Post on 11-Apr-2017
transcript
May 2, 2023
Silver Front Contact Paste for Photovoltaic Cells Nathan SchaeferStevens Institute of Technology
Outline• Objective • Photovoltaic Operation• Fabrication Process• Testing Methods
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Objective• Invent the next industry standard front contact paste
by manipulating inorganic additives to improve overall performance
– Minimize parasitic resistance– Boost fill factor and efficiency
Photovoltaic Operation
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Energy Bands• Fermi Energy
– Difference in electron energy between the conduction and valance bands
– 1.12 eV at 300K• Incident light
– Wavelength is inversely proportional to photo energy
– Wafer pyramid texture increases changes of successful refraction
– If Eph Ef with a proper incident angel then an electron-hole pair is created
– Charge carriers facilitate conduction• Recombination
– Excited electron falls back down to valence band
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P-N Junction• N-type doped silicon
– Doped with phosphorous (5 valence electrons) compare to silicon with 4
– Excess free electrons• P-type doped silicon
– Doped with boron (3 valence electrons) compare to silicon with 4
– Excess electron holes• The P-N junction creates a voltage
– Excess electrons in n-type silicon diffuse to the p-type, leaving behind the exposed positive ion cores.
– Excess holes of the p-type silicon diffuse to the n-type, leaving behind exposed negatively charged ions
– Creates an electric field.
Fabrication Process
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Formulating• Solvents, Vehicle, and Dispersants– Required for printability– Provide green strength (Pre-firing cohesive/adhesive strength)– Burned off after firing, not present in completed cell
• Glass Frits– Come in a variety of compositions
Examples: PbO-TeO2, PbO-Al2O3-SiOX, Bi2O3-B2O3– Etch through antireflective coating during firing process– Precipitates silver to the surface of the silicon
• Metallic & Oxide Additives– Reduce glass flow temperature– Minimize glass bleeding– Reduce electron contact recombination– Improve silver contact formation
• Silver– Bulk conductor and the vast majority of the paste
• Nano-Silver– Lowers sintering temperature of glass– Broadens firing window
Bare Wafer
Precipitated Silver
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Paste Preparation• Prepare a premix
– Additives that are held constant through paste series
– Vehicle and dispersant • Measure additives
– +/- 10 wt% per formulation– Accuracy to a thousandth of a gram
• Measure paste viscosities– Target viscosity: ~140 Pa*s– Partial and full deficient let-downs adjust paste
as necessary• Mill pastes
– De-agglomerate the particles to less than 15 um
– Homogeneous the paste
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Printing Aluminum• Aluminum functions as the back
electrical contact– Paste formulation deemed sufficiently
optimized– Electrons flow into the aluminum
• P+ Region– Al (group III element) in higher
concentration than Boron in base– Creates electric field that traps
electrons in base• Target weights
– Six-inch ~ 1.4 grams– Five-inch ~ 0.9 grams
• Printing Parameters– Rear squeegee speed– Squeegee pressure– Screen-wafer gap Six-inch square Five-inch pseudo square
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Printing Front Contacts
• Silver paste is screen printed onto front side of wafer
– Screen has 45μm openings– Line widths of silver on cell spread
to around 50-70μm– Wire diameter: 0.6 mm
• Target weights– Five inch ~ 100 mg – Six inch ~ 140 mg
• Printability as a design criteria– Finger line breaks– Interior breaks worse than breaks outside
the bus bars
Six inch screen
Moiré pattern defect behind bus bar
Screen emulsion qualification
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Drying and Firing• After printing cell is dried
– Series of 3 zones– Solvents removed
• Dried cell is fired– Series of 4 zones – Sinters front contacts
down through nitride antireflective coating to make contact with p-doped layer
– Organics removed and glass melted
– Three sintering outcomes
Dryer
Furnace
Nitride antireflective
coatingEmitter(N-type)
Under-fired High Rs and Rsh
Well-firedLow Rs and High Rsh
Over-fired Low Rs and Rsh
Base(P-type)
Testing Methods
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IV Testing
• Short Circuit Current– Theoretical cell resistance
of zero– Slope inversely proportional
to shunt resistance• Open Circuit Voltage
– Theoretical infinite resistance in cell
– Slope inversely proportional to series resistance
• Fill Factor– P=I*V– Ratio of maximum power point
to theoretical power– Best cells ~ 80%
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SunsVOC
• SunsVOC measurement provides the IV curve of the diode without the effects of series resistance
• J02 – Measured by SunsVOC – Recombination factor (carrier losses) from the
fingers back into the emitter region
Two Diode Equation
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CoRescan• Bulk Resistance
– Excited electron travels from substrate base to emitter region– Wafer property
• Sheet Resistance– Electron travels in emitter region next to a
finger– Wafer property
• Contact Resistance– Electron escapes from emitter into finger lines– Paste property
• Grid Resistance– Electron travels down finger line and into bus
bar– Paste property