Junction Evolution During Fabrication 

of CdS/CdTe Thin–film PV Solar Cells

Timothy A. Gessert

Principal Scientist, Group Manager 

CdTe Thin Film PV Materials and Devices Research

National Center for Photovoltaics, NREL 

tim_gessert@nrel.gov, 303-384-6451

Presented at the China New Energy International Forum and Fair 2010

28-30 September 2010, Chengdu, China

This work was supported by the U.S. Department of Energy under Contract No. DE-AC36-08GO28308 with the National Renewable Energy Laboratory.

NREL/PR-520-49387

NREL is a national laboratory of the U.S. Department of Energy, Office of Energy Efficiency and Renewable Energy, operated by the Alliance for Sustainable Energy, LLC.
Tim Gessert

General PV Energy Talk, Date

Disclaimer and Government License

The Alliance for Sustainable Energy, LLC (Alliance), is the manager and operator of the National Renewable Energy 

Laboratory (NREL). Employees of the Alliance, under Contract No. DE-AC36-08GO28308 with the U.S. Dept. of 

Energy, have authored this work. The United States Government retains and the publisher, by accepting the article for 

publication, acknowledges that the United States Government retains a non-exclusive, paid-up, irrevocable, 

worldwide license to publish or reproduce the published form of this work, or allow others to do so, for United States 

Government purposes.
Tim Gessert

General PV Energy Talk, Date

Solar Land Area Requirements

For 2004 US Electricity Requirements

Sources: K. Zweibel, “PV Past the Tipping Point” (www.nrel.gov/ncpv/thin_film)

PVWATTS Calculator, Version 1 (http://rredc.nrel.gov/solar/codes_algs/PVWATTS/)

Photovoltaics

Phoenix, AZ

2100 

KWH/m2

Boulder, CO

1900 KWH/m2

Madison, WI

1600 KWH/m2

Caribou, ME

1550 KWH/m2

Nebraska

~1800 KWH/m2

Note:

Munich, Germany

~1100 KWH/m2
Tim Gessert

General PV Energy Talk, Date

Background

Example - Size of PV Market for Commercial Glass

Source: Brent Nelson, NREL 

Area of PV Installed

1.00E+05

1.00E+06

1.00E+07

1.00E+08

1.00E+09

1.00E+10

1.00E+11

1.00E+12

1.00E+13

1980 2000 2020 2040 2060 2080 2100

Year

20 TW-yr 10% efficient

Actual

Area to Install

USA Roofing Production

Roofs in USA

World Glass Production

World Paved Roads

PV Use of Glass

Could Exceed 2005

World Glass Production

By ~2020!
Tim Gessert

General PV Energy Talk, Date

Some CIGS-alloy, CdTe, & a-Si Laboratory Cell

NREL-Confirmed Record Efficiencies

Area

(cm2)

CIGSe

CIGSe

CIGS

CIAS

CdTe

CdTe

a-Si

Area

(cm2)

0.410

0.402

0.409

–

1.03

–

-

VOC

(V)

0.697

0.67

0.83

0.621

0.845

0.814

-

JSC

(mA/cm2)

35.1

35.1

20.9

36.0

25.9

23.56

-

FF

(%)

79.52

78.78

69.13

75.50

75.51

73.25

73.25

Efficiency

(%)

20.0

18.5

12.0

16.9

16.7

14.0

12.1

Comments

CIGSe/CdS/Cell

NREL, 3-stage process

CIGSe/ZnS (O,OH)

NREL, Nakada et al

Cu(In,Ga)S2/CdS

Dhere, FSEC

Cu(In,Al)Se2/CdS

IEC, Eg = 1.15eV

CTO/ZTO/CdS/CdTe

NREL, CSS

ZnO/CdS/CdTe/Metal

U. of Toledo, sputtered

United Solar, Stabilized 

Efficiency

Sources: Updated from R. Noufi and K. Zweibel, Proc. 4th WCPEC, Waikola, Hawaii, 5/2006, Photon International, October 2004
Tim Gessert

General PV Energy Talk, Date

Polycrystalline Thin Film PV Modules

(Standard conditions, Aperture-area, *NREL Confirmed)

Ranked by Power

Sources: Updated From R. Noufi and K. Zweibel, Proc. 4th WCPEC, Waikola, Hawaii, 5/2006, Photon International October 2004, 

Company Device Aperture

(cm2)

Efficiency*(%) Power

(W)

Date

Miasole CIGS 9762 13.6-13.8 - 4/10

Global Solar CIGS 8390 10.2* 88.9 8/02

First Solar CdTe 6623 12.6* 84.4 8/08

Wurth Solar CIGS 6500 13.0 84.6 6/04

United Solar a-Si 4519 7.9* 35.7 6/97

Shell Solar CIGSS 3626 12.8* 46.5 3/03

Showa Shell CIGS 3459 13.5 - 8/02
Tim Gessert

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Goal 

Increase Average Production Module Efficiency

0

5

10

15

20

25

CIGS CdTe a-Si

Efficiency (%)

Technology

Average Module

Champion Large Module

Champion Cell

Approx Practical Max

DOE CdTe Roadmap Goal 

15% Average Production 

Efficiency
Tim Gessert

General PV Energy Talk, Date

Demonstrated

Efficiency

Perceived 

Production 

Advantage

Perceived 

Materials 

Abundance/

Low Toxicity

a-Si Strength

CdTe Strength

CIS Strength

Present Strengths of Each Thin-film PV Technology

Thoughts:

Each technology has different advantages

Its not clear which (if any) advantage will yield a long-term product advantage

This situation could remain true for many years!
Tim Gessert

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How Do These Things Work?

PV Device Operation 101

Hole

Atoms

Electron

Semiconductor Material with “Band-gap” energy

of 0.8-2.0 electron volts

Carrier

“Lifetime”
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PV Device Operation 101

N Type

P-Type

Hole

Atoms

Electron

Built-In

Electric

Field
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PV Device Operation 101

N

P

Hole

Atoms

Electron

Atoms

Built-In

Electric

Field
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PV Device Operation 101

N

P

Hole

Atoms

Electron

Atoms

Front Electrical Contact (Transparent Conducting Oxide-TCO)

Back Electrical Contact (Metal)

(-) (+)

Built-In

Electric

Field
Tim Gessert

General PV Energy Talk, Date

PV Device Operation 101

N

P

Hole

Atoms

Electron

Atoms

Front Electrical Contact (Transparent Conducting Oxide-TCO)

Back Electrical Contact (Metal)

(-) (+)

Built-In

Electric

Field

Glass (CdTe Device)
Tim Gessert

General PV Energy Talk, Date

PV Device Operation 101

N

P

Hole

Atoms

Electron

Atoms

Front Electrical Contact (Transparent Conducting Oxide-TCO)

Back Electrical Contact (Metal)

(-) (+)

Built-In

Electric

Field

Glass (CIGS Device)
Tim Gessert

General PV Energy Talk, Date

Thin Film CdTe Solar Cells

Glass

CdS (600-2000 Å)

CdTe

2-8 μm

Cu-Containing 

Interface

(Paste, Pretreatments,

ZnTe:Cu) + Metals

η = 16.7% 

(NREL)

TCO
Tim Gessert

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How do CdTe Thin-Film PV Junctions Form??

What this really means:

How do we get the electric field to form in the device at the correct location and strength -

- without destroying the minority-carrier lifetime? 

Motivation

40

20

0

-20

Current Density (mA/cm 2

)

-0.2 0.0 0.2 0.4 0.6 0.8 1.0

Voltage (V)

 612A-1, 0Å

 614A-2, 500Å

 615A-4, 1000Å'

 616A-1L, 2000Å

 613A-2, 3000Å

 611A-2, 4200Å

 617A-3, 5000Å
Tim Gessert

General PV Energy Talk, Date

Lower

Temp.

Downstream

Processes

The ZnTe:Cu/Ti Contact Process

(All-Dry, High-Temperature [~300°C]) 

ZnTe:Cu/Ti Contact Process

Higher 

Temp.

Upstream

Processes

Photolithography

Heating

CdTe CdTe CdTe CdTe

ZnTe:Cu ZnTe:Cu

Ti

Glass Glass Glass Glass

Ion-Beam 

 Milling

 r.f. Sputter 

 Deposition 

 (ZnTe:Cu)

d.c. Sputter 

Deposition 

 (Ti)

CdTe

ZnTe:Cu

Ti

Glass

Some Research Advantages

Precise control of junction performance

High device stability

Can achieve very low Cu incorporation

Easy to make large, identical sample sets
Tim Gessert

General PV Energy Talk, Date

Some Insight from Past Studies

30

20

10

0

-10Current Density (mA cm 2

)

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

 ~320°C (32V, Optimum)

 ~340°C (34V)

 ~360°C (36V)

Cu Diffusion Less Than Optimum Cu Diffusion Greater Than Optimum

30

20

10

0

-10Current 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)

40

30

20

10

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

Effect of

Contacting 

Temperature

Effect of

ZnTe:Cu

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

Comparison of Profiling C-V Analysis 

10 13

10 14

10 15

10 16

10 17

10 18

Na-N d (cm

-3

)

0 1 2 3 4 Depletion Width (um)

 ZnTe:Cu Thickness

Contact Temperature 360°C

 100 KHz, 10 mV

 Sweep -8.0 V - +0.6 Volts

 1 μm

 0.5 μm

 0.2 μm

 0.1 μm

0 V Bias

10,000 Å 

0 V Bias 

5,000 Å

0 V Bias 

2,000 Å

0 V Bias 

1,000 Å

1013

1014

1015

1016

1017

1018

N

A-N

D (cm-3)

1 2 3 4

Depletion Width (μm)

 ZnTe:Cu Depositon Temperature

 240C

 300C

 320C

 340C

 360C

340C 320C

240C

360C

300C

Vary Contact Temperature Vary ZnTe:Cu Thickness

Some Insight from Past Studies

T.A. Gessert, S. Smith, T. Moriatry, M. Young, S. Asher, S. Johnston, A. Duda, C. DeHart

Proc. 31st. IEEE Photovoltaics Specialists Conf. (IEEE, Piscataway, NJ, 2005), pp. 291-294

T.A. Gessert, S. Asher, S. Johnston, A. Duda, M.R. Young, T. Moriarty, 

Proc. 4th World Conf. on PV Energy Convr. (IEEE, Piscataway, NJ, 2006) pp. 432-435
Tim Gessert

General PV Energy Talk, Date

10

17

10

18

10

19

10

20

10

21

Cu Concentration (cm-3) 

-1 0 1 2 3 4

Depth (microns)

ZnTe:Cu CdTe CdS

 1.0 μm, 360C

 0.5 μm, 360C

 0.2 μm, 360C

 Previous, 1.0 μm, 360

 Previous, 1.0 μm, 320

1017

1018

1019

1020

1021

Cu Concentration (cm-3)

0 1 2 3 4 5

Depth (μm)

ZnTe:Cu CdTe CdS

 240C

 300C

 320C

 340C

 360C

Vary Contact Temperature

Excessive Cu

Optimum Cu

Insufficient Cu

Vary Contact Thickness

Excessive Cu

??

Optimum Cu

Insufficient Cu

Some Insight from Past Studies

High-Resolution 

Compositional

Depth-Profiling 

Secondary Ion

Mass Spectrometry

Analysis

(SIMS)

T.A. Gessert, W.K. Metzger, P. Dippo, S.E. Asher, R.G. Dhere, M.R. Young, 

Thin Solid Films, 517, 2370-2373 (2009)
Light and Voltage-Dependent Quantum Efficiency

100

80

60

40

20

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 Bias QE Confirms

Photoconductive CdS

Only For “Excessive” Cu Devices

120

100

8 0

6 0

4 0

2 0

0

Apparent Quantum Efficienty (%)

400 500 600 700 800 9 0

W a v e le n g th (n m )

U n c o rre c te d f o r A ct u a l J s c

Red-Light Bias AQE

Tim Gessert

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T.A. Gessert, W.K. Metzger, S.E. Asher, M.R. Young, S. Johnston, R. Dhere, T. Moriarty, A. Duda

Proc. 33rd Photovoltaics Specialists Conf. (IEEE, Piscataway, NJ, 2008) Manuscript No. 14.
Tim Gessert

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10 13

10 14

10 15

10 16

10 17

10 18

NA (cm -3

)

0 1 2 3 4

Depletion Width (μm)

1e14

1e11

2e13

1e17

4e13

1e16

8e13

1e15

1e14

1e14

1e14

1e13

 SCAPS Simulation

 100 kHz, Sweep -0.7V to +0.1 Volt

 NA Nd NA ND

 CC46_2e13_1e17 CC46_1e14_1e14

 CC46_4e13_1e16 CC46_1e14_1e13

 CC46_8e13_1e15 CC46_1e14_1e11

CdTe NA

CdS ND

4th World Conf. PV 

SCAPS1D Simulation Energy Conversion, pp. 432-435

30

20

10

0

-10Current Density (mA cm 2

)

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

 ~320°C (32V, Optimum)

 ~340°C (34V)

 ~360°C (36V)

40

30

20

10

0

-10

-20 Current Density (mA cm -2 )

0.0 0.2 0.4 0.6 0.8 1.0

Voltage (Volts)

 SCAPS Modeled LIV/DIV

 CdTe N A_ CdS ND

 LIV_2e13_1e17, 844 mV, 22.9, 83.1%

 LIV_1e14_1e14, 893 mV, 24.0, 72.4%

 LIV_1e14_1e12, 894 mV, 23.9, 64.7%

 LIV_1e14_1e11, 887 mV, 23.8, 60.8%

 DIV46_1e14_1e14 

 DIV46_1e14_1e12

 DIV46_1e14_1e11

1013

1014

1015

1016

1017

1018

N

A-ND (cm-3)

1 2 3 4

Depletion Width (μm)

 ZnTe:Cu Depositon Temperature

 240C

 300C

 320C

 340C

 360C

340C 320C

240C

360C

300C

T.A. Gessert, S. Asher, S. Johnston, A. Duda, M.R. Young, T. Moriarty, 

Proc. 4th World Conf. on PV Energy Convr. (IEEE, Piscataway, NJ, 2006) pp. 432-435
Tim Gessert

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CdS CdTe

Ti

Ti

ZnTe:Cu

CdS CdTe

Ti

Ti

ZnTe:Cu

Constant Temperature ~300°C

Vary ZnTe:Cu Thickness

Constant Thickness ~0.5 μm

Vary ZnTe:Cu Temperature

Cu Cu

Some Insight from Past Studies

But...Assumes Minority Carrier Lifetime Does Not Change

T.A. Gessert, M.J. Romero, S. Johnston, B. Keys, and P. Dippo

Proc. 29th IEEE Photovoltaics Specialists Conf. (Piscataway, NJ, 2002) pp. 535-538
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General PV Energy Talk, Date

Minority Carrier Lifetime

Previous TRPL studies indicate 

minority carrier lifetime decreases with Cu

Metzger, Albin, Levi, Sheldon, Li, 

Keyes, Ahrenkeil, 

JAP 94(6) (2003)

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

Metzger, Demtsu, Wei, Noufi, 

TSF, 515, 5798 (2007)

Demtsu, Albin, Sites, 

Metzger, Duda, 

TSF, 516 p. 2251 (2008)
Tim Gessert

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Minority Carrier Lifetime

Question

30

20

10

0

-10Current 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)

Glass

Glass

CdTe

If both carrier lifetime and space 

charge width decreases with Cu 

incorporation, why does voltagedependant collection decrease?
Minority Carrier Lifetime

Time Resolved Photoluinescence (TRPL)

Glass CdTe

650 nm Excitation

Two Power Levels

250 μW and 2500 μW

820 nm PL Measurement

Room Temperature

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Minority Carrier Lifetime

TRPL Study of Cu Diffusion 

and Contact Temperature

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 20 30 40 50n(s)

Time

 UC700 (25°C)

 UC699 (200°C)

 UC697 (240°C)

 UC694 (280°C)

 UC702 (300°C)

 UC696 (320°C)

 UC695 (340°C)

 UC698 (360°C)

250 μW Power 2500 μW Power
Minority Carrier Lifetime

TRPL Study of Cu Diffusion 

and Contact Temperature (2500 μW Power)

1

10

100

1000

PL counts

0 10 20 30 40 50n(s)

Time

RT, 200°C, & 240°C

 700-900 ps

 Insufficient Cu

280°C & 300°C

1000-1200 ps

 Optimum Cu

340°C & 360°C

 300-350 ps

 Excessiv e Cu

Tim Gessert

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T.A. Gessert, W.K. Metzger, P. Dippo, S.E. Asher, R.G. Dhere, M.R. Young, 

Thin Solid Films, 517, 2370-2373 (2009)
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Minority Carrier Lifetime

Time Resolved Photoluinescence (TRPL)

10000

8000

6000

4000

2000

0

Minority Carrier Lifetime (ps)

0 50 100 150 200 250 300 350

Approximate Substrate Temperature During Contact (°C)

Insufficient Cu Excessive Cu

Optimum Cu

 t 1 2.5 mW

 t 1 0.25 mW

 t 2 2.5 mW

 t 2 0.25 mW

T.A. Gessert, W.K. Metzger, P. Dippo, S.E. Asher, R.G. Dhere, M.R. Young, 

Thin Solid Films, 517, 2370-2373 (2009)
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Minority Carrier Lifetime

Low-Temperature Photoluinescence (LTPL)

Increasing Contact 

Temperature 

produces peak that 

that has been 

associated with a 

defect complex

Cui

-OTe , and an 

unidentified peak at 

~1.53 eV

Reference for Cui-OTe

Corwine, Sites, Gessert, 

Metzger, and Duda, Appl. 

Phys Lett. 86 (1) 221909 

(2005)

10

3

10

4

10

5

10

6

PL (Arb. Units)

1.35 1.40 1.45 1.50 1.55 1.60

Energy (eV)

1.463 eV

~1.53 eV

4.25K, 632.8 nm, 1 mW

 PL from Glass Side

 UC700 (25°C)

 UC697 (240°C)

 UC694 (280°C)

 UC696 (320°C)

 UC698 (360°C)

Cui-O Te

T.A. Gessert, W.K. Metzger, P. Dippo, S.E. Asher, R.G. Dhere, M.R. Young, 

Thin Solid Films, 517, 2370-2373 (2009)
Tim Gessert

General PV Energy Talk, Date

Minority Carrier Lifetime

Time Resolved Photoluinescence (TRPL)

1500

1000

500

0

Minority Carrier Lifetime (ps)

0 2000 4000 6000 8000 10000

ZnTe:Cu Thickness (Å)

 NREL Tau 1 (2.5 mW, 360°C)

 CSU Tau 1 (2.5 mW, 320°C)

 VTD Tau 1 (2.5 mW, 320°C)

 VTD (5000 Å, 25°C)

 Wu (No Cu)

 Demtsu (No Cu)

T.A. Gessert, W.K. Metzger, P. Dippo, S.E. Asher, R.G. Dhere, M.R. Young, 

Thin Solid Films, 517, 2370-2373 (2009)
Tim Gessert

General PV Energy Talk, Date

What Does it all Mean??

1500

1000

500

0

Minority Carrier Lifetime (ps)

0 2000 4000 6000 8000 10000

ZnTe:Cu Thickness (Å)

 NREL Tau 1 (2.5 mW, 360°C)

 CSU Tau 1 (2.5 mW, 320°C)

 VTD Tau 1 (2.5 mW, 320°C)

 VTD (5000 Å, 25°C)

 Wu (No Cu)

 Demtsu (No Cu)

40

30

20

10

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

30

20

10

0

-10Current 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)

30

20

10

0

-10Current Density (mA cm 2

)

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

 ~320°C (32V, Optimum)

 ~340°C (34V)

 ~360°C (36V)

Constant Cu, Vary Temp Constant Temp, Vary Cu

2000

1500

1000

500

0

Minority Carrier Lifetime (ps)

0 50 100 150 200 250 300 350

Approximate Substrate Temperature During Contact (°C)

Insufficient Cu Excessive Cu

Optimum Cu

 t 1 2.5 mW
Tim Gessert

General PV Energy Talk, Date

Conclusions

CdTe PV Devices Conclusions

•Net acceptor level increases during contacting, control produces optimum Wdepletion

•Minority carrier lifetime can decrease and/or increase during contacting

•Primarily depends on temperature during Cu diffusion

•Also depends on amount of Cu diffused

•Reason(s) remain uncertain

•For the ZnTe:Cu/Ti contact, the longest lifetimes occur at contact temperature of 

~280-320°C

•This may explain various contact-process functionalities!

•Strategy for optimization of Cu-containing contacts:

•Understand effect of temperature on lifetime

•Adjust Cu to optimize depletion width and Voc (and limit Cu diffusion into CdS)

•Future work – maintain lifetime in quazi-neutral region during contacting!
Tim Gessert

General PV Energy Talk, Date

Thank You!