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University Center of Excellence for Photovoltaics

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2002 - 2004 7
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Author: Kim, Dong Seop × Author: Rohatgi, Ajeet × End date: 2005 × Start date: 2000 × Item Type: Publication × Has files: Yes × search.filters.applied.f.resourcesubtype: Proceedings ×

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Now showing 1 - 7 of 7
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    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.
    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.
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    Record-High-Efficiency Solar Cells on Multicrystalline Materials Through Understanding and Implementation of RTP-Enhanced SiNx-induced Defect Hydrogenation
    (Georgia Institute of Technology, 2004-01) Rohatgi, Ajeet ; Kim, Dong Seop ; Yelundur, Vijay ; Nakayashiki, Kenta ; Upadhyaya, A. D. ; Hilali, Mohamed M. ; Meemongkolkiat, Vichai
    This paper presents results on five record-high-efficiency 4 cm(2) solar cells on three different multicrystalline silicon materials through effective hydrogen passivation of bulk defects during cell processing. Silicon ribbon solar cell efficiencies of 18.2% and 17.9% were achieved on EFG and String Ribbon Si cells fabricated with photolithography front contacts, screen-printed Al-doped back surface field, and double layer anti-reflection coating. In addition, high-efficiency, screen-printed, manufacturable cells were achieved on HEM (16.9%), EFG (16.1%), and String Ribbon (15.9%) Si. It is found that proper implementation of a fast co-firing of front and back screen-printed contacts in a belt furnace can significantly enhance the bulk lifetime to ~100 μs and simultaneously produce high quality contacts with fill factors approaching 0.78. The firing process involves fast ramp-up and cooling rates to enhance PECVD SiN(x)-induced hydrogen passivation of defects and the quality of Al back surface field.
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    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
    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.
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    Hydrogenation of Si from SiN(x):H Films: How Much Hydrogen Is Really in the Si?
    (Georgia Institute of Technology, 2003-05) Stavola, Michael ; Jiang, Fan ; Rohatgi, Ajeet ; Kim, Dong Seop ; Holt, J. ; Atwater, H. ; Kalejs, Juris P.
    A promising method to introduce H into Si solar cells in order to passivate bulk defects is by the post-deposition annealing of an H-rich, SiN(x) surface layer. It previously has been difficult to characterize the small concentration of H that is introduced by this method. IR spectroscopy has been used together with marker impurities in the Si to determine the concentration and depth of H introduced into Si from an annealed SiN(x) film.
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    Light Induced Degradation in Promising Multi-Crystalline Silicon Materials for Solar Cell Fabrication
    (Georgia Institute of Technology, 2003-05) Damiani, Benjamin Mark ; Nakayashiki, Kenta ; Kim, Dong Seop ; Yelundur, Vijay ; Ostapenko, Sergei ; Tarasov, Igor ; Rohatgi, Ajeet
    Light induced degradation (LID) in boron doped Czochralski (Cz) silicon with high oxygen content is known to degrade solar cell efficiency. Multicrystalline Si crystals also have oxygen and use B doping, but LID effects are largely unknown. In this paper, ribbon, Cz, and cast multi-crystalline Si crystals with a resistivity of 1-3 Ωcm were investigated for LID. 15-16% efficient EFG, String Ribbon, and cast mc-Si solar cells, fabricated by manufacturable screen printed technology, show small but measurable LID (0.2% absolute efficiency loss). In less than 15% efficient devices, LID was not detectable in ribbon Si crystals. However, >16% efficient photolithography ribbon Si degraded >0.5% absolute. Analysis of the bulk lifetime using photoluminescence mapping, after cell processing, supports the presence of LID in the good regions of the ribbon materials while the defective regions remained essentially unaffected.
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    Implementation of Rapid Thermal Processing to Achieve Greater Than 15% Efficient Screen-Printed Ribbon Silicon Solar Cells
    (Georgia Institute of Technology, 2003-05) Rohatgi, Ajeet ; Yelundur, Vijay ; Jeong, Ji-Weon ; Kim, Dong Seop ; Gabor, A. M.
    This paper summarizes our progress in fabricating record-high efficiency ribbon Si solar cells with screen-printed and photolithography defined contacts. We have developed and optimized rapid thermal processing enhanced SiN(x)-induced hydrogenation to achieve record-high efficiency screen-printed EFG (15.9%) and String Ribbon (15.6%) cells and a high-efficiency String Ribbon cell (17.8%) with photolithography defined contacts. A low-frequency SiN(x) film and a two-step RTP firing process were critical in achieving high-efficiency screenprinted cells. Step 1 provides SiN(x) induced hydrogenation and forms an aluminum doped back surface field. Step 2 is designed for Ag grid firing and includes rapid cooling to retain hydrogen introduced in Step 1.
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    Rapid Thermal Processing for Front and Rear Contact Passivation
    (Georgia Institute of Technology, 2002-05) Bowden, S. ; Kim, Dong Seop ; Honsberg, Christiana ; Rohatgi, Ajeet
    Rapid firing processes are well known to allow improvements in solar cell contacts, particularly for the rear contact. Previous results characterizing the quality of a rear aluminum-alloyed back surface field have measured the effective surface recombination velocity, which depends not only on the material parameters of the back surface field, but also on the base doping. This paper shows that the determination of the recombination current density in the back surface field via photoconductance measurements is an accurate technique to measure the back surface field, independent of the base resistivity. Results show that fast firing conditions give the lowest recombination, but that the firing conditions can be altered substantially while still allowing high open circuit voltages.