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Design features of high efficiency cells

Design Features Technology

For high absorption:

AR coatings Double layer coating (e.g., ZnS/MgF2)

Thick Si wafer Sawing 300 m thick wafers from ingot

Inverted pyramids on front Oxidation, photolithography, etching

Back reflector Oxide layer perforated by

photolithography

For low recombination:

High lifetime Si Monocrystalline (FZ) and highly purified

Si

Passivated surfaces Oxidation

Small contacts Photolithography to open oxide in front

and back

High doping at contacts Photolithography for local diffusion at

front and back

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PERL Cell

Passivated emitter, rear locally diffused cell

Developed at UNSW by Martin Green, 1990

J. Nelson, The Physics of Solar Cells, Imperial College Press, 2007

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The Honda Dream

Winner of 1996 World Solar Challenge

20000 PERL cells were fabricated for three cars

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PERL Cell

24.7% record efficiency, 4 cm2 cell

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PERL Cell

Shallow p-n junction

n-doping on top of cell followed by p-doping since minority

electron mobility is greater in p-type material compared to minority

hole mobility in n-type material

Heavily doped emitter (~1019

cm-3

) for low sheet resistance, ohmiccontacts, and BSF

Lightly doped base (~1016 cm-3) to improve minority carrier

diffusion length

High quality FZ substrate

Thick cell for high absorption

Minimized top reflection by AR coating

Reflection at bottom electrode by metal contact and oxide TIR

Heavy doping at rear for BSF and ohmic contacts

Elaborate evaporated Ti-Pd-Ag front metallization for low contact

resistances

Thin (20 m) grid lines by photolithography

Limited to concentrator, space, or PV-car racing applications

http://pvcdrom.pveducation.org/

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Industrial Cells

Must consider cost and throughput: Single layer rather than double layer ARC

May omit surface texture

Replace monocrystalline (FZ) wafer with cast multicrystalline wafer

or Cz wafer

Wider grid contacts (100-200 m) by screen printing to replace

photolithography (10-20 m)

Avoid costly high temperature oxidation Local diffusion may not be applied

Contacts formed by screen printing and firing of metal pastes (Al, Ag)

replace Ti/Pd/Ag deposition

Lower cost but also lower efficiency (12-16%)

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Industrial Cells

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Industrial Cell Processing

POCl3 spray followed by RTP

10 m removed by alkaline etch

NaOH

Laser cutting or plasma etch

CVD deposited SiN or TiO2

600-800 C

Al for BSF then Ag for soldering

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Saw Damage Etch

Wafers loaded into cassettes for etching in bath of NaOH

Wafer cassettes are rinsed and spun dry

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Dopant Diffusion

Diffusion in quartz furnaces at 900-950 C

Bubble N2 through liquid POCl3

OR Spin-on liquid, or screen-printed pastes followed by RTP

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Edge Isolation

Isolation by diode-pumped solid-state laser,

chemical etching, or plasma etching

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ARC

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Screen-printing

Wire mesh: 200 wires/in, wire diameter~10m, mesh

opening~30m, total thickness (wires + emulsion)~100m

Pastes: Ag powder (n-type contact) or Al+Ag (p-type contact)

powder in solvent; Ag used for solderability; Al forms BSF in p-

type material

Dried at 200 C after printing

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Screen-printing

http://pvcdrom.pveducation.org/

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Screen-printing

http://pvcdrom.pveducation.org/

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Screen-printing

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Testing and Sorting

http://pvcdrom.pveducation.org/

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Throughput

Modern fab line: ~1000 wafers/h

2-3 s per cell operation

1.5 Wp/cell * 1000 cells/h x 24 h/day * 365 days/year = 13 MWp/year