ZnO/Graphene Quantum Dot Solid-State Solar CellMrinal Dutta,†,‡ Sanjit Sarkar,† Tushar Ghosh, and Durga Basak*

Department of Solid State Physics, Indian Association for the Cultivation of Science, Jadavpur, Kolkata-700032, India

*S Supporting Information

ABSTRACT: Graphene quantum dots (GQDs) synthesized by a directchemical method have been used in combination with ZnO nanowires(NWs) to demonstrate their potential as a solar harvesting material inphotovoltaic cells exhibiting an open circuit voltage of 0.8 V. The excitedstate interaction between the photoexcited GQDs and the ZnO NWs hasbeen verified from the charge-transfer process by both emissionspectroscopy and photovoltaic measurements. This work has implicationsfor less expensive and efficient next generation solid-state solar cells.

■ INTRODUCTION

Graphene, a flat monolayer of carbon atoms in a two-dimensional (2D) honeycomb lattice, has become the pin-upamong all of the carbon materials since its discovery byNovoselov and his group in 2004.1 It is now considered awonder kit among all of the promising building blocks forfuture nanodevices because of the superior electronic, thermal,and mechanical properties as well as chemical stability.1a,2

Currently available micrometer-sized graphene sheets (GSs)produced by reduction of exfoliated graphene oxide (GO),micromechanical cleavage, solvothermal synthesis, and otherphysical and chemical routes generally are highly conducting,3

which makes them suitable for flexible conductors in electroniccircuits. GSs are also used in solar cells as electrodes forefficient charge transfer.4 However, because these GSs do nothave an energy band gap, their direct application inoptoelectronics is limited. Theoretical and experimentalworks5 show that quantum confinement and edge effects mayintroduce a band gap in narrow graphene nanoribbons (GNRs)with widths of <∼10 nm and smooth edges. On the other hand,quantum confinement and edge effects are found to be morepronounced in graphene quantum dots (GQDs) from whichmany new fascinating phenomena are expected.6 The synthesisof GQDs has been done successfully by different methods. Panand co-workers7 presented a hydrothermal method for cuttingpreoxidized GSs into GQDs (approximately 10 nm in size),while Shen et al.6b have prepared GQDs by hydrazine hydratereduction of GO by a surface-passivation technique withpolyethylene glycol (PEG). An electrochemical route has beenshown to be successful for the synthesis of GQDs by Li and co-workers.8 GQDs have also been synthesized from smallmolecule precursors such as 3-iodo-4-bromoaniline and othersubstituted benzene derivatives by solution chemistry in abottom up approach.9 However, depending on the synthesis,the physical properties of GQDs widely vary; thus, theirapplications in opto-electronic devices have become a greatchallenge. The major area in which GQDs have shown promiseis luminescent materials for bioimaging,10 single electrontransistors,11 light-harvesting assemblies,12 and electron-trans-

porting materials in photovoltaic devices.8,13 The size-depend-ent band gap of graphene5a,14,15 and large optical absorptivity16

are particularly interesting for its application as a photosensitiz-ing material in photovoltaic devices. Theoretical results showthat the broad range of the whole solar spectrum can becovered only by tuning the band gap of graphene achieved byvarying their sizes.5a,14 Thus, exploring GQDs to work as alight-harvesting material becomes very important and timely inthe present scenario of energy crisis, especially when solidsensitized solar cell devices are found to be more reliable andsustained for a long time with respect to the dye-based solarcells. Despite the recent progress in quantum dot-sensitizedphotovoltaic devices,17 the use of mostly higher cost, toxic, andhazardous acceptor materials (CdSe and PbTe) becomes aserious impediment for large-scale applications. On the otherhand, ZnO has been widely used in organic as well as hybridsolar cells18 due to its salient characteristics such as low cost,easy synthesis of 1D nanostructures, nontoxicity, high stability,and good optoelectronic properties.19,20 The applications ofZnO in solar cells also include its use as electrode buffer layersor transparent electrodes.21 Graphene-based materials haveshown promising applications for energy materials as well asenergy storage.22 The GQDs have a great potential to be usedas a sensitizer for the solar cells as demonstrated by Yan et al.12

for the first time. Therefore, because they are nontoxic,biocompatible, and cheaper, these QDs might be the rightchoice as a replacement for benign sensitizing materials forZnO in solar cells. In this paper, we thus report a combinationGQDs with ZnO NWs that can be efficiently harnessed in solarcells. Most notably, use of this combination in the solar cell isunique and novel. The emission spectroscopy shows that acharge-transfer process takes place at the interface betweenGQDs and NWs. This charge separation at the interface hasfurther been confirmed from the photovoltaic performance ofthe cells made up by ZnO NWs-GQD-based composite. The

Received: March 29, 2012Revised: August 23, 2012

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photovoltaic cell comprised of these GQDs shows a high opencircuit voltage (VOC) of ∼0.8 V. The results demonstrate thepotential of these GQDs to work as efficient light-harvestingmaterials.

■ EXPERIMENTAL SECTIONGQDs were synthesized from GO by a simple one-stephydrazine reduction method following a technique as reportedearlier.6b Graphite oxide was first prepared from naturalgraphite powder (Sigma-Aldrich) by the modified Hummersmethod.23 In this method, graphite (0.2 g) was treated withsodium nitrate (2 g) in 60 mL of H2SO4 with the addition ofpotassium permanganate (6 g) under constant stirring for 4days and processed further to produce a yellow-brown-coloredgraphite oxide. The exfoliation of graphite oxide to GO wasachieved by adding graphite oxide (35 mg) to water (100 mL)and ultrasonication of the dispersion for 3 h, yielding ahomogeneous yellow-brown dispersion. By centrifugation ofthe dispersion at 2000 rpm, the unreacted graphite wasseparated from GO. The chemical conversion of GO tographene was obtained by mixing the resulting homogeneousdispersion in water (100 mL) with hydrazine solution (1.5 mL)in a 250 mL round-bottomed flask. The solution was thenheated in an oil bath at 120 °C under a water-cooled condenserfor 30 h with the addition of aqueous ammonia after 24 h,wherein the reduced GO gradually precipitated out as a blacksolid mass.Vertical arrays of ZnO NWs grown on the Al-doped ZnO

(AZO) thin films (details of the growth technique of NWs andAZO can be found in the Supporting Information) wereinfiltrated and covered with the synthesized GQDs. This wasdone by repeated spin-casting of an ethanolic suspension ofGQDs on the NWs arrays until the space between them wasfilled up completely and a thin layer of QDs was formed on topof the NWs. Then, we deposited a 60−70 nm thin layer of N-N′-diphenyl-N-N′-bis(3-methylphenyl)-1,1′-biphenyl)-4,4′-dia-mine (TPD), which acts as a hole-transporting layer24 by spin-casting its solution in chloroform (10 mg/mL). After that, thedevice was annealed in an inert atmosphere at 110 °C for 30min. Then, the top Au electrode was sputtered on the TPDlayer.A high-resolution transmission electron microscopy

(HRTEM) (JEOL, model: JSM-2010) attached with the energydispersive X-ray analysis (EDAX) facility (INCA, Oxford), afield-emission scanning electron microscopy (FESEM) (JEOLJSM-6700F), and an atomic force microscope (AFM; Nanosurf,easyScan2) were used for the microstructural characterizations.The X-ray powder diffraction pattern (XRD) was obtained byX-ray diffractometer (Bruker axs, model: D8 Advanced). TheUV−Vis transmission spectra were recorded in the wavelengthrange 300−800 nm using UV−vis−NIR spectrophotometer(Hewlett-Packard, model: 8453). Fourier transform infrared(FTIR) spectra were collected using a Perkin-Elmerspectrophotometer (model: Spectrum 100). The Ramanspectroscopy was done using a Raman Microscope (RenishawinVia; serial no. 12W143). The energy-dependent roomtemperature photoluminescence (PLE) was measured byfluorescence spectrophotometer (Hitachi; model F-4500) andalso by employing a He−Cd laser (Kimmon Koha Co. Ltd.;model: KR1801C) with a 325 nm excitation source and a high-resolution spectrometer (Horiba Jobin-Yvon, model: iHR 320)together with a photomultiplier tube. The current−voltage (I−V) characteristics of the photovoltaic cell and incident photon-

to-current conversion efficiency (IPCE) were measured using aKeithley 2400 series source meter and 6485 Pico ammeter. Theinternal quantum efficiency (IQE) was estimated by dividingIPCE by the fraction of absorbed light in the active film. Thecells were illuminated with white light from a 300 W Xe arclamp (Oriel) after filtering the ultraviolet light. The intensity ofthe incident white light on the electrode surface correspondedto a power of 100 mW.

■ RESULTS AND DISCUSSIONThe microstructure and size distribution of the synthesizedGQDs as observed by the TEM are shown in Figure 1a. Their

sizes vary within 4−15 nm with an average diameter of ∼8.5nm (the inset in Figure 1a). The major Raman features as seenfrom the Raman spectra of these GQDs in Figure 1b are the Dband at around 1350 cm−1 as a breathing mode of k-pointphonons of A1g symmetry, which is assigned to the local defectsand disorder, especially at the edges of graphene, and the Gband at around 1593 cm−1, which is usually assigned to the E2gphonon of C sp2 atoms.23,25 The ratio of intensity, ID/IG, is∼1.4, which is similar to the value of the GQDs synthesized byhydrothermal method, indicating enhanced edge effects inGQDs.26 Previous investigations have shown that the peakposition of the G band (1585 cm−1) shifts about 6 cm−1 towarda lower wavenumber after stacking more graphene layers (for2−6 layers, G band shifts to 1579 cm−1).5b,c,25b,27 However, inour case, the G band position of GQDs (1−3 layers as observedby AFM studies) rather shifts to a higher wavenumber side by 8cm−1, which can be attributed to the confinement effect andalso due to the strain present in the QDs as confirmed from theXRD pattern.28 However, a detailed theoretical and exper-imental investigation on this confinement and strain effect bystudying the Raman spectra is beyond the scope of the presentwork and hence will be presented elsewhere. Details of the X-ray diffraction and FTIR characterization results can be seen inFigures S1 and S2 in the Supporting Information. The AFM

Figure 1. (a) TEM image of GQDs. The inset shows thecorresponding size distribution. (b) Raman spectrum of GQDs. (c)AFM image of the GQDs deposited on cleaned glass substrate. (d)Height distribution of the GQDs. (Inset) Height profile along the linesAB, CD, EF, and GH in panel c.

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images of the GQDs are shown in Figure 1c. The topographicheight distribution obtained from the AFM studies variesbetween 1 and 4 nm as shown in Figure 1d, which is at par withthe previous reports.6b,26 The height distribution suggests thatamong these GQDs, more than 50% are single layer, 30% arebilayer, and 11% are trilayer graphene.Figure 2a shows the absorbance spectra of GQDs, ZnO

NWs, and ZnO NWs-GQDs composites. It shows that the

GQDs exhibit an absorbance in the visible region, and thecomposite shows further enhanced absorbance in that region.The synthesized GQDs also exhibit an excitation-dependent PLbehavior as shown in Figure 2b, which is quite similar to thepreviously reported amorphous29 and disordered luminescentcarbons.30 The maximum of the emission band shifts towardhigher wavelength side as the excitation wavelength is changedfrom 280 to 420 nm and the intensity of the emission banddecreases rapidly. From Figure 2b, it can be seen that onexcitation at the absorption band of 300 nm, the PL spectrumshows a strong peak at 346 nm with a Stokes shift of 46 nm(0.55 eV). The evolution of the luminescence behavior with achange in the excitation energy is a result of the fact that theband gap in these QDs depends on the size and shape of thesp2 domains that result from the size distribution of the GQDs.The absorption and emission spectra of the GQDs in Figure 2are comparable to the recently reported absorption andemission spectra for GQDs produced by hydrothermal andelectrochemical methods.8,26 The creation of localized sp2

structures and structural defects during reduction31 are morelikely to be responsible for the origin of this PL behavior. ThePL spectra of GQDs, ZnO, and ZnO NWs-GQDs compositesunder the excitation of 325 nm from a He−Cd laser source areshown in Figure 2c. Bare GQDs show a broad PL emission witha peak at 346 nm. When these GQDs are combined with ZnONWs, the emission from these QDs is quenched fully in thevisible region as shown in Figure 2c. The emission due to ZnOcomponent in the composite material has been decreased as

compared to the emission of bare ZnO because of the blockingeffect by the black-colored GQDs layer on the NWs. Suchquenching of the emission process indicates an interfacialcharge separation through the additional pathways created bythe interactions between the excited GQDs and the ZnONWs.32 Recently, using ultraviolet (UV) photoelectron yieldspectroscopy (PYS), Yan et al.15 have determined the HOMOlevel of the graphene QDs to be varied within 5.1−5.4 eV withrespect to the vacuum level, which is similar to other recentmeasurements.8,33 Thus, using the result of the PLE spectrum,the position of the LUMO levels can be determined as 1.8−2.8eV. The HOMO and LUMO levels of GQDs as well as thevalence (7.6 eV) and conduction bands (4.35 eV)34 of ZnOrelative to the vacuum level are shown schematically in Figure2d. The band alignment indicates that the electron transfer isquite feasible at the interface of these QDs and ZnO NWs.The charge transfer process at the interface has been suitably

utilized in photovoltaic cells based on GQDs-ZnO NWs bulkheterojunction. The device model is shown in the inset inFigure 3a. The IQE of the devices made up of GQDs-ZnO

NWs and ZnO NWs is shown in Figure 3a. The IQE ofcomposite cell closely resembles the absorption spectrum ofGQDs and reaches a value of 87% at the absorption maximum.The IQE in the visible region of composite cell is much higherthan that of the control sample. The I−V measurement carriedout in the dark reveals diode behavior (the inset in Figure 3b).When illuminated, this device exhibits a clear shift in thevoltage axis, which is evidence for the photovoltaic effect.Figure 3b shows the photocurrent action spectra of the devicemade by the composite (ZnO NWs-GQDs) and the control

Figure 2. (a) Absorbance spectra of GQDs, ZnO NWs, and ZnONWs-GQDs composites. (b) Excitation-dependent PL spectra of theGQDs. (c) PL spectra of ZnO NWs, GQDs, and ZnO NWs-GQDscombined structure excited at 325 nm. (d) Schematic energy levelband diagram of the device with energy levels in eV relative to vacuum.

Figure 3. (a) IQE of the samples. The inset shows the schematicmodel of the device. (b) Solar cell I−V characteristics under whitelight illumination. The inset shows the logarithmic plot of dark andphoto current of the GQDs-ZnO cell.

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sample (ZnO NWs only). While there is no photovoltaic effectshown by the control sample, a clear photovoltaic effect isobserved from the I−V curve of the device made with thecomposite. This indicates and confirms that there is an electroninjection into the NWs from the excited GQDs. The controldevice shows a short-circuit current, JSC = 5.9 μA/cm2 and VOC= 0.1 V, whereas the composite device shows JSC = 0.45 mA/cm−2 with open-circuit voltage, VOC = 0.8 V and FF = 0.5,leading to an estimated power conversion efficiency of 0.2%.So, there is a 75 times increase in the JSC value, which is also atpar with the difference in the IQE values between ZnO andZnO NWs-GQDs composites in the visible region (Figure 3a).The power conversion efficiency is, however, low due to theinefficient hole collection by TPD from GQDs. Because thegaps in between the NWs were filled with the GQDs, and theTPD layer exists only at the top, the hole collection efficiencymay not be good, leading to a poor cell performance. Now, weare trying to improve the hole conduction by regulating thethickness of GQDs layer and spacing between NWs. The VOCof the bulk-heterojunction photovoltaic device depends uponthe difference between the position of the conduction band ofthe acceptor material and the valence band (HOMO level) ofthe donor material.35 The electron affinity value of ZnO (χZnO)is usually reported to be from ∼4.1 to 4.5 eV,36 and theHOMO level of GQDs lies within 5.1−5.4 eV.8,33 Therefore,the theoretical VOC may be calculated to be in the range of 0.6−1.3 eV. The present experimental value of VOC ∼ 0.8 V is fairlywithin the expected theoretical range. However, the Fermi levelof n-type ZnO NWs is chosen as 4.35 eV (average electronaffinity of 4.25 eV plus a donor level of 0.1 eV), and consideringa constant of 0.3 eV (the difference between built in potential,VBI and VOC) as proposed by an empirical formula by Scharberet al.37 for a conjugated polymer-PCBM-based solar cell, theVOC is expected to be lower than the value obtained in thisstudy, although the value of the constant would largely dependon the cell fabrication as well as its composition. The ISC valueobtained in this case is comparable to the ISC values of theearlier reported CdS, CdTe, CdSe, PbS Qds, and CdSe-ZnSecoreshell sensitized or P3HT infiltrated ZnO NWs solar cells.38

Our results show that the environmentally friendly GQDs canbe potentially harnessed as a replacement of toxic semi-conductor QDs sensitizers along with an advantage of holetransport. However, the efficiency of the present GQDs-sensitized solar cell is quite low, but it is noteworthy to mentionthat our study is an initial effort toward demonstrating thepotential of nontoxic GQDs as a light-absorbing and hole-transporting material for the next generation solid-state solarcells. By further optimizing the device configuration andimproving the light absorption properties of the GQDs with theband gap in the visible and infrared regions, we expect toimprove the performance of these GQDs-sensitized solar cells,which is at present in progress.

■ CONCLUSIONSIn conclusion, ZnO NWs-GQDs composites have been formedby infiltrating ZnO NWs with GQDs synthesized by thehydrazine reduction of GO. Both emission spectroscopy andphotovoltaic measurements confirm charge transfer at theinterface of photoexcited GQDs and ZnO NWs. Solid-statesolar cell device performance of the ZnO NWs-GQDscomposites without any optimization exhibits VOC ∼ 0.8 V.The concept of using this composite in the solar cell is novel,and thus, these results will extend the application of graphene

in the next generation solar cells and other optoelectronicdevices.

■ ASSOCIATED CONTENT*S Supporting InformationDetailed synthesis procedure of AZO and ZnO NWs, FTIRspectroscopy, and X-ray diffraction spectra. This material isavailable free of charge via the Internet at http://pubs.acs.org.

■ AUTHOR INFORMATIONCorresponding Author*E-mail: sspdb@iacs.res.in.Present Address‡International Center for Materials Nanoarchitectonics(MANA), National Institute for Materials Science, 1-1 Namiki,Tsukuba, 305-0044, Japan.Author Contributions†Both authors have contributed equally to this work.NotesThe authors declare no competing financial interest.

■ ACKNOWLEDGMENTSWe acknowledge the funding support of the Department ofScience and Technology, India, for the HRTEM instrumentunder the Nanoinitiative programme. S.S. thanks CSIR, India,for providing a junior research fellowship.

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dx.doi.org/10.1021/jp302992k | J. Phys. Chem. C XXXX, XXX, XXX−XXXE