Stuart Bowden BAPVC January 12, 2011 1

Crystalline Silicon Solar Cells Future Directions Stuart Bowden BAPVC January 2011

Stuart Bowden Co-Director of Solar Power Labs at ASU

Work relevant to BAPVC: • Pilot solar cell production line on

industry standard 6” wafers to advance processes, manufacturing science and metrology.

• Heterojunction solar cells for low temperature high efficiency cells

• Kerfless production of wafers at 10 to 100 µm using lasers.

• Nanostructured solar cells to achieve the 86.8% efficiency limit

• Tandems on InGaN.

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For QESST ERC: co-lead on Thrust 1 Terawatt PV and its implementation via the student pilot line testbed

Scale of the Problem - Motivation

• When I started in PV there was the hope that temperatures would return to normal

• Continually rising temperature makes our job more urgent

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Graph from 1996 Is the world heating up?

Graph of Today Just kept getting hotter

Scale of the solution

1.E-02

1.E-01

1.E+00

1.E+01

1.E+02

1.E+03

1.E+04

1995 2000 2005 2010 2015 2020 2025 2030

elct

ricity

gen

erat

ion,

PV

prod

uctio

n (G

W)

Year

World Electricity production

US Electricity Production

Cumulative PV production

New Generation (US)

PV production

New Generation (Wor

World Electricity production

US Electricity Production

Cumulative PV production

New Generation (US)

PV production

New Generation (Wor

• At Historic 40% growth

1. All new US generation ~5years

2. All new World generation ~ 10 years

3. Total US Production ~ 15 years

4. Total World Production ~ 20 years

Learning Curves

Plots from QESST and 1366

BOS and Efficiency

• With BOS prices equivalent to module prices higher efficiency modules reduce the cost of PV.

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Present

Current Silicon PV Market

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• Silicon prices have declined dramatically recent years

• Crystalline silicon continues to dominate the PV industry at 87% of the market in 2010

Current Silicon Device Technology

• Efficiency 15 – 22 % • Diffused junction emitter • SiN AR coating and

passivation • Screen printed contacts • Aluminum back surface

field • Large interaction

between component parts and processes

• Many Variations Solar Power Laboratory 8

IC Processes Solar Processes

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Process Past IC Heritage Present Solar cells Future Solar Cells

Feed stock Siemens process Siemens process Fluidized bed reactor

Ingot Cz Mono and/or multi Direct growth, ribbons

Wafering blade sawn wire sawn Kerf-less

Thickness 600 um ~200um 50-100 um

Doping High temperature diffusions

High temperature diffusions

Low temperature depositions of a-Si

Passivation Thermally grown silicon dioxide

Silicon nitride Al2O3, -ve SiN

Contacts Photolithography/ lift-off

Screen printed silver Copper Plating

Feedstock

• Electronic grade silicon fluctuates widely but at 40 $/kg :

• Better use of the Si feedstock is a key cost driver

* crystallization costs are in addition

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Current Future Efficiency 15% 20% Thickness 220 um 100 um Kerf 280 um Kerfless Si only cost* 0.3 $/W 0.05 $/W Wafer cost 0.7 $/W 0.1 $/W?

Feedstock

• Even when electronic grade silicon is cheap, new production methods needed to meet scalability requirements.

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Photo: REC

Wafering

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• Wafering wastes half the silicon in cutting and there is a limit to the device thickness

• Alternative wafering with ion implantation and peeling.

• At ASU we’ve adopted laser cleaving.

• Direct growth of substrates for ribbons etc

http://www.evergreensolar.com/

Surface Passivation

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present future

• Silicon solar cell peak efficiency is 10 - 100 µm.

• Surface activity dominates device performance as we go thinner

• Many options for surface passivation such as SiN, Al2O3, organics

• Need to tailor the surface passivation to the doping concentration and type.

Light Trapping

• Essential for wafers • Usually combined with

the surface texturing • Needs development

alongside surface passivation as sharp tips are recombination sites.

• Photonics. • Light trapping at the

module level?

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Metallization

• Silver price is being dragged up by the price of gold

• We can do roughly 10 times bigger industry than today.

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Fraunhofer-ISE

Material Abundance

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Metallization option Cu plating

• Copper is 100 times cheaper than silver

• Plating gives a much denser metal finger

• Wet process • Good for thin wafers • Cu diffuses in Si at

room temp

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Photo: Fraunhofer-ISE

UNSW

Silicon Heterojunction

Wide bandgaps such as aSi

give a junction as well as surface passivation.

Heterojunction reduces recombination, enabling high Voc

Low current due to absorption in the top a-Si and transparent conducting oxide (TCO)

Low fill factor

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Depletion Region

Surface Inversion

∆EV

Novel Device Design

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IBC solar cell has junction and all the contacts on the rear of the cell High JSC, low Rseries

Ease of module manufacture

Visually appealing

Multiple process steps

http://www.sunpowercorp.com

Manufacturability

• Critical to any new process technology is throughput.

• Many producers are above or near 1 GWp = 500 million wafers/year = 15 wafers/sec

• All inline processing vs batch processing • Solar cells are more like CDs than ICs • Module costs are increasingly important so need to

think how cell are to be encapsulated. Rear contacts are very attractive.

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Ideal Bulk Silicon Solar Cell

• 10 – 100 um thickness • N-type to tolerate impurities and low cost

feedstock • Surfaces are critical recombination sites • Low temperature processing for junctions and

metallization • Rear contact for ease of metallization and

incorporation in module • Efficiency ~ 25 % for reduced BOS

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Acknowledgements

Much of this material was taken from: • www.pveducation.org • QESST engineering research center kickoff

meeting • University PV courses especially

Honsberg (ASU) and Buonassisi (MIT)

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Advanced Silicon at Solar Power Lab.

• Nanostructures allow new physical mechanisms, which can be used to achieve solar cells with higher efficiency or new functionality

• Goal is to transition nanostructures to existing technologies

Conclusions

• Leverage points for lowering the cost of PV electricity

• 1) Increase efficiency to lower BOS • 2) lower direct cost with earth abundant

materials. • 3) increase scalability. • Decrease of waste in productions line.

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