable process techniques. Annealing of SiN coated EFG, without AI on the back, shows very little defect passivation with maximum lifetime enhancement at 700°C. However, annealing of the SiN film, in the presence of AI, significantly increases the defect passivation and moves the optimum temperature to above 800°C. This increase in the optimum temperature is the result of tradeoff between the increase in the release of hydrogen from the SiN film and the decrease in the retention of hydrogen at defects at high temperatures. A higher annealing temperature (>800°C) is desirable because it produces a superior AIBSF without sacrificing defect passivation. Finally, it is shown that the efficacy of the gettering and hydrogenation process is a strong function of the as-grown lifetime, which dictates the final lifetime as well as cell efficiency.
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    Development of RIE-Textured Silicon Solar Cells
    (Georgia Institute of Technology, 2000-09) Damiani, Benjamin Mark ; Ludemann, R. ; Ruby, D. S. ; Zaidi, S. H. ; Rohatgi, Ajeet ; School of Electrical and Computer Engineering ; College of Engineering ; University Center of Excellence for Photovoltaics ; Georgia Institute of Technology. University Center of Excellence for Photovoltaic Research and Education
    A maskless plasma texturing technique using Reactive Ion Etching for silicon solar cells results in a very low reflectance of 5.4 % before and 3.9 % after SiN deposition. A detailed study of surface recombination and emitter properties was made, then solar cells were fabricated using the DOSS solar cell process. Different plasma damage removal treatments are tested to optimize low lifetime solar cell efficiencies. Highest efficiencies are observed for little or no plasma-damage removal etching on mc-Si. Increased & due to the RIE texture proved superior to a single layer anti-reflection coating. This indicates that RIE texturing is a promising texturing technique, especially applicable on lower lifetime (multicrystalline) silicon. The use of non-toxic, non-corrosive SFS makes this process attractive for mass production.
  • Item
    Self-Aligned Self-Doping Selective Emitter for Screen-Printed Silicon Solar Cells
    (Georgia Institute of Technology, 2001-10) Rohatgi, Ajeet ; Hilali, Mohamed M. ; Meier, D. L. ; Ebong, Abasifreke ; Honsberg, Christiana ; Carroll, A. F. ; Hacke, P. ; School of Electrical and Computer Engineering ; College of Engineering ; University Center of Excellence for Photovoltaics
    A self-aligned selective emitter for screen-printed solar cells is described in which phosphorus dopant is incorporated into a silver paste and diffused into the silicon. This produces an ohmic contact on 70-100 Ω/􀂆 emitter due to the doping of silicon underneath the Ag grid. Alloying is performed in a belt furnace at 900 degrees C for 2 min, above the Ag-Si eutectic temperature of 835 degrees C. SIMS analysis showed a surface doping concentration of 1x10(21) cm(-3) for a fritless paste (Dupont PV167) and 2x10(19) cm(-3) for the fritted paste (PV 168). Sheet resistance due to self-doping alone was quite high 121 Ω/􀂆 and 700 Ω/􀂆 for the PV167 and PV168 pastes, respectively. Therefore, a light diffusion is required underneath the Ag to achieve good ohmic contact. Fritted paste was successfully fired through the SiN(x) AR coating producing a reasonable ohmic contact to the n+ layers doped up to 100 Ω/􀂆 sheet resistance. PC1D model calculations revealed that a selective emitter induced performance enhancement is a function of base resistivity, front and back-surface recombination velocities, and bulk lifetime. For example, if the front-surface recombination velocity (FSRV) is very high (>100,000 cm/s), then the selective emitter under-performs the conventional 40 Ω/􀂆 homogeneous emitter cell. However, if the FSRV is 10000 cm/s the selective emitter gives a 0.6% (absolute) increase in cell efficiency. Selective emitter cells fabricated with 70-80 Ω/􀂆 sheet resistance between the gridlines produced approximately 16% and 15% efficient cells on float-zone and cast multicrystalline Si materials. Series resistance of 0.75 Ω-cm(2) and a fill factor of ~0.76 were achieved. Selective emitter cells were about 0.3% (absolute) more efficient than the conventional cells with 45 Ω/􀂆 homogeneous emitter. Cell analysis revealed that a reduced FSRV could result in greater improvement.
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    Development of a Low Temperature Silver Paste for High Efficiency Screen-Printed Solar Cells
    (Georgia Institute of Technology, 2006-09) Ebong, Abasifreke ; Zhang, W. ; Bokalo, P. ; Rohatgi, Ajeet ; School of Electrical and Computer Engineering ; College of Engineering ; University Center of Excellence for Photovoltaics
    The screen-printing technology provides a low cost high-throughput approach to good contacts for silicon solar cells. However, currently screen-printed contacts are formed at the expense of slight performance and fill factor loss. The front grid contact is particularly important and requires low contact resistance, high shunt resistance, and low junction recombination for high fill factor. Often contacts are fired in the moderate to high temperature range (750-800 degrees C) to achieve low series resistance. However, high temperature firing can lead to junction shunting and recombination, which degrades fill factor. Moreover shallow or higher sheet resistance emitters (50-100 Ω/sq) are desirable for high performance, which makes devices even more vulnerable to high temperature firing. Therefore, in this study, we modify the paste composition by adding some dopants and additives to lower the peak firing temperature for good ohmic contacts. This also reduces the wafer bowing and enhances SiN-induced defect hydrogenation in multicrystalline silicon substrates. The results show that increasing the additives concentration lowered the optimum firing temperature from 780 to 720 degrees C. In addition, the ideality factor is reduced significantly at the lower firing temperature. Thus additives used in this study were able to lower the peak firing temperature and increase the fill factor without hurting the series resistance. Fill factor of 0.774 on textured CZ was obtained at ~720 degrees C peak firing temperature for paste G (SOL9807). These pastes were formulated at Heraeus. Heraeus paste formulation differs in the nature and the amount of additives in the pastes.
  • Item
    2D-Modeling and Development of Interdigitated Back Contact Solar Cells on Low-Cost Substrates
    (Georgia Institute of Technology, 2006-05) Kim, Dong Seop ; Meemongkolkiat, Vichai ; Ebong, Abasifreke ; Rounsaville, Brian ; Upadhyaya, V. ; Das, A. ; Rohatgi, Ajeet ; School of Electrical and Computer Engineering ; College of Engineering ; University Center of Excellence for Photovoltaics
    Two-dimensional numerical simulations were performed to derive design rules for low-cost, high-efficiency interdigitated back contact (IBC) solar cells on a low-cost substrate. The IBC solar cells were designed to be fabricated using either the conventional screen printing or photolithography metallization processes. Bulk lifetime, bulk resistivity, contact spacing (pitch), contact opening width, recombination in the gap between the p(+) BSF and n(+) emitter, and the ratio of emitter width to pitch have been used as key variables in the simulations. It is found that short circuit current density (J(sc)) is not only a strong function of the bulk lifetime but also the emitter coverage of the rear surface. Fill factor (FF) decreases as the emitter coverage increases because the majority carriers need to travel a longer distance through the substrate for longer emitter width. The simulated IBC results were compared with those for conventional screen printed solar cells. It was found that the IBC solar cell outperforms the screen printed (SP) solar cell when the bulk lifetime is above 50 μs due to higher V(oc) and J(sc), which suggests that higher performance can be realized on low-cost substrates with the IBC structure.
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    18% Large Area Screen-Printed Solar Cells on Textured MCZ Silicon with High Sheet Resistance Emitter
    (Georgia Institute of Technology, 2006-05) Ebong, Abasifreke ; Upadhyaya, V. ; Rounsaville, Brian ; Kim, Dong Seop ; Tate, K. ; Rohatgi, Ajeet ; School of Electrical and Computer Engineering ; College of Engineering ; University Center of Excellence for Photovoltaics
    In this paper we report on high efficiency screen-printed 49 cm(2) solar cells fabricated on randomly textured float zone (1.2 Ω-cm) and magnetic Czochralski (MCz) silicon with resistivities of 1.2 and 4.8 Ω-cm, respectively. A simple process involving POCl3 diffused emitters, low frequency PECVD silicon nitride deposition, Al back contact print, Ag front grid print followed by co-firing of the contacts and forming gas anneal produced efficiencies of 17.6% on 1.2 Ω-cm textured float zone Si, 17.9% on 1.2 Ω-cm MCz Si and 18.0% on 4.8 Ω-cm MCz Si. A combination of high sheet resistance emitter (~95 Ω-/ ) and the surface texturing resulted in a short circuit current density of 37.8 mA/cm(2) in the 4.8 Ω-cm MCz cell, 37.0 mA/cm(2) in the 1.2 Ω-cm(2) MCz cell and 36.5 mA/cm(2) in the 1.2 Ω-cm(2) float zone cell. The open circuit voltages were consistent with the base resistivities of the two materials. The fill factors were in the range of 0.760-0.770 indicating there is considerable room for improvement. Detailed modeling and analysis is performed to explain the cell performance and provide guidelines for achieving 20% efficient screen-printed cells on MCZ Si.
  • Item
    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 ; School of Electrical and Computer Engineering ; College of Engineering ; University Center of Excellence for Photovoltaics
    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.
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    Implied-V(oc) and Suns-V(oc) Measurements in Multicrystalline Solar Cells
    (Georgia Institute of Technology, 2002-05) Bowden, S. ; Yelundur, Vijay ; Rohatgi, Ajeet ; School of Electrical and Computer Engineering ; College of Engineering ; University Center of Excellence for Photovoltaics
    Identifying loss mechanisms and predicting device performance are key goals of device and process characterization. Photoconductance measurements allow the extraction of the Implied V(oc) and Suns V(oc), which together can be used for process monitoring, for loss analysis and to identify the potential device performance in the absence of unwanted defects. In this paper, we measure the Implied V(oc) and Suns V(oc) from solar cells with a range of different substrates and at different stages in processing. These measurements are used to analyze the correlation with the actual V(oc) to determine the impact of both non-idealities such as depletion region recombination, and expected effects such as lifetime changes, both during processing and in the final devices.
  • Item
    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 ; School of Electrical and Computer Engineering ; College of Engineering ; University Center of Excellence for Photovoltaics ; 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.