The document outlines the objective, operation, fabrication process, and testing methods for developing an improved front contact paste for photovoltaic cells. The objective is to minimize parasitic resistance and boost efficiency by manipulating inorganic additives. The fabrication process involves formulating the paste, printing it on the front of silicon wafers, then drying and firing the cells. Various testing methods are used to analyze short circuit current, open circuit voltage, fill factor, recombination factors, bulk resistance, sheet resistance, contact resistance, and grid resistance.

1. 4 January 2016
Silver Front Contact Paste
for Photovoltaic Cells
Nathan Schaefer
Stevens Institute of Technology

2. Outline
• Objective
• Photovoltaic Operation
• Fabrication Process
• Testing Methods

3. 3
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

4. Photovoltaic Operation

5. 5
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

6. 6
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.

7. Fabrication Process

8. 8
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

9. 9
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

10. 10
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

11. 11
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

12. 12
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
coating
Emitter
(N-type)
Under-fired
High Rs and Rsh
Well-fired
Low Rs and High Rsh
Over-fired
Low Rs and Rsh
Base
(P-type)

13. Testing Methods

14. 14
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%

15. 15
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

16. 16
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