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Dr. Paul D. Warren, NSG

European Technical Centre - Lancashire, U.K.

Industrial perspectives on large-area

TCOs

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Outline

(1) Transparent conductors – all around us!

(2) Basic TCO characteristics

(3) Large-area production – CVD and PVD

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TCOs for windows

Wind

Heat Loss

20 W m-2

Thermal

Radiation

@~10mm

Conduction

Radiation

‘U’ value ~>1W/m2/K

(Compare ~6W/m2/K for single glazing)

TCOs for thin-film PV systems

Front contact - SnO2:F

Absorber

Glass

Back contact - metal

CdTe

Sheet resistance ~10-15 ohms/sq

Roughness not wanted

hn

hn

a-Si

Sheet resistance ~7-10 ohms/sq

Roughness necessary for light

scattering

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Other applications for TCOs

Frost-free – so you can

still see the product

Heat-mirror for oven

Control of em signals

in and out of buildings

Touch-screens

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Outline

(1) Transparent conductors – all around us!

(2) Basic TCO characteristics

(3) Large-area production – CVD and PVD

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TCO characteristics - transmission

At low l – low T, high A - controlled by band-gap.

At high l – low T, high R – controlled by n (Steepness of curve controlled

by m)

Rather unusual to have conductivity and transparency in the same material

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TCO characteristics

ITO – ‘best’, FTO – cheapest, ZnO - intermediate

‘Best’ is defined in terms of a figure of merit – various can be chosen but

all usually involve just two properties – an optical property (absorption,

transmission, weighted transmission etc) and an electrical property

(resistivity, conductivity).

Sheet resistance 10 ohms/square, 80% visible transmission (inc. substrate)

Ideal TCO – low carrier concentration (high plasma wavelength) coupled

with high carrier mobility.

From an industrial perspective – ease of manufacture, durability, cost

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TCO characteristics - morphology

• Si-based systems are poor absorbers in red/NIR – so light trapping schemes necessary.

• Easiest way is to use rough interfaces – layers grown on rough TCO

TCO for a-Sieg NSG TECTM A7

TCO for CdTeeg NSG TECTM C15

• CdTe system – no need for light trapping schemes as very good absorber at higher wavelengths.

• Interfaces can be smooth –so use smooth TCO.

10mm

0.5mm

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Outline

(1) Transparent conductors – all around us!

(2) Basic TCO characteristics

(3) Large-area production – CVD and PVD

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Float glass manufacture - the process

Batch in

Furnace, 1600°C

Tin bath, 1100°C

Lehr, 600°CWarehouse,

20°C

• ‘On-line’ TCO coating is done towards the end of the tin bath.

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On-line coatingsCVD in tin bath

Silane SiH4 Nitrogen N2

Cross section of coating head

Surface of moving glass ribbon 600oC

Reflective silicon coatingSilicon

Waste gases for

treatment

Si+H2

Hydrogen

• What you use the glass for depends on the material – mirrors as above, TEC glass with tin oxide, Pilkington Activ self-cleaning glass with titania

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FTO characteristics – TEC glass

Glass

Buffer layer – reduce optical mismatch with subsequent layers, alter

interfacial chemistry/crystallography/band structure.

SnO2:F (300nm-500nm)

SiO2 (25nm)

SnO2 (25nm)

Buffer layer

Growth layers, Anti-reflection

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Best transparent conductor - Ag

Thin (~10nm) layer of sputtered Ag grown on sputtered ZnO.

Much lower absorption than other highly conducting metals

Surface roughness is negligible – important for thin functional devices

Absorbance of Glass/10nm Metal

0

0.1

0.2

0.3

0.4

0.5

0.6

0.7

0.8

0.9

1

360 410 460 510 560 610 660 710

Wavelength nm

Ab

so

rban

ce

Cu

Au

Al

Ag

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Off-line coatingsMagnetron sputtering

• Large-scale sputter coating of glass for architectural applications

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Summary

Large-area deposition of TCO is now an important market for flat-glass

companies – windows for thermal efficiency and a whole host of up-and-

coming opto-electronic applications.

Optical and electrical properties important – but a large number of other

considerations need to be taken into account.

Significant R&D effort going into this area worldwide – and a wide range

of new materials being evaluated.

Event/meeting 17Date