C A R B O N 5 0 ( 2 0 1 2 ) 6 6 8 – 6 7 3

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Photo-thermal chemical vapor deposition growth of graphene

Y.Y. Tan, K.D.G.I. Jayawardena, A.A.D.T. Adikaari, L.W. Tan, J.V. Anguita, S.J. Henley,V. Stolojan, J.D. Carey, S.R.P. Silva *

Advanced Technology Institute, University of Surrey, Guildford GU2 7XH, United Kingdom

A R T I C L E I N F O

Article history:

Received 28 June 2011

Accepted 1 September 2011

Available online 17 September 2011

0008-6223/$ - see front matter � 2011 Elsevidoi:10.1016/j.carbon.2011.09.025

* Corresponding author.E-mail address: s.silva@surrey.ac.uk (S.R.

A B S T R A C T

The growth of graphene on Ni using a photo-thermal chemical vapor deposition (PT-CVD)

technique is reported. The non-thermal equilibrium nature of PT-CVD process resulted in a

much shorter duration in both heating up and cooling down stages, thus allowing for a

reduction in the overall growth time. Despite the reduced time for synthesis compared

to standard thermal chemical vapor deposition (T-CVD), there was no decrease in the qual-

ity of the graphene film produced. Furthermore, the graphene formation under PT-CVD is

much less sensitive to cooling rate than that observed for T-CVD process. Growth on Ni also

allows for the alleviation of hydrogen blister damage that is commonly encountered during

growth on Cu substrates and a lower processing temperature. To characterize the film’s

electrical and optical properties, we further report the use of pristine PT-CVD grown graph-

ene as the transparent electrode material in an organic photovoltaic device (OPV) with

poly(3-hexyl)thiophene (P3HT)/phenyl-C61-butyric acid methyl ester (PCBM) as the active

layer where the power conversion efficiency of the OPV cell is found to be comparable to

that reported using pristine graphene prepared by conventional CVD.

� 2011 Elsevier Ltd. All rights reserved.

1. Introduction

Graphene is a two dimensional material with an in-plane sp2

hybridization of carbon atoms that leads to a robust lattice

structure and a p orbital perpendicular to the graphene plane

that results in a delocalized p electron system. The linear en-

ergy dispersion relation associated with massless electrons

(Dirac fermions) results in a zero bandgap material with a lin-

ear density of states at low energies [1]. Recently, graphene

has been utilized for a wide variety of applications, including

as a transparent electrode for organic solar cells [2] and fuel

cells [3], amongst others. While the popular fabrication tech-

nique is to produce single layer graphene by mechanical exfo-

liation, this method is not amenable to large scale production

of a large area film. Chemical routes, such as those based on

Hummers’ oxidation method [4] have also been explored as a

process for large scale graphene production. However, the

quality of the resultant film is lower than that obtained using

er Ltd. All rights reservedP. Silva).

mechanical exfoliation due to the presence of residual oxygen

moieties on the surface. Graphene synthesis by sublimation

of single crystal 4H–SiC [5] is another popular alternative

which has been demonstrated to produce high quality single

layered graphene. However, the cost of this process is very

high due to the use of single crystalline SiC and the size is

limited. By contrast chemical vapor deposition (CVD) has

emerged as the leading growth method that is inherently

scalable for large area film production. A second key advan-

tage of using CVD based methods is the ability to dissolve

the underlying metal for transferring graphene to an arbitrary

substrate [6]. To date Ni [6] and Cu [7] are two common

choices, among the metals used in CVD growth of graphene.

Previously, we reported carbon nanotube synthesis by a

photo-thermal chemical vapor deposition (PT-CVD) method

[8,9]. PT-CVD can offer several technological advantages over

conventional CVD and to conventional thermal CVD

methods. The experience gained using rapid thermal

.

C A R B O N 5 0 ( 2 0 1 2 ) 6 6 8 – 6 7 3 669

processing in Si fabrication has shown that unlike conven-

tional resistive heating, the use of a rapid optical heating un-

der vacuum ensures the growth process is a non-thermal

equilibrium process with a large thermal gradient between

the sample and surrounding processing chamber [10]. Since

the surface radiant energy can be affected by a variety of fac-

tors such as wavelength dependent absorption of the thermal

energy, reflections of thermal energy from the enclosing vac-

uum chamber among others, the mechanism is still not well

understood [10]. The PT-CVD design uses a rapid optical infra-

red (IR) heating source to deliver the energy from the top to

the substrate below and a thermal barrier on the substrate

which helps to absorb and reflect the IR radiation back to

the metal sample resulting in more efficient energy usage

for the growth process [8]. While most reported graphene

CVD growth uses a hot-wall system, there is an advantage

of using cold-wall CVD instead as deposition on side-walls

leading to sample contamination can be minimized [10]. Fur-

thermore, we observed that photo-induced rapid thermal pro-

cessing reduces the overall process time due to much shorter

heating up and cooling down stages, and that the graphene

formation under PT-CVD seems much less affected by the

much faster cooling rate. In the present work, we use the

PT-CVD to demonstrate growth of graphene on a polycrystal-

line Ni substrate using acetylene as the hydrocarbon source

with an overall much faster turnaround process time than

standard thermal CVD. Ni rather than Cu has been used in

this work since the combination of Ni with acetylene has

been reported to result in a lower growth temperature for

graphene [11], and furthermore, growth on Ni also allows

for the alleviation of hydrogen blister damage that is com-

monly encountered during growth on non-oxygen free Cu

substrates.

2. Experiment

The schematic of the PT-CVD system (Surrey NanoSystem

SNS NanoGrowth 1000) is shown in Fig. 1. The thermal barrier

on the substrate support is made from a bilayer of Ti/TiN

which helps to enhance the surface temperature [9]. Ni foil

(Alfa Aesar, product 10254) was first cleaned by acetone, IPA

and methanol and placed on the substrate holder. At a base

pressure of better than 10�2 Torr, a mixture of Ar/H2 (total of

100 sccm) was introduced. The IR lamp is switched on simul-

taneously for duration of 5 min to anneal the Ni foil to reduce

any oxide on the surface. Maintaining the temperature at

Fig. 1 – Schematic of the PT-CVD system.

700 �C, the Ar/H2 mixture was switched off and C2H2 (2 sccm)

was introduced for 5 s. We note that increasing the duration

of C2H2 tends to result in multi-layer graphene growth domi-

nating the surface of Ni. After the growth period, both the IR

lamp and the C2H2 flow were switched off and Ar/H2 mix was

re-introduced in order to cool the sample down to room tem-

perature at a rate of approximately �100 �C/min. The effect of

dilution of C2H2 with H2 was also carried out but no observa-

ble difference in the film quality or surface coverage was de-

tected by subsequent Raman spectroscopy. The as-grown

graphene sample on Ni foil was examined first both optically

and using Raman spectroscopy (Renishaw microRaman,

514 nm Ar+ laser). Afterwards, the graphene layer was trans-

ferred onto SiO2 (100 nm)/Si substrate using either the PMMA

based transfer technique [6] or through direct lift off from a Ni

etchant solution (Alfa Aesar commercial Ni etchant). Further

Raman spectroscopic analysis were carried out on the graph-

ene upon transferred to SiO2. Graphene was also transferred

onto a TEM holey carbon grid (Agar) for examination using a

Hitachi HD2300 Scanning Transmission electron microscope

(STEM). Optical transmission properties of the film were

investigated using a Cary 5000 UV–Vis spectrometer and elec-

trical characterization was performed using a Kiethley 4200

semiconductor characterization system. Finally, the graphene

film prepared by PT-CVD was used as the transparent elec-

trode in an organic photovoltaic (OPV) device fabrication.

The OPV device is fabricated using our previously reported

process [12]. In brief, first graphene is transferred onto a glass

substrate, followed by using the ‘spin and drop’ technique to

coat the graphene’s surface with a PEDOT:PSS. After this, a

P3HT:PCBM layer is spin-coated on top, to act as the active

layer. BCP and Al were then thermally evaporated sequen-

tially to form a hole blocking layer and as the electrode

respectively. The OPV device was characterized with a solar

simulator operating under AM 1.5G illumination, calibrated

with a Newport reference cell. A Kiethley 2400 sourcemeter

was used as the electronic load.

3. Results and discussion

Fig. 2a and b show the optical image (Leica microscope) of

graphene on the Ni foil upon PT-CVD growth and its corre-

sponding Raman spectra (respectively). Dark boundaries are

observed on the surface of the Ni foil that were previously ab-

sent in the pristine Ni foil as received from the supplier (inset

of Fig. 2a). The straight lines running across the pristine Ni foil

is believed to be due to the cold-rolling process used for the

production of the metallic foil. We observed ‘boundaries’ in

the foil after the growth process. While the origins of these

await further investigations, these boundaries may poten-

tially lead to a discontinuous graphene film upon cooling after

the growth process [6,13]. The Raman spectrum at the bound-

ary line as indicated in the optical image (Fig. 2b) shows a

higher I(G)/I(2D) ratio. Furthermore, the FWHM of the 2D peak

is wider (some of these sites’ FWHM approximated to

�90 cm�1) compared to a region away from the boundary.

This may be indicative of a higher graphene layer count at

the boundaries than at sites away from the boundaries as is

often observed in CVD growth of graphene using Ni as cata-

lyst [13].

Fig. 2 – (a) Optical image of graphene as grown on Ni foil. Inset is the Ni foil before undergoing the growth process. The film

surface seems to exhibit ‘boundaries’ that maybe related to the grain size of the underlying Ni foil. (b) Raman spectra for

points A and B of the optical image in (a). Note the wider FWHM in the Raman 2D peak measured on the darker pattern (A) on

the sample which may be indicative of few layered graphene. The scale bar is 100 microns.

670 C A R B O N 5 0 ( 2 0 1 2 ) 6 6 8 – 6 7 3

The as-grown graphene was transferred onto SiO2 (300 nm

thickness)/Si for further examination. Optical image of the

film and Raman spectra at selected sites on the film are

shown in Fig. 3. A FWHM of 38 cm�1 (by fitting with a single

Lorentzian) for the 2D peak (2695 cm�1) of the Raman spec-

trum indicates the presence of single layered graphene [14],

while some sites on the film also indicated a higher defect

(more pronounced D) peak which may be due to the exposure

to acid etchants used to remove the underlying metal catalyst

[15]. While there remains a possibility that the graphene may

be damaged during the transfer process, the observation of a

low I(D)/I(G) ratio of graphene on Ni foil compared to the D

peak of the same graphene film upon transferring to SiO2/Si

provides some support for the hypothesis of acid induced

damage. However, in comparing our film to graphene films

formed by chemical routes, the Raman I(D)/I(G) is still lower

[16] and is comparable to graphene grown by the more

Fig. 3 – (a) Optical image of graphene after transferred on to a 3

and C on (a). The Raman spectra indicated that the film has few

completely single layer. Furthermore, some places have a higher

is not clear if the defects are inherent in the growth process or

from Ni to SiO2. The scale bar is 100 microns.

commonly reported hot wall CVD method [6,13]. The position

of the Raman 2D peak is also higher compared to exfoliated

graphene from HOPG (�2640 cm�1) but is still consistent with

other reported CVD grown graphene [6,7]. One reason for the

difference in the thermal expansion between graphene and

Ni during the cooling stage causes a strain in the graphene

lattice, resulting in a shift in Raman 2D’s peak position [17].

There have been other reports that cite the doping of graph-

ene by HNO3 (a component in our commercial Ni etchant

solution) [18] and the substrate effect [19] as additional causes

contributing to this shift. It is also observed that our graphene

film also exhibits Raman spectra characteristic of a few lay-

ered graphene at other sites, indicating that the film is not a

single layer at all sites. It is also observed that in some of

these sites, the Raman 2D peak exhibits a single Lorentzian

peak but with a wider FWHM (up to �70 cm�1). It has been

argued that CVD grown few-layered graphene may exhibit

00 nm SiO2 wafer, (b) Raman spectra collected at points A, B

layered graphene on certain places, thus the film may not be

Raman D peak indicating defects in the graphitic structure. It

the result of damages caused during the graphene transfer

C A R B O N 5 0 ( 2 0 1 2 ) 6 6 8 – 6 7 3 671

characteristics similar to turbostratic graphene resulting from

the absence of long term order in the c axis as compared to

HOPG’s ordered AB stacking [20–23].

It was reported that alternative methods like I(G) Raman

intensity, ratios of I(G)/I(2D) and AFM can help in the estima-

tion of the number of graphene layers [6,24]. Using the same

methodologies, we approximated that at some of these sites,

the layer of graphene may be approximately 3–4 layers thick.

Fig. 4 shows selected area electron diffraction (SAED) patterns

of our graphene film on a TEM holey carbon grid. This con-

firms the earlier observations that the PT-CVD grown film

has the hexagonal symmetry that is characteristic of gra-

phitic material as there is in general a lack of ring-like diffrac-

tion patterns which are associated with random stacking in

either amorphous polycrystalline or thick graphitic carbons.

The existence of faint secondary diffraction spots (as indi-

cated by the arrow) may be attributed to mis-orientation be-

tween adjacent layers providing further support to the

turbostratic nature of our PT-CVD grown few layered graph-

ene [6]. However, the possibility that this may also be a result

of the electron beam damage during electron microscopic

analysis cannot be dismissed, as the electron beam incidence

is not perpendicular to the sample.

It has been previously noted that using hot wall CVD for

graphene growth, the cooling rate was deemed to be critical

in controlling the number of graphene layers formed on the

Ni surface. Our PT-CVD system exhibits a nominal cooling

rate of �100 �C/min under the flow of H2 and Ar. While this

cooling rate is an order of magnitude faster than the more

commonly reported rate of �10 �C/min for standard thermal

CVD [13], we did not observe similar increase in the Raman

D peak which have been partially attributed to a rapid cooling

(>20 �C/min) that did not allow sufficient time for the segre-

gated carbon to diffuse and achieve better crystallinity [13].

We do note that since our growth process is essentially in

non-thermal equilibrium unlike standard thermal CVD, a di-

rect comparison between the two different processes is

inaccurate.

To further characterize the graphene film’s electrical and

optical properties, we also fabricated an organic solar cell

(OPVs) using the pristine (non-chemically modified) graph-

ene. Previous works [3,25] have indicated that the hydropho-

bic nature of pristine graphene and its inherently higher

sheet resistance [26] can hinder the performance of graphene

Fig. 4 – The electron diffraction patterns (SAED) measured at thr

grid, showed the hexagonal symmetry that is characteristic of

(arrows in image c) may have resulted from mis-orientation of

few layered graphene may be turbostratic.

based OPV cells unless additional methods like chemical

modification of graphene with PBASE [2], AgCl3 [25] and HCl/

HNO3 compounds [18] are used. Since our aim was to estab-

lish if our pristine graphene can have comparable perfor-

mance to graphene grown using standard thermal CVD, no

process optimizations were used to maximize the OPV’s per-

formance in this study.

We first investigated the optical transmission of the graph-

ene film by transferring the as grown film onto a glass sub-

strate using the PMMA technique as before and then

examined using UV–Vis absorption/transmission studies.

We observed that the optical transmission of the graphene

film is above 90% within the range 300–800 nm which is a sig-

nificant improvement over ITO [27]. The sheet resistance of

the graphene film was also measured to be �790 X/sq (Kulicke

and Soffa four point measurement system). This falls within

the range of the previously reported sheet resistance values

[7,27]. Investigations into using CVD grown graphene as OPV’s

transparent electrode have been reported earlier using both

bulk hetero-junction and bilayer OPV structures [25]. In both

cases, PEDOT:PSS has been utilized as a hole transport/plana-

rising layer. As mentioned, one common issue is that graph-

ene surface tends to be hydrophobic thus spin-coating of

PEDOT-PSS on the surface faces considerable difficulty. We at-

tempted to minimize this issue by utilizing an ethanol/PED-

OT:PSS (1:3) solution for the spin coating process instead of

just spin-coating with the original water based PEDOT:PSS

solution. Apart from acting as a hole transport layer, in the

case of transferred graphene electrodes, PEDOT:PSS also acts

as a reasonable planarising material.

Fig. 5 shows the J–V characteristics of the graphene elec-

trode based OPV, with the inset showing the PEDOT:PSS refer-

ence. The performance parameters are summarized in

Table 1. The graphene electrode based device shows 0.2% effi-

ciency, which is comparable to other reported result that uses

CVD grown pristine graphene on a similar OPV device struc-

ture [2]. It has been previously reported that PEDOT:PSS has

been experimented as an transparent electrode in solar cell

[28]. To ensure that the photovoltaic effect observed in our

graphene OPV cell is not due to PEDOT:PSS layer acting as

the transparent electrode in place of graphene, we also fabri-

cated another OPV cell structure: PEDOT:PSS/P3HT:PCBM/

BCP/Al on glass substrates. As seen in Fig. 5 inset, there is

no observable difference in the I–V characteristics under both

ee random points of a graphene film on a TEM holey carbon

graphitic materials. Note the secondary diffraction spots

adjacent graphene layers indicating that the CVD growth of

Fig. 5 – (a) The J–V characteristics of OPV cell fabricated using graphene as transparent electrode (graphene/PEDOT:PSS/

P3HT:PCBM/BCP/Al) under dark and light (AM1.5G) conditions. (Inset) shows the J–V characteristics of a ‘reference’ OPV cell of

PEDOT:PSS/P3HT:PCBM/BCP/Al. (b) The proposed flat band diagram for the device incorporating graphene as an electrode. (c)

The schematic of the graphene based OPV device.

Table 1 – Comparison of performance between graphene based OPV against a control device without graphene. The area ofthe OPV is 0.032 cm2. Compared to a device consisting of PEDOT and Al electrodes only, the device containing grapheneelectrode shows an improved performance.

Voc Jsc (mA/cm2) FF (%) g (%)

Glass/Graphene/PEDOT:PSS/P3HT:PCBM/BCP/Al 0.52 1.160 31.5 0.2Glass/PEDOT:PSS/P3HT:PCBM/BCP/Al 0.26 0.001 26.0 0.0

672 C A R B O N 5 0 ( 2 0 1 2 ) 6 6 8 – 6 7 3

dark and light illumination, thus indicating the role of graph-

ene as the transparent electrode for the OPV.

We have previously reported a standard ITO based

P3HT:PCBM bulk heterojunction solar cell [12] of 2.5% efficient

with open circuit voltage mainly governed by the donor

acceptor energy levels �0.6 V, and short circuit current densi-

ties in excess of 6 mAcm�2. From Table 1, it can be seen that

while Voc is comparable for both ITO and graphene based

OPV devices, the short circuit current density is lower for

the device with the graphene electrode in comparison to

our previously published ITO based OPVs [12], leading to

poorer efficiency. Beside the earlier mentioned issues

regarding using graphene as electrodes for OPVs, we further

postulate that the low conversion efficiency can be attributed

to a number of other reasons. The work function of graphene

(4.5 eV) is lower compared with ITO (4.8 eV), thereby affecting

the built in electric field of the diode, resulting in lower charge

collection as well as a slightly lower open circuit voltage. From

the J–V curves (Fig. 5a) it can be seen that the shunt resistance

of the device is finite (slope at Jsc), possibly due to sub optimal

planarising of the layer after transfer, leaving abundant

leakage paths for charges. This fact along with poor sheet

resistance lowers the fill factor, reducing the device efficiency

further.

In conclusion, the growth of graphene on Ni using a photo-

thermal CVD system (PT-CVD) has been demonstrated. By

C A R B O N 5 0 ( 2 0 1 2 ) 6 6 8 – 6 7 3 673

using PT-CVD system, the overall growth time has been

greatly reduced compared to the standard thermal CVD

technique while still maintaining a comparable quality. This

is demonstrated in similar performance in resistance and

optical transmission as compared to those reported in liter-

ature. Furthermore, investigations into applying our graph-

ene film as a transparent electrode for OPV to further

characterize the film’s optical and electrical properties had

found comparable performance to other reported pristine

graphene based devices. Through this work, we demon-

strated an alternative CVD technology that made mass pro-

duction of graphene more attractive. The speed of operation

coupled with the optical energy is ideally suited for mono

and multiple layer material growth required in graphene

synthesis.

Acknowledgment

The authors would like to acknowledge the editors and

reviewers for their kind comments and suggestions.

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