700

720

740

760

780

800

VO

C [

mV

]

0

10

20

30

40

50

60

70

Fill F

acto

r [%

]

0

5

10

15

20

25

30

1 10 50

Contact Area [%]

JS

C [

mA

/cm

2]

No CuCu-doped

0

5

10

15

1 10 50

Contact Area [%]

Eff

icie

ncy [

%]

Jason M. Kephart1, Anna Kindvall1, Desiree Williams1, Darius Kuciauskas2, Pat Dippo2, Amit Munshi1, and W.S. Sampath1

1Department of Mechanical Engineering, Colorado State University, Fort Collins, CO USA2National Renewable Energy Laboratory, Golden, CO USA

Sputter-deposited Oxides for Interface Passivation

of CdTe Photovoltaics

Al K Zn L

Mg K SE

Interface Passivation Strategy

Oxides have shown interface passivation in c-SI,

CIGS, CZTS, and other materials systems. Sputter

deposition is favored by industry as a cost-

effective deposition method.

Oxides were screened to identify the best

passivation. Patterning of the oxide on the scale of

the diffusion length is necessary to collect carriers

through the insulator.

Oxide material screening (front passivation)

Candidate materials were chosen and deposited via RF magnetron

sputter deposition (MSD) or ion beam sputter deposition (IBSD):

• MgO (MSD)

• Mg0.35Zn0.65O (MSD)

• SiO2 (IBSD)

• TiO2 (IBSD)

• Ta2O5 (IBSD)

• Al2O3 (IBSD)

Micropatterning

• Patterning of oxide layers was performed with

photolithography

• A lift-off process was developed for materials that

would be difficult to etch on top of MZO

• A an etching process was developed for Al2O3 using

tetramethylammonium hydroxide, which can be

used for front or rear passivation

Acknowledgments

The authors would like to acknowledge James

Sites and Alex Huss for LBIC and PL assistance and

Chungho Lee and Gang Xiong for advice and TRPL.

This work was funded under the US Department of

Energy Photovoltaics R&D “Small Innovative

Projects in Solar” Program (DE-EE0007365).

Contact

Jason Kephart, Research Scientist, CSU

jason.kephart@colostate.edu

Background and Motivation

Cadmium Telluride is the lowest cost PV

technology and most widely deployed thin-film PV

technology. Further efficiency gains require

improved passivation of the bulk absorber

material, grain boundaries, and interfaces.

A highly resistive MgxZn1-xO (x = 0.23) window

layer has previously been demonstrated to replace

CdS as the CdTe front contact with 18.3%

efficiency. Its bandgap of 3.7 eV nearly eliminates

optical losses from the window layer. Oxide

materials have the potential to further passivate

the front and rear interface.

Front-patterned Device Results

CdTe was deposited on patterned oxides

with MZO point contacts. Here, devices with

20 nm Al2O3 are shown.

Conclusions

• Al2O3 passivates CdTe and CdSeTe interfaces

• CdSeTe/Al2O3 can produce lifetimes over 500 ns

• Front and rear patterning can be performed

with 3-μm point size

• Improved doping and contact resistance are

needed for high efficiency

Time-resolved photoluminescence (TRPL) and photluminescence emission intensity (PL) were used to

compare passivation due to oxide layers (full coverage) relative to baseline MZO/CdTe devices. MgO and

Mg0.35Zn0.65O had little effect compared to standard MZO. TiO2 and Ta2O5 caused a decrease in lifetime,

while SiO2 and Al2O3 caused an increase. CdTe growth on SiO2 was poor.

-30

-20

-10

0

10

20

30

-0.4 -0.2 0.0 0.2 0.4 0.6 0.8 1.0

Voltage [V]

Cu

rre

nt

De

nsity [

mA

/cm

2] No Al2O3

Al2O3 , 6 m hole spacing

Al2O3 , 10 m hole spacing

Al2O3 , 20 m hole spacing

Al2O3 , Full Coverage

-30

-20

-10

0

10

20

30

-0.4 -0.2 0.0 0.2 0.4 0.6 0.8 1.0

Voltage [V]

Cu

rre

nt

De

nsity [

mA

/cm

2] No Al2O3

Al2O3 , 6 m hole spacing

Al2O3 , 10 m hole spacing

Al2O3 , 20 m hole spacing

Al2O3 , Full Coverage

Laser beam induced current (LBIC) scans a focused laser (638 nm) across part

of the cell to map photocurrent. In forward bias (0.4V), current collection is

much more uniform for the device with a CdSexTe1-x graded absorber,

attributed to its higher lifetime.

-30

-20

-10

0

10

20

30

-30 -20 -10 0 10 20 30

x [ m]

y [

m]

0.6

1.0

I/Imax

-30

-20

-10

0

10

20

30

-30 -20 -10 0 10 20 30

x [ m]

y [

m]

0.6

1.0

I/Imax

CdTe CdSexTe1-x/ CdTe

CdTe CdTe/CdSexTe1-x

101

102

103

104

0 5 10 15 20 25

Time [ns]

Counts

MZO/CdTeMZO/TiO2/CdTeMZO/Ta2O5/CdTeMZO/SiO2/CdTe

101

102

103

104

0 5 10 15 20 25

Time [ns]

Counts

MZO/CdTeMZO/Al2O3/CdTe

Front Al2O3 passivation

Photoresist on TEC10/MZO for lift-off 3-μm holes in Al2O3 patterned by lift-off

0

200

400

600

800

1000

1200

0 1 5 10 20 50 100

Al2O3 Thickness [nm]

PL I

nte

nsity [

counts

]

-30

-20

-10

0

10

20

30

-0.8 -0.4 0.0 0.4 0.8 1.2

Voltage [V]

J [

mA

/cm

2]

0 nm1 nm5 nm10 nm

20 nm50 nm100 nm

101

102

103

104

0 10 20 30 40 50 60 70 80

Time [ns]

Co

un

ts

MZO/CdTe

MZO/CdTe/Al2O3

Al2O3/CdTe/Al2O3

Al2O3/CdSeTe/Al2O3

Al2O3 (20 nm) or none

CdTe or CdSexTe1-x (2 μm)

Al2O3 (20 nm) or none

MZO

TEC10 TCO-coated glass

Al2O3 (20 nm) or none

CdSexTe1-x (2.5 to 2.7 μm)

Al2O3 or MZO (100 nm)

Soda-lime glass

102

103

104

0 100 200 300 400 500 600 700 800

Time [ns]

Co

un

ts

MZO/CdSeTe (2.7 m)

Al2O3/CdSeTe (2.7 m)

MZO/CdSeTe (2.5 m)/Al2O3

Al2O3/CdSeTe (2.5 m)/Al2O3

Rear-patterned Device Results

-30

-20

-10

0

10

20

30

-0.4 -0.2 0.0 0.2 0.4 0.6 0.8 1.0

Voltage [V]

Cu

rre

nt D

en

sity [m

A/c

m2]

4.5 m

6 m

8 m

10 m

14 m

20 m

30 m

CdTe/CdCl2 CdTe/Al2O3/CdCl2 CdTe/Al2O3/CdCl2/Etch

Rear Al2O3 passivation

Lifetimes of 100 ns and 599 ns have been measured on TEC

10 and glass, respectively for front- and rear-passivation

• Rear passivation has a

stronger effect on lifetime

• MZO/CdSeTe/Al2O3

structures exhibit high

lifetime

• Double heterostructures

on glass exceed 500 ns

5 ns18 ns

100 ns

410 ns

599 ns

CdCl2 is performed after Al2O3 deposition

Aluminum oxide

exhibits field-effect

passivation, which has

been shown for p-type c-

Si, CIGS, and CZTS.

Negative fixed charge

repels minority carriers

from the interface.

negative fixed charge

repels minority

carriers from interface

------

+++++

e-

oxide

absorber

A backscattered

electron image shows

CdTe point contacts

(light) surrounded by

aluminum oxide (dark).

• Slight upward trend in VOC

• Maintenance of FF and JSC with

significant Al2O3 coverage

• Improved contacting/doping

needed Glass/TCO

MZO

1.5-μm CdSeTe/CdTe

telluriumNickel paint

Al2O3 CdTe point

contact

0

2

4

6

8

100

101

102

Lifetime [ns]

Ap

pro

x.

Diffu

sio

n L

en

gth

[ m

] Electron Mobility [cm2/V-s]

50100200