Ajay Upadhyaya

Chemical etching of boron-rich layer and its impact on high efficiency n-type silicon solar cells

This paper reports on an effective chemical etching treatment to remove a boron-rich layer which has a significant negative impact on n-type silicon (Si) solar cells with boron emitter. A nitric acid-grown oxide/silicon nitride stack passivation on the boron-rich layer-etched boron emitter markedly decreases the emitter saturation current density J0e from 430 to 100 fA/cm2. This led to 1.6% increase in absolute cell efficiency including 22 mV increase in open-circuit voltage Voc and 1.9 mA/cm2 increase in short-circuit current density Jsc. This resulted in screen-printed large area (239 cm2) n-type Si solar cells with efficiency of 19.0%.

High-Efficiency Large-Area Rear Passivated Silicon Solar Cells With Local Al-BSF and Screen-Printed Contacts

This paper describes the cell design and technology on large-area (239 cm2) commercial grade Czochralski Si wafers using industrially feasible oxide/nitride rear passivation and screen-printed local back contacts. A combination of optimized front and back dielectrics, rear surface finish, oxide thickness, fixed oxide charge, and interface quality provided effective surface passivation without parasitic shunting. Increasing the rear oxide thickness from 40 to 90 Å in conjunction with reducing the surface roughness from 1.3 to 0.2 μm increased the Voc from 640 mV to 656 mV. Compared with 18.6% full aluminum back surface field (Al-BSF) reference cell, local back-surface field (LBSF) improved the back surface reflectance (BSR) from 65% to 93% and lowered the back surface recombination velocity (BSRV) from 310 to 130 cm/s. Two-dimensional computer simulations were performed to optimize the size, shape, and spacing of LBSF regions to obtain good fill factor (FF). Model calculations show that 20% efficiency cells can be achieved with further optimization of local Al-BSF cell structure and improved screen-printed contacts.

Fully Ion-Implanted and Screen-Printed 20.2% Efficient Front Junction Silicon Cells on 239 cm

In this study, we present fully ion-implanted screen-printed high-efficiency 239 cm2 n-type silicon solar cells that are fabricated on pseudosquare Czochralski wafers. Implanted boron emitter and phosphorous back-surface field (BSF) were optimized to produce n-type front junction cells with front and back SiO2 /SiNx surface passivation and rear point contacts. Average efficiency of 19.8%, with the best efficiency of 20.2%, certified by Fraunhofer ISE, Freiburg, Germany, was achieved. In addition, the planarized rear side gave better surface passivation, in combination with optimized BSF profile, raised the average efficiency to ~20% for the fully implanted and screen-printed n-type passivated emitter, rear totally diffused cells.

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Formation of a well-passivated boron emitter for mass production of low-cost and high-efficiency n-type silicon solar cells is a major challenge in the photovoltaic industry. In this letter, we report on a novel and commercially viable method, inkjet printing, to create boron emitters. Phosphorus diffusion was used on the rear to form a back-surface held in conjunction with chemically grown oxide/silicon nitride (SiNx) stack on the front and back for surface passivation. Finally, front and back screen-printed contacts were formed through the dielectric stacks to fabricate large-area (239 cm2) n-type cells. This technology resulted in 19.0%-efficient p+-n-n+ cells with a Voc of 644 mV, a Jsc of 38.6 mA/cm2, and a fill factor of 76.3%. This demonstrates for the hrst time the promise of boron-inkjet-printing technology for low-cost and high-performance n-type Si cells.

n-Type CZ Substrate

In this paper we report on the formation of high quality contacts to HHSE with a sheet resistance variation of ≥30 Ω/sq using a commercial front-side Ag paste, PV16A from DuPont. We fabricated and characterized solar cells with emitter sheet resistances of 65, 75, 85, 95 and 105 Ω/sq. We found that emitter sheet resistances in the range of 65–95 Ω/sq can be contacted with low average series resistance of 0.63–0.77 Ω-cm² and high fill factor (FF) of 77.4–78.8%. The 75 Ω/sq emitter gave the best average efficiency of 18.3%, followed by 18.2% for the 65 Ω/sq, 18.0% for the 85 Ω/sq, and finally 17.7% for the 95 Ω/sq. The 105 Ω/sq emitter gave a low FF due to high series resistance, but the shunt resistance and ideality factor were excellent, which suggests the paste was not encroaching the shallow emitter junction.

High-Efficiency n-Type Si Solar Cells With Novel Inkjet-Printed Boron Emitters

High-efficiency boron- and gallium-doped multicrystalline silicon (mc-Si) cells were fabricated and compared in this paper. The quality of three different boron-doped mc-Si ingots and one gallium-doped mc-Si ingot was investigated and compared by means of lifetime measurements and solar cell efficiencies. Untextured screen printed 4-cm2 cell efficiencies in excess of 16% were achieved in this paper when the lifetime after gettering and hydrogenation exceeded 100 mus. This was true for most wafers from top, middle, and bottom regions of the boron-doped ingots. Lifetimes in excess of 300 mus were achieved from the middle region of some boron- and gallium-doped mc-Si ingots. High efficiencies in excess of 16.7% were attained from the middle region of most ingots investigated in this paper regardless of gallium or boron dopant. Light-induced degradation in efficiency (2%-3% relative) was observed in some of the boron-doped mc-Si wafers in which oxygen concentration was high (15 ppm). In contrast, gallium-doped solar cells were found to be very stable under illumination irrespective of their location in the ingot. Device characterization and modeling were performed to show that the combined effect of large variation in resistivity and lifetime along the gallium-doped mc-Si ingots results in variation in the cell efficiency from different regions of the gallium-doped ingots. Design rules were established to determine the optimum thickness of the solar cell for extracting maximum efficiency when the bulk lifetime and resistivity vary along the length of the ingot for a better utilization of the whole ingot

Formation of high quality screen-printed contacts to homogeneous high sheet resistance emitters (HHSE)

High-efficiency 4cm2 untextured screen-printed solar cells were achieved on cast multicrystalline silicon. These cells were fabricated using a simple manufacturable process involving POCl3 diffusion for emitter, PECVD SiNx: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 <1Ωcm2, low backsurface recombination velocity of <500cm∕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 cycl...

A Comparison of Bulk Lifetime, Efficiency, and Light-Induced Degradation in Boron- and Gallium-Doped Cast mc-Si Solar Cells

This paper demonstrates the Direct Gas to Wafer™ technology to produce high quality epitaxial kerfless mono crystalline n-type and p-type silicon wafers. The key aspects of the approach involve anodic etching to form porous Si release layer, growing epitaxial wafers, separation of the epitaxial wafers from the substrate and substrate reuse. The advantages of epitaxial wafers over conventional Cz wafers are discussed. With 156 mm epitaxial wafers, p-type PERC cell has achieved an efficiency of 19.7% and n-type cell has achieved an efficiency above 20%.

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

In this paper we report on the impact of mc-Si wafer thickness on efficiency. We have obtained 16.8%, 16.4%, 16.2% and 15.7% efficient screen printed 4 cm2 solar cells on 280 mum, 170 mum, 140 mum and 115 mum thick cast mc-Si respectively. Analysis of these cells showed that the efficiency of the 115 mum thick cell is limited by a BSRV of 750 cm/s, FSRV of 120,000 cm/s and a BSR of 67%. A module manufacturing cost model for a 25 MW plant was used to demonstrate that 15.7% efficient cells on 115 mum thick wafers are more cost effective than 16.8% cells on 280 mum wafers. The module manufacturing cost reduced from

High efficiency solar cells on direct kerfless 156 mm mono crystalline Si wafers by high throughput epitaxial growth

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