ARTICLE IN PRESS

Solar Energy Materials & Solar Cells 94 (2010) 1196–1200

Contents lists available at ScienceDirect

Solar Energy Materials & Solar Cells

0927-02

doi:10.1

n Corr

E-m

yshwan

journal homepage: www.elsevier.com/locate/solmat

Solution-processable functionalized graphene in donor/acceptor-typeorganic photovoltaic cells

Zhiyong Liu, Dawei He n, Yongsheng Wang n, Hongpeng Wu, Jigang Wang

Key Laboratory of Luminescence and Optical Information, Ministry of Education, Institute of Optoelectronic Technology, Beijing Jiaotong University, Beijing 100044, PR China

a r t i c l e i n f o

Article history:

Received 19 December 2009

Received in revised form

28 February 2010

Accepted 6 March 2010Available online 9 April 2010

Keywords:

Graphene

P3HT

HOMO

LUMO

48/$ - see front matter & 2010 Elsevier B.V. A

016/j.solmat.2010.03.004

esponding authors.

ail addresses: 08118340@bjtu.edu.cn (Z. Liu)

g@bjtu.edu.cn (Y. Wang).

a b s t r a c t

An organic photovoltaic device based on an acceptor of solution-processable functionalized graphene

(SPFGraphene) was designed. The devices were based on heterostructure polymer-graphene composite

layers. The structural configuration of devices is ITO/PEDOT:PSS/P3HT:SPFGraphene/LiF/Al. Due to the

functional groups of the graphene, a homogeneous blend of graphene–polymer composite could be

obtained. In the graphene–polymer composite, the graphene acted as exciton dissociation sites and

provided the transport pathway of LUMO-graphene-Al. Doping of graphene into P3HT resulted in

appropriate energetic distance between HOMO and LUMO of the donor/acceptor for a high open circuit

voltage and provided higher exciton dissociation volume mobility of carrier transport for a large short-

circuit current density. The device containing only 10 wt% of graphene shows the best performance

with a power conversion efficiency of 0.88%, an open-circuit voltage of 0.77 V, and a short-circuit

current density of 3.72 mA/cm2.

& 2010 Elsevier B.V. All rights reserved.

1. Introduction

The photovoltaic of inorganic materials based on the ZnO, TiO2,

CdSe and CdS, has attracted much interest of researcher all overthe world [1]. But the photovoltaic devices based on inorganicmaterials offer great disadvantage because of their high cost andenvironment-pollute manufacturing methods. Organic photovol-taics (OPVs) are a promising low cost alternative to silicon solarcells, thus a great deal of effort is being devoted to increase thepower conversion efficiency and to scale-up the productionprocesses [2]. An attractive feature of the organic photovoltaicsbased on conjugated polymers is that they can be fabricated by acoating process (e.g. spin coating or inkjet printing) to cover largeareas, and may be formed on flexible plastic substrates [3]. Thephotovoltaic devices based on organic materials have attractedmuch interest of researcher including materials, processes, anddevices [4]. Power efficiency of organic photovoltaic devices isstill low compared with the traditional inorganic devices [5]. Themain factor is structural traps in the form of dead ends, isolateddomains, and incomplete pathways in the random percolationnetwork [6], which has resulted in inefficient hopping chargetransport and electron transport. Therefore, the challenge here isto provide continuous pathways within each component and thusto allow charges to transport efficiently to the electrodes before

ll rights reserved.

, dwhe@bjtu.edu.cn (D. He),

recombination occurs [7]. So far, a research effort in solution-processed OPV materials has been dominated by the use of PCBMas the electron acceptor. In addition, the solubility and stability ofboth donor and acceptor are critically important. The mostsuccessful OPV cells are those with BHJ architecture based onsoluble poly (3-hexylthiophene) (P3HT) and poly (3-octylthio-phene) (P3OT) as the donor and PCBM as the acceptor [8,9], theExternal Quantum Efficiency (EQE) of P3HT/PCBM hybrid solarcell is nearly 80%, the power conversion efficiency (PCE) of organicphotovoltaic cells has surpassed 6% [9]. However, the powerconversion efficiency of these OPV devices is still low comparedwith conventional inorganic devices [5]. The commonly acceptedmechanism for the light-to-electricity conversion process is lightabsorption exciton generation, exciton diffusion, exciton dissocia-tion and charge formation and charge transport and chargecollection [10]. The main factor of low power efficiency comparedwith conventional inorganic devices is the absorption spectrum ofP3HT. Thus, new materials for both donor and acceptor withbetter HOMO/LUMO matching, stronger light absorption, andhigher charge mobility with good stability are much needed. Thishas led to studies of other allotropic forms of carbon nanomater-ials, including single walled carbon nanotubes (SWCNTs) andmultiwalled carbon nanotubes (MWNTs) as acceptors [11].However, some unfavorable factors, such as their insolubility,impurities, and bundling structure, have greatly hindered thedevice performance of carbon nanotubes [12].

Graphene has different electronic and optical characteristiccompared with CNT and C60. Graphene is a gapless semiconduc-tor with unique electronic properties [13]. It shows high electron

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Z. Liu et al. / Solar Energy Materials & Solar Cells 94 (2010) 1196–1200 1197

mobility and displays as high as 100,000 cm2/V s [14]. The uniquestructure and excellent electronic properties, particularly its highmobility, and the ready availability of functionalized graphene,render it a competitive alternative as the electron-acceptingmaterial in PV device applications. In this paper, graphene notonly acts as electron acceptors but also provides high field at thepolymer/graphene interfaces for exciton dissociation.

0.5

0.6

0.7

0.8

0.9

1.0

nsity

(a.u

.)

SPFGraphene contents 15% SPFGraphene contents 10% SPFGraphene contents 5% SPFGraphene contents 0%

2. Experimental

2.1. Synthesis of solution-processable functionalized graphene

(SPFGraphene)

In this paper, graphene has been prepared by exfoliatedgraphene oxide sheets. The first step is the preparation ofgraphite oxide by modified hummer method [15]. Five grams ofcrystalline flake graphite, 30 g KMnO4, and 15 g of NaNO3 (purity99%) were placed in a flask. Then, 300 ml of H2SO4 (purity 98%)was added, a stirrer chip was placed in the mixture, and themixture was stirred while being cooled in an ice water bath. Theliquid was added to 1000 cm3 of deionized water over about 1 hwith stirring. Then, 30 ml of H2O2 (30 wt% aqueous solution) wasadded to the above liquid and the mixture was stirred for 2 h.

In order to remove Mn2 +, the resultant liquid was purified byrepeating the following procedure: centrifugation, removal of thesupernatant liquid, addition of a mixed aqueous solution of 3 wt%H2SO4/0.5 wt% H2O2, and shaking to disperse. The procedure wascycled using aqueous HCl solution (5 wt%) and using H2O, andthen drying process in vacuum. The molecular structure ofgraphite oxide is shown in Fig. 1b.

Isocyanate functionalization of graphene oxide: dried graphiteoxide (200 mg) was suspended in deionized water (20 ml), andtreated with phenyl isocyanate (20 g) for 24 h and remove theimpurities, and finally the isocyanate-treated graphene oxide wasobtained [7]. The second step is to exfoliate graphite oxideultrasonically. Then a phenyl isocyanate treatment resulted inSPFGraphene that can dissolve in organic solvent [16].

0.0

0.1

0.2

0.3

0.4

PL

inte

Wavelength (nm)500 550 600 650 700 750

Fig. 2. PL spectra of P3HT and P3HT/SPFGraphene (SPFGraphene content: 2.5 wt%,

5 wt%, 10 wt% and 15 wt%) composite films at an excitation wavelength of 422 nm.

2.2. Fabrication and characterization of optoelectronic devices

The organic photovoltaic (OPV) was made using a commonfabrication process. The hole-injections buffer layer of (polyethylene dioxythiophene) doped with polystyrene sulfonic acid(PEDOT:PSS) was spin-coated on the indium tin oxide (ITO)coated glass substrate. Then PEDOT:PSS-coated substrate wasannealed for 20 min at 120 1C in vacuum. And then spin-coating asolution of 15 mg/ml poly(3-hexylthiophene-1,3-diyl) (P3HT) inchlorobenzene with SPFGraphene content of 0 wt%, 1 wt%,

Fig. 1. (a) Schematic of the devices with P3HT/SPFGraphene a

2.5 wt%, 5% wt%, 10 wt%, 12.5 wt%, and 15 wt% onto indium tinoxide (ITO) glass substrate. Then the device was annealed for10 min at 120 1C in vacuum. LiF and Al were vapor deposited onthe active layer. Fig. 1a shows the schematic of the devices withP3HT/Graphene as the active layer.

The current–voltage (J–V) was determined using a Keithley2410 source measure unit. A 150 mW Xenon lamp acted as abroadband light source. The photoluminescence was measuredusing a Fluolog-3fluoresvent spectrometer. The absorption spec-tra of P3HT:Graphene were measured using a Shimadzu UV-3101PC spectrometer. All measurements were at atmospheric pressureand room temperature.

3. Results and discussion

Photoluminescence (PL) in conjugated thiophenes is wellknown to arise from radiative recombination of polaron–excitonpairs into Franck–Condon (FC) states, and the reduction of the PLof an appropriate donor polymer by a suitable acceptor gives anindication of an effective photoinduced charge transfer from thedonor to the acceptor [17], which indicates an effective photo-induced charge transfer from the donor to the acceptor, asdescribed by Sariciftci [18]. Thus, the PL spectra of the P3HT/SPFGraphene (P3HT: 5 mg/ml, SPFGraphene content: 0 wt%,5 wt%, 10 wt%, and 15%) mixture solution in chlorobenzene andthe P3HT (5 mg/ml) solution in chlorobenzene were investigated,as shown in Fig. 2.

s the active layer. (b) The chemical structure of graphene.

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From Fig. 2, we can see that the pure P3HT solution showsstrong photoluminescence between 525 and 750 nm, withexcitation at 422 nm. However, the photoluminescence is re-markably reduced after the SPFGraphene was introduced, show-ing efficient charge/energy transfer along the P3HT/SPFGrapheneinterface. More significant reduction in PL intensity for the P3HT/SPFGraphene films is due to the efficient electron transfer fromP3HT to SPFGraphene. The trend of reduction of PL intensity withlarger SPFGraphene content manifests that the efficiency ofcharge separation is improved in the roughened P3HT/SPFGra-phene configuration. These results show that the excitedfluorophore in the P3HT backbone decreased due to the electronicinteractions at the P3HT/SPFGraphene interfaces. The otherreason is the increased interfacial areas that facilitate chargeseparation within the bulk instead of just at the planar interfacefor the bilayer structure. By referring to previous work with PCBMand carbon nanotubes [19,20], this efficient reduction of PLintensity shows that graphene is expected to be an effectiveelectron-acceptor material for organic photovoltaic applications.

For study on absorption spectra of P3HT/SPFGraphene compo-site film, mixed solution of P3HT and SPFGraphene dissolved inchlorobenzene was used. Fig. 3 shows the absorption spectra ofP3HT/SPFGraphene (SPFGraphene content: 10 wt%), as well asthe reference solution of P3HT (5 mg/ml) in chlorobenzene. Theabsorption characteristics of P3HT in the range of 300–800 nm,the original absorption of P3HT centred at 550 nm. However,absorption of P3HT/SPFGraphene mixed solution is almost thesame as the scope and absorption peaks, but the absorptionpeak of the P3HT/SPFGraphene slightly increases, and enhancedabsorption ranging from 320 to 570 nm. This may explainthe absorption of P3HT/SPFGraphene composite film. Despitethe SPFGraphene content of 10 wt%, the absorption spectra ofP3HT/SPFGraphene did not show significant changes. This shouldbe the result of P3HT/SPFGraphene mixed in solution, with nosignificant ground state interaction between the two materials.Therefore, there is no charge transfer in the ground state ofP3HT/SPFGraphene composite [21].

The power conversion efficiency (Z) was calculatedaccording to

Z¼ VocIscFF

Pin

where Voc, Isc, Pin, and FF are the open-circuit voltage, the short-circuit current density, the incident light power, and the fill factor(FF). The FF measures the quality of the solar cell as a power

0.0

0.2

0.4

0.6

0.8

1.0 P3HT P3HT-Graphene

Abs

orpt

ion

(a.u

.)

Wavelength (nm)200 300 400 500 600 700 800

Fig. 3. Absorption spectra of P3HT and P3HT-Graphene.

source and can be defined as the ratio between the maximumpower delivered to an external circuit and the potential power.The fill factor (FF) is given by

FF ¼VmaxImax

VocIsc

where Vmax and Imax are the values of the voltage and currentdensity for maximizing for the product of I–V curve in the fourthquadrant, where the device operates as an electrical power source.

Fig. 4 shows the current–voltage (J–V) of photovoltaic devices inthe dark and AM 1.5 100 mW simulated solar radiation for P3HT,P3HT/SPFGraphene and annealing treating P3HT/SPFGraphenedevices. Fig. 4 There is no reaction in the dark of pure P3HT,P3HT/SPFGraphene (SPFGraphene content: 5 wt%), and P3HT/SPFGraphene (SPFGraphene content: 5 wt%) annealed devices.Under simulated 100 mW AM 1.5G illumination, open-circuitvoltage (Voc) of pure P3HT active layer before annealing is 0.45 V,short-circuit current density (Jsc) of pure P3HT active layer beforeannealing is 0.31 mA/cm2 and FF of pure P3HT active layer beforeannealing is 0.22. In contrast, Voc of P3HT/SPFGraphene(SPFGraphene content: 5 wt%) has increased to 0.67 V, Jsc hasincreased to 1.6 mA/cm2, and FF has increased to 0.24. Improvementof the overall photovoltaic performance can be attributed to anincrease in SPFGraphene. After annealing treatment, Fig. 4 showsthat Voc of the heterojunction device increased to 0.76 V, Jsc

increased to 2.77 mA/cm2 and FF increased to 0.28.After functionalization, the SPFGraphene sheet introduced

many functional groups and the p-conjugated structure waspartly isolated by the functional groups. Therefore, the organicfunctional groups decrease charge transport properties andmobility of the graphene sheets. The SPFGraphene is much lessconductive than graphene. This will limit the performance of theabove P3HT/SPFGraphene based device. In view that the functionalgroups can be removed from the SPFGraphene sheet in an elevatedtemperature under vacuum, the conductivity of the SPFGraphenesheet can be recovered [22]. The other affect of introducedfunctional groups is bandgap. Graphene has zero bandgap; somepaper has reported that the solution-processable functionalized ofgraphene band gap is 0.4 eV [23]. Clearly, improvement of theoverall photovoltaic performance is due to annealing process.Therefore, we will anneal all the optoelectronic devices.

Then we will study the different graphene content (0%, 1%,2.5%, 5%, 10% 12.5%, and 15%) based on optical and electricalproperties of P3HT/SPFGraphene composite, shown in Fig. 5.

1E-6

1E-5

1E-4

1E-3

0.01

0.1

1

10

J sc (

mA

/cm

2 )

Voc (V)

P3HT darkP3HT/graphene darkP3HT/graphene dark annealingP3HT light P3HT/graphene lightP3HT/graphene light annealing

-1.0 -0.5 0.0 0.5 1.0 1.5

Fig. 4. J–V characteristics of PV devices based on P3HT, P3HT/SPFGraphene

(SPFGraphene content is 5 wt%) and P3HT/SPFGraphene(SPFGraphene content is

5 wt%) annealing treatment in the dark and light.

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0.400.450.500.550.600.650.700.750.800.850.90

Voc FF

Weight fraction of Graphene on P3HT:PCBM solution (%)

Voc

(V)

0.24

0.26

0.28

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0.34

FF (%

)

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Jsc

PCE

Weight fraction of Graphene on P3HT:Graphene solution (%)

J sc (

mA

/cm

2 )

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1.0

Pow

er c

onve

rsio

n ef

ficie

ncy

(%)

0 2 4 6 8 10 12 140 2 4 6 8 10 12 14

Fig. 5. (a) Dependence of the short-circuit current density, the power conversion efficiency on the different graphene concentrations. (b) Dependence of the open-circuit

voltage and the FF on the graphene concentration on the different graphene concentrations.

Fig. 6. Energy band diagram of the fabricated device showing band alignment for

graphene

Table 1Performance details (Voc, Jsc, FF and Z) of the photovoltaic devices having a

structure of ITO/PEDOT:PSS/P3HT:graphene/LiF/Al after annealing under a simu-

lated AM 1.5G 100 mW illumination.

Graphene content (wt%) Voc (V) Jsc (mA/cm2) FF Z (%)

0 0.41 0.41 0.25 0.04

1 0.57 1.5 0.25 0.21

2.5 0.73 2.54 0.26 0.48

5 0.76 2.77 0.28 0.58

10 0.77 3.72 0.31 0.88

12.5 0.76 3.55 0.3 0.81

15 0.58 1.7 0.26 0.26

Z. Liu et al. / Solar Energy Materials & Solar Cells 94 (2010) 1196–1200 1199

For SPFGraphene content of 0 wt%, 1 wt%, 2.5 wt%, 5 wt%, 10 wt%12.5 wt%, and 15 wt%, P3HT/SPFGraphene on the basis ofcomposite materials shows photoelectric power conversionefficiencies of 0.04%, 0.21%, 0.48%, 0.58%, 0.88% 0.81%, and0.26%. Fig. 5 shows that with the increase of SPFGraphenecontent, the overall performance reached its peak; when thecontent was 10 wt%, the power efficiency was 0.88%.

Voc of the pure P3HT is 0.41 V and the Voc of composite filmP3HT/SPFGraphene is 0.77 V. There are different models describ-ing the Voc of the pure P3HT [18,19]. A single layered organicphotovoltaic cell is composed of a pure conjugated polymer andthe Voc was principally determined by the work functiondifference between the two metal electrodes. The configurationof organic photovoltaic devices is the metal of the electrode–insulator–metal (MIM) model [24], i.e, ITO–active layer–Al. In thispaper, Voc of pure P3HT is 0.41 V, well matching the work functiondifference between the ITO anode (4.7 eV) and Al cathode (4.3 eV)[25]. However, as the P3HT/SPFGraphene has a BHJ structure, theMIM model is not applicable and Fermi level pinning is the mainfactor behind this [26]. Therefore, the upper limit of Voc can bedetermined by the difference of the electron acceptor’s LUMO andthe polymer’s HOMO [24,25]. The pinning mechanism betweenthe negative electrode and the single-walled nanotubes (SWNTs)has been applied in the solar cells based on polymer–carbonnanotube composites. The work function of carbon nanotube aswell as the HOMO level of the conjugated polymer governs Voc

[25]. Thus, in our P3HT/SPFGraphene-based device, the piningmechanism may be applied to describe the origin of Voc value. Thecalculated work function of pristine graphene is 4.5 eV [27].Therefore, the upper limit of Voc has been determined by thedifference of the work function of graphene (4.5 eV) and theHOMO level (5.25 eV) of P3HT [28]. Energetically favorable chargetransportation and band diagram is shown in Fig. 6.

The increase in FF can be attributed to an improvement of theseries and/or the shunt resistance due to a rougher surface of thephotoactive layer compared with the pristine layer, resulting in anincrease in the contact area between the photoactive layer and theAl, and consequently to a reduction in the series resistance [29].

From Table 1, we can find J–V characteristics underillumination of the ITO/PEDOT:PSS/P3HT:graphene/Al with theoptimum graphene concentration of 10%, compared with ITO/PEDOT:PSS/P3HT/Al photovoltaic devices. The photovoltaiccharacteristics of both devices, open-circuit voltage (Voc), short-

circuit current density (Jsc), FF, and power conversion efficiency(Z), are extracted from Fig. 5 and are given in Table 1.

Table 1 shows different SPFGraphene contents (0 wt%, 1 wt%,2.5 wt%, 5 wt%, 10 wt% 12.5 wt%, and 15 wt%) of the J–V curve.Power conversion efficiencies are 0.04%, 0.21%, 0.48%, 0.58%, 0.88%0.81%, and 0.26%, respectively. SPFGraphene content of 10 wt%shows the best results. If the SPFGraphene content is lower than10 wt%, along with the increase of SPFGraphene content thepower conversion efficiency increases. SPFGraphene content isthe main factor improving the power conversion efficiency. ForSPFGraphene for smaller concentrations, such as 1 wt% and 2.5%,the SPFGraphene film is too small to form a continuous donor/acceptor interface and the transport pathway for the active layerP3HT matrix. Therefore, the electron cannot effectively meet the

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donor/acceptor interface and transported smoothly through theactive layer. However, if SPFGraphene is further increased to aconcentration of 10 wt%, the SPFGraphene film can form acontinuous donor/acceptor interface and produce a better wayto be transported smoothly through P3HT matrix. This willimprove the electronic transport and the transport pathway ofLUMO–SPFGraphene–Al. If there is a further increase in theconcentration of SPFGraphene, such as 12.5 wt% and 15 wt%, thenthe aggregation of SPFGraphene may occur. We consider that thehigh series resistance should come from inefficient stackingmorphology of SPFGraphene sheets, and chemical modificationmay also destroy the p-conjugation of the SPFGraphene sheet andthen result in a decrease in charge carrier mobility.

For a high photocurrent value, we require sufficient interfaces toensure efficient exciton dissociation and continuous conductingpaths for electrons and holes to the appropriate electrodes [30]. Inthe active layer, the exciton generation takes place only in thepolymer. However, SPFGraphene concentration beyond 10 wt%, theaverage distance between individual SPFGraphene has decrease andthe photogeneration rate has reduced. The maximum intensity ofthe solar spectrum is at a wavelength of about 550 nm within thegreen band. Otherwise, the inevitable presence of SPFGrapheneenhances recombination. The SPFGraphene has no bandgap and actsas trapping and recombination centres in the bandgap of thecomposite semiconductor medium. Upon increasing the SPFGra-phene concentration, it is likely that the SPFGraphene will alignparallel to each other and pack into crystalline ropes due to strongvan der Waals attraction. In this way, the percentage of SPFGraphenewill significantly increase, since only one SPFGraphene is sufficientto convert an entire bundle to a quasi-metallic state. In this way, thenegative impact of the SPFGraphene is boost, since in a given bundleonly one SPFGraphene is adequate for transformation of the entirebundle to a quasi-metallic state. The reduced hole mobility due toincreased trapping observed for higher than 10% SPFGrapheneconcentrations also leads to suppressed carrier extraction. As aresult, as the SPFGraphene content increases beyond 10 wt%, thephotocurrent decreases, confirming that the number of extractedcarriers decreases. Nevertheless, more charge transport experimentsare required to clarify this argument.

4. Conclusions

In this paper, SPFGraphene acted as the acceptor material in theorganic photovoltaic cells. In the photovoltaic device withgraphene, P3HT acts as the photoexcited electron donors andSPFGraphene acts as electron acceptor and provides percolationpaths. When the SPFGraphene content is 10 wt%, the best Jsc

reaches 3.72 mA/cm2, the best Voc reaches 0.77 V and the best FFreaches 0.31 compared with the pure P3HT devices. We get thebest power conversion efficiency of 0.88% for the device whenSPFGraphene content is 10 wt% in the active layer after anannealing process.

Acknowledgements

We are grateful for the financial support of National Out-standing Youth Science Foundation under Contract no. 60825407,National Natural Science Fund Project under Contractno. 60877025, Beijing Science and Technology ommittee underContract no. Z08000303220803, Beijing Science and TechnologyCommittee under Contract no. D090803044009001 and BeijingNatural Science Fund Project under Contract no. 2092024

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