Effects of Cu Diffusion From ZnT

Contacts on Carrier Lifetime of C

Thin Film Solar Cells

T. A. Gessert, W.K. Metzger, S.E. Asher

S. Johnston, R.G. Dhere, and A. 

National Center for Photovoltaic

National Renewable Energy Labora

This work was supported by the U.S. Departme

under Contract No. DE-AC36-99GO10

with the National Renewable Energy Labo

NREL/PR-520-43314Presented at the 33rd IEEE Photovoltaic Specialist Conference held May 11-16, 2008 
Tim Gessert, 4WCPEC, 5/11/06

The ZnTe:Cu/Ti Contact Proce

(All-Dry, High-Temperature [~300°C

ZnTe:C

High Temp.

Vacuum

CdTe/CdS +

Vapor CdCl2

Photolithography

Heating

CdTe CdTe CdTe 

ZnTe:Cu 

Glass Glass Glass 

Ion-Beam  Milling

 r.f. Sputt Depositi (ZnTe:C

CdTe

ZnTe:Cu

Ti

Glass

Some Research A

Precise control of junc

High device s

Can achieve very low 

Easy to make large, ide
Tim Gessert, 4WCPEC, 5/11/06

CdS CdTe

CdS CdTe

Ti

Ti

ZnTe:Cu

Constant Temperature ~300°C

Vary ZnTe:Cu Thickness

Constant T

Vary ZnTe

Cu 
Tim Gessert, 4WCPEC, 5/11/06

Some Insig

302010

0

-10

Current Density (mA cm

2

)

-0.2 0.0 0.2 0.4 0.

Voltage (Volts)

 ~320°C (32V, Opti ~340°C (34V) ~360°C (36V)

Cu Diffusion Less Than Optimum Cu Diffusion Greater T

302010

0

-10

Current Density (mA cm

2

)

-0.2 0.0 0.2 0.4 0.6 0.8 1.0 Voltage (Volts)

 ~240°C (24V) ~280°C (28V) ~320°C ( 32V, Optimum)

40302010

0

-10-20

Current Density (mA cm-2)

0.0 0.2 0.4 0.6 0.8 1.0

Voltage (Volts)

ZnTe:Cu Thickness

Devices Contacted at 360°C

 1 μm 0.5 μm 0.2 μm 0.1 μm 0.04 μm

Ef

Zn

Thi
Tim Gessert, 4WCPEC, 5/11/06

(100 kHz, 10 mV, -8.0 to +0.6 Sweep)

Capacita

Comparison of Previous and Present C-V 

10 1310 1410 1510 1610 1710 18

N

a-Nd (cm -3

)

0 1 

 ZnTe:Cu ThiContact Temperat 100 KHz, 10  Sweep -8.0 V - +

 1 μm0.5 μm 0.2 μ 0.1 μ

0 V Bias10,000 Å 

101310141015101610171018

N

A-ND (cm-3

)

1 2 3 4

Depletion Width (μm)

 ZnTe:Cu Depositon Temperature

 240C 300C 320C 340C 360C

340C 320C

240C

360C

300C

Vary Contact Temperature Vary Zn
Tim Gessert, 4WCPEC, 5/11/06

Co

10

17

10181019102010

21

Cu Concentration (cm-3) 

-1 0 1 2 3 4

Depth (microns)

ZnTe:Cu CdTe CdS

 1.0 μm, 360C 0.5 μm, 360C 0.2 μm, 360C Previous, 1.0 μm, 360 Previous, 1.0 μm, 320

10171018101910201021

Cu Concentration (cm-3

)

0 1 2 3 4 5

Depth (μm)

ZnTe:Cu CdTe CdS

 240C 300C 320C 340C 360C

Contact TemperExcessive CuOptimum CuInsufficient Cu

Contact Tempe

Excessive Cu??Optimum CuInsufficient Cu
Tim Gessert, 4WCPEC, 5/11/06

Light and Voltage-Dependen

100

80604020

0

Quantum Efficiency (%)

400 500 600 700 800 900

Wavelength (nm)

 24 Volt Heater

 Insufficient Cu

 32 Volt Heater

 Optimum Cu

 36 Volt Heater

 Excessive Cu

1-Sun Bias+0.4 Volts

QE Comparison Suggests

Narrowing Junction 

for Excessive Cu Devices

Red-Light 

Photoc

Only For “Ex

120100

80604020

0

Apparent Quantum Efficienty (%)

400 500 
Tim Gessert, 4WCPEC, 5/11/06

Comparison of C-V Measuremen

and SCAPS-1D C-V Simulation

10 1310 1410 1510 1610 1710 18

N

a-Nd (cm -3

)

0 1 2 3 4

Depletion Width (um)

 ZnTe:Cu ThicknessContact Temperature 360°C 100 KHz, 10 mV Sweep -8.0 V - +0.6 Volts

 1 μm0.5 μm 0.2 μm 0.1 μm

0 V Bias10,000 Å 

0 V Bias 5,000 Å

0 V Bias 2,000 Å

0 V Bias 1,000 Å

10

13

10

14

10

15

10

16

10

17

10

18

N

A (cm -3

)

0 1 

1e141e11

1e141e13

S

100 kHz

 NA 

 CC46_2e13 CC46_4e13 CC46_8e13

Gessert, et. al, 4th World Conf. PV Energy Conversion, pp.4
Tim Gessert, 4WCPEC, 5/11/06

Comparison of LIV/DIV Measure

and SCAPS-1D Simulation

40302010

0

-10-20

Current Density (mA cm -2)

0.0 0.2 

 SCAPS 

CdTe N

 LIV_2e13 LIV_1e14 LIV_1e14 LIV_1e14 DIV46_1e DIV46_1e DIV46_1e

40302010

0

-10-20

Current Density (mA cm

-2)

0.0 0.2 0.4 0.6 0.8 1.0

Voltage (Volts)

ZnTe:Cu Thickness

Devices Contacted at 360°C

 1 μm 0.5 μm 0.2 μm 0.1 μm 0.04 μm

But...

Modeling Assumes Minority Carrier Lifetime D

Gessert, et. al, 4th World Conf. PV Energy Conversion, pp.4
Tim Gessert, 4WCPEC, 5/11/06

Min

Previous TRPL studies indic

minority carrier lifetime decrease

Metzger, Albin, Levi, Sheldon, Li, 

Keyes, Ahrenkeil, 

JAP 94(6) (2003)

QuickTimeTM and a

TIFF (LZW) decompressor

are needed to see this picture.

Wu, Zhou, Duda, Yan, Teeter, Asher, 

Metzger, Demtsu, Wei, Noufi, 

TSF, 515, 5798 (2007)

a
Tim Gessert, 4WCPEC, 5/11/06

Min

Quandary

302010

0

-10

Current Density (mA cm

2

)

-0.2 0.0 0.2 0.4 0.6 0.8 1.0 Voltage (Volts)

 ~240°C (24V) ~280°C (28V) ~320°C ( 32V, Optimum)

GlassGlass

If both carrier lifetime and space charge width decreases with Cu incorporation, why does voltagedependant collection decrease?
Tim Gessert, 4WCPEC, 5/11/06

Min

Time Resolved Photoluinescence 

Glass CdT

650 nm Excitation

Two Power Levels

250 μW and 2500 μW

820 nm PL Measurement

Room Temperature
Tim Gessert, 4WCPEC, 5/11/06

Min

TRPL Study of Cu Diffusio

and Contact Temperatur

1

10

100

1000

PL counts

0 10 20 30 40 50n(s)

Time

 UV700 (25°C) UC699 (200°C) UC697 (240°C) UC694 (280°C) UC702 (300°C) UV696 (320°C) UC695 (340°C) UC698 (360°C)

1

10

100

1000

PL counts

0 10 

250 μW Power 2500 
Tim Gessert, 4WCPEC, 5/11/06

Min

TRPL Study of Cu Diffusio

and Contact Temperature (2500 μ

1

10

100

1000

PL counts

0 10 20 30 40 

Time

RT, 200°C, & 240°C 700-900 ps Insufficient Cu

280°C & 1000-120 Optimum 

340°C & 360°C 300-350 ps Excessive Cu
Tim Gessert, 4WCPEC, 5/11/06

Min

Time Resolved Photoluinescence 

10000

8000600040002000

0

Minority Carrier Lifetime (ps)

0 50 100 150 200 250 

Approximate Substrate Temperature During Contact (°

Insufficient Cu 

Opt

 t 1 2.5 mW t 1 0.25 mW t 2 2.5 mW t 2 0.25 mW
Tim Gessert, 4WCPEC, 5/11/06

Min

Time Resolved Photoluinescence 

15001000

500

0

Minority Carrier Lifetime (ps)

0 2000 4000 6000 8000 

ZnTe:Cu Thickness (Å)

 NREL Tau 1 (2.5  CSU Tau 1 (2.5 m VTD Tau 1 (2.5 m VTD (5000 Å, 25° Wu (No Cu) Demtsu (No Cu)
Tim Gessert, 4WCPEC, 5/11/06

Conclusions

•Minority carrier lifetime can decrease or increase 

•Depends on temperature during Cu diffusion•Depends on amount of Cu diffused

•For the ZnTe:Cu/Ti contact, the longest lifetimes temperature of ~280-320°C

•This may explain why other contact processes are ofthis this temperature range

•Strategy for optimization of Cu-containing conta

•Examine effect of temperature on lifetime•Adjust Cu amount of Cu in contact to maximize net 

maximize Voc, optimize depletion width, and limit Cu 
Tim Gessert, 4WCPEC, 5/11/06

Author Contact Informat

Tim Gessert

National Renewable Energy Labora

Golden, Colorado 80401

303-384-6451

tim_gessert@nrel.gov