Ajay Upadhyaya

Publications (2)3.84 Total impact

• Article: High-efficiency screen-printed belt co-fired solar cells on cast multicrystalline silicon

  Ajay Upadhyaya, Manav Sheoran, Ajeet Rohatgi
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  ABSTRACT: High-efficiency 4 cm ² untextured screen-printed solar cells were achieved on cast multicrystalline silicon. These cells were fabricated using a simple manufacturable process involving POCl ₃ diffusion for emitter, PECVD SiN _x: H deposition for a single-layer antireflection coating and rapid co-firing of Ag grid, Al backcontact, and Al-BSF in a belt furnace. An optimized process sequence contributed to effective impurity gettering and defect passivation, resulting in high average bulk lifetimes in the range of 100–250 μs after the cell processing. The contact firing contributed to good ohmic contacts with low series resistance of ≪1 Ω cm ² , low backsurface recombination velocity of ≪500 cm / s , and high fill factors of ∼0.78 . These parameters resulted in 16.9% and 16.8% efficient untextured screen-printed cells with a single layer AR coating on heat exchanger method (HEM) and Baysix mc-Si. The identical process applied to the untextured float zone wafers gave an efficiency of 17.2%. The same optimized co-firing cycle, when applied to HEM mc-Si wafers with starting lifetimes varying over a wide range of 4–70 μs, resulted in cell efficiencies in the range of 16.5%–17%.
  Applied Physics Letters 02/2005; · 3.84 Impact Factor
• Article: Bulk lifetime and efficiency enhancement due to gettering and hydrogenation of defects during cast multicrystalline silicon solar cell fabrication

  Manav Sheoran, Ajay Upadhyaya, Ajeet Rohatgi
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  ABSTRACT: Cast multicrystalline silicon (mc-Si) shows a significant variation in quality depending on the location of the brick in the ingot and the location of the wafer in the brick. Variation also occurs in ingots from different suppliers, which is attributed to the difference in the cleanliness of the crucible used for growth and the quality of the silicon feedstock used. Process-induced lifetime investigation conducted in this paper showed that wafers from the top region of mc-Si ingot grown by Heat Exchanger Method (HEM) benefited most from the gettering step during phosphorus diffusion to form the n+ junction. Wafers from the bottom of the ingot, however, benefited most from the hydrogenation taking place from the SiNx film during the co-firing cycle used to form simultaneous front and back contacts and aluminum back surface field. Wafers from the middle region benefited from both, the diffusion-gettering, and the SiNx-hydrogenation. Un-textured, 4 cm2, screen-printed, best solar cell efficiencies of 15.9% and above were achieved on wafers from top, middle, and bottom regions of most of the ingots used in this study because the bulk lifetime exceeded 100 μs after gettering and hydrogenation. Lifetimes in excess of 300 μs were achieved from the middle region of some mc-Si ingots. Solar cell efficiencies of 16.7% were attained from the middle regions of two out of the three ingots investigated in this study. Device modeling was performed to provide guidelines to reduce the efficiency variation across different regions of the ingots and to obtain the highest possible efficiency with a given bulk lifetime and device structure.
  Solid-State Electronics.