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School of PV and RE Engineering

Lecture 9 – Ag Metallisation Andre Augusto

Lecture Overview

1. Metallisation Overview 2. Screen-Printing 3. Rear Surface Al Electrode 4. Front Surface Ag Grid Electrode 5. Localised Rear Contacts

Today

Screen-Printed Si Solar Cells Saw-damage removal

Texturing

Emitter diffusion

PSG Removal

Edge isolation

SiNx antireflection coating

Al rear SP

Ag front contact SP

Co-firing

• Metal electrodes enable current to be collected from cells • Aluminium (Al) metal paste is used for the rear surface to make electrical connection to the

rear p-type wafer. The Al usually covers most of the rear surface. • Silver (Ag) metal paste is used for the front surface to make connection to the front n-type

diffused emitter. The Ag is usually patterned with an “H” shaped grid pattern. • Screen printing is used to place the metal onto the surfaces. Thus, the metal is initially formed

into a viscous liquid “paste” that can be screen printed. • A “firing” process then sinters and alloys the metal paste together and to the silicon wafer so

that a low-resistance, “ohmic” contact is formed with good adhesion to the silicon wafer • The “firing” process also forms an Al-Si alloy at the rear surface that is beneficial in reducing

recombination at the rear surface (so-called “Back Surface Field”)

Metallisation Overview

Screen printing screen “H” grid pattern of front electrode

• Current practice now is to screen print silver (Ag) paste on top of a silicon nitride ARC coating and then fire this paste through the ARC so that it makes electrical contact to the underlying n-diffused Si.

• The Ag paste contains glass frits that, during the firing step:

(1) etch through the ARC coating

(2) reduce the melting point of Ag

(3) promote adhesion to Si during the firing cycle

• During the firing step, Ag crystallites nucleate at the glass/Si interface to form small crystals that make the electrical contact with the emitter.

Formation of Ag Finger Contacts

Lecture Overview

1. Metallisation Overview 2. Screen-Printing 3. Rear Surface Al Electrode 4. Front Surface Ag Grid Electrode 5. Localised Rear Contacts

• Ag is an excellent conductor. It doesn’t readily oxidise. • Resistivity:

Ø Fired Ag paste: 3 – 9 µΩ.cm but can be as low as 2.7 µΩ.cm with new pastes. Ø Pure Ag metal: 1.6 µΩ.cm at 20 0C

• Printed pattern typically has 2 or 3 busbars and many fine fingers spaced 1- 2 mm apart.

Ferro, 2001

Front Surface Ag Grid

Ag Firing Sequence A) For T < 600ºC, organic components

burn off. B) For 600ºC < T< 850ºC, glass frit

melts, etches ARC and reduces melting point of the Ag. Pb (Lead) or Bi (Bismuth) in the glass frit reduce the melting temp of Ag and wet the surface.

C) Molten glass dissolves both Ag and Si while the metal particles sinter together.

D) As the glass cools, Ag and possibly also Ag-Pb, crystallites precipitate from the glass and grow on/into the Si surface. Glass promotes adhesion mechanical strength to the underlying Si.

Ag Contact Mechansim

Source: T. Lin (2009) High efficiency paste metallisation systems (DuPont).

Figure (a). Direct interconnection at isolated spots between the Ag crystallites and Ag grid. Tunnelling through ultra-thin glass regions (probably the dominant transport mechanism).

Figure (b). Conduction through the glass layer by multi-step tunnelling via metal precipitates in the glass layer.

Source: Hilali, “A review and understanding of screen-printed contacts and selective-emitter formation.” www.nrel.gov, 2004.

Ag Contacts: Current Transport Mechanism

• The final fired finger width is not the same as designed finger width (ie. opening width in the screen).

• The fired Ag finger width depends on the properties of the screen, paste and the printing process.

Ø Higher mesh densities result in better fired line resolution and fired finger widths that are closer to screen opening widths.

Ø If the viscosity of the paste is too low then the fired fingers will be wider (than designed) due to slump behaviour.

• Modern-day pastes can result in final fired fingers that are 60-80 µm wide and 20 µm high and can make good mechanical and electrical contact to lightly-doped emitters (as high as 80 Ω/sq�).

Ø Can’t do this in PVF, where 100 um wide fingers and < 50 Ω/sq emitter diffusions are more practical.

Ag Screen Design Issues

Screen Design for New Pastes

DuPont is a US company so mesh counts are given in wires/inch. Confusing that they use mm for wire diameter and emulsion thickness.

(mesh density (count/inch), strand diameter (um), emulsion thickness (um))

1. Bulk resistance: ρ =1 Ω cm 2. Rear contact resistance ~very small (Al/Si alloy) 3. Rear metal resistance ~negligible (full coverage at rear) 4. Lateral emitter sheet resistance 5. Front specific contact resistivity between silicon and fired Ag

paste (approx 3 × 10-3 Ω cm2) 6. Front metal grid resistance

p-Si1

Al 2

6 5

n+-Si

RS Components

4. Lateral emitter sheet resistance – depending on topography (pitch, sheet resistance, length) and sheet resistance of the diffused

emitter 5. 6. Front metal grid resistance

– depending of geometry (height, width and length) and conductivity of fired Ag paste.

RS Components

p-Si1

Al 2

6 5

n+-Si

4. Lateral emitter sheet resistance

The power lost in a unit cell of the front grid design due to lateral current conduction in the diffused emitter is

where Jmp (A/cm2) is the current density ( = Imp / A), 𝜌o is the sheet resistance of the diffused emitter (ohm/sq), L is the length of the finger and p is the pitch.

The total available power in the unit cell is

So, the fraction of power lost due to lateral emitter sheet resistance is

RS Components

Silicon Wafer

Finger Finger

Pitch, p

Width

Height

𝑃#$%,'() = 1 12 - 𝐽/0

1 - 𝜌 - 𝐿 - 𝑝4

Diffused Emitter

𝑃(5(6' = 𝑝 - 𝐿 - 𝐽/0 - 𝑉/0

𝑃8 = 𝑃#$%,'() 𝑃(5(6'

http://www.pveducation.org/pvcdrom/design/emitter-resistance

4. Lateral emitter sheet resistance and Grid finger shading

A key trade-off in the front grid design is power lost due to lateral emitter sheet resistance and grid finger shading

The power lost due to grid finger shading is

where w is the finger width, L is the finger length and Jmp, Vmp are the max power point current density and voltage.

The total available power in the unit cell is

So, the fraction of power lost due to shading:

RS Components

Silicon Wafer

Finger Finger

Pitch, p

Width

Height

Diffused Emitter

𝑃(5(6' = 𝑝 - 𝐿 - 𝐽/0 - 𝑉/0

𝑃8 = 𝑃#9(: 𝑃(5(6'

𝑃#9(: = 𝑤 - 𝐿 -𝐽/0 - 𝑉/0

6. Front metal grid resistance

The power lost in a unit cell of the front grid design due to the metal grid resistivity

where Jmp (A/cm2) is the current density, 𝜌f is the finger metal resistivity (ohm.cm2), L is the length of the finger, p is the pitch, w,h are the finger width and height.

The total available power in the unit cell is

So, the fraction of power lost due to lateral emitter sheet resistance is

RS Components

Silicon Wafer

Finger Finger

Pitch, p

Width

Height

𝑃#$%,86< = 1 3 -𝐽/0

1 - 𝐿4 - 𝑝1 - 𝜌> 𝑤 - ℎ

Diffused Emitter

𝑃(5(6' = 𝑝 - 𝐿 - 𝐽/0 - 𝑉/0

𝑃8 = 𝑃#$%,86< 𝑃(5(6'

Optimizing the grid design, for example, for the pitch, is done choosing the value for pitch that results in the minimum total fractional power lost

RS Components

𝑃8,)@) = 𝑃#$%,'() 𝑃(5(6'

+ 𝑃#$%,86< 𝑃(5(6'

+ 𝑃#9(: 𝑃(5(6'

Ag Finger Spacing Key limitation of screen printed solar cells:

Wide metal fingers à Space them further apart to minimise shading losses à Need for heavily-doped emitter to minimise lateral resistance à Poor response in short wavelength due to dead layer.

NARROW Spacing WIDER Spacing

Low RS High shading à low JSC

Can tolerate more lightly-doped emitter

High RS Less shading à high JSC

Need more heavily-doped emitter for low lateral resistance

• The Ag firing process is more critical than the Al firing process. Fire just enough to make contact to Si at the surface.

• If too deep: Ø Contact lighter doped n-Si à high RCONTACT (RS component #5) Ø Worst case scenario: shunts the p-n junction

p-Si

n+-Si ~ 0.5um

Ag Firing

Ag Firing Temperature / Time

Ag firing T too HIGH/ Fire Ag too LONG

Ag firing T too LOW/ Fire Ag too SHORT

Fire close to the junction à high RS Extreme: Fire through the junction à shunts

Metal not properly sintered à high RS Not fired through ARC à high RS

• Screen-printed contacts are now typically 80-100 μm wide, which gives rise to high shading losses. Ø Trend is for narrower fingers (approaching 60-80 µm).

• Fill factors were typically low (~75%) because of high contact resistance, low metal conductivity, and junction shunting. Ø FFs are increasing to ~79% with new pastes.

• Heavily-doped emitters (45-50 Ω/sq ) are used to achieve a reasonable contact resistance (3 mΩ cm2) and low lateral series resistance but they also result in poor short-wavelength response (due to dead layer). Ø Higher sheet resistance emitters now possible (60-80 Ω/sq and maybe even

higher in near future).

Losses Associated with SP Front Contacts

Lecture Overview

1. Metallisation Overview 2. Screen-Printing 3. Rear Surface Al Electrode 4. Front Surface Ag Grid Electrode 5. Localised Rear Contacts

Localised Rear Contacts • Localised rear metal contacts can increase cell efficiencies by resulting by:

Ø Increasing Voc and Jsc due to improved rear surface passivation; Ø Increasing Jsc due to reduced absorption of long wavelength light in the rear

aluminium layer (only ~ 65% of the IR light reaching the rear Al is reflected back into the cell).

• However spreading resistance (“crowding”) can be significant if the contact regions are spaced too far apart.

Localised Rear Contacts • Simplest way to introduce localised rear contacts into an existing screen-

print production line is to introduce a rear dielectric layer and pattern that into openings (lines or holes) using, for example, a laser.

• Alloying process is very dependent on the patterning dimensions and the type of paste used. Special “local BSF” pastes are used.

• Lines are easier to pattern, but problems can occur.

QE Results for Locally Rear Contacted Cells

• Jsc improvement of up to 1.5 mA/cm2 due to improved rear passivation and improved rear reflectance.

• Voc of 650-660 mV compared to 620-630 mV for full area BSF.

Source: Dullweber, “High-efficiency rear-passivated screen-printed silicon solar cells.” PV-Tech, August 2011.

Next Lecture

Cell Testing