Stability Of CIS/CIGS Modu

Outdoor Test Facility Over Tw

J. A. del Cueto 

National Renewable Energy Laboratory, G

33rd IEEE PVSC, San Diego, 

Thursday, May 15, 2008

NREL/PR-520-43255 1:30 –3:00 PMPresented at the 33rd IEEE Photovoltaic Specialist Conference held May 11-16, 20
Outline

 Introduction

3⁄4 Rationale for thin-film CIS PV modules: cost 3⁄4 Loss modes in CIS/CIGS modules

 Experimental Tests at NREL OTF

3⁄4 Multiple modules from two manufacturers, ty3⁄4 Modules deployed on 3 separate testbeds

 Analyses

3⁄4 3 types of data and data analysis

 Data Conclusions

3⁄4 FF degradation is predominant loss mode

Type ‘A’ can show very low loss rate to moderatel

9 Series-resistance increases emblematic of A module

Type ‘B’ can show very low loss rate to nominal lo

9 Shunt increases & other changes emblematic of B m

3⁄4 Some transient behavior observed especially 

 Acknowledgements
Introduction

 Thin-film PV technologies (CIGS, CdT

expected to achieve and compete for lowvs. bulk technologies (c-Si, poly-c-Si) laeconomy in and costs of semiconductor 

 Copper indium diselenide (CIS) and/or 

CIGS photovoltaic (PV) modules achievhighest PV conversion efficiency of the 

3⁄4Current state-of-the-art CIGS efficien

Test Conditions (STC):

 cells attain 19.9% modules attain ~12% 

 CIGS PV module stability is a key issu

be addressed (as well by other thin-film order to achieve low levelized cost of ele
Introduction: Stability Heat/Hum

 FF losses:

3⁄4 Rse increases may result due to: 

degradation of top TCO (ZnO) resistivity due to c

(especially if doped with Al)

Increase in CB offset/barrier height at CdS/CIS in

electrons must travel

3⁄4 Gsh changes: may either increase/decrease d

Voc losses:

3⁄4 Change in doping density in CIS3⁄4 Induction of deep acceptor states/traps in bu3⁄4 Decrease in VB offset/barrier height at CdS

increase in interface recombination

Isc losses:

3⁄4 Not typically observed, but can arise if : 

transparency of top TCO degradesRse increases are very large
Experimental Tests

 Two types of modules ‘A’ & ‘B’

3⁄4glass/Mo/CIGS/CdS/ZnO/glass lamin3⁄4type A began to deploy in array field 3⁄4type B began deploying at OTF in 200

 Study CIS/CIGS modules deployed on 3 

3⁄4Single, free-standing, long-term expo

Pmax (STC) with fixed resistor, 8 tota

3⁄4High Voltage Stress Testbed (HVST2

 consists of bipolar strings, nominally ± 12 type ‘A’ CIGS modules per string, 24 I-V traces monitored & loaded continuou

3⁄4Performance & Energy Ratings Testb

I-V traces monitored & loaded continuouA module 1997, B module 2002
Analysis of Data

 Single I-V curves at STC or dark at 25°

3⁄4Module data reduced to unit area cell 

 dividing voltage by series cell count (Nc dividing current by area per cell (Acell = 

3⁄4Standard PV device diode circuit mod

series resistance (Rse) and shunt cond

3⁄4 determined Rse, Gsh (dark) allow raw 

corrected and then to derive A, J0 

J = J

0 * [ e(V-RseJ)/AkT - 1 ] - GSHV 

dV/dJ = Rse + (AkT/q) / [ J

L + J ]

V- Rse* J = (AkT/q) * Log[ 1 + (J + 

dV/dJ = Rse + (AkT/q) / J

V- Rse* J = (AkT/q) * Log[ 1 + (J 

Dark J-V

Light J-V
Analysis of Data

 PERT real-time outdoor data measured 

3⁄4I-V power parameters (Voc, FF, etc.) d

from traces segregated into narrow irra500±25, 1000±25 W/m

2.

3⁄4Linear temperature corrections determ

parameters by performing regression ointervals

3⁄4Changes in power parameters vs. time 

 HVST2 array real-time outdoor data mea

with DAS for each string done same as Pat one irradiance window 1000±25 W/m

2
Analysis of Data

 HVST2 array: PVUSA Test Conditions 

3⁄4Perform regression of power vs. irradia

wind speed conditions for coefficients monthly, for data where irradiance > 8

3⁄4 monthly calculated coefficients (A, B, 

to evaluate rated power (PPTC) at PVUS

E0 = 1000 W/m2 , Tair = 20°C, Ws = 1 m/

PPTC = E0 * (A + B*E0 + C*Tair + D*WS )

Pmax (E, Tair, Ws) / E = (A + B*E0 + C* T
Data: single module stability 

 Type A 1988, 

1998; type B 2

3⁄4 Isc, Voc, F

STC on the 

3⁄4 Type A ini

from 8% ((1998)

3⁄4 stability of 

more of an 

 FF losse Voc inc

partly osubsequ

 Type B modul

~11% show sliFF, is also offs

1997 A1998 A

2002 

B

1988 A1990 A1992 A1994 A

1997 A1998 A

2002 

B

1988 A1990 A1992 A1994 A

Taken out of serv ice

Change in SPIRE

1/88 1/90 1/92 1/94 1/96 1/98 1/00 1/02 1/04 1/06 1/08

0.50.6

56789

101112

455055606570

1.92.02.12.22.32.42.52.6

22232425262750515253

Efficiency (%) FF (%) Isc (A) A modules

Voc (V)

B modules

Change

in SPIRE
Data: series resistance changes sin

Dark & Light Slopes 

3⁄4 1/J for dark data re

ordinate axis

3⁄4 1/(J+JLight) for ligh

upper ordinate axi

3⁄4 2002 B in upper p

1998 A in lower p

3⁄4 For 2002 B no inc

intercept in both dover time

curvature suggestiv

3⁄4 For 1998 A substa

Rse intercept in da

some in light (1-2 

time

1/J (cm 2/A)

dV/dJ (Ω-cm2)

05

10152025

CIS B: 2002

Lit-08Lit-02

Drk-05 Lit-05Drk-02Drk-08

0 100 200 300 400

400 300 200 100

0 1/[J + J L] (cm2/A)

CIS A: 1998

 dV/dJ (Ω-cm2)

05

10152025

1/[J + JL] (cm2/A)

0 100 200 300 400

1/J (cm2/A)

Lit-99Lit-02Lit-07

Drk-99

Drk-02

Drk-07

400 300 200 100

0
Data: series resistance vs. time (sin

 Dark (filled symbo 1988 A 

3⁄4 Dark increase 3⁄4 Light increase 

 1994 A #1 & #2

3⁄4 Dark increase 3⁄4 Light increase 

 1998 A

3⁄4 Dark increase: 3⁄4 light increase: 

 2002 B

3⁄4 Dark 1.5 to 2.3 3⁄4 Light 0.6 to 1.

 2002 PERT B (200

3⁄4 Dark nearly no 3⁄4 light nearly no 

 Rse increases impa

because of higher J

3⁄4 ~30 mA/cm

2 f

3⁄4 ~24 mA/cm

2 f

6/99 6/00 6/01 6/02 6/03 6/04 6/05 6/06 6/07 6/08

0.00.51.01.52.02.53.03.54.04.55.05.56.06.5

Rse (ohm-cm2)

1988 A Drk 1988 A Lit1994 A #1 Drk 1994 A #1 Lit1994 A #2 Drk 1994 A #2 Lit1998 A Drk 1998 A Lit2002 B (PERT) Drk 2002 B (PERT) Lit2002 B Drk 2002 B Lit
Data: HVST2 Array PTC Re

 24 Type A 2004 CIGS modules

3⁄4 12 per string in positive (+) & 

negative (–) configuration

 PTC rated power: start out with 

~425 W each string in Feb. 2005

Degradation rate is un-even:

3⁄4 + string ~ -2.5 %/yr (relative) 3⁄4 – string ~ -3.8 %/yr (relative)

PTC regression analysis rating 

mitigates environmentally-induced fluctuations in performance (like temperature cycles) but not entirely, as evidence of higher/lower power cycles in winter/summer are still observed

300350400450

Jan'05 Jul'05 Ja

HVST2 PTC Regression Rated Power (W)
Data: HVST2 Array data at 1000 

 24 Type A 2004 CIGS modules  Bipolar (+/-) strings I-V power 

parameters, corrected to 25°C temperature vs. time shown:

3⁄4 Isc, Voc, FF & Eff top to bottom

Efficiency of each string clearly 

declining between 2005 and 2008, relative loss rates:

3⁄4 + string ~ -2.9 %/year3⁄4 – string ~ -4.7%/year

FF losses account for most of 

changes:

3⁄4 + string ~ -2 %/year3⁄4 – string ~ -4 %/year

Voc declines ~ -0.2%/yr to -0.4%/yr

Efficiency (%)

7.07.58.08.59.09.5

10.0

5055606570

FF(%)

270275280285290295

Voc (V)

2.22.32.42.52.62.7

Isc (A)

2.8

4/2/05 10/2/05 4
Data: PERT type ‘A’ at 500 & 10

1997 A I-V pow

corrected to 25°

 Isc, Voc, FF & 

from top to bott

3⁄4 FF losses lea3⁄4 similar loss 

500 & 1000 

windows, -0%/yr , respe

consistent w

source

3⁄4 Data gap in 

indoors

Transient im

just after re

Jan'02 Jan'03 Jan'04 Jan'05 Jan'06 Jan'07 Jan'08

Efficiency (%)

789

10

5759616365

FF(%)

242526

Voc (V)

500 ± 25 W/m2

1000 ± 25 W/m2

Isc(A)

1.01.11.21.32.22.32.4

1000 ± 25 W/m 2

500 ± 25 W/m2

1997 A
Data: PERT type ‘B’ at 500 & 10

 2002 B I-V po

corrected to 25°

3⁄4 Isc, Voc, FF 

from top to b

3⁄4 Uneven loss 

500 & 1000 

windows:

 -0.93%/yr 

respectively

Consistent 

increases w

3⁄4 Transient im

by 1-2 V wiafter low lig2002 & 200

Jan'02 Jan'03 Jan'04 Jan'05 Jan'06 Jan'07 Jan'08Efficiency (%)

9

101112

FF(%)

6567697173505152535455

Voc (V)

0.80.91.01.1 Isc(A) 1.92.02.1

1000±25 W/m2

 500 ±25 W/m2

1000±25 W/m2

 500±25 W/m2

 500 ±25 W/m2

2002 B

1000±25 W/m2
Conclusions: Performance L

 Loss rates type A modules:

3⁄4 range from negligible (-0.3%/yr), to nominal 

(1%/yr) and moderate (2%/yr) for later types

3⁄4 FF worsens due to series resistance increases3⁄4 Moderate larger loss rates observed when 

deployed at high-voltage bias:

 + HV bias ~ -2.5%/yr to -2.8%/yr – HV bias ~ -3.8%/yr to -4.7%/yr

 Loss rates type B modules:

3⁄4 Not significant from STC data, due to

 opposing FF & Voc trends Not as much Rse increase & lower Jsc

3⁄4 FF loss mode more tied to shunt conductance 

increases:

 Nominal ~ -1%/yr from PERT data around 

1000 W/m2 irradiance

 Slightly larger ~ -1.8%/yr from PERT data at 

500 W/m

2 irradiance

Type

ΔEff/Eff0

(%/yr)

1988 A -0.90 1990 A -0.27 1992 A -0.43 1994 A -1.01 1998 A -2.19 2002 B -0.67 1997 A -2.10 1997 A -1.35 1997 A -1.27 2002 B -1.80 2002 B -0.89 2004 A -2.87 2004 A -4.68 2004 A -2.55 2004 A -3.77 
Acknowledgement

 Contributing Coauthors

3⁄4 Ben Kroposki3⁄4 Carl Osterwald3⁄4 Steve Rummel3⁄4 Alan Anderberg

 This work was supported by the U.S

Energy under Contract No. DE-AC3
Thank you for your conside