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University Center of Excellence for Photovoltaics

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Now showing 1 - 4 of 4
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    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
    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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    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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    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.