High Efficiency CdTe and CIGS Thin F

Highlights of the Technolog

Challenges

Acknowledgement: Work performed at NREL for US 

under contract No. DE-AC36-99GO-10337

National Renewable Energy Laboratory • National Center fo

Rommel Noufi

NREL/PR-520-39773Presented at the 2006 IEEE 4th World Conference on Photovoltaic Energy Conversion (WCPheld May 7-12, 2006 in Waikoloa, Hawaii.
Disclaimer and Government License

This work has been authored by Midwest Research Institute (MRI) under Contract No. DE-AC36Department of Energy (the “DOE”). The United States Government (the “Government”) retains athe work for publication, acknowledges that the Government retains a non-exclusive, paid-up, irrepublish or reproduce the published form of this work, or allow others to do so, for Government puNeither MRI, the DOE, the Government, nor any other agency thereof, nor any of their employeeor implied, or assumes any liability or responsibility for the accuracy, completeness, or usefulnesproduct, or process disclosed, or represents that its use would not infringe any privately owned ri

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Outline

IntroductionHighlights

- summary of device performanc- how devices are structured- properties of thin film layers- summary of module performan

Key Challenges
Acknowledgement

Polycrystalline Thin Film Group – NRELMeasurement and Characterization – NK. Zweibel, H. Ullal, B. von Roedern – NDale Tarrant – Shell Solar IndustriesRobert Birkmire – IEC, U. of DelewareBernhard Dimmler – Wurth SolarPeter Meyers – First SolarDennis Hollars – MIASOLEJeff Britt, Scott Wiedeman – Global SolaTim Anderson – U. of FloridaW.S. Sampath – AVA TECH
Introduction

• CdTe and CIGS PV modules have the pot

cost effective PV-generated electricity.

• They have transitioned from the laboratory 

market place.

• Pilot production/first-time manufacturing (U• CdTe technology ramping to 75 MW.• Enjoying a flux of venture capital funding.• Transitioning from the lab to manufacturin

much more difficult than anticipated.
CIS and CdTe PV Comp

Shell Solar, CAGlobal Solar Energy, AZEnergy Photovoltaics, NJISET, CAITN/ES, CONanoSolar Inc., CADayStar Technologies, NY/CAMiaSole, CAHelioVolt, TxSolyndra, CASoloPower, CA

Wurth Solar, GSULFURCELLCIS SolartechnSolarion, GermSolibro, SwedCISEL, FranceShowa Shell, Honda, Japan

CIS

CdTe

First Solar, OHSolar Fields, OHAVA TECH, CO

CANRON, NY Antec Solar, G
Laborator

Solar Cell

Laborator

Solar Cell
Thin Film CIGS Solar C

Efficiency

Area

(cm

2)

CIGSeCIGSeCIGSCIASCdTeCdTeCdTe

Area

(cm

2)

0.4100.4020.409

–

1.03

–

0.16

VOC

(V)

0.6970.670.830.6210.8450.8400.814

JSC

(mA/cm

2)

35.135.120.936.025.924.4

23.56

FF

(%)

79.5278.7869.1375.5075.5165.0073.25

Efficiency

(%)

19.518.512.016.916.513.314.0

NR

C

N

C

Sn

Zn

U. 
CIGS 

ZnO/CdSCIGSMoGlass

Glass

CdS

CdTe

C-Paste/Cu

Cd2SnO4/ZnSnOx
SEM Micrographs - Sputte

Thin Films

100 nm 

100 nm 

100 nm

Rate: 25Å/sec. 

H. Althani Thesis
CIGS Thin Film with E

g=1.1

26% Ga/(In+Ga) 
AES Depth Profile

605040302010

0

10 

Spu

605040302010

0

10 20 30 40

Sputter Time (min)

 Cu Ga Se In

S2212
CIGS Deposition System
CIGS Formation Pathways

Deposition Methods

Evaporation of the Elements VaSputtering of the Elements VaNanotechnology/Nano-particles-(Inks) PCVD-based (lab. R&D) Lo

1. Cu + In + Ga CCu:In:Ga intermetallic + H

2Se (or Se) C

2. Cu

2Se + (In,Ga)

2Se

3 C

3. Cu + In + Ga + Se C
Deposition of CdS

Solution (CBD): CdSO

4, NH

4OH, N

2

H

4CS 

Temperature: 60°C to 85°CTime: 4 to 20 min.

Sputtered CdSTs 150-200°C
Optical Transmission 

RF-sputteredTarget: ZnO, ZnO:AlRate: 1.5Å/sec.

3.3 ohm/square1.4 microns
Parameters of High Effic

CIGS Solar Cells

Sample Number

Voc (V)

Jsc (mA/cm

2) Fill facto

C1812-11 0.692 35.22 79.87 S2212-B1-4 0.704 34.33 79.48 S2232B1-3 0.713 33.38 79.54 S2232B1-2 0.717 33.58 79.41 S2229A1-3 0.720 32.86 80.27 S2229A1-5 0.724 32.68 80.37 S2229B1-2 0.731 31.84 80.33 S2213-A1-3 0.740 31.72 78.47 

Tolerance to wide range of moleculari

Cu/(In+Ga) 0.95 to 0.82

Ga/(In+Ga) 0.26 to 0.31

Yields device efficiency of 17.5% to 19.
Quantum Efficienc

1.00.80.60.40.20.0

QE, R, A

400 600 800 1000 

Wavelen

gth (nm)

S221226% Ga

S221331% Ga
Diode Quality

J

o and n (light curve

Bandgap (eV) J

0 (A/cm

2) 

(Diode 

1.10 5x10-11 

1.12 6x10-11 1.12 6x10-11 1.21 4x10-10 1.22 5x10-10 

R = 0.25 

Ω cm

2

G = 0.10 mS cm-2 (or Rsh = 10 k

Ω cm
Efficiency and Voc vs 

0.90.80.70.60.5

Open Circuit voltage (V)

1.0 1.1 1.2 1.3 1.4 1.5 1.

Absorber band gap (eV)

1816141210

Efficiency (%)

1.0 1.1 1.2 1.3 1.4 1.5 1

Absorber band gap (eV)
High Quality TCO – Cd

2SnO

CdTe bandgap
Solution (CBD): CdSO

4, NH

4OH, N

2

H

4CS 

Temperature: 60°C to 85°CTime: 15 to 30 min.

Deposition of CdS

Vapor Transport Deposition of CdS
Zn

2μm

CdTe

CdTe Thin Film Morph
IEC VTD

4”x6” Pre-Heater, 600°C 2”x6” Post-Heater 600°C

4”x4” Substrate

Heater/Enclosure(Hot-Press boron nitride (BN) with borate binder)

Ta Wire Confined in BN

Non-Heated RegionConstrains Deposit

CdTe

Heater

CdTe

Source

Substrate

H

Close Space 

Sc
High-Efficiency

CTO/ZTO/CdS/CdTe Ce

Cell #

Voc

(mV)

Jsc

(mA/cm

2)

FF

(%)

W547-A 847.5 25.86 74.45 W553-A 849.9 25.50 74.07 W566-A 842.7 25.24 76.04 W567-A 845.0 25.88 75.51 W597-B 835.6 25.25 76.52 W608-B 846.3 25.43 74.24 W614-B 842.2 25.65 74.67 
Thin Film

Modules

Thin Film

Modules
Company Device Aperture 

Area (cm2) Efficiency (%) Po

Global Solar CIGS 8390 10.2* Shell Solar CIGSS 7376 11.7* Wrth Solar CIGS 6500 13.0 First Solar CdTe 6623 10.2* Shell Solar GmbH CIGSS 4938 13.1 Antec Solar CdTe 6633 7.3 Shell Solar CIGSS 3626 12.8* Showa Shell CIGS 3600 12.8 

Polycrystalline Thin Film PV M

(standard conditions, apertur

Ranked by Power

* NREL Confirmed
CIGS and CdTe Devices and Mod

Similar Structure and Process S

Substrate Base Electrode 

First Scribe

Third

Scribe

Top

Electrode

Second

Scribe

External

Contacts Encapsulation
Module Monolithic Interconne

Monolithic integration of TF solar cells can lead to manufacturing cost reduction; e.g., fewer processinautomation, lower consumption of materials.Shared characteristics lead to similar cost per unit 

Efficiency discriminating factor for cost per wat

⇐

Courtesy 
Challenges

Lack of adequate science and engineering kno

• Measurable material properties that are predictive o

performance

• Relationship between materials delivery and film gr• Develop control and diagnostics based on material 

growth

• Coupling of this knowledge to industrial processes

Benefits:

• High throughput and high yield at every step of the • High degree of reliability and reproducibility• Higher Performance
Challenges (cont.)

Long-Term Stability (Durability)

• Both technologies have shown long-term stability.

However, performance degradation has also been 

• CdTe and CIGS devices have different sensitivity to 

e.g., oxidation of metal contact, change in propertie- Thin Film Barrier to Water Vapor- New encapsulants and less aggressive applicatio

• Need for better understanding degradation mechan

level and prototype module level.


· Product Durability:

Environmental, Lifetime Tests

Recent Effort at GSE

GSE CIGS-Glass (2.3 kW at Springerville, AZ)

0.02.04.06.08.0

10.012.0

1-Aug-2005 1-Sep-2005 2-Oct-2005 2-Nov-2005 3-Dec-2005 3-Jan-20

Date

DC Efficiency (%, at Temp.) ** Not at STC (Temp. is un

** Load Point Determined 

S. Wiedeman, TFPP 2006 Global Solar Global Solar -
Temperature Dependent Deg

Temp Time to Failure

60 24000080 940

100 168

120 62

1210

8

ln(Time to Fail)

6

2.6 

Ea = 0.63 eV

arb. assign 10% as “time to fail”

120 °C

Different mechanisms dominate degradation at different temperatures ( ~90-120°C associated with Cu diffusion)

Cu diffusivity in CdTe: 

D = 3.7 x 10-4 

-25-20-15-10

-5

0

0 500 1000 1500 2000

Stress Time (hrs)

%Change in Eff

T=120°C

T=100°C

T=80°C

T=60°C

Stress Data Fit: y = b · x a
CIGS Stability Dry/1-Sun/85

°

After some initial “equilibration”, CIGS deviceexcellent stability (dry/1-Sun/85

°C/Voc bias)

1816141210

8

Total-Area Efficiency (%)

0 100 200 300 400 500

Time (hrs) @ 1 Sun, 85C, dry

Three-Stage CIGS (Contreras)Three-Stage CIGS (Ramanathan)Three-Stage CIGS (Contreras) + ZnS (Bhattacharya)

Industrial; Small-Area Device (Ag contact)
Lamination Losses w

Different Encapsulan

-30-25-20-15-10

-5

05

10

% Change

DVoc 

DJsc

DFF Dh
Challenges (cont.)

Thinner CIS and CdTe layers

• Current thickness is 1.3 to 8 μm• Target <0.5μm thick layers• Maintain state-of-the-art performance• Requires modification of deposition parameters reg• Need for models that relate film growth to material d• Device structure that maximizes photon absorption

Benefits:

• Addresses the issue of 

In and Te availability

• Higher throughput• Less material usage• Cost??

Risks:

• potential for • changes in d

and structure

• Non-uniform• lower yield?
1 μm

Voc= 0.676 V

Jsc= 32 mA/cm2

FF= 79.5%; Eff = 17.2%

Thinner Absorbers

0

Voc= 0.565 V

Jsc= 21.3 mA/

FF= 75.7%; E
Thin Cells Summar

t (μm) Voc (V) Jsc (mA/cm

2) FF (%

1.0 CIGS 0.676 31.96 79.40.75 CIGS 0.652 26.0 74.0 0.40 CIGS 0.565 21.3 75.7 0.47 CIGS 0.576 26.8 64.2 1.3 25.26 2.66 69.2 0.87 CdTe 0.772 22.0 69.7 

CIGSSModule
Challenges (cont.)

Need for Low-cost processes

• More relevant to CIGS technology• Relatively slow throughput and poor material utilizat

of complex processes

• High cost of In; ~$1000/kg• High rate co-sputtering from the elements (in the pre• Non-vacuum or low vacuum, simple equipment• Innovative processes:

- CVD-based- Nanotechnology utilizing nano-components to ma

e.g. printable CIGS
480-kW Thin Film CdTe Sol

Tucson Electric/First Solar
245-kW Thin Film CIGSS Roo
Wales CIGS - 84 kW

San Diego CIGS - 4 
· Roll-Roll production of CIGS PV

· Web-based processes for all Mat’l

Deposition

 Stainless Foil or Polyimide Film  1000-ft x 1-ft Process lots

Flexible PV Technology

S. Wiedeman, TFPP 2006 Global Solar Global Solar -
Finally

Thin Film CIGS and CdTe technologiebecome cost competitive with Si.Challenge: obtain large investment for facility/equipment to take advantage othroughput and simplified manufacturi
End
QE

100

80604020

0

Realtive External QE (%)

400 600 800 1000 

Wavelength (nm)

 S2232B12 S2265B12, thin 3 stage S2297B12, thin codep
0.4 μm Cell - Optic

80604020

0

%T, A, QE

400 600 800 

Wavelength 

QE

Absorptio

R of Cell
The Best One-of-a-Kind Labor

Cell Efficiencies for Thin Film

(Standard Conditions)

Efficiency (%)

CuInGaSe

2

CdTeAmorphous silicon (stabilized)

Univ. of Maine

Boeing

Boeing

Boeing

Boeing

ARCO

AMETEK

N

Boeing

EuroCIS

Univ. of So. Florida

Univ. of So. FL

BP Solar

Kodak

Kodak Monosolar

Matsushita

12

840

1975 1980 1985 1990 1995 

1620

RCA

ECD

Photon Energy
Cu

Increases

NA in CdTe

Reduces Height and

Width of

Back Barrier

dJ/dV

(FF Loss)

Grain 

Boundary

Shunts

CdS:Cu

(Donor Compensation)

Cd

CdS

Cu

Cu in CdTe PV Devic

The Good, Bad, and the Ug
High Efficiency CdTe C

Replaced SnO

2

with Cd

2SnO4 in 

CdTe cells, yielding improved Jsc and FF

Hi

gh-efficienc

y CdTe cells 

Cell

# 

Voc 

(mV) 

Jsc 

(mA/cm

2) 

(

1 847.5 25.86 72 845.0 25.88 7

High-efficiency CdTe cell

Cell

# 

Voc 

(mV) 

Jsc 

(mA/cm

2) 

1 842.1 24.12 72 848.1 23.97 7
Effect of Zn2SnO4 Buffer 

Integrated highresistivity Zn2SnO4 (ZTO) 

buffer layer, yielding improved device performance and reproducibility

0

102030405060708090

100

200 300 400 500 600 

Wavelength 

CTO/CdS/CdTCTO/ZTO/Cd
VOC Improvement

• To achieve CdTe cell with efficiency hig

needs Voc improvement

• Voc improvement : 

(1) Optimize device process to improv

(reduce A & J0) and reduce back b

(2) Study defects that limit doping and 

device

• Achieved an NREL-confirmed Voc of 85

cell with an efficiency of 15.6%
Sputtered 

Cd

2SnO

4

 ~20°C

Sputtered Zn

2SnO

4

 ~20°C

Sputtered  CdS  ~20°C

CSS-CdTe 550-620°C

CdCl

2 treatment 

 ~ 400°C

Back contact  formation  ~ 270°C

C V D-c-SnO2500-600°C

C V D-i-SnO2

500-600°C

 CSS-C dS 500-600°C

C SS-C dTe 550~620°C

CdCl2treatm ent ~400°C

Back contact  form ation 

or C BD-CdS ~270°C ~100°C

or CV D -C dS ~430°C

Time

Temperature

• CT

depby 

• Sin• Cry

and occdep

• Co

devthic

• Mix 

pro

• Sev

dow(coinc

Improvement to the Deposition 
CIGS Deposition Pro

0

2

46

8

101214

0 10 20 30 40 50

Metal rate (Å/s)

60

Runtime (min)

100200

300

400500600

Monitor temperature (°C) 

Se

In Cu

Ga

Back of 

substrate

temperature
Global Solar