Surfactant-treated graphene oxide in organic solvents and its application in photovoltaic cells

Accepted Manuscript Surfactant-treated graphene oxide in organic solvents and its application in photovoltaic cells Yishan Wang, Shengyi Yang, Haowei Wang, Li Zhang, Haijuan Cheng, Bo He, Weile Li, Bingsuo Zou PII:

S1567-1739(16)30370-4

DOI:

10.1016/j.cap.2016.12.017

Reference:

CAP 4403

To appear in:

Current Applied Physics

Received Date: 15 September 2016 Revised Date:

20 November 2016

Accepted Date: 19 December 2016

Please cite this article as: Y. Wang, S. Yang, H. Wang, L. Zhang, H. Cheng, B. He, W. Li, B. Zou, Surfactant-treated graphene oxide in organic solvents and its application in photovoltaic cells, Current Applied Physics (2017), doi: 10.1016/j.cap.2016.12.017. This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.

ACCEPTED MANUSCRIPT

Surfactant-treated graphene oxide in organic solvents and its application in photovoltaic cells †

Yishan Wang1, 2, Shengyi Yang1, 3 , Haowei Wang4, Li Zhang4, Haijuan Cheng1, 2, Bo He1, Weile Li1, Bingsuo Zou1

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(1 Beijing Key Lab of Nanophotonics and Ultrafine Optoelectronic Systems, School of Physics, Beijing Institute of Technology, Beijing 100081, P.R. China) (2 Kunming Institute of Physics, Kunming 650000, P. R. China) (3 State Key Lab of Transducer Technology, Chinese Academy of Sciences, Beijing 100081, P. R. China) (4 Beijing Key Lab of Nanophotonics and Ultrafine Optoelectronic Systems, School of Materials Science and Engineering, Beijing Institute of Technology, Beijing 100081, P.R. China)

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Abstract:

In this paper, a simple and non-poisonous "surfactant treatment" method to prepare graphene oxide (GO) in organic solvents with good dispersibility was presented. As the surfactant molecules, didodecyldimethyla-mmonium bromide (DDAB) was attached onto the GO sheets via ionic interactions by mild sonication, the obtained nanocomposites

were

then

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GO:DDAB

blended

into

copolymer

Poly(3-

hexylthiophene-2,5-diyl) (P3HT): [6,6]-Phenyl C61 butyric acid methyl ester (PC61BM) as the active layer to fabricate bulk-heterojunction (BHJ) organic solar cells

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ITO/PEDOT:PSS/P3HT:PC61BM:(GO:DDAB)/Ca/Al.

The

concentration

of

GO:DDAB in the active layer, a maximum power conversion efficiency (PCE) of

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3.67 % was obtained by blending 0.5 mg/mL GO:DDAB in the active layer, showing an efficiency increment of 13.35 % as compared with that of the control device without doping GO:DDAB. The optimized OPVs with PTB7:PC71BM by adding GO:DDAB shows the PCE of 7.96 %. Therefore, it paves a way to get high efficiency organic photovoltaic cells by directly blending surfactant-treated graphene oxide in organic solvent.

Keywords: Graphene oxide (GO), organic photovoltaic cells (OPVs), acceptor materials 1

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1. Introduction Organic photovoltaic cells (OPVs) are promising alternatives to inorganic counterparts due to their advantages such as low cost, light weight, the versatility of their components, beneficial optical and mechanical properties[1-3]. Their inherent

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features, such as the low cost, less energy resources needed for production and reduced installation costs, have pushed the researches on OPVs forward in the last decades[4]. Incessant and creative efforts have led to a significant enhancement in power conversion efficiency (PCE), and currently the world record PCE in the lab is

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over 10 %[5].

For OPVs, people always encounters certain problems such as the structural traps,

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isolated domains, and incomplete pathways in the percolation network[6], and these problems can be solved through optimizing the thickness of both hole- and electron-transporting layers, and currently the surface roughness of metal electrodes is under extensive studying. However, it is important to synthesize different acceptor and donor materials in order to obtain more stable, better matching between the

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highest occupied molecular orbit (HOMO) energy level and the lowest unoccupied molecular orbit (LUMO) energy level and stronger light absorption. Till now, the acceptor materials for OPVs are much less as compared to donor materials. Being a

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single layer with a two-dimensional carbon hexagons network structure, especially for its outstanding mechanical, optical and electronic properties, graphene has attracted

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great attention for its potential applications in energy conversion and storage devices[7-9] such as supercapacitors, fuel cells and organic photovoltaic cells[10-13]. Particularly, graphene has successfully been employed as the electrodes, the active layers and the interfacial layers in OPVs[14]. As the acceptor used in OPVs, graphene has three advantages: (i) high electron mobility; (ii) the thickness of single atom and the two-dimensional planar structure of large specific surface area; (iii) large interfaces to improve excitons diffusion[15]. However, pristine graphene shows some defects such as its insolubility and inclined agglomeration, which is not suitable for solution-processed devices. Usually, reduced graphene oxides (r-GO) were used as the 2

ACCEPTED MANUSCRIPT electron acceptor in OPVs[16]. As we know, GO is the oxidized form of graphene in which its surfaces and edges were modified with hydroxyl, carboxyl and epoxide groups[17,

18]

. In this way, such a kind of modifications will affect the electronic

structure of the graphene plane, however, GO could be acted as an excellent electron

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acceptor in OPVs after its partial reduction to graphene[19]. On the other hand, even GO has good solubility and dispersion in aqueous solution, most organic materials were dissolved in organic solvents, therefore GO must be functionized before its usage in OPVs. In 2011, such a kind of OPVs, in which pheny isocyanate

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functionalized GO acted as the acceptor and P3OT as the donor, was reported with a maximum PCE of 1.4 %[14]. Afterwards, the functionalized GO was blended with

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P3HT as the active layer in OPVs and usually a PCE just over 1 % was obtained[20], furthermore, in this way the organic functionalized process of GO always influence the electrical properties of the pre-prepared GO.

Therefore, in this paper we demonstrated a direct method to improve the PCE of OPVs by doping GO in organic solvent as acceptor materials. In our experiments,

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didodecyldimethylammonium bromide (DDAB) was used as the surfactant, and it was attached to the GO sheets via ionic interactions by mild sonication. Then, GO aqueous solution was transferred into organic solvent. By characterizing the performance and

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reliability of OPV ITO/PEDOT:PSS/P3HT:PC61BM:(GO:DDAB)/Ca/Al in which P3HT:PC61BM was blended with GO:DDAB in different concentrations as the active layer. We found the device in which P3HT:PC61BM was blended with 0.50 mg/mL

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GO:DDAB exhibits a highest PCE of 3.67 %, and it is much higher than those ever reported, showing an efficiency enhancement of 13.35 % as compared with that of the control devices without doping GO:DDAB. Also, the maximum PCE of the device ITO/PEDOT:PSS/PTB7:PC71BM:(GO:DDAB)/Ca/Al in which PTB7:PC71BM was blended with GO:DDAB as the active layer reached 7.96 %.

2. Experimental 2.1. Preparation of GO and its aqueous solution 3

ACCEPTED MANUSCRIPT GO was fabricated from pure graphite (Alfa Aesar, 99.999 % in purity) by a modified Hummers method[21]. Firstly, 9:1 mixture of concentrated H2SO4/H3PO4 (360 mL H2SO4, 98.08 % and 40 mL phosphoric acid, 98 %) was mixed with 3.0 g of the natural graphite flakes. Secondly, 18.0 g of 99.3 % KMnO4 was slowly added in

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and the reaction was continued overnight. After that, 3 mL of 30 wt% H2O2 was added into the resulting suspension on ice batch and stirred for l h. The suspension was sonicated for 30 min. At last, the resulting suspension was centrifuged at 3000 rpm for 10 min and then redispered in 10 wt% HCl to rinse the unreacted residues.

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This rinsing step was repeated twice, followed by rinsing twice with DI water. To filter the impurities further, the resulting suspension was dialyzed against DI water.

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The final concentration of the GO stock solution was adjusted to 2 mg/mL.

2.2. Transfer GO in aqueous solution into organic solution. Didodecyldimethylammonium bromide (DDAB) was added into the stock solution of GO in aqueous solution to yield a mass ratio of about 1:4 for GO to

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DDAB, respectively[19]. This solution was sonicated for 2 h to keep the sufficient interaction between the GO sheets and the surfactants. The solution was then centrifuged at 10000 rpm for 10 min. This process was repeated three times. After oven

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pouring off the supernatant, the solid GO:DDAB product was dried in a 100

overnight. Then the solid powders were subsequently diluted into o-Dichlorobenzene

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in different concentrations of 0.05 mg/mL, 0.50 mg/mL, 2.50 mg/mL and 7.50 mg /mL, respectively.

2.3. Device fabrication and characterization Firstly, ITO substrate was cleaned for 15 min by ultrasonication with DI water, acetone

and

isopropyl

alcohol

(IPA),

consequently.

Then,

a

poly-(3,4-ethylenedioxythiophene):poly(styrenesulfonate) (PEDOT:PSS) solution was spin-coated on the ITO at 3000 rpm for 50 s after an oxygen plasma treatment to the ITO substrates. The PEDOT: PSS layer was annealed at 140 °C for 10 min. Then the 4

ACCEPTED MANUSCRIPT device was transferred to a nitrogen glove box, where the blend active layer of the poly(3-hexylthiophène) (P3HT) and [6,6]-phenyl-C61-butyric acid methyl ester (PC61BM) was spin-coated onto the PEDOT:PSS layer to complete the OPV device ITO (350 nm)/PEDOT:PSS (60 nm)/P3HT:PC61BM:(GO:DDAB) (310 nm)/Ca (40

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nm)/Al (100 nm). The active layer was formed by spin-coating 15 mg/mL o-dichlorobenzene (ODCB) solution at 600 rpm for 18 s, followed by a solvent annealing at room temperature for 40 min, then it was thermally annealed at 110

for

10 min. The PTB7:PC71BM was dissolved (1:1.5 w/w, 20 mg/mL) in chlorobenzene

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(99.8 %) and stirred overnight at 60 °C in a nitrogen-filled glovebox. In the end, 3 vol % solvent additive DIO was added into the blend solution. The active layer was

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formed by spin-coating at 900 rpm for 60 s. The methanol solvent was then subsequently spun off from the active layer via spin-coating at 2500 rpm for 40 s. Finally, Ca and Al electrodes were deposited successively in a vacuum onto the active layer at a pressure of ca. 3.0×10-4 Pa. The active area of the device was

4.4 mm2.

The current density-voltage (J-V) characteristics were measured on a

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computer-controlled Keithley 236 Source Meter. A xenon lamp coupled to AM 1.5 solar spectrum filter was used as the light source, and the optical power intensity was 100 mW/cm2. The XPS measurements were performed in a Kratos Ultra Spectrometer

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(@ a base pressure of 1×10-9 Torr) using monochromatized Al Ka X-ray photons (hν= 1486.6 eV) discharge lamp. The AFM images were acquired using an Innova

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Microscope with a silicon cantilever in tapping mode. The film thickness was characterized by a scanning electron microscopy (SEM) (Hitachi S-4800). The UPS measurements were acquired with a Kratos Ultra Spectrometer. The UV-vis spectra were recorded using JASCOV-570 spectrophotometer. The PL spectra were measured using JASCO FP-6600 spectrophotometer with a Xenon flash lamp. The phase identification was determined by X-ray diffraction (XRD) (Rifaku D/MAX-2004) with Cu Kα radiation (λ=1.54178 Å) operating at 40 kV and 60 mA. The J-V characteristics of the devices were measured in dark or under illumination with a computer-controlled Keithley 236 Source Meter which was kept in the glove box. 5

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3. Results and discussion Fig. 1 shows the XPS spectra of GO and GO:DDAB nanocomposites. As shown in Fig. 1 (a), carbon and oxygen peaks are observed in both the XPS spectrum of GO

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and that of GO:DDAB nanocomposites. Meanwhile, an additional 0.59 % nitrogen and 2.02 % bromine peak are observed in the XPS spectra of GO:DDAB nanocomposites, as shown in Fig. 1 (b) and (c). As we know, the elemental

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compositions of DDAB include nitrogen and bromine. From here, therefore, one can see the peaks of nitrogen and bromine appear in the XPS spectra of GO:DDAB nanocomposites but not in that of GO, certifying that the surfactant DDAB was

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attached to GO sheets surface via ionic interactions[19].

After its blending into GO organic solvent, DDAB was attached to the GO surface via ionic interactions and, in this way, GO:DDAB nanocomposites formed and it can be readily dispersed into o-dichlorobenzene[22]. The molecular structures of

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GO and DDAB are shown in Fig. 2 (a). Fig. 2 (b) shows the photos of GO:DDAB dispersion in different concentrations from 0.05 mg/mL to 7.50 mg/mL at the initial stage and after one week. Although GO can be dispersed in o-dichlorobenzene under ultrasonication even for the concentration of 7.50 mg/mL, the precipitation appeared

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after one week storage in atmospheric environment even for the low concentration of 0.05 mg/mL. These results suggest that it is necessary to keep GO:DDAB to be well

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dispersed in solvent before its use. Fig. 3 shows the SEM images of GO:DDAB in different concentrations on silicon substrate. From here, one can see that the image of different concentration of GO:DDAB dispersion from 0.05 mg/mL to 2.50 mg/mL is quite uniform. However, some aggregations of nanosheets appeared for the concentration of 7.50 mg/mL (see Fig. 3 (d), the inset shows the HR-SEM image). Fig. 4 (a) shows the X-ray diffraction (XRD) patterns of GO and GO:DDAB thin films deposited on glass substrate. From here, one can see that there is a strong diffraction peak at 2θ =12.37° for GO, and the reason is that the hydroxyl, epoxy and carboxyl groups are all attached to GO sheets, and the GO sheets still stack with each 6

ACCEPTED MANUSCRIPT other regularly. After GO is attached with the surfactant DDAB, a broad and much lower diffraction peak appeared at 2θ =23.5° for GO:DDAB. The broad and small diffraction peaks for the GO:DDAB indicate that the GO:DDAB sheets are corrugated in structure and they are disorderly stacked. Fig. 4 (b) shows the absorption spectra of

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P3HT:PC61BM and GO:DDAB in organic solvent. The GO:DDAB nanocomposites displayed an absorption with a tail to 800 nm, which is consistent with that of a water-soluble GO deposited film[23]. The absorption of P3HT:PC61BM lies in the wide range of 400-800 nm, and it peaks at around 460 nm.

P3HT:PC61BM

in

o-dichlorobenzene

doped

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Fig. 5 (a) shows the normalized photoluminescence (PL) spectra of the with

GO:DDAB

in

various

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concentrations at the excitation wavelength of 450 nm, showing the PL peaks at 584 nm. One can see that the PL peak intensity decreased significantly with increasing the concentration of GO:DDAB, showing efficient excitons dissociation happens at the P3HT/GO:DDAB interface. This so-called PL quenching indicates efficient electron transfer at the donor/acceptor interfaces[24]. The trend of PL quenching with increasing

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the concentration of GO:DDAB indicates the enhanced efficiency due to charge carriers separation. Fig. 5 (b) shows the absorption spectra of P3HT:PC61BM films in different concentration of GO:DDAB. From here, one can see that P3HT:PC61BM film in different concentration of GO:DDAB exhibit an absorption peak at 510 nm

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and it mainly origins from P3HT[25]. The absorption band of P3HT:PC61BM film lies in the range of 400 nm-600 nm, and there is no significant shift for the absorption

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spectrum after blending GO:DDAB into P3HT:PC61BM. This could be the result of no significant interaction between the ground states of the two components after blending GO:DDAB, as well as no charge transfer happens between the ground states of two components[25, 26]. As we know, film morphology plays a key role in determining the device performance of OPVs. For OPVs, efficient excitons dissociation occurs at the interfaces between donors and acceptors[27, 28]. The AFM images of P3HT:PC61BM blended with different concentration of GO:DDAB are shown in Fig. 6. The surface of pristine P3HT:PC61BM thin film shows bright and dark features that can be 7

ACCEPTED MANUSCRIPT associated with phase-separated domains. The P3HT phase tends to crystallize forming aggregates, while the PCBM molecules could easily penetrate into the bulk of active layer. By increasing the concentration of GO:DDAB from 0.05 mg/mL to 2.50 mg/mL

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in the P3HT:PC61BM, the distribution process of GO:DDAB in the P3HT matrix was recorded (see Fig.6). From Fig.6, one can see that the GO:DDAB sheets are homogeneously dispersed in the blend, and there are no large GO:DDAB domains. However, the phenomenon of agglomeration can be observed when the concentration

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of the GO:DDAB increased to 7.50 mg/mL in the blend, and such a kind of agglomeration in the active layer can affect the charge carriers separation/transport,

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therefore, a suitable concentration of GO:DDAB could be well dispersed into the P3HT:PC61BM solution to form a homogeneous solution, in this way, to improve excitons dissociation and more efficient transport pathways will be formed for charge carriers.

Fig. 7 (a) shows the schematic architecture of OPV ITO/PEDOT:PSS/

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P3HT:PC61BM:(GO:DDAB)/Ca/Al in which the blended active layer is doped with different concentrations of GO:DDAB. Fig. 7 (b) shows the SEM images of the cross-section of the device. The work function of ITO surface modified with GO and GO:DDAB were investigated by ultraviolet photoelectron spectroscopy (UPS), and it

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was obtained from the following equation[29], EHOMO=hν- (ECutoff - EFermi)

(1)

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where EHOMO, ECutoff, and EFermi are the work function, secondary electron cut-off and Fermi level, respectively. The photon energy of the UPS light source (helium I radiation) was 21.2 eV. As shown in Fig. 7 (c), the work function of GO:DDAB is -4.61 eV, implying there is an increment of 0.21 eV as compared with the work function of GO ( -4.82 eV). As shown in Fig. 7 (d), the increased work function of modified GO:DDAB could result in better energy alignment in the device, resulting to a more effective transfer efficiency. Fig. 8 (a) shows the J-V characteristics of the photovoltaic device ITO/PEDOT:PSS/P3HT:PC61BM:(GO:DDAB)/Ca/Al in which the active layer 8

ACCEPTED MANUSCRIPT contains pure GO and GO:DDAB in different concentrations, respectively. Fig. 8 (a) shows the EQE spectra of photovoltaic device with the active layer. The open-circuit voltage (Voc), short-circuit current (Jsc), fill factor (FF) and PCE can be extracted from these curves and these parameters are summarized in Table. 1. The reference

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photovoltaic cell based on P3HT:PC61BM:GO showed a much lower efficiency of 1.78 % with Voc = 0.588 V, Jsc = 8.04 mA/cm2 and FF = 37.62 %. For the control device without doping GO:DDAB, the PCE is only 3.18 % with Voc = 0.602 V, Jsc = 7.54 mA/cm2 and FF = 65.69 %. From Table 1, one can see the PCE increases with

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increasing the concentration of GO:DDAB and it reaches a maximum value of 3.67 % for the device in which P3HT:PC61BM was doped with 0.50 mg/mL of GO:DDAB.

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Therefore, the photovoltaic cells based on P3HT:PC61BM:(GO:DDAB) clearly out-performed its counterpart P3HT:PC61BM:GO as active layer. For higher concentration of GO:DDAB (i.e. 7.50 mg/mL), the PCE decreased to 2.11 % again. From here, we can see that a suitable GO:DDAB dopant can increase the device performance. The donor/acceptor of nanocomposites have considerable large

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interfaces to improve the efficiency of the excitons disassociation[30,31]. Furthermore, donor/acceptor interface in the mixed film formed interpenetrating network structure, which can facilitate the transport of electrons and holes after excitons dissociating at

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the P-N interfaces, as well as the collection at the electrodes. The dependence of Jsc and FF on the concentration of GO:DDAB can be governed by the more efficient carriers transport[32]. For a higher concentration of GO:DDAB, it will aggregates on

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the film surface and it will trap charge carriers or act as recombination centers for carriers, and excitons accumulation within the active layer will hinder the transfer of electrons and holes[33, 34]. Although the obtained PCE of 3.67 % is still moderate, there is still more space to improve the PCE of solar cells, such as to further functionalize GO:DDAB or to optimize the chemical structure of the donor polymer and the device configuration.

Fig.

8

(c)

shows

the

device

performance

of

ITO/PEDOT:PSS/PTB7:PC71BM:(GO:DDAB)/Ca/Al, and it shows a maximum PCE of 7.96 % by adding 0.5 mg/mL GO:DDAB. Therefore, our experimental results show that GO:DDAB not only can be used for the traditional P3HT:PC61BM system, but 9

ACCEPTED MANUSCRIPT also can apply for many other polymer:fullerene systems.

VOC (V)

JSC (mA/cm2)

FF %

PCE (max) (%)

0.00 mg/mL

0.602±0.01

7.54±0.11

65.69±0.06

3.18

0.05 mg/mL

0.609±0.01

7.82±0.09

69.16±0.04

3.26

0.50 mg/mL

0.609±0.01

8.53±0.12

70.73±0.01

3.67

2.50 mg/mL

0.601±0.01

7.51±0.10

67.18±0.02

3.04

7.50 mg/mL

0.579±0.03

7.56±0.12

48.10±0.04

2.11

P3HT:PC61BM:GO

0.588±0.12

8.04±0.15

37.62±0.16

1.78

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Different concentration of GO:DDAB

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Table 1. Photovoltaic parameters of photovoltaic device ITO/PEDOT:PSS/P3HT:PC61BM:(GO:DDAB)/Ca/Al in which P3HT:PC61BM was doped with pure GO and GO:DDAB in different concentrations as the active layer. The average parameters are based on more than 15 devices.

Table 2. Photovoltaic parameters of photovoltaic device in which PTB7:PC71BM was doped with GO:DDAB as the active layer.

JSC (mA/cm2)

FF %

PCE (max)(%)

0.738±0.01

14.51±0.10

69.53±0.06

7.45

0.737±0.01

15.19±0.11

71.11±0.08

7.96

VOC (V)

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Active layer PTB7:PC71BM only

*

PTB7:PC71BM:(GO:DDAB)

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* Here the concentration of GO:DDAB is 0.5 mg/mL.

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4. Conclusion

In summary, we have demonstrated the method to synthesize GO:DDAB

nanocomposites by attaching the surfactant DDAB molecules to GO sheets via ionic interactions and its application in organic photovoltaic cells. After transferring from aqueous solution into organic solvent, GO sheets can act as an efficient acceptor material for highly efficient and stable OPVs since GO:DDAB can be well dispersed in organic solvents. In this way, it could enhance excitons dissociation and provides the transport pathways for electron species, a maximum PCE of 3.67 % was obtained 10

ACCEPTED MANUSCRIPT for 0.50 mg/mL of GO:DDAB in solution, showing an efficiency enhancement of 13.35 % as compared with that of the control device without doping GO:DDAB. Furthermore,

the

maximum

PCE

of

7.96

%

was

obtained

for

PTB7:PC71BM:(GO:DDAB) as active layer, our experimental results clearly

candidates as acceptor materials for high-efficiency OPVs.

Acknowledgements

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demonstrated that GOs by surfactant treatment in organic solvent can be excellent

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This project was partially funded by the project of State Key Laboratory of Transducer Technology (SKT1404), the project of the Key Laboratory of

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Photoelectronic Imaging Technology and System (2015OEIOF02), Beijing Institute of Technology, Ministry of Education of China, and Key Project of Chinese National

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Programs for Fundamental Research and Development (2013CB329202).

Figure Captions: 11

ACCEPTED MANUSCRIPT Figure 1 (a) XPS spectra of GO (black line) and GO:DDAB (red line), (b) The N1s spectrum of GO:DDAB, (c) The Br3d spectrum of GO-DDAB. Figure 2 (a) Chemical structures of GO and DDAB. (b) Photos of GO:DDAB dispersion in different concentrations at the initial stage and after one week.

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Figure 3 SEM images of GO:DDAB in different concentrations on silicon substrate: (a) 0.05 mg/mL, (b) 0.50 mg/mL, (c) 2.50 mg/mL and (d) 7.50 mg/mL. The inset in (d) shows its HR-SEM image.

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Figure 4 (a) X-ray diffraction patterns of GO and GO:DDAB thin film deposited on ITO substrate, (b) Normalized absorption spectra of P3HT:PC61BM and GO:DDAB in o-dichlorobenzene.

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Figure 5 (a) Normalized PL of P3HT:PC61BM in o-dichlorobenzene in various concentrations of GO:DDAB at excitation wavelength of 450 nm, (b) Absorption spectra of P3HT:PC61BM thin film with various concentrations of GO:DDAB. Figure 6 AFM images (100 nm×100 nm) of P3HT:PC61BM with different concentration of GO:DDAB: (a) 0.00 mg/mL, (b) 0.05 mg/mL, (c) 0.50 mg/mL, (d) 2.50 mg/mL and (e) 7.50 mg/mL.

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Figure 7 (a) Schematic architecture of the OPVs. (b) SEM images of the cross-section of the OPVs. (c) UPS of GO and GO:DDAB, and the inset shows the magnified UPS spectra data. (d) Schematic diagram of the energy levels for the OPVs.

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Figure 8 (a) J-V characteristics of the photovoltaic device ITO/PEDOT:PSS/P3HT:PC61BM:(GO:DDAB)/Ca/Al in which P3HT:PC61BM was doped with GO and GO:DDAB in different concentrations as the active layer. (b) EQE spectra of photovoltaic device with the active layer. (c) The J-V curves of the photovoltaic device ITO/PEDOT:PSS/ PTB7:PC71BM:(GO:DDAB)/Ca/Al in which the concentration of GO:DDAB is 0.5 mg/mL.

12

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O1S

(a)

GO GO:DDAB

Inensity (a.u.)

C1S

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Fig.1

Si2p

O1S

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C1S N1s

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Si2p Br3d

700 600 500 400 300 200 100 Binding energy (eV)

404 402 400 Binding energy (eV)

398

Figure 1

13

Br3d

Inensity (a.u.)

(b)

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406

N1s

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Inensity (a.u.)

(b)

0

74

72

70 68 66 64 Binding energy (eV)

62

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Figure 2

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Fig.2

14

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Fig.3

Figure 3

15

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Diffraction Intensity(a.u.)

(a)

GO

10

20

30 40 50 2-Theta (degrees)

GO:DDAB

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1.6 (b)

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GO:DDAB

1.2

P3HT:PC BM 61

0.8 0.4

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Abs (a.u.)

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Fig.4

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0.0

360

450 540 630 Wavelength (nm)

Figure 4

16

720

60

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3000 (a) 2500

1500

0.00 mg/mL

1000

0.05 mg/mL 0.50 mg/mL

500

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2000

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Fluorescence Intensity (a.u.)

Fig.5

2.50 mg/mL 7.50 mg/mL

0 500

550

600

650

700

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Wavelength (nm)

1.4

(b)

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Abs (a.u.)

1.2 1.0

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0.8

0.00 mg/mL 0.05 mg/mL 0.50 mg/mL 2.50 mg/mL 7.50 mg/mL

0.6

400

450 500 550 Wavelength (nm)

Figure 5

17

600

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Fig.6

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Figure 6

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Fig.7

Figure 7

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(a)

6

0 -3

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3

-6 -9 -0.2

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GO:DDAB 0.00 mg/mL GO:DDAB 0.05 mg/mL GO:DDAB 0.50 mg/mL GO:DDAB 2.50 mg/mL GO:DDAB 7.50 mg/mL P3HT:PCBM:GO

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Current Density (mA/cm2)

Fig.8

0.0

0.2

0.4

0.6

Voltage (V)

70

40 30

GO:DDAB 0.00 mg/mL GO:DDAB 0.05 mg/mL GO:DDAB 0.50 mg/mL GO:DDAB 2.50 mg/mL GO:DDAB 7.50 mg/mL

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50

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60 (b)

20

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Wavelength (nm)

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700

4 (C) PTB7:PC71BM PTB7:PC71BM:GO-DDAB

0

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-4 -8 -12 0.2

0.4 0.6 Voltage (V)

0.8

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Current Density (mA/cm2)

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Figure 8

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Highlights

(1) "Surfactant treatment" method to prepare GO in organic solvents is presented.

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(2) DDAB was attached onto the GO sheets via ionic interactions by mild sonication. (3) A PCE of 3.67 % was obtained by doping 0.5 mg/mL GO:DDAB in P3HT:PC61BM.

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(4) A PCE of 7.96 % was obtained by doping GO:DDAB in PTB7:PC71BM.

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(5) Surfactant-treated GO in organic solvent paves a way for high-efficiency OSC.