Record High Efficiency Screen-Printed Belt Co-Fired Cells on Cast Multi-Crystalline Silicon
Presented at the 19th European Photovoltaic Solar Energy Conference and Exhibition, Paris, France; June 7-11, 2004. Record-high efficiency screen-printed 4 cm(2) solar cells were achieved on HEM and Baysix cast multi-crystalline silicon. These cells were fabricated using a simple, manufacturable process involving POCl3 diffusion for a 45 Ω/ ٱ emitter, PECVD SiN(x):H deposition for a single-layer antireflection coating and rapid co-firing of an Ag grid, an Al back contact, and Al-BSF formation in a belt furnace. This process scheme resulted in effective impurity gettering and defect passivation. It also contributed to good ohmic contacts with series resistance of < 1Ω-cm(2), back surface recombination velocity of < 500 cm(2)/s, high average bulk lifetimes in the range of 100-250 μs after cell processing and fill factors of ~0.78. These parameters resulted in record high, 16.9% and 16.8% efficient screen-printed cells on HEM (Heat Exchanger Method) and Baysix mc-Si (confirmed by NREL). The identical process applied to the un-textured Float zone (FZ) wafers gave an efficiency of 17.2%. The optimized co-firing cycle, when applied to HEM mc-Si wafers with starting lifetimes varying over a wide range from 4 - 70 μs, resulted in a very tight efficiency range of 16.6% to 16.8% as a result of efficient defect gettering and passivation. Model calculations performed using the simple cell design and measured cell parameters agreed well with the experimental cell efficiency.
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ABSTRACT: This paper shows that direct low-frequency (LF) deposition of SiN films at 425 °C by PECVD followed by a conventional screen-printed contact firing cycle is more effective than a high-frequency (HF) SiN film deposited at 300 °C in passivating both bulk defects and the emitter surface. The emitter saturation current density (J<sub>oe</sub>), was found to be higher for LF SiN compared to the HF SiN just after deposition. J<sub>oe</sub> values for LF SiN reduced dramatically after contact firing to 100-200 fA/cm<sup>2</sup>, well below the J<sub>oe</sub> for HF SiN passivated emitters. Solar cells fabricated on float zone (FZ) Si and mc-Si grown by the heat exchanger method (HEM) yielded efficiencies as high as 17.2% and 16.8%, respectively, when coated with LF SiN. The enhanced cell performance is corroborated by a higher short wavelength IQE response in FZ and HEM cells and a higher post hydrogenation lifetime in HEM mc-Si cells coated with LF SiN.
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ABSTRACT: 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 cm<sup>2</sup> 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 $1.82/W to $1.63/W when the wafer thickness was reduced from 280 mum (efficiency 16.8%) to 115 mum (efficiency 15.7%). A roadmap is developed for 115 mum thick wafers to demonstrate how cell efficiency can be increased to greater than 18% resulting in a module cost of less than $1.40/W