Polycrystalline Thin Film Pho

From the Laborato

Solar Fields

National Center for Photovoltaics

National Renewable Energy Laborato

Golden, Colorado USA

IEEE 4th World Conference on Photovoltaic Energ

Waikoloa, HI • May 8-12, 2006

Bolko von Roedern,

Harin Ullal, and Ken Zweibe

NREL/PR-520-39837
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Presentation Outli

• Scientific Base for Polycrystal

Thin Film Photovoltaics

• CIGS Technology• CdTe Technology• Commercialization
The Best One-of-a-Kind Lab

Cell Efficiencies for Thin 

(Standard Conditions)

Efficiency (%)

CuInSe

2

CdTeAmorphous silicon (stabilized)

Univ. of Maine

Boeing

Boeing

Boeing

Boeing

ARCO

AMETEK

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
Why is Better Fundam

Understanding Requir

Helpful? 

(a) For better process control and o(b) Understanding the limits of a giv
Why better understan

• Not only an academic issue, but a

requirement for achieving maximucommercial product performance a

• Assist in selecting necessary proc

best performance (when to “tweak

to upgrade a process)

• Realistic assessment of ultimate p

of a given technology
Activities and To

• In-house characterization at NREL• New characterization group using 

“near-surface” analyses at UNLV

• Much work on the effect of micro- 

non-uniformities (Y. Yan at NREL, at U. Toledo)
Are there limits t

understanding?

• Materials with different chemistry an

structure can result in both high and cells! (So what is to be learnt from sparticular details of a specific device

• As often the case is being made tha

is known about the silicon semicondcompared to CIGS and CdTe, why tsolar cells and modules generally of efficiencies than devices made from silicon (<50 

μm-thick)?
CIGS PV Technology 

• Thin (<1

μm) -absorber cells

• (Cd)ZnS(O,OH) junction layer• Alternative fabrication method

“paints,” –non-vaccum) hybrid selenization and sulfurization

• Wider bandgap CuGaSe

2 or C
Area 

(cm2)

VOC

(V)

JSC

(mA/cm2)

FF

(%)

Effic.

(%)

0.410 0.6975 35.078 79.52 19.5 S0.408 0.7052 35.515 77.90 19.5 C0.402 0.6698 35.112 78.78 18.5 0.409 0.6782 31.93 79.20 17.1 

0.5 0.774 21.6 73.7 12.3 

0.409 0.8305 20.88 69.13 11.99 

C
New Buffer Layer

• CdZnS and ZnS junction cells hav

champion level cell performance

• Even the ZnS junctions are best w

“wet” chemical bath deposition (CBprocesses

• All high performance junction mate

found to have very high electrical r
CIS R&D Focus

• Since champion efficiency cells we

be co-evaporation, this seems the deposition method of choice, woulsupport of commercial processes 

• Industry also uses selenization/su

(Shell Solar), hybrid processes (EPnon-vaccum “ink” or “paint” proces

and Nanosolar)
CdTe PV Technolog

• Micro non-uniformity of CdTe films 

on device performance

• Thin CdTe absorber layers• Interdiffusion at the CdS/CdTe inte• (Vapor) CdCl

2 heat treatments

• Role of Cu “doping” during back-co

• Can VOC be increased to above 85

without loss in JSC and FF?
“Champion” Modu

(

* = NREL confirmed)

Company Device Aperture 

Area (cm

2)

Eff.

(*NREL 

confirmed)

Würth Solar CIGS 6500 13.0 Shell Solar GmbH CIGSS 4938 13.1 

Shell Solar CIGSS 3626 12.8* 

Showa Shell CIGS 3600 12.8 Shell Solar CIGSS 7376 11.7* Global Solar CIGS 8390 10.2* First Solar CdTe 6623 10.2* 
(a) Long-term module performance

(b) Customer acceptance issues (e(c) Optimized packaging schemesNREL-led team working on these iss

Module Reliability a

Customer Accepta
Reliability and Pack

• CIGS and CdTe were found to be m

sensitive, often require better module encapsulation schemes than x-Si (or Si) PV modules 

• Improved module package schemes 

and CdTe modules to pass the “dam(1000-hour exposure at 85 

oC and 8

humidity)

• Processing affects the performance 

“stressing,” — light exposure at elev

65 to 100 

oC device temperature) in 

atmospheric ambients
Reliability and Pack

• To study cell/cell-interconnect/pac

interactions, it is planned to “stressminimodules

• In CdTe cells, the details of the en

process as well as the details of bacontacting affect potential stability

• Warranties up to 25 years (80% of 

guaranteed) now available from somanufacturers (First Solar) 
Hot & Humidity Tes
Commercialization 

Systems

• Thin-Film Modules are used for so

field installations in Germany

• Module cost per Watt should be ~7

modules, for installed system cost90% of x-Si systems

• Glass-to-glass laminates most suit

field installations
Commercialization 

Systems

PV Market segments:(1) Large Fields(2) Retail Modules(3) BIPV

Thin Film Modules have addressapplications, but manufacturers ncontinue to improve modules to mmarket’s needs
Polycrystalline Thin Fi

Global Solar Installa

Location Material Size (k

Dimbach, Germany CdTe 1,400 Reussenkoge, Germany CdTe 1,040 Fellber, Germany CdTe 800 Sinzheim, Germany CdTe 800 Tepfhelm, Germany CdTe 778 Springerville, AZ, USA CdTe 500 Florsheim, Germany CdTe 440 Camarillo, CA, USA CIS 245 
Large Solar Field

First Solar 1,425 kW

p System Deponie Sinzhe
U.S. Thin Film Produc

(Figure from 31st (01/2005) IEEE PVSC 

05

10152025

1996 1999 2002 

a-Si CIS CdTe

MW/year
US-owned, US manufactured modules by te

crystalline Si and Thin Films (incl. a-Si) 19
Conclusions

• Many issues how thin-film solar cells work remain u

further fundamental R&D effort

• Commercial thin-film PV module production reached 

the U.S., indicating much more rapid growth than cry

• Commercial module performance is increasing based 

knowledge. More R&D will lead to further improvem

• Stability of thin-film modules is acceptable (≤ 1% pe

the right manufacturing processes are used for manuf