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CIGS Material and Device Stabi

Processing Perspective

Kannan Ramanathan, NCPVPV Module Reliability Workshop, March 1, 2

Golden, Colorado

NREL/PR-5200-54569
CIGS landscape• Multiple companies trying to get to high

volume, low‐cost manufacturing. Challen

increase efficiency, control variability and

ensure reliability. Efficiency bar is rising.

• Diverse approaches, cell designs. Differen

stages of maturity. Process details largelyproprietary.

• Process control and understanding of ‘cau

effect’ still needed, desired.

• Precursor selenization/sulfurization and c

evaporation based processes have an edg
Connecting the pieces• Solar cell fabrication method, tool, pro

details

• Process to property correlation

• Cause and effect analysis of variability

• Performance improvement

• Device level changes and mitigation

• Packaging/ Protection of circuits

• Above pieces are connected, must wor

together to address stability issues.
Stability Topics• Light soaking• Post lamination loss

• Changes due to moisture ingress

• Reverse bias leakage

• Shunts• Hot spots

• Weak diodes
Outline• CIGS Material Properties: Basics• CIGS Devices: Basic features

• Cell level changes• Examples of previous work

• What do we need to measure? Interpr

Improve?
CIGS(S) Absorber• Quaternary and pentenary alloys derived 

base compound CuInSe2. Band gap is incrby alloying with Ga and/or S.

• Band gap may not be uniform across the 

of the film, often graded.

• Phase purity and stoichiometry are impo

control.

• Single crystal/ epi knowledge base is wea

• Adequate working knowledge of physical 

electronic properties, bear great resemblII‐VI ‘parents’.
Absorber: desired properties, proces• Durable metal contact to the p‐side (M

o Minimally reactive, ohmic contact stabilize

MoSe2.

o Needs proper process conditions to be the 

• P‐type absorber

o Doping by native defects (close compensao Some elements enhance p‐type doping (No Higher temperature growth preferredo Chalcogen rich growth preferredo Crystal quality = efficiency (stability?)
Absorber: Electrical• CuInSe2 can be n‐ or p‐type

• Thin films are p‐type when grown Cu‐poor in 

conditions.

• With Ga and Na included, p‐type is likely stab

• If grown in Se‐poor conditions, material can b

resistivity p‐type or even n‐type (more compe

low lifetime).

• Electrical properties are a sensitive function o

growth method, tool, recipe.

• No direct measure of absorber’s electrical pro
Junction• Chemically grown CdS layers form the 

emitter. Preferred junction partner.

• CBD bath induces change in electronic

properties in addition to the growth of 

compatible “buffer layer”

• Alternative emitter layers (ZnOS, In2S3)

promising, come with unique characte

• ZnO conductivity can degrade upon ca

compensation.
Device stability/ Metastability• 1992: Siemens Solar asked for help in

understanding “transient effects”

o Device properties changed dramatically when 

to light, voltage bias etc.

• 2012: Similar products in vogue, exhibit s

characteristics.

• Device characteristics are a function of ho

are made. NREL ≠ Miasole ≠ SƟon. Specifi

each device to be taken into account whesolving cell/ module optimization.
Prior NREL work: D. Albin

All devices show attainment of a “stabilized” level

identical 3‐very differerecovery bedistinguishModified “characterissubstrate“industrial” biggest sprbehavior

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)
Cell in DH; no encapsulation

M. Schmidt et al. / Thin Solid Films 361±
PL of cells after damp heat exposure

DH effects:• Decrease in absorbe

(increase in defect leve

• Increase in junction 
Light soaking: early Siemens cells

D
Process understanding/ quality

improvement:

Case studies from past NREL wo
605040302010

0

10 20 30 40

Sputter Time (min)

 Cu Ga Se In

S2212

Comparison of NREL and SSI absorbers
Comparison of J-V Curves

Example 1: SSI Absorber deviation

Common absorLower performaNREL CdS/ZnO

(not typical)

K. Ramanathan, CIS Na
J-V curve: NREL absorber CdS and ZnO processed in same runs as SSI

NRELabsorbwindo
PL Spectra

SSI windows

NREL windows

Left most NREL Cdanneal.Green: 5’anneal afRight mosPL from 3 SSI windo

NRELCdS/ZnO

SSI CdS/ZnO
External QE Comparison 

Long wave edge •Poor Diffusion L•Drift assisted co•ZnO reflectance •Band gap gradi•Extracting band straightforward iThere appears tSame direction ashift.

Quantum efficiency
Compositional analysis

Revealed a large dro

the Cu ratio for the bof absorbers.
Example 2: Junction anneal to improve performance

K. Ramanathan, NR
A Shell Renew

Shell Solar

Thermal Degradation Characteristics

ST40 Module - Daystar Outdoor Tests

0

0.5

1

1.5

2

2.5

3

0 5 10 15 20 25

Voltage (V)

Current (A) 

Efficiency: Initial = 11.2%200h = 9.9%1000h = 8.6%

No Loss

20% Loss

5% Loss
A Shell Renew

Shell Solar

Modified Processing for Thermal StabilityDry Heat Test Only

What wa• Incre

thick

• Low 

 10W Laminates - LAPSS Test

Each data point represents the average of 21 laminates

0123456789

101112

0 200 400 600 800 1000 1200

Exposure (h)

Efficiency (%)

Std Product DryNew Process Dry
Summary• Proper encapsulation of CIGS devices can alleviate m

the moisture driven performance degradation.

• It is possible the high efficiency devices exhibit few

metastable effects. Efficiency improvement efforts off in stability.

• A case by case approach is needed to optimize devi

performance and long term stability.
Note added March 5, 2012• Important questions were raised in the afternoon discussion session 

clarifications and further work on how CIGS devices are affected by m

• Siemens/ Shell Gen II arrays have demonstrated stable operation at t

• A recent NREL study of Shell’s Eclipse 80 modules showed excellent s

negligible effect of moisture because of improved packaging and edg

paper that just appeared [Solar Energy Materials & Solar Cells 98 (20

] showed that a new edge seal design enabled stable performance fo

damp heat.

• It is not possible to draw definitive conclusions about the moisture s

CIGS based on the available reports on unencapsulated cells.