RESEARCH PAPER

Enhanced polymer light-emitting diode property usingfluorescent conducting polymer-reduced graphene oxidenanocomposite as active emissive layer

Jyoti Prakash Singh • Uttam Saha •

Rimpa Jaiswal • Raghubir Singh Anand •

Anurag Srivastava • Thako Hari Goswami

Received: 21 May 2014 / Accepted: 4 October 2014

� Springer Science+Business Media Dordrecht 2014

Abstract The present article reports the polymer

light-emitting diode property of the nanocomposite

comprising poly 9,9-dioctyl fluorene-alt-bithiophene

and reduced graphene oxide used as an emissive layer.

Two times repetition of Hummers oxidation and

hydrazine hydrate reduction method produce reduced

graphene oxide (term as rGO2) with more uniform

distribution in size and thickness. In addition, this

uniquely synthesized rGO2 induces favorable shift in

balance of electron and hole recombination zone

toward the center of emissive layer owing to increase

in in-plane crystallite size and high localize aromatic

confinement. Five times increase in maximum device

efficiency (Cd/A) and three times increase in maxi-

mum brightness (Cd/m2) are achieved with the LED

device using nanocomposite as emissive layer com-

pared to neat polymer. Also, the fabricated device

requires relatively low turn-on voltage (4 V) because

of low energy barrier between PEDOT work function

(-5.0 eV) and HOMO levels of bi-thiophene copoly-

mer -5.67 eV) and nanocomposite (-5.66 eV).

Keywords Polymer light-emitting diode �Fluorescent conducting polymer � Reduced graphene

oxide � Emissive layer � Nanocomposite

Introduction

The remarkable physico-chemical properties

which include large theoretical-specific surface area

(2,630 m2 g-1), high intrinsic mobility (2,000 m2

v-1 s-1), unprecedented pliability and impermeabil-

ity, high Young’s modulus (*1.0 TPa), thermal

conductivity (*5,000 Wm-1 K-1), optical transmit-

tance (*97.7 %) and good electrical conductivity of

graphene, the one-atom-thick planar sheet comprising

an sp2-bonded carbon structure, merit attention for

applications in large variety of opto-electronic areas,

which include solar cells, field-effect transistors, light-

emitting diodes (LEDs), super-capacitors, fuel cells,

sensors, and actuators (Wan et al. 2012; Chen et al.

2012; Zhu et al. 2010; Eda and Chhowalla 2010;

Ghosh et al. 2013). Potential advantages of using

transparent conductive graphene electrode as alterna-

tive to traditional indium tin oxide (ITO) and dye-

sensitized solar cells, as competitive low cost alterna-

tive for costly fullerene-based electron-accepting

materials (PCBM) and possible alternative to

Electronic supplementary material The online version ofthis article (doi:10.1007/s11051-014-2693-7) contains supple-mentary material, which is available to authorized users.

J. P. Singh � U. Saha � R. Jaiswal � A. Srivastava �T. H. Goswami (&)

Defence Materials and Stores Research and Development

Establishment, DMSRDE P.O., G.T Road,

Kanpur 208013, India

e-mail: thgoswami@yahoo.co.uk

R. S. Anand

Indian Institute of Technology, Kanpur, Kanpur 208916,

India

123

J Nanopart Res (2014) 16:2693

DOI 10.1007/s11051-014-2693-7

conventional toxic, acidic, and hygroscopic PE-

DOT:PSS polymer interface layer material in organic

photovoltaic (OPV), have already been demonstrated.

A review covering vivid description of all the aspect of

OPVs has appeared recently (Wan et al. 2011).

Very little attention has, however, been paid toward

LED application of these materials, although the

presence of conducting character of graphene/graph-

ene-based materials significantly suppresses the effec-

tive hole mobility and favors the formation of

recombination pathways (Kymakis et al. 2007). Few

recent reports indicate tremendous possibility of using

soluble graphene oxides as transparent electrodes (Wu

et al. 2010; Han et al. 2012) and the hole transport

layer (HTL) (Lee et al. 2012; Zhong et al. 2011) in

LED applications. The transparent electrode in elec-

troluminescent thin-film device has different require-

ments and must considered both carrier injection and

light out-coupling efficiency and radiation pattern.

The ITO benefits from optical interference effects in

the ITO layer to control the light out-coupling

efficiency and radiation pattern (Greenham et al.

1994; Madigan et al. 2000; Agrawal et al. 2007).

OLED developed by applying small molecule AlQ3 on

thin transparent graphene film electrode demonstrates

comparable electrical and optical performance to that

of control ITO devices (Wu et al. 2010).

Use of PEDOT:PSS as HTL in the interfaces

between ITO and emissive semiconducting layer

reduces the contact barrier between the ITO and

active semiconducting polymers and also reduces the

ITO roughness upon coating. But the acidic nature and

significant quenching of radiative excitons at the

interface between PEDOT:PSS and emissive semi-

conductor (Kim et al. 2005) demand replacement with

better alternatives. The use of 4-octyloxyphenyl

diazonium tetrafluoroborate functionalized GO as a

HTL shows 150 % enhancement of OLEDs efficiency

compared to PEDOT:PSS reference device (Lee et al.

2012). An optimum thickness of solution processable

graphene oxide (GO) is required for preventing the

significant quenching of radiative excitons between

the emissive polymer and GO layer. 220 % Increase in

luminous efficiency and 280 % increase in power

conversion efficiency are achieved at 4.3 nm GO

thickness compared to PEDOT:PSS (Han et al. 2012).

The progress in LED work using graphene-based

materials is summarized in Table 1.

Table 1 Summary of updated information in LED work using Graphene/graphene-based materials

S. no. Chemical structure of

graphene/CMG

Process of synthesis of

graphene/CMG

Application References

1. Graphene Graphene by CVD technique Flexible LED using graphene

in place of transparent ITO

anode

Han et al. (2012)

2. Graphene oxide (GO) and

reduced graphene oxide

(rGO)

GO and rGO by chemical route GO and rGO in place of hole

injecting layer

Lee et al. (2012)

3. Reduced graphene oxide Thermally reduced graphene

oxide

PEDOT:PSS/Graphene

Nanocomposite as hole

injecting layer

Lin et al. (2012)

4. Surface-grafted graphene oxide Graphene oxide was surface

grafted with long chain

organic molecule

As a hole injecting buffer

material

Zhong et al. (2011)

5. Phenyl isocyanate

functionalized GO (SPF-

graphene)

Graphene oxide treated with

phenylisocyanate

SPF-graphene and MEH-PPV

nanocomposite for active

emissive layer

Liu et al. (2010)

6. Reduced graphene oxide (rGO) rGO produced by thermal

reduction of GO

rGO inplace of transparent

anode

Wu et al. (2010)

7. Reduced graphene oxide

adopting oxidation and

reduction process for two

times. Reduced graphene

oxide does not contain any

functional group.

Synthesized by chemical route Reduced graphene oxide and

fluorine-bi-thiophene

copolymer nanocomposite as

active emissive layer

Present work

2693 Page 2 of 20 J Nanopart Res (2014) 16:2693

123

The soluble fluorene-based (PFs) p-conjugated

fluorescent conducting polymers have generated con-

siderable research interest in LEDs owing to their

good solubility, better thermal and oxidative stability,

high photoluminescence (PL) and electrolumines-

cence (EL) efficiency, good life time, improved

efficiency, and tunable properties to cover the entire

visible range (Donat-Bouillud et al. 2000; Fujinami

et al. 2012; Chen et al. 2008; Charas et al. 2002).

Various fluorene copolymers, mostly comprising aryl

(Molina et al. 2009; Mallavia et al. 2005), thiophene

and thieno-arenes (Lim et al. 2003; Lim et al. 2006a, b),

benzooxadiazole, benzothiadiazole and carbazole

(Beaupre et al. 2010; Bouffard and Swager 2008),

3,4-ethylenedioxythiophene (EDOT) and pyridine

(Aubert et al. 2004), are tested to enhance stability,

efficiency and to achieve tunable EL properties.

Proper designs of charge injection/transport layers to

achieve an adequate and balanced transport of both

injected electrons and holes for efficient recombina-

tion in the luminescent chromospheres zone have

paramount requirement for achieving enhanced

PLED/OLED efficiency. Often a hole injection and

transport layer adjacent to the anode and an electron

injection and transport layer adjacent to the cathode on

both sides of an emitting layer are desired. The PF

polymers possessing a hole and/or an electron-trans-

porting (hole blocking) unit or both in the main chain

and the side group are reported to produce highly

efficient polymer LEDs without complex fabrication

process (Lim et al. 2006a, b). A molecular charge-

transporting layer in-between light-emitting materials

and corresponding electrode can also improve the

LED devices performance and efficiency. Various

hole-transporting materials, i.e., triphenylamine

(TPA), N,N-diphenyl-N,N-bis(3-methylphenyl)-1,1-

biphenyl-4,4-diamine (TPD), and electron-transport-

ing (hole blocking) materials—oxadiazole, triazine,

triazole, and benzothiadiazole (Greeham et al. 1993;

Parker et al. 1994; Son et al. 1995)—are tried.

In this report, nanocomposite comprising 9,9-

dioctyl fluorene-alt-bithiophene copolymer and

reduced graphene oxides is used as an active emissive

layer. The familiar Hummers and hydrazine hydrated

methods (Hummers and Offeman 1958; Stankovich

et al. 2007) of oxidation and reduction are adopted for

two successive times for the preparation of reduced

graphene oxide (rGO2). This unique method of

preparation of reduced graphene oxide produces more

uniform distribution in size and thickness of reduced

graphene oxide and thereby forms homogeneous film

through easy dispersion in polymer matrix. In addi-

tion, the rGO2 induces favorable shift in balance of

electron and hole recombination zone toward the

center of emissive layer owing to an increase in in-

plane crystallite size and high localize aromatic

confinement. The high-electrical conductivity of

reduced graphene oxide (rGO2) acts as an electron-

transporting (hole blocking) unit in the nanocompos-

ite. Device made with this nanocomposite shows five

times increase in power efficiency and three times

enhancement of brightness compared to neat polymer.

Experimental

Measurements

Gel permeation chromatography (GPC) analysis was

carried out using waters BREEZE 2 high performance

liquid chromatography (HPLC) System attached with

a refractometric detection system. Analysis was

performed using terahydrofuran (THF) as the eluent.

Calibration is based on polystyrene standards obtained

from polymer standard services. Fourier transformed

infra red spectroscopy (FT-IR) spectra were recorded

from a KBR pellet of the sample in a Perkin Elmer FT-

IR instrument (model RX1). The spectra over the

range of 400–4,000 cm-1 are obtained at a resolution

of 4 cm-1. RAMAN spectra are recorded in RENI-

SHAW Raman Spectrophotometer using a single

514 nm laser source over the range of

500–3,000 cm-1. Atomic force microscopy (AFM)

images were taken on MultiTask; Veeco Instruments.

Imaging was done in tapping mode using a V-shape

probe (tip radius = 10 nm). All images were collected

under ambient conditions at 50 % relative humidity

and 25 �C. AFM images were obtained by dispersions

of reduced graphite oxides on a clean glass surface.

Wide-angle X-ray scattering (WAXS) experiments are

carried out using Phillips X’pert wide-angle X-ray

diffractometer in reflection mode with a parallel beam

optics attachment. Instrument is operated at 45 kV

voltage and 40 mA current and is calibrated against a

standard silicon sample. Ni-filtered Cu-Ka radiation

(k = 1.54 A) is used to scan samples from 2h = 4�–

40� at the step scan mode (step size 0.02�, preset time

2 s) to record the diffraction pattern. Surface

J Nanopart Res (2014) 16:2693 Page 3 of 20 2693

123

morphology of polymers was analyzed with scanning

electron microscopy (SEM) Carl-Zeiss Model: EVO-

50 XVP instrument at different magnifications at

10 kV. Samples are first mounted on the stub and then

gold–platinum coating is done by sputtering method.

Thermal stability is measured using Thermo Gravi-

metric Analysis (TGA) 2950 TA instruments under

nitrogen atmosphere. Samples are heated at a scan rate

of 10 �C/min from room temperature to 900 �C.

Dynamic scanning calorimetry (DSC) study is carried

out using DSC Q200 TA instruments under argon

atmosphere. All the samples are scanned at a heating

rate of 10 �C/min from room temperature to 300 �C.

Absorption spectra are recorded in an Ultra Violet–

Visible (UV–Vis) spectrophotometer (Analytik jena

SpecordR 200 plus) from 200 to 900 nm. photolumi-

nescence spectroscopy (PL) emission data are col-

lected in a backscattering configuration, focused into a

TRIAX 320 mono-chromator and detected by Ham-

amatsu R928 photomultiplier.

Synthesis

Materials

All the starting materials were purchased from Sigma

Aldrich and used as received. N-bromosuccinamide is

re-crystallized before use. All the solvents are purified

and dried following standard procedures.

4,40-dibromo bithiophene

Freshly re-crystallized N-bromosuccinamide (2.14 g,

12 m mole) was added in multiple aliquots over the

course of the reaction to the bithiophene (1 g, 6.0 m

mole) solution taken in 50 ml dichloromethane. The

reaction mixture was stirred for 3 h to complete the

reaction. The crude reaction mixture was evaporated on

rotary evaporator and purified by column chromatogra-

phy packed with neutral alumina using hexane as eluent.

Poly 9,9-dioctyl fluorene-alt-bithiophene

The synthesis of poly 9,9-dioctyl fluorene-alt-bithi-

ophene copolymer employing Suzuki cross-coupling

method is outlined in Scheme 1. In a typical synthesis,

4,40-dibromo bithiophene (1 g, 3.09 m mol) dissolved

in 150 ml THF was taken in a 250 ml two necks round

bottom flask equipped with water cooling condenser.

9,9-dioctylfluorene-2,7-diboronic acid bis (1,3-pro-

panediol) ester (1.73 g, 3.1 m mol) was then added

into it. Prior to addition of di-boronic ester, the reaction

vessel is degassed for 10 min with argon and again

degassed for 10 min after addition. The degassed (with

argon) potassium carbonate (20.8 mg, 0.018 m mol,

0.6 %) dissolved in water (1 ml) was added to the

reaction mass. Finally, the reaction mixture was

refluxed for 24 h after adding tetrakis (triphenyl

phosphine) palladium (0) catalyst (5.1 mg, 0.0044 m

mol) followed by evacuation and purging with argon.

After completion of polymerization reaction, the reac-

tion mixture was poured into 500 ml cold methanol.

Solid crude polymer is precipitated out, filtered and

dissolved in 20 ml chloroform. Re-precipitation pro-

cess was repeated twice by adding 500 ml of cold

methanol and finally, dried under vacuum to collect the

pure polymer. 1H NMR: d = 7.7, 7.36, 7.28, 2.06, 1.27,

1.2, 1.1, 0.90 0.83, 0.74. IR (cm-1): 2,926, 2,846, 1,630,

1,409, 996, 782. GPC: Wt Av mol wt (Mw) = 49,048;

No. Av mol wt (Mn) = 28,096, PDI = 1.75.

Synthesis of reduced graphene oxides (rGO1

and rGO2)

Graphite (Sigma Aldrich) was oxidized by modified

Hummers method (Hummers and Offeman 1958). In a

typical synthesis, 2.5 g of graphite powder is taken in a

conical flask and 175 ml conc. sulfuric acid is added into

it in stirring ice-bath environment so that the temperature

must not rise above 10 �C. 1.25 g sodium nitrate

(NaNO3) is then placed into the flask, and finally, 7.5 g

potassium permanganate (KMNO4) is added slowly in a

span of 1 h. The reaction mixture is stirred vigorously for

6 days at room temperature. Hydrogen peroxide (10 ml

of 50 wt% aqueous solution) is added to terminate the

reaction. The resultant mixture is purified by repeated

washing with 600 ml 10 vol% HCl/H2O followed by

ultra sonication (140 W for 30 min). Excess acid is

removed by centrifugation and filtration till pH *6.5

and then dried for a week at 60 �C under vacuum.

Familiar hydrazine-hydrated reduction (Stankovich

et al. 2007) is carried out to obtain reduced graphene

oxides (rGOs). In a typical reaction set up, 250 mg

graphite oxide (GO) was sonicated in 200 ml water, and

then hydrazine hydrate (2.5 ml, 80.25 mmol) was added

into it. The whole reaction mass was heated at 100 �C

for 24 h in the round bottom flask equipped with water-

2693 Page 4 of 20 J Nanopart Res (2014) 16:2693

123

cooled condenser. The progress of reaction is easily

gagged from the precipitation of reduced graphite oxide

(rGO1) as black solid. This black solid product cake was

isolated by filtration, washed copiously with water

(5 9 100 mL), and dried on the funnel under continu-

ous air flow. The above oxidation and reduction process

were repeated twice to yield rGO2.

Preparation of rGO2-polymer nanocomposite

The rGO2 (2.5 mg) was sonicated in degassing mode in

THF (250 ml) under continuous flow of argon to obtain

uniform dispersion of reduced GO into THF. The

uniformly dispersed rGO2 was then added into the poly

9,9-dioctyl fluorene-alt-bithiophene (250 mg) solution,

also dissolved in THF (100 ml). The mixture was stirred

for 2 h and then sonicated for 10 min. The solvent was

slowly evaporated and the residue was dried under

vacuum for 6 h to obtain green color solid mass and also

in the form of uniform homogeneous film.

Device structure

Hole-injection/transport layer (HIL/HTL), the poly

(3,4-ethylenedioxy-thiophene) doped with poly (styrene

sulfonic acid) (PEDOT:PSS), was spin coated on the

pre-patterned ITO anode and dried. Active emissive

materials, i.e., polymer/nanocomposite dissolved in

anhydrous dichloro benzene (6 mg/ml), were then spin

coated onto the PEDOT:PSS layer at 1,000 rpm and

dried. The LiF electron injection/transport layer (EIL/

ETL) and Al cathode (LiF/Al 1 nm/100 nm) were

vacuum-deposited onto the polymer film through a

shadow mask. The EL spectra were recorded with a

Minolta CS-1000. Current–voltage and luminescence-

voltage characteristics were recorded on a current–

voltage source (Keithley-236) and a radiometer lumi-

nescence detector, (Minolta LS-1000) respectively.

Results and discussion

Product integrity and structure identification

Suzuki polymers

The Suzuki cross-coupling poly-condensation using

palladium catalyst is very sensitive to purity of the

precursors and depends on lot of other parameter, such

as, choice of palladium source and its associated base,

C8H17 C8H17

BB

O

OO

O

S

SBr Br

C8H17 C8H17

**

n

S

C8H17 C8H17

* S*

n

(PPh3)4Pd(0)

(THF, K2CO3)Reflux for 24 hrs

S S BrBr

(a)

(b)

Scheme 1 Outline of reaction scheme for the synthesis of fluorene-alt-thiophene copolymers by Suzuki cross-coupling reaction

J Nanopart Res (2014) 16:2693 Page 5 of 20 2693

123

the solvent, nature of boronic substituent, and the

temperature used during the reaction and any devia-

tion may drop molecular weights of the polymers

drastically (Singh et al. 2012; Littke et al. 2000). The

Suzuki poly-condensation was carried out in THF/

H2O/EtOH under nitrogen using Pd(PPh3)4 catalyst

and a weak base (potassium carbonate), essentially at

absolute oxygen-free conditions and maintaining strict

stoichiometry of the co-monomers, as the first one

poisoning the Pd complex and the second one

dramatically decreasing the molecular weight of the

polymer. The as-obtained Suzuki copolymers are

reddish yellow crystalline solid soluble in organic

solvents (like CHCl3, THF etc.), and GPC analysis

determines the average molecular weight (Mw &49,048 and Mn & 28,096) and poly-dispersity index

(PDI & 1.75). Physical data of Suzuki copolymers

are summarized in Table 2, and the structures eluci-

dated through spectroscopic techniques (FT-IR and 1H

NMR spectroscopy) are in agreement with reported

values (Donat-Bouillud et al. 2000; Lim et al. 2003;

Lim et al. 2006a, b).

Reduced graphene oxides (rGO1 and rGO2)

Considerable reduction in size and thickness and

restoration of sp2 conjugate domain in reduced

graphene oxides (rGO1 and rGO2) are primarily

assessed from Raman spectroscopy, AFM, and XRD

spectral analysis of the samples. It is proposed that

Raman analysis can distinguish the ‘quality’ of

graphene and determine the number of layers for

n-layer graphene (for n up to 5) from the shape, width,

and position of 2D peak, the second-order Raman

feature of the D band. The 2D band at *2,700 cm-1 is

very sensitive to the stacking order of the graphene

sheets along the c-axis as well as to the number of

layers and shifts to higher wave number values (blue-

shift) and becomes broaden (often a doublet) with

increasing number of layers (Ferrari et al. 2006;

Ferrari 2007; Pimenta et al. 2007). Significant changes

in the Raman spectra are observed on reduction of

graphene oxides; the G band becomes sharper and

shifted to lower frequencies (red-shift), and the D band

intensity is also reduced (Stankovich et al. 2007;

Kudin et al. 2008). Typical Raman spectra for the

reduced graphene oxide samples are presented in

Fig. 1, and the assignments of Raman band positions

are summarized in Table 3. The Raman spectrum of

rGO1 contains a G and D band at 1,582 and

1,350 cm-1, respectively, and the second-order fea-

tures, i.e., 2D peak at 2,689 and D ? G peak at

2,933 cm-1. Raman spectral features are changed on

two times repetition of oxidation and reduction, and

the G and D band are shifted to around 1,593 and

1,346 cm-1, respectively, in rGO2. In addition, the 2D

peak is completely absent and a very weak

D ? G appears (Fig. 1). Normally, the D band arises

from disorder and is very weak in a single-layer

graphene, but increases its intensity with the increas-

ing number of layers (Subrahmanyam et al. 2008). The

G band position (1,582 cm-1) of rGO1 is in good

agreement with the reported literature (Stankovich

et al. 2007), but the position is 12 cm-1 higher in

rGO2 compared to graphite (1,581 cm-1) (Stankovich

et al. 2007). This type of shift was observed while

breaking of graphite crystal to a single graphene sheet,

in which the G band is shifted to 3–6 cm-1 higher

value than the bulk graphite (Gupta et al. 2006). The

ratio of intensities of the two bands, the D (ID) and the

G (IG), is often used as a means of determining the

number of layers in a graphene sample and its overall

stacking behavior as a metric of disorder which arises

from ripples, edges, charged impurities, presence of

domain boundaries, and others; high ID/IG ratio

indicates a high degree of exfoliation/disorder (Das

et al. 2008; Ferrari 2007). Contrary to earlier report

(Stankovich et al. 2007; Subrahmanyam et al. 2008),

the D band intensity is lower than the G band, and this

decrease in intensity ratio is normally expected as the

disorder associated with the amorphous graphene

oxide diminishes on removal of sp3-oxygenated

functional groups (epoxy groups) and the sp2-conju-

gated domain is restored in the basal plane. The blue-

shift in G band, relatively lower D band intensity and

less than unity ID/IG intensity ratio, indicates small

stacks, fewer defects, and increased sp2 domain

graphene sheets in rGO2. The in-plane crystallite size

Table 2 Physical properties of fluorene Bi-thiophene

copolymer and its nanocomposite with reduced graphene oxide

(rGO2)

Mn Mw PDI

(Mw/

Mn)

Tg

(�C)

TCr

(�C)

Tm

(�C)

Polymer 28,096 49,048 1.75 105 163 241

Nanocomposite 108 164 230

2693 Page 6 of 20 J Nanopart Res (2014) 16:2693

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(La), calculated by employing the relation

La = 4.4(IG/ID) (Pimenta et al. 2007; Subrahmanyam

et al. 2008; Ferrari 2007), indicates value of about

5 nm for rGO2 (Table 3). It is noteworthy to mention

here that the position of the G band varied in the order

rGO2 [ rGO1 (*11 nm higher in rGO2) confirming

decreasing number of layers in rGO2 compared to

rGO1. It is known that the G band position is shifted to

higher frequencies (blue-shift) on decreasing the

number of layers (Pimenta et al. 2007; Kudin et al.

2008; Subrahmanyam et al. 2008; Gupta et al. 2006;

Das et al. 2008; Ferrari 2007). The specific second-

order Raman feature, namely the 2D band, is absent

and a very weak D ? G combination band induced by

disorder at *2,927 cm-1 is observed in rGO2. Thus,

it is conceivable that rGO2 is thinner than rGO1, and

the relatively higher disorder and randomly arranged

graphene sheets in rGO1 are transformed into more

ordered and arranged structure in rGO2 (Srinivas et al.

2010).

AFM images are recorded in tapping mode by

dispersing reduced graphene oxides on a glass surface

(Fig. 2). The AFM images of rGO1 indicate a very

rough non-uniform aggregate of large cluster spread

over the surface (Fig. 2a). The average depth of the

surface roughness is 150 nm as measured by AFM

profilometry. Two successive oxidation and reduction

of GO dramatically reduces the roughness inducing a

better structural homogeneity and improved surface

morphology (Fig. 2b). The surface roughness is mea-

sured to be 50 nm by AFM profilometry. The differ-

ences in step heights in the height profile give total

average thickness of rGOs which is used to calculate

number of layers, assuming *1 nm thickness for

single-layer graphene sheet (McAllister et al. 2007)

and determining the inter-lamellar (002) d-spacing by

X-ray diffraction. Thus, the rGO1 gives an apparent

thickness of *1.41 nm for single sheet, on determin-

ing the inter-lamellar (002) d-spacing of *0.41 nm

by X-ray diffraction (Saha et al. 2014) (Table 4). Total

average thickness obtained at different locations

varies from 33.7 to 57.3 nm, giving an average

number of layers of 24–40 for rGO1. The average

thickness is substantially reduced and the inter-lamel-

lar (002) d-spacing is also diminishes (*0.38 nm),

giving an apparent thickness of *1.38 nm for single

rGO2 sheet. Total average thickness is reduced by

about ten times (obtained value at different locations

varies from 3.5 to 4.8 nm), giving an average number

of layers of 2–3 for rGO2. The surface thickness is

more uniform and homogeneous in rGO2 compared to

rGO1. Interestingly, the average size of rGO sheet

does not vary significantly (lies within 1.4–0.6 nm) in

both cases. Thus, two successive oxidation and

reduction offers more uniform thin few layer graphene

sheets (rGO2) compared to single oxidation and

reduction (rGO1).

Polymer-reduced graphene oxides (rGO2)

nanocomposite

Two successive oxidation and reduction processes

reduce the platelet thickness and provide more

uniform and homogeneous surface in rGO2 compared

to rGO1. Intimate mixing with polymer permits easy

transfer of electron from hole transporting polymer

layer to electron-transporting graphene layer. The

electron-transporting (ETL) or hole blocking property

of rGO2 in nanocomposite (used as emissive layer) is

useful in shifting the hole and electron recombination

zone toward the center to increase the probability of

desired radiative emissive recombination (Mullen and

Scherf 2005). Interestingly, the recorded brighter

picture in SEM micrograph of reduced graphene

oxide-polymer nanocomposite compared to neat poly-

mer at similar experimental condition (Fig. 3) reaf-

firms better conductivity in nanocomposite. It is

proven that the conductive samples appear brighter

as compared to less conductive or nonconductive

samples in SEM. The enhanced conductivity is

recorded in the I–V characteristics of composite after

preparing the LED device. Recent reports also indicate

that replacing conventional transparent electrode

(ITO) and hole transporting layer (PEDOT:PSS) with

Fig. 1 Comparative Raman spectra of reduced graphene oxides

obtained at different oxidation–reduction stages (rGO2 and

rGO1) using 320 nm laser

J Nanopart Res (2014) 16:2693 Page 7 of 20 2693

123

large band gap (*3.6 eV) negatively charged GO

(behaves like an insulator) maximizes hole–electron

recombination within emissive layer, enhancing

PLEDS device performance and efficiency (Wu

et al. 2010; Han et al. 2012; Lee et al. 2012; Zhong

et al. 2011).

Comparative FTIR spectra of polymer and its

nanocomposites (Fig. 4) provide valuable informa-

tion regarding the CH–p/p–p interactions between

polymer and graphitic layer. Higher frequency

shifting (blue-shift) of CH-stretching frequency

region (*2,925–2,940 cm-1) by *7–15 cm-1

indicates CH–p interaction in the nanocomposite.

Decreases in intensity and down shifting in position

of anti-symmetric (1,505 cm-1) and symmetric

(1,457 cm-1) C=C stretching vibration of polymer

by *7–17 cm-1 are due to the presence of p–pinteraction between polymer and graphite unit in

composite (Saha et al. 2011, 2013). Two possible

explanations are put forward to corroborate the

observed downshift: first, the effect of electron

donor–acceptor type charge addition between rGO

and PF polymer decreases the electron density in

the polymer’s bonding orbital on electron injection

to rGO due to weakening of average C=C bond

length. It is apparently looks like from the observed

downshift that the electronic interaction originates

from hole-conducting polymer and transfers to

hole-trapping nature of rGO (Schwartz 2003).

Second, the increased probability of enhanced

coverage of the graphitic surface with conducting

polymer affects the freedom of C=C vibration on

the graphene plane due to CH–p/p–p interactions.

The total CH–p/p–p interactions effect observed

between graphitic surface and polymer are stronger

than the p–p interactions occurred between stacked

graphitic layer surfaces. In addition, the character-

istic reduced graphene oxide peak (rGO) at

*1,256 cm-1 is also observed in FTIR spectra of

the composite.

Thermal analysis: TGA and DSC thermograms

The thermal stabilities of the copolymer and nano-

composite were evaluated under nitrogen atmosphere

by thermo gravimetric analysis (TGA) at a heating rate

10 �C min-1 (Fig. 5a). Polymer shows good thermal

stability with less than 5 % weight loss on heating to

about 338 �C. The nanocomposite, on the other hand,

shows relatively lower thermal stability (286 �C)

which is of course sufficient enough to be used in

optoelectronic devices. The thermal stability of

reduced graphite oxides (rGO1, rGO2) is also evalu-

ated separately which shows a lower onset of degra-

dation in rGO2 (84 �C) compared to rGO1 (148 �C).

Also, the first derivative TGA traces indicate multi-

step degradation for rGO1 and distinct three sharp

peaks for rGO2 (233, 545, and 845 �C). Relatively

smaller distinct dimension of platelet size could

possibly responsible for lowering of thermal stability

in reduced graphene oxide (rGO1 and rGO2).

The differential scanning calorimetry (DSC) study

was carried out to elucidate the thermal properties of

copolymer and nanocomposite across melting and

crystalline region. Thermally induced phase transition

behaviors were investigated in nitrogen atmosphere at

a heating rate of 10 �C min-1 after the samples being

heated little below the degradation temperature and

then allowing slow cooling to room temperature

(Fig. 5b). The DSC heating scans show a glass

transition (Tg) at around 105 and 108 �C for polymer

and nanocomposite, respectively. The typical liquid

crystalline (LC) characteristics, i.e., both crystalliza-

tion and melting processes, were clearly observed in

the DSC curve of the polymer with crystallization

exothermal peak (Tcr) and endo-thermal melting peak

(Tm) appeared at 163 and 241 �C, respectively

(Fig. 5b; Table 2). These characteristic temperatures

are around 100 �C higher than those for the corre-

sponding homopolymer probably because of the

introduction of the un-substituted bi-thiophene moiety

Table 3 Raman peaks assignment of reduced graphene oxides of rGO1 and rGO2, their D/G and G/D intensity ratios and in-plane

crystallite size

S. no. Materials Peak Position (cm-1) ID/IG ratio IG/ID ratio La (crystallite size) (nm0

D G 2D D ? G

1. rGO1 1,350 1,582 2,689 2,933 0.79 1.27 6

2. rGO2 1,346 1,593 2,927 0.87 1.15 5

2693 Page 8 of 20 J Nanopart Res (2014) 16:2693

123

Fig. 2 AFM images of

reduced graphene oxides

a rGO1 and b rGO2 showing

roughness and average

thickness at different

locations

J Nanopart Res (2014) 16:2693 Page 9 of 20 2693

123

and nicely corroborated the previous result (Lim et al.

2006a, b). Very little changes in DSC pattern are

observed in nanocomposite with exothermal

crystallization peak (Tcr) and endo-thermal melting

peak (Tm) nearly matching with neat polymer (Fig. 5b;

Table 2).

Electrochemical properties

The electrochemical properties are essentially inves-

tigated to gage the electronic properties of the polymer

and nanocomposite and are carried out from the films

prepared by drop-casting the polymer and nanocom-

posite solutions onto Pt wire. Cyclic voltammograms

are recorded with a computer-controlled Autolab

model 302 Potentiostat at a constant scan rate of

100 mV/s. Measurements were performed under N2

purging in 0.2 M (0.8 g/10 ml) solution of tetra-butyl

ammonium hexaflorophosphate as supporting electro-

lyte dissolved in dichloromethane. A three-electrode

configuration undivided cell was used: platinum disk

working electrode, platinum wire counter electrode,

and Ag/AgCl (3 M KCl and saturated Ag/Cl) sepa-

rated with a diaphragm used as reference electrode.

Typical cyclic voltammogram of Suzuki polymer

Fig. 3 SEM micrographs of a Polymer and b nanocomposite

3000 2500 2000 1500 1000 500 048

50

52

54

56

58

60

62

64

66

68

70

%T

cm-1

Composite POLYMER

Fig. 4 Comparative FTIR spectra of polymer and nanocom-

posite using rGO2

Table 4 Average thickness, size, and number of layers of rGO1 and rGO2 calculated from XRD and AFM results

S. no. Name XRD result AFM result

2h (�) Sinh d-Spacing (nm) Size (nm) Thickness (nm) Average number

of layers

1. rGO 1 22.3 0.19 0.41 0.74, 1.4 33.7, 57.3 24, 40

2. rGO 2 22.6 0.20 0.38 0.63, 1.4 3.5, 4.8 2, 3

2693 Page 10 of 20 J Nanopart Res (2014) 16:2693

123

(scan window -2.4 to1.6 V) and nanocomposite (scan

window -2.5 to 2.5 V) is displayed in Fig. 6, and

electrochemical data are summarized in Table 5. The

onset oxidation potential (E0ox) and reduction potential

(E0red), relative to Ag/AgCl reference electrode, for

polymer and composite are recorded to be 1.28 and

-1.52 and 1.27 and -1.41 V, respectively. The

measured redox behavior is transposed to estimate

the ionization potential (Ip) and electron affinity (Ea)

by relating the electrochemical potentials to the

vacuum level relative to which Ip and Ea are defined.

The empirical relationship proposed by Bredas et al.

(1983) on the basis of a detail comparison between

valence effective Hamiltonian calculation and exper-

imental electrochemical measurement is used to

calculate the Ip and Ea:

Ip HOMOð Þ ¼ � E0ox þ 4:39

� �eV; and

Ea LUMOð Þ ¼ � E0red þ 4:39

� �eV

Eg Band Gapð Þ ¼ LUMO� HOMOð Þor E0ox�E0

red

� �eV

The ionization potential (HOMO level) of polymer

and composite was calculated to be as -5.67 and

-5.66 eV, and electron affinity (LUMO level) was

0

10

20

30

40

50

60

70

80

90

100

110W

eigh

t (%

)

Temperature (ºC)

Polymer Composite

(a)

0 200 400 600 800 1000

100 150 200 250 300

-0.80

-0.75

-0.70

-0.65

-0.60

Heat

Flo

w (W

/g)

Temperature (oC)

Polymer Composite

(b)

Fig. 5 Thermograms of polymer and nanocomposites recorded

at 10 �C/min under nitrogen atmosphere: a Thermogravimetric

analysis, b DSC analysis

-0.00003

-0.00002

-0.00001

0.00000

0.00001

0.00002Polymer

i/A

E/V

(a)

-3 -2 -1 0 1 2

-3 -2 -1 0 1 2

-0.0007

-0.0006

-0.0005

-0.0004

-0.0003

-0.0002

-0.0001

0.0000

0.0001

0.0002Nanocomposite

i/A

E/V

(b)

Fig. 6 Cyclic voltammograms recorded at constant scan rate of

100 mV/s under N2 purging using 0.2 M (0.8 g/10 ml) solution

of tetrabutylammonium hexaflorophosphate as supporting

electrolyte dissolved in dichloromethane of a Polymer and

b nanocomposite

Table 5 Cyclic voltammograms results of polymer and

nanocomposite to calculate HOMO, LUMO, and band gap

Eonsetox

(V)

Eonsetred

(V)

HOMO

(eV)

LUMO

(eV)

Eg

(eV)

Polymer 1.28 -1.52 -5.67 -2.87 2.8

Nanocomposite 1.27 -1.41 -5.66 -2.98 2.68

J Nanopart Res (2014) 16:2693 Page 11 of 20 2693

123

estimated to be -2.87 and -2.98 eV, respectively,

relative to Ag/AgCl from vacuum level (de Leeuw

et al. 1997). The difference in energy between LUMO

(Ea) and HOMO (Ip) yields the band gap of the

material (Pei et al. 2000; Chen et al. 2000), and these

values are 2.8 and 2.68 eV, indicative of greenish

yellow and yellow light emission, respectively. The

result clearly indicates a lowering of electrochemical

band gaps of the nanocomposite compared to neat

polymer. The electrochemical band gaps are however

slightly higher than the optically determined ones

(Tables 5, 6) due to interface barrier for charge

injection (Chen et al. 2000; Janietz et al. 1998). The

incorporation of the electron-rich reduced graphene

oxides into the nanocomposite leads to higher HOMO

and lower LUMO than those of the neat polymer

(Janietz et al. 1998).

Optical properties

UV–Vis absorption property

UV–Vis absorption spectra of polymer and nanocom-

posite recorded at equal concentration in chloroform

solution and in films at room temperature are

presented in Fig. 7 and optical data are summarized

in Table 6. Absorption spectra of reduced graphene

oxides, obtained at two different stages, are also

recorded in aqueous media at room temperature. The

nature of UV–Vis absorption spectral pattern of

reduced graphite/graphene oxides is very similar to

that of pristine graphite, although the absorption peak

at around 345 nm is missing in rGO2. The rGOs show

strong absorption in the entire visible region of

spectrum, typically matching with other allotropes of

carbon, i.e., fullerene and nanotubes, and definitely

indicates its powerfulness in optoelectronic applica-

tions. Pure polymer shows a strong absorption band at

445 nm corresponding to the p–p* transition (elec-

tronic transition) of its conjugated segments. Nearly

overlapping absorption spectra are recorded in poly-

mer nanocomposite with identical absorption maxima

(445 nm). This shows that low loading of rGO does

not affect the absorption behavioral pattern of the

polymer so significantly (Table 6; Fig. 7a). Interest-

ingly enough, the peak intensity of absorption maxima

is increased in nanocomposite possibly indicating an

increase in electron density at ground state on addition

of rGO. The optical band gap (Eg) of polymer and

nanocomposite is estimated from the absorption onset

wavelengths of the UV–Vis spectrum and band gap is

found to be 2.42 eV (514 nm) in both the cases. The

absorption spectra of the polymer and nanocomposite

in film state are more red-shifted and broaden than

those of the solution state (Table 6; Fig. 7b) because

of the increased intermolecular interactions between

neighboring molecules in the film state. The absorp-

tion maxima appear at 458 and 461 nm and also there

appear a shoulder peak at around 283 and 285 nm,

respectively. The optical band gap (Eg), estimated

from the absorption onset wavelengths of the UV–Vis

spectrum, is lower (*2.22 eV, 560 nm) in the both

cases.

Emission property

The normalized photoluminescence (PL) emission

spectra of the polymer and nanocomposite at room

temperature recorded in chloroform solution are

presented in Fig. 8 and optical data are summarized

in Table 6. Both absorption and emission peaks of the

copolymer are clearly red-shifted compared to pristine

fluorene polymer. This spectral shift can be under-

stood in terms of the more planar backbone and

smaller band gap produced by the introduction of

flexible conductive bi-thiophene rings (Lim et al.

Table 6 Summary of UV–Vis absorption and PL emission spectral data of the polymer and nanocomposite and calculation optical

band gap

Solution kmax (nm) Film kmax (nm)

Absorption

kmax (nm)

Absorption

konset (nm)

Band gap

(e.v.)

Emission

kmax (nm)

Absorption

kmax (nm)

Absorption

konset (nm)

Band gap

(eV)

Polymer 446 514 2.42 451,497, 531 458, 461 559 2.22

Nanocomposite 446 514 2.42 451,497, 533 461,485 560 2.22

2693 Page 12 of 20 J Nanopart Res (2014) 16:2693

123

2003). The PL emission spectra of the polymer

solution show typical vibronically structured bands

on excitation at 320 nm wave length (kex = 320 nm)

and comprise a maximum, a shoulder, and a tail

appeared at 450, 497, and 531 nm, respectively. The

strong electron transition emission peak at 497 nm for

fluorene copolymer is closely comparable with optical

band gap in solution obtained from the absorption

onset wavelengths. Besides electronic transition, the

weak shoulder peak at around 531 nm is due to the first

vibronic band of pristine polymer. The photolumines-

cence (PL) spectrum of nanocomposite also shows

overlapping emission spectral pattern with identical

peak positions as that of pristine polymer. The

emission peak intensities are, however, substantially

quenched in nanocomposite. Photoluminescence

behavior suggests an electronic interaction between

rGO and polymer; the electronic interaction originates

from a hole-conducting polymer to the electron-

conducting nature of rGO. A plausible explanation

could be the p–p interaction of fluorene copolymer

with rGO forming additional decaying paths of the

excited electrons through the sp2 conjugation of rGO.

The p–p interaction between fluorene copolymer and

rGO allows the formation of strong charge transfer

(CT) exciplex between donor (fluorene copolymer)

and acceptor (rGO), and dissociation of photo-excited

exciton or exciton–polaron (Ex–P) upon collision with

rGO causes the quenching of PL intensity (Singh and

Goswami 2007). The most likely requirement for

efficient electron injection from polymer to reduced

graphene oxide in composite material is the close

physical contact of polymer, preferably in some

crystalline or ordered monolayer form at reduced

graphene oxide interfaces throughout the composite

films. A larger PL quenching is previously been

observed at the rGO/Super Yellow [poly(phenyl

vinylene): super yellow (SY, Merck Co.,

Mw = 1,950 000 g mol-1)] interface due to intercon-

nectivity of the localized sp2 sites in the reduction

process of GO when the conventional PEDOT:PSS

interface layer is replaced by rGO in LED application

(Lee et al. 2012). Both polymer and nanocomposite

show about 45 nm Stokes shift between absorption

and emission peaks. The Stokes shift can be ascribed

to a possible exciton migration from short to long

conjugation segments in the composite in excited

state.

0.0

0.2

0.4

0.6

Abs

orba

nce

Wavelength (nm)

Composite Polymer

(a)

400 600 800

400 500 600 700 800 900 1000

0.0

0.1

0.2

0.3

0.4

0.5

Abs

orba

nce

Wavelength (nm)

Composite Polymer

(b)

Fig. 7 Comparative absorption spectra of polymer and nano-

composite recorded in a chloroform solution and b solid state as

film

Table 7 EL emission spectral positions and characteristic device data of the polymer and nanocomposite

Emission

kmax (nm)

CIE coordinate color:

greenish yellow

Turn on

voltage (V)

Max. efficiency

(Cd/A)

Max. brightness

(Cd/m2)

Max. current

density (mA/cm2)

Polymer-B 538 X = 0.40 and Y = 0.56 4 0.018 17.0 112.0

Nanocomposite 540 X = 0.42 and Y = 0.54 4 0.084 55.0 106.0

J Nanopart Res (2014) 16:2693 Page 13 of 20 2693

123

Device fabrication and characterization

The polymer LEDs (PLEDs) using neat polymer and

polymer-rGO2 nanocomposite as emissive layers were

fabricated. Fluorene-alt-thiophene copolymer (polymer-

A) does not yield uniform homogeneous film, that is why

discarded during device analysis. All device properties

were evaluated using fluorene-alt-bithiophene copoly-

mer (polymer-B) and its nanocomposite with reduced

graphene oxides obtained after two times repetition of

oxidation/reduction steps (rGO2). Hole-injection/trans-

port layer (HIL/HTL), the poly (3,4-ethylenedioxy-

thiophene) doped with poly (styrene sulfonic acid)

(PEDOT:PSS), was spin coated on the pre-patterned ITO

anode and dried. Polymer/composite (6 mg/ml) dis-

solved in anhydrous dichlorobenzene was then spin

coated onto the PEDOT:PSS layer at 2,400 rpm and

dried. The LiF electron injection layer (EIL) and Al

cathode [LiF(0.8 nm)/Al(200 nm)] were vacuum-

deposited onto the polymer film through a shadow mask.

The devices were sealed using UV-cured glass plates.

The CIE coordinates and the EL spectra of the

devices having neat polymer and nanocomposite are

shown in Figs. 9 and 10, respectively. Nearly identical

EL spectral pattern emitting greenish yellow light with

EL maximum at 538 and 540 nm was observed in case

of nanocomposite and neat polymer, respectively

(Fig. 10a, b). Although there is no significant change

of emission wavelength due to addition of rGO2, the

EL intensity is about three times higher in nanocom-

posite than the neat polymer at 10 V. The quality of

emission spectra can be specified by their Commission

Internationale de L’Eclairage (CIE) chromaticity

coordinates (x, y) and the full width at half maximum

(FWHM) in EL spectra. Interestingly, the color

coordinates (0.40, 0.56) of polymer are almost similar

to that of previously reported polymer, e.g., (0.40,

300 400 500 600 700 800

0

100

200

300

400

500

600

700cp

s

Wavelength (nm)

Polymer-B Polymer-B rGO

Fig. 8 Comparative emission spectra of polymer and its

reduced graphene oxides nanocomposite recorded in chloroform

solution at 320 nm excitation

Fig. 9 a CIE coordinate of polymer–GO nanocomposites: X = 0.42 and Y = 0.54 at 4 V and b CIE coordinate of polymer: X = 0.40

and Y = 0.56 at 4 V

2693 Page 14 of 20 J Nanopart Res (2014) 16:2693

123

0.58) (Lim et al. 2003). There is a slight change in

color (wavelength) from green to yellow, a shift

toward better excitation purity (saturation). The rela-

tively broader FWHM (*80 nm, the ideal being the

50 nm) and appearance of an additional shoulder

emission peaks (*600 nm) in EL emission spectrum

may be a point of concern on the quality of color in this

type of polymer (Lim et al. 2003). The FWHM is

however approaching relatively closer to ideal value in

nanocomposite (*69 nm, the ideal being the 50 nm).

0.0000

0.0002

0.0004

0.0006

0.0008

0.0010

0.0012

0.0014E

L In

tens

ity(a

.u.)

Wavelength (nm)

4V 5V 6V 7V 8V 9V 10V

400 500 600 700 800 400 500 600 700 800

0.0000

0.0001

0.0002

0.0003

0.0004

EL

Inte

nsity

(a.u

.)

Wavelength (nm)

4V 5V 6V 7V 8V 9V 10V

(a) (b)

Fig. 10 Comparative EL intensity verses wavelength graphs at different applied voltages of a polymer–GO nanocomposites and

b virgin polymer

10864-505

1015202530354045

Current Light

Voltage (V)

Cur

rent

(mA

)

-5051015202530354045505560

Lig

ht In

tens

ity (C

d/m

2 )

4 6 8 10

05

101520253035404550

Current Light

Voltage (V)

Cur

rent

(mA

)

0

5

10

15

20

Lig

ht In

tens

ity (C

d/m

2 )

(a)

(b)

Fig. 11 Comparative I–V–L graphs of a polymer–GO nano-

composite and b virgin polymer

0.04

0.05

0.06

0.07

0.08

0.09E

ffici

ency

(Cd/

A)

Voltage (V)

Efficiency

ITO/PEDOT:PSS/Ex-2/LiF/Al

4 5 6 7 8 9 10

4 5 6 7 8 9 100.008

0.010

0.012

0.014

0.016

0.018 ITO/PEDOT:PSS/Ex-3/LiF/Al

Effic

ienc

y (C

d/A

)

Voltage (V)

(a)

(b)

Fig. 12 Comparative EL efficiency vs Voltage graphs of

a polymer–GO nanocomposite and b virgin polymer

J Nanopart Res (2014) 16:2693 Page 15 of 20 2693

123

2693 Page 16 of 20 J Nanopart Res (2014) 16:2693

123

The voltage(V)–luminance (L)–current density

(I) characteristics of the devices are shown in

Fig. 11 and their related performances including their

EL spectral data are summarized in Table 7. The turn-

on voltage (the voltage needed for brightness of 1 cd/

m2) in both cases is 4 volt and showed relatively lower

turn-on voltages than that of pristine polyfluorene

polymer (5.5 V). The maximum electro-luminance is

recorded to be 17.69 and 54.31 Cd/m2 for neat

polymer and nanocomposite, respectively. The effi-

ciency (Cd/A vs V) graphs are shown in Fig. 12 and

the maximum efficiency is 0.018 and 0.084 from the

devices made with neat polymer and composite,

respectively. The luminance and efficiency have

significantly improved by addition rGO2 and there is

about three times increase of luminance/brightness

and 4.7 times increase in efficiency in PLEDs device

made of composite compared to neat polymer.

Mechanism

As the holes are moving faster through the emitting

polymer layer than the electrons, the hole and electron

recombination zone is generally shifted toward cath-

ode which usually leads to a non-radiative loss of

energy (Rothberg and Lovinger 1996) and conse-

quently decreases the device efficiency. In order to

attain high quantum efficiency (%) (photon emitted/

charge injected) for EL, it is necessary to have a

mechanism to achieve an efficient charge injection

from electrodes at low drive voltage, good charge

balance, and the confinement of the injected charge

carriers within the emitting layer so that the probabil-

ity of the desired emissive recombination is increased

(Gupta et al. 2006). The high electrical conductivity of

reduced graphene oxides (alternatively, should act as

hole blocking agent) is expected to increase the

electron transport property in emissive layer on

incorporation as nanocomposite and should induce a

favorable shift in balance of electron and hole

injection toward the center of emissive zone, making

this nanocomposite a suitable candidate to be used as

emitting layer (EML) in PLED devices. In addition,

considerable thickness and size reduction, better

surface smoothness and greater homogeneity allow

easy dispersion of rGO2 into polymer matrix. Stronger

CH–p/p–p interactions owing to intimate mixing

make the nanocomposite film smoother and more

uniform compared to neat polymer. The lower turn-on

voltages are a consequence of the lower energy

barriers of the copolymers between the PEDOT work

function (-5.0 eV) and the HOMO levels of the bi-

thiophene copolymer (-5.67 eV) and nanocomposite

(-5.66 eV) (Janietz et al. 1998). Moreover, the

nanocomposite exhibits higher EL efficiency

(Fig. 12), despite its strong PL intensity quenching

(Fig. 8). The EL efficiency of an OLED depends not

only on the PL efficiency of the active polymer but

also on its charge injection balance and its carrier

mobility (Lim et al. 2003). The electron injection

seems to improve with better alignment of composite

LUMO (2.98 eV) level with cathode than that of neat

polymer (2.87 eV) as shown in energy band diagram

(Fig. 13). This leads to more e–h recombination

current resulting in better luminance. Use of graph-

ene-based nanocomposite as emissive layer (EML) to

improve the luminance and efficiency is a matter of

discovery and established a point for further investi-

gation to find out the exact cause and physics behind it.

Further improvement of EL characteristics can thus

possibly be achieved by modifying the polymer main

back bone structure, incorporating a third co-monomer

(hole and electron transport moiety) in the backbone/

side chain and optimizing reduced GO and polymer

composition ratios in nanocomposites in order to

enhance and balance proper electron and hole trans-

port/recombination within polymer emissive layer.

Conclusions

The device fabricated with the use of poly(fluorene-

alt-bi-thiophene copolymer) and reduced graphene

oxide (rGO2) nanocomposite as active emissive layer

shows enhanced luminescence and efficiency com-

pared to neat polymer. The copolymer is synthesized

by Suzuki cross-coupling method, and physical data

were comparable with reported values. The size and

thickness of the reduced graphene oxide obtained after

two times repetition of oxidation and reduction

sequence ideally suit for easy dispersion into polymer

matrix and thereby provide better film forming

b Fig. 13 The construction of typical device diagram; ITO/

PEDOT:PSS/Active Layer/LiF/Al. a Schematic representation

of the device using polymer/nanocomposite as active emissive

layer and working LED using nanocomposite; band diagram of

b polymer and c nanocomposite

J Nanopart Res (2014) 16:2693 Page 17 of 20 2693

123

property. Improved in-plane crystallinity enhances

electron transport properties of the nanocomposite and

induces a favorable shift in balance of electron and

hole injection toward the center of emissive zone

making nanocomposite suitable for use as active

emitting layer. As the EL efficiency of an LED

depends not only on the PL efficiency of the active

polymer but also on its balanced charge injection and

its carrier mobility, research is initiated toward the

synthetic manipulation of polymer back bone to

prepare thieno[3,2-b]thiophene-based conjugated

copolymer and incorporation of hole and electron

blocking agent in the main/side chain. Optimization of

nanocomposite compositions and use of additional

hole blocking agents, such as, 2,9-dimethyl-4,7-

diphenyl-1,10-phenanthroline (BCP) during device

fabrication to compare the efficacy of electron trans-

port properties (with respect to reduced graphene

oxides), have also been initiated.

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