Silicon Wafer Solar Cells

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Silicon Wafer Solar Cells

Armin Aberle

Solar Energy Research Institute of Singapore (SERIS)

National University of Singapore (NUS)

April 2009

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1 PV – Some background

1. PV – Some background

Photovoltaics (PV):

• Direct conversion of solar energy to electrical energy via solar cells

Advantages: Advantages: • Clean energy

• Uses an inexhaustible renewable energy source

• Modular (from mW to TW) • Very low safety risks

2

y y

• Reliable; low maintenance cost

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The PV market

4

The PV market

• The PV market is booming (> 30 %/a since 1999) 3 n [G W ] ( 30 %/a since 1999)

• Market share of silicon in 2007: Approx 97% (!) 2 p ro d u c tio n Si wafers ~93%, Si thin-films ~4% 1 A nnual PV p 1 A 0 1980 1990 2000 2010 Year

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Evolution of the global PV market

1 GW_p / a

Mono-Si Multi Si 2003

2000

Mono-Si Multi-Si 2003

Thin film (a-Si) Ribbon Si

1990

1980 Graph: G. Willeke, 2006

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Why is silicon so dominant in PV?

Si dominates the semiconductor industry (microelectronics,

Why is silicon so dominant in PV?

Si dominates the semiconductor industry (microelectronics, displays) → Large variety of machines for industrial production exists already.

Al t id l b d f PV ( ffi i li it 29% t 1 )

Almost ideal bandgap for PV (efficiency limit = 29% at 1 sun). Excellent PV efficiency already realised in industry (> 22%). Good electronic and mechanical properties

Good electronic and mechanical properties. Abundant and non-toxic material.

PV modules are long-term stable (> 20 years).g ( y )

Si can be made as a wafer or as a ribbon or as a thin-film on rigid or flexible substrates.

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Future growth of the silicon PV industry

Aim: To lower the $/W cost of PV modules, via higher

PV ffi i i d/ l f t i t ($/ 2₎

PV efficiencies and/or lower manufacturing costs ($/m2_).

Two strategies:

Si thin-film

technologies

Larger & thinner &

cheaper Si wafers

technologies

cheaper Si wafers

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2 Silicon wafer solar cells

2. Silicon wafer solar cells

PV module with Si wafers:

Module assembly

0.92 US$/W Si wafer (mc-Si)

1.37 US$/W$

Cost distribution of a PV module with Cost = 3-6 US$/W

Solar cell process 0.72 US$/W

Cost distribution of a PV module with mc-Si wafers (13%, 3 US$/W)

Fantastic technology but: Need further cost reductions ($/W) Cost = 3-6 US$/W

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Fantastic technology, but: Need further cost reductions ($/W) Î Major R&D efforts required

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Structure of a simple Si wafer solar cell

Front contact Light beam

Front contact Antireflection coating g

+

-Emitter (n-type) Base (p-type) Rear contact

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Photogeneration of electron-hole pairs in a

semiconductor

Conduction

E

-band

-hf

_red

hf

green Bandgap

energy

+

V l gy Valence band

+

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Main loss mechanisms in single-bandgap

solar cells (Example: Silicon)

1400 1600 #1 P f f m -1 ] 1000 1200

Available energy for PV conversion

#1: Poor usage of energy of short-wavelength photons [W m -2 µ m 400 600 800 #2: Non-absorbed photons a ab e e e gy o co e s o

using a c-Si solar cell

e r density 0 200 400 Pow e 500 1000 1500 2000 2500 0 Wavelength [nm]

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Single-bandgap p-n junction solar cells

under one-sun illumination

Theoretical limit for PV efficiency of such cells: ~31% at 25ºC (W. Shockley and H.-J. Queisser, 1960/61, calculated using

thermodynamic principles) thermodynamic principles)

Best such solar cell realised as yet: 26.1% (GaAs, 2009, Radboud University Nijmegen)

( , , y j g )

Best such silicon solar cell realised as yet: 25.0% (1999, UNSW)

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Bulk recombination

a major problem in

Bulk recombination – a major problem in

standard industrial Si wafer cells (~250 µm

thi k)

thick)

+

-+

+

-+

PV efficiency:

15-16%

15 16%

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Step 1 towards improved PV efficiency:

Use of a thinner wafer (~150 µm)

+

₊

-

-PV efficiency: 15% (Ouch!) 15% (Ouch!)

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Step 2 towards improved PV efficiency:

Thin wafer with optimised rear surface (and

f

t fi

)

narrower front fingers)

Narrower f f front finger

-- +

+

PV efficiency:

~20% (nice!)( ) _Reduced _Optical _Passivating_Passivating

film Reduced

contact area

Optical mirror

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Highly efficient laboratory solar cells using

thin monocrystalline Si wafers

Wafer thickness 42 µm Wafer thickness 42 µm PV efficiency 20.2% Cell area 1.0 cm2

(Fra nhofer ISE) (Fraunhofer ISE)

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Laser-grooved buried-contact cells

Invented in 1980s at UNSW

M ll t ti f t ff ti hi h ff PV

Many excellent properties for cost-effective high-eff PV Lab cells up to 21%, factory cells up to 18% (BP Solar)

Narrow copper contact Narrow copper contact

SiO₂ coating

lightly diffused emitter

p-type locally diffused contact

BSF

16 BSF

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RISE cell

(Rear Interdigitated Single Evaporation)

RISE cell

(Rear Interdigitated Single Evaporation)

Invented at ISFH in 2005 Invented at ISFH in 2005

Only 1 diffusion process, only 1 metallisation process

“All-back-contact” cell

Holes are laser drilled through the Si wafer to connect the front the Si wafer to connect the front junction with the corresponding electrode

Source: ISFH, 2006

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3 Summary Silicon Wafer PV

3. Summary Silicon Wafer PV

Si wafer PV is booming (> 25% p a ) Si wafer PV is booming (> 25% p.a.) Its market share is approx 90%

3 technologies: mono, multi, ribbon Modules are long-term stable

Good price/performance ratio Wafers are getting thinner Wafers are getting thinner

Trend towards high-eff structures Cost of modules are falling ($/W_p)

“Si wafer PV is the benchmark PV

technology and a moving target for any competing technology”

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