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University Center of Excellence for Photovoltaics
University Center of Excellence for Photovoltaics
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ItemUnderstanding the Role of Forming Gas on the Screen-Printed Crystalline Silicon Solar Cell Front Grid(Georgia Institute of Technology, 2006-09) Ebong, Abasifreke ; Kim, Dong Seop ; Yelundur, Vijay ; Upadhyaya, V. ; Rounsaville, Brian ; Upadhyaya, A. D. ; Tate, K. ; Rohatgi, AjeetIn this paper we report on the role of forming gas anneal on the fill factor of a small area cell and efficiency loss due to scaling the cell area. Solar cells that are under-fired and those fired at the optimum peak firing cycle showed very marginal response to forming gas anneal. Forming gas anneal is most effective for over-fired cells. The high temperature for the over-fired cells is believed to enhance Ag crystallites growth and the formation of a thick glass layer between the Ag front grid and silicon material. The forming gas anneal aids in reducing the glass to its metal, increase the conductivity of the glass and decrease the contact resistance. Solar cells with four different areas (4-cm(2), 49-cm(2), 100-cm(2) and 156-cm(2)) that were fired at the optimized peak firing temperature showed excellent fill factors without the forming gas anneal treatment. The fill factor was not a strong function of the area even though individually the n-factor and series resistance varied due to edge recombination. The efficiency and short circuit current density showed a quadratic relation with the cell area. The short circuit current density showed a difference of 3.2 mA/cm2 between the 4-cm2 and 156-cm2 cells. The short circuit current density decreased with area due to shading, diffusion length and back surface recombination velocity or Leff, front surface recombination velocity, and area loss due to edge isolation. Improved understanding of these effects coupled with grid design and process optimization can bridge the gap between the small and large area cells.
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ItemRecord-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, VichaiThis 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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ItemString 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, AjeetWe 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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ItemLight 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, AjeetLight 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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ItemImplementation 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.