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Applied Surface Science 379 (2016) 206–212

Contents lists available at ScienceDirect

Applied Surface Science

journa l h om epa ge: www.elsev ier .com/ locate /apsusc

ensely-packed graphene/conducting polymer nanoparticle papersor high-volumetric-performance flexible all-solid-stateupercapacitors

hao Yanga, Liling Zhanga, Nantao Hua,∗, Zhi Yanga, Hao Weia, Zhichuan J. Xub,∗,anyan Wangc, Yafei Zhanga,∗

Key Laboratory for Thin Film and Microfabrication Technology of the Ministry of Education, School of Electronics, Information and Electrical Engineering,hanghai Jiao Tong University, Dong Chuan Road No. 800, Shanghai 200240, PR ChinaSchool of Materials Science & Engineering, Nanyang Technological University, 50 Nanyang Avenue Block, 639798 Singapore, SingaporeCollege of Physics, Optoelectronics and Energy, Soochow University, Suzhou 215006, PR China

r t i c l e i n f o

rticle history:eceived 21 January 2016eceived in revised form 4 March 2016ccepted 8 April 2016vailable online 11 April 2016

eywords:olyaniline

a b s t r a c t

Graphene-based all-solid-state supercapacitors (ASSSCs) are one of the most ideal candidates for high-performance flexible power sources. The achievement of high volumetric energy density is highly desiredfor practical application of this type of ASSSCs. Here, we present a facile method to boost volumetricperformances of graphene-based flexible ASSSCs through incorporation of ultrafine polyaniline-poly(4-styrenesulfonate) (PANI-PSS) nanoparticles in reduced graphene oxide (rGO) papers. A compact structureis obtained via intimate contact and �–� interaction between PANI-PSS nanoparticles and rGO sheets.The hybrid paper electrode with the film thickness of 13.5 �m, shows an extremely high volumetric spe-

3 3

rapheneolumetric capacitancelexibleupercapacitor

cific capacitance of 272 F/cm (0.37 A/cm in a three-electrode cell). The assembled ASSSCs show a largevolumetric specific capacitance of 217 F/cm3 (0.37 A/cm3 in a two-electrode cell), high volumetric energyand power density, excellent capacitance stability, small leakage current as well as low self-dischargecharacteristics, revealing the usefulness of this robust hybrid paper for high-performance flexible energystorage devices.

© 2016 Elsevier B.V. All rights reserved.

. Introduction

With the increased demand for portable and wearable energytorage devices, flexible all-solid-state supercapacitors (ASSSCs)ave been attracting much attention because of their excel-

ent electrochemical stability [1] and rate capability [2] with themployment of solid-state electrolyte. Compared with commer-ial supercapacitors using aqueous electrolyte, ASSSCs based onel electrolyte can take the advantages of avoiding possible elec-rolyte leakage and reducing cost of packaging materials [3]. Recentesearch has mainly focused on improving the volumetric perfor-ances of ASSSCs, because volumetric capacitance is an important

erformance metric for practical application [4]. However, theolumetric capacitance of carbon-based electrodes is limited toe ∼60 F/cm3 for commercial devices [5]. An efficient strategy to

∗ Corresponding author.E-mail addresses: hunantao@sjtu.edu.cn (N. Hu), xuzc@ntu.edu.sg (Z.J. Xu),

fzhang@sjtu.edu.cn (Y. Zhang).

ttp://dx.doi.org/10.1016/j.apsusc.2016.04.058169-4332/© 2016 Elsevier B.V. All rights reserved.

improve volumetric capacitance is to develop porous conductiveelectrode materials with high packing density to maximize the uti-lization of the limited volume [6]. With high ion-accessible surfacearea and low ion transport resistance, the chemically convertedgraphene (CCG) hydrogel films can be compressed irreversibly bycapillary pressure, resulting in a packing density of 1.33 g/cm3

and a volumetric capacitance of 255.5 F/cm3, better than thoseof activated carbons (0.5 g/cm3 and 50–80 F/cm3) [7]. The spon-taneous intercalation of cations into layered TiC MXene can offerthe volumetric capacitance in excess of 300 F/cm3 [8]. However, thefabrication of this electrode material required the handling of con-centrated HF and a laborious multi-step procedure [9]. Therefore,it is really a challenge to develop a safe and facile route to fabricateASSSC electrodes with high volumetric performances.

With excellent conductivity and high mechanical flexibil-ity, graphene-based papers are widely regarded as a promising

electrode material for flexible ASSSCs, but usually exhibit poorvolumetric performances due to the parallel restacking ofgraphene sheets, which greatly increases the packing densitybut reduces accessible surface area and ion diffusion rate of

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C. Yang et al. / Applied Surf

olid-state electrolytes [10]. Pseudocapacitive polyaniline-poly(4-tyrenesulfonate) (PANI-PSS) nanoparticles have been previouslysed for inkjet-printed sensors [11] and supercapacitors [12].hese ultrafine PANI–PSS nanoparticles show high dispersiontability and narrow particle-size distribution, serving as the inter-ayer spacer to partially prevent the layer-to-layer aggregationf graphene sheets. However, none of work has been reportedo focus on enhancing the volumetric capacitance of graphene-ased ASSSCs via incorporation of PANI-PSS nanoparticles. Herein,e prepare an ink composed of the water-dispersible PANI-PSSanoparticles and the rGO sheets, and subsequently use this inko form densly-packed hybrid papers through the vacuum fil-ration method. With compact and hierarchical structures, thes-fabricated hybrid papers exhibit �–� interaction and chargeransfer between PANI and rGO. With good electrical conductiv-ty, excellent mechanical flexibility and high packing density, theGO/PANI-PSS paper electrodes possess an extremely high volu-etric specific capacitance. In addition, flexible ASSSC devices are

ssembled in order to further evaluate the electrochemical perfor-ances of the hybrid paper electrodes for practical applications.

. Experimental

.1. Materials

Graphite powder (500 meshes) and sodium 4-styrenesulfonateere purchased from Shangdong Jinrilai Co., Ltd. and Sinopharmhemical Reagent Co., Ltd., respectively. Sodium dodecyl benzeneulfonate (SDBS), hydrazine hydrate aqueous solution (85%), Tri-on X-100, a,a′-azobisisobutyronitrile (AIBN), aniline, ammoniumersulfate (APS) and polyvinyl alcohol (PVA) were purchased fromhanghai Chemical Reagents Co., Ltd., and used without furtherurification.

.2. Fabicaton of rGO/PANI-PSS papers

GO was prepared by a modified Hummers’ method [13], whichad been mentioned in our previous report [14]. The reduction ofO was performed as follows: 600 mg GO and 1.5 g SDBS were dis-ersed in 600 mL deionized water. 3 mL hydrazine hydrate aqueousolution (85%) was added and kept stirring and refluxing at 100 ◦Cor 14 h. After cooling down, the product was filtered and washedith deionized water. Finally, the as-prepared rGO was dispersed in

vol.% Triton X-100 solution for further use. The water-dispersibleANI-PSS nanoparticles were chemically polymerized [11] as fol-ows: 1 g sodium 4-styrenesulfonate was added into deionized

ater (40 mL) and stirred for 2 h. Then AIBN (0.03 g) was used as annitiator. The polymerization was conducted for 5 h at 60 ◦C, result-ng in the aqueous PSS solution. Aniline monomer (0.53 mmol) wasissolved in 35 mL 0.5 M HCl solution. The PSS solution (0.6 g) wasdded and stirred for 1 h. Then a solution of APS (0.69 mmol) pre-issolved in 5 mL 0.5 M HCl solution was added to the mixturend stirred for 12 h at room temperature. The final product wasashed with deionized water for three times. Then the obtained

ANI-PSS dispersion was mixed with the rGO suspension. After son-cation, the rGO/PANI-PSS suspension was vacuum filtered throughitrocellulose membrane. By adjusting the amounts of the rGOuspension and the PANI-PSS dispersion, the final rGO/PANI-PSSapers were obtained after drying at room temperature.

.3. Assembly of flexible ASSSCs

The PVA-H2SO4 gel electrolyte was prepared as follows: 1 g2SO4 and 1 g PVA were added into 10 mL deionized water, and

hen heated to 85 ◦C under stirring until the mixture became clear.he as-prepared rGO/PANI-PSS paper was cut into rectangular

ience 379 (2016) 206–212 207

strips and pressed on carbon paper with poly(ethylene terephtha-late) (PET) substrate to form ASSSC electrodes. Then PVA-H2SO4gel electrolyte was slowly poured on two ASSSC electrodes andair-dried at room temperature to evaporate excess water. The twoelectrodes were pressed together to form an integrated ASSSCdevice.

2.4. Characterization

The morphologies of the samples were observed by field-emission scanning electron microscope (FE-SEM, Carl Zeiss Ultra55). Fourier Transform Infrared Spectroscopy (FTIR) was recordedon a Bruker (Germany) VERTEX 70 spectrometer (KBr pellets).Raman scattering was performed on a Renishaw inVia ReflexRaman spectrometer using a 532 nm laser source. X-ray photo-electron spectrometry (XPS) was carried out on a Kratos AxisUltra DLD using monochromated Al K� X-ray beams as the exci-tation source (1486.6 eV). Conductivity was recorded using RTS-8four-point probes resistivity measurement system. Thermal Grav-ity Analysis (TGA, PerkinElmer Pyris 1) was carried out by heatingthe samples from 30 ◦C to 800 ◦C at a heating rate of 5 ◦C/min inN2. The tensile strength and Young’s modulus of the samples wereevaluated by DMA (TA-Q800, TA Instruments-Waters LLC).

2.5. Electrochemical measurements

The electrochemical experiments were carried out on an elec-trochemical workstation (CHI 760E). Cyclic voltammetry (CV) andgalvanostatic charge/discharge measurements were conductedfrom −0.2 V to 0.8 V in 1 M H2SO4. The electrochemical impedancespectroscopy (EIS) measurements were performed over a fre-quency range from 105 to 10−2 Hz. For three-electrode test, theareal specific capacitance (Ca) and volumetric specific capacitance(Cv) were calculated as follows: Ca = (I�t)/(S�V) and Cv = Ca/d,where I is the discharge current, �t is the discharge time, S is thearea of one electrode, �V represents the voltage window and d isthe film thickness. For the ASSSCs, the gravimetric specific capaci-tance (Csm), areal specific capacitance (Csa) and volumetric specificcapacitance (Csv) of single electrode were calculated accordingto the following equations [15]: Csm = 2(I�t)/(m�V), Csa = Csmm/Sand Csv = Csa/d, where m is the mass of one electrode. The arealenergy density (Ea) and areal power density (Pa) were calculatedby using the equations [16]: Pa = (I�V)/S and Ea = Pa�t. The volu-metric energy density (Ev) and volumetric power density (Pv) werecalculated by using the equations: Pv = Pa/d and Ev = Ea/d.

3. Results and discussion

From SEM image, the uniform PANI-PSS nanoparticles wereobserved to display a narrow size distribution of mainly 20–40 nmin diameter (Fig. S1a in the Supporting information). An aqueousink could be easily obtained by mixing the nanoparticles with rGOnanosheets followed by sonication treatments. The resultant highlydispersible inks were used to prepare flexible papers through thevacuum filtration method. With the hybrid papers as flexible elec-trodes, symmetric ASSSC devices were assembled as shown in Fig. 1.PVA-H2SO4 gel and carbon papers were used as solid-state elec-trolyte and current collector. The photographs in Fig. 1 reflectedthe water-dispersible polymer solutions could keep a good disper-sion for over a week and the thin device was highly flexible underbending state. These well dispersed nanoparticles were able to keepstable because of a strong electrostatic interaction between PANI

and PSS [11]. In top-view SEM images, the PANI-PSS nanoparticlescould be found on the surface of hybrid papers (Fig. 2a), differentfrom the morphology of pure rGO paper (Fig. S1b in the Supportinginformation). The film thicknesses of rGO/PANI-PSS papers with

208 C. Yang et al. / Applied Surface Science 379 (2016) 206–212

Fig. 1. A schematic diagram of fabrication of the ASSSC (insets: photographs of (A) rGO, (B) PANI-PSS and (C) rGO/PANI-PSS dispersions kept still for a week and photographof a device under bending state).

F f rGOa

PoaSsra

wasrot[tdbo

ig. 2. (a) Top-view, (b) low- and (c) high-magnification cross-section SEM images ond rGO/PANI-PSS-40%-1 paper.

ANI-PSS content of 40% (denoted as rGO/PANI-PSS-40%) werebtained from the cross-section SEM images (Figs. 2 b and Fig. S1cnd d in the Supporting information) and listed in Table S1 in theupporting information. In Fig. 2c, the rGO/PANI-PSS-40%-1 paperhowed the same compact and hierarchical structures as otherGO/PANI-PSS-40% papers with different film thicknesses (Fig. S1end f in the Supporting information).

FTIR spectra of rGO, PANI-PSS and rGO/PANI-PSS-40%-1 paperere shown in Fig. 2d. The rGO paper showed the peak at 1083 cm−1

scribing to the C O stretching band. The PANI-PSS nanoparticleshowed typical bands of PANI, C C stretchings of the quinonoiding and benzenoid ring at 1574 and 1493 cm−1, C N stretchingf secondary aromatic amines at 1306 cm−1, and C H bendings ofhe benzenoid ring and the quinonoid ring at 1240 and 1144 cm−1

17]. However, there were some differences in the spectrum ofhe composite paper. Compared with that of the PANI-PSS pow-

er, the C C stretching intensity ratio of the quinonoid ring andenzenoid ring was increased, indicating that the quinonoid ringf PANI was promoted and stabilized by the composite structure

/PANI-PSS-40%-1 paper. (d) FTIR spectra of pure rGO paper, PANI-PSS nanoparticles

[17,18]. The Raman spectrum of the PANI-PSS nanoparticles inFig. 3a showed the characteristic peaks of PANI, C H bending inthe quinonoid ring and benzenoid ring at 807 and 1175 cm−1, C Nstretching mode of the polaronic units at 1255 cm−1, the vibrationof the semiquinone radical at 1343 cm−1, C C stretching and C Cstretching in the quinonoid ring at 1568 and 1600 cm−1 [17]. In thespectrum of the composite paper, some peaks were overlapped bythe D band at 1340 cm−1 and G band at 1580 cm−1 of rGO. How-ever, the C H bending in the benzenoid ring at 1175 cm−1 indicatedthe existence of the PANI-PSS nanoparticles. The atomic composi-tion of rGO, PANI-PSS and the hybrid paper was analyzed by XPS(Fig. S2 in the Supporting information). The PANI-PSS nanoparti-cles had a higher N content (6.15%) than that of the hybrid paper(2.10%). The curve fitting of C 1s spectra were shown in Fig. 3b–d.C C bonding at 285.0 eV was dominating in the spectra of rGO andPANI-PSS. The characteristic peak of C N backbone bonding with

the binding energy of 285.8 eV demonstrated the existence of thePANI-PSS nanoparticles in the composite paper. Conductivity wasobtained by a four-point probe method and shown in Table S1 in the

C. Yang et al. / Applied Surface Science 379 (2016) 206–212 209

F I-PSS-a

SptcSPf4Ypm

cfirucgetap1tucamvtrp∼q

ig. 3. (a) Raman spectra of pure rGO paper, PANI-PSS nanoparticles and rGO/PANnd (d) rGO/PANI-PSS-40%-1 paper.

upporting information. The conductivities of the rGO/PANI-PSSapers with different film thicknesses were 356–366 S/m, higherhan that of pure rGO paper (47.5 S/m), attributed to the excellentonductivity of PSS-doped PANI. TGA curves were shown in Fig.3 in the Supporting information. The char yield of the rGO/PANI-SS-40%-1 paper was ∼30.6%, close to the result (36.8%) calculatedrom the char yields of rGO and PANI-PSS. The rGO/PANI-PSS-0%-3 paper showed a maximum tensile strength of 39.7 MPa andoung’s modulus of 7.40 ± 1.35 GPa by DMA in Fig. S4 in the Sup-orting information, indicating these composite papers had enoughechanical strength as flexible ASSSC electrodes.A three-electrode test was conducted to evaluate the electro-

hemical performances of the rGO/PANI-PSS papers with differentlm thicknesses. In Fig. 4a, the CV curves were deviated fromectangular shape with the increase of the scan rate. Areal and vol-metric specific capacitances were calculated from galvanostaticharge/discharge tests (Fig. S5 in the Supporting information). Thealvanostatic charge/discharge curves of the rGO/PANI-PSS paperlectrodes showed a significant characteristic of pseudocapaci-ance, greatly different from those of pure rGO paper electrode with

symmetric triangular shape. In Fig. 4b, the rGO/PANI-PSS-40%-3aper electrode had a maximum areal specific capacitance, from350 mF/cm2 at 0.5 mA/cm2 to 282 mF/cm2 at 10 mA/cm2. In Fig. 4c,he rGO/PANI-PSS-40%-1 paper electrode showed a maximum vol-metric specific capacitance with an excellent rate capability, 58.0%apacitance retention from 272 F/cm3 at 0.37 A/cm3 to 158 F/cm3

t 7.4 A/cm3, higher than that of the reported liquid electrolyte-ediated CCG film (255.5 F/cm3) which was recorded as the best

olumetric capacitance of carbon-based electrodes [7], attributedo the high packing density of about 1.35 g/cm3 close to that of pure

3

GO paper (1.39 g/cm ). In Fig. 4d, the Nyquist plots showed all theaper electrodes had a small equivalent series resistance (ESR) of1.2 � and a low charge transfer resistance (Rct). In the low fre-uency region, the impedance curve of the rGO/PANI-PSS-40%-1

40%-1 paper. Fitted C 1s spectra of (b) pure rGO paper, (c) PANI-PSS nanoparticles

paper electrode was more parallel to the imaginary axis than thoseof the rGO/PANI-PSS-40%-2 paper electrode and the rGO/PANI-PSS-40%-3 paper electrode, indicating the film thickness could affact theinfiltration of electrolyte ion.

With the highest volumetric specific capacitance and thebest rate capability among the rGO/PANI-PSS-40% papers, therGO/PANI-PSS-40%-1 paper electrode was used to assemblesymmetric ASSSC devices in order to further investigate the electro-chemical performances of the hybrid paper electrodes for practicalapplications. The CV curves at low scan rates showed nearly rect-angular shape (Fig. 5a). Galvanostatic charge/discharge curveshad nearly symmetric triangular shape as shown in Fig. 5b. Asa result, the ASSSC had a high areal specific capacitance of 293mF/cm2 at 0.5 mA/cm2 and a large volumetric specific capaci-tance of 217 F/cm3 at 0.37 A/cm3 (Fig. 5c), higher than those of thereported SWNT/PANI (76.7 F/cm3) [19], MWNT/MnOx (50 F/cm3)[20], rGO/PVP (62.2 F/cm3) [21], MnOOH/nitrogen-doped graphene(26.3 F/cm3) [22], CNT/PPy (4.9 F/cm3) [23] and MnO2//Fe2O3(1.2 F/cm3) [24]. Furthermore, Ragone plots of the rGO/PANI-PSSASSSC were calculated from areal and volumetric specific capac-itances, and compared with other reports as shown in Table S2in the Supporting information. In Fig. 5d, the rGO/PANI-PSS-40%-1 ASSSC showed a high areal energy density, from 40.7 �Wh/cm2

at 0.5 mW/cm2 to 12.5 �Wh/cm2 at 4 mW/cm2, higher thanthose of the reported ASSSCs based on graphene/cellulose paper(15 �Wh/cm2 at 0.04 mW/cm2) [25], graphene/cellulose nanofiber(20 �Wh/cm2 at 15.5 mW/cm2) [26] and graphene/manganese(II)phosphate nanosheets (0.17 �Wh/cm2 at 0.046 mW/cm2) [27]. InFig. 5e, the ASSSC had a large volumetric energy density, from30.1 mWh/cm3 at 0.37 W/cm3 to 9.28 mWh/cm3 at 3.0 W/cm3, bet-

ter than those of the reported ASSSCs based on MnOOH/nitrogen-doped graphene (2.34 mWh/cm3 at 0.04 W/cm3) [22], CNT/PPy(0.26 mWh/cm3 at 0.15 W/cm3) [23] and MnO2//Fe2O3 (from0.41 mWh/cm3 at 0.06 W/cm3 to 0.35 mWh/cm3 at 0.1 W/cm3)

210 C. Yang et al. / Applied Surface Science 379 (2016) 206–212

Fig. 4. (a) CV curves of rGO/PANI-PSS-40%-1 paper electrode at different scan rates. (b) Areal specific capacitance, (c) volumetric specific capacitance and (d) Nyquist plotsof pure rGO paper electrode and rGO/PANI-PSS paper electrodes in 1 M H2SO4 electrolyte. The areal and volumetric specific capacitances were obtained from galvanostaticcharge/discharge curves at different current densities.

F s at dd ic Rag

[t(EIpissi

ig. 5. Electrochemical performances of rGO/PANI-PSS-40%-1 ASSSC. (a) CV curveensities. (c) Areal and volumetric specific capacitances. (d) Areal and (e) volumetr

24]. EIS measurement was performed in order to investigatehe interior resistance of the as-fabricated flexible ASSSC deviceFig. 5f). In the high frequency region, the ASSSC showed a smallSR of 0.84 �, indicating a small solid-state electrolyte resistance.n the low frequency region, the impedance curve tended to bearallel to the imaginary axis, indicating an ideal capacitive behav-

or. Furthermore, the flexible ASSSC exhibited excellent capacitance

tability under various bending states (Fig. 6a). The CV curveshowed an invariable shape at 100 mV/s, indicating the same capac-tive performance. Cycling stability was examined by galvanostatic

ifferent scan rates. (b) Galvanostatic charge/discharge curves at different currentone plots. (f) Nyquist plots (inset: a magnification of high frequency region).

charge/discharge tests at 10 mA/cm2 for 10,000 cycles, as shown inFig. 6b. Compared with the three-electrode test of flexible paperelectrode in aqueous electrolyte, the ASSSC had a much highercapacitance retention rate (80.0% vs. 52.9%) and reflected a smallerfluctuation after 10 000 cycles.

For practical application of portable and flexible energy storagedevices, it is important to evaluate the leakage current and the self-

discharge characteristic (Fig. S6 in the Supporting information). TheASSSC had a small leakage current of ∼33 �A with high stabilityafter 2 h. After being charged at 1.0 V for 15 min, the open-circuit

C. Yang et al. / Applied Surface Science 379 (2016) 206–212 211

Fig. 6. (a) CV curves of rGO/PANI-PSS-40%-1 ASSSC under different bending angles at 100 mV/s. (b) Cycling stability of rGO/PANI-PSS-40%-1 ASSSC compared with that ofs s at 10t et: a pr f this

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A

d

ingle paper electrode in aqueous electrolyte by galvanostatic charge/discharge testhree rGO/PANI-PSS-40%-1 ASSSCs connected in series at 100 mV/s and 0.5 mA (inseferences to colour in this figure legend, the reader is referred to the web version o

oltage of the device exhibited the retention of ∼0.21 V after 24 h,ndicating a low self-discharge characteristic. Moreover, severalSSSCs with the same mass of rGO/PANI-PSS-40%-1 papers wereonnected in series to evaluate the performance of the solid-stateevices. Fig. 6c showed the potential window could be extended to.0 V for three ASSSCs connected in series. With each ASSSC unit athe same current density, the charge/discharge time of the deviceased on two or three ASSSCs was nearly the same as that of the

ndividual ASSSC, indicating the capacitive performance of eachSSSC unit was well maintained in series (Fig. 6d). Furthermore, the

andem device based on three ASSSCs could power a blue LED, sug-esting its realistic application potential in flexible energy storageelds.

. Conclusions

In summary, rGO/PANI-PSS papers, with compact and hierar-hical structures, have been successfully fabricated by vacuumltration of the hybrid inks. The ultrafine PANI-PSS nanoparti-les can be well dispersed and incorporated in rGO papers. Theesultant hybrid papers show excellent conductivity, high packingensity and good mechanical robustness. Also, the electrode basedn this type of hybrid papers can deliver high-volumetric capacitiveerformances. The further-assembled ASSSC based on the hybridapers exhibits a large volumetric specific capacitance, outstand-

ng capacitance stabilities, as well as high volumetric energy andower densities. Hence, the as-fabricated hybrid paper electrodesnd the assembled flexible ASSSCs hold a great potential for high-erformance energy storages.

cknowledgments

The financial supports from the National Natural Science Foun-ation of China (no. 51102164, no. 61376003, and no.51302179),

mA/cm2 for 10,000 cycles. (c) CV and (d) galvanostatic charge/discharge curves ofhotograph of a blue LED powered by the tandem device). (For interpretation of the

article.)

the Natural Science Foundation of the Jiangsu Higher EducationInstitutions of China (no. 13KJB430018), Medical-Engineering (Sci-ence) cross-Research Fund of Shanghai Jiao Tong University (no.YG 2014QN01 and no. YG2012MS37), and the Foundation for SMCExcellent Young Teacher in Shanghai Jiao Tong University are grate-fully acknowledged. We also acknowledge analysis supports fromthe Instrumental Analysis Center of Shanghai Jiao Tong Universityand the Center for Advanced Electronic Materials and Devices ofShanghai Jiao Tong University.

Appendix A. Supplementary data

Supplementary data associated with this article can be found, inthe online version, at http://dx.doi.org/10.1016/j.apsusc.2016.04.058.

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