Effective Work Function Modulation of Graphene/Carbon Nanotube Composite Films As Transparent Cathodes for Organic Optoelectronics

June 28, 2011

C 2011 American Chemical Society

E

ffective Work Function Modulation of

Graphene/Carbon Nanotube

Composite Films As Transparent

Cathodes for Organic Optoelectronics

Jen-Hsien Huang,

_{Jheng-Hao Fang,}

_{Chung-Chun Liu,}

_{and Chih-Wei Chu}

_*

†

_{Research Center for Applied Sciences, Academia Sinica, Taipei, Taiwan 11529 and}

_{Department of Photonics, National Chiao-Tung University, Hsinchu,}

Taiwan 30010

F

lexible optoelectronic devices have

spurred immense interest because of

their applications in displays, solar

cells, sensors, and hand-held

communica-tion systems.

Highly conductive indium

tin oxide (ITO) is frequently used as the

transparent electrode in optoelectronic

de-vices; diminishing indium resources,

com-plicated processing procedures, and poor

mechanical

flexibility have, however,

lim-ited its widespread applications in

flexible

systems.

Graphene, a molecule comprising a

sin-gle layer of carbon, has been touted for its

excellent electrical conducting properties

ever since its experimental isolation in

2004.

Graphene-based

films have been

investigated extensively for use as

transpar-ent electrodes because of their excelltranspar-ent

optical and electrical properties.

At

pre-sent, most large-scale graphene thin

films

are prepared through chemical vapor

de-position (CVD), followed by transfer printing

on a target substrate.

Unfortunately, the

high processing temperature required for

CVD restricts the growth of graphene to a

limited number of substrates. The transfer

procedure is also too complicated to scale

up, and the resulting graphene samples are

often readily contaminated and damaged.

Although an electrochemical exfoliation

method has been proposed

_{for the}

production of higher-quality graphene on

a large scale, this approach does not provide

stable, dispersed graphene solutions for

solution processing. An alternative method

for preparing large-area graphene thin

films

is through solution processing of

suspen-sions of stable graphene oxide (GO).

Although the conjugated networks can be

restored upon reduction in hydrazine vapor

or through thermal annealing after

deposi-tion, their conductivities do not rival those

of systems prepared through CVD, by virtue

of the large resistance originating from the

stack of individual GO pieces. In this regard,

a strategy of using carbon nanotubes (CNTs)

to tailor the electronic properties of GO has

been proposed.

In addition to requiring excellent

conduc-tivity and transparency of their electrodes,

the performance of organic optoelectronic

devices also relies on efficient carrier

injec-tion between the active layers and the

electrodes. Modulating the work function

(

Φ

) of the electrode to form an ohmic

contact with the active layer can enhance

the degree of charge injection, thereby

improving device performance.

Accord-ingly, work function engineering of

electro-des has been investigated extensively as a

means of improving optoelectronic

perfor-mance. In general, transparent electrodes

based on highly conductive polymers [e.g.,

polyaniline, poly(3,4-ethylenedioxythiophene)]

or graphene possess high values of

Φ

(ca. 5.0

5.2 eV).

Nevertheless,

func-tional electrodes with such high values of

Φ

can be used only as hole transport/

bu

ffer anodes in organic optoelectronic

* Address correspondence to [email protected]. Received for review April 4, 2011 and accepted June 28, 2011. Published online 10.1021/nn201253w

ABSTRACT

In this study, we found that the work functions (Φ

) of solution-processable,

functional graphene/carbon nanotube-based transparent conductors were readily manipulated,

varying between 5.1 and 3.4 eV, depending on the nature of the doping alkali carbonate salt. We

used the graphene-based electrodes possessing lower values of

Φ

as cathodes in

inverted-architecture polymer photovoltaic devices to e

ffectively collect electrons, giving rise to an optimal

power conversion e

fficiency of 1.27%.

KEYWORDS: graphene . transparent electrode . solar cells . surfactant . work function . cathode

devices; applying them in di

fferent configurations,

such as inverted or tandem solar cells, requires

mod-i

fication of the interface through insertion of a

func-tional interfacial layer or chemical doping to form a

low-

Φ

cathode. For graphene, the variation in the

work function can be achieved through chemical

dop-ing or modi

fication of the surface with an interfacial

layer to form interface dipoles.

Recently, it was

demonstrated that the presence of salts, particularly

alkali-based salts, in an n-type interfacial layer can

improve the e

fficiency of solar cells and thin film

transistors.

These results suggested the possibility

of controlling the values of

Φ

of graphene-based

electrodes. In the present study, we prepared highly

conductive and transparent graphene-based

electro-des with tunable work functions by combining

single-walled carbon nanotubes (SWCNTs) with chemically

reduced graphene (rGO). To produce hybrid

suspen-sions of rGO and CNTs, we dispersed dry powders of

GO and SWCNTs directly in anhydrous hydrazine. The

work functions of these carbon composite

films could

be tuned through doping with alkali carbonates. Upon

thermal annealing, the alkali carbonates decomposed

into low-Φ

alkali oxides that covered the

carbon-based materials. We used X-ray photoelectron

spectro-metry (XPS)/UPS spectra to measure the correlation

between the work function and the nature of the

doped alkali carbonates. Finally, we fabricated organic

solar cells having an inverted structure to demonstrate

the potential use of carbon composite electrodes with

tunable values of

Φ

.

RESULTS AND DISCUSSION

The signals in the Raman spectra of SWCNTs and rGO

directly re

flect the stacking and functionality of the

graphitic frameworks. We recorded Raman spectra of

films cast from each constituent and from their hybrid.

In the spectrum of the carbon composite

film recorded

with excitation of 514 nm, we observe (Figure 1)

dominant features at 1596 and 1587 cm

(associated

with the phonon transition within the 1-D SWCNTs)

and at 1350 cm

(the D peak of the rGO). These signals

suggest that the blended

films were mixed well

with-out severe aggregation and that they maintained the

unique electronic properties of their individual carbon

components.

To better understand the structural features, Figure

2 presents representative transmission electron

micro-scopy (TEM) images of the rGO, the SWCNTs, and their

all-carbon hybrid. The image of the rGO reveals its

paper-like structure, with several stacking layers of

mon-atomic rGO sheets (Figure 2a). SWCNTs can form stable

solutions in hydrazine through the creation of

hydrazi-nium

compounds,

comprising

negatively

charged

SWCNTs surrounded by N

H

counterions.

Although

our SWCNTs formed a stable solution in hydrazine without

precipitation, the TEM image in Figure 2b reveals

severe aggregation and many SWCNT bundles. These

SWCNT aggregates and bundles underwent

disper-sion after sonicating with rGO for several hours

(Figure 2c and d). This rGO-SWCNT colloidal

disper-sion was stable for several months without visible

precipitation, suggesting that the rGO acted as a

molecular surfactant to disperse the SWCNTs

because its paper-like structure possessed a large

surface area for interaction with the SWCNTs. The

abundance of highly conjugated structures on the

surface of the rGO allowed it to adhere readily onto

the cylindrical planes of the SWCNTs through

π
π

interactions. Figure 2e displays a central region of the

surface of the rGO-SWCNT hybrid; it reveals many

SWCNTs running across the rGO surface to form a

conductive network. We believe that this

surfactant-free method of dispersing SWCNTs might spur

re-search into their potential uses in optoelectronic and

electronic applications.

We also used atomic force microscopy (AFM) and

scanning electron microscopy (SEM) to characterize

the morphologies of the single-component and

com-posite carbon-based thin

films (Figure 3). The AFM and

SEM images indicate that the rGO thin

film consisted of

one to three layers of rGO sheets stacked on the

substrate with a broad size distribution of 1

7 μm

Figure 1. Raman spectroscopic characterization of chemi-cal compositions. Representative Raman spectra offilms of (a) rGO, (b) SWCNTs, and (c) the rGO-SWCNT composite. The spectrum of the carbon composite exhibits the character-istic peak originating from the SWCNTs, indicating that the electronic structure of the SWCNTs was preserved in the hybrid dispersion.

Figure 2. TEM images of dispersed solutions of rGO, SWCNTs, and rGO-SWCNTs. TEM images of (a) rGO, (b) SWCNTs, and (c
e) rGO-SWCNT composites. The low-magni_{fication (scale bar: 50 nm) TEM images of the rGO-SWCNT composites in (c) and (d)} indicate that rGO acted as a surfactant to disperse the SWCNTs throughπ
π interactions. The high-resolution (scale bar: 20 nm) TEM image of the rGO-SWCNT composite in (d) indicates that the SWCNTs underwent debundling to become physically attached to the rGO sheets. (e) Central region of the surface of the rGO-SWCNTs, revealing many SWCNTs extended across the rGO surface.

Figure 3. Representative images of the surface morphologies of the carbon-basedfilms. (a
c) AFM images with height profiles of films of (a) rGO, (b) SWCNTs, and (c) the rGO-SWCNT composite. (d
f) Corresponding SEM images. The AFM images reveal that rGO effectively debundled the SWCNTs. The average surface roughness of the SWCNTs decreased from ca. 35 nm to ca. 5 nm after interacting with the rGO.

(Figure 3a and d). The AFM pro

file revealed that the film

was approximately 1

3 nm thick, consistent with one

to three layers of the rGO. For the pristine SWCNTs

films, the surface morphology was uneven, featuring

coarse SWCNT bundles, with a roughness of

approxi-mately 7.34 nm (Figure 3b and e). In a transparent

electrode, such bundles would be problematic for

device fabrication because they might protrude through

the active layers and, thereby, result in shorting.

Con-sistent with the TEM images, the SWCNTs appeared to

adhere to the rGO sheets through

π
π interactions

originating from their graphitic structure. In addition,

the rGO acted as a blanket, covering the SWCNTs to

smooth out the surface. In the hybrid, the SWCNTs

acted as a conductive bridge to connect the rGO

sheets, thereby minimizing the barrier for charge

transfer between the rGO sheets and decreasing the

sheet resistance.

Both a smooth surface and high transparency are

necessary if rGO-SWCNT

films are to become effective

substitutes for transparent metal oxide electrodes.

We characterized the optical and electrical properties

of our

films using UV
vis spectroscopy (at normal

incidence) and a four-point probe. The number of

spin-cast layers had the most direct e

ffect on

transmit-tance, as revealed in the spectra and photographs

presented in Figure 4. Images depicting the

film

thick-nesses are provided in the Supporting Information. As

expected, increasing the number of spin-cast layers

from one to

five provided thicker carbon composite

films, thereby decreasing the optical transmittance at

550 nm from 88.8% to 58.7%. The inset in Figure 4b

displays the corresponding sheet resistances of the

rGO-SWCNT

films. A lower number of spin-cast layers

resulted in less material and, hence, fewer conduction

pathways, leading to higher sheet resistances. For the

film formed from five layers, the sheet resistance reached

as low as 254

Ω sq

. Control experiments revealed

that the sheet resistances of the single-component

SWCNTs and rGO

films cast from hydrazine were much

higher than those of the rGO-SWCNT composites. We

suspect that the sheet resistance would decrease

further if we were to optimize the content ratio

be-tween the rGO and SWCNTs.

Grati

fied by the performance of the rGO-SWCNT

films during electrical characterization, we

incorpo-rated these materials as transparent electrodes for

the fabrication of P3HT/PCBM photovoltaic (PV)

de-vices. Table 1 summarizes the effect of the thickness

of the rGO-SWCNT layer on the performance of the

organic PVs; the Supporting Information presents

the corresponding current
voltage characteristics.

The

fill factor (FF) underwent a monotonic increase

from 34.2% to 46.4% upon increasing the number

of layers, due to the decrease in the sheet

resis-tance. The short-circuit current density (J

)

gradu-ally increased from 2.76 mA cm

for the single-layer

rGO-SWCNT device to 4.77 mA cm

for the

four-layer rGO-SWCNT device, decreasing thereafter to

3.89 mA cm

(

five-layer) because of insufficient

trans-parency. Therefore, an optimal rGO-SWCNT thickness

exists for optimal PV performance. The power

conver-sion e

fficiency (PCE) of the P3HT:PCBM PV device

incorporating the four-layer rGO-SWCNT

film was

1.27%.

Figure 4. Transmittances and resistances of the carbon compositefilms. (a) Transmission spectra of rGO-SWCNT films featuring di_{fferent numbers of deposited layers. Inset: photograph of the corresponding samples. (b) Sheet resistances of} rGO, SWCNTs, and the hybrid combination as a function of the number of layers. The sheet resistance of rGO decreased significantly after adhering to the SWCNTs, forming an extended conjugated network with individual SWCNTs connecting the gaps between the rGO sheets. The sheet resistance was measured using a four-point probe.

TABLE 1.Photovoltaic Characteristics of Inverted P3HT/ PCBM Solar Cells Incorporating rGO-SWCNTs As Transparent Electrodes; Transmittance (T) at 550 nm and Sheet Resistance (Rsh) Data for the rGO-SWCNTs

sample T550 nm(%) Rsh(Ω sq
1) JSC(mA cm
2) VOC(V) FF (%) PCE (%)

1 layer 88.6 954 2.76 0.55 34.2 0.51 2 layer 81.3 631 3.63 0.60 36.7 0.80 3 layer 73.3 478 4.37 0.60 39.3 1.03 4 layer 65.8 331 4.77 0.60 44.4 1.27 5 layer 58.1 254 3.89 0.58 46.4 1.18

ARTICLE

To tune the work functions of the rGO-SWCNT

films and thereby develop versatile electrodes for

optoelectronics, we used alkali carbonates to dope

the carbon composites and modify their surfaces. We

dissolved the rGO-SWCNTs and alkali carbonates in

hydrazine and deionized water, respectively, and then

blended them together at a 1:0.25 ratio (w/w) to form a

0.2 wt % solution. Figure 5a displays XPS spectra

revealing the chemical compositions of the doped

carbon composite

films; these spectra featured

pro-nounced alkali metal peaks, indicating successful

dop-ing. Figure 5b presents the absolute values of

Φ

,

determined by measuring the shift in the secondary

electron cuto

ff, of the rGO-SWCNT films incorporating

di

fferent doping alkali carbonates. We observe a clear

trend: the values of

Φ

of the rGO-SWCNT electrodes

decreased from 4.6 eV when doped with Li

CO

to

3.4 eV when doped with Cs

CO

. These results are

consistent with those reported previously by the Yang

group.

_{The doped alkali carbonates formed}

interfa-cial layers on the rGO-SWCNTs, inducing interfainterfa-cial

dipoles and, thereby, decreasing the work function of

the carbon composites.

The pristine rGO-SWCNT

electrode exhibited a value of

Φ

of 5.1 eV (Supporting

Information); therefore, the large changes in

Φ

ap-pear to result directly from the surface modi

fication of

the rGO-SWCNTs.

Figure 6 presents an SEM image and corresponding

EDS map for the Cs

CO

-doped rGO-SWCNT electrode

(Cs:rGO-SWCNTs). The Cs atoms were distributed

homogeneously throughout the sample, suggesting

that the surface of the rGO-SWCNTs had been modi

fied

with the alkali carbonate. AFM imaging of the surface

morphologies of the Cs

CO

-doped and undoped

rGO-SWCNTs revealed that the rGO sheets and rGO-SWCNTs

were slightly rougher and coarser in the formed

sam-ple, suggesting that the surface of the carbon

compo-site was decorated with Cs

CO

. The root-mean-square

roughnesses of the doped and undoped

films were

3.23 and 4.01 nm, respectively, suggesting that the

doping Cs

CO

layer on the surface of the carbon

composite was only a few monolayers thick (i.e., the

surface roughness was almost unchanged). To study

the e

ffect of alkali carbonate on electrode conductivity,

we also measured the sheet resistances of the various

electrodes (Supporting Information),

finding that the

graphene-based electrodes maintained their

conduc-tivity in the presence of a trace amount of dopant

(0.05 wt %).

To demonstrate the e

ffect of the shift in the values of

Φ

on the PV performance, we used the doped

rGO-SWCNT

films to fabricate P3HT:PCBM PVs possessing

an inverted architecture. We thermally annealed all of

the doped rGO-SWCNT

films at 200 °C for 30 min prior

to deposition of the active layer. Figure 7a reveals that

the presence of a thin layer of each of the alkali

carbonates decreased the value of

Φ

of the

rGO-SWCNTs, but to di

fferent extents. Upon varying the

dopant of the rGO-SWCNTs from Li

CO

to Cs

CO

, the

open-circuit voltage (V

) and FF of the corresponding

devices increased from 0.26 V/25.4% to 0.57 V/42.9%,

respectively, consistent with the shift in the work

Figure 5. Measurements of the values ofΦwof the rGO-SWCNTfilms after doping with various alkali carbonates. (a) XPS and

(b) UPS spectra of the rGO doped with various alkali carbonates. The pronounced peaks for the alkali metals indicate the successful doping of the alkali carbonates. The work functions of the rGO-SWCNTfilms after doping with Li2CO3and Cs2CO3

were 4.6 and 3.4 eV, respectively. The values ofΦwof the rGO-SWCNTfilms were determined from the UPS secondary electron

cutoff.

function. The device incorporating the Cs

CO

-doped

electrode exhibited a PCE of 1.13%, indicating that

enhanced charge injection occurred as a result of the

shift in the value of

Φ

.

Figure 7b displays the energy

Figure 6. Elemental distribution and surface morphology of the rGO-SWCNTfilm after doping with Cs2CO3. (a) SEM image and

(b) corresponding EDS map for the Cs:rGO-SWCNTfilm (scale bar: 1 μm). The red dots represent signals from Cs elements. The EDS elementary map reveals that the Cs atoms were distributed homogeneously throughout the basal plane of the rGO sheet and the circumferential plane of the SWCNTs. AFM images offilms of (c) rGO-SWCNTs and (d) Cs:rGO-SWCNTs, revealing that the SWCNTs were slightly thicker after doping and that the surface of the rGOfilm was covered with a thin layer of Cs2CO3.

Figure 7. Effect of dopant on inverted cell performance. (a) J
V characteristics of the inverted P3HT/PCBM solar cells incorporating rGO-SWCNT doped with various alkali carbonates as the cathode. (b) Energy level diagrams of inverted solar cells featuring alkali carbonate-doped carbon-based cathodes.

levels of the individual layers and illustrates the

me-chanism of the solar cells. The value of

Φ

of the Li:

rGO-SWCNT device (4.6 eV) decreased to 3.4 eV for the

Cs:rGO-SWCNT device. The work function tuning

im-proved the match between the values of

Φ

of the

transparent electrode and the lowest unoccupied

mo-lecular orbital (LUMO) of PCBM (3.7 eV), leading to a

stronger built-in

field and increased charge collection.

To better understand the mechanism responsible

for the variation of the work function after doping with

alkali carbonates, we also used UPS to evaluate the

values of

Φ

of Cs:rGO-SWCNT devices that we had

prepared using di

fferent annealing temperatures

(Figure 8a). Without thermal annealing, the value of

Φ

of the Cs:rGO-SWCNT device was 4.9 eV, close to

that of the pristine rGO-SWCNTs (5.1 eV). The work

function of the Cs:rGO-SWCNT device decreased from

4.9 to 3.4 eV upon increasing the annealing temperature

to 250

°C. To gain more insight into the chemical

reac-tion between Cs

CO

and rGO-SWCNT, we used XPS to

monitor the chemical shift of the Cs 3d signals. Figure 8b

reveals that the Cs 3d signal shifted to higher binding

energy upon annealing, consistent with decomposition

to form stoichiometric Cs

O doped with Cs

O

and

CsO

.

The thermal decomposition of Cs salts leads

to the formation of cesium oxide with higher ion

valence level,

_{resulting in cesium oxide behaving as}

an electron-transporting layer having a relatively low

work function; therefore, the electron injection

proper-ties improved after annealing treatment. Moreover, we

also recorded the C 1s signals to study the properties of

the interface between rGO-SWCNT and Cs

CO

. For this

measurement, we spin-cast a very thin layer of Cs

CO

onto rGO-SWCNT

films and then subjected them to

di

fferent annealing temperatures. This approach allowed

us to monitor the reaction occurring at the interface

Figure 8. Determining the mechanism for the variation in work function of rGO-SWCNTfilms after doping with Cs2CO3. (a)

UPS spectra and (b) XPS spectra (Cs 3d) of Cs:rGO-SWCNTfilms that had been subjected to annealing at various temperatures. The signature peak of the Cs 3d energy level shifted to higher binding energy after thermal annealing, a conclusive indication that Cs2CO3decomposed into cesium oxide. (c) XPS spectra of rGO-SWCNTfilms presenting a thin layer of Cs2CO3that had

been subjected to annealing at various temperatures. After thermal annealing, the increased intensity of the shoulder located at higher binding energy indicates the formation of C
O
Cs complexes. (d) Performances of P3HT/PCBM inverted cells incorporating Cs:rGO-SWCNTfilms that had been subjected to annealing at various temperatures.

between the contact layer of rGO-SWCNT and Cs

CO

.

Figure 8c presents spectra displaying the C 1s signal.

After thermal annealing of the thin

films, a shoulder

appeared at high binding energy, indicating the

for-mation of strong chemical bonds between the carbon

materials and Cs

CO

layer.

We suspect that the

interfacial monolayer of C

O
Cs complexes that

forms during treatment at high temperature

acted

as an additional dipole to further reduce the value of

Φ

. This interfacial complex has an even lower work

function than that of decomposed Cs

CO

.

The

pre-sence of similar complexes, including W

O
Cs,

Si

O
Cs,

Ga

O
Cs,

and Al

O
Cs,

can also

produce low-

Φ

surfaces. Figure 8d presents the

performance parameters of the inverted cells

incorpor-ating Cs:rGO-SWCNT electrodes that had been

an-nealed at various temperatures. For the

Cs:rGO-SWCNTs that had not experienced thermal annealing,

the PCE was 0.05%. The performance of each device

improved when the Cs:rGO-SWCNT layer had been

subjected to annealing. When the annealing

tempera-ture of the Cs:rGO-SWCNT layer increased from room

temperature to 200

°C, the PCE increased from 0.05%

to 1.27%. In addition, all of the other device

character-istics

;J

, V

, and FF

;improved as well. The

signifi-cant

improvements

in

the

inverted

devices'

performance resulted from the lowering of the work

function of the Cs:rGO-SWCNT electrodes.

To investigate the

flexibility of the rGO-SWCNT

electrodes, we used the sheet resistance as a

para-meter for exploring the stability of the rGO-SWCNT

and ITO

films under various bending conditions.

Figure 9 displays the correlation between bending

angle and conductance for the rGO-SWCNT and ITO

films. The ITO film underwent an irreversible loss of

electrical conductivity, due to the propagation of

cracks throughout its crystalline network; the sheet

resistance increased from 24 to 1123

Ω sq

after

blending at 60

°. Meanwhile, the rGO-SWCNT film

exhibited comparable sheet resistances before and

after performing the bending test cycle (247 and 244

Ω sq

_{, respectively), by virtue of its high}

_flexibility

and mechanical strength (see the Supporting

Information). These measurements suggest that the

carbon-based hybrid composite

film possessed much

greater mechanical

flexibility than that of the ITO film,

making it well-suited for use as a conductive platform

for

flexible electronics.

CONCLUSIONS

We have developed a high-throughput, facile

strat-egy for tuning the value of

Φ

of solution-processed

rGO-SWCNT

films through systematic doping with

alkali carbonates, potentially opening up a new route

toward

Φ

-tunable graphitic materials. The

alkaline-doped carbon-based hybrid electrodes are readily

employed as functional electrodes in inverted PVs

exhibiting much-improved efficiencies. The ability to

tailor the electrical properties of graphene-based

transparent electrodes should boost the development

of

flexible and ITO-free optoelectronics.

EXPERIMENTAL SECTION

Materials. All chemicals were purchased from Sigma
Aldrich and used as received. GO was prepared from graphite powder (Bay Carbon, SP-1) using a modified version of Hum-mers' method.46Briefly, graphite (2 g), NaNO3(1 g), and H2SO4

(46 mL) were stirred together in an ice bath, and then KMnO4

(6 g) was added slowly. Once mixed, the solution was trans-ferred to a water bath and stirred at 35°C for ca. 1 h, forming a thick paste. Water (80 mL) was added, and then the solution was stirred for 1 h at 90°C. Finally, more water (200 mL) was added, followed by the slow addition of H2O2(30%, 6 mL). The warm

solution was filtered and washed sequentially with 10% HCl (3 200 mL) and water (200 mL). The filter cake was dispersed in water through mechanical agitation and stirred overnight. The dispersion was left to settle; the supernatant (clear yellow dispersion) was subjected to dialysis for 1 month, resulting in a stock solution having a GO concentration of ca. 0.17 mg mL
1. The stable dispersion was filtered through an alumina mem-brane and left to dry for several days. The GO paper was then

carefully peeled from the filter and stored under ambient conditions. To produce hybrid suspensions of rGO and SWCNTs, dry powders of GO and SWCNTs were dispersed directly in anhydrous hydrazine and left to stir for 1 week. Hydrazine bubbled violently upon contact with the GO and SWCNT powders, but soon formed a uniform dark gray suspension with no visible precipitation. XPS spectra of the GO and rGO (dispersed in hydrazine) are provided in the Supporting Infor-mation. Once the stirring was complete, the stable dispersion was centrifuged to separate out any SWCNT bundles and aggregated rGO. After centrifugation, the uniformity of a given rGO-SWCNT dispersion was ensured through heating to 60°C with repeated ultrasonic agitation (VWR model 250D sonicator; set at level 9) for 30 min.

Solar Cell Devices. The polymer solar cells consisted of a layer of the P3HT/PCBM blend thin film sandwiched between the rGO-SWCNT electrode and a metal cathode. Precleaned glass substrates were treated with O2plasma to activate the surface.

Typically, a mixture of GO (1 mg mL
1) and SWCNTs (5 mg mL
1) Figure 9. Bending tests offilms of ITO and rGO-SWCNTs on

flexible PET substrates. (a) Sheet resistances of rGO-SWCNT and ITOfilms on PET substrates, plotted with respect to the bending angle. (b) Representative photographs of an rGO-SWCNTflexible electrode subjected to bending.

in hydrazine was used for spin-coating on the hydrophilic glass substrates. The device structure included a thin PEDOT:PSS buffer layer, upon which was spin-coated and“slow-grown” a layer of P3HT/PCBM (1:1, w/w; 2% in dichlorobenzene).47_Finally,

thermal evaporation of Al and Ca provided the reflective cathodes. For the inverted cell, rGO-SWCNT films incorporating alkali carbonates were spin-cast onto the precleaned glass substrates. The P3HT/PCBM layer was then cast from dichlor-obenzene onto the carbon-based electrode. Finally, thermal evaporation of Al and V2O5provided the reflective anodes.

Characterization. An alpha 300 Raman spectrometer (WITec Instruments, Germany) was used to analyze the chemical compositions of the carbon composite films with a fixed wavelength of 514.5 nm. The surface morphologies of the carbon composite films were investigated using AFM (Digital Instrument NS 3a controller equipped with a D3100 stage) and SEM (Hitachi S-4700). The transmittance spectra of the carbon composite films were obtained using a Jasco-V-670 UV
vis spectrophotometer. The conductivities of the transparent elec-trodes were analyzed using a four-point probe. XPS/UPS spectra were recorded using a PHI 5000 VersaProbe (ULVAC-PHI, Chi-gasaki, Japan) system with He(I) (hν = 21.2 eV) as the energy source. Solar cell testing was performed inside a glovebox under simulated AM 1.5G irradiation (100 W cm
2) using a Xe lamp-based solar simulator (Thermal Oriel 1000 W). The light source was a 450 W Xe lamp (Oriel Instrument, model 6266) equipped with a water-based IR filter (Oriel Instrument, model 6123NS). The light output from the monochromator (Oriel Instrument, model 74100) was focused onto the tested PV cell. Electrical characteristics were measured at room temperature under a N2environment using an HP 4156C apparatus placed

within a glovebox.

Acknowledgment. We thank the National Science Council (NSC) of Taiwan (NSC 99-2221-E-001-012) and the Academia Sinica Research Program on Nanoscience and Nanotechnology for financial support. We thank Dr. Vincent Tung and Prof. Jiaxing Huang at Northwestern for technical assistance and helpful discussions.

Supporting Information Available: J
V characteristics of P3HT/PCBM solar cells incorporating rGO-SWCNTfilms as trans-parent electrodes; UPS spectrum of an rGO-SWCNTfilm spin-coated on a Si substrate; optical images of the ITO/PETfilm before and after bending; XPS spectra of the GO and rGO reduced through treatment with hydrazine; images of the thicknesses of carbon-basedfilms possessing different numbers of cast layers; sheet resistances of graphene-based electrodes incorporating different dopants. This material is available free of charge via the Internet at http://pubs.acs.org.

REFERENCES AND NOTES

1. Wang, L.; Swensen, J. S.; Polikarpov, E.; Matson, D. W.; Bonham, C. C.; Bennett, W.; Gaspar, D. J.; Padmaperuma, A. B. Highly Efficient Blue Organic Light-Emitting Devices with Indium-Free Transparent Anode on Flexible Sub-strates. Org. Electron. 2010, 11, 1555–1560.

2. Qi, Y.; McAlpine, M. C. Nanotechnology-Enabled Flexible and Biocompatible Energy Harvesting. Energy Environ. Sci. 2010, 3, 1275–1285.

3. Krebs, F. C.; Nielsen, T. D.; Fyenbo, J.; Wadstrømc, M.; Pedersen, M. S. Manufacture, Integration and Demonstra-tion of Polymer Solar Cells in a Lamp for the“Lighting Africa” Initiative. Energy Environ. Sci. 2010, 3, 512–525. 4. Lian, K.; Li, R.; Wanga, H.; Lu, Z.; Zhang, J. DC and AC

Analyses of a Printed Flexible Memory Device. Org. Elec-tron. 2010, 11, 1141–1144.

5. Zhang, F.; Funahashi, M.; Tamaoki, N. Flexible Field-Effect Transistors from a Liquid Crystalline Semiconductor by Solution Processes. Org. Electron. 2010, 11, 363–368. 6. Novoselov, K. S.; Geim, A. K.; Morozov, S. V.; Jiang, D.;

Zhang, Y.; Dubonos, S. V.; Grigorieva, I. V.; Firsov, A. A. Electric Field Effect in Atomically Thin Carbon Films. Science 2004, 306, 666–669.

7. De Arco, L. G.; Zhang, Y.; Schlenker, C. W.; Ryu, K.; Thompson, M. E.; Zhou, C. Continuous, Highly Flexible, and Transparent Graphene Films by Chemical Vapor Deposition for Organic Photovoltaics. ACS Nano 2010, 4, 2865–2873.

8. Choe, M.; Lee, B. H.; Jo, G.; Park, J.; Park, W.; Lee, S.; Hong, W. K.; Seong, M. J.; Kahng, Y. H.; Lee, K.; et al. Efficient Bulk-Heterojunction Photovoltaic Cells with Transparent Multi-Layer Graphene Electrodes. Org. Electron. 2010, 11, 1864– 1869.

9. Geng, J.; Liu, L.; Yang, S. B.; Youn, S. C.; Kim, D. W.; Lee, J. S.; Choi, J. K.; Jung, H. T. A Simple Approach for Preparing Transparent Conductive Graphene Films Using the Con-trolled Chemical Reduction of Exfoliated Graphene Oxide in an Aqueous Suspension. J. Phys. Chem. C 2010, 114, 14433–14440.

10. Kalita, G.; Matsushima, M.; Uchida, H.; Wakita, K.; Umeno, M. Graphene Constructed Carbon Thin Films as Transpar-ent Electrodes for Solar Cell Applications. J. Mater. Chem. 2010, 20, 9713–9717.

11. Hong, T. K.; Lee, D. W.; Choi, H. J.; Shin, H. S.; Kim, B. S. Transparent, Flexible Conducting Hybrid Multilayer Thin Films of Multiwalled Carbon Nanotubes with Graphene Nanosheets. ACS Nano 2010, 4, 3861–3868.

12. Chang, H.; Wang, G.; Yang, A.; Tao, X.; Liu, X. A Transparent, Flexible, Low-Temperature, and Solution Processable Gra-phene Composite Electrode. Adv. Funct. Mater. 2010, 20, 2893–2902.

13. Park, H.; Rowehl, J. A.; KangKim, K.; Bulovic, V.; Kong, J. Doped Graphene Electrodes for Organic Solar Cells. Na-notechnology 2010, 21, 505204–505210.

14. Li, X.; Zhu, Y.; Cai, W.; Borysiak, M.; Han, B.; Chen, D.; Piner, R. D.; Colombo, L.; Ruoff, R. S. Transfer of Large-Area Graphene Films for High-Performance Transparent Con-ductive Electrodes. Nano Lett. 2009, 9, 4359–4363. 15. Kim, K. S.; Zhao, Y.; Jang, H.; Lee, S. Y.; Kim, J. M.; Kim, K. S.;

Ahn, J. H.; Kim, P.; Choi, J. Y.; Hong, B. H. Large-Scale Pattern Growth of Graphene Films for Stretchable Transparent Electrodes. Nature 2009, 457, 706–710.

16. Li, X.; Cai, W.; An, J.; Kim, S.; Nah, J.; Yang, D.; Piner, R.; Velamakanni, A.; Jung, I.; Tutuc, E.; et al. Large-Area Synth-esis of High-Quality and Uniform Graphene Films on Copper Foils. Science 2009, 324, 1312–1314.

17. Hernandez, Y.; Nicolosi, V.; Lotya, M.; Blighe, F. M.; Zhenyu, S.; De, S.; McGovern, I. T.; Holland, B.; Byrne, M.; Gun, Y.; et al. High-Yield Production of Graphene by Liquid-Phase Ex-foliation of Graphite. Nat. Nanotechnol. 2008, 3, 563–568. 18. Su, C. Y.; Lu, A. Y.; Xu, Y.; Chen, F. R.; Khlobystov, A. N.; Li, L.-J. High-Quality Thin Graphene Films from Fast Electroche-mical Exfoliation. ACS Nano 2011, 5, 2332–2341. 19. Eda, G.; Fanchini, G.; Chhowalla, M. Large-Area Ultrathin Films

of Reduced Graphene Oxide as a Transparent and Flexible Electronic Material. Nat. Nanotechnol. 2008, 3, 270–274. 20. Wu, J.; Agrawal, M.; Becerril, H. A.; Bao, Z.; Liu, Z.; Chen, Y.;

Peumans, P. Organic Light-Emitting Diodes on Solution-Processed Graphene Transparent Electrodes. ACS Nano 2010, 4, 43–48.

21. Tung, V. C.; Chen, L. M.; Allen, M. J.; Wassei, J. K.; Nelson, K.; Kaner, R. B.; Yang, Y. Low-Temperature Solution Processing of Graphene-Carbon Nanotube Hybrid Materials for High-Performance Transparent Conductors. Nano Lett. 2009, 9, 1949–1955.

22. Cox, P. A. The Electronic Structure and Chemistry of Solids; Oxford University Press: New York, 1987; p 259. 23. Shi, Y. M.; Luo, S. C.; Fang, W. J.; Zhang, K. K.; Ali, E. M.; Boey,

F. Y. C.; Ying, J. Y.; Wang, J. L.; Yu, H. H.; Li, L. J. Work Function Engineering of Electrodes via Electropolymerization of Ethylenedioxythiophenes and its Derivatives. Org. Elec-tron. 2008, 9, 859–863.

24. Jung, J. W.; Lee, J. U.; Jo, W. H. High-Efficiency Polymer Solar Cells with Water-Soluble and Self-Doped Conducting Polyaniline Graft Copolymer as Hole Transport Layer. J. Phys. Chem. C 2010, 114, 633–637.

25. Chang, M. Y.; Wu, C. S.; Chen, Y. F.; Hsieh, B. Z.; Huang, W. Y.; Ho, K. S.; Hsieh, T. H.; Han, Y. K. Polymer Solar Cells

Incorporating One-Dimensional Polyaniline Nanotubes. Org. Electron. 2008, 9, 1136–1139.

26. Li, S. S.; Tu, K. H.; Lin, C. C.; Chen, C. W.; Chhowalla, M. Solution-Processable Graphene Oxide as an Efficient Hole Transport Layer in Polymer Solar Cells. ACS Nano 2010, 4, 3169–3174.

27. Schedin, F.; Geim, A. K.; Morozov, S. V.; Hill, E. W.; Blake, P.; Katsnelson, M. I.; Novoselov, K. S. Detection of Individual Gas Molecules Adsorbed on Graphene. Nat. Mater. 2007, 6, 652–655.

28. Dong, X. C.; Fu, D. L.; Fang, W. J.; Shi, Y. M.; Chen, P.; Li, L. J. Doping Single-Layer Graphene with Aromatic Molecules. Small 2009, 5, 1422–1426.

29. Benayad, A.; Shin, H. J.; Park, H. K.; Yoon, S. M.; Kim, K. K.; Jin, M. H.; Jeong, H. K.; Lee, J. C.; Choi, J. Y.; Lee, Y. H. Controlling Work Function of Reduced Graphite Oxide with Au-Ion Concentration. Chem. Phys. Lett. 2009, 475, 91–95. 30. Shi, Y.; Kim, K. K.; Reina, A.; Hofmann, M.; Li, L. J.; Kong, J.

Work Function Engineering of Graphene Electrode via Chemical Doping. ACS Nano 2010, 4, 2689–2694. 31. Jo, G.; Na, S. I.; Oh, S. H.; Lee, S.; Kim, T. S.; Wang, G.; Choe, M.;

Park, W.; Yoon, J.; Kim, D. Y.; et al. Tuning of a Graphene-Electrode Work Function to Enhance the Efficiency of Organic Bulk Heterojunction Photovoltaic Cells with an Inverted Structure. Appl. Phys. Lett. 2010, 97, 213301– 213301-3.

32. Huang, J.; Watanabe, T.; Ueno, K.; Yang, Y. Highly Efficient Red-Emission Polymer Phosphorescent Light-Emitting Diodes Based on Two Novel Tris(1-phenylisoquinolinato-C2,N)iridium(III) Derivatives. Adv. Mater. 2007, 19, 739– 743.

33. Chu, C. W.; Sung, C. F.; Lee, Y. Z.; Cheng, K. Improved Performance in n-Channel Organic Thin Film Transistors by Nanoscale Interface Modification. Org. Electron. 2008, 9, 262–266.

34. Mitzi, D. B.; Copel, M.; Chey, S. J. Low-Voltage Transistor Employing a High-Mobility Spin-Coated Chalcogenide Semiconductor. Adv. Mater. 2005, 17, 1285–1289. 35. Cote, L. J.; Kim, F.; Huang, J. X. Langmuir
Blodgett

Assem-bly of Graphite Oxide Single Layers. J. Am. Chem. Soc. 2009, 131, 1043–1049.

36. Kim, F.; Cote, L. J.; Huang, J. X. Graphene Oxide: Surface Activity and Two-Dimensional Assembly. Adv. Mater. 2010, 22, 1954–1958.

37. Huang, J.; Li, G.; Yang, Y. A Semi-Transparent Plastic Solar Cell Fabricated by a Lamination Process. Adv. Funct. Mater. 2008, 20, 415–419.

38. Huang, J.; Xu, Z.; Yang, Y. Low-Work-Function Surface Formed by Solution-Processed and Thermally Deposited Nanoscale Layers of Cesium Carbonate. Adv. Funct. Mater. 2007, 17, 1966–1973.

39. Yuan, Y.; Reece, T. J.; Sharma, P.; Poddar, S.; Ducharme, S.; Gruverman, A.; Yang, Y.; Huang, J. Efficiency Enhancement in Organic Solar Cells with Ferroelectric Polymers. Nat. Mater. 2011, 10, 296–302.

40. Liao, H. H.; Chen, L. M.; Xu, Z.; Li, G.; Yang, Y. Highly Efficient Inverted Polymer Solar Cell by Low Temperature Anneal-ing of Cs2CO3Interlayer. Appl. Phys. Lett. 2008, 92, 173303–

173303-3.

41. Sommer, A. H. Hypothetical Mechanism of Operation of the Ag-O-Cs (S-1) Photocathode Involving the Peroxide Cs2O2. J. Appl. Phys. 1980, 51, 1254–1255.

42. Pickett, W. E. Negative Electron Affinity and Low Work Function Surface: Cesium on Oxygenated Diamond (100). Phys. Rev. Lett. 1994, 73, 1664–1667.

43. Desplat, J. L. Auger Electron Spectroscopy of Cesium Adsorbed on Clean and Oxygen Covered (100) Tungsten. Surf. Sci. 1973, 13, 689–691.

44. Martinelli, R. U. Thermionic Emission from the Si/Cs/O (100) Surface. J. Appl. Phys. 1974, 45, 1183–1190. 45. Levine, J. D.; Gelhaus, F. E. Oxygen as a Beneficial Additive

in Cesium Thermionic Energy Converters. J. Appl. Phys. 1967, 38, 892–893.

46. Hummers, W. S.; Offeman, R. E. Preparation of Graphitic Oxide. J. Am. Chem. Soc. 1958, 80, 1339–1339.

47. Li, G.; Shrotriya, V.; Huang, J.; Yao, Y.; Moriarty, T.; Emery, K.; Yang, Y. High-Efficiency Solution Processable Polymer Photovoltaic Cells by Self-Organization of Polymer Blends. Nat. Mater. 2005, 4, 864–868.