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University Center of Excellence for Photovoltaics

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Author: Nakayashiki, Kenta × End date: 2006 × Start date: 2003 ×

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Investigation of Modified Screen-Printing Al Pastes For Local Back Surface Field Formation

2006-05 , Meemongkolkiat, Vichai , Nakayashiki, Kenta , Kim, Dong Seop , Kim, Steve , Shaikh, Aziz , Kuebelbeck, Armin , Stockum, Werner , Rohatgi, Ajeet

This paper reports on a low-cost screen-printing process to form a self-aligned local back surface field (LBSF) through dielectric rear surface passivation. The process involved formation of local openings through a dielectric (SiNx or stacked SiO(2)/SiN(x)) prior to full area Al screenprinting and a rapid firing. Conventional Al paste with glass frit degraded the SiN(x) surface passivation quality because of glass frit induced pinholes and etching of SiN(x) layer, and led to very thin LBSF regions. The same process with a fritless Al paste maintained the passivation quality of the SiN(x), but did not provide an acceptably thick and uniform LBSF. Al pastes containing appropriate additives gave better LBSF because of the formation of a thicker and more uniform Al-BSF region. However, they exhibited somewhat lower internal back surface reflectance (<90%) compared to conventional Al paste on SiN(x). More insight on these competing effects is provided by fabrication and analysis of complete solar cells.

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The Effect of the Variation in Resistivity and Lifetime on the Solar Cells Performance along the Commercially Grown Ga- and B-Doped Czochralski Ingots

2005-01 , Meemongkolkiat, Vichai , Nakayashiki, Kenta , Rohatgi, Ajeet , Crabtree, Geoffrey , Nickerson, Jeff , Jester, Theresa L.

A systematic study of the variation in resistivity and lifetime on cell performance, before and after light-induced degradation (LID), was performed along the B- and Ga-doped Czochralski (Cz) ingots. Screen-printed solar cells with Al-back surface field were fabricated and analyzed from different locations on the ingots. Despite the large variation in resistivity (0.57 Ω-cm to 2.5 Ω-cm) and lifetime (100-1000 μs) in the Ga-doped Cz ingot, the efficiency variation was found to be ≤ 0.5%. No LID was observed in the cells fabricated from the Ga-doped ingot. In contrast with the Ga-doped ingot, the B-doped ingot showed a very tight resistivity range (0.87 Ω-cm to 1.22 Ω-cm), resulting in very tight lifetime and efficiency distributions. However, the LID effect reduced the efficiency of these B-doped cells by about 1.1% absolute. Additionally, the use of thinner substrate and higher resistivity B-doped Cz is shown to effectively reduce the LID effect.

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Record-High-Efficiency Solar Cells on Multicrystalline Materials Through Understanding and Implementation of RTP-Enhanced SiNx-induced Defect Hydrogenation

2004-01 , Rohatgi, Ajeet , Kim, Dong Seop , Yelundur, Vijay , Nakayashiki, Kenta , Upadhyaya, A. D. , Hilali, Mohamed M. , Meemongkolkiat, Vichai

This 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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Implementation of a Homogenous High-Sheet-Resistance Emitter in Multicrystalline Silicon Solar Cells

2005-01 , Yelundur, Vijay , Nakayashiki, Kenta , Hilali, Mohamed M. , Rohatgi, Ajeet

Solar cell efficiency enhancement resulting from the implementation of a high-sheet-resistance emitter (95 Ω/sq.) in multicrystalline silicon solar cells with screenprinted contacts is demonstrated in this paper. Solar cells on low-cost String Ribbon Si from Evergreen Solar, Baysix mc-Si from Deutsche Solar, and high-quality float zone silicon with 45 Ω/sq. and 95 Ω/sq. phosphorus-doped n+- emitters are fabricated with RTP-fired screen-printed contacts and characterized to asses the impact of a highemitter-sheet resistance emitter on cell performance. Screen-printed mc-Si solar cells show an improvement in Voc of 4-5 mV in most cases that is attributed to the use of the high-sheet-resistance emitter. An appreciable increase in Jsc by as much as 1.0 mA/cm(2) is also observed due to enhanced blue response identified by internal quantum efficiency measurement.

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Investigation of Spatially Non-Uniform Defect Passivation in EFG Si by Scanning Photoluminescence Technique

2005-01 , Nakayashiki, Kenta , Rohatgi, Ajeet , Tarasov, Igor , Ostapenko, Sergei , Gedvilas, Lynn , Keyes, Brian , Bathey, Bala R. , Kalejs, Juris P.

This paper shows that both hydrogenation of defects from SiN(x) coating and thermally-induced dehydrogenation of defects are rapid and occur simultaneously in EFG Si during cell processing. Room-temperature scanning photoluminescence mappings, before and after the SiN(x) induced hydrogenation, revealed that hydrogenation of defective regions is effective and pronounced, with more than an order of magnitude increase in lifetime, compared to the rest of the bulk. In addition, FTIR measurements showed the concentration of bonded hydrogen in the SiN(x) film decreases with the increase in annealing temperature and time. However, the rate of release of hydrogen from the SiN(x) film decreases sharply after the first few seconds. Based on this understanding, a process was developed for a co-firing of SiN(x) film and screen-printed Al and Ag in RTP unit, which produced 4 cm(2) EFG Si cell with highest efficiency of 16.1%.

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Light Induced Degradation in Promising Multi-Crystalline Silicon Materials for Solar Cell Fabrication

2003-05 , Damiani, Benjamin Mark , Nakayashiki, Kenta , Kim, Dong Seop , Yelundur, Vijay , Ostapenko, Sergei , Tarasov, Igor , Rohatgi, Ajeet

Light 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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Investigation of High-Efficiency Screen-Printed Textured SI Solar Cells with High Sheet-Resistance Emitters

2005-01 , Hilali, Mohamed M. , Nakayashiki, Kenta , Ebong, Abasifreke , Rohatgi, Ajeet

In this study it is found that the efficiency enhancement (Δη) resulting from the use of a 100 Ω/sq emitter instead of a conventional 45 Ω/sq emitter is substantially enhanced further by surface texturing. This enhancement is greater for textured cells by at least ~0.4% absolute over the enhancement for planar cells, and is mainly due to the greater difference in the front-surface recombination velocity (FSRV) between the high and low-sheet-resistance emitter textured cells. A FSRV of 60,000 cm/s resulted in a reasonably good V(oc) of ~642 mV for the 100 Ω/sq emitter textured cell. Our investigation of the Ag-Si contact interface shows a more regular distribution of Ag crystallite precipitation for the textured emitter (mainly at the peaks of the texture pyramids). The high contact-quality resulted in a series resistance of 0.79 Ω-cm, a junction leakage current of 18.5 nA/cm(2) yielding a FF of 0.784. This resulted in a record high-efficiency 4 cm(2) screen-printed cell of 18.8% (confirmed by NREL) on textured 0.6 Ω-cm FZ, with single-layer antireflection coating.

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Understanding of the RTP-assisted Reduction of Hydrogen Dissociation from Defects in EFG Si

2004-01 , Nakayashiki, Kenta , Kim, Dong Seop , Rohatgi, Ajeet , Bathey, Bala R.

This paper shows that very short, one second, firing of screen-printed Al on the back and SiN(x) anti-reflection coating on the front can significantly enhance the bulk lifetime in EFG Si through SiN(x)-induced hydrogenation of defects. This process improved average minority carrier lifetime from 3 μs to 93 μs, resulting in the open-circuit voltages as high as 613 mV. It is proposed that rapid firing at an appropriate temperature enhances the retention of hydrogen at defect sites by minimizing the hydrogen dissociation from defects. This is supported by a combination of simulations and experiments which reveal that the dissociation of hydrogen is extremely rapid at or below firing temperature of 700°C.

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