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