Single-walled carbon nanotube transparent conductive films fabricated by reductivedissolution and spray coating for organic photovoltaicsAminy E. Ostfeld, Amélie Catheline, Kathleen Ligsay, Kee-Chan Kim, Zhihua Chen, Antonio Facchetti, SiânFogden, and Ana Claudia Arias Citation: Applied Physics Letters 105, 253301 (2014); doi: 10.1063/1.4904940 View online: http://dx.doi.org/10.1063/1.4904940 View Table of Contents: http://scitation.aip.org/content/aip/journal/apl/105/25?ver=pdfcov Published by the AIP Publishing Articles you may be interested in Conductive nanocomposite films based on functionalized double-walled carbon nanotubes dispersed inPEDOT:PSS AIP Conf. Proc. 1459, 259 (2012); 10.1063/1.4738462 Transparent conductive thin films of single-wall carbon nanotubes encapsulating dopant molecules Appl. Phys. Lett. 100, 063121 (2012); 10.1063/1.3684811 Organic solar cells using few-walled carbon nanotubes electrode controlled by the balance between sheetresistance and the transparency Appl. Phys. Lett. 94, 123302 (2009); 10.1063/1.3103557 Conducting and transparent single-wall carbon nanotube electrodes for polymer-fullerene solar cells Appl. Phys. Lett. 87, 203511 (2005); 10.1063/1.2132065 Conducting transparent thin films based on Carbon Nanotubes — Conducting Polymers AIP Conf. Proc. 723, 591 (2004); 10.1063/1.1812156

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Single-walled carbon nanotube transparent conductive films fabricatedby reductive dissolution and spray coating for organic photovoltaics

Aminy E. Ostfeld,1 Am�elie Catheline,1,2 Kathleen Ligsay,2 Kee-Chan Kim,2 Zhihua Chen,3

Antonio Facchetti,3,4 Sian Fogden,2 and Ana Claudia Arias1,a)

1Department of Electrical Engineering and Computer Sciences, University of California, Berkeley,California 94720, USA2Linde Nanomaterials, Linde LLC, 1970 Diamond Street, San Marcos, California 92078, USA3Polyera Corporation, 8045 Lamon Avenue, Skokie, Illinois 60077, USA4Center of Excellence for Advanced Materials Research, King Abdulaziz University, Jeddah 21589,Saudi Arabia

(Received 28 October 2014; accepted 9 December 2014; published online 22 December 2014)

Solutions of unbundled and unbroken single-walled carbon nanotubes have been prepared using a

reductive dissolution process. Transparent conductive films spray-coated from these solutions show

a nearly twofold improvement in the ratio of electrical conductivity to optical absorptivity versus

those deposited from conventional aqueous dispersions, due to substantial de-aggregation and

sizable nanotube lengths. These transparent electrodes have been utilized to fabricate P3HT-PCBM

organic solar cells achieving power conversion efficiencies up to 2.3%, comparable to those of

solar cells using indium tin oxide transparent electrodes. VC 2014 AIP Publishing LLC.

[http://dx.doi.org/10.1063/1.4904940]

One of the great challenges in the field of organic elec-

tronics has been the replacement of the ubiquitous transpar-

ent electrode material, indium tin oxide (ITO), with an earth-

abundant, low-cost, solution processable, and flexible alter-

native.1–5 Several transparent conducting films (TCFs) have

emerged as potential replacements for ITO, including con-

ducting polymers,6 silver nanowires,7 graphene,5,8 and

single-walled carbon nanotubes (SWNTs). SWNTs are a

promising alternative to ITO due to the great abundance of

carbon; the potential for cost reduction due to economies of

scale; exceptional flexibility and conductivity; and the ability

to deposit nanotube networks from solution using additive,

low-temperature printing and coating processes.5,8–13 Carbon

nanotube TCFs have been successfully utilized in organic

optoelectronic devices, including light-emitting diodes,9

photodiodes,10 and photovoltaics.12–16

However, while SWNTs have the potential to form net-

works with simultaneously high conductivity and high trans-

parency, experimentally measured conductivity of transparent

SWNT networks tends to be much lower than that of ITO.

This is in large part because the typical ink fabrication

method, dispersing SWNTs in water with the aid of sonica-

tion, damages and shortens the SWNTs.16,17 Conductivity in

thin SWNT networks follows the percolation theory and is

further limited by contact resistance at the tube-tube junctions.

Therefore, to achieve high conductivity, a SWNT network

should consist of individual tubes or small bundles, which

have low contact resistance and a large number of percolation

pathways, and the SWNTs should be of maximum length to

minimize the number of junctions.18–20 An alternative ink fab-

rication method known as reductive dissolution has been

developed to produce solutions of highly dispersed and unbro-

ken SWNTs.21–23 In these processes, SWNTs are chemically

reduced in the presence of an alkali metal either in liquid am-

monia or in a naphthalenide solution in tetrahydrofuran. The

reactive media is then removed, leaving a nanotubide salt that

dissolves with only gentle mechanical stirring, without sonica-

tion, in a polar organic solvent such as dimethylsulfoxide

(DMSO) or dimethylformamide. In this work, the reductive

dissolution technique using liquid ammonia is employed to

produce SWNT TCFs with improved conductivity versus

those deposited from aqueous dispersion. We then demon-

strate the use of these SWNT TCFs in organic photovoltaic

(OPV) devices as the transparent anode.

CVD-grown SWNTs were purified and processed at

Linde Nanomaterials. The organic SWNT ink was prepared

using the liquid ammonia reductive dissolution technique

developed by Fogden et al.21,22 In this process, SWNTs were

chemically reduced in the presence of sodium and liquid am-

monia, and the resulting nanotubide salt was dissolved in

DMSO with magnetic stirring. Processing and deposition of

the organic ink was performed in a glovebox; after deposi-

tion by spray coating, the films were maintained under clean

dry air overnight to oxidize the nanotubide to the neutral spe-

cies. An aqueous SWNT dispersion was also produced, by

sonicating SWNTs in water with sodium dodecylbenzenesul-

fonate surfactant, and deposited under ambient conditions by

spray coating. Various SWNT film thicknesses were depos-

ited onto glass substrates, using multiple layers to produce

thicker films ranging in thickness from about 20 to 100 nm.

Thickness of the SWNT films was determined by scratching

the films and measuring the step height with atomic force

microscopy (AFM). Both aqueous and organic SWNT films

were immersed in a solution of nitric acid (at least 1 M)

diluted to 10% in isopropanol, to remove residual surfactant

or alkali metal salt and to p-dope the nanotubes. UV-visible

transmittance and Raman spectroscopy showed no signifi-

cant changes after the nitric acid treatment, indicating no sig-

nificant damage to the nanotubes.

a)Author to whom correspondence should be addressed. Electronic mail:

acarias@eecs.berkeley.edu

0003-6951/2014/105(25)/253301/4/$30.00 VC 2014 AIP Publishing LLC105, 253301-1

APPLIED PHYSICS LETTERS 105, 253301 (2014)

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Figure 1 shows transmission electron micrographs

(TEM) of SWNTs cast from the aqueous (a) and organic (b)

inks; several micrographs from each ink were used to deter-

mine the bundle size distributions (c). Many individual nano-

tubes can be seen in the micrograph of the organic ink, and

the vast majority of bundles were less than 10 nm in diame-

ter. The aqueous ink, on the other hand, showed larger bun-

dle sizes, very few individual tubes, and the presence of

residual surfactant between the nanotubes even after the

washing step. Dynamic light scattering, known as an effi-

cient technique to analyze the SWNT length as described by

Lucas et al.,17 confirmed that the SWNTs in the organic ink

maintained their original length of �20 lm, while those in

the aqueous dispersion were shortened to 1.9 6 0.2 lm, con-

siderably shorter than their original length.

The performance of a TCF is often quantified by the fig-

ure of merit (FOM) given by the ratio of the optical absorp-

tivity to the electrical conductivity [¼ �RsheetlnðTÞ], where

Rsheet is the sheet resistance in X/� and T is the optical

transmittance at k¼ 550 nm. Thus, a lower FOM value indi-

cates a higher performing TCF. For organic photovoltaics,

an FOM of 10 is considered adequate if a metal grid is used

in conjunction with the TCF, while without a metal grid, the

FOM should be �10� lower to minimize resistive losses.24

To determine the FOM, the sheet resistance of the

SWNT TCFs was measured using the 4-point probe tech-

nique, while the transmittance was measured by UV-visible

spectroscopy. Sheet resistance and transmittance values are

shown in Figure 2(a) for SWNT films of various thicknesses:

the thinner the film, the more transparent. Due to the consid-

erable SWNT length, smaller bundle size, and absence of

surfactant, TCFs from the reductive process ink consistently

afforded higher performance (average FOM¼ 15, minimum

12) than that from the aqueous dispersion (average

FOM¼ 26, minimum 24). The performance of the aqueous

SWNT film is in good agreement with that previously

reported in the literature for spray-deposited aqueous disper-

sions,10 while the organic SWNT is competitive with some

of the best performing carbon nanotube films.25–29 AFM,

transmittance, and spot-to-spot Raman measurements show

good uniformity and consistent results for each film. An

AFM micrograph for an organic SWNT film, showing the to-

pographical uniformity of the film, is given in Figure 2(b).

The suitability of the organic SWNT films as a transparent

electrode was assessed through their integration into OPV

devices. OPVs were fabricated using thin (311 X/�, 93%

transmittance) and thick (78 X/�, 78% transmittance) SWNT

films, and compared with control devices fabricated on ITO-

coated glass slides (Thin Film Devices, 20 X/�, ITO film

transmittance of 96.5%). To complete the solar cell structure, a

40-nm-thick hole transporting layer of poly(3,4-ethylenedioxy-

thiophene):poly(styrenesulfonate) (PEDOT:PSS, Clevios P VP

AI 4083, mixed with 3% DMSO and 22% isopropanol to

improve its conductivity and ability to wet the SWNT film)

was spin-cast onto the TCFs and baked at 130 �C for 10 min.

The addition of PEDOT:PSS reduced the SWNT film sheet re-

sistance by about 30% and the transmittance by about 4%. The

active layer, a 220-nm-thick bulk heterojunction of 1:1 poly(3-

hexylthiophene):[6,6]-phenyl-C61-butyric acid methyl ester

(P3HT:PCBM) was spin-cast from a 1,2-dichlorobenzene solu-

tion and solvent annealed at room temperature under a petri

dish overnight. Aluminum cathodes were deposited by thermal

evaporation, and the solar cells were annealed at 150 �C for

10 min. All active layer solution preparation and film process-

ing were carried out in a glovebox. Each sample was patterned

with several small cells (3 mm2 each) with three different dis-

tances to the anode contact, as shown in Figure 3(a), to investi-

gate the impact of this distance on the series resistance and fill

factor (FF) of the resulting devices.

Table I summarizes the performances obtained for the

ITO- and SWNT-based solar cells, as measured under

FIG. 2. (a) Sheet resistance and trans-

mittance at 550 nm wavelength of

SWNT films deposited from aqueous

and organic inks, compared to a com-

mercially available ITO film. The

black and red solid lines correspond to

constant FOM of 15 and 26, respec-

tively. (b) AFM image of SWNT film

spray-cast from organic ink. For inter-

pretation of the references to color in

this figure, the reader is referred to the

online version of this article.

FIG. 1. Transmission electron micrographs of SWNTs deposited from (a)

aqueous and (b) organic inks. (c) Bundle size distributions for organic and

aqueous inks.

253301-2 Ostfeld et al. Appl. Phys. Lett. 105, 253301 (2014)

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simulated air mass 1.5 global illumination. The ITO and thin

SWNT each gave an average power conversion efficiency of

2.2% and maximum efficiencies of 2.5% and 2.3%, respec-

tively. Figure 3(b) shows typical I-V characteristics for solar

cells with these two transparent electrodes, indicating that

the performance of cells with the thin SWNT transparent

electrode lies well within the typical range of the ITO solar

cell performances. The devices using the thick SWNT film,

however, had increased leakage currents due to the higher

SWNT film roughness, resulting in a lower open-circuit volt-

age, as shown in Figure 3(c). The lower transmittance of the

thicker SWNT film also reduced the short-circuit current

density slightly.

It is important to note that the highest OPV performance

with the thin SWNT film was obtained with the shortest dis-

tance to the anode contact, which is 1 mm. The disadvantage

of a high transparent electrode sheet resistance such as this

SWNT film is that it increases the series resistance of the so-

lar cell, reducing the fill factor. This effect is amplified when

charge carriers must travel a long distance within the trans-

parent electrode before reaching a lower-resistance, opaque

metal contact. Figures 3(d) and 3(e) show the increase in se-

ries resistance and reduction in fill factor, respectively, as the

distance to the anode contact is increased. With the longest

distance (7 mm), the effect of the series resistance is so

severe for the solar cell with the thin SWNT film that the ef-

ficiency decreases to 2.0%, below that of the thick SWNT

film. Cells with the lower-resistance TCFs (thick SWNT and

ITO), on the other hand, have lower series resistance and a

less significant reduction in FF for the longer distances.

For applications in large area modules, OPV devices

should have higher overall efficiencies and larger active area

than those presented in this work. Higher efficiencies can be

achieved through the use of optimized processing conditions

for the P3HT:PCBM, a lower-bandgap polymer in place of the

P3HT (see, e.g., Pierre et al.30), or a lower work function cath-

ode material. To achieve larger active areas while minimizing

losses due to series resistance, it will be necessary to add a

metal grid to the SWNT film, as has been proposed in the past

to supplement SWNT,15,31 PEDOT:PSS,6 and ITO32 TCFs.

This requirement will become even more stringent if a lower-

bandgap polymer is used in the active layer, as the increase in

photocurrent density would increase the resistive losses.

Nevertheless, the enhancement in TCF conductivity achieved

with the reductive dissolution process represents an important

step toward the viability of SWNT films as an ITO replace-

ment for organic optoelectronics. We expect these SWNT

films to become increasingly competitive with further optimi-

zation of the processing and deposition techniques, particularly

on flexible plastic substrates, and as the continued scaling up

of SWNT production lowers the manufacturing costs.

Portions of this work were performed as a User Project,

#1289, at the Molecular Foundry, supported by the Office of

Science, Office of Basic Energy Sciences, of the U.S.

Department of Energy under Contract No. DE-AC02-

05CH11231. This material is based upon work supported by

the National Science Foundation Graduate Research

Fellowship under Grant No. DGE 1106400. This work was

supported in part by the National Science Foundation under

Cooperative Agreement No. ECCS-1202189. A.F. thanks

KAU for financial support. A.C. thanks Linde Nanomaterials

for funding. A.E.O. and A.C. also thank Professor Rachel

Segalman and Professor Vivek Subramanian and the

Electron Microscope Lab at UC Berkeley for access to

characterization equipment.

FIG. 3. (a) Illustration of the structure

of organic solar cells used in this work.

(b) Current density-voltage character-

istics of solar cells using ITO and

SWNT transparent electrodes. The

light blue area corresponds to the typi-

cal I-V curve range for ITO-based so-

lar cells and the crosshatched area

corresponds to the typical range for

cells with the thin SWNT transparent

electrode. (c) Solar cell current

density-voltage characteristics on a

logarithmic scale, in the dark and

under illumination. (d) Series resist-

ance and (e) fill factor versus distance

to the anode contact. For interpretation

of the references to color in this figure,

the reader is referred to the online ver-

sion of this article.

TABLE I. Average solar cell performance parameters using ITO, thin

SWNT (311 X/�, 93%T), and thick SWNT (78 X/�, 78%T) transparent

electrodes. Highest values of power conversion efficiency achieved for each

TCF are given in parentheses.

TCF Voc (V) Jsc (mA/cm2) FF Efficiency (%)

ITO 0.504 9.4 0.47 2.2 (2.5)

SWNT, thin 0.501 9.6 0.45 2.2 (2.3)

SWNT, thick 0.467 8.6 0.52 2.1 (2.1)

253301-3 Ostfeld et al. Appl. Phys. Lett. 105, 253301 (2014)

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