Manav Sheoran

Hydrogen diffusion in silicon from plasma-enhanced chemical vapor deposited silicon nitride film at high temperature

The stable hydrogen isotope deuterium (D), which is released during the annealing of deuterated silicon nitride films, diffuses through the crystalline silicon and is captured by a thin, amorphous layer of silicon sputtered on the rear surface. We report on the measurement of the concentration of “penetrated” D by secondary ion mass spectrometry to monitor the flux of D diffusing through single-crystalline silicon wafers. The penetrated D content in the trapping layer increases with the annealing time. However, the flux of D injected into the silicon from the silicon nitride layer decreases as annealing time increases.

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

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

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

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...

Greater Than 16% Efficient Screen Printed Solar Cells on 115-170 μm Thick 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

Hydrogen diffusion in silicon from PECVD silicon nitride

1.82/W to

Study of direct PECVD SiN/sub x/-induced surface emitter and bulk defect passivation in p-type silicon solar cells

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

Investigation of the Effect of Resistivity and Thickness on the Performance of Cast Multicrystalline Silicon Solar Cells

1.40/W

Bulk lifetime and efficiency enhancement due to gettering and hydrogenation of defects during cast multicrystalline silicon solar cell fabrication

Hydrogen (H) released during the annealing of hydrogenated amorphous silicon nitride (SiNx:H) films diffuses through the crystalline silicon and passivates the defects. This study shows that the stable H isotope deuterium (D), which is released during the annealing of deuterated amorphous silicon nitride (SiNx:D) films, diffuses through the crystalline silicon and is subsequently captured by a thin, highly defective amorphous layer of silicon (a-Si) sputtered on the rear surface. We report on the measurement of the concentration of “penetrated” deuterium (hydrogen), by secondary ion mass spectrometry (SIMS) to monitor the flux of D diffusing through a defect-free single-crystalline silicon wafer. The penetrated D content in the trapping layer increases with the annealing time. However, the flux of D injected into the silicon from the SiNx layer decreases as annealing time increases. At an annealing temperature of 750 °C, D was found to penetrate through a 575 μm thick wafer in as little as 1 second peak annealing time in a Rapid Thermal Processing (RTP) system. Lifetime measurements on defective Si show that higher flux of H during the short RTP anneal is crucial for enhanced hydrogenation of the defects in Si.

Erratum: “High-efficiency screen-printed belt co-fired solar cells on cast multicrystalline silicon” [Appl. Phys. Lett. 86, 054103 (2005)]

This paper shows that direct low-frequency (LF) deposition of SiN films at 425 /spl deg/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 /spl deg/C in passivating both bulk defects and the emitter surface. The emitter saturation current density (J/sub oe/), was found to be higher for LF SiN compared to the HF SiN just after deposition. J/sub oe/ values for LF SiN reduced dramatically after contact firing to 100-200 fA/cm/sup 2/, well below the J/sub oe/ 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.

Record High Efficiency Screen-Printed Belt Co-Fired Cells on Cast Multi-Crystalline Silicon

A low resistivity of 0.2-0.3 Omegacm has been shown to be optimum for high quality single crystal silicon for solar cells. However, for lower quality cast mc-Si, this optimum resistivity increases owing to a dopant-defect interaction, which reduces the bulk lifetime at lower resistivities. In this study, solar cells fabricated on 225 mum thick cast multicrystalline silicon wafers showed very little or no enhancement in efficiency with the decrease in resistivity. However, Voc enhancement was observed for the lower resistivity cells despite significantly lower bulk lifetimes compared to higher resistivity cells. After gettering (during P diffusion) and hydrogenation (from SiNx) steps used in cell fabrication, the bulk lifetime in 225 mum thick wafers from the middle of the ingot decreased from 253 mus to 135 mus when the resistivity was lowered from 1.5 Omegacm to 0.6 Omegacm. This paper shows that solar cells fabricated on 175 mum thick, 1.5 Omegacm, wafers showed no appreciable loss in the cell performance when compared to the 225 mum thick cells, consistent with PC1D modeling