Fraunhofer Technology Center Semiconductor Materials Institute for Experimental Physics TU...

FraunhoferTechnology CenterSemiconductor Materials

Institute for Experimental PhysicsTU Bergakademie Freiberg

Industrial aspects of silicon material research for photovoltaic

applications

Hans Joachim Möller



General development of photovoltaics

Crystalline silicon technology

Thin film technologies

Feedstock ressources

Summary

Outline



Market share of different solar cell technologies

EU - Prognosis for future development

2010 c - Si 80 - 90% multi, Cz, ribbons2020 c - Si 50% multi, ribbons, thin films

PV - market based on the silicon technology



System cost 1,0 - 1.5 €/WpModule cost 0.5 - 1.0 €/WpSystem lifetime 20 - 30 yearsSystem efficiency 15% - 30%Electricity cost 0.06 - 0.1 €/kWh

Source: Study of M. Green 2002

Goals of future developments

Grid parity for < 0.1 €/kWhPeak current parity for 0.3 - 0.5 €/kWh

PV goals for 2020 - 2030

3,50 €/Wp

1,00 €/Wp

0,50 €/Wp

2,00 €/Wp

0,20 €/Wp

c-Si

Thin film

PV - system cost depend on efficiency and cost per area



Cost and efficiency per area for different technologies

Source: I. Schwirtlich, Schott Solar 2006

100 - 200 200 - 500 500 - 1000Cost per m2

Cost per Wp

New concepts

Nanocryst. Dye

Efficiency

Technologies

EFGCIS

Newconcepts

Thin films







Summary



Formation of defects during crystal growth

DislocationsMelt

precipitation

Transition elements

Solid impurity precipitation

Carbon

Oxygen

New Donors

ThermalDonors

DislocationsOxygenNitrogenCarbonBoronMetals

Defect interactions



Internal quantum efficiency (IQE) - topogram Dislocation density - topogram

Correlation between dislocations and lifetime

Analysis of dislocation activity in solar cells requires a modified Donolato model



Theoretical description with modified Donolato's theory

Experimental results yield similar recombination strengths compared to wafers but higher volume - diffusion lengths L



Cost reduction through more efficient use of silicon



Cost distribution

Wafer

Solar cell

Ingot crystal



spec. Si-consumption [g/Wp] 8.2 7.5 6.5

Development of wafer thickness and silicon consumption

Wafers below 100 µm thickness become very flexible and fragile

60 µm wafer



Radial/median cracks

Lateral cracks

Subsurface microcracks from multi-wire sawing

SEM images of wafer cross sections

1 µm

3 µm



Etch removal [µm] 0 [MPa]

as-sawn 83etched 2 6.2 407etched 3 7.7 429etched 4 7.9 429etched 8 15.3 632etched 9 16.9 562

Surface damage by microcracks determines fracture toughness

Weibull distribution and fracture strength

Biaxial bending test







Summary



front cover

foil

substrate

active layera-Si or CIS

back side cover

foil

superstrate

active layera-Si or CdTe

Substrate: glass, metal, polymerFoil: EVA or PVBFront cover: glass, polmer, varnish

Superstrate: glassFoil: EVA or PVBBack side cover: glass, polmer, metal

3 mm

0.5 - 1 mm0.5 - 3 µm

3 mm 3 mm

0.5 - 1 mm0.5 - 3 µm

3 mm

Substrate Superstrate

Principle of thin film cells



Thin film solar cells

Flexible CIS - cell

Today‘s thin film materials

Cadmium telluride CdTeKupfer-Indium/Gallium-Diselenide CIGSAmorphous Silicon a-Si:H

Application



Status thin film efficiencies

Module efficiency only about 60 - 80% of cell efficiency



Analyses of the cost reduction potential

From 1.5 to 1.0 €/kWp

Material and energyYield increaseReduction of glass fracture

EfficiencyFrom 8% to 12%

Production optimization

From 1.0 to 0.5 €/kWp

cheaper TCO and foilsnew substrate glassNew absorber material

EfficiencyFrom 20% to 40%

Production optimization

Normal cost reduction and efficiency increases are not suffcient

to reach the goals of the EU roadmap

Cost per Wp converges to fixed cost

Material and energy cost cannot be reduced arbitrarily

Efficiency has to be increased disproportionately



New generation thin film cells

Losses in the solar spectrum

More efficient use of the spectrum by multi-junction solar cells with different band gap

Tandem-junction efficiency (theoretical)> 45% (Si: 33%)

Triple-junction cell > 51% (WR 41,1%)

Four junction cell> 54%

Thin film technologies allow flexible formation of multi-junction cells







Summary



Comparison of material consumption for c-Si and thin films

Wafer technology Si - wafer thickness 150 µm

(mono- or multi-Si) Wafer size 0.01 m2 to 0.04 m2

3 kg silicon for 1 kWp solar power

Thin films deposition on substrate

(a-Si/µ-Si,tf-cSi,CdTe, CIS) 0.3 - 5 µm layer thickness

Substrate size 0.5 m2 to 1.43 m2

0.03 - 0.2 kg material for 1 kWp solar power

Cost advantage for electronic metals only, if prices are below 1 000 Euro/kg



SiliconMassive expansion of the crystalline technology requires separate feedstock

supply. Long term supply secured

CIGS, CdTe und GaInAs/GaInP/Ge

Feedstock shortage for InProblem with toxicity of Cd and As-compounds

Prices for electronic materialsare high because of small markets

Development of new solar cell concepts necessary

Technological development of the thin film technology in industrial scale still difficult

Summary



Thank you for your attention

Fraunhofer Technology Center Semiconductor Materials Institute for Experimental Physics TU...

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