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Cite this: DOI: 10.1039/c4nr03658a

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Facile synthesis of analogous graphene quantum dots with sp2 hybridized carbon atom dominant structures and their photovoltaic application† Zhengcheng Huang, Yongtao Shen, Yu Li, Wenjun Zheng, Yunjia Xue, Chengqun Qin, Bo Zhang, Jingxiang Hao and Wei Feng* Graphene quantum dot (GQD) is an emerging class of zero-dimensional nanocarbon material with many novel applications. It is of scientific importance to prepare GQDs with more perfect structures, that is, GQDs containing negligible oxygenous defects, for both optimizing their optical properties and helping in their photovoltaic applications. Herein, a new strategy for the facile preparation of “pristine” GQDs is reported. The method we presented is a combination of a bottom-up synthetic and a solvent-induced interface separation process, during which the target products with highly crystalline structure were selected by the organic solvent. The obtained organic soluble GQDs (O-GQDs) showed a significant

Received 1st July 2014, Accepted 27th August 2014 DOI: 10.1039/c4nr03658a www.rsc.org/nanoscale

1.

difference in structure and composition compared with ordinary aqueous soluble GQDs, thus leading to a series of novel properties. Furthermore, O-GQDs were applied as electron-acceptors in a poly(3-hexylthiophene) (P3HT)-based organic photovoltaic device. The performance highlights that O-GQD has potential to be a novel electron-acceptor material due to the sp2 hybridized carbon atom dominant structure and good solubility in organic solvents.

Introduction

Graphene quantum dots (GQDs) have triggered tremendous attention as a new class of “zero-dimensional” carbon nanomaterials.1 In comparison with the traditional inorganic nanoparticles, the light-emitting quantum sized graphene materials are superior in many aspects, such as their biocompatibility, green and facile synthesis, high stability without photobleaching, and tunable photoluminescence (PL) emission depending on the excitation wavelength,1–7 thus making them fascinating in applications ranging from biosensing to photocatalysis and photovoltaics.5–7 Recently, several approaches have been developed to prepare GQDs. Most of them can be classified into two main groups:8 top-down1,5,9–13 and bottom-up.2–4,6,7 The former routes consist of the decomposition of suitable precursors, including micrometer-sized graphene sheets,10 graphite nanoparticles and graphene oxide,11,12 carbon nanotubes and fibers,1,13 nanodiamonds,14 wherein the precursors are “broken off” and GQDs are formed. Nevertheless, this non-

School of Materials Science and Engineering, Tianjin Key Laboratory of Composite and Functional Materials, Tianjin University, Tianjin 300072, P. R. China. E-mail: [email protected] † Electronic supplementary information (ESI) available. See DOI: 10.1039/ c4nr03658a

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selective “top-down” chemical cutting process does not allow precise control over the morphology.15 They are usually of low throughput and produce GQDs with non-ideal or nonuniformly distributed lateral size and thickness (layer number).16 Additionally, in spite of the term “graphene quantum dots”, the synthesized GQDs, strictly speaking, are graphene oxide quantum dots (GOQDs) or partially reduced GOQDs (rGOQDs).11 Despite treating with the harsh reduction process, the oxygenous sites of rGOOD still remain at a non-ignorable level.17 Inevitably, these residual defects would affect their optical characteristics and thereby limit their performance in many applications. The bottom-up methods rely on assembly of small natural or artificial cyclic molecules (e.g., glucose, hexa-peri-hexabenzocoronene, cetylpyridinium chloride)2,15,18 under suitable conditions that consist, for example, of combustion/thermal,19–21 ultrasonic3 and microwave-assisted pyrolysis.2,22–25 However, various oxygen-containing functional groups, including carbonyl, carboxyl, hydroxyl, and epoxy groups, were introduced into the edges and onto the basal plane during the formation process.10 So typically, this approach cannot prepare GQDs with a “defect-free” structure either. Based on the fact that the obtained products contain abundant sp3 hybridized carbon atoms and are filled with defects, they are usually called “carbon dot” rather than GQD. Herein, we developed a new strategy for facile synthesis of high quality GQDs with few surface defects by a modified

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bottom-up method. The initial products were synthesized through an ordinary bottom-up microwave pyrolysis of the carbohydrate molecules.26 Then they were treated by a solventinduced interface separation process, during which they were separated into two sections. The structures and properties of aqueous soluble GQDs (A-GQDs) almost remained unchanged during the separation. In contrast, the oxygenous contents in O-GQDs are dramatically reduced, and they show a relatively more regular structure dominated by the sp2 graphite carbon atoms. As a consequence of the structural disparities, O-GQDs show a different PL mechanism compared to the carbon dots prepared by microwave treatment.2,25 Moreover, O-GQDs show similar optical properties to the GQD directly made from graphene sheets and carbon fibers.10,13 In consideration of their structural properties and organic solubility, a conceptual device was fabricated employing P3HT/O-GQDs as electrondonors/acceptors. The performance revealed that O-GQD is an effective semiconductor material as the electron-acceptor.

2. Experimental section 2.1.

Preparation of initial GQDs for further separation

The initial GQDs for further separation were synthesized via a microwave-assist pyrolysis method, together with a tiny amount of inorganic additives as the catalyst for the formation of the aromatic carbon skeleton.26 In a typical experiment, 7 mL glycerol and 3 mL ultrapure water (70% (v/v)) were mixed with a suitable amount (10 mM) of sulfuric acid or a double dose of hydrochloric acid. Then the mixtures were heated in a microwave chemical reactor (640 W) for 8 minutes. Finally the color-changed solution was diluted and purified with ultrapure water after cooling down to room temperature. For removing the impurities, the above solution was dialyzed through a dialysis membrane (MWCO of 1 kDa) for 72 hours. After all the purifications, the hydrosolvent was removed by freeze-drying. The weight of the obtained GQDs solid sample was 0.87 g. According to the weight of the carbon source (7 mL glycerol, 8.84 g), the calculated yield of this process was 9.8%. 2.2. Solvent-induced separation process for preparing A- and O-GQDs In a typical experiment for the separation process, 50 mg GQD solid sample was redissolved by ultrapure water to form the solution with a volume of 50 mL and a concentration of 1 mg mL−1. Then the solution was added into a separating funnel, followed by the addition of chloroform in an equal volume. The mixed solutions were well blended by shaking the funnel for 1 minute. After the stratification, the two sections were collected respectively, the upper half was the A-GQDs solution and the bottom half was the solution of O-GQDs. For obtaining enough amounts of O-GQDs, the above separation procedure was repeated 3 times. After this, the chloroform solvent was removed by vacuum rotary evaporation and the corresponding O-GQDs were collected after completely vacuum drying. The

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weight of O-GQDs obtained for each time was 3.2 mg, giving a yield of 6.4% based on the initial GQDs (50 mg). 2.3.

Characterization

The topographic morphology and microstructures of the obtained GQDs were observed by AFM (Bruker Dimension 3100) and TEM (Philips Tecnai G2F20). Chemical structures were analyzed by FTIR (Bruker Tensor 27, using KBr pellets), XRD (Rigaku D/max-2500 with Cu Kα radiation) and XPS (PERKIN ELMZR PHI 3056 spectrometer with an Al anode source operated at 15 kV and an applied power of 350 W). Raman spectra were recorded with a DXR microscope (ThermoFisher). UV-vis absorption and PL were studied using a spectrophotometer (UV-3600, Shimadzu) and a spectroflurometer (Fluorolog3, HORIBA, Jobin Yvon), respectively. 2.4.

Electrochemical measurements

To estimate the HOMO and LUMO energy levels of O-GQDs, cyclic voltammetry (CV) was carried out using a standard three-electrode system, which consists of a glassy carbon disk as the working electrode, a platinum wire as the counter electrode, and an Ag/AgCl reference electrode. The O-GQD based electrode was prepared by drop-casting O-GQD/chloroform suspensions (5 mg mL−1) onto the glassy carbon electrode. CV was recorded in acetonitrile containing 0.1 M tetrabutylammonium hexafluorophosphate (TBAPF6, Aldrich) as the supporting electrolyte. Before each measurement, the electrochemical cell was purged with a high-purity argon gas for 15 min. 2.5.

Fabrication of the photovoltaic devices

Indium tin oxide (ITO) coated glass plates were used as the substrate and they were ultrasonicated consecutively with distilled water, acetone, and ethanol in advance. After drying at 60 °C for 20 minutes, 150 μL of O-GQD/chloroform suspensions (5 mg mL−1) was spin coated onto the clean substrates and dried naturally. Then 50 μL P3HT chloroform solution (15 mg mL−1) was spin coated onto the pre-existing O-GQD thin film and allowed to dry naturally. The photoresponse was investigated using a CHI 660D electrochemical analyser with a time interval (ON/OFF) of 40 s, a light intensity of 230 mW cm−2 and a bias voltage of 0.2 V.

3. Results and discussion The hydroxyl groups in the molecule of glycerol (C3H8O3) make it easy for a dehydration reaction under microwave conditions. Through this dehydration, the glycerol molecules were pyrolyzed, cross-linked, and finally converted into carbon nanoparticles. In the process of microwave heating, the solution changed color (e.g., from transparent to dark brown) as a result of formation of GQDs. Moreover, this process would lead to a result that various oxygenous groups were introduced along the surface of the carbon core. As illustrated in Fig. 1, the obtained crude products contained various shapes and

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Fig. 1

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Schematic and the corresponding photographs of the interface separation process and detailed structures of the obtained A- and O-GQDs.

nanosized sp2 carbon domains localized by the sp3 carbon structures, resulting in a relatively amorphous nature compared with GQDs directly scissored from graphene-based carbon precursors.7,10,12 According to previous reports,1–7,8–13 no matter which route to carbon nanodots or graphene quantum dots is chosen, inevitably there would be residues, which included the starting reactants and the by-products. Most of the impurities were small molecules, so a dialysis process was utilized for a pure GQD aqueous solution. Then the hydrosolvent was removed by freeze-drying and the initial GQD sample was obtained. The initial pure GQDs were acquired based on the above work, but the surface structures (oxygen-related groups) and components of these “pure” GQDs were still complicated. Many oxygen-related groups were formed along the surface of most GQDs due to the effect of dehydration. Although the obtained GQDs actually showed the properties of photoluminescence and their water solubility may facilitate the application in bioimaging, the oxygen-rich groups on the surfaces would affect their solubility in common organic solvents such as chloroform and methylbenzene, thus limiting their applications in non-aqueous phase devices. We think that the distributions of oxygenous components in these GQDs were random; more specifically speaking, most of the GQD surfaces were filled with various oxygenous defects; however, a small proportion of GQDs with a tiny amount of oxygenous groups still existed. This result was due to the complex reactions during the microwave treatment. The purpose of the next step is separation and selection. A solvent-induced interface separation was introduced to realize this target. After the mixing of the organic solvent (chloroform), the interface between two phases provided a place for processing the separation and selection of GQDs. The GQDs

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with more defects created by the oxygen-containing groups are water-soluble. However, GQDs which have the sp2 carbon dominant structure with much fewer (even negligible) oxygenous groups show a hydrophobic property but they can disperse well in the organic solvent. Thus, they would transfer from the aqueous phase to the organic phase. Interestingly, A-GQDs exhibit an aqua green color while O-GQDs emit a pure blue color under ultraviolet irradiation respectively, as shown in Fig. 1. According to our observation, the newly obtained O-GQDs show a relatively high stability without photobleaching under ambient conditions for several months. The topographic morphology and height distribution of the initial GQDs directly obtained from microwave pyrolysis were investigated by atomic electron microscopy (AFM) as shown in Fig. S1a of the ESI.† The height profile of the line in Fig. S1b† shows that the thickness of the GQDs is less than 4 nm, implying that the simple microwave process will result in the largescale synthesis of several-layered GQD with regular shape. Fig. 2a and 2c present the transmission electron microscopy (TEM) images of the two kinds of GQDs produced in this work. These figures reveal that ca. 1–5 nm sized GQDs are uniformly distributed without agglomeration. It is worth noting that the O-GQDs exhibit a narrower size distribution than that of A-GQDs, probably due to their more regular structures selected by the organic solvents. The graphitic lattice of the two kinds of GQD can be clearly resolved under high-resolution TEM (HRTEM) images and the corresponding fast Fourier transform (FFT) patterns, as shown in the insets of Fig. 2b and 2d. The two kinds of lattice spacing (0.22 nm and 0.25 nm) observed correspond to the hexagonal lattice plane spacing of d1100 27 and d1120 13 respectively. The disparities of bonding compositions and functional groups of the two kinds of GQDs were investigated by Fourier

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Fig. 2 (a) and (c) TEM images of A- and O-GQDs and their corresponding diameter distributions. (b) and (d) High resolution TEM images of A- and O-GQDs (insets: a typical single A- and O-GQD and the corresponding 2D FFT image).

transform infrared (FTIR) spectroscopy (Fig. 3a). Both A- and O-GQDs display a strong and broad hydroxyl peak at 3430 cm−1 and the sharp absorption peaks at 1360 and 2965 cm−1 reveal the existence of C–H. However, the typical carbonyl peak at 1730 cm−1 and epoxy peak at 1090 cm−1 are observed from A-GQDs while there are no distinct corresponding peaks on the O-GQDs. Moreover, from the FTIR spectra of O-GQDs, an obvious absorption peak centered at 1647 cm−1 can be observed, which is caused by CvC stretching, but this peak is unobvious as seen in A-GQDs. Under microwave treatment, the precursor molecules were dehydrated to form CvC which acts as the elementary unit of the GQDs core. These results demonstrate that the main difference between A- and O-GQDs is the content and type of the oxygen-related groups along their surfaces. To further probe the differences of chemical compositions of the two kinds of GQDs, X-ray photoelectron spectroscopy (XPS) characterization was carried out. As shown in Fig. 3b, the XPS results show a dominant graphitic C1s peak at 284.6 eV and O1s peak at 532 eV for A- and O-GQDs. The oxygen content in A- and O-GQDs is 30.8% and 8.8%, respectively. The comparison of the high-resolution spectra of C1s (Fig. 3c and 3d) demonstrates the obvious disparities in chemical compositions for A- and O-GQDs. A-GQDs show a dominant sp2 C1s peak at 284.5 with a fraction of 46.90%, sp3 carbons including a hydroxyl peak at 286.6 eV (45.03%) and a carbonyl peak at 288.3 eV (8.07%), as shown in Fig. 3c. In contrast to A-GQDs, O-GQDs only display a major peak at 284.5 eV (90.92%) which is derived from sp2 carbon. Besides, a very

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small peak at 286.5 eV (9.08%) can still be observed. According to ref. 11, this small peak belongs to the residual negligible oxygenous peaks, as shown in Fig. 3d. The above results confirm that the structure of O-GQD is just like a smooth core mostly consisting of sp2 carbons (CvC) with very few oxygenous groups on the surface. This kind of GQDs may be formed in the complicated reactions during the whole process and only took a very small proportion, but the organic solvent filtered them out. It is a method taking advantage of the solubility disparities of GQDs in different solvents. The surface components of the A- and O-GQDs as determined by the XPS are in good agreement with FTIR results. They suggest that the as-prepared O-GQDs have a higher crystalline structure and less amounts of oxygenous defects compared with A-GQDs. Furthermore, the proportion of sp2 hybridized carbon in O-GQDs is almost the same level as that in pristine GQDs exfoliated from graphite nanoparticles.11 It can be anticipated that “pristine” GQDs can also be acquired by this separation method. As analyzed in the literature,11 for GQD, a “defectfree” structure is an extraordinarily significant character to understand the relationship between the optical PL origin and their surface states, which will be discussed later. Raman spectroscopy was also performed to confirm the quality of the as-prepared GQDs. As shown in Fig. 4a, both of them show a typical Raman spectrum of carbon nanoparticles with broad peaks.7,11,13,28–30 Besides, A-GQDs show a disordered (D) band at 1358 cm−1 and a crystalline (G) band at 1600 cm−1, and the intensity ratio of the D to the G band (ID/IG) is 0.892. On the other hand, unlike the A-GQDs with a relatively more intense D-band, the ID/IG value for O-GQDs is only 0.837. Fig. 4b shows the typical X-ray diffraction (XRD) profiles for A- and O-GQDs. The 2θ diffraction peaks of A-GQDs and O-GQDs centered at 19.2° and 20.2° correspond to a 002 peak of the graphitic structure. The peaks are broad but not sharp because of their nanometer size (1–5 nm),2,13 which is a difference often shown between the nanoparticles and the bulk materials. The calculated d spacing for A- and O-GQDs is 4.63 Å and 4.39 Å respectively. They are in good agreement with the reported values (3.40–4.81 Å) of GQDs prepared by other researchers.2,7,10,13 The difference in d spacing between them has also resulted from the disparity of surface functional groups. In addition, both A- and O-GQDs show a d spacing which is broader than that of graphite (3.35 Å) but smaller than that of graphene oxide (GO, 8.49 Å).31,32 The results may be mainly attributed to the oxygen-containing groups that were introduced during the formation process, which enhanced the interlayer distance. However, compared with GO, GQDs were only oxidized on the edges due to the very small size.13 Raman and XRD spectra further demonstrate that GQDs with higher crystallinity were acquired by chloroform through this solventinduced separation. A unique structure will lead to the variation of the optical properties of O-GQDs. To further study the novel properties of O-GQDs and clarify the responsible PL mechanisms, several related tests, including UV-visible (UV-vis) absorbance, PL

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Fig. 3 (a) FTIR spectra of A- and O-GQDs. (b) XPS survey spectrum of A- and O-GQDs. (c) High-resolution XPS C 1s spectra of A-GQDs. (d) The XPS C 1s spectra of O-GQD.

emission, time-resolved PL (TRPL) and PL excitation (PLE) spectroscopy, were performed. As shown in Fig. 5a, the A-GQDs exhibit two deep UV absorption peaks centered at 220 and 269 nm. The absorption peaks of the A-GQD samples are similar to that of the GQDs prepared from the microwaveassisted hydrothermal method (228 and 282 nm).2,25 These peaks are attributed to π electron transition in these oxygencontaining GQDs. The 220 nm peak has resulted from π→π* transition of CvC and the 269 nm peak is caused by n→π* transition of the CvO bond.2,33 For the O-GQDs, a typical absorption peak at 240 nm is observed, which is assigned to the π→π* transition of aromatic sp2 domains.34,35 Notably, apart from the above-mentioned strong π→π* absorption peak, only a weak shoulder at 275 nm is observed, which is parallel to the previously reported highly crystalline GQDs,12,13,15 indicating an unobvious n→π* transition. The PL of GQD solutions is excitation wavelength (λex) dependent as shown in Fig. S2a and S2b.† With the excitation wavelength increased from 300 to 450 nm, the PL peak of A-GQDs shifts to longer wavelengths with a maximum peak at

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490 nm when excited at 375 nm. Similarly, the emission peak of O-GQDs shows an obvious red shift accompanied by the increase of the excitation wavelength. The maximum emission intensity of O-GQDs is achieved at 440 nm. The luminescence decay profiles of the two kinds of GQDs are shown in Fig. 5b. The decay was recorded for the GQD transitions at 490 nm for green and 440 nm for blue emission excited at 375 nm and 366 nm, respectively. Both samples show bi-exponential decay curves with fast and slow decay components. The lifetimes of A-GQDs are τ1 = 3.0 ns and τ2 = 9.2 ns, whereas for O-GQDs, lifetimes τ1 = 1.9 ns and τ2 = 6.6 ns were observed. Generally, the emission that originates from defect states shows a longer recombination lifetime than that from intrinsic states.11,36,37 So the two components probably respectively correspond to recombination from the intrinsic state and the defect states. Moreover, since the defects are in varying proportions in the two kinds of GQDs, they show some difference in the observed lifetimes. The PLE spectra recorded with the strongest luminescence as well as the corresponding PL spectra are shown in Fig. 6a

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Fig. 5 Optical properties of GQDs. (a) UV-vis absorbance of A- and O-GQDs. (b) TRPL spectra of A- and O-GQDs.

Fig. 4 Raman and XRD data for GQDs. (a) Raman spectra of A- and O-GQDs. (b) XRD pattern of A- and O-GQDs.

and 6b. The detection emission wavelength (λde) for A- and O-GQDs is 490 nm and 440 nm, respectively. Unlike A-GQDs with a single peak centered at 375 nm, O-GQDs show two sharp peaks at 275 and 366 nm, which could be correlated with the two new transitions rather than the commonly observed π→π* transition.10,13 The PLE spectra clearly demonstrate that the observed luminescence of the two GQDs may have different origins, as will be discussed next. Since Zhu et al. explained the PL mechanism of GOQDs using the defect state emission, which involves the presence of oxygenous functional groups on the surface, while that of rGOQD is derived from the intrinsic state emission caused by the quantum size effect or by zigzag sites,30 it can be inferred that the photoluminescent properties of A-GQDs were attributed to the defect states caused by the oxygen-related functional groups located on the surface.25 In this way, the absorption peaks at 220 and 269 nm are caused by the electron transitions of π→π* of CvC and n→π* of the CvO bond respectively; the excited electrons may emit light by means of radiative recombination that induces PL.2,25 The excitation wavelength

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dependent PL is also attributed to the surface functional groups which have various energy levels,2 as illustrated in Fig. 7a. Nevertheless, due to the lack of oxygenous groups on O-GQDs, the fluorescence of the O-GQDs may mainly originate from the emissive free zigzag sites with a carbene-like triplet ground state.10,30 Prof. Hoffmann determined that for a triplet ground state, the energy difference (δE) between the σ and π orbitals should be below 1.5 eV.38 The two electronic transitions of 366 nm (3.39 eV) and 275 nm (4.50 eV) observed in the PLE spectra (Fig. 6b) can be regarded as transitions from the σ and π orbitals (highest occupied molecular orbitals, HOMOs) to the lowest unoccupied molecular orbital (LUMO), as demonstrated in Fig. 7b. The δE is calculated to be 1.11 eV, within the critical value (35 under a bias voltage of 0.2 V. Further

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Fig. 9 (a) Transient photocurrent responses of the device at a light intensity of 230 mW cm−2 and a bias voltage of 0.2 V. (b) I–V characteristics of the device as a function of light intensity ranging from 110 to 340 mW cm−2. Fig. 8 (a) Illustration of the O-GQDs/P3HT device (1 : 1 in weight). (b) Energy-level diagram of an O-GQDs/P3HT device.

experiments show that the photocurrent is sensitive to the intensity of the incident light, as shown in Fig. 9b. The I–V curve displays a nonlinear behavior and the photocurrent was found to increase with increasing light intensity, which is a typical feature of the semiconductors. To better understand the role of O-GQDs in this system, a blank test was also performed. As shown in Fig. S5a,† for the device based on pure P3HT, due to the poor electron mobility in the polymer and the lack of interfaces for dissociation of photogenerated excitons, a low photocurrent was obtained.7 So, the change in photocurrent that originates from a pure P3HT device is relatively low. However, in the P3HT/O-GQD composite device, the O-GQDs provide a large surface area for the formation of p–n interfaces.7 To further show the effect of O-GQD intuitively, a contrastive test employing PCBM as the photoelectron acceptor was also performed. All the parameters of the P3HT/PCBM device and the testing conditions were completely the same as those of the P3HT/O-GQD device.

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As to this device, an on/off photoresponse performance was observed (Fig. S5b†). In addition, the result showed that although there was still a gap in the performance of photosensitivity for O-GQD compared with PCBM, the photoresponse and photocurrent of the O-GQD-device and the PCBM-device were of the same order of magnitude, despite the absence of device optimization in this primary study. It can be inferred that the O-GQD can be applied as an electron-acceptor material, which may be a suitable substitute for PCBM. This is of great value to the fabrication of PV devices since O-GQD is much easier to prepare compared with PCBM and the cost is really low.

4.

Conclusions

In summary, we have developed a facile solvent-induced interface separation approach for the preparation of high-quality organic phase GQDs. Thorough characterization indicates that the new-style O-GQDs are of uniform distribution and high crystallinity with a sp2 carbon dominant structure, and they

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have much fewer oxygenous defects compared to GQDs from bottom-up synthesis. By selecting and separating GQDs with different oxygenous contents, we have provided a good platform to study the relationship between the PL origin and the structural composition, which is significant to clarify the PL mechanism of this fluorescent carbon material. Furthermore, the application of O-GQDs with this particular structure is also probed; a preliminary examination of O-GQDs doped with P3HT shows that they can effectively function as an electron acceptor material. The performance of O-GQDs is comparable to that of PCBM when applied to the same P3HT-based system. This work highlights that these organic-soluble GQDs hold great promise for a wide range of potential applications in the PV field. What’s more, O-GQD may become a feasible substitution for PCBM by further optimization.

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Acknowledgements This work was financially supported by the National Natural Science Foundation of China (grant no. 51173127, 51103094, 51273144, 51373116, 51473116 and 51411140036).

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