Study of graphene doped zinc oxide nanocomposite as transparent conducting oxide electrodes for solar cell applications

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<ul><li><p>J. Shanghai Jiaotong Univ. (Sci.), 2014, 19(3): 378-384</p><p>DOI: 10.1007/s12204-014-1512-8</p><p>Study of Graphene Doped Zinc Oxide Nanocomposite asTransparent Conducting Oxide Electrodes for Solar Cell Applications</p><p>LI Pan-pan1 (), MEN Chuan-ling1 (), LI Zhen-peng1 ()CAO Min1 ( ), AN Zheng-hua2 ()</p><p>(1. School of Energy and Power Engineering, University of Shanghai for Science and Technology,Shanghai 200093, China; 2. Institute of Advanced Materials and State Key Laboratory of</p><p>Surface Physics, Fudan University, Shanghai 200433, China)</p><p> Shanghai Jiaotong University and Springer-Verlag Berlin Heidelberg 2014</p><p>Abstract: The graphite oxide (GO) was prepared based on the modified Hummers method, then reacted withzinc acetate aqueous, sodium hydroxide aqueous and hydrazine hydrate, and was doped into ZnO eventually toform graphene doped ZnO, an alternative transparent conducting oxide (TCO) for solar cell applications. Thesamples were characterized by Raman spectrometer, X-ray diffractometer, Fourier transform infrared spectroscopyand scanning electron microscope, and compared with widely used aluminum doped ZnO (AZO) in resistivity andtransmissivity. The results show that the transmissivity of graphene doped ZnO reaches the same level as thatof AZO in visible light band. In ultraviolet light wave band, the transmissivity of graphene doped ZnO reachesas high as 50%, exceeding that of AZO which is only 20%. The resistivity of optimized graphene doped ZnO is1.03 105 m, approaching AZO resistivity which is about 104106 m. As a result, graphene dopedZnO may have potential applications in the area of TCO due to its low cost and high performance.Key words: graphene doped ZnO, graphite oxide (GO), aluminum doped ZnO (AZO), transmissivity, resistivityCLC number: TK 511 Document code: A</p><p>0 Introduction</p><p>Transparent conducting oxide (TCO) thin films arewidely used in various fields including photovoltaic solarcells, flat panel displays, transparent thin film transis-tors (TFTs),and light emitting diodes (LEDs) of semi-conductor lasers. TCO thin films of Sn doped In2O3(ITO) and F doped SnO2 (FTO) are the preferable ma-terials in these areas. However, the expanding use ofTCO materials, especially for the production of trans-parent electrodes for optoelectronic device applications,is endangered by the scarcity and high price of In. Thissituation drives the urgent search for alternative TCOmaterials to replace ITO. The best candidates are ZnO-based thin films such as aluminum doped ZnO (AZO)thin films which have low resistivity on the order of106 m and good optical transparency. AZO is in-expensive source material, and is non-toxic. So far,AZO thin films have been used as windows and con-tact layers for thin film solar cells with absorber materi-</p><p>Received date: 2013-06-14Foundation item: the Natural Science Foundation of</p><p>Shanghai (No. 13ZR1428200), and the National Projectof University of Shanghai for Science and Technology(No. 14XPM06)</p><p>E-mail:</p><p>als, such as amorphous silicon, CuIn1xGaxSe2 (CIGS),and CdTe. Nevertheless, AZO suffers from low ultra-violet and near-infrared transmittance and rather poorstability in hostile environment containing acidic andalkali solutions, oxidizing and reducing atmospheres,and elevated temperature.Since the first discovery of graphene, the outstand-</p><p>ing electrical, optical, mechanical and chemical prop-erties of graphene make it very attractive for applica-tions in optoelectronics[1-2]. Recent achievements onhigh-throughput of large area graphene highlight itsgreat potential for industrial applications[2]. Bae etal.[3] demonstrated that graphene itself can be used astransparent conducting electrodes for flat panel display,grown by chemical vapor deposition method on 76.2 cmcopper substrate. This appears to be a very elegantway to make high-performance devices. However, it re-quires rather sophisticated device process, with ratherhigh production cost. It is natural to consider whethercheaper method such as chemical synthesis can be uti-lized to produce graphene for transparent electronicapplications.In this paper, we attempted to dope the chemically</p><p>synthesized graphene into ZnO matrix which was alsoprepared by chemical methods. With the optimizeddoping concentration, we found that the graphene</p></li><li><p>J. Shanghai Jiaotong Univ. (Sci.), 2014, 19(3): 378-384 379</p><p>doped ZnO can exhibit low resistivity and high opticaltransparency up to ultraviolet region. Our work sug-gests that graphene doped ZnO prepared by low-costchemical method may find possible applications in op-toelectronics field.However, application of materials depends greatly on</p><p>their intrinsic properties. Hybridization of different ma-terials offers a powerful way to enhance the applicationof graphene by enabling versatile and tailor-made prop-erties with high performance far beyond those of the in-dividual materials. Recently, graphene decorated withmetal nanoparticles or metal oxides has also attracteda great deal of attention due to potential applicationsin many technological fields[4-6]. At the same time, asan important wide-bandgap semiconductor, zinc oxide(ZnO) has shown excellent performances in electronics,optics and photonics systems[7]. Compared with indi-vidual counterparts, the graphene doped ZnO compos-ites might possess unusual properties and present somespecial features, such as high electrical property, goodoptical transmittance, improved field emission and ca-pacitive properties[8]. So we try to prepare graphenedoped ZnO composites which can improve conductivityand transmittance of TCO glass, and compare graphenedoped ZnO with AZO, a kind of fast-growing TCOglass.Some methods could be used to prepare graphene</p><p>doped ZnO. Lu et al.[9] successfully synthesizedgraphene doped ZnO electrode materials by ultrasonicspray pyrolysis. The graphene doped ZnO electrodematerials exhibited a specific capacitance of 61.7F/gand maximum power density of 4.8W/g. Tung et al.[4]</p><p>reported the production of graphene doped ZnO hybridarchitectures. The graphene doped ZnO hybrid archi-tectures composed of regular arrays of ZnO nanorods(formed on graphene films) exhibited a high electricalproperty.Solution method employed in this paper is a con-</p><p>venient and low cost approach for the synthesis ofZnO nanocrystals[10] and the development of var-ious graphene-based composites. Firstly, graphenenanosheets were synthesized via using hydrazine as areducing agent[11]. Then, ZnO was directly grown ontoconducting graphene nanosheets to obtain graphenedoped ZnO composites. The small ZnO particles ho-mogeneously anchor onto graphene sheets, and performas spacers to keep neighboring sheets separate.</p><p>1 Experiment</p><p>1.1 Synthesis of Graphite Oxide Nanocompos-ites</p><p>Graphite oxide (GO) was synthesized from natu-ral flake graphite powder by a modified Hummersmethod[12-14]. In a typical synthesis, 2.0 g of graphitepowder (44 m) was put into cold concentrated H2SO4</p><p>(50mL, 0C, 98.3% of mass fraction). Then, 8.0 g ofKMnO4 was added gradually under stirring and thetemperature of the mixture was kept to be 8C by cool-ing. Successively, the mixture was stirred at 35C for1 h and then diluted with 350mL of deionized (DI) wa-ter (stirred for 1 h). After that, 20mL of H2O2 (30% ofmass fraction) was added to the mixture to reduce theresidual KMnO4. The mixture released a large amountof bubbles and the colour of the mixture changed intobrilliant yellow. The mixture was filtered and washedwith HCl aqueous solution (1L, 5% of mass fraction) toremove metal ions, and finally washed with 1.0 L of DIwater to remove the acid. The resulting solid was driedat 60C for 24 h.1.2 Synthesis of Graphene-ZnO (Graphene</p><p>Doped ZnO) NanocompositesFirstly, 50mg of graphite oxide was dispersed in</p><p>30mL ethylene glycol with ultrasonication for 30min.Meanwhile, 1 g of zinc acetylacetonate was dissolved in30mL ethylene glycol under stirring. The above twosolutions were mixed under stirring. After that, properNaOH was added into mixed suspension until pH = 9and stirred for 30min. Then, 30L hydrazine hydratewas added to reduce GO. Subsequently, the mixturewas put into an autoclave and heated at 160C for 16 h.The as-synthesized product was isolated by centrifuga-tion, washed three times with water, and finally driedin a vacuum oven at 60C for 24 h.1.3 Synthesis of Graphene Doped ZnO FilmIn order to synthesize graphene doped ZnO film, we</p><p>dispersed 50mg of graphene doped ZnO nanocompos-ite into 30mL ethylene glycol with ultrasonication for30min. Then graphene doped ZnO was spun on the sil-icon chip with Lacquering machine (Smart Coater 100,USA). The rotational speed is 30 r/min and the rota-tional time is 15 min.1.4 Characterization MethodsThe crystallographic structures of the materials</p><p>were characterized by X-ray diffraction (XRD) systemequipped with CuK radiation. Raman measurementswere performed by a micro-Raman spectrometer with aresolution of 1.2 cm1. The microstructure of the sam-ples was investigated by scanning electron microscope(SEM). Fourier transform infrared spectroscopy (FT-IR) analysis was carried out on a spectrophotometerwith a resolution of 0.5 cm1. The ultraviolet-visible(UV-vis) spectroscopy measurement was performed ona Lam750(S) four-probe conductor and the semiconduc-tor resistivity was performed in 106106 region.</p><p>2 Results and Discussion</p><p>Figure 1 illustrates the fabrication process and for-mation mechanism for graphene doped ZnO compos-ites. As shown by previous studies, GO sheets havetheir basal planes covered mostly with epoxy and</p></li><li><p>380 J. Shanghai Jiaotong Univ. (Sci.), 2014, 19(3): 378-384</p><p>Zn(CH3COO)22H2OStirring 30min</p><p>NaOHStirring 30 min</p><p>ZnO2</p><p>ZnO2</p><p>ZnO2</p><p>ZnO2</p><p>ZnOZnO</p><p>2</p><p>ZnO2</p><p>OH</p><p>HO</p><p>Hydrazine hydrate</p><p>Step (1)</p><p>OOHOH</p><p>OH</p><p>OH</p><p>OHOH</p><p>OH</p><p>OH</p><p>OHHO</p><p>HO</p><p>HO</p><p>HO</p><p>HO</p><p>HO</p><p>HO</p><p>HOOH</p><p>OO</p><p>OO</p><p>O</p><p>O</p><p>O O O</p><p>OO</p><p>O</p><p>O</p><p>O</p><p>O</p><p>O</p><p>O</p><p>O</p><p>O</p><p>O</p><p>HO</p><p>OHOHO</p><p>O CH</p><p>Zn2+Zn2+</p><p>Zn2+</p><p>Zn2+Zn2+</p><p>Zn2+</p><p>Zn2+</p><p>HOHOHO</p><p>O</p><p>OOH</p><p>OH</p><p>O</p><p>HO</p><p>O</p><p>O</p><p>O</p><p>O</p><p>ZnO</p><p>OH</p><p>HO O</p><p>Step (2)</p><p>Fig. 1 Schematic of the fabrication process and formation mechanism for graphene doped ZnO composite</p><p>hydroxyl groups, while carbonyl and carboxyl groupsare located at the edges. These functional groups, act-ing as anchor sites, enable the subsequent in situ for-mation of nanostructures to attach to the surfaces andedges of GO sheets. However, these oxygen-containingfunctional groups impair the conductivity of GO sheetsto such an extent that the GO sheets are not suitable forelectrode materials. Graphene has an excellent conduc-tivity, with ideal single-atom thick substrate for growthof functional nanomaterials to render GO electrochem-ically active and electrically conductive. Next, Step(1) in Fig. 1 shows the preparation of graphene basedon hydrazine hydrate reducing GO, in which hydrazinehydrate as a reductant will remove most of the oxygen-containing groups and recover the electrical conductiv-ity. At Step (1), with the addition of Zn(CH3COO)2solution to GO dispersion solution, Zn2+ ions will bindwith the O atoms of the negatively charged residualoxygen-containing functional groups on GO via an elec-trostatic force. At Step (2), with the addition of a so-lution of NaOH, a large number of nuclei are formed in</p><p>a short time from the reactions:</p><p>Zn2+ + 4OH Zn(OH)24 ,Zn(OH)24 ZnO+ H2O+ 2OH2.</p><p>The Zn atoms of ZnO octahedron may form bondswith the O atoms of functional groups via a covalentcoordination bond, acting as anchor sites for the crys-tals to grow. Finally, ZnO particles grow larger alongthe planes and the edges of graphene sheets to formgraphene doped ZnO composites.XRD measurements were employed to investigate the</p><p>phase and structure of the synthesized samples. Asshown in Fig. 2, XRD pattern of the as-synthesizedgraphite oxide (intensity curve II of Fig. 2) shows asharp peak at diffraction angle 2 = 10.6 correspond-ing to the (001) reflection of GO. From intensity curveI of Fig. 2, there is a wide peak at 2 = 26.5 assignedto reflection of graphene, and the peak of GO totallydisappears, which indicates GO has been reduced tographene. From intensity curve III of Fig. 2, all the</p></li><li><p>J. Shanghai Jiaotong Univ. (Sci.), 2014, 19(3): 378-384 381</p><p>diffraction peaks of grapheme doped ZnO nanocom-posite can be indexed to hexagonal ZnO. And thereare nine main peaks at 2 = 31.7, 34.4, 36.2, 47.5,56.6, 62.8, 66.3, 67.9 and 69.1 which correspond tothe (100), (002), (101), (102), (110), (103), (200), (112)and (201) crystalline planes of ZnO, respectively. Thecharacteristic peak of GO or graphite (the characteris-tic peak at around 2 = 26.5) was not observed. Therestacking of the as-reduced graphene sheets was effec-tively prevented[15-16]. Recent studies have shown that,if the regular stacks of GO or graphite are destroyed, forexample, by exfoliation, their diffraction peaks becomeweak or even disappear.</p><p>0 20 40 60</p><p>IGrophene, IIGO, IIIGraphene doped ZnO</p><p>80</p><p>200</p><p>400</p><p>600</p><p>800</p><p>1000</p><p>IIIII</p><p>Inte</p><p>nsity </p><p>(a.u</p><p>.)</p><p>2/()</p><p>I</p><p>(100)(002)</p><p>(101)</p><p>(102)</p><p>(110)(103)</p><p>(200)</p><p>(112)(201)</p><p>(001)</p><p>Fig. 2 Typical XRD patterns of graphene, GO andgraphene doped ZnO</p><p>Raman spectrum is one of the best ways to char-acterize graphite and graphene materials, as shown inFig. 3. The Raman spectrum of GO displays two peaksat 1 350 cm1 (peak D) and at 1 590 cm1 (peak G)which are usually assigned to the breathing mode of-point phonons of A1g symmetry and E2g phonon ofC sp2 atoms, respectively. The Raman spectrum ofgraphene doped ZnO also contains peak D and peakG, but the ratio of peak D intensity to peak G inten-sity , denoted as I(D)/I(G), increases, indicating theexistence of reduction procedure of GO.Given by SEM, the direct evidence of the formation</p><p>of GO and graphene doped ZnO is shown in Fig. 4.As shown in Fig. 4(a), the corrugated and scrolled GOsheets resemble crumpled silk veil. Maybe, it is ow-ing to spinning. As shown in Fig. 4(b), it is obviouslyobserved that widespread ZnO nanoparticles closelyanchor onto the edges and planes of curled graphenesheets due to the remaining oxygen-containing groups.Because of the content of ZnO is much more thangraphene, the layered graphene is almost encircledby ZnO which displays a good combination betweengraphene sheet and ZnO naoparticles. When moreZnO particles deposite onto the edges and planes of</p><p>0</p><p>2000</p><p>4000</p><p>6000</p><p>Inte</p><p>nsity </p><p>(a.u</p><p>.)</p><p>0 500 1000 1500</p><p>I</p><p>D</p><p>G</p><p>IGO, IIGraphene doped ZnO</p><p>II</p><p>2000 2500 3000 3500</p><p>Raman shift/cm1</p><p>Fig. 3 Raman spectra</p><p>(a) GO (b) Graphene doped ZnO</p><p>5m 1m</p><p>Fig. 4 SEM images</p><p>graphene, the graphene sheets work as spacers to pre-vent the restacking of graphene sheets.Further evidence from FT-IR spectra is shown in</p><p>Fig. 5 to describe the formation of graphene in thegraphene doped ZnO nanocomposites. The represen-tative FT-IR peaks of GO (Fig. 5) at about 3 391 cm1</p><p>of high frequency region correspond to the stretchingvibrations of OH. The wide peak is within the rangeof 2 0003700 cm1, owing to the hydrone absorbedby GO. It shows that GO has a strong accessibility. Inmiddle frequency region, the absorb peak at 1 615 cm1</p><p>corresponds to the remaining sp2 character. The absorbpeaks at 1 728 an...</p></li></ul>


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