Journal of Colloid and Interface Science 419 (2014) 142–147
Contents lists available at ScienceDirect
Journal of Colloid and Interface Science www.elsevier.com/locate/jcis
Manipulating surface ligands of Copper Sulfide nanocrystals: Synthesis, characterization, and application to organic solar cells Jun Li a,b, Tonggang Jiu b,⇑, Guo-Hong Tao a, Guojie Wang b, Chunming Sun b, Pandeng Li b, Junfeng Fang b,⇑, Ling He a,⇑ a b
College of Chemistry, Sichuan University, Chengdu, Sichuan 610065, PR China Institute of New Energy Technology, Ningbo Institute of Material Technology and Engineering (NIMTE), Chinese Academy of Science (CAS), Ningbo, Zhejiang 315201, PR China
a r t i c l e
i n f o
Article history: Received 19 October 2013 Accepted 21 December 2013 Available online 31 December 2013 Keywords: CuS Nanocrystals Ligand-exchange Interfacial application
a b s t r a c t CuS NCs were synthesized via a facile sol–gel method without post-thermal treatment. The as-prepared CuS NCs were analyzed and confirmed by XRD, HR-TEM, EDS and XPS as hexagonal covellite CuS. The average diameter of the samples was about 3 nm with narrow size distribution. CuS NCs can form a thin and smooth film without ligand-exchange that can be used as hole transport layer in organic solar cell. These hydrophilic CuS NCs with pyridine ligands can be exchanged with OAm and OA rapidly at room temperature and present hydrophobic characteristic, resulting in forming oil-soluble CuS NCs. This makes it possible tuning the surface property of CuS NCs and has the potential application for different fields. Ó 2014 Elsevier Inc. All rights reserved.
1. Introduction Nanoscale semiconductors have attracted a tremendous interest due to their size- and shape-dependent properties compared with their bulk counterparts [1]. Since the size decreases, the energy gap (Eg) increases, resulting in a blueshift of the absorption wavelength which is known as quantum size effect. As a well-known p-type semiconductor, CuS has been intensively studied in numerous fields such as photovoltaic devices, nonlinear optical materials [2], room temperature ammonia gas sensors [3], catalysts [4]. Nowadays, various shapes and sizes of CuS nanocrystals (CuS NCs) have been synthesized to investigate their properties and to expand their applications [5–8]. Cao and his colleagues synthesized 200 nm CuS spheres with strong opticallimiting properties [9]. Chen and Hu synthesized 500–800 nm flower-like CuS as an efficient 980 nm laser-driven photothermal agent for ablation of cancer cells [10]. On the other hand, uniform small sized (less than 10 nm) CuS NCs have been rarely reported. Erkey and his coworkers synthesized CuS NCs in water-in-carbon dioxide microemulsions [11]. And Iwahori synthesized CuS NCs in the protein cage [12]. Most of the CuS NCs obtained in the reported literature were capped with long hydrocarbon molecules such as oleic acid (OA) and oleylamine (OAm). However, the existence of such bulky hydrophobic molecules created barrier around each NC which greatly impacts ⇑ Corresponding authors. Fax: +86 2885470368 (L. He). E-mail addresses: [email protected] (T. Jiu), [email protected] (J. Fang), [email protected] (L. He). 0021-9797/$ - see front matter Ó 2014 Elsevier Inc. All rights reserved. http://dx.doi.org/10.1016/j.jcis.2013.12.057
their electrical and catalytic properties [13]. Although these can be solved by a ligand-exchange procedure, exchange is generally incomplete with such weak ligands and allows NCs forming aggregates easily [14–16]. In recent years, considerable attention has been focused on heterojunction organic solar cell (OSCs) which is one of potential alternatives to expensive silicon solar cells, due to the advantages of their light weight, flexibility, low cost and cheaper solution based process [17,18]. As the most common material used as hole transport layer (HTL), poly(3,4-ethylenedioxythiophene): poly (styrenesulfonate) (PEDOT:PSS) can affect interfacial stability negatively on account of its high hygroscopicity and acidity [19]. Semiconductors such as MoO3, V2O5, NiO have been investigated to replace PEDOT:PSS to overcome these disadvantages [20–22]. There is little research on utilizing CuS NCs as hole transport layer, though various properties of CuS NCs have been widely explored. In our work, uniform small sized CuS NCs were prepared via a facile sol–gel method without using template and complicated post-treatment. Our method utilizes low temperature, no protective gas and cheap chemicals that make our method advantageous. Compared with common synthetic routes, the as-synthesized CuS NCs are capped with pyridine which is weak and hydrophilic ligand. Furthermore, it is easily exchanged with OA and OAm resulting in oleophilic surface property. This opens selectable pathways to study the surface properties of CuS NCs for numerous fields such as biological and catalytic applications. More interestingly herein we find that their potential capacity applied as photovoltaic interfacial materials by fabricating polymer photovoltaics in the configuration of ITO/CuS/P3HT:PC61BM/Ca/Al.
J. Li et al. / Journal of Colloid and Interface Science 419 (2014) 142–147
2. Materials and methods 2.1. Chemicals The following chemicals were purchased and used as received. Copper acetate monohydrate (analytical reagent (AR)), pyridine (AR), sodium sulfide nonahydrate (AR), acetone (AR), and DMSO (AR) were obtained from Sinopharm Chemical Reagent. OA (85%) and OAm (90%) were purchased from Aladin and 1, 2-dichlorobenzene (AR) was purchased from Sigma–Aldrich. P3HT and PC61BM were both obtained from Luminescence Technology Corp.
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over the spectral range of 300–1500 nm. X-ray photoelectron spectroscopy (XPS) and ultraviolet photoelectron spectroscopy (UPS) were performed using a Kratos AXIS ULTRADLD UPS/XPS system (Kratos analytical, Manchester, UK). Raman spectra were measured using a Renishaw in Via-reflex spectrometer with the 532 nm line of the Ar ion laser as an excitation source in a quasi-backscattering configuration. The current density–voltage characteristics of the photovoltaic devices were recorded using a computer-controlled Keithley 2400 source meter under air mass (AM) 1.5G simulated solar light. 3. Results and discussion
2.2. Synthesis and Purification of CuS NCs In a typical experiment, 5.0 mmol of copper acetate monohydrate was dissolved into 40 mL deionized water/pyridine mixture (VDI-water:Vpy = 1:3) and heated to 80 °C; 5.0 mmol of sodium sulfide nonahydrate was dissolved into 20 mL of deionized water. Under vigorous magnetic stirring, sodium sulfide solution was dropped into copper acetate solution in 20 min. The mixture reacted for 2 h at 80 °C. At last, the reaction product was collected and washed twice followed by precipitation with acetone and redispersion in deionized water or other organic solvents (e.g., pyridine, DMSO). 2.3. Ligand-exchange experiments of pyridine coated CuS NCs 2.3.1. Ligand-exchange with OAm In a typical process, 2 mL DMSO solution of CuS NCs (20 mg/ mL) was added with 2 mL OAm at room temperature. The resulting mixture was shaken gently for 60 s and stood for 2 min at room temperature. Then CuS NCs were moved to OAm phase which indicated that ligand-exchange is completed. The OAm capped CuS NCs were collected by precipitation with ethanol and redispersion in hexane. 2.3.2. Ligand-exchange with OA In a similar process, 40 mg of CuS NCs redispersion in the mixture of DI-water and DMSO was added with 2 mL OA at room temperature. The rest of process was the same with OAm treatment. 2.4. Devices’ fabrication Indium tin oxides (ITO) were washed by ITO cleaning agent, DIwater, acetone and isopropanol in turns. The 2 mg/mL of CuS NCs DMSO solution was spun at 4000 rpm for 120 s on dry ITO substrates, then followed by a 15 min anneal at 140 °C in air. A solution of P3HT:PC61BM with 1:1 ratio in 1, 2-dichlorobenzene (40 mg/mL in total) was stirred at 60 °C for 12 h. The cooling active solution was spin-coated at 600 rpm for 60 s and the devices were separately dried in small covered petri dishes for 2 h in a glove box. The devices were completely fabricated after thermally depositing 20 nm Ca followed by 100 nm Al cathodes.
Fig. 1 shows the XRD pattern of the as-synthesized CuS NCs. All diffraction peaks can be readily indexed to the hexagonal covellite CuS crystal structure with lattice constants a = 3.792 Å, b = 16.34 Å (JCPDS No. 06-0464). To be emphasized, the disappearance of the (1 0 7) diffraction peak from the XRD pattern indicated preferentially orientation effects of the growth direction along the (1 1 0) plane of hexagonal copper sulfide crystals [23]. No other crystalline phase, such as Cu2S, Cu1.8S and Cu1.96S, was observed, indicating the high purity of the products. The sharp diffraction peaks suggest that the obtained CuS NCs were well crystalline, even the CuS NCs were formed at 80 °C. From the half-width of XRD peaks, the average particle size is estimated as 3 nm by the Scherer’s formula, which agrees well with the values from the TEM observation in the later part. The diffraction peaks are broadened because of their very small size. Fig. 2a presents a TEM image of the as-synthesized CuS NCs at a high magnification. As can be seen, CuS NCs sample capped with pyridine surface ligands shows favorable dispersity and most of the CuS NCs were irregular in shape, with sizes varying from 2 nm to 5 nm. It was observed in the high-resolution transmission electron microscopy (HR-TEM) image (Fig. 2b) that the particle was single crystal with clear interplanar distances of 0.28 nm, which correspond to that of the (1 0 3) plane of the hexagonal covellite CuS. EDX analysis (Fig. 2c) was also performed for as-prepared CuS NCs. Copper and sulfur elements were detected and their atomic ratio is close to 1:1, which agrees with the stoichiometric ratio of CuS. The size distribution of CuS NCs is shown in Fig. 2d. The CuS NCs are not standard spherical and the size referrers to average of the horizontal and vertical diameters. The average size was measured by randomly counting 80 particles and was determined to be 3.0 nm.
2.5. Characterization The morphology and size of CuS NCs were determined by FEI Tecnai F20 Transmission Electron Microscope (TEM) coupled with energy dispersive X-ray spectroscopy (EDX). Fourier Transform Infrared (FTIR) spectra were detected on a Thermo Nicolet 6700 Spectroscopy, which operated from 4000 to 400 cm 1, to characterize the ligand binding to CuS NCs surface. X-ray diffraction (XRD) was studied by using Bruker D8 Advance diffractometer with Cu Ka radiation (k = 1.54060 Å). UV/vis spectra were collected with a Perkin Elmer Lambda 950 scan UV/vis spectrophotometer
Fig. 1. The X-ray diffraction (XRD) pattern of the CuS NCs. The vertical tick indicate the positions of the reflexes from the hexagonal phase CuS (JCPDS No. 06-0464).
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J. Li et al. / Journal of Colloid and Interface Science 419 (2014) 142–147
Fig. 2. TEM image of CuS NCs (a). HR-TEM image of CuS NCs (b). EDX of CuS NCs (c). Size histograms of CuS NCs (d).
To further investigate the materials composition, CuS NCs were analyzed by XPS. Fig. 3a shows the XPS spectrum of the Cu 2p state. An intense peak is observed at about 932.4 eV, and an additional feature appears at the binding energies of 933.8 eV. The structure at 932.4 eV is concerned with Cu+, whereas the structure at
933.8 eV is formally described as Cu2+ [24]. The coexistence of Cu+ and Cu2+ in covellite CuS has been reported by the study of XPS [25–27]. Fig. 3b presents the XPS spectrum for the S 2p state, which can be split into two apparent peaks. Firstly, the peak located at a binding energy of 162.5 eV corresponds to single sulfur
Fig. 3. Cu 2p XPS acquisition for CuS NCs (a). S 2p XPS acquisition for CuS NCs (b). O 1s XPS acquisition for CuS NCs (c). Raman spectrum of the CuS NCs (d).
J. Li et al. / Journal of Colloid and Interface Science 419 (2014) 142–147
atom bonding with copper atom [24]. Secondly, the peak located at a binding energy of 163.7 eV showed that the binding energy for S 2p electrons for S-S dimers [28]. The two peaks of S 2p3/2 directly reflected the inequivalent environment of the two types of sulfur atom in the CuS crystals. Fig. 3c shows the XPS spectrum for the O 1s state. The O 1s peak centered at 532 eV assigns to absorbed oxygen. It must be emphasized that the O 1s spectrum measured by us shows no exclusive peak at 530 eV was observed which could corresponds to Cu2+ of CuO [29]. Fig. 3d displays a Raman spectrum of CuS. A distinct peak wavenumber around 467 cm 1 was ascribed to S-S stretching which in good agreement with the XPS data of the S 2p state [30]. UV–vis absorption measurement is one of the most important approaches to investigate the optical properties of semiconductor nanocrystals and has been used to study as-synthesized CuS NCs. The UV–vis spectrum of CuS NCs dispersed in DMSO is shown in Fig. 4a. Copper sulfide has several stable phases with varied stoichiometry, and each phase has its distinct optical absorption feature. The spectrum shows the NCs absorb light in the region of 300–500 nm within an absorption shoulder at short wavelength 400 nm. Furthermore one broad and intensity absorbance with a maximum at 1167 nm in the near-IR region is obviously observed. Such data indicate that the characteristic of covellite CuS origins from the inter band transitions (absorption) from valence states to the unoccupied states [31]. The direct band gap value of CuS NCs, is evaluated according to the equation aEp = K(Ep Eg)1/2 (where a is the adsorption coefficient, K is a constant, Ep is the discrete photo energy, Eg is the band gap energy), and (aEp)2 versus Ep is shown in Fig. 4b. By extrapolating the straight portion of the graph on Ep axis at a = 0, the optical band gap is calculated to be 2.2 eV, which is larger than the value of bulk CuS (Eg = 2.0 eV) [32]. The increase in the band gap indicates the quantum size effect of as-prepared CuS NCs and their size is smaller than Bohr exciton radius of CuxS [33].
3.1. Ligand-exchange The ligand exchange opens the pathway to disperse uniform small size CuS NCs in oleic media. In our synthesis the resultant CuS NCs were capped with weak ligand of pyridine so as to be replaced easily. The change from hydrophilic surface to hydrophobic surface suggests the removal of the original ligands, shown in the inset photos of Fig. 5, which is further confirmed by FTIR
Fig. 4. Optical absorption spectrum of the CuS NCs (a). Plot of the square of absorbance versus photon energy for the CuS NCs (b).
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Fig. 5. FTIR spectra of CuS NCs (a). CuS NCs exchanged with OAm (b). CuS NCs exchanged with OA (c).
spectroscopy. After ligand exchange the OAm (or OA) capped CuS NCs dispersed uniformly in the top of the layered solution. Fig. 5a presents the IR spectrum of CuS NCs. The peaks at 1600– 1650 cm 1 and 1400 cm 1 are separately assigned to C@C stretching vibrations and C@N stretching vibrations of pyridine. In the case of ligand exchange, Fig. 5b shows that the intensive characteristic CAH stretching vibrations at 2800–3000 cm 1 ascribed to OAm molecules and Fig. 5c shows similar characteristic CAH stretching vibrations at 2800–3000 cm 1 and C@O stretching vibrations at 1711 cm 1 ascribed to OA. These characteristic peaks did not shift obviously suggesting that ANH2 and ACOOH were weakly bonded to NCs. Both of the spectra under 1500 cm 1 region are apparently different from corresponding spectrum capped with original pure ligand which indicate a successful exchange between the organic ligands with long alkyl chain and pyridine.
Fig. 6. Energy level diagrams of device components referenced to the vacuum level.
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J. Li et al. / Journal of Colloid and Interface Science 419 (2014) 142–147
3.2. Devices
4. Conclusion
The as-prepared CuS NCs have been applied in organic solar cell to expand their potential application. Fig. 6 illustrates the energy levels of the materials used in the devices. The energy level of as-synthesized CuS NCs was determined by UPS (see Fig. S1). And the energy levels of P3HT and PC61BM were obtained from literature [34,35]. As the HOMO level ( 5.1 eV) of CuS is nearly the same as P3HT, holes can transport from P3HT favorably to ITO electrode through CuS. Meanwhile, the LUMO level ( 2.9 eV) of CuS is higher than that LUMO ( 3.0 eV) of P3HT, which blocks the electron transport from P3HT to the ITO electrode. Thus, assynthesized CuS NCs have the potential to be used as HTL candidate. Herein we demonstrated the capacity of CuS as hole transport layer by the organic photovoltaic devices configuration of ITO/ CuS/P3HT:PC61BM/Ca/Al. Our CuS NCs were chosen as hole transport layer in photovoltaic devices because it is advantageous for its small uniform size, volatile ligand and easy solution processability which are convenient for preparing smooth and thin films. As shown in Fig. 7a, the device with CuS NCs indicates an open-circuit voltage (Voc) of 0.57 V, short-circuit current density (Jsc) of 10 mA/cm2, and fill factor (FF) of 0.54, yielding a power conversion efficiency (g) of 3.1% under AM 1.5G illumination. In contrast, the PV device without CuS HTL shows depressed device performance which gives Voc of 0.43 V, Jsc of 9.2 mA/cm2, and FF of 0.48, only yielding g of 1.9%. The device efficiency is significantly decreased due to the low Voc and FF. These results indicate the application potentials of CuS NCs in organic photovoltaics. The external quantum efficiency (EQE) spectrum of CuS modified device is shown in Fig. 7b, which was used to verify Jsc.
In summary, we have introduced a facile solution approach for CuS colloidal nanocrystals synthesis. Via our approach, uniform small sized CuS NCs were obtained with easy processability. Our method which used cheap chemicals and processed in low temperature without protective gas is promising for mass production. Surface chemistry investigation of as-prepared CuS NCs shows the obtained OAm (or OA) capped CuS NCs can disperse well in hexane which presents the widespread applied potential in various fields. Initial photovoltaic devices testing based on CuS NCs as hole transport layer gave efficiency of 3.1% with a 63% improvement compared to that without anode buffer layer under AM 1.5G illuminations. The synthesized CuS NCs have the superior interfacial property such as water stability, non-corrosiveness and the matched energy level. And the NCs can be processed in lower temperature than any other metal oxide based hole transport layer, such as NiO and MoO3. These results should be appreciated for improving our understanding of basic properties and applicability of CuS NCs. Acknowledgments The authors gratefully acknowledge the support of the National Natural Science Foundation of China (Nos. 51202264 and 51273208), and the Specialized Research Fund for the Spring Buds Talent Program (No. Y20804RA02). The work was also supported by Hundred Talent Program of Chinese Academy of Science; the Starting Research Fund of Team Talent (Y10801RA01) in NIMTE. Appendix A. Supplementary material Supplementary data associated with this article can be found, in the online version, at http://dx.doi.org/10.1016/j.jcis.2013.12.057. References
Fig. 7. J–V characteristics of the OPV devices without HTL and with CuS as HTL (a). EQE of the same device (b).
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Contents lists available at ScienceDirect
Journal of Colloid and Interface Science www.elsevier.com/locate/jcis
Manipulating surface ligands of Copper Sulfide nanocrystals: Synthesis, characterization, and application to organic solar cells Jun Li a,b, Tonggang Jiu b,⇑, Guo-Hong Tao a, Guojie Wang b, Chunming Sun b, Pandeng Li b, Junfeng Fang b,⇑, Ling He a,⇑ a b
College of Chemistry, Sichuan University, Chengdu, Sichuan 610065, PR China Institute of New Energy Technology, Ningbo Institute of Material Technology and Engineering (NIMTE), Chinese Academy of Science (CAS), Ningbo, Zhejiang 315201, PR China
a r t i c l e
i n f o
Article history: Received 19 October 2013 Accepted 21 December 2013 Available online 31 December 2013 Keywords: CuS Nanocrystals Ligand-exchange Interfacial application
a b s t r a c t CuS NCs were synthesized via a facile sol–gel method without post-thermal treatment. The as-prepared CuS NCs were analyzed and confirmed by XRD, HR-TEM, EDS and XPS as hexagonal covellite CuS. The average diameter of the samples was about 3 nm with narrow size distribution. CuS NCs can form a thin and smooth film without ligand-exchange that can be used as hole transport layer in organic solar cell. These hydrophilic CuS NCs with pyridine ligands can be exchanged with OAm and OA rapidly at room temperature and present hydrophobic characteristic, resulting in forming oil-soluble CuS NCs. This makes it possible tuning the surface property of CuS NCs and has the potential application for different fields. Ó 2014 Elsevier Inc. All rights reserved.
1. Introduction Nanoscale semiconductors have attracted a tremendous interest due to their size- and shape-dependent properties compared with their bulk counterparts [1]. Since the size decreases, the energy gap (Eg) increases, resulting in a blueshift of the absorption wavelength which is known as quantum size effect. As a well-known p-type semiconductor, CuS has been intensively studied in numerous fields such as photovoltaic devices, nonlinear optical materials [2], room temperature ammonia gas sensors [3], catalysts [4]. Nowadays, various shapes and sizes of CuS nanocrystals (CuS NCs) have been synthesized to investigate their properties and to expand their applications [5–8]. Cao and his colleagues synthesized 200 nm CuS spheres with strong opticallimiting properties [9]. Chen and Hu synthesized 500–800 nm flower-like CuS as an efficient 980 nm laser-driven photothermal agent for ablation of cancer cells [10]. On the other hand, uniform small sized (less than 10 nm) CuS NCs have been rarely reported. Erkey and his coworkers synthesized CuS NCs in water-in-carbon dioxide microemulsions [11]. And Iwahori synthesized CuS NCs in the protein cage [12]. Most of the CuS NCs obtained in the reported literature were capped with long hydrocarbon molecules such as oleic acid (OA) and oleylamine (OAm). However, the existence of such bulky hydrophobic molecules created barrier around each NC which greatly impacts ⇑ Corresponding authors. Fax: +86 2885470368 (L. He). E-mail addresses: [email protected] (T. Jiu), [email protected] (J. Fang), [email protected] (L. He). 0021-9797/$ - see front matter Ó 2014 Elsevier Inc. All rights reserved. http://dx.doi.org/10.1016/j.jcis.2013.12.057
their electrical and catalytic properties [13]. Although these can be solved by a ligand-exchange procedure, exchange is generally incomplete with such weak ligands and allows NCs forming aggregates easily [14–16]. In recent years, considerable attention has been focused on heterojunction organic solar cell (OSCs) which is one of potential alternatives to expensive silicon solar cells, due to the advantages of their light weight, flexibility, low cost and cheaper solution based process [17,18]. As the most common material used as hole transport layer (HTL), poly(3,4-ethylenedioxythiophene): poly (styrenesulfonate) (PEDOT:PSS) can affect interfacial stability negatively on account of its high hygroscopicity and acidity [19]. Semiconductors such as MoO3, V2O5, NiO have been investigated to replace PEDOT:PSS to overcome these disadvantages [20–22]. There is little research on utilizing CuS NCs as hole transport layer, though various properties of CuS NCs have been widely explored. In our work, uniform small sized CuS NCs were prepared via a facile sol–gel method without using template and complicated post-treatment. Our method utilizes low temperature, no protective gas and cheap chemicals that make our method advantageous. Compared with common synthetic routes, the as-synthesized CuS NCs are capped with pyridine which is weak and hydrophilic ligand. Furthermore, it is easily exchanged with OA and OAm resulting in oleophilic surface property. This opens selectable pathways to study the surface properties of CuS NCs for numerous fields such as biological and catalytic applications. More interestingly herein we find that their potential capacity applied as photovoltaic interfacial materials by fabricating polymer photovoltaics in the configuration of ITO/CuS/P3HT:PC61BM/Ca/Al.
J. Li et al. / Journal of Colloid and Interface Science 419 (2014) 142–147
2. Materials and methods 2.1. Chemicals The following chemicals were purchased and used as received. Copper acetate monohydrate (analytical reagent (AR)), pyridine (AR), sodium sulfide nonahydrate (AR), acetone (AR), and DMSO (AR) were obtained from Sinopharm Chemical Reagent. OA (85%) and OAm (90%) were purchased from Aladin and 1, 2-dichlorobenzene (AR) was purchased from Sigma–Aldrich. P3HT and PC61BM were both obtained from Luminescence Technology Corp.
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over the spectral range of 300–1500 nm. X-ray photoelectron spectroscopy (XPS) and ultraviolet photoelectron spectroscopy (UPS) were performed using a Kratos AXIS ULTRADLD UPS/XPS system (Kratos analytical, Manchester, UK). Raman spectra were measured using a Renishaw in Via-reflex spectrometer with the 532 nm line of the Ar ion laser as an excitation source in a quasi-backscattering configuration. The current density–voltage characteristics of the photovoltaic devices were recorded using a computer-controlled Keithley 2400 source meter under air mass (AM) 1.5G simulated solar light. 3. Results and discussion
2.2. Synthesis and Purification of CuS NCs In a typical experiment, 5.0 mmol of copper acetate monohydrate was dissolved into 40 mL deionized water/pyridine mixture (VDI-water:Vpy = 1:3) and heated to 80 °C; 5.0 mmol of sodium sulfide nonahydrate was dissolved into 20 mL of deionized water. Under vigorous magnetic stirring, sodium sulfide solution was dropped into copper acetate solution in 20 min. The mixture reacted for 2 h at 80 °C. At last, the reaction product was collected and washed twice followed by precipitation with acetone and redispersion in deionized water or other organic solvents (e.g., pyridine, DMSO). 2.3. Ligand-exchange experiments of pyridine coated CuS NCs 2.3.1. Ligand-exchange with OAm In a typical process, 2 mL DMSO solution of CuS NCs (20 mg/ mL) was added with 2 mL OAm at room temperature. The resulting mixture was shaken gently for 60 s and stood for 2 min at room temperature. Then CuS NCs were moved to OAm phase which indicated that ligand-exchange is completed. The OAm capped CuS NCs were collected by precipitation with ethanol and redispersion in hexane. 2.3.2. Ligand-exchange with OA In a similar process, 40 mg of CuS NCs redispersion in the mixture of DI-water and DMSO was added with 2 mL OA at room temperature. The rest of process was the same with OAm treatment. 2.4. Devices’ fabrication Indium tin oxides (ITO) were washed by ITO cleaning agent, DIwater, acetone and isopropanol in turns. The 2 mg/mL of CuS NCs DMSO solution was spun at 4000 rpm for 120 s on dry ITO substrates, then followed by a 15 min anneal at 140 °C in air. A solution of P3HT:PC61BM with 1:1 ratio in 1, 2-dichlorobenzene (40 mg/mL in total) was stirred at 60 °C for 12 h. The cooling active solution was spin-coated at 600 rpm for 60 s and the devices were separately dried in small covered petri dishes for 2 h in a glove box. The devices were completely fabricated after thermally depositing 20 nm Ca followed by 100 nm Al cathodes.
Fig. 1 shows the XRD pattern of the as-synthesized CuS NCs. All diffraction peaks can be readily indexed to the hexagonal covellite CuS crystal structure with lattice constants a = 3.792 Å, b = 16.34 Å (JCPDS No. 06-0464). To be emphasized, the disappearance of the (1 0 7) diffraction peak from the XRD pattern indicated preferentially orientation effects of the growth direction along the (1 1 0) plane of hexagonal copper sulfide crystals [23]. No other crystalline phase, such as Cu2S, Cu1.8S and Cu1.96S, was observed, indicating the high purity of the products. The sharp diffraction peaks suggest that the obtained CuS NCs were well crystalline, even the CuS NCs were formed at 80 °C. From the half-width of XRD peaks, the average particle size is estimated as 3 nm by the Scherer’s formula, which agrees well with the values from the TEM observation in the later part. The diffraction peaks are broadened because of their very small size. Fig. 2a presents a TEM image of the as-synthesized CuS NCs at a high magnification. As can be seen, CuS NCs sample capped with pyridine surface ligands shows favorable dispersity and most of the CuS NCs were irregular in shape, with sizes varying from 2 nm to 5 nm. It was observed in the high-resolution transmission electron microscopy (HR-TEM) image (Fig. 2b) that the particle was single crystal with clear interplanar distances of 0.28 nm, which correspond to that of the (1 0 3) plane of the hexagonal covellite CuS. EDX analysis (Fig. 2c) was also performed for as-prepared CuS NCs. Copper and sulfur elements were detected and their atomic ratio is close to 1:1, which agrees with the stoichiometric ratio of CuS. The size distribution of CuS NCs is shown in Fig. 2d. The CuS NCs are not standard spherical and the size referrers to average of the horizontal and vertical diameters. The average size was measured by randomly counting 80 particles and was determined to be 3.0 nm.
2.5. Characterization The morphology and size of CuS NCs were determined by FEI Tecnai F20 Transmission Electron Microscope (TEM) coupled with energy dispersive X-ray spectroscopy (EDX). Fourier Transform Infrared (FTIR) spectra were detected on a Thermo Nicolet 6700 Spectroscopy, which operated from 4000 to 400 cm 1, to characterize the ligand binding to CuS NCs surface. X-ray diffraction (XRD) was studied by using Bruker D8 Advance diffractometer with Cu Ka radiation (k = 1.54060 Å). UV/vis spectra were collected with a Perkin Elmer Lambda 950 scan UV/vis spectrophotometer
Fig. 1. The X-ray diffraction (XRD) pattern of the CuS NCs. The vertical tick indicate the positions of the reflexes from the hexagonal phase CuS (JCPDS No. 06-0464).
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Fig. 2. TEM image of CuS NCs (a). HR-TEM image of CuS NCs (b). EDX of CuS NCs (c). Size histograms of CuS NCs (d).
To further investigate the materials composition, CuS NCs were analyzed by XPS. Fig. 3a shows the XPS spectrum of the Cu 2p state. An intense peak is observed at about 932.4 eV, and an additional feature appears at the binding energies of 933.8 eV. The structure at 932.4 eV is concerned with Cu+, whereas the structure at
933.8 eV is formally described as Cu2+ [24]. The coexistence of Cu+ and Cu2+ in covellite CuS has been reported by the study of XPS [25–27]. Fig. 3b presents the XPS spectrum for the S 2p state, which can be split into two apparent peaks. Firstly, the peak located at a binding energy of 162.5 eV corresponds to single sulfur
Fig. 3. Cu 2p XPS acquisition for CuS NCs (a). S 2p XPS acquisition for CuS NCs (b). O 1s XPS acquisition for CuS NCs (c). Raman spectrum of the CuS NCs (d).
J. Li et al. / Journal of Colloid and Interface Science 419 (2014) 142–147
atom bonding with copper atom [24]. Secondly, the peak located at a binding energy of 163.7 eV showed that the binding energy for S 2p electrons for S-S dimers [28]. The two peaks of S 2p3/2 directly reflected the inequivalent environment of the two types of sulfur atom in the CuS crystals. Fig. 3c shows the XPS spectrum for the O 1s state. The O 1s peak centered at 532 eV assigns to absorbed oxygen. It must be emphasized that the O 1s spectrum measured by us shows no exclusive peak at 530 eV was observed which could corresponds to Cu2+ of CuO [29]. Fig. 3d displays a Raman spectrum of CuS. A distinct peak wavenumber around 467 cm 1 was ascribed to S-S stretching which in good agreement with the XPS data of the S 2p state [30]. UV–vis absorption measurement is one of the most important approaches to investigate the optical properties of semiconductor nanocrystals and has been used to study as-synthesized CuS NCs. The UV–vis spectrum of CuS NCs dispersed in DMSO is shown in Fig. 4a. Copper sulfide has several stable phases with varied stoichiometry, and each phase has its distinct optical absorption feature. The spectrum shows the NCs absorb light in the region of 300–500 nm within an absorption shoulder at short wavelength 400 nm. Furthermore one broad and intensity absorbance with a maximum at 1167 nm in the near-IR region is obviously observed. Such data indicate that the characteristic of covellite CuS origins from the inter band transitions (absorption) from valence states to the unoccupied states [31]. The direct band gap value of CuS NCs, is evaluated according to the equation aEp = K(Ep Eg)1/2 (where a is the adsorption coefficient, K is a constant, Ep is the discrete photo energy, Eg is the band gap energy), and (aEp)2 versus Ep is shown in Fig. 4b. By extrapolating the straight portion of the graph on Ep axis at a = 0, the optical band gap is calculated to be 2.2 eV, which is larger than the value of bulk CuS (Eg = 2.0 eV) [32]. The increase in the band gap indicates the quantum size effect of as-prepared CuS NCs and their size is smaller than Bohr exciton radius of CuxS [33].
3.1. Ligand-exchange The ligand exchange opens the pathway to disperse uniform small size CuS NCs in oleic media. In our synthesis the resultant CuS NCs were capped with weak ligand of pyridine so as to be replaced easily. The change from hydrophilic surface to hydrophobic surface suggests the removal of the original ligands, shown in the inset photos of Fig. 5, which is further confirmed by FTIR
Fig. 4. Optical absorption spectrum of the CuS NCs (a). Plot of the square of absorbance versus photon energy for the CuS NCs (b).
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Fig. 5. FTIR spectra of CuS NCs (a). CuS NCs exchanged with OAm (b). CuS NCs exchanged with OA (c).
spectroscopy. After ligand exchange the OAm (or OA) capped CuS NCs dispersed uniformly in the top of the layered solution. Fig. 5a presents the IR spectrum of CuS NCs. The peaks at 1600– 1650 cm 1 and 1400 cm 1 are separately assigned to C@C stretching vibrations and C@N stretching vibrations of pyridine. In the case of ligand exchange, Fig. 5b shows that the intensive characteristic CAH stretching vibrations at 2800–3000 cm 1 ascribed to OAm molecules and Fig. 5c shows similar characteristic CAH stretching vibrations at 2800–3000 cm 1 and C@O stretching vibrations at 1711 cm 1 ascribed to OA. These characteristic peaks did not shift obviously suggesting that ANH2 and ACOOH were weakly bonded to NCs. Both of the spectra under 1500 cm 1 region are apparently different from corresponding spectrum capped with original pure ligand which indicate a successful exchange between the organic ligands with long alkyl chain and pyridine.
Fig. 6. Energy level diagrams of device components referenced to the vacuum level.
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3.2. Devices
4. Conclusion
The as-prepared CuS NCs have been applied in organic solar cell to expand their potential application. Fig. 6 illustrates the energy levels of the materials used in the devices. The energy level of as-synthesized CuS NCs was determined by UPS (see Fig. S1). And the energy levels of P3HT and PC61BM were obtained from literature [34,35]. As the HOMO level ( 5.1 eV) of CuS is nearly the same as P3HT, holes can transport from P3HT favorably to ITO electrode through CuS. Meanwhile, the LUMO level ( 2.9 eV) of CuS is higher than that LUMO ( 3.0 eV) of P3HT, which blocks the electron transport from P3HT to the ITO electrode. Thus, assynthesized CuS NCs have the potential to be used as HTL candidate. Herein we demonstrated the capacity of CuS as hole transport layer by the organic photovoltaic devices configuration of ITO/ CuS/P3HT:PC61BM/Ca/Al. Our CuS NCs were chosen as hole transport layer in photovoltaic devices because it is advantageous for its small uniform size, volatile ligand and easy solution processability which are convenient for preparing smooth and thin films. As shown in Fig. 7a, the device with CuS NCs indicates an open-circuit voltage (Voc) of 0.57 V, short-circuit current density (Jsc) of 10 mA/cm2, and fill factor (FF) of 0.54, yielding a power conversion efficiency (g) of 3.1% under AM 1.5G illumination. In contrast, the PV device without CuS HTL shows depressed device performance which gives Voc of 0.43 V, Jsc of 9.2 mA/cm2, and FF of 0.48, only yielding g of 1.9%. The device efficiency is significantly decreased due to the low Voc and FF. These results indicate the application potentials of CuS NCs in organic photovoltaics. The external quantum efficiency (EQE) spectrum of CuS modified device is shown in Fig. 7b, which was used to verify Jsc.
In summary, we have introduced a facile solution approach for CuS colloidal nanocrystals synthesis. Via our approach, uniform small sized CuS NCs were obtained with easy processability. Our method which used cheap chemicals and processed in low temperature without protective gas is promising for mass production. Surface chemistry investigation of as-prepared CuS NCs shows the obtained OAm (or OA) capped CuS NCs can disperse well in hexane which presents the widespread applied potential in various fields. Initial photovoltaic devices testing based on CuS NCs as hole transport layer gave efficiency of 3.1% with a 63% improvement compared to that without anode buffer layer under AM 1.5G illuminations. The synthesized CuS NCs have the superior interfacial property such as water stability, non-corrosiveness and the matched energy level. And the NCs can be processed in lower temperature than any other metal oxide based hole transport layer, such as NiO and MoO3. These results should be appreciated for improving our understanding of basic properties and applicability of CuS NCs. Acknowledgments The authors gratefully acknowledge the support of the National Natural Science Foundation of China (Nos. 51202264 and 51273208), and the Specialized Research Fund for the Spring Buds Talent Program (No. Y20804RA02). The work was also supported by Hundred Talent Program of Chinese Academy of Science; the Starting Research Fund of Team Talent (Y10801RA01) in NIMTE. Appendix A. Supplementary material Supplementary data associated with this article can be found, in the online version, at http://dx.doi.org/10.1016/j.jcis.2013.12.057. References
Fig. 7. J–V characteristics of the OPV devices without HTL and with CuS as HTL (a). EQE of the same device (b).
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