Electrostatic fabrication of RGO-g-SSS/CdTe graphene/quantum dot nanocomposites with enhanced optoelectronic properties

Abstract

We report a simple and facile strategy for fabricating RGO-g-SSS/CdTe graphene/quantum dot nanocomposites via electrostatic interactions at room temperature. Firstly, sodium p-styrenesulfonate (SSS) was successfully grafted on the reduced graphene oxide (RGO) surface via in situ free radical polymerization. Then, through the electrostatic interactions between positively charged amino groups of CdTe QDs and negatively charged sulfonic acid groups of the RGO-g-SSS surface, we have successfully fabricated RGO-g-SSS/CdTe nanocomposites. We have systematically analysed the structures and photoluminescence behaviors of RGO-g-SSS/CdTe nanocomposites. Finally, by virtue of their special electronic properties, we extended the as-prepared RGO-g-SSS composites as an excellent conductive scaffold to employ in quantum dot sensitized solar cells. These RGO-g-SSS/CdTe hybrids present enhanced optoelectronic properties compared with the pure CdTe QDs, especially in power conversion efficiency, indicating an improvement of 56%.

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1. Introduction

Graphene–quantum dot (QD) nanocomposites have triggered considerable research interests since their potential applications in solar energy conversion, optoelectronic devices, catalysis and sensing.^1–4 Graphene–QD nanocomposites not only inherit the high speed electron transport property of the intrinsic graphene, but also possess the quantum size effect originating from the special structure of QDs.^5,6 To date, a wide range of methods, including the phase-transfer method, solvothermal method, hydrothermal method and electrochemical template method have been proposed for graphene–QD nanocomposites.^7–10 Although various methods have been explored, reports concerning the electrostatic compound strategies of graphene–QD nanocomposites are relatively scarce.⁸

Besides, there also is a great challenge to directly connect the graphene with QDs due to the poor dispersibility and aggregation of graphene in common solvents, which greatly hindered their applications. This obstacle could be overcome through noncovalent or covalent functionalization. Many kinds of aromatic molecules and polymers have been reported to functionalize graphene by “graft on”, “graft from” or in situ polycondensation strategies.^11–13 The functionalization of graphene provided an effective way to endow graphene with tailorable optoelectronic properties, to improve their solubility property and to afford targets for the assembly of QDs. Gao's group have reported a general, facile approach to 2D molecular brushes by conventional free radical polymerization (FRP) using graphene and graphene oxide (GO) nanosheets as flat macromolecular backbones.¹⁴ Nutt's group have demonstrated the ability to systematically tune the grafting density and chain length of polystyrene polymer molecules covalently bonded to single-layer graphene nanosheets by combining diazonium addition and atomic transfer radical polymerization.¹⁵

Herein, we presented a simple and facile method to synthesize RGO-g-SSS/CdTe graphene/QDs nanocomposites via electrostatic interactions. Firstly, polyelectrolyte sodium p-styrenesulfonate (SSS) was successfully grafted on the reduced graphene oxide (RGO) surface via the in situ free radical polymerization. Remarkably, the graft and reduction of graphene oxide were simultaneously carried out in this work. After RGO nanosheets were modified with SSS, the obtained RGO-g-SSS composites were hydrophilic and soluble in the presence of a static repulsion force. Then we attached the positively charged cysteamine capped CdTe QDs onto the negatively charged RGO-g-SSS nanosheets via one step ultrasound at room temperature. Their structural changes have been investigated by SEM, TEM and Raman spectroscopy. We also have analysed the decays of the fluorescence of QDs in RGO-g-SSS/CdTe nanocomposite by steady state and time-resolved fluorescence spectroscopy. Finally, we extended the as-prepared RGO-g-SSS composites as an excellent conductive scaffold to employ in quantum-dot-sensitized solar cells (QDSSCs).

2. Experimental

2.1 Materials

Cadmium chloride (CdCl₂·2.5H₂O), 2-aminoethanethiol (AET), tellurium powder, sodium borohydride (NaBH₄), sodium p-styrenesulfonate (SSS), N,N-dimethylformamide (DMF) and hydrazine hydrate (80%) were purchased from Aldrich and used as received. Azobisisobutyronitrile (AIBN) was employed after twice recrystallization. Graphite (120 mesh) and hydrogen peroxide (H₂O₂) (AR, 30 wt% in water) were received from Aladdin Industrial Corporation and used without any further purification. The water used in this study was deionized water with an electrical resistance of 18.2 MΩ cm⁻¹.

2.2 Synthesis of amino-modified CdTe QDs

Briefly, 1.8 mmol of NaBH₄ was initially added into a 5 mL vial, and then 3 mL of water was transferred, followed by the addition of 0.75 mmol of tellurium powder. The vial equipped with a small outlet was placed in an ice water bath for a certain time to obtain a NaHTe solution. Then, the CdTe precursor solution was prepared by adding oxygen-free NaHTe solution to a N₂ deaerated CdCl₂ solution at pH of 6 in the presence of AET ([Cd] = 0.015 mol L⁻¹, [AET] = 0.0225 mol L⁻¹). The reaction was carried out at the temperature of 95 °C for 1 h. CdTe QDs were precipitated by the addition of absolute ethanol. By repeated washing and drying, the CdTe powders for further characterization were obtained. Finally, the CdTe QDs solution was obtained by redispersion of the CdTe powders into deionized water.

2.3 Synthesis of RGO-g-SSS composites

RGO-g-SSS composites were prepared via the in situ free radical polymerization. In a typical procedure, 2 g of SSS monomer was added to 25 mg of GO dispersed in 40 mL DMF and sonicated (40 kHz) for 30 min to obtain a homogeneous solution. Then hydrazine hydrate (0.5 mL) and 15 mg AIBN were added. The solution was purged with N₂ for 30 min and then reacted in a 65 °C oil bath for 24 h. Finally, the black resultant solution was precipitated in methanol, centrifuged and washed with DMF (at least five times), and then redispersed in deionized water ready for use. The obtained RGO-g-SSS aqueous dispersion possessed good stability due to electrostatic repulsion, and there was no sign of sediment of graphene sheets after more than 2 months.

2.4 Preparation of RGO-g-SSS/CdTe nanocomposites

The cysteamine capped CdTe QDs were added to the RGO-g-SSS stock dispersion with concentrations between 0.05 and 0.4 mg mL⁻¹, along with adjustment of the pH value to 5. The mixture was sonicated (40 kHz) for 30 min, then the positively charged CdTe QDs were attached with negatively charged RGO-g-SSS sheet surface. The obtained RGO-g-SSS/CdTe nanocomposites were centrifuged at the speed of 12 000 rpm for 30 min and washed with water for at least three times to remove RGO-g-SSS and CdTe QDs that did not electrostatically attach. Then the resulting RGO-g-SSS/CdTe nanocomposites were dispersed in DMF ready for further characterization.

2.5 Characterizations

2.5.1 Measurement method for structure and composition. RGO-g-SSS and RGO-g-SSS/CdTe nanocomposites for atomic force microscopy (AFM) were prepared by drop-casting the suspensions onto the freshly cleaved mica. Imaging was accomplished under ambient conditions with Bruker Dimension Icon scanning probe microscope in the tapping mode of operation. The morphology of the cross section of the resultant RGO-g-SSS was investigated by SEM with a QUANTA 200 (Philips-FEI, Holland) at 30.0 kV. Low-resolution Transmission Electron Microscopy (TEM) images were recorded on a JEOL JEM 1011 microscope operating at 100 kV. The high-resolution transmission electron microscopy (HRTEM) observations were performed on a JEOL JEM-2010UHR instrument at an acceleration voltage of 200 kV. The particle diameter was estimated by using Image software analysis of the TEM micrographs. Raman spectra were recorded by the single scan generated by the Horiba HR 800 Raman system equipped with a 514.5 nm laser. The XRD patterns were recorded on a BrukerD8 Advance X-ray diffractometer (40 kV, 25 mA, Cu Kα radiation, λ = 1.5418 Å) at room temperature. The data were collected in the range of 5° < 2θ < 60° with the scan rate of 2° min⁻¹ and step width of 0.02°. XPS spectra of the RGO-g-SSS/CdTe nanocomposites were collected on an ES-CAIAB250 XPS system with Al/Kα as the source, and the energy step size was set as 0.100 eV. Fourier-transform infrared (FT-IR) analysis was performed using a Nicolet-6700 spectrometer from Thermo Electron at room temperature. Selected area energy dispersive X-ray analysis (SAEDX) were performed with an energy-dispersive X-ray spectrometer attached to the S-4800 scanning electron microscope.

2.5.2 Measurement of optical properties. UV-vis absorption spectra were recorded by a UV-vis spectrometer (Lambda 950, Perkin-Elmer). Photoluminescence (PL) measurements were carried out on a Varian Cary Eclipse spectrophotometer. Fluorescence decay time was measured based on the Leica SP5 FLIM system using a 405 nm laser as the excitation source.

2.5.3 Photovoltaic measurement. Photocurrent–voltage measurements were performed using a Keithley 2420 source meter (Keithley, USA). A 450 W xenon lamp (Oriel, USA) was used as the light source and the light intensity was AM 1.5 G one sun (100 mW cm⁻²) calibrated with a standard Si solar cell. The active area of the tested solar cells was 0.16 cm².

3. Results and discussion

3.1 Characterization of RGO-g-SSS

Free radical polymerization (FRP) had been successfully used to graft polymer chains with vinyl groups on graphene oxide or graphene surface by macromolecular radical addition to the reactive carbon–carbon double bonds.^14,16 In this paper, we choose GO as backbones and polyelectrolyte SSS with hydrophilic sulfonic acid groups as vinyl monomer. After initiating of SSS vinyl monomers by FRP, macromolecular radicals are formed immediately and added to double bonds of graphene oxide, and resulting SSS polymer chains were directly grafted onto the graphitic surface. The ungrafted free polymers were removed from the resulting RGO-g-SSS samples by repeated washing and centrifugation. Notably, as illustrated in Scheme 1a, the concurrent reduction process of GO to reduced graphene oxide (RGO) was also carried out during the polymerization for removal of oxygen functionalities of aromatic graphene network. After RGO sheets were modified with negatively charged SSS, the obtained sheets were hydrophilic (Fig. S1c ESI†) and soluble in the presence of a static repulsion force (Fig. S1d ESI†).

Scheme 1 Schematic representation of preparation procedure of RGO-g-SSS/CdTe nanocomposites, including the graft and reduction of graphite oxide (GO) (a) and the preparation of RGO-g-SSS/CdTe nanocomposites via electrostatic interactions (b).

The typical sheet structure and morphology of the RGO-g-SSS composites were observed and confirmed by SEM, TEM and AFM measurements. Well-packed layers are observed for the RGO-g-SSS composites at a low magnification in SEM image (Fig. 1a and b), validating the successful synthesis of 2D molecular brushes.¹⁴ From TEM observation (Fig. 1c), we can find that the flat surface is coated with a rough polymer layer.¹⁷ The representative AFM measurement shows that the average thickness of a RGO-g-SSS nanosheet is 3 nm (Fig. 1d and S2 ESI†). This implies that the GO nanosheet (1.05 nm in thickness) was covered uniformly by the SSS on bilateral sides. These results indicate that the SSS polymer chains have been successfully grafted on RGO sheets through the radical polymerization approach.

Fig. 1 (a, b) SEM, (c) TEM and (d) AFM patterns of the RGO-g-SSS composites.

3.2 Characterization of RGO-g-SSS/CdTe nanocomposites

The charge of the RGO-g-SSS surface and CdTe QDs was confirmed by the zeta potential value. The zeta potential value of RGO-g-SSS is −40.3 mV, indicating high negative charged surface of RGO (Fig. S3a ESI†). On the other hand, the zeta potential value of amino capped CdTe QDs is +44.2 mV (Fig. S3b ESI†), indicating the positive charge of QDs. As shown in the Scheme 1b, the negatively charged polyelectrolyte SSS with sulfonic acid groups acted as a primer on the graphene surface for the subsequent homogeneous adsorption of amino-modified CdTe QDs through electrostatic interactions. It is clearly seen from AFM image (Fig. 2a and S4 ESI†) that the average thickness of a RGO-g-SSS/CdTe nanosheet is 8.87 nm, which implies that the RGO-g-SSS nanosheet (3 nm in thickness) was covered uniformly by the QDs (3.2 nm in size). Transmission electron microscopy (TEM) images give direct evidence of CdTe QDs on graphene sheet. Fig. 2b shows that the RGO-g-SSS/CdTe nanocomposites consisted of 2D graphene sheet decorated with a large number of CdTe QDs. And wrinkles of the single-layer RGO-g-SSS/CdTe nanocomposites sheet are also observed. High resolution TEM (HRTEM) images (Fig. 2c) show that the CdTe nanoparticles are well spread out on the RGO-g-SSS sheet.¹⁸ Fig. 2d clearly shows well-resolved lattice fringes for an individual CdTe nanocrystal, indicating an excellent crystalline structure, meaning the QDs are well preserved in the RGO-g-SSS sheet. The above results clearly demonstrate the excellent distribution of QDs on graphene sheets, which guarantees the good optoelectronic property of RGO-g-SSS/CdTe nanocomposites.

Fig. 2 (a) AFM, (b–c) TEM and (d) HRTEM patterns of the RGO-g-SSS/CdTe nanocomposites.

To confirm the electrostatic interactions between the CdTe QDs and RGO-g-SSS, the FT-IR spectra of the CdTe, RGO-g-SSS and RGO-g-SSS/CdTe nanocomposites are shown in Fig. 3a, respectively. As depicted in the FT-IR spectrum of amino-stabilized CdTe QDs, strong absorption peaks at 1576 cm⁻¹ (N–H) and 2921 cm⁻¹ (C–H) are indicative of the presence of amino groups on the surfaces of CdTe QDs, and there is no characteristic peak of the mercapto group ranging from 2500 to 2600 cm⁻¹, suggesting the formation of robust bonding between Cd²⁺ and AET. The characteristic peaks of RGO-g-SSS at 1237, 1176 and 1032 cm⁻¹ are assigned to unit of sulfonic acid. In the FT-IR spectrum of RGO-g-SSS/CdTe nanocomposites, the characteristic absorption peaks at 1597 cm⁻¹, 1165 and 1020 cm⁻¹ are all observed, supporting that the CdTe nanoparticles have been successfully grafted onto the surface of RGO-g-SSS sheet via electrostatic interactions.^8,19 The composition of RGO-g-SSS/CdTe nanocomposites was further confirmed by means of selected area energy dispersive X-ray analysis (SAEDX), as shown in Fig. 3b. The strong signals from C, O, S, Te and Cd elements indicated the presence of QDs and RGO-g-SSS.

Fig. 3 (a) FT-IR spectra, (c) Raman spectra and (d) XRD patterns of the GO, RGO-g-SSS and RGO-g-SSS/CdTe composites. (b) The EDX analysis of RGO-g-SSS/CdTe nanocomposites.

The structural change of RGO-g-SSS composites due to attachment of QDs has been investigated by Raman spectroscopy. As shown in Fig. 3c, Raman spectrum of GO sheets displays two prominent peaks at around 1589 cm⁻¹ and 1352 cm⁻¹, which correspond to the G and D bands, respectively. A general feature of the reduction of GO is that the G band shifts to lower wavenumber. The Raman spectrum of RGO-g-SSS synthesized from GO also contains the G and D bands, but the G band peak of hydrazine-reduced graphene band shifts to 1585 cm⁻¹, indicating the reduction of GO. It's well in agreement with most reports on the reduction of GO. And the G band peak of RGO-g-SSS/CdTe nanocomposites synthesized from hydrazine-reduced graphene locates at 1579 cm⁻¹. The shifting of G band to the lower frequencies also confirmed the electron transfer from QDs to graphene.^20,21 In Fig. 3d, the XRD peak at around 10.48° refers to a symbol of GO (002) plane. The peak of 002 plane shift to 21°, indicating the reduction of GO in RGO-g-SSS. XRD peaks of CdTe structure in RGO-g-SSS/CdTe show scattering angle of 24°, 39° and 46°, corresponding to crystal planes (111), (220) and (311) of CdTe, respectively.^21–23 The results suggest that CdTe QDs decorated on graphene sheets are in blende form. Meanwhile, the broad diffraction peaks in the XRD patterns indicate the small particle size of the CdTe QDs. Also the size of the CdTe QDs in the hybrids can be calculated as approximately 4 nm from the major diffraction peak (111) (2θ = 24°) by Scherrer's equation,²⁴ which is in good agreement with the result of TEM measurement.

X-ray photoelectron spectroscopy (XPS) findings were also used to investigate the surfaces of these samples, which further confirm the presence of N–H, C–S, C–N and S–O groups. The full spectrum presented in Fig. 4a indicates that the RGO-g-SSS/CdTe nanocomposites are mainly composed of Te, Cd, S, C, O and N. In the high-resolution spectra (Fig. 4b), the C 1s band can be deconvoluted into two peaks, corresponding to sp² carbons (C–C, 284.5 eV) and sp³ carbons (C–S/C–N, 285.5 eV). This evidently demonstrates the removal of hydroxyl and the epoxy groups from the GO, correlating well to the FT-IR spectra, which reveals the reduction of GO. The high-resolution spectra of N1s reveal the presence of N–H (400.8 eV) (Fig. 4c), and the high-resolution spectra of S2p (Fig. 4d) reveal the presence of S=O and S–O units of RGO-g-SSS.^18,25 All these facts imply the graft reaction and reduction process of GO in the presence of SSS and hydrazine hydrate, and the electrostatic interaction between CdTe QDs and RGO-g-SSS sheet.

Fig. 4 (a) XPS spectra of the RGO-g-SSS/CdTe. High-resolution C 1 s (b), N 1 s (c), and S 2p (d) peaks of the RGO-g-SSS/CdTe nanocomposites.

3.3 Photoluminescence behaviours of the RGO-g-SSS/CdTe nanocomposites

Fig. 5 displays the absorption and photoluminescence (PL) spectra of CdTe QDs and RGO-g-SSS/CdTe nanocomposites. The absorption maximum and PL emission peak of these amino-modified CdTe QDs at 550 nm and 610 nm, respectively. After depositing the CdTe QDs on the RGO-g-SSS sheets, there is no obvious change of the absorption peak of the QDs in the RGO-g-SSS/CdTe hybrids, which indicates that a dense coating and well dispersion of QDs on the graphene sheets, corresponding to the TEM observations. The PL emission spectrum of RGO-g-SSS/CdTe nanocomposites (Fig. 5b) shows a dramatic decrease in intensity compared with that of pure QDs. Due to nonradiative relaxation of exciton, the PL quenching of QDs occurs during their interaction with graphene by energy/electron transfer process.²⁶ The PL quenching of the QDs is observed with increase of RGO-g-SSS concentration. As shown in Fig. 5c, by changing the concentration of RGO-g-SSS from 0.05 to 0.4 mg mL⁻¹, the photoluminescence quenching of QDs varies from 20% to 80%.

Fig. 5 (a) Absorption spectra and (b) emission spectra of the CdTe QDs solution, the RGO-g-SSS/CdTe and supernatant of RGO-g-SSS/CdTe. (c) Emission spectra of the pure CdTe QDs and after addition of 0.05, 0.066, 0.1, 0.2 and 0.4 mg mL⁻¹ concentration of RGO-g-SSS. (d) Decay curves of CdTe QDs fluorescence with gradual increase of RGO-g-SSS concentration 0.05, 0.1, 0.2 mg mL⁻¹ concentration of RGO-g-SSS and the solid lines are the fitted decay curves by a triexponential decay.

To further investigate the energy/electron transfer process from QDs to RGO-g-SSS, a time-correlated single-photon counting (TCSPC) methodology is also conducted for measuring their exciton lifetimes. Fig. 5d shows the time-resolved fluorescence decay curves of CdTe QDs without and with changing the concentration of RGO-g-SSS. The values are 15.23, 14.48, 12.69 and 9.75 ns for pure QDs and QDs with 0.05, 0.1, 0.2 mg mL⁻¹ RGO-g-SSS, respectively. It is clearly seen that the decay times of the QDs decrease gradually with the increasing concentration of RGO-g-SSS, which is in accordance with the PL quenching. The corresponding decay time parameters are given in Table 1. Therefore, the PL quenching and the shortened decay time further definitely indicate their electronic interactions.^27,28

Table 1 Decay time parameters of CdTe QDs with gradual increasing concentration of RGO-g-SSS

c (mg mL^−1)	τ1 (ns)	α1 (%)	τ2 (ns)	α2 (%)	χ^2	[small tau, Greek, macron]
0	3.87	58.7	31.36	41.3	1.08	15.23
0.05	3.65	48.4	24.63	51.6	1.17	14.48
0.1	3.30	55.3	24.29	44.7	1.22	12.69
0.2	2.29	60.8	21.33	39.2	1.10	9.75
0.4	1.83	77.4	21.59	22.6	1.08	6.30

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3.4 RGO-g-SSS/CdTe nanocomposites as sensitizers in QDSSCs

Quantum-dot-sensitized solar cells (QDSSCs) have attracted considerable attention because of the extraordinary optical and electrical properties of QDs, and been regarded as a promising alternative to conventional solid-state semiconductor solar cells. The most important issue for improving the photovoltaic properties of QDSSCs is to increase effectively generation of carrier, separation and transport of electrons and holes. Graphene with special electronic properties and structures could be as an excellent conductive scaffolds to capture and transport electrons from the excited QDs and also effectively separate the electron–hole pair.^29,30

Fig. 6b presents the J–V curves of CdTe QDs and RGO-g-SSS/CdTe (graphene/QDs) sensitized solar cells, respectively. The open circuit voltage (V_oc), short circuit current density (J_sc), fill factor (FF), and overall power conversion efficiencies (η) of those QDSSCs are summarized in Table 2. It is evident that the graphene treatment during the process resulted in the increase of both J_sc and V_oc, and thus led to enhanced performance. The QDSSCs made of graphene/QDs nanocomposites showed a J_sc of 0.473 mA cm⁻², V_oc of 0.766 V and η of 0.209%, whereas the electrode made of CdTe QDs achieved a J_sc of 0.408 mA cm⁻², V_oc of 0.601 V and η of 0.134%, indicating an improvement of 56% in the power conversion efficiency compared to the CdTe QDSSCs.³¹ The increase of J_sc is mainly attributed to more effective generation of photoexcited electrons, while the improvement of V_oc and FF can be explained by the reduced charge recombination for the CdTe QDs treated with graphene.

Fig. 6 (a) Schematic presentation of the graphene/QDs based solar cell, (b) J–V characteristics of the CdTe QDs and graphene/QDs based solar cells measured under one-sun illumination (AM 1.5 G, 100 MW cm⁻²), (c) Nyquist plots of the CdTe QDs and graphene/QDs nanocomposites sensitized solar cells, respectively.

Table 2 Results obtained from the photocurrent-voltage (J–V) measurements of two QDSSCs

Sample	Jsc (mA cm^−2)	Voc (V)	FF (%)	η (%)
CdTe	0.408	0.601	54	0.134
RGO-g-SSS/CdTe	0.473	0.766	58	0.209

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To further evaluate the charge recombination processes, electrochemical impedance spectroscopy (EIS) measurement was performed. Fig. 6c shows the impedance spectra of CdTe and graphene/CdTe sensitized QDSSCs. As the same counter electrode and electrolyte are used, there is no apparent differences of the electron ejection and transport at the electrode/electrolyte interface (small semicircle) in these QDSSCs. The bigger semicircle corresponded to the electron transfer at the TiO₂/QDs/electrolyte interface and transport in the TiO₂ film.^32–34 As shown in Fig. 6c, the radius of semicircle of graphene/CdTe sensitized QDSSCs is bigger than that of the pure CdTe QDSSCs, as a result, the charge transfer resistance for the electron transfer process at the TiO₂/graphene/CdTe/electrolyte interface is larger than that of the pure CdTe QDSSCs, demonstrating the reduced interfacial recombination and contributing to the improvement in J_sc and FF.

4. Conclusions

We have synthesized sodium p-styrenesulfonate (SSS)-protected reduced graphene oxide (RGO) via free radical polymerization. The obtained RGO-g-SSS sheets have better dispersion property and stability in the presence of a static repulsion force. Cysteamine capped CdTe QDs were successfully attached onto RGO-g-SSS sheets through electrostatic interactions. The changes of structures and photoluminescence behaviors of RGO-g-SSS/CdTe nanocomposites due to electron transfer have been confirmed in this study. The as-prepared RGO-g-SSS/CdTe nanocomposites have been employed in quantum-dot-sensitized solar cells. The CdTe QDs treated with graphene can effectively separate the electron–hole pair and reduce the charge recombination, as a result, an improvement of 56% in the power conversion efficiency has been observed. This research displays the powerful utility of graphene in quantum-dot-sensitized solar cells studies, and provides a convenient and effective method for fabricating graphene/QDs nanocomposites.

Acknowledgements

This work was supported by National Natural Science Foundation of China (21076103 and 21474052), and Priority Academic Program Development of Jiangsu Higher Education Institutions (PAPD).

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Footnote

† Electronic supplementary information (ESI) available: Materials, experimental section, characterization and additional figures. See DOI: 10.1039/c6ra11104a

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