National Renewable Energy Laboratory 

Innovation for Our Energy Future 

A national laboratory of the U.S. Department of Energy

Office of Energy Efficiency & Renewable Energy

NREL is operated by Midwest Research Institute ● Battelle Contract No. DE-AC36-99-GO10337 

Application of Single Wall 

Carbon Nanotubes as 

Transparent Electrodes in 

Cu(In,Ga)Se2-Based Solar Cells 

Preprint 

M. Contreras, T. Barnes, J. van de Lagemaat, 

G. Rumbles, and T.J. Coutts 

National Renewable Energy Laboratory 

C. Weeks, P. Glatkowski, and J. Peltola 

EIKOS Inc.

Presented at the 2006 IEEE 4th World Conference on 

Photovoltaic Energy Conversion (WCPEC-4) 

Waikoloa, Hawaii 

May 7–12, 2006 

Conference Paper 

NREL/CP-520-39914 

May 2006 
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Fig. 1. Morphology of Invisicon (100 ohm/sq sheet 

resistance) deposited onto a CdS/CIGS/Mo 

substrate. 

APPLICATION OF SINGLE-WALL CARBON NANOTUBES AS TRANSPARENT 

ELECTRODES IN Cu(In,Ga)Se2-BASED SOLAR CELLS 1

Miguel Contreras,†

 Teresa Barnes,† Jao van de Lagemaat,† Garry Rumbles,† Timothy J. Coutts,† 

Chris Weeks,*

 Paul Glatkowski,* Igor Levitsky,* and Jorma Peltola*

†

National Renewable Energy Laboratory, Golden, Colorado, USA *EIKOS Inc., Franklin, Massachusetts, USA 

ABSTRACT 

 We present a new thin-film solar cell structure in which 

the traditional transparent conductive oxide electrode 

(ZnO) is replaced by a transparent conductive coating 

consisting of a network of bundled single-wall carbon 

nanotubes. Optical transmission properties of these 

coatings are presented in relation to their electrical 

properties (sheet resistance), along with preliminary solar 

cell results from devices made using CuIn1-xGaxSe2 thinfilm absorber materials. Achieving an energy conversion 

efficiency of >12% and a quantum efficiency of ~80% 

demonstrate the feasibility of the concept. A discussion of 

the device structures will be presented considering the 

physical properties of the new electrodes comparing 

current-voltage results from the new solar cell structure 

and those from standard ZnO/CdS/Cu(In,Ga)Se2/Mo solar 

cells. 

INTRODUCTION 

State-of-the-art thin-film photovoltaic (PV) devices 

incorporate a transparent conductive oxide (TCO) as a 

“window” layer in their device structure. TCO materials 

such as ZnO:Al and In2O3:Sn (ITO) work relatively well in 

thin-film PV applications, demonstrated by energy 

conversion efficiencies of 19.5% attained in CuIn1-

xGaxSe2-based (CIGS) solar cells [1]. However, TCOs limit 

the device photo-generated current due to a rather strong 

free-carrier absorption of infrared (IR) photons. Eliminating 

this absorption is difficult because it requires reducing the 

free-carrier concentration [2]. 

New transparent conductive coatings are needed in 

thin-film PV technology to provide higher transmittance in 

the IR portions of the spectrum, while maintaining high 

electrical conductivity. Other desirable qualities of these 

new materials are simplicity of deposition, run-to-run 

reproducibility, and low cost. One type of coating that may 

be able to achieve this consists of randomly oriented 

bundles of single-wall carbon nanotubes (SWCNT). 

EXPERIMENTAL AND RESULTS 

 Invisicon® is a transparent conductive coating, 

produced by EIKOS, consisting of networked bundles of 

SWCNTs that can be encapsulated in a durable polymer. 

The coating is made from arc-produced SWCNTs purified 

by acid reflux, washing, and centrifugation to remove 

metal catalyst particles and non-tubular carbon. The 

purified SWCNTs are then dispersed in water and alcohol 

to form an ink that can be spray coated onto a heated 

substrate to form an essentially pure SWCNT coating. The 

morphology of these coatings is characterized by a 

SWCNT network with random distribution and a high 

density of voids between bundles of nanotubes. Figure 1 

shows a micrograph of a typical Invisicon layer deposited 

on a CdS/CIGS/Mo/glass substrate. 

SWCNT-based coatings have already been used as 

transparent electrodes in organic PV devices [3]. However, 

to the best of our knowledge, they have not been 

employed in inorganic PV devices. The SWCNT-based 

coatings are highly conductive in thin layers, and optical 

measurements, such as the transmission data shown in 

Fig. 2, indicate good transmission (>80%) in the IR 

(wavelengths >1200 nm) and the absence of free-carrier 

1 This work has been authored by an employee or employees of the Midwest Research Institute under Contract No. DE-AC36-99GO10337 with the 

U.S. Department of Energy. The United States Government retains and the publisher, by accepting the article for publication, acknowledges that the 

United States Government retains a non-exclusive, paid-up, irrevocable, worldwide license to publish or reproduce the published form of this work, or 

allow others to do so, for United States Government purposes.

1
100

80

60

40

20

0

Percent Transmission

500 1000 1500 2000

Wavelength (nm)

 CNT 50 ohm/sq

 CNT 100 ohm/sq

 ZnO:Al 22 ohm/sq

 ZnO:Al 76 ohm/sq

 

Fig. 2. Optical transmission data in selected SWCNT 

and n-ZnO layers. 

Table 1. Device results for solar cells with structure of Invisicon®/i-ZnO/CdS/CIGS/Mo using varied sheet 

resistance values for the SWCNT layers. State-of-the-art ZnO/CdS/CIGS values shown in first row for reference 

SWCNT 

(ohm/sq)

Voc 

(V)

Jsc

(mA/cm2

)

FF 

(%)

Eff. 

(%)

Rs

(ohm-cm)

Rsh

(ohm-cm)

n/a 0.693 35.34 79.4 19.5% 1.449 8798.10 

100 0.663 30.43 63.16 12.74 5.072 736.30

100 0.674 31.21 61.66 12.98 5.826 596.70

100 0.680 29.75 63.26 12.79 5.643 1158.84

 

75 0.650 28.34 61.67 11.3 3.2 165.47

75 0.621 29.71 62.23 11.4 3.25 216.92

75 0.622 29.12 64.93 11.7 3.22 510.78

 

50 0.598 25.02 62.13 9.30 3.656 265.05

50 0.593 25.03 61.75 9.17 4.08 244.17

50 0.593 25.10 61.65 9.18 4.141 242.90

Fig. 3. Cross section SEM micrograph for device 

structure Invisicon/i-ZnO/CdS/CIGS/Mo.

absorption in that region. This characteristic of SWCNT 

layers is a significant qualitative difference as compared to 

traditional TCOs such as ZnO and ITO. Figure 2 shows 

the optical transmission data for comparable sheet 

resistances of SWCNTs and ZnO. These values of sheet 

resistances (20–100 ohm/sq) were selected to represent 

typical values used in solar cells that incorporate a topcontact grid typically made by evaporating Ni (~50 nm) 

and Al (~3 μm) and deposited over the TCO using a 

shadow mask. 

Our standard ZnO/CdS/CIGS/Mo solar cells actually 

incorporate a bilayer ZnO made of 50–100-nm-thick 

intrinsic ZnO capped with a 100–130-nm-thick n-type ZnO.

To evaluate Invisicon as a replacement for ZnO, we 

have fabricated (and tested) device structures composed 

of varied materials, compositions, and layer thicknesses. 

SWCNT/i-ZnO/CdS/CIGS/Mo device structure 

 We used an Invisicon/i-ZnO/CdS/CIGS/Mo device 

structure (see cross-section view in Fig. 3) to preliminarily 

evaluate the coatings as a replacement for the n-type 

ZnO. The CIGS absorber layer was grown by evaporation 

from elemental sources onto molybdenum coated sodalime glass. CdS was deposited by chemical-bath 

deposition. After rf sputtering a thin layer of i-ZnO, the 

devices were shipped to EIKOS to be coated with 50–100 

ohm/sq Invisicon. The coated devices were then returned 

to NREL, where metal grids were applied and the cells 

were mechanically scribed and evaluated. 

 Current density-voltage (J-V) data obtained in initial 

experiments are summarized in Table 1. We point out that 

2
0.8

0.6

0.4

0.2

Internal QE

400 600 800 1000

nm

 19.5% Eff. cell (std. cell)

 SWNT 100 ohm/sq

 SWNT 50 ohm/sq

Fig. 4. Internal quantum efficiency of SWCNT/iZnO/CdS/CIGS/Mo structures using Invisicon layers 

with different sheet resistance values. The 19.5%-

efficiency cell data are for a standard solar cell (using 

n-ZnO) and are shown for reference. 

-40

-30

-20

-10

0

10

20

30

current density [mA/cm2

]

-0.4 -0.2 0.0 0.2 0.4 0.6 0.8 1.0

Voltage [V]

SWCNT/CdS/CIGS/Mo

 60 nm CdS

 90 nm CdS

Voc Jsc FF (%) Eff. (%)

0.341 28.9 18.5 1.8

0.542 30.6 24.4 4.1

 Fig. 5. Effect of CdS thickness on device performance 

for the 100 ohm/sq SWCNT/CdS/CIGS/Mo structures. 

the device structure, absorber film, and device processing 

have not yet been optimized, and the data presented 

herein should be considered preliminary in nature. 

 The data in Table 1 show two shortcomings in the new 

device structure: (a) lower short-circuit current density (Jsc) 

values as compared to standard and similar devices 

incorporating n-ZnO, and (b) low shunt resistance, 

possibly associated with the thickness of the SWCNT 

coating (usually <100 nm for those sheet resistance 

values) and the morphology (voids) that could allow short 

circuiting between the metal grid and the layers 

underneath the Invisicon. 

 The lower Jsc values can be directly attributed to the 

overall lower internal quantum efficiency (QE) of the new 

structures (see Fig. 4). Although the SWCNT coatings are 

more transparent in the IR than n-ZnO:Al, they are less 

transparent in the visible and ultraviolet (UV) regions for 

equivalent sheet resistance (see Fig. 2) . The lower optical 

transmission across the UV, visible, and up to ~1200 nm 

contributes strongly to the loss in QE. In other words, 

some of the photons that could generate photocurrent are 

absorbed in the SWCNT layer before they reach the 

electronic junction of the device. 

Invisicon /CdS/CIGS/Mo device structure 

 It is of technological interest to simplify the device 

fabrication process by completely replacing the ZnO 

bilayer using another robust, low-cost, high-throughput, 

and scalable approach, such as the one described here. 

For this purpose, we have fabricated 

Invison/CdS/CIGS/Mo test structures incorporating varying 

CdS thickness and obtained the J-V characteristics (a 

selected set of J-V curves are shown in Fig. 5). 

 In general, the J-V data obtained indicate a much 

lower performance (efficiency <4%) and a different 

behavior as compared to the previous structure 

incorporating i-ZnO. The lower performance arises from a 

significant reduction in the open-circuit voltage (Voc) and fill 

factor (FF) parameters (as seen in Fig. 5). It seems the iZnO (absent in these structures) plays a role in mitigating 

some of the shunting and recombination issues observed. 

This argument is also supported by the J-V data obtained 

for devices using thicker (than standard) CdS layers. The 

thicker CdS also mitigates, to a certain extent, the 

shunting and loss of Voc and FF, as seen in Fig. 5. 

Emulating the electrical characteristics of the ZnO 

bilayer by using SWCNT layers and Parylene-N 

 For the reasons mentioned above, we decided to build 

a device structure that mimics the electrical qualities of the 

ZnO bilayer; that is, we deposited a high sheet resistance 

layer on top of the CdS, followed by a highly conducting 

layer. To achieve this effect, test structures were built 

using 1000 ohm/sq SWCNT layers on top of the CdS, 

capped by a thin layer (100–200 nm) of Parylene-N. 

Parylene-N is a highly insulating polymer used in 

conjunction with the 1000 ohm/sq SWCNT layer to replace 

the i-ZnO film. This high sheet resistance composite layer 

was subsequently capped with highly conductive SWCNT 

(100 ohm/sq). 

 The J-V data obtained (e.g., see Fig. 6) indicate a 

significant improvement in performance when compared to 

using no Parylene-N (as in the previous case). The best 

3
-30

-20

-10

0

10

20

30

J (mAcm2

)

-0.4 0.0 0.4 0.8

V (Volts)

100 ohm CNT/100 nm Parylene/1000 ohm CNT/CdS/CIGS

 C2050-33 #4 

Fig. 6. J-V characteristics of solar cell test structure 

incorporating Parylene-N. 

cells we made using this approach showed energy 

conversion efficiency >8%. As an example, the J-V 

parameters for the device data shown in Fig. 6 are: 

Voc=0.676 V, Jsc=23.9 mA/cm2

, FF=45.48%, and 

efficiency=8.2%. 

 Evidently, the performance is still lower than structures 

incorporating i-ZnO; but, as previously mentioned, these 

structures have not been optimized (neither in thickness 

nor in sheet resistance values), and they should be 

considered preliminary. Nevertheless, the approach shows 

potential for additional improvements and demonstrates 

that the ZnO bilayer can be completely replaced by 

SWCNT-based layers. 

CONCLUSIONS AND FINAL REMARKS 

 SWCNT electrodes warrant further study in thin-film 

PV devices. It may be possible to increase the optical 

transmission with a correlated reduction of resistivity in the 

SWCNT coatings by diameter, length, and metallic 

selection and improvement in the network morphology. 

Alternatively, the simplicity of the deposition process for 

SWCNTs and the low cost of the coatings may offset the 

loss in efficiency. Manufacturing complexity is a significant 

challenge to the commercialization of CIGS thin-film PV 

technology. If SWCNT coatings could replace multiple 

layers of the device (n-ZnO:Al/i-ZnO/CdS), they would 

greatly enhance the manufacturability and reduce overall 

module cost. 

 We have demonstrated that ZnO can indeed be 

replaced by the approach described in this contribution. 

Furthermore, due to the unusual optoelectronic properties 

of the SWCNTs, particularly the high conductivity and 

superior optical transmission in the IR (>1200 nm) as 

compared to traditional TCOs, it may be possible to apply 

them in other PV technologies that utilize more of the IR 

portion of the solar spectrum. For example, thermophotovoltaic (TPV) devices and bottom cells in 

multijunction tandem structures. 

 Perhaps a more fundamental question is whether 

SWCNTs can be used as absorber materials as well, that 

is, as the active material of a solar cell. For this to happen, 

the tailoring of optical properties (bandgap) and electrical 

properties (conductivity type) must be addressed. 

ACKNOWLEDGEMENTS 

 The authors wish to thank the following personnel at 

NREL: Dr. Rommel Noufi, Dr. Dave Ginley and Dr. Philip 

Parilla for their critical reviews, comments, suggestions 

and technical discussions; Mr. James Dolan and Mr. 

James Keane for their technical assistance in device 

processing, equipment and hardware maintenance. 

 This work has been authored by an employee of the 

Midwest Research Institute under Contract No. DE-AC36-

99GO10337 with the U.S. Department of Energy. The 

United States Government retains and the publisher, by 

accepting the article for publication, acknowledges that the 

United States Government retains a non-exclusive, paidup, irrevocable, worldwide license to publish or reproduce 

the published form of this work, or allow others to do so, 

for United States Government purposes. 

REFERENCES 

[1] M.A. Contreras, K. Ramanathan, J. AbuShama, F. 

Hasoon, D. L. Young, B. Egaas, and R. Noufi. Prog. 

Photovolt: Res. Appl. 13: 2005, 209–216. 

[2] T.J. Coutts, D.L. Young, and X. Li, Characterization 

of Transparent Conducting Oxides. MRS Bulletin 25(8): 

2000, p.58. 

[3] A.D. Pasquier et al., “Conducting and transparent 

single-wall carbon nanotube electrodes for polymerfullerene solar cells.” Applied Physics Letters 87(20), 

2005, 203511-1-3 

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