Letter pubs.acs.org/NanoLett
Synthesis of Unstable Colloidal Inorganic Nanocrystals through the Introduction of a Protecting Ligand Xiaoyong Liang,†,⊥ Qing Yi,†,⊥ Sai Bai,† Xingliang Dai,† Xin Wang,† Zhizhen Ye,† Feng Gao,§ Fengling Zhang,§ Baoquan Sun,∥ and Yizheng Jin*,†,‡ †
State Key Laboratory of Silicon Materials, Cyrus Tang Center for Sensor Materials and Applications, Department of Materials Science and Engineering and ‡Center for Chemistry of High-Performance and Novel Materials, Zhejiang University, Hangzhou 310027, People’s Republic of China § Biomolecular and Organic Electronics, Department of Physics, Chemistry, and Biology, Linköping University SE-581 83 Linköping, Sweden ∥ Jiangsu Key Laboratory for Carbon-Based Functional Materials and Devices, Institute of Functional Nano and Soft Materials (FUNSOM), Soochow University, 199 Ren’ai Road, Suzhou 215123, People’s Republic of China S Supporting Information *
ABSTRACT: We demonstrate a facile and general strategy based on ligand protection for the synthesis of unstable colloidal nanocrystals by using the synthesis of pure p-type NiO nanocrystals as an example. We find that the introduction of lithium stearate, which is stable in the reaction system and capable of binding to the surface of NiO oxide nanocrystals, can effectively suppress the reactivity of NiO nanocrystals and thus prevent their in situ reduction into Ni. The resulting ptype NiO nanocrystals, a highly demanded hole-transporting and electron-blocking material, are applied to the fabrication of organic solar cells and polymer light-emitting diodes, demonstrating their great potential as an interfacial layer for low-cost and large-area, solution-processed optoelectronic devices. KEYWORDS: Protecting ligand, colloidal inorganic nanocrystal, nickel oxide, hole-transporting interlayer, solution-processed optoelectronic device
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the reaction system, leading to the formation of Ni particles. The instability of colloidal NiO nanocrystals at high temperatures has been reported by many groups.31,32 For instance, O’Brien and co-workers found that the reactions between nickel acetylacetonate and hexadecylamine were rapid and difficult to control, resulting in the formation of Ni particles exclusively.31 Attempts to obtain pure NiO nanocrystals by making the reaction slower have not succeeded so far.31 Here we demonstrate a strategy based on ligand protection for eliminating the unwanted side reaction involved in the synthesis of colloidal NiO nanocrystals. Instead of manipulating the reaction conditions, we propose to reduce the reactivity of the colloidal NiO nanocrystals. Lithium stearate (LiSt), which is stable in the reaction system and can readily bind to the surface of a NiO nanocrystal, is employed to test our hypothesis. The resulting pure NiO nanocrystals are utilized in bulk heterojunction organic photovoltaics (OPVs) and polymer light-emitting diodes (PLEDs), demonstrating their great potential as high-quality hole-transporting interlayers (HTLs) for solution-processed optoelectronic devices. This
olloidal inorganic nanocrystals are attractive functional materials with a unique combination of solid-state properties and excellent solution dispersibility. They have been intensively investigated as building blocks for the fabrication of low-cost, large-area and solution-processed optoelectronic devices.1−8 In many cases, the applications can be greatly advanced by developing new synthetic approaches to produce colloidal nanocrystals with the desired properties.9−19 A good example can be found in NiO, which is an intrinsic ptype and wide bandgap semiconductor with high ionization potential and low electron affinity.20,21 The p-type conductivity of NiO is different from those of several other high work function metal oxides, such as WOx, MoOx, and VOx, which are n-type and cannot serve as hole-transporting and electronblocking materials simultaneously.22,23 The unique electronic structure of NiO offers impressive charge selective holetransporting properties, making NiO thin films attractive for many optoelectronic devices.24−30 However, the synthesis of pure colloidal NiO nanocrystals has not been achieved so far, which hinders their applications in solution-processed devices. The synthesis of pure colloidal NiO nanocrystals has been plagued by an unwanted side reaction. It is well-known that NiO nanocrystals can be readily reduced by alcohol or amine in © 2014 American Chemical Society
Received: January 18, 2014 Published: May 12, 2014 3117
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(Supporting Information Figure S1) while the crystal structure of the larger particles matched the face-centered cubic structure of Ni (Figure 1C). The formation of metal Ni particles can be attributed to the in situ reduction of NiO nanocrystals by ODA. If the reaction time was sufficiently long, eventually all the NiO nanocrystals were converted to Ni particles (Supporting Information Figure S2). We have tried to avoid the unwanted side reaction by adjusting experimental parameters, including reaction temperature, reaction time, and the molar ratio of ODA to Ni(St)2. However, all these efforts failed because a high reaction temperature and an adequate amount of ODA are required to initiate the conversion of Ni precursor. For example, our FTIR analysis indicates that the alcoholysis reaction could only take place at a minimum temperature of 235 °C. At this temperature, the formation of Ni particles was found to be inevitable. We have also tried to modify the reaction system by introducing free stearate acid and this strategy did not work either (Supporting Information Figure S3). We believed that the in situ reduction of colloidal NiO nanocrystals could be suppressed by protecting their surfaces with a proper capping ligand. To this end, LiSt, an alkali fatty acid salt, was employed to achieve this goal. When LiSt was introduced into the reaction system, we obtained pure NiO nanocrystals (Figure 1D−F). No metallic Ni phase could be identified in the XRD pattern of the products (Figure 1D). In this case, we only observed irregular nanoparticles during TEM imaging (Figure 1E). HRTEM characterization (Figure 1F) indicated that all the particles were NiO nanocrystals with decent crystalline features. The NiO nanocrystals could be readily dispersed in organic solvents, such as chloroform, forming suspensions that were stable for months under ambient conditions. A remarkable feature of this synthesis is that the mass yield counted by nickel ions was over 85%. Further studies showed that pure NiO nanocrystals could also be obtained under various conditions, such as a high reaction temperature of 280 °C (Supporting Information Figure S4) and a large molar ratio of ODA to Ni(St)2 at 10 (Supporting Information Figure S5). We also carried out further studies to explore the role of LiSt in enabling the synthesis of pure NiO nanocrystals. The results can be summarized as the following. First, LiSt was stable in the reaction system. We monitored the LiSt-assisted alcoholysis reaction by Fourier transform infrared spectroscopy (FTIR). The appearance of the ester peak (1740 cm−1),15,33 which became stronger as the reaction proceeded (Figure 2A), could be attributed to the conversion of metal carboxylates. The broad peak at 1562 cm−1 corresponding to −COO− vibrations from both Ni(St)2 and LiSt gradually evolved to doublet peaks at 1580 and 1560 cm−1, which could be assigned to the asymmetric stretch vibrations of the unreacted LiSt.36 This result indicates that LiSt is much less reactive than Ni(St)2, which is consistent with the results of control experiments that involved reactions between pure LiSt and ODA or pure Ni(St)2 and ODA at 235 °C. In the case of LiSt, after 3 h the ester peak was not evident while the doublet peaks at 1580 and 1560 cm−1 originating from LiSt36 remained in the FTIR spectrum (Supporting Information Figure S6A). In contrast, FTIR analyses (Supporting Information Figure S6B) show that under identical conditions all the Ni(St)2 was consumed after 3 h. Second, LiSt readily binds to the surface of the NiO nanocrystal. In literature, sodium oleate and potassium oleate
new strategy has also been extended to other systems, suggesting its general applicability for the synthesis of colloidal inorganic nanocrystals. We employed a simple noninjection method based on alcoholysis of metal carboxylates, a general strategy for generating oxide nanocrystals,15,16,33−35 to synthesize NiO nanocrystals. Nickel stearate (Ni(St)2) and 1-octadecanol (ODA) were selected as the reagents. X-ray diffraction (XRD) measurements (Figure 1A) on the products indicated
Figure 1. Synthesis of NiO nanocrystals (A−C) without and (D−F) with the introduction of LiSt. (A) XRD pattern, (B) a typical TEM image, and (C) a typical HRTEM image of the larger and faceted particles in the products of a reaction without the introduction of LiSt. The interplanar spacing of 0.20 and 0.21 nm shown in (C) correspond to the {011} and {002} lattice planes of the face-centered cubic structured Ni (unit cell shown in the inset), respectively. (D) XRD pattern, (E) a typical TEM image, and (F) a typical HRTEM image of the nanocrystals from a reaction with the introduction of LiSt. The interplanar spacing of 0.21 and 0.24 nm shown in (F) correspond to the {200} and {111} lattice planes of the rock salt structured NiO (unit cell shown in inset), respectively.
the coexistence of both NiO (JSPDS card no. 65-2901) and Ni (JSPDS card no. 45-1027) phases. Transmission electron microscopy (TEM) observations (Figure 1B) revealed that the products consisted of two different types of particles. The majority of the products were irregular nanoparticles that often attached to each other, forming nanoflowers. Larger and faceted particles with a higher bright-field contrast were also observed. High-resolution transmission electron microscopy (HRTEM) analyses indicate that the crystal structure of the irregular nanoparticles matched the rock salt structure of NiO 3118
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Figure 2. FTIR analysis of the role of LiSt in the synthesis of colloidal NiO nanocrystals. (A) Temporal evolution of FTIR spectra recorded from a LiSt-assisted reaction. (B) FTIR spectra of the nanocrystals from a reaction without the introduction of LiSt (top), the nanocrystals from the LiStassisted reaction and purified by the first method (middle) and the nanocrystals from the LiSt-assisted reaction and purified by the second method (bottom), respectively.
in Supporting Information Figure S8, when the NiO nanocrystals purified by the second method were intensively washed by hot methanol to remove the surface-bound LiSt, the peak at 1560 cm−1 disappeared in the corresponding FTIR spectrum. Furthermore, refluxing the NiO nanocrystals purified by the first method, that is, NiO nanocrystals without surface-bound LiSt, in a solution containing LiSt led to the adsorption of LiSt and hence the emergence of the peak at 1560 cm−1 in the corresponding FTIR spectrum (Supporting Information Figure S9). We conclude that LiSt can serve as a protecting ligand for the synthesis of pure colloidal NiO nanocrystals (Scheme 1). The
were used as capping ligands to synthesize iron oxide nanocrystals with controlled shapes,37,38 indicating that the ionic alkali carboxylates could readily bind to the surface of the oxide nanocrystals. Here we demonstrate the binding of LiSt to the surface of the NiO nanocrystals by elemental analyses and FTIR measurements (the FTIR assignments are summarized in Supporting Information Table S1) on carefully purified products. Considering that LiSt is soluble in hot methanol and is not soluble in a nonpolar solvent such as hexane, two purification methods were used (refer to the experimental section for details). In the first method, the NiO nanocrystals were purified by extraction three times using a biphase system of hexane and methanol at 50 °C. Elemental analyses on the products showed a low lithium content ([Li]/([Li] + [Ni])) of less than 1.0%, indicating that both free LiSt in the reaction mixtures and majority of the surface-bound LiSt were removed. In the second method, the use of methanol was avoided. Free LiSt in the hexane suspension of nanocrystals was removed by aging the sample overnight, followed by centrifugation and filtration. FTIR measurements on the insoluble precipitates revealed the presence of free LiSt (Supporting Information Figure S7), suggesting efficient separation of unbounded LiSt from the dispersible NiO nanocrystals. Elemental analyses on the nanocrystals purified by the second method revealed a lithium content of 9.0%, much higher than that of the sample purified by the first method. We suggest that the higher lithium content in the samples purified by the second method is due to surface-bound LiSt instead of residue free LiSt based on the following characterizations. FTIR analyses (Figure 2B middle) on the nanocrystals purified by the first method showed that the surface ligands were stearate ions, as indicated by the asymmetric stretch vibration at 1550 cm−1.15,33 A similar vibration band was also found in the FTIR spectra of the products from the reactions without the introduction of LiSt (Figure 2B top). FTIR measurements on the NiO nanocrystals purified by the second method revealed the existence of an extra peak at 1560 cm−1 (Figure 2B bottom). This peak is not due to free LiSt, which has doublet peaks at 1580 and 1560 cm−1, with the intensity of the former peak stronger than that of the latter (Supporting Information Figure S7). We attribute the extra peak at 1560 cm−1 to the surface-bound LiSt. This assignment is confirmed by our control experiments. As shown
Scheme 1. Synthesis of Colloidal NiO Nanocrystals with LiSt As a Protecting Ligand
binding of inert LiSt to the surfaces of NiO nanocrystals significantly suppressed their reactivity. In other words, the LiSt protected the NiO nanocrystals, preventing them from being reduced to Ni. In this way, pure NiO nanocrystals were obtained in high yields, over 85%. The protecting effect of LiSt for NiO nanocrystals is confirmed by the fact that once the surface-bound LiSt is removed, the reactivity of the NiO nanocrystals is recovered. As shown in Supporting Information Figure S10, the deprotected NiO nanocrystals, that is, the NiO nanocrystals without surface-bound LiSt, were not resistant to alcohol at high temperatures and were readily reduced to Ni particles. We emphasize that the binding of LiSt to the surfaces of NiO nanocrystals is essential to reducing their reactivity. Molecules that are stable in the reaction system and do not 3119
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Figure 3. Nanocrystals from the reactions with Cu(acac)2 and oleylamine as the reagents. (A) XRD profiles of the nanocrystals from the reactions without (top) or with (bottom) the introduction of LiSt. In the top profile, the peaks at 36.5 and 42.4°, which correspond to the Cu2O phase, were due to the oxidation of Cu nanocrystals prior to the XRD measurements. (B) A typical TEM image of the nanocrystals from a reaction without the introduction of LiSt. (C) A typical TEM image of the nanocrystals from the LiSt-assisted reactions.
Figure 4. Thin films of NiO nanocrystals as HTLs for solution-processed devices. (A) AFM height topography, (B) UPS spectrum (light source: He I 21.2 eV), and (C) transmission spectrum of the NiO nanocrystal thin films (on ITO-glass substrates). The spectrum of a ITO-glass substrate was also plotted. (D) Device structure, (E) J−V curves, and (F) log plots of the dark J−V characteristics of the organic solar cells. (G) Device structure and (H) curves of current density and luminance versus applied bias of the PLED devices. (I) A digital picture of a flexible PLED device.
bind to the surfaces of NiO nanocrystal cannot act as a protecting ligand. For example, our control experiments show that simply adding n-octadecane, which is structurally similar to LiSt in terms of the length of the alkyl chain, into the reaction mixture did not prevent the reduction of NiO nanocrystals (Supporting Information Figure S11). Following the successful demonstration of LiSt-assisted synthesis of NiO nanocrystals and the role of LiSt as a protecting ligand, we investigated the applicability of this strategy to other systems. First, we show that using LiSt to
suppress the reactivity of NiO nanocrystals is valid for aminolysis reactions, another general route to the synthesis of oxide nanocrystals.34,39−42 When oleylamine was employed to activate Ni(St)2, the introduction of LiSt into the reaction system resulted in pure NiO nanocrystals (Supporting Information Figure S12A,B). If LiSt was absent, as shown in Supporting Information Figure S12C,D, the NiO nanocrystals were converted to Ni particles. Second, we demonstrate that the choice of protecting ligand is not limited to LiSt. The use of NaSt also led to pure colloidal NiO nanoparticles (Supporting 3120
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PCEs of the devices with NiO nanocrystal HTLs arose from enhanced fill factor (FF) and short-circuit current (Jsc). In addition, as shown in Figure 4F, the reverse saturation dark current of the devices with NiO nanocrystal HTLs were 1 order of magnitude lower than that of the devices with PEDOT:PSS interlayers. These impressive characteristics reveal the charge selective nature and superior hole extraction properties of NiO nanocrystal HTLs.46,47 A device structure of ITO/NiO/poly [2-methoxy-5-(2ethylhexyloxy)-1,4-phenylenevinylene] (MEH-PPV)/Ca/Al (Figure 4G) was used to fabricate PLEDs. PLEDs with PEDOT:PSS HTLs were also fabricated. The turn-on voltage (Von) (the voltage that a luminance of 1 cd/m2 was observed) for both devices is same, 2.3 V. The current of the device with NiO nanocrystal HTLs increased steeply just after turn-on, yielding a maximum luminance of 13 500 cd/m2 at 8 V. The peak external quantum efficiency (EQE) and maximum current efficiency were 1.08% and 1.45 cd/A, respectively. These values were comparable with those of the devices with PEDOT:PSS interlayers (Figure 4H and Table 2).
Information Figure S13). FTIR analyses (Supporting Information Figure S13C) showed that the NaSt-assisted reactions exhibited features similar to those of the LiSt-assisted ones. Third, we show that the concept of protecting ligand is feasible for controlling the synthesis of other colloidal nanocrystals. The in situ reduction of oxide nanocrystals by alcohol or amine at elevated temperatures has been identified to be a key intermediate step in the formation of a number of metal nanoparticles.43,44 For example, Huaman and co-workers reported a hydroxyl ion-assisted alcohol reduction method for the synthesis of Cu nanocrystals through intermediate steps corresponding to the formation of copper oxides.44 Therefore, the synthesis assisted by a protecting ligand, which significantly influences the reactivity of oxide nanocrystals, is readily applied to these material systems. As a demonstration, we conducted reactions with copper(II) acetylacetonate (Cu(acac)2) and oleylamine as the reagents. Pure Cu nanocrystals were generated (Figure 3A,B). If LiSt was introduced into the reaction system, Cu2O nanocrystals were obtained and the conversion of Cu2O nanocrystals to metal particles was inhibited (Figure 3A,C). Next we show that the colloidal p-type NiO nanocrystals from the LiSt-assisted synthesis are promising candidates as HTLs for solution-processed devices. The NiO nanocrystals, purified by the first method to remove the surface-bound LiSt, were deposited onto ITO-glass substrates by spin-coating, followed by mild annealing at 130 °C and UV-ozone treatment. SEM characterizations of the as-deposited films, annealed films, and UV-ozone treated films show that all the films were crackfree and exhibited uniform surface features (Supporting Information Figure S14). Atomic force microscopy (AFM) measurements (Figure 4A) indicated that the thin film of NiO nanocrystals exhibited a flat surface with a root-mean-square roughness of 3.6 nm. Ultraviolet photoelectron spectroscopy (UPS) results shown in Figure 4B suggested that the work function of the NiO nanocrystal thin films was 5.2 ± 0.1 eV. The valence band edge was ∼0.4 eV below the Fermi level, consistent with the p-type nature of NiO. Optical transmission spectrum (Figure 4C) of the thin films of NiO nanocrystals on ITO-glass substrates showed an average transmittance of around 90% in the visible and near-infrared region, revealing excellent optical transparency. The NiO nanocrystal thin films were applied as HTLs in OPVs. A model system of poly[2,3-bis(3-octyloxyphenyl)quinoxaline-5,8-diyl-alt-thiophene-2,5-diyl] (TQ1)/[6,6]-phenyl-C71-butyric acid methyl ester (PC71BM)45 was used for the fabrication of solar cells (Figure 4D). Control devices with poly(3,4-ethylenedioxythiophene):poly(styrene-sulfonate) (PEDOT:PSS) interlayers were also fabricated. The devices with NiO nanocrystal HTLs exhibited an average power conversion efficiency (PCE) of 6.1% and a champion PCE of 6.3% (Figure 4E). In contrast, the average PCE for the devices with PEDOT:PSS was 5.0%. The electrical output characteristics of the solar cell devices are summarized in Table 1. The improved
Table 2. Device Parameters of the PLEDs Based on MEHPPV Emissive Layers HTLs NiO nanocrystal PEDOT:PSS
NiO nanocrystal PEDOT:PSS
Voc (V)
JSC (mA cm−2)
FF
best PCE (%)
average PCE (%)
0.88
10.2
0.71
6.3
6.1 + 0.2
0.86
9.4
0.64
5.2
5.0 + 0.2
maximum LV @ 8 V (cd/m2)
peak EQE (%)
maximum CE (cd/A)
2.3
13 550
1.08
1.45
2.3
11 200
1.04
1.42
In addition, the processing temperature as low as 130 °C makes it possible to integrate NiO nanocrystal HTLs into flexible electronics. MEH-PPV PLED devices with NiO nanocrystal interlayers spun on polyethylene-naphthalate (PEN)/ITO substrates were fabricated (Figure 4I). The resulting flexible PLED devices exhibited decent performance with a Von of 2.4 V, a maximum luminance of 8840 cd/m2 at 8 V, and a peak EQE of 1.08% (Supporting Information Figure S15). Note that the annealing temperatures in most reports involving the deposition of NiO thin films by using nickel precursor solutions were higher than 250 °C.27,47,48 The nanocrystal approach presented here, which decoupled the high-temperature synthesis from the deposition of thin films, provides an attractive solution for applying NiO HTLs in flexible electronics. In conclusion, we have demonstrated a general strategy based on ligand protection for the synthesis of pure p-type NiO colloidal nanocrystals. The key finding is that the reactivity of the NiO nanocrystals could be suppressed by the introduction of LiSt, which was stable in the reaction system, readily bind to the surfaces of oxide nanocrystals, and could be removed under mild conditions by simple purification methods. The LiSt served as a protecting ligand for colloidal NiO nanocrystals. In this way, the unwanted in situ reduction of NiO nanocrystals was avoided and high-quality pure p-type NiO nanocrystals were obtained in high yields. The high work function, excellent optical transparency, flat surface features, charge selective nature, and the compatibility with flexible electronics of the NiO nanocrystal thin films are attractive for HTL applications in solution-processed optoelectronic devices. Considering the unique properties of NiO, we expect that further investigation on the material synthesis (e.g., alloying or doping of NiO nanocrystals) and film processing (e.g., ligand exchange or
Table 1. Device Parameters of the OPV Cells Based on TQ1/PC71BM HTLs
Von (V)
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postdeposition treatments) will lead to a variety of highperformance solution-processed devices. We further demonstrated that the strategy based on protecting ligand is a general approach to controlling the reactivity of colloidal nanocrystals, which may shed light on future rational design of synthetic schemes. Our study provides an excellent example that new concept for synthetic chemistry of colloidal inorganic nanocrystals expands the synthesis-by-design capabilities, leading to new functional materials with targeted properties that fulfill the requirements of practical applications. Methods. Synthesis of NiO Nanocrystals (without the Introduction of LiSt). A noninjection approach was used. For a typical synthesis, Ni(St)2 (0.5 mmol), ODA (3 mmol), and 1octadecene (ODE, 5 mL) were loaded in a three-neck flask and degassed at 80 °C for 30 min. The reaction mixture was then heated to 235 °C and kept at this temperature for 3 h under argon flow. The products were precipitated out by adding a mixture of ethanol and ethyl acetate and further purified by dispersing/precipitating twice using the combination of hexane/ethanol. LiSt-Protected Synthesis of NiO Nanocrystals. The protocol was the same as above except for the addition of 0.2 mmol of LiSt. When the reaction was completed, the product was purified by two methods. In the first method, a procedure of extraction using a biphase system of hexane and methanol at 50 °C was applied three times. Next, the nanocrystals were precipitated out by adding a mixture of ethanol and ethyl acetate, followed by dispersing/precipitating twice using the combination of hexane/ethanol. In the second method, the nanocrystals were precipitated out by adding a mixture of acetone and ethyl acetate. Next the nanocrystals were dispersed in hexane and then precipitated by adding ethanol to remove the remaining ODA. Then the nanocrystals were dispersed in hexane and allowed to stay overnight. The insoluble precipitates were removed by centrifugation. The nanocrystal suspension was further purified by filtering through PTFE membranes (0.22 μm). NaSt-Protected Synthesis of NiO Nanocrystals. The synthetic procedure was similar to what was used for the synthesis of NiO nanocrystals with the introduction of LiSt except that LiSt was replaced by NaSt (0.2 mmol). Synthesis of Cu and Cu2O Nanocrystals. Cu(acac)2 (0.5 mmol) and oleylamine (10 mL) were loaded in a three-neck flask and degassed at 80 °C for 30 min. Next the reaction mixture was heated to 200 °C and kept at this temperature for 1 h under argon flow. The products were precipitated out by adding a mixture of ethanol and ethyl acetate and further purified by dispersing/precipitating twice using the combination of hexane/ethanol. In a control experiment, additional LiSt (0.5 mmol) was introduced into the reaction flask while all other parameters were kept the same. Materials, characterization techniques, as well as details of device fabrication and characterization are provided in the Supporting Information.
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Letter
AUTHOR INFORMATION
Corresponding Author
*E-mail: [email protected]. Author Contributions ⊥
X.L. and Q.Y. contributed equally to this work.
Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS We thank Professor Xiaogang Peng, Professor Chao Gao, Dr. Hang Qi, and Dr. Zhen Xu (Zhejiang University, China) for discussions. We thank Professor Liwei Chen and Mr. Qi Chen (Suzhou Institute of Nanotech and Nanobionics, Chinese Academy of Science) for the assistance with AFM characterization and Dr. Ergang Wang (Chalmers University of Technology, Sweden) for providing the TQ1 polymer. This work was financially supported by the National High Technology Research and Development Program of China (2011AA050520), the National Basic Research Program of China (973 Program) (2012CB932402), the National Natural Science Foundation of China (51172203), the Natural Science Funds for Distinguished Young Scholar of Zhejiang Province (R4110189) and the Public Welfare Project of Zhejiang Province (2013C31057). F.G. acknowledges the financial support of the European Commission under a Marie Curie Intra-European Fellowship for Career Development.
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ASSOCIATED CONTENT
S Supporting Information *
Materials, characterization techniques, details of device fabrication and characterization, and additional data (including TEM, HRTEM, XRD, and FTIR) associated with the synthesis of oxide nanocrystals. This material is available free of charge via the Internet at http://pubs.acs.org. 3122
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(20) Mrowec, S.; Grzesik, Z. J. Phys. Chem. Solids 2004, 65, 1651− 1657. (21) Hufner, S. Adv. Phys. 1994, 43, 183−356. (22) Meyer, J.; Hamwi, S.; Kröger, M.; Kowalsky, W.; Riedl, T.; Kahn, A. Adv. Mater. 2012, 24, 5408−5427. (23) Greiner, M. T.; Lu, Z.-H. NPG Asia Mater. 2013, 5, e55. (24) Irwin, M. D.; Buchholz, D. B.; Hains, A. W.; Chang, R. P. H.; Marks, T. J. Proc. Natl. Acad. Sci. U.S.A. 2008, 105, 2783−2787. (25) Caruge, J. M.; Halpert, J. E.; Wood, V.; Bulovic, V.; Bawendi, M. G. Nat. Photonics 2008, 2, 247−250. (26) Steirer, K. X.; Ndione, P. F.; Widjonarko, N. E.; Lloyd, M. T.; Meyer, J.; Ratcliff, E. L.; Kahn, A.; Armstrong, N. R.; Curtis, C. J.; Ginley, D. S.; Berry, J. J.; Olson, D. C. Adv. Energy Mater. 2011, 1, 813−820. (27) Manders, J. R.; Tsang, S.-W.; Hartel, M. J.; Lai, T.-H.; Chen, S.; Amb, C. M.; Reynolds, J. R.; So, F. Adv. Funct. Mater. 2013, 23, 2993− 3001. (28) Chan, I.-M.; Hsu, T.-Y.; Hong, F. C. Appl. Phys. Lett. 2002, 81, 1899−1901. (29) Ohta, H.; Hirano, M.; Nakahara, K.; Maruta, H.; Tanabe, T.; Kamiya, M.; Kamiya, T.; Hosono, H. Appl. Phys. Lett. 2003, 83, 1029− 1031. (30) Bai, S.; Cao, M. T.; Jin, Y. Z.; Dai, X. L.; Liang, X. Y.; Ye, Z. Z.; Li, M.; Cheng, J. P.; Xiao, X. Z.; Wu, Z. W.; Xia, Z. H; Sun, B. Q.; Wang, E. G.; Mo, Y. Q.; Gao, F.; Zhang, F. L. Adv. Energy Mater. DOI: 10.1002/aenm.201301460. (31) Li, Y.; Afzaal, M.; O’Brien, P. J. Mater. Chem. 2006, 16, 2175− 2180. (32) Lee, W.; Kim, I.; Choi, H.; Kim, K. Surf. Coat. Technol. 2013, 231, 93−97. (33) Chen, Y. F.; Kim, M.; Lian, G.; Johnson, M. B.; Peng, X. G. J. Am. Chem. Soc. 2005, 127, 13331−13337. (34) Niederberger, M.; Garnweitner, G. Chem.Eur. J. 2006, 12, 7282−7302. (35) Narayanaswamy, A.; Xu, H. F.; Pradhan, N.; Kim, M.; Peng, X. G. J. Am. Chem. Soc. 2006, 128, 10310−10319. (36) Shoeb, Z. E.; Hammad, S. M.; Yousef, A. A. Grasas Aceites. 1999, 50, 426−434. (37) Kovalenko, M. V.; Bodnarchuk, M. I.; Lechner, R. T.; Hesser, G.; Schäffler, F.; Heiss, W. J. Am. Chem. Soc. 2007, 129, 6352−6353. (38) Shavel, A.; Rodriguez-Gonzalez, B.; Spasova, M.; Farle, M.; LizMarzan, L. M. Adv. Funct. Mater. 2007, 17, 3870−3876. (39) Zhang, Z. H.; Zhong, X. H.; Liu, S. H.; Li, D. F.; Han, M. Y. Angew. Chem., Int. Ed. 2005, 44, 3466−3470. (40) Jin, Y. Z.; Yi, Q.; Zhou, L. M.; Chen, D. D.; He, H. P.; Ye, Z. Z.; Hong, J. H.; Jin, C. H. Eur. J. Inorg. Chem. 2012, 2012, 4268−4272. (41) Jin, Y. Z.; Yi, Q.; Ren, Y. P.; Wang, X.; Ye, Z. Z. Nanoscale Res. Lett. 2013, 8, 153. (42) Kanehara, M.; Koike, H.; Yoshinaga, T.; Teranishi, T. J. Am. Chem. Soc. 2009, 131, 17736−17737. (43) Zhang, D. Q.; Li, G. S.; Yu, J. C. Cryst. Growth Des. 2009, 9, 2812−2815. (44) Cuya Huaman, J. L.; Sato, K.; Kurita, S.; Matsumoto, T.; Jeyadevan, B. J. Mater. Chem. 2011, 21, 7062−7069. (45) Wang, E. G.; Hou, L. T.; Wang, Z. Q.; Hellström, S.; Zhang, F. L.; Inganäs, O.; Andersson, M. R. Adv. Mater. 2010, 22, 5240−5244. (46) Ratcliff, E. L.; Garcia, A.; Paniagua, S. A.; Cowan, S. R.; Giordano, A. J.; Ginley, D. S.; Marder, S. R.; Berry, J. J.; Olson, D. C. Adv. Energy Mater. 2013, 3, 647−656. (47) Ratcliff, E. L.; Meyer, J.; Steirer, K. X.; Armstrong, N. R.; Olson, D.; Kahn, A. Org. Electron. 2012, 13, 744−749. (48) Chen, S.; Manders, J. R.; Tsang, S.-W.; So, F. J. Mater. Chem. 2012, 22, 24202−24212.
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Synthesis of Unstable Colloidal Inorganic Nanocrystals through the Introduction of a Protecting Ligand Xiaoyong Liang,†,⊥ Qing Yi,†,⊥ Sai Bai,† Xingliang Dai,† Xin Wang,† Zhizhen Ye,† Feng Gao,§ Fengling Zhang,§ Baoquan Sun,∥ and Yizheng Jin*,†,‡ †
State Key Laboratory of Silicon Materials, Cyrus Tang Center for Sensor Materials and Applications, Department of Materials Science and Engineering and ‡Center for Chemistry of High-Performance and Novel Materials, Zhejiang University, Hangzhou 310027, People’s Republic of China § Biomolecular and Organic Electronics, Department of Physics, Chemistry, and Biology, Linköping University SE-581 83 Linköping, Sweden ∥ Jiangsu Key Laboratory for Carbon-Based Functional Materials and Devices, Institute of Functional Nano and Soft Materials (FUNSOM), Soochow University, 199 Ren’ai Road, Suzhou 215123, People’s Republic of China S Supporting Information *
ABSTRACT: We demonstrate a facile and general strategy based on ligand protection for the synthesis of unstable colloidal nanocrystals by using the synthesis of pure p-type NiO nanocrystals as an example. We find that the introduction of lithium stearate, which is stable in the reaction system and capable of binding to the surface of NiO oxide nanocrystals, can effectively suppress the reactivity of NiO nanocrystals and thus prevent their in situ reduction into Ni. The resulting ptype NiO nanocrystals, a highly demanded hole-transporting and electron-blocking material, are applied to the fabrication of organic solar cells and polymer light-emitting diodes, demonstrating their great potential as an interfacial layer for low-cost and large-area, solution-processed optoelectronic devices. KEYWORDS: Protecting ligand, colloidal inorganic nanocrystal, nickel oxide, hole-transporting interlayer, solution-processed optoelectronic device
C
the reaction system, leading to the formation of Ni particles. The instability of colloidal NiO nanocrystals at high temperatures has been reported by many groups.31,32 For instance, O’Brien and co-workers found that the reactions between nickel acetylacetonate and hexadecylamine were rapid and difficult to control, resulting in the formation of Ni particles exclusively.31 Attempts to obtain pure NiO nanocrystals by making the reaction slower have not succeeded so far.31 Here we demonstrate a strategy based on ligand protection for eliminating the unwanted side reaction involved in the synthesis of colloidal NiO nanocrystals. Instead of manipulating the reaction conditions, we propose to reduce the reactivity of the colloidal NiO nanocrystals. Lithium stearate (LiSt), which is stable in the reaction system and can readily bind to the surface of a NiO nanocrystal, is employed to test our hypothesis. The resulting pure NiO nanocrystals are utilized in bulk heterojunction organic photovoltaics (OPVs) and polymer light-emitting diodes (PLEDs), demonstrating their great potential as high-quality hole-transporting interlayers (HTLs) for solution-processed optoelectronic devices. This
olloidal inorganic nanocrystals are attractive functional materials with a unique combination of solid-state properties and excellent solution dispersibility. They have been intensively investigated as building blocks for the fabrication of low-cost, large-area and solution-processed optoelectronic devices.1−8 In many cases, the applications can be greatly advanced by developing new synthetic approaches to produce colloidal nanocrystals with the desired properties.9−19 A good example can be found in NiO, which is an intrinsic ptype and wide bandgap semiconductor with high ionization potential and low electron affinity.20,21 The p-type conductivity of NiO is different from those of several other high work function metal oxides, such as WOx, MoOx, and VOx, which are n-type and cannot serve as hole-transporting and electronblocking materials simultaneously.22,23 The unique electronic structure of NiO offers impressive charge selective holetransporting properties, making NiO thin films attractive for many optoelectronic devices.24−30 However, the synthesis of pure colloidal NiO nanocrystals has not been achieved so far, which hinders their applications in solution-processed devices. The synthesis of pure colloidal NiO nanocrystals has been plagued by an unwanted side reaction. It is well-known that NiO nanocrystals can be readily reduced by alcohol or amine in © 2014 American Chemical Society
Received: January 18, 2014 Published: May 12, 2014 3117
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(Supporting Information Figure S1) while the crystal structure of the larger particles matched the face-centered cubic structure of Ni (Figure 1C). The formation of metal Ni particles can be attributed to the in situ reduction of NiO nanocrystals by ODA. If the reaction time was sufficiently long, eventually all the NiO nanocrystals were converted to Ni particles (Supporting Information Figure S2). We have tried to avoid the unwanted side reaction by adjusting experimental parameters, including reaction temperature, reaction time, and the molar ratio of ODA to Ni(St)2. However, all these efforts failed because a high reaction temperature and an adequate amount of ODA are required to initiate the conversion of Ni precursor. For example, our FTIR analysis indicates that the alcoholysis reaction could only take place at a minimum temperature of 235 °C. At this temperature, the formation of Ni particles was found to be inevitable. We have also tried to modify the reaction system by introducing free stearate acid and this strategy did not work either (Supporting Information Figure S3). We believed that the in situ reduction of colloidal NiO nanocrystals could be suppressed by protecting their surfaces with a proper capping ligand. To this end, LiSt, an alkali fatty acid salt, was employed to achieve this goal. When LiSt was introduced into the reaction system, we obtained pure NiO nanocrystals (Figure 1D−F). No metallic Ni phase could be identified in the XRD pattern of the products (Figure 1D). In this case, we only observed irregular nanoparticles during TEM imaging (Figure 1E). HRTEM characterization (Figure 1F) indicated that all the particles were NiO nanocrystals with decent crystalline features. The NiO nanocrystals could be readily dispersed in organic solvents, such as chloroform, forming suspensions that were stable for months under ambient conditions. A remarkable feature of this synthesis is that the mass yield counted by nickel ions was over 85%. Further studies showed that pure NiO nanocrystals could also be obtained under various conditions, such as a high reaction temperature of 280 °C (Supporting Information Figure S4) and a large molar ratio of ODA to Ni(St)2 at 10 (Supporting Information Figure S5). We also carried out further studies to explore the role of LiSt in enabling the synthesis of pure NiO nanocrystals. The results can be summarized as the following. First, LiSt was stable in the reaction system. We monitored the LiSt-assisted alcoholysis reaction by Fourier transform infrared spectroscopy (FTIR). The appearance of the ester peak (1740 cm−1),15,33 which became stronger as the reaction proceeded (Figure 2A), could be attributed to the conversion of metal carboxylates. The broad peak at 1562 cm−1 corresponding to −COO− vibrations from both Ni(St)2 and LiSt gradually evolved to doublet peaks at 1580 and 1560 cm−1, which could be assigned to the asymmetric stretch vibrations of the unreacted LiSt.36 This result indicates that LiSt is much less reactive than Ni(St)2, which is consistent with the results of control experiments that involved reactions between pure LiSt and ODA or pure Ni(St)2 and ODA at 235 °C. In the case of LiSt, after 3 h the ester peak was not evident while the doublet peaks at 1580 and 1560 cm−1 originating from LiSt36 remained in the FTIR spectrum (Supporting Information Figure S6A). In contrast, FTIR analyses (Supporting Information Figure S6B) show that under identical conditions all the Ni(St)2 was consumed after 3 h. Second, LiSt readily binds to the surface of the NiO nanocrystal. In literature, sodium oleate and potassium oleate
new strategy has also been extended to other systems, suggesting its general applicability for the synthesis of colloidal inorganic nanocrystals. We employed a simple noninjection method based on alcoholysis of metal carboxylates, a general strategy for generating oxide nanocrystals,15,16,33−35 to synthesize NiO nanocrystals. Nickel stearate (Ni(St)2) and 1-octadecanol (ODA) were selected as the reagents. X-ray diffraction (XRD) measurements (Figure 1A) on the products indicated
Figure 1. Synthesis of NiO nanocrystals (A−C) without and (D−F) with the introduction of LiSt. (A) XRD pattern, (B) a typical TEM image, and (C) a typical HRTEM image of the larger and faceted particles in the products of a reaction without the introduction of LiSt. The interplanar spacing of 0.20 and 0.21 nm shown in (C) correspond to the {011} and {002} lattice planes of the face-centered cubic structured Ni (unit cell shown in the inset), respectively. (D) XRD pattern, (E) a typical TEM image, and (F) a typical HRTEM image of the nanocrystals from a reaction with the introduction of LiSt. The interplanar spacing of 0.21 and 0.24 nm shown in (F) correspond to the {200} and {111} lattice planes of the rock salt structured NiO (unit cell shown in inset), respectively.
the coexistence of both NiO (JSPDS card no. 65-2901) and Ni (JSPDS card no. 45-1027) phases. Transmission electron microscopy (TEM) observations (Figure 1B) revealed that the products consisted of two different types of particles. The majority of the products were irregular nanoparticles that often attached to each other, forming nanoflowers. Larger and faceted particles with a higher bright-field contrast were also observed. High-resolution transmission electron microscopy (HRTEM) analyses indicate that the crystal structure of the irregular nanoparticles matched the rock salt structure of NiO 3118
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Figure 2. FTIR analysis of the role of LiSt in the synthesis of colloidal NiO nanocrystals. (A) Temporal evolution of FTIR spectra recorded from a LiSt-assisted reaction. (B) FTIR spectra of the nanocrystals from a reaction without the introduction of LiSt (top), the nanocrystals from the LiStassisted reaction and purified by the first method (middle) and the nanocrystals from the LiSt-assisted reaction and purified by the second method (bottom), respectively.
in Supporting Information Figure S8, when the NiO nanocrystals purified by the second method were intensively washed by hot methanol to remove the surface-bound LiSt, the peak at 1560 cm−1 disappeared in the corresponding FTIR spectrum. Furthermore, refluxing the NiO nanocrystals purified by the first method, that is, NiO nanocrystals without surface-bound LiSt, in a solution containing LiSt led to the adsorption of LiSt and hence the emergence of the peak at 1560 cm−1 in the corresponding FTIR spectrum (Supporting Information Figure S9). We conclude that LiSt can serve as a protecting ligand for the synthesis of pure colloidal NiO nanocrystals (Scheme 1). The
were used as capping ligands to synthesize iron oxide nanocrystals with controlled shapes,37,38 indicating that the ionic alkali carboxylates could readily bind to the surface of the oxide nanocrystals. Here we demonstrate the binding of LiSt to the surface of the NiO nanocrystals by elemental analyses and FTIR measurements (the FTIR assignments are summarized in Supporting Information Table S1) on carefully purified products. Considering that LiSt is soluble in hot methanol and is not soluble in a nonpolar solvent such as hexane, two purification methods were used (refer to the experimental section for details). In the first method, the NiO nanocrystals were purified by extraction three times using a biphase system of hexane and methanol at 50 °C. Elemental analyses on the products showed a low lithium content ([Li]/([Li] + [Ni])) of less than 1.0%, indicating that both free LiSt in the reaction mixtures and majority of the surface-bound LiSt were removed. In the second method, the use of methanol was avoided. Free LiSt in the hexane suspension of nanocrystals was removed by aging the sample overnight, followed by centrifugation and filtration. FTIR measurements on the insoluble precipitates revealed the presence of free LiSt (Supporting Information Figure S7), suggesting efficient separation of unbounded LiSt from the dispersible NiO nanocrystals. Elemental analyses on the nanocrystals purified by the second method revealed a lithium content of 9.0%, much higher than that of the sample purified by the first method. We suggest that the higher lithium content in the samples purified by the second method is due to surface-bound LiSt instead of residue free LiSt based on the following characterizations. FTIR analyses (Figure 2B middle) on the nanocrystals purified by the first method showed that the surface ligands were stearate ions, as indicated by the asymmetric stretch vibration at 1550 cm−1.15,33 A similar vibration band was also found in the FTIR spectra of the products from the reactions without the introduction of LiSt (Figure 2B top). FTIR measurements on the NiO nanocrystals purified by the second method revealed the existence of an extra peak at 1560 cm−1 (Figure 2B bottom). This peak is not due to free LiSt, which has doublet peaks at 1580 and 1560 cm−1, with the intensity of the former peak stronger than that of the latter (Supporting Information Figure S7). We attribute the extra peak at 1560 cm−1 to the surface-bound LiSt. This assignment is confirmed by our control experiments. As shown
Scheme 1. Synthesis of Colloidal NiO Nanocrystals with LiSt As a Protecting Ligand
binding of inert LiSt to the surfaces of NiO nanocrystals significantly suppressed their reactivity. In other words, the LiSt protected the NiO nanocrystals, preventing them from being reduced to Ni. In this way, pure NiO nanocrystals were obtained in high yields, over 85%. The protecting effect of LiSt for NiO nanocrystals is confirmed by the fact that once the surface-bound LiSt is removed, the reactivity of the NiO nanocrystals is recovered. As shown in Supporting Information Figure S10, the deprotected NiO nanocrystals, that is, the NiO nanocrystals without surface-bound LiSt, were not resistant to alcohol at high temperatures and were readily reduced to Ni particles. We emphasize that the binding of LiSt to the surfaces of NiO nanocrystals is essential to reducing their reactivity. Molecules that are stable in the reaction system and do not 3119
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Figure 3. Nanocrystals from the reactions with Cu(acac)2 and oleylamine as the reagents. (A) XRD profiles of the nanocrystals from the reactions without (top) or with (bottom) the introduction of LiSt. In the top profile, the peaks at 36.5 and 42.4°, which correspond to the Cu2O phase, were due to the oxidation of Cu nanocrystals prior to the XRD measurements. (B) A typical TEM image of the nanocrystals from a reaction without the introduction of LiSt. (C) A typical TEM image of the nanocrystals from the LiSt-assisted reactions.
Figure 4. Thin films of NiO nanocrystals as HTLs for solution-processed devices. (A) AFM height topography, (B) UPS spectrum (light source: He I 21.2 eV), and (C) transmission spectrum of the NiO nanocrystal thin films (on ITO-glass substrates). The spectrum of a ITO-glass substrate was also plotted. (D) Device structure, (E) J−V curves, and (F) log plots of the dark J−V characteristics of the organic solar cells. (G) Device structure and (H) curves of current density and luminance versus applied bias of the PLED devices. (I) A digital picture of a flexible PLED device.
bind to the surfaces of NiO nanocrystal cannot act as a protecting ligand. For example, our control experiments show that simply adding n-octadecane, which is structurally similar to LiSt in terms of the length of the alkyl chain, into the reaction mixture did not prevent the reduction of NiO nanocrystals (Supporting Information Figure S11). Following the successful demonstration of LiSt-assisted synthesis of NiO nanocrystals and the role of LiSt as a protecting ligand, we investigated the applicability of this strategy to other systems. First, we show that using LiSt to
suppress the reactivity of NiO nanocrystals is valid for aminolysis reactions, another general route to the synthesis of oxide nanocrystals.34,39−42 When oleylamine was employed to activate Ni(St)2, the introduction of LiSt into the reaction system resulted in pure NiO nanocrystals (Supporting Information Figure S12A,B). If LiSt was absent, as shown in Supporting Information Figure S12C,D, the NiO nanocrystals were converted to Ni particles. Second, we demonstrate that the choice of protecting ligand is not limited to LiSt. The use of NaSt also led to pure colloidal NiO nanoparticles (Supporting 3120
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PCEs of the devices with NiO nanocrystal HTLs arose from enhanced fill factor (FF) and short-circuit current (Jsc). In addition, as shown in Figure 4F, the reverse saturation dark current of the devices with NiO nanocrystal HTLs were 1 order of magnitude lower than that of the devices with PEDOT:PSS interlayers. These impressive characteristics reveal the charge selective nature and superior hole extraction properties of NiO nanocrystal HTLs.46,47 A device structure of ITO/NiO/poly [2-methoxy-5-(2ethylhexyloxy)-1,4-phenylenevinylene] (MEH-PPV)/Ca/Al (Figure 4G) was used to fabricate PLEDs. PLEDs with PEDOT:PSS HTLs were also fabricated. The turn-on voltage (Von) (the voltage that a luminance of 1 cd/m2 was observed) for both devices is same, 2.3 V. The current of the device with NiO nanocrystal HTLs increased steeply just after turn-on, yielding a maximum luminance of 13 500 cd/m2 at 8 V. The peak external quantum efficiency (EQE) and maximum current efficiency were 1.08% and 1.45 cd/A, respectively. These values were comparable with those of the devices with PEDOT:PSS interlayers (Figure 4H and Table 2).
Information Figure S13). FTIR analyses (Supporting Information Figure S13C) showed that the NaSt-assisted reactions exhibited features similar to those of the LiSt-assisted ones. Third, we show that the concept of protecting ligand is feasible for controlling the synthesis of other colloidal nanocrystals. The in situ reduction of oxide nanocrystals by alcohol or amine at elevated temperatures has been identified to be a key intermediate step in the formation of a number of metal nanoparticles.43,44 For example, Huaman and co-workers reported a hydroxyl ion-assisted alcohol reduction method for the synthesis of Cu nanocrystals through intermediate steps corresponding to the formation of copper oxides.44 Therefore, the synthesis assisted by a protecting ligand, which significantly influences the reactivity of oxide nanocrystals, is readily applied to these material systems. As a demonstration, we conducted reactions with copper(II) acetylacetonate (Cu(acac)2) and oleylamine as the reagents. Pure Cu nanocrystals were generated (Figure 3A,B). If LiSt was introduced into the reaction system, Cu2O nanocrystals were obtained and the conversion of Cu2O nanocrystals to metal particles was inhibited (Figure 3A,C). Next we show that the colloidal p-type NiO nanocrystals from the LiSt-assisted synthesis are promising candidates as HTLs for solution-processed devices. The NiO nanocrystals, purified by the first method to remove the surface-bound LiSt, were deposited onto ITO-glass substrates by spin-coating, followed by mild annealing at 130 °C and UV-ozone treatment. SEM characterizations of the as-deposited films, annealed films, and UV-ozone treated films show that all the films were crackfree and exhibited uniform surface features (Supporting Information Figure S14). Atomic force microscopy (AFM) measurements (Figure 4A) indicated that the thin film of NiO nanocrystals exhibited a flat surface with a root-mean-square roughness of 3.6 nm. Ultraviolet photoelectron spectroscopy (UPS) results shown in Figure 4B suggested that the work function of the NiO nanocrystal thin films was 5.2 ± 0.1 eV. The valence band edge was ∼0.4 eV below the Fermi level, consistent with the p-type nature of NiO. Optical transmission spectrum (Figure 4C) of the thin films of NiO nanocrystals on ITO-glass substrates showed an average transmittance of around 90% in the visible and near-infrared region, revealing excellent optical transparency. The NiO nanocrystal thin films were applied as HTLs in OPVs. A model system of poly[2,3-bis(3-octyloxyphenyl)quinoxaline-5,8-diyl-alt-thiophene-2,5-diyl] (TQ1)/[6,6]-phenyl-C71-butyric acid methyl ester (PC71BM)45 was used for the fabrication of solar cells (Figure 4D). Control devices with poly(3,4-ethylenedioxythiophene):poly(styrene-sulfonate) (PEDOT:PSS) interlayers were also fabricated. The devices with NiO nanocrystal HTLs exhibited an average power conversion efficiency (PCE) of 6.1% and a champion PCE of 6.3% (Figure 4E). In contrast, the average PCE for the devices with PEDOT:PSS was 5.0%. The electrical output characteristics of the solar cell devices are summarized in Table 1. The improved
Table 2. Device Parameters of the PLEDs Based on MEHPPV Emissive Layers HTLs NiO nanocrystal PEDOT:PSS
NiO nanocrystal PEDOT:PSS
Voc (V)
JSC (mA cm−2)
FF
best PCE (%)
average PCE (%)
0.88
10.2
0.71
6.3
6.1 + 0.2
0.86
9.4
0.64
5.2
5.0 + 0.2
maximum LV @ 8 V (cd/m2)
peak EQE (%)
maximum CE (cd/A)
2.3
13 550
1.08
1.45
2.3
11 200
1.04
1.42
In addition, the processing temperature as low as 130 °C makes it possible to integrate NiO nanocrystal HTLs into flexible electronics. MEH-PPV PLED devices with NiO nanocrystal interlayers spun on polyethylene-naphthalate (PEN)/ITO substrates were fabricated (Figure 4I). The resulting flexible PLED devices exhibited decent performance with a Von of 2.4 V, a maximum luminance of 8840 cd/m2 at 8 V, and a peak EQE of 1.08% (Supporting Information Figure S15). Note that the annealing temperatures in most reports involving the deposition of NiO thin films by using nickel precursor solutions were higher than 250 °C.27,47,48 The nanocrystal approach presented here, which decoupled the high-temperature synthesis from the deposition of thin films, provides an attractive solution for applying NiO HTLs in flexible electronics. In conclusion, we have demonstrated a general strategy based on ligand protection for the synthesis of pure p-type NiO colloidal nanocrystals. The key finding is that the reactivity of the NiO nanocrystals could be suppressed by the introduction of LiSt, which was stable in the reaction system, readily bind to the surfaces of oxide nanocrystals, and could be removed under mild conditions by simple purification methods. The LiSt served as a protecting ligand for colloidal NiO nanocrystals. In this way, the unwanted in situ reduction of NiO nanocrystals was avoided and high-quality pure p-type NiO nanocrystals were obtained in high yields. The high work function, excellent optical transparency, flat surface features, charge selective nature, and the compatibility with flexible electronics of the NiO nanocrystal thin films are attractive for HTL applications in solution-processed optoelectronic devices. Considering the unique properties of NiO, we expect that further investigation on the material synthesis (e.g., alloying or doping of NiO nanocrystals) and film processing (e.g., ligand exchange or
Table 1. Device Parameters of the OPV Cells Based on TQ1/PC71BM HTLs
Von (V)
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postdeposition treatments) will lead to a variety of highperformance solution-processed devices. We further demonstrated that the strategy based on protecting ligand is a general approach to controlling the reactivity of colloidal nanocrystals, which may shed light on future rational design of synthetic schemes. Our study provides an excellent example that new concept for synthetic chemistry of colloidal inorganic nanocrystals expands the synthesis-by-design capabilities, leading to new functional materials with targeted properties that fulfill the requirements of practical applications. Methods. Synthesis of NiO Nanocrystals (without the Introduction of LiSt). A noninjection approach was used. For a typical synthesis, Ni(St)2 (0.5 mmol), ODA (3 mmol), and 1octadecene (ODE, 5 mL) were loaded in a three-neck flask and degassed at 80 °C for 30 min. The reaction mixture was then heated to 235 °C and kept at this temperature for 3 h under argon flow. The products were precipitated out by adding a mixture of ethanol and ethyl acetate and further purified by dispersing/precipitating twice using the combination of hexane/ethanol. LiSt-Protected Synthesis of NiO Nanocrystals. The protocol was the same as above except for the addition of 0.2 mmol of LiSt. When the reaction was completed, the product was purified by two methods. In the first method, a procedure of extraction using a biphase system of hexane and methanol at 50 °C was applied three times. Next, the nanocrystals were precipitated out by adding a mixture of ethanol and ethyl acetate, followed by dispersing/precipitating twice using the combination of hexane/ethanol. In the second method, the nanocrystals were precipitated out by adding a mixture of acetone and ethyl acetate. Next the nanocrystals were dispersed in hexane and then precipitated by adding ethanol to remove the remaining ODA. Then the nanocrystals were dispersed in hexane and allowed to stay overnight. The insoluble precipitates were removed by centrifugation. The nanocrystal suspension was further purified by filtering through PTFE membranes (0.22 μm). NaSt-Protected Synthesis of NiO Nanocrystals. The synthetic procedure was similar to what was used for the synthesis of NiO nanocrystals with the introduction of LiSt except that LiSt was replaced by NaSt (0.2 mmol). Synthesis of Cu and Cu2O Nanocrystals. Cu(acac)2 (0.5 mmol) and oleylamine (10 mL) were loaded in a three-neck flask and degassed at 80 °C for 30 min. Next the reaction mixture was heated to 200 °C and kept at this temperature for 1 h under argon flow. The products were precipitated out by adding a mixture of ethanol and ethyl acetate and further purified by dispersing/precipitating twice using the combination of hexane/ethanol. In a control experiment, additional LiSt (0.5 mmol) was introduced into the reaction flask while all other parameters were kept the same. Materials, characterization techniques, as well as details of device fabrication and characterization are provided in the Supporting Information.
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Letter
AUTHOR INFORMATION
Corresponding Author
*E-mail: [email protected]. Author Contributions ⊥
X.L. and Q.Y. contributed equally to this work.
Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS We thank Professor Xiaogang Peng, Professor Chao Gao, Dr. Hang Qi, and Dr. Zhen Xu (Zhejiang University, China) for discussions. We thank Professor Liwei Chen and Mr. Qi Chen (Suzhou Institute of Nanotech and Nanobionics, Chinese Academy of Science) for the assistance with AFM characterization and Dr. Ergang Wang (Chalmers University of Technology, Sweden) for providing the TQ1 polymer. This work was financially supported by the National High Technology Research and Development Program of China (2011AA050520), the National Basic Research Program of China (973 Program) (2012CB932402), the National Natural Science Foundation of China (51172203), the Natural Science Funds for Distinguished Young Scholar of Zhejiang Province (R4110189) and the Public Welfare Project of Zhejiang Province (2013C31057). F.G. acknowledges the financial support of the European Commission under a Marie Curie Intra-European Fellowship for Career Development.
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ASSOCIATED CONTENT
S Supporting Information *
Materials, characterization techniques, details of device fabrication and characterization, and additional data (including TEM, HRTEM, XRD, and FTIR) associated with the synthesis of oxide nanocrystals. This material is available free of charge via the Internet at http://pubs.acs.org. 3122
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