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

Office of Energy Efficiency & Renewable Energy

National Renewable Energy Laboratory 

Innovation for Our Energy Future 

Optimizing Carbon Nanotube 

Contacts for Use in Organic 

Photovoltaics 

Preprint 

T.M. Barnes, J.L. Blackburn, R.C. Tenent, 

M. Heben, and T.J. Coutts 

National Renewable Energy Laboratory 

A. Morfa 

University of Colorado-Boulder

Presented at the 33rd IEEE Photovoltaic Specialists Conference 

San Diego, California 

May 11–16, 2008 

Conference Paper 

NREL/CP-520-42550 

May 2008 

NREL is operated by Midwest Research Institute ● Battelle Contract No. DE-AC36-99-GO10337 
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OPTIMIZING CARBON NANOTUBE CONTACTS FOR USE IN ORGANIC 

PHOTOVOLTAICS 

Teresa M. Barnes1, Jeffrey L. Blackburn1, Robert C. Tenent1, Anthony Morfa2, Michael Heben1, and Timothy J. Coutts1

1

National Renewable Energy Laboratory, Golden, CO, 2Dept. of Chemistry, University of Colorado, Boulder, CO 

ABSTRACT 

Transparent electrical contacts (TCs) play an important 

role in thin film photovoltaics (PV), including in organic 

devices (OPV). OPV devices have different TC 

requirements than traditional thin film devices, due to the 

need to make them flexible, cost and manufacturing 

constraints, and the potential benefits that may be realized 

from using a hole conducting TC. Additionally, the active 

layer in an OPV device is far thinner than in an inorganic 

“thin-film” device, causing TC morphology to have a 

significant effect on device performance. Here, we 

present data on optimizing carbon nanotube networks for 

use as TCs in OPV. We look at network deposition 

techniques and required post-treatment methods along 

with electrical, optical, and morphological data to devise a 

fabrication process for a high quality network that is 

compatible with OPV device production. 

INTRODUCTION 

Several groups have proposed replacing traditional 

transparent conducting oxides (TCOs) with thin films made 

of single-wall carbon-nanotube (SWCNT) networks.[1-4] 

These SWCNT films can be deposited successfully on 

flexible substrates using low-cost, low-temperature 

deposition methods. One major advantage of the SWCNT 

films over TCOs such as In2O3:Sn (ITO), ZnO, and SnO2

is that their fabrication process readily produces a 

preferentially hole-conducting contact.[5] This is 

particularly advantageous in OPV devices, and we have 

achieved efficient devices in which the SWCNT network 

replaces both ITO and PEDOT:PSS in a typical bulk 

heterojunction device.[6] Others have also had success 

replacing the ITO layer in a bulk heterojunction device on 

a flexible substrate[7] and augmenting the ITO layer on 

glass with a SWCNT film.[8] In addition to our work on 

OPV, we have also successfully replaced the ZnO bi-layer 

in an efficient CuIn2Se3/CdS (CIGS) device and replaced 

the ITO back contact in a transparent CdTe device.[9, 10] 

Previous work on p-GaN indicates that SWCNT network 

electrodes can produce an ohmic contact to a p-type 

semiconductor. [5] Transparent SWCNT networks have 

also been incorporated into flexible, transparent 

transistors,[11] organic light-emitting diodes (OLEDs),[12] 

and chemical sensors.[13] 

Bulk SWCNTs contain a random distribution of differing 

lengths, diameters, and chiralities. This distribution 

generally consists of about two-thirds semiconducting 

tubes and one-third metallic tubes. The tubes can be 

made by a variety of methods including chemical-vapor 

deposition, laser ablation of a graphite target, and electricarc decomposition of a target. As produced, the tubes are 

intermixed with large amounts of amorphous carbon and 

metallic impurities (e.g. Ni, Co), both of which must be 

removed before the tubes are useful for electronic 

applications. [14] A variety of purification procedures are 

used, but most rely on an acid reflux that can 

unintentionally (but often beneficially) dope the 

semiconducting tubes p-type. [15] This, along with doping 

from atmospheric impurities, [16] is thought to influence 

the optoelectronic performance of the films. The carrier 

concentration in the networks is very difficult to assess, 

and work is ongoing in this area.[17] It may be possible to 

determine carrier density from a free carrier absorption 

edge in the IR, and optical probes of mobility are being 

investigated. Preliminary data indicate that the mobility is 

low. 

Transport in SWCNT networks is poorly understood, but it 

seems that some basic concepts from traditional TCOs 

may prove helpful in improving their performance. 

SWCNT networks can be doped to improve conductivity, 

but the actual function of chemical dopants in the networks 

is not well understood. TCO performance data suggest 

that improving mobility in the networks should be a major 

thrust of future research. 

It is important to compare the optical and electrical 

properties of SWCNT networks with the TCOs that they 

may potentially replace in OPV including ZnO and SnO2 in 

addition to ITO. Figure 1 illustrates the differences in 

optical and electrical properties between a commercial 

SWCNT network (produced by Eikos, Inc.), SWCNT 

networks made at NREL by different techniques, and the 

most commonly used TCOs for PV devices. There are 

several critical differences between the TCOs and the 

SWCNT TCs. First, the visible transmittance of the 

SWCNT TCs is generally lower than that of the TCOs, and 

many groups are working to improve this. Also, the 

transmission of the SWCNT TCs in the near infrared is 

dramatically higher than that of the TCOs because the 

SWCNT TCs do not exhibit free carrier absorption at these 

wavelengths. The transmission of the NREL produced 

electrodes is rapidly approaching that of the commercial 

material. In addition to comparing Rs and transmission, it 

is critical to keep in mind any deposition constraints 

imposed by the device or manufacturing process when 

comparing different TCs. For example, it is misleading to 

compare a flexible SWCNT network deposited at room -

1
temperature to a high-temperature sputtered or CVD 

grown TCO film on glass if the desired application requires 

an electrode grown at room temperature on a flexible 

substrate. 

In this report, we study film morphology, conductivity, and 

optical properties of the networks in order to produce a 

high-performance transparent contact that is well suited to 

deployment in high-efficiency OPV devices. We also 

study the effects of different device processing parameters 

EXPERIMENTAL METHODS 

We compare two deposition methods for SWCNT network 

TCs. One fabrication procedure is a straightforward 

adaptation of a literature method.[1] First, carbon 

nanotubes are produced at NREL by pulsed laser 

vaporization (PLV). The as-produced tubes are then 

purified via acid reflux and sonicated with various 

surfactants to disperse them into solution. These 

solutions are then diluted in large quantities of DI water 

and carefully poured onto a mixed-cellulose ester 

membrane in a filtration flask. The solution is filtered 

through the membrane and a glass frit and then rinsed 

with 100 ml of DI water to remove the surfactant. The 

membrane can then be acid treated (as will be discussed 

later) or used directly to produce an electrode. The 

SWCNT network can be transferred to a substrate or an 

existing device by wet pressing or an acetone vapor bath. 

Acetone vapor bath transfer is preferable for transferring 

directly onto existing polymer layers because it does not 

rely on water to accomplish the transfer. 

The second method for producing the networks uses 

ultrasonic spray deposition. Ultrasonic spray is readily 

scalable and amenable to patterning and deposition on 

flexible substrates, which makes it potentially suitable for 

large-scale manufacturing. We use the same purified 

PLV grown SWCNT as for the membrane transfer. The 

tubes are dispersed into an ink solution using one of 

several surfactants into an aqueous solution using an 

ultrasonic bath. We,[18] and others,[19] have seen that 

the mixing conditions in the ultrasonic bath can affect the 

electrode properties. The ink is then sprayed onto a 

gently heated substrate to form the TC and dry the film. To 

date, we have deposited only on glass substrates. 

Network TCs made by both deposition processes 

generally require some sort of post-treatment process to 

improve their conductivity. In the membrane process, the 

surfactant is removed by copious rinsing through the 

membrane before it is used to transfer the film. The 

networks made this way can also be doped by soaking in 

acid, and this can be done before or after the film is 

transferred to the substrate. Some films/membranes were 

rinsed before use in OPV devices to study the effects of 

residual acids on device performance. The spray 

deposited networks are generally soaked in nitric acid 

after deposition, and then rinsed in DI water. The network 

TCs were characterized using a variety of techniques 

including spectrophotometry, four-point probe, atomic 

force microscopy (AFM), and optical microscopy. 

Figure 1: Optical transmission as a function 

of wavlength for several TCOs and nanotube 

networks made by membrane transfer and 

spray at NREL and one commerical SWCNT 

network from Eikos, Inc. 

These devices were all made in the standard configuration 

where the SWCNT network was deposited directly on a 

glass substrate. The network TCs were ultrasonically 

cleaned in isopropyl alcohol for 5 minutes. The ITO 

reference device was ultrasonically cleaned in acetone 

and then isopropyl alcohol for 5 minutes each, followed by 

a 5 minute oxygen plasma clean. Poly (3,4-

ethylenedioxythiophene) poly (styrenesulfonate) 

(PEDOT:PSS, Baytron 4083) is often used as a hole 

transport layer in OPV devices on ITO coated substrates, 

and we deposited it on several of the SWCNT networks by 

spin-coating twice at 4000 rpm for 60 seconds each. 

Some of the coated networks and the ITO reference 

device were then annealed. The devices were transferred 

into the glovebox, where the active layers were deposited. 

The best devices were made using 1:1 mixtures of poly(3-

hexylthiophene): phenyl-C61-butyric acid methyl ester 

(P3HT:PCBM) in dichlorobenzene at a concentration of 25 

mg/mL spun on at 200 rpm and slow dried, or “solvent 

annealed”,{Li, 2007 #924} in a covered glass petri dish for 

several hours. The devices were then annealed at 110o

C 

for 10 minutes in the glove box and transferred to a 

thermal evaporator where they were coated with 20 nm of 

Ca and 100 nm of Al to form the back contact. The 

devices were then tested on a solar simulator in the glove 

box. 

RESULTS AND DISUSSION 

SWCNT Network Characterization 

SWCNT network TCs produced by the membrane transfer 

process were used for our initial attempts to produce OPV 

devices. The networks were highly conductive, especially 

after treatment with HNO3 or SOCl2. However, the 

devices always failed when made on the acid treated 

networks due to wetting problems and adhesion failures of 

the active layer. Building devices on networks that had 

not been acid treated was also generally unsuccessful. 

These networks were more resistive by a factor of about 

two (140 Ω/sq for an “as-produced” network and about 70 

Ω/sq for a SOCl2 treated network), which should have 

reduced efficiency, but not killed the devices entirely. The 

2
devices all function as resistors, meaning there is likely a 

direct short between the top and bottom contacts. 

Figure 2a shows an AFM image of a typical SWCNT 

network produced by membrane transfer over a 20 μm2

area. Although the networks are generally flat over this 

area, there are several large-scale features that are over 

500 nm in height. The features range in size from small 

chunks to long thin wrinkles are likely induced by 

imperfections in the membrane transfer process. It is 

difficult to tell whether texturing from the membrane itself 

plays a role in the film morphology. Remembering that the 

active layers in these devices are only 200-300 nm thick, 

an electrode with 500 nm features could clearly induce 

shorts in the devices. 

The AFM image in Figure 2b illustrates the difference in 

morphology between sprayed and membrane transferred 

films produced at NREL. The sprayed film shown in a 10 

μm scan in Figure 2b is smooth and almost featureless 

with a maximum feature height on the order of 50 nm. It is 

wrinkle free with a very uniform morphology that is typical 

of our spray deposited films. This film was taken from the 

set used to make the devices described in this work. 

The sprayed SWCNT network TCs used in efficient 

devices were first soaked in HNO3 overnight, then rinsed 

three times in DI water to remove residual acid. Acidsoaked films have a sheet resistance of about 60 Ω/sq, 

and this value increases to about 108 Ω/sq after the water 

rinsing. Transmission spectra for a doped and 

subsequently rinsed film are given in Figure 4. The 

spectrum for the rinsed film shows an average 

transmission of 77% across the visible range. Absorption 

peaks due to fundamental excitonic transitions of the 

SWCNTs around 650 nm and 950 nm are completely 

bleached in the as-doped films, but the peaks at 650 and 

950 nm return upon rinsing. This indicates a high level of 

doping in the films, which slightly decreases upon rinsing. 

This decrease is commensurate with the observed change 

in sheet resistance.[17] 

OPV Device Fabrication 

Several sets of OPV devices were produced on sprayed 

SWCNT electrodes. Table 1 contains efficiency (η), opencircuit voltage (Voc), current density (Jsc), and fill factor 

data for the devices described here. This data is not 

NREL certified and there are certain caveats associated 

with measuring small OPV devices like the 0.1cm2

 devices 

discussed here. They are not masked, and they are highly 

susceptible to collection outside the active area of the 

device, which may artificially inflate Jsc and η. However, 

the data for the FF and Voc are fairly reliable. We are 

working on building larger devices that can be effectively 

masked off for more reliable efficiency measurements. 

The first devices made on the sprayed electrodes used 

drop-cast active layers, yielding devices with good 

efficiency and very poor reproducibility due to a highly 

non-uniform active layer thickness. The data for the best 

drop cast device, with an efficiency of approximately 2.9%, 

is included in Table 1. This device contained a thin layer 

of PEDOT:PSS as well.

The best OPV devices in this study were made using the 

sprayed electrode and very slow spin coating for the active 

layers (200 rpm). This yields a thicker layer, and the films 

are very wet after spin coating. Data for these devices is 

presented in Table 1, including data for an ITO reference 

and a SWCNT network device without PEDOT:PSS. 

Current density vs. Voltage curves for these devices are 

presented in Figure 5. Note that although the Jsc values 

(a) (b)

ransferred 

 nm, 

Figure 2: (a) AFM image of membrane t

film. The maximum height in the image is 500

and the dimensions are 20μm2

. (b) AFM image 

of a sprayed film with a maximum height of 42 nm 

and dimensions of 10 μm2. 

Figure 3: Transmission spectra for acid treated 

(as-doped) and subsequently rinsed sprayed 

SWCNT network films used in the OPV devices.

80

60

40

20

0

Transmission (%)

1000 1500 2000

 rinsed

Rs as-doped = 60 Ω/sq

Rs rinsed = 108 Ω/sq

500

Wavelength (nm)

 as-doped

he 

d is 

Device Voc (V) Jsc (mA cm-2) FF (%) η (%) 

Table 1: Device Voc, Jsc, FF, and η for a drop cast 

device on SWCNT and three spin coated devices. T

first is on sprayed SWCNT with PEDOT, the secon

on an ITO reference substrate, and the third is on a 

sprayed SWCNT electrode with no PEDOT. 

Drop cast 0.561 9.803 52.95 2.91 

spray 0.572 10.65 57.6 3.5 

ITO 0.571 11.09 55.0 3.5 

Spray w/o 

PEDOT 0.491 5.4 39.2 1.05 

3
are likely inflated by collection outside the active area, that 

the FF and Voc values are quite high. All of the values for 

the SWCNT device (with PEDOT:PSS) and the ITO 

reference are very similar, and the data are highly 

reproducible. 

The device without PEDOT:PSS shows poor performance 

compared with the others, but it is a rectifying device with 

measureable efficiency. The device made without 

PEDOT:PSS exhibits a much lower Voc and FF than the 

devices made with it. This may be due a combination of 

the lower workfunction of the SWCNT bare electrode 

compared to PEDOT:PSS and a reduced shunt resistance 

caused by shorting between the electrode and the active 

layer. 

CONCLUSIONS 

SWCNT network electrodes can work well in OPV devices 

using relatively thick active layers. The SWCNT network 

must be very smooth, and we find that this is best 

achieved using ultrasonic spray deposition. The electrode 

sheet resistance can be relatively high (~100 Ω/sq vs. ~10 

Ω/sq for ITO) without a significant drop in device 

performance provided that the network is smooth enough 

to prevent shorting through the active layer. It is also 

necessary to thoroughly rinse out any residual acid from 

the network to prevent damage to the active layer. 

Currently, PEDOT:PSS is required to yield an efficient 

device, but it may be eliminated in future devices once 

alternative means of altering the SWCNT network work 

function are achieved. 

Current Density (mA/cm2

)

ACKNOWLEDGEMENTS 

This work was supported by the U.S. Department of 

Energy under Contract No. DE-AC36-99GO10337 with the 

National Renewable Energy Laboratory.

REFERENCES 

[1] Z. Wu, C. Zhihong, D. Xu, J. M. Logan, J. Sippel, M. 

Nikolou, K. Kamaras, J. R. Reynolds, D. B. Tanner, A. F. 

Hebard, and A. G. Rinzler, "Transparent, Conductive 

Carbon Nanotube Films," Science, 305, 2004, 1273-1276. 

[2] G. Fanchini, H. E. Unalan, and M. Chhowalla, 

"Optoelectronic Properties of Transparent and Conducting 

Single-Wall 

Carbon Nanotube Thin Films," Applied Physics Letters, 

88, 2006, 191919. 

[3] H. Haiping, Z. Fei, Y. Zhizhen, Z. Liping, Z. Binghui, 

and H. Jingyun, "Defect-Related Vibrational and 

Photoluminescence Spectroscopy of a Codoped Zno:Al: N 

Film," Journal of Physics D (Applied Physics), 39, 2006, 

2339-2342. 

[4] G. Gruner, "Carbon Nanotube Films for Transparent 

and Plastic Electronics," Journal of Materials Chemistry, 

16, 2006, 3533-3539. 

[5] K. Lee, Z. Wu, Z. Chen, F. Ren, S. J. Pearton, and A. 

G. Rinzler, "Single Wall Carbon Nanotubes for p-Type 

Ohmic Contacts to Gan Light-Emitting Diodes," Nano 

Letters, 4, 2004, 911-914. 

[6] J. van de Lagemaat, T. M. Barnes, G. Rumbles, S. E. 

Shaheen, T. J. Coutts, C. Weeks, I. Levitsky, J. Peltola, 

and P. Glatkowski, "Organic Solar Cells with Carbon 

Nanotubes Replacing In2O3 : Sn as the Transparent 

Electrode," Applied Physics Letters, 88, 2006, 233503. 

40

20

0

-1.0 -0.5 0.0 0.5 1.0

Voltage (V)

ITO Reference

η = 3.5%

Jsc = 11.09

Voc = 570.6

FF = 55%

60

40

20

0

Current Density (mA/cm2

)

-1.0 -0.5 0.0 0.5 1.0

Voltage (V)

SWCNT

η = 3.5%

Jsc = 10.65

Voc = 571.8

FF = 57.6%

15

10

5

0

-5

Current Density (mA/cm2

)

-1.0 -0.5 0.0 0.5 1.0

Voltage (V)

NO PEDOT

η = 1.05%

Jsc = 5.4

Voc = 491.1

FF = 39.2%

40

20

0

ITO Reference

η = 3.5%

Jsc = 11.09

Voc = 570.6

FF = 55%

-1.0 -0.5 0.0 0.5 1.0

Voltage (V)

60

40

20

0

Current Density (mA/cm2

)

-1.0 -0.5 0.0 0.5

SWCNT

η = 3.5%

Jsc = 10.65

Voc = 571.8

FF = 57.6%

1.0

Voltage (V)

15

10

5

0

-5

Current Density (mA/cm2

)

-1.0 -0.5 0.0 0.5

NO PEDOT

η = 1.05%

Jsc = 5.4

Voc = 491.1

FF = 39.2%

1.0

Voltage (V)

Figure 5: Current Density vs. voltage plots 

where the dashed line is the dark current and 

the solid line is the light current for (a) the ITO 

reference device, (b) SWCNT device, and (c) 

the SWCNT device without PEDOT:PSS. 

4
[7] M. W. Rowell, M. A. Topinka, M. D. McGehee, H. J. 

Prall, G. Dennler, N. S. Sariciftci, L. B. Hu, and G. Gruner, 

"Organic Solar Cells with Carbon Nanotube Network 

Electrodes," Applied Physics Letters, 88, 2006, 233506. 

[8] R. Ulbricht, S. B. Lee, X. Jiang, K. Inoue, M. Zhang, S. 

Fang, R. H. Baughman, and A. A. Zakhidov, "Transparent 

Carbon Nanotube Sheets as 3-D Charge Collectors in 

Organic Solar Cells," Solar Energy Materials and Solar 

Cells, 91, 2007, 416-419. 

[9] M. A. Contreras, T. Barnes, J. vandeLagemaat, G. 

Rumbles, T. J. Coutts, C. Weeks, P. Glatkowski, I. 

Levitsky, J. Peltola, and D. A. Britz, "Replacement of 

Transparent Conductive Oxides by Single-Wall Carbon 

Nanotubes in Cu(in,Ga)Se2-Based Solar Cells," Journal of 

Physical Chemistry C, 111, 2007, 14045-14048. 

[10] T. M. Barnes, X. Wu, J. Zhou, A. Duda, J. van de 

Lagemaat, T. J. Coutts, C. L. Weeks, D. A. Britz, and P. 

Glatkowski, "Single-Wall Carbon Nanotube Networks as a 

Transparent Back Contact in Cdte Solar Cells," Applied 

Physics Letters, 90, 2007, 243503. 

[11] E. Artukovic, M. Kaempgen, D. S. Hecht, S. Roth, 

and G. GrUner, "Transparent and Flexible Carbon 

Nanotube Transistors," Nano Letters, 5, 2005, 757-760. 

[12] C. M. Aguirre, S. Auvray, S. Pigeon, R. Izquierdo, P. 

Desjardins, and R. Martel, "Carbon Nanotube Sheets as 

Electrodes in Organic Light-Emitting Diodes," Applied 

Physics Letters, 88, 2006, 183104. 

[13] E. S. Snow, F. K. Perkins, E. J. Houser, S. C. 

Badescu, and T. L. Reinecke, "Chemical Detection with a 

Single-Walled Carbon Nanotube Capacitor," Science, 307, 

2005, 1942-1945. 

[14] R. Saito, G. Dresselhaus, and M. S. Dresselhaus, 

Physical Properties of Carbon Nanotubes, 1 ed. London: 

Imperial College Press, 1998. 

[15] U. Dettlaff-Weglikowska, V. Skakalova, R. Graupner, 

S. H. Jhang, B. H. Kim, H. J. Lee, L. Ley, Y. W. Park, S. 

Berber, D. Tomanek, and S. Roth, "Effect of Socl2 

Treatment on Electrical and Mechanical Properties of 

Single-Wall Carbon Nanotube Networks," Journal of the 

American Chemical Society, 127, 2005, 5125-5131. 

[16] P. G. Collins, K. Bradley, M. Ishigami, and A. Zettl, 

"Extreme Oxygen Sensitivity of Electronic Properties of 

Carbon Nanotubes," Science, 287, 2000, 1801. 

[17] J. L. Blackburn, T. M. Barnes, M. C. Beard, Y.-H. 

Kim, R. C. Tenent, T. J. McDonald, T. J. Coutts, B. To, 

and M. J. Heben, "Transparent Conductive Single-Walled 

Carbon Nanotube Networks with Precisely Tunable Ratios 

of Semiconducting and Metallic Nanotubes," ACS Nano, 

in press, 2008. 

[18] R. C. Tenent, J. L. Blackburn, T. M. Barnes, and M. 

J. Heben, "Optimizing Ink Preparation for Ultrasonic Spray 

Deposition of Highly Conductive and Transparent SWCNT 

Networks " In preparation, 2008. 

[19] M. Kaempgen, M. Lebert, M. Haluska, N. Nicoloso, 

and S. Roth, "Sonochemical Optimization of the 

Conductivity of Single Wall Carbon Nanotube Networks," 

Advanced Materials, 20, 2008, 616. 

[20] G. Li, Y. Yao, H. Yang , V. Shrotriya, G. Yang, and Y. 

Yang, "“Solvent Annealing” Effect in Polymer Solar Cells 

Based on Poly(3-Hexylthiophene) and 

Methanofullerenes," Advanced Functional Materials, 17, 

2007, 1636-1644. 

5
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Optimizing Carbon Nanotube Contacts for Use in Organic 

Photovoltaics: Preprint 

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14. ABSTRACT (Maximum 200 Words)

Transparent electrical contacts (TCs) play an important role in thin film photovoltaics (PV), including in organic 

devices (OPV). OPV devices have different TC requirements than traditional thin film devices, due to the need to 

make them flexible, cost and manufacturing constraints, and the potential benefits that may be realized from using a 

hole conducting TC. Additionally, the active layer in an OPV device is far thinner than in an inorganic “thin-film” 

device, causing TC morphology to have a significant effect on device performance. Here, we present data on 

optimizing carbon nanotube networks for use as TCs in OPV. We look at network deposition techniques and 

required post-treatment methods along with electrical, optical, and morphological data to devise a fabrication process 

for a high quality network that is compatible with OPV device production. 

15. SUBJECT TERMS

PV; transparent electrical contacts; thin film; organic devices; optimizing; carbon nanotube; 

16. SECURITY CLASSIFICATION OF: 19a. NAME OF RESPONSIBLE PERSON 

a. REPORT 

Unclassified 

b. ABSTRACT 

Unclassified

c. THIS PAGE 

Unclassified

17. LIMITATION 

OF ABSTRACT

UL 

18. NUMBER 

OF PAGES 

19b. TELEPHONE NUMBER (Include area code)

Standard Form 298 (Rev. 8/98) 

Prescribed by ANSI Std. Z39.18