Solar Energy Materials & Solar Cells 94 (2010) 2148–2153

Contents lists available at ScienceDirect

Solar Energy Materials & Solar Cells

0927-02

doi:10.1

n Corr

E-m

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

Improving photovoltaic properties by incorporating both SPFGraphene andfunctionalized multiwalled carbon nanotubes

Zhiyong Liu, Dawei He n, Yongsheng Wang n, Hongpeng Wu, Jigang Wang, Haiteng 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 8 May 2010

Received in revised form

20 June 2010

Accepted 5 July 2010

Keywords:

SPFGraphene

f-MWCNTs

Charge

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

016/j.solmat.2010.07.001

esponding authors.

ail addresses: dwhe@bjtu.edu.cn (D. He), yshw

a b s t r a c t

Solution-processable functionalized graphene (SPFGraphene) and functionalized multiwalled carbon

nanotubes(f-MWCNTs) are introduced for heterojunction solar cell. The performance of the device has

improved by the incorporation of both SPFGraphene and f-MWCNTs. The open-circuit voltage (Voc),

short-circuit current density (Jsc), fill factor (FF) and power conversion efficiency (Z) were 0.67 V,

4.7 mA/cm2, 32%, and 1.05%, respectively. Here, we expect that SPFGraphene acts as exciton dissociation

and provide percolation paths for electron transfer, whereas f-MWCNTs provide efficient hole

transportation. SPFGraphene and f-MWCNTs incorporation yields better carrier mobility, easy exciton

splitting, and suppression of charge recombination, thereby improving photovoltaic action.

& 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]. However, the photovoltaic devices based oninorganic materials offer great disadvantage because of their highcost and environment-pollute manufacturing methods. Organicphotovoltaics (OPVs) are a promising low-cost alternative tosilicon solar cells, thus a great deal of effort has been devoted toincrease the power conversion efficiency and to scale up theproduction processes [2]. An attractive feature of the organicphotovoltaics based on conjugated polymers is that they can befabricated by a coating process (e.g. spin coating or inkjetprinting) to cover large areas, and may be formed on flexibleplastic substrates [3]. The photovoltaic devices based on organicmaterials have attracted much interest of researcher includingmaterials, processes and devices [4]. Some lab has reported themanufacture of polymer solar cells using full roll-to-roll proces-sing [5]. It has developed the full manufacture, integration anddemonstration of polymer solar cells [6]. Power efficiency oforganic photovoltaic devices is still low compared to thetraditional inorganic devices [7]. The main factor is structuraltraps in the form of dead ends, isolated domains and incompletepathways in the random percolation network [8], which hasresulted in inefficient hopping charge transport and electrontransport. Therefore, the challenge here is to provide continuouspathways within each component and thus to allow charges to

ll rights reserved.

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

transport efficiently to the electrodes before recombinationoccurs [9]. So far, the research effort of OPV materials hasdominated on the PCBM as the electron acceptor. In addition, thesolubility and stability of both donor and acceptor are criticallyimportant. The most successful OPV cells are based on solublepoly(3-hexylthiophene) (P3HT) and poly(3-octylthiophene)(P3OT) as the donor and PCBM as the acceptor [10,11].Some paper had reported the external quantum efficiency (EQE)of P3HT/PCBM hybrid solar cell to be nearly 80%; thepower conversion efficiency (PCE) of organic photovoltaiccells has surpassed 7.4%. The structure of devices is based onITO/PEDOT:PSS/PTB7: PC71BM/Ca/Al, the PTB7 acted as donormaterials and the PC71BM acted as acceptor materials [11,12].However, the power conversion efficiency of these OPV devices isstill low compared to conventional inorganic devices [7]. Thecommonly accepted mechanism for the light-to-electricity con-version process is light absorption exciton generation, excitondiffusion, exciton dissociation and charge formation and chargetransport and charge collection [13]. The main factor of low-power efficiency compared with conventional inorganic devices isthe absorption spectrum of P3HT. Thus, new materials for bothdonor and acceptor with better HOMO/LUMO matching, strongerlight absorption and higher charge mobility with good stabilityis needed. This has led to studies of other allotropic forms ofcarbon nanomaterials, including single-walled carbon nanotubes(SWCNTs) and multiwalled carbon nanotubes (MWNTs) asacceptors [14]. Functionalized multiwalled carbon nanotubes(f-MWCNTs), SWCNTs and PCBM have shown better powerconversion efficiency than pristine samples without CNTs orPCBM [15,16]. In such solar cells, it is suggested that MWCNTsenhance hole transport, whereas SWCNTs enhance electron

Fig. 1. (a) Schematic of the devices with P3HT/f-MWCNT-SPFGraphene as the

active layer. (b) The chemical structure of SPFGraphene.

Z. Liu et al. / Solar Energy Materials & Solar Cells 94 (2010) 2148–2153 2149

transport. However, practically, the solubility and stability of bothdonor and acceptor are critically important.

Graphene, as a very recent rising star in materials science withtwo-dimensional (2D) structure consisting of sp2-hybridized car-bon, exhibits remarkable electronic and mechanical properties thatqualify it for application in future optoelectronic devices [17]. It is agapless semiconductor with unique electronic properties and itselectron mobility reaches 200,000 cm2/V s at room temperature[18]. Its one-atom thickness and large 2D plane lead to a largespecific area, and therefore, very large interfaces can form when itwas added to a polymer matrix. A conducting film and a transparentanode for PV device applications have also been developed [19,20].The unique structure and excellent electronic properties, particu-larly its high mobility, and the ready availability of solution-processable functionalized graphene (SPFGraphene), render it acompetitive alternative as the electron-accepting material in PVdevice applications [21]. In this paper, the SPFGraphene not onlyacts as electron acceptors, but also provide high field at thepolymer/SPFGraphene interfaces for exciton dissociation.

2. Experimental

2.1. Synthesis of functionalized multiwalled carbon nanotubes

(f-MWCNTs)

In this work, we aimed to study the role of incorporation of bothf-MWCNTs and graphene with conducting polymer to makeheterojunction photovoltaic device. Purified MWCNTs was sus-pended in mixture of concentrated H2SO4/HNO3 (H2SO4:HNO3 is3:1) and sonicated in a water bath for a few hours. The suspensionis diluted by deionized water. A functionalized multiwalled carbonnanotube (0.1 mg) is dispersed in chloroform solvent (1 ml) [14].

2.2. Synthesis of solution-processable functionalized graphene

(SPFGraphene)

In this paper, SPFGraphene is been prepared by exfoliatedgraphene oxide sheets. The first step is the preparation of graphiteoxide by the modified hummer method [21]. 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 added to 1000 cm3 of deionized water over about 1 h ofstirring. Then, 30 ml of H2O2 (30% aqueous solution) was added tothe above liquid and the mixture was stirred for 2 h.

In order to remove Mn2+, the resultant liquid purified by repeatingthe following procedure: centrifugation, removal of the supernatantliquid, addition of a mixed aqueous solution of 0.5% H2O2, and shakingto disperse. The procedure was cycled using aqueous HCl solution(5%) and using H2O, and then drying process in vacuum. Themolecular structure of graphite oxide been 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 the impuritieswere removed, and finally the isocyanate-treated graphene oxidewas obtained [9]. The second step is to exfoliate graphite oxideultrasonically. Then a phenyl isocyanate treatment resulted inSPFGraphene that can dissolve in organic solvent [22].

2.3. Fabrication and characterization of optoelectronic devices

The organic photovoltaic (OPV) was made using a commonfabrication process. The hole-injections buffer layer of (polyethy-

lene 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 various SPFGraphene contents (0, 1, 2.5, 5, 10,12.5 and 15 wt%) and 2% f-MWCNTs content onto indium tinoxide (ITO) glass substrate. Then the devices annealed for 10 minat 180 1C in vacuum. LiF and Al were vapor deposited on theactive layer. Fig. 1a shows the schematic of the devices withP3HT/SPFGraphene as the active layer.

The current–voltage (J–V) was determined using a Keithley 2410source measure unit. A 150 W xenon lamp acted as a broadbandlight source and the intensity of incident light is 100 mW/cm2. Thephotoluminescence been measured using a Fluolog-3fluoresventspectrometer. The absorption spectra been measured using aShimadzu UV-3101 PC spectrometer. All measurements were atatmospheric pressure and room temperature.

3. Results and discussion

The power conversion efficiency (Z) was calculated according to

Z¼ VocIscFF

Pin

where Voc, Jsc, Pin and FF are the open-circuit voltage, the short-circuit current density, the incident light power and the fill factor(FF), respectively. The fill factor (FF) of definition is

FF¼VmaxImax

VocIsc

The FF measures the quality of solar cell as a power source and isdefined as the ratio between the maximum power delivered to anexternal circuit and the potential power. Vmax and Imax are,respectively, the values of the voltage and current densities formaximizing for the product of I–V curve in the fourth quadrant,where the device operates as an electrical power source.

After functionalization, the SPFGraphene sheet and multi-walled carbon nanotubes introduced many functional groups andthe structure been partly isolated by the functional groups.Therefore, the organic functional groups decrease charge trans-port properties and mobility of the SPFGraphene sheets andf-MWCNTs. This will limit the performance of the above P3HT/SPFGraphene-f-MWCNTs based device. In view that the functional

Fig. 3. Energy band diagram of the fabricated device showing band alignment for

SPFGraphene.

500-0.1

0.0

0.1

0.2

0.3

0.4

0.5

0.6

0.7

0.8

0.9

1.0

PL in

tens

ity (a

.u.)

Wavelength (nm)

SPFGraphene content 12%SPFGraphene content 8%SPFGraphene content 5%SPFGraphene content 0%

550 600 650 700 750 800

Fig. 4. PL spectra of P3HT and P3HT/SPFGraphene (SPFGraphene contents: 0%, 5%,

8% and 12%) composite films at an excitation wavelength of 422 nm.

Z. Liu et al. / Solar Energy Materials & Solar Cells 94 (2010) 2148–21532150

groups can be removed from the SPFGraphene sheet andf-MWCNTs in an elevated temperature under vacuum, theconductivity of the SPFGraphene sheet and f-MWCNTs can berecovered [14,23]. The other affect of introduced functionalgroups is band gap. Graphene has zero band gap; some paperhas reported that the solution-processable functionalized ofgraphene band gap is 0.4 eV [24]. Clearly, improvement of theoverall photovoltaic performance is due to annealing process.Therefore, we will anneal all the optoelectronic devices.

For study on absorption spectra of P3HT/SPFGraphene compo-site film, mixed solution of P3HT/f-MWCNTs, P3HT/f-MWCNTs-SPFGraphene (P3HT: 1 mg/ml, SPFGraphene content: 5%) and P3HTdissolved in chlorobenzene was used. Fig. 2 shows the absorptionspectra of P3HT/f-MWCNTs, P3HT/f-MWCNTs-SPFGraphene, as wellas the reference solution of P3HT in chlorobenzene. The absorptioncharacteristics of P3HT in the range of 300–800 nm, the originalabsorption of P3HT centred at 550 nm. However, the absorptionspectra of P3HT/f-MWCNTs, P3HT/f-MWCNTs-SPFGraphene mixedsolution is almost the same as the scope and absorption peaks, butthe absorption peak of the P3HT/f-MWCNTs-SPFGraphene slightlyincreases, and enhanced absorption ranging from 340 to 550 nm.This may explain the absorption of P3HT/f-MWCNTs-SPFGraphenecomposite film. Despite the SPFGraphene content of 5%, theabsorption spectra of P3HT/f-MWCNTs-SPFGraphene did not showsignificant changes. This should be the result of P3HT/f-MWCNTs-SPFGraphene mixed in solution, with no significant ground stateinteraction between the two materials. Therefore, there is no chargetransfer in the ground state of P3HT/SPFGraphene composite [21].

SPFGraphene could also exhibit strong donor/acceptor interac-tions for the conjugated polymers. We will investigate the characterof electron acceptor between SPFGraphene and P3HT by photo-luminescence (PL). Thus, we will investigate the PL spectra thatP3HT/SPFGraphene (P3HT: 5 mg/ml, SPFGraphene content: 0%, 5%,8% and 10%) mixture solution in chlorobenzene and P3HT (5 mg/ml)solution in chlorobenzene. From Fig. 4 we can see that the pureP3HT solution shows strong photoluminescence between 525and 750 nm, with excitation at 422 nm. However, introduction ofSPFGraphene into the P3HT has remarkably reduced thephotoluminescence intensity. It has shown efficient charge/energytransfer along the P3HT/SPFGraphene interface. This efficientquenching of PL emission is due to the efficient electron transferfrom P3HT to SPFGraphene. The trend of reduction in PL intensityalong with an increase in SPFGraphene content has shown that theefficiency of charge separation has improved in the roughened

300

0.0

0.2

0.4

0.6

0.8

1.0

Abs

orpt

ion

(a.u

.)

Wavelength (nm)

P3HT/f-MWCNT-SPFGrapheneP3HT/f-MWCNTP3HT

400 500 600 700

Fig. 2. Absorption spectra of P3HT, P3HT/f-MWCNT and P3HT/f-MWCNT-

SPFGraphene.

P3HT/SPFGraphene configuration. These results show that thequench of fluorophore is due to the electronic interactions at theP3HT/SPFGraphene interfaces. The relative position of donor LUMOand acceptor LUMO is crucial for the aimed charge transfer. Fig. 3shows that there is a difference between LUMO of P3HT and workfunction of SPFGraphene. Energy band diagram favored thephotoexcited P3HT to transfer electron to SPFGraphene molecule.Therefore, P3HT acted as electron donor and SPFGraphene acted aselectron acceptor to prepare donor/acceptor solar cells. Thequenching of PL of an appropriate donor polymer by a suitableacceptor gives an indication of an effective donor–acceptor chargetransfer from the donor to the acceptor, as described by Sariciftciet al. [25] for composites of p-conducting polymers andSPFGraphene derivatives. The other reason is the increasedinterfacial areas that facilitate charge separation within the bulkinstead of just at the planar interface for the bilayer structure. Byreferring to previous work with PCBM and carbon nanotubes[26,27], this efficient reduction in PL intensity shows thatSPFGraphene expected to be an effective electron acceptormaterial for organic photovoltaic applications.

Fig. 5 shows the current–voltage (J–V) of photovoltaic devicesin the dark and AM 1.5 100 mW simulated solar radiation for P3HT/f-MWCNTs and P3HT/f-MWCNTs-SPFGraphene (SPFGraphene con-tent is 5%) devices. There is no reaction in the dark of P3HT/f-MWCNTs and P3HT/f-MWCNTs-SPFGraphene (SPFGraphenecontent is 5%) devices. Under simulated 100 mW AM 1.5 Gillumination, open-circuit voltage (Voc) of P3HT/f-MWCNTs activelayer is 0.65 V, short-circuit current density (Jsc) of P3HT/f-MWCNTs

Z. Liu et al. / Solar Energy Materials & Solar Cells 94 (2010) 2148–2153 2151

active layer is 0.27 mA/cm2, FF of P3HT/f-MWCNTs active layer is0.27 and power conversion efficiency (Z) of P3HT/f-MWCNTs activelayer is 0.65%. In contrast, Voc of P3HT/f-MWCNTs-SPFGraphene(SPFGraphene content is 5%) has increased to 0.67 V, Jsc hasincreased to 3.2 mA/cm2, FF has increased to 0.32 and powerconversion efficiency (Z) of P3HT/f-MWCNTs-SPFGraphene activelayer is 0.9%. Improvement of the overall photovoltaic performancecan be attributed to an increase in SPFGraphene.

Then we will study the optical and electrical properties ofdifferent SPFGraphene contents (0%, 1%, 5%, 8%, 10% and 12%)based on P3HT/f-MWCNTs-SPFGraphene composite, as shown inFig. 6. The different SPFGraphene contents (0%, 1%, 5%, 8%, 10% and12%) show different power conversion efficiencies (0.65%, 0.75%,0.9%, 1.05%, 0.82% and 0.58%), respectively. Fig. 6 shows that alongwith an increase in SPFGraphene content, the overall performancereached its peak; the best content was 8% and the powerefficiency, 1.05%.

Voc of the P3HT+f-MWCNTs is 0.65 V and the Voc of compositefilm P3HT/f-MWCNTs-SPFGraphene is 0.67 V. There are differentmodels describing the Voc of the pure P3HT [25,28]. A singlelayered organic photovoltaic cell is composed of a pure conjugated

-1.01E-6

1E-5

1E-4

1E-3

0.01

0.1

1

10

Cur

rent

Den

sity

(mA

/cm

2 )

Voltage (V)

P3HT/f-MWCNT dark

P3HT/f-MWCNT-SPFGraphene dark

P3HT/f-MWCNT light

P3HT/f-MWCNT-SPFGraphene light

-0.5 0.0 0.5 1.0 1.5

Fig. 5. J–V characteristics of PV devices based in P3HT/f-MWCNT,

P3HT/f-MWCNT-SPFGraphene (SPFGraphene content is 5%) in the dark and light.

03.4

3.6

3.8

4.0

4.2

4.4

4.6

4.8

5.0

JscPCE

Weight fraction of SPFGraphene on P3HT/f-MWCNTs solution (%)

Jsc

0.6

0.8

1.0

Pow

er c

onve

rsio

n ef

ficie

ncy

(PC

E)

2 4 6 8 10 12

Fig. 6. (a) Dependence of the short-circuit current density and the power convers

open-circuit voltage and the FF on the different SPFGraphene concentrations.

polymer and the Voc principally determined by the work functiondifference between the two metal electrodes. The configuration oforganic photovoltaic devices is the electrode–insulator–metal(MIM) model [29], i.e, ITO–active layer–Al. However, the P3HT/f-MWCNTs-SPFGraphene has a BHJ structure, the MIM model is notapplicable and Fermi level pinning is the main factor [30].Therefore, the upper limit of Voc can determine by the differencebetween the work function of SPFGraphene and f-MWCNTs. Somepaper has reported that the work function of as-preparedSPFGraphene is 4.5 eV. The work function of MWCNTs ranges from4.6 to 5.1 eV. After acid oxidation, carboxylic acid groups wereintroduced onto the surface of MWCNTs, which produced higherwork function (5.1 eV) [14,21]. Energetically favorable chargetransportation and band diagram are shown in Fig. 3.

Increase in FF can be attributed to the introduction ofSPFGraphene in P3HT. Introduced SPFGraphene into P3HT increasedthe built-in electric field and field-dependent exciton dissociationrate. The SPFGraphene will improve the electron transport andbalance the electron-hole pairs transport. Another reason is animprovement of the series and/or the shunt resistance. IntroducedSPFGraphene into P3HT will roughen the interface and increase thecontact area between the photoactive layer and the Al, andconsequently reduce the series resistance [31].

Table 1 shows the J–V curve of different SPFGraphene contents(0%, 1%, 5%, 8%, 10% and 12%). Power conversion efficiencies are0.65%, 0.75%, 0.9%, 1.05%, 0.82% and 0.58%, respectively.SPFGraphene content of 8% has shown the best results. If theSPFGraphene content is lower than 8%, along with an increasein SPFGraphene content, the power conversion efficiency increases.SPFGraphene content is the main factor improving the power

00.52

0.54

0.56

0.58

0.60

0.62

0.64

0.66

0.68

VocFF

Weight fraction of SPFGraphene on P3HT/f-MWCNTs solution (%)

Voc

0.26

0.28

0.30

0.32FF

2 4 6 8 10 12

ion efficiency on different SPFGraphene concentrations. (b) Dependence of the

Table 1Performance details (Voc, Jsc, FF and Z) of the P3HT/f-MWCNT-SPFGraphene based

photovoltaic devices.

SPFGraphene

content (%)

Voc (V) Jsc (mA/

cm2)

FF Z (%)

0 0.65 3.7 0.27 0.65

1 0.66 3.9 0.29 0.75

5 0.67 4.4 0.31 0.9

8 0.67 4.7 0.32 1.05

10 0.65 4.2 0.3 0.82

12 0.54 3.5 0.31 0.58

Z. Liu et al. / Solar Energy Materials & Solar Cells 94 (2010) 2148–21532152

conversion efficiency. While SPFGraphene concentrations are lower,such as 1%, the SPFGraphene film is too small to form a continuousdonor/acceptor interface and the transport pathway for the activelayer P3HT matrix. Therefore, the electron cannot effectively meetthe donor/acceptor interface and transported smoothly through theactive layer. However, SPFGraphene concentration further increasedto 8%, the SPFGraphene film can form a continuous donor/acceptorinterface and produce a better way to transport smoothly throughP3HT matrix. This will improve the electronic transport to form thetransport pathway of LUMO–SPFGraphene–Al. The work functionsof SPFGraphene are closer to the work functions of Al; this willdecrease the barrier of Al/LUMO to form the transport pathway ofLUMO–SPFGraphene–Al. In this phase, the SPFGraphene acted asthe percolation paths of electron. The work functions of f-MWCNTsare closer to the work functions of ITO; this will decrease the barrierof ITO/HOMO to form the transport pathway of HOMO–f-MWCNTs–ITO; the hole will be transported from HOMO of P3HTto f-MWCNTs, and then transported from f-MWCNTs to ITO.In this phase, the f-MWCNTs acted as the percolation paths ofhole. The other reason is that the SPFGraphene acted as an electronacceptor. In the pure conjugated polymer, the excitons candissociate at the interface of polymer. As can be seen inthe band diagram of Fig. 3, while introducing SPFGraphene andf-MWCNTs into the polymers, the excitons can dissociate at thepolymer/SPFGraphene and the polymer/f-MWCNTs interfaces. Theelectrons were captured by the SPFGraphene and transferred tothe Al, and the hole was captured by the f-MWCNTs and transferredto the ITO, which is energetically favoured. This results in a fasterelectron transport than could be achieved in the pristine device byhopping only through the polymer molecule.

If there is a further increase in the concentration ofSPFGraphene, such as 10% and 12%, then the aggregation ofSPFGraphene may occur; the average distance between individualSPFGraphene has decreased and the photogeneration rate hasreduced. For a high photocurrent value, we require sufficientinterfaces to ensure efficient exciton dissociation and continuousconducting paths for electrons and holes to the appropriateelectrodes [32]. In the active layer, the exciton generation takesplace only in the polymer. However, SPFGraphene concentrationbeyond 8%, the average distance between individual SPFGraphenehas decreased and the photogeneration rate has reduced.The maximum intensity of the solar spectrum is at a wavelengthof about 550 nm within the green band. Otherwise, the inevitablepresence of SPFGraphene enhances recombination. SPFGraphenehas no band gap and acts as trapping and recombination centresin the band gap of the composite semiconductor medium. Onincreasing the SPFGraphene concentration, it is likely that theSPFGraphene will align parallel to each other and pack intocrystalline ropes due to strong van der Waals attraction. Thepercentage of SPFGraphene will significantly increase, since onlyone SPFGraphene is sufficient to convert an entire bundle to aquasi-metallic state. In this way, the negative impact of theSPFGraphene is boosted, since in a given bundle only oneSPFGraphene is adequate for transformation of the entire bundleto a quasi-metallic state. It will reduce hole mobility due toincreased trapping observed and suppressed carrier extraction.As a result, SPFGraphene content increases beyond 8%, thephotocurrent decreases, confirming that the number of extractedcarriers decreases. Nevertheless, more charge transport experi-ments are required to clarify this argument.

4. Conclusion

In this paper, SPFGraphene acted as the acceptor material inthe organic photovoltaic cells. In the photovoltaic device based on

ITO/PEDOT:PSS/P3HT-f-MWCNTs-SPFGraphene/LiF/Al, P3HT actsas the photoexcited electron donors; SPFGraphene act as electronacceptor and provide percolation paths of electron; f-MWCNTsprovide percolation paths of hole. When the SPFGraphene contentis 8 wt%, the best Jsc has reach 4.7 mA/cm2, the best Voc has reach0.67 V, the best FF has reach 0.32 and the power conversionefficiency is 1.05% compared to the other devices.

Acknowledgements

We gratefully acknowledge the financial support of NationalOutstanding Youth Science Foundation under Contractno. 60825407, National Natural Science Fund Project underContract no. 60877025, Beijing Science and Technology Committeeunder Contract no. Z08000303220803, Beijing Science and Tech-nology Committee under Contract no. D090803044009001 andBeijing Natural Science Fund Project under Contract no. 2092024.

References

[1] I.G. Valls, M.L. Cantu, Vertically-aligned nanostructures of ZnO for excitonicsolar cells: a review, Energy Environ. Sci. 2 (2009) 19–34.

[2] T. Ameri, G. Dennler, C. Lungenschmied, C.J. Brabec, Organic tandem solarcells: a review, Energy Environ. Sci. 2 (2009) 347–363.

[3] F.C. Krebs, Fabrication and processing of polymer solar cells: a reviewof printing and coating techniques, Sol. Energy Mater. Sol. Cells 93 (2009)394–412.

[4] M. Helgesen, R. Søndergaard, F.C. Krebs, Advanced materials and processes forpolymer solar cell devices, J. Mater. Chem. 20 (2010) 36–60.

[5] F.C. Krebs, T. Tromholt, M. Jørgensen, Upscaling of polymer solar cellfabrication using full roll-to-roll processing, Nanoscale 2 (2010) 873–886.

[6] F.C. Krebs, T.D. Nielsen, J. Fyenbo, M. Wadstrøm, M.S. Pedersen, Manufacture,integration and demonstration of polymer solar cells in a lamp for the‘‘Lighting Africa’’ initiative, Energy Environ. Sci. 3 (2010) 512–525.

[7] S.A. Backer, K. Sivula, D.F. Kavulak, J.M.J. Frechet, High efficiency organicphotovoltaics incorporating a new family of soluble fullerene derivatives,J. Chem. Mater. 19 (2007) 2927–2929.

[8] W.U. Huynh, J.J. Dittmer, A. Paul, Hybrid nanorod-polymer solar cells, Science295 (2002) 2425–2427.

[9] Q. Liu, Z.F. Liu, X.Y. Zhang, L.Y. Yang, N. Zhang, G.P. Pan, S.G. Yin, Y.S. Chen,J. Wei, Polymer photovoltaic cells based on solution-processable graphemeand P3HT, Adv. Funct. Mater. 19 (2009) 894–904.

[10] W.L. Ma, C.Y. Yang, X Gong, K. Lee, A.J Heeger, Thermally Stable, Efficientpolymer solar cells with nanoscale control of the interpenetrating networkmorphology, Adv. Funct. Mater. 15 (2005) 1617–1622.

[11] J.Y. Kim, K. Lee, N.E. Coates, D. Moses, T.Q. Nguyen, M. Dante, A.J. Heeger,Efficient tandem polymer solar cells fabricated by all-solution processing,Science 317 (2007) 222–225.

[12] Y.Y. Liang, Z. Xu, J.B. Xia, S.T. Tsai, Y. Wu, G. Li, C. Ray, L.P. Yu, For the brightfuture—bulk heterojunction polymer solar cells with power conversionefficiency of 7.4%, Adv. Mater. 22 (2010) 1–4.

[13] C.J. Brabec, N.S. Sariciftci, J.C. Hummelen, Plastic solar cells, Adv. Funct. Mater.11 (2001) 15–26.

[14] I. Khatri, S. Adhikari, H.R. Aryal, T. Soga, T. Jimbo, M. Umeno, Improvingphotovoltaic properties by incorporating both single walled carbon nano-tubes and functionalized multiwalled carbon nanotubes, Appl. Phys. Lett. 94(2009) 093509–093511.

[15] B. Pradhan, S.K. Batabyal, A.J. Pal, Functionalized carbon nanotubes indonor/acceptor-type photovoltaic devices, Appl. Phys. Lett. 88 (2006)093106–093108.

[16] C. Li, Y. Chen, Y. Wang, Z. Iqbal, M. Chhawalla, S. Mitra, A fullerene–singlewall carbon nanotube complex for polymer bulk heterojunction photovoltaiccells, J. Mater. Chem. 17 (2007) 2406–2411.

[17] H.B. Heersche, P.J. Herrero, J.B. Oostinga, L.M.K. Vandersypen, A.F. Morpurgo,Bipolar supercurrent in graphene, Nature 446 (2007) 56–59.

[18] K.I. Bolotina, K.J. Sikesb, Z. Jiang, M. Klimac, G. Fudenberga, J. Honec, P. Kima,H.L. Stormer, Ultrahigh electron mobility in suspended graphene, Solid StateCommun. 146 (2008) 351–355.

[19] S. Gilje, S. Han, M. Wang, K.L. Wang, R.B. Kaner, A chemical route to graphenefor device applications, Nano Lett. 7 (2007) 3394–3398.

[20] X. Wang, L. Zhi, K. Mullen, Transparent, conductive graphene electrodes fordye-sensitized solar cells, Nano Lett. 8 (2008) 323–327.

[21] Z.F. Liu, Q. Liu, Y. Huang, Y.F. Ma, S.G. Yin, X.Y. Zhang, W. Sun, Y.S. Chen,Organic photovoltaic devices based on a novel acceptor materical: graphene,Adv. Mater. 20 (2008) 3924–3930.

[22] S. Stankovich, D.A. Dikin, G.H.B. Dommett, K.M. Kohlhaas, E.J. Zimney,E.A. Stach, R.D. Piner, S.T. Nguyen, R.S. Ruoff, Graphene-based compositematerials, Nature 442 (2006) 282–286.

Z. Liu et al. / Solar Energy Materials & Solar Cells 94 (2010) 2148–2153 2153

[23] D. Wegner, R. Yamachika, Y.Y. Wang, V.W. Brar, B.M. Bartlett, R.J. Long,F.M. Crommie, Single-molecule charge transfer and bonding at an organic/inorganic interface: tetracyanoethylene on noble metals, Nano Lett. 8 (2008)131–135.

[24] X.L. Li, X.R. Wang, L. Zhang, S.W. Lee, H.J. Dai, Chemically derived, ultrasmoothgraphene nanoribbon semiconductors, Science 319 (2008) 1229–1232.

[25] N.S. Sariciftci, L. Smilowitz, A.J. Heeger, F. Wudl, Photoinduced electrontransfer from a conducting polymer to buckminster fullerene, Science 258(1992) 1474–1476.

[26] S. Bertho, W.D. Oosterbaan, V Vrindts, J. D’Haen, T.J. Cleij, L. Lutsen, J. Manca,D Vanderzande, Controlling the morphology of nanofiber-P3HT:PCBMblends for organic bulk heterojunction solar cells, Org. Electron. 10 (2009)1248–1251.

[27] S. Berson, R.D. Bettignies, S. Bailly, S. Guillerez, B. Jousselme, Elaboration ofP3HT/CNT/PCBM composites for organic photovoltaic cells, Adv. Funct.Mater. 17 (2007) 3363–3370.

[28] M.A. Ibrahim, H.K. Roth, U. Zhokhavets, G. Gobsch, S. Sensfuss, Flexible largearea polymer solar cells based on poly(3-hexylthiophene)/fullerene, Sol.Energy Mater. Sol. Cells 85 (2005) 13–20.

[29] C.J. Brabec, A. Cravino, D. Meissner, N.S. Sariciftci, T. Fromherz, M.T. Rispens,L. Sanchez, J.C. Hummelen, Origin of the open circuit voltage of plastic solarcells, Adv. Funct. Mater. 11 (2001) 374–380.

[30] K. Yoshina, S. Nakajima, D.H. Park, R.I. Sugimoto, Thermochromism,photochromism and anomalous temperature dependence of luminescencein poly(3-alkylthiophene) film, Jpn. J. Appl. Phys. Part 27 (1998) 716–718.

[31] C. Li, Y. Chen, Y. Wang, Z. Iqbal, M. Chhowalla, M. Mitra, A fullerene-singlewall carbon nanotube complex for polymer bulk heterojunction photovoltaiccells, J. Mater. Chem 17 (2007) 2406.

[32] E. Kymakis, P. Servati, P. Tzanetakis, E Koudoumas, N. Kornilios,I. Rompogiannakis, Y. Franghiadakis, G.A.J. Amaratunga, Effective mobilityand photocurrent in carbon nanotube–polymer composite photovoltaic cells,Nanotechnology 18 (2007) 435702–435707.