OPTIMIZING CARBON NAN

CONTACTS FOR USE IN OR

PHOTOVOLTAICS 

Teresa Barnes, Jeff Blackburn, Robe

Anthony Morfa, Mike Heben, and Ti

PVSC 2008

5/13/2008

NREL/PR-520-43262Presented at the 33rd IEEE Photovoltaic Specialist Conference held May 11-16, 2
Transparent Contacts 

• Thin Film Devices

– CdTe (16.5%) – TC sees 

600

o

C

– CIGS (20%) – TC must be 

grown at room T

• TCOs

– ZnO:Al, SnO

2:F, In

2

O

3:Sn, 

Cd

2SnO

4

• TCs must meet different 

requirements in each technology

• Successful demonstration of 

SWNT electrodes in thin film devices

www.nrel.gov/ncpv/thin

13% w/C

12% w/C

CIGS - Contreras, JCdTe - Barnes, APL
Carbon Nanotube Bas

• Many s

method

– CVD

vapo

• Extens

proces

• Impurit

• Multi v

• Rando

distribu

– 1/3 m

semi

Armchair (m)Zig-zagChiral

Multi-wall tubes Single-wall bundle 

Baughman, Science 287 2002
OPV Device Structur

Bulk Heterojunction

OPV Device Results:J. van de Lagemaat, T. M. Barnes, G. Rumbles et al., Applied Physics Letters 88 (23), 3 (2006).

Short diffusions lengths would require impracticalllayers in a “stacked” heterojunction (BHJ ~ 300nm Mix P3HT and PCBM in a 1:1 ratio to create a “buP3HT carries holes to the ITO side, PCBM transpo

ITO
Early SWCNT Production 

• All “TCO” work so far uses laser tubes• 1nm < d < 1.4 nm; all SWCNT

• PLV tubes are very long with low defect 

density (Raman D/G ~1/190)

• Films formed by vacuum filtration/ 

membrane transfer

W

NREL Pulsed Laser Vaporization
Scalable Production: Ultra-so

Metrics: Transparency, Conductivity, StabilityParameter MatrixPurification: metals, amorphous CSurfactant: SWCNT dispersion, bundle sizeSonication: SWCNT length, bundle size, defectsPost Processing: surfactant removal, dopingSurface Functionalization: wetting, drop formation
Ink Formulation for Ultrason

• Aqu• Ink • Tub

pursta

• Sur

cho

– M– C– R

Best OK Bad
Sprayed vs. Membrane Tran

• La

roume

• Po

fro

• Wr

• Sprayed films ar• ~70 

Ω/sq, 80% T 

nm)

• Smooth and feat

– Potential for few

devices

– Fewer apparent 
Compare TCOs with SWC

• Transparent SWCNT

– Spaghetti-like network

– ~50 nm thick

• Hole conducting

• Function

performa

• Need to 

– Dopin

– improv

purific

302010

0

Seebeck V (μV)

0.0 0.5 

Sample: 50 Ω/sq CNTTemp: 300.4 KSeebeck coeff.: 16.5 ± 1.4 μV/K 
Conductivity in “Bulk” Net

• Mixture of metallic (1/3) and semiconducting (2• Tubes/bundles have high aspect ratio in “good• Measured “p” is low, but networks behave like 

• Mobility of the network is very different from in

μH, tube ~ 1000-10000 cm

2/V-s

μΗ, Network ~ 0.02-0.1cm

2/V-s 

Junctions between tubes and bundles domina
SWCNT Network Condu

• Networks a

HNO

3 after 

– Network c– Conductiv

Improvem

–

R

s = 60 

Ω

• Rinsing aft

– Promotes – Improves –

R

s increas

Ω/sq

80604020

0

Transmission (%)

500 1000 1500 2000

W avelen

gth (nm)

 rinsed as-doped

Absorption peaks at 650 and 950 are due to fundamental excitonic transitions of 

SWCNT
Devices on NREL SWCNT N

• Ultrasonic spray deposition• Several ~ 3% devices 

• Thick active layers - spun at 

200 rpm 

• Reducing electrode 

roughness is key

• PEDOT can be eliminated

4020

0

Current Density (mA/cm2)

-1.0 

604020

0

Current Density (mA/cm2)

-1.0 

1510

50

-5

Current Density (mA/cm2)

-1.0 

40302010

IPCE

400 500 600 700 800

Wavelength (nm)

 CNT  ITO ref
Conclusions

• SWCNT networks are the best hole-co

available 

– Enables new device structures using “p” TC

• SWCNT is a good flexible TC• Optoelectronic performance is improvin

– SWCNT processing, deposition, doping, m/

• Improve process compatibility for bette

performance, new architectures (no PE

• Device performance likely to improve fo

printed active layers

• SWCNT networks could be a model for 

– If a completely black material works as a TC

could we use??
Acknowledgements

• Jao van de Lagemaat, Nikos Kopidakis

Rumbles, Glenn Teeter, Bobby To, Tim Duenow, Matt Reese, Xerxes Steirer, Mand many others at NREL

• This work was supported by the U

Department of Energy under ConDE-AC36-99GO10337 with the NRenewable Energy Laboratory.

• for materials and add

funding