DOI: 10.1002/chem.201404220
Communication
& Nanoparticles
Synthesis and Characterization of Ag8(Ge1 x,Snx)(S6 y,Sey) Colloidal Nanocrystals Bin Zhou, Yedi Xing, Shu Miao, Mingrun Li, Wen-Hua Zhang,* and Can Li*[a] Abstract: A facile colloidal approach to synthesize Ag8(Ge1 x,Snx)(S6 y,Sey) nanocrystals (NCs) in a highly controlled way across the entire compositional ranges (0 x 1, 0 y 6) has been developed. The NCs exhibit a uniform size distribution, highly crystalline structure, over 1 g scalable synthesis, and tunable band gaps in the range of 0.88–1.45 eV by varying their chemical compositions. The Ag8GeS6 NCs with a band gap of approximately 1.45 eV were employed as a model light harvester to assess their applicability in solar cells by a full solution-processing device, yielding an efficiency of 0.28 % under AM1.5 illumination, demonstrating their application potential in solar energy utilization.
Colloidal semiconductor nanocrystals (NCs) have attracted remarkable attention due to their novel properties that are distinct from their bulk counterparts and offer great application potential, for example, in bioimaging,[1] light-emitting devices,[2] photovoltaics,[3] and thermoelectrics.[4] They exhibit interesting properties that can be controlled by varying their size,[5] structure,[6] or shape,[1b, 7] which are difficult to achieve in their bulk analogues. The field of semiconductor NC synthesis has been dominated by binary systems, in which precise control over their dimensions and crystal phase has been well developed. It is currently highly desirable to exploit more complex multicomponent nanocrystals (MC-NCs).[8–10] MC-NCs have the ability to tune their optical and electronic properties by controlling chemical compositions, which may affect significantly the performance of devices made from them. Moreover, the band gaps of the MC-NCs can be consecutively tailored by alloying the constituents in a wide range, providing an alternative to the quantum confinement effect. Therefore, it is currently very appealing to study systematically the synthesis, properties, and application of MC-NCs. For instance, successes have been achieved with a number of chalcogenide MC-NCs by alloying two constituents to show consecutively tunable properties, for example, Cu2(S,Se),[8] Pb(S,Se),[9] (Zn,Cd)(S,Se),[10] [a] B. Zhou, Y. Xing, Prof. S. Miao, Prof. M. Li, Prof. W.-H. Zhang, Prof. C. Li State Key Laboratory of Catalysis Dalian Institute of Chemical Physics, Chinese Academy of Sciences Dalian National Laboratory for Clean Energy, Dalian 116023 (China) E-mail: [email protected] [email protected] Supporting information for this article is available on the WWW under http://dx.doi.org/10.1002/chem.201404220. Chem. Eur. J. 2014, 20, 12426 – 12431
Cu2Ge(S,Se)3,[11] Cu2ZnSn(S,Se)4,[12] Cu2Zn(Sn,Ge)Se4,[13] and Cu(In,Ga)(S,Se)2.[14] Nevertheless, in comparison with the binary systems, there is still the major challenge of balancing the reactivity of each precursor in a multicomponent system, and thus it remains a challenge to achieve the controlled synthesis of targeted MC-NCs. The ever increasing demand for clean and sustainable energy necessitates the development of solar energy utilization. The colloidal NCs offer scalable synthesis, wet-chemical processing, and tunable photoelectric properties, thereby providing a possible strategy to address the challenge for developing cost-effective optoelectrical devices, for example, the solution-processing solar cells. In this regard, I–III–VI NCs [e.g., CuInS2/CuInSe2 and Cu(In,Ga)(S,Se)2] and I–II–IV–VI NCs [such as CuZnSn(S,Se)4] have shown significant promise; hence, extensive research has been conducted to understand their synthesis and their application in solution-processed NC solar cells with impressive performance,[15] demonstrating the potential in photovoltaic devices derived from semiconductor NCs. Along the above lines, I–IV–VI semiconductors are potentially interesting, yet not well studied for solar energy utilization. They exhibit band gaps suitable for sunlight harvesting, high absorption coefficients, and carrier mobility in bulk materials. The current studies of I–IV–VI materials were mainly focused on Cu2GeSe3,[16] Cu2SnS3,[17] and Cu2SnSe3,[7b, 18] whereas Agbased I–IV–VI NCs have been largely overlooked. Qian and coworkers reported the synthesis of Ag8SnS6 and Ag8SnSe6 nanomaterials using a solvothermal process, which resulted in products exhibiting agglomeration and broad size distributions.[19] To the best of our knowledge, the synthesis of Ag8(Ge1 x,Snx)(S6 y,Sey) (0 x 1, and 0 y 6) NCs in a controlled way has not been reported to date, hampering the studies of their properties and potential applications. Herein we demonstrate a facile solution approach to the synthesis of Ag-based I–IV–VI NCs with the nominal formula of Ag8(Ge1 x,Snx)(S6 y,Sey) (0 x 1, 0 y 6), and presented their fundamental properties. The chemical compositions of the resulting NCs are highly tunable in the entire compositional ranges (0 x 1, 0 y 6), and the band gaps of the NCs were tuned in the range of 0.88–1.45 eV, and the latter one is ideal for solar light absorption. Therefore, we have assessed their applicability as photovoltaic materials by fabricating solar cells using Ag8GeS6 NCs (with a band gap of 1.45 eV) as a light harvester by a low-cost solution process. The synthesis of Ag8(Ge1 x,Snx)(S6 y,Sey) colloidal NCs was performed in oleylamine (OLA) solution by a simple non-injection synthetic approach. In a typical synthesis, the OLA-AgNO3
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Communication solution, OLA-S solution, Ge[N(SiMe3)2]2, Sn[N(SiMe3)2]2, bis(trimethyl- silyl)selenide (TMSe), and bis(trimethylsilyl)sulfide (TMS) with specific ratios were loaded into a Schlenk tube under an inert atmosphere. The reaction tube was then inserted into a preheated oil bath at 220 8C and held at this temperature for 15 min to produce Ag8(Ge1 x,Snx)(S6 y,Sey) NCs. We additionally demonstrated the possibility of up-scaling this method by performing the gram-scale synthesis of Ag8Ge(S2,Se4) NCs. Full experimental details can be found in Experimental Section. Figure 1 presents powder X-ray diffraction (XRD) patterns of the as-synthesized Ag8Ge(S6 y,Sey) NCs with 0 y 6. The
Figure 1. XRD patterns of the Ag8Ge(S6 y,Sey) NCs with various S/Se ratios (0 y 6). a) Ag8GeS6, b) Ag8Ge(S4,Se2), c) Ag8Ge(S2,Se4), and d) Ag8GeSe6
broadening of the reflection peaks on the background occurs, which is typical of small NCs. All the diffraction peaks of the Ag8GeS6 NCs and Ag8GeSe6 NCs match well with the major peaks of Canfieldite-structured Ag8GeS6 (JCPDS No. 44-1416) and Ag8GeSe6 (JCPDS No. 71-1690), respectively. Additionally, as the Se content increases, the major reflection peaks systematically shift toward lower angles, which is attributed to the replacement of the smaller S atoms (1.84 ) by the larger Se atoms (1.98 ) in the lattices of the resulting NCs. Very importantly, no additional peak or peak splitting can be detected by XRD, thus precluding the possibility of phase separation that often takes place in the preparation of MC-NCs. Therefore, pure phase Ag8Ge(S6 y,Sey) NCs were successfully obtained, with the characteristics that the chemical compositions can be consecutively tailored across the entire compositional range (0 y 6). This is in line with Vegard’s law,[20] demonstrating the formation of alloyed Ag8Ge(S6 y,Sey) NCs in a highly controlled way. We next applied transmission electron microscopy (TEM) to gain more insight into the microstructures of the Ag8Ge(S6 y,Sey) NCs (0 y 6) (see Figure 2). The low-magnification TEM images reveal a uniform size distribution of the Ag8Ge(S6 y,Sey) NCs. The polycrystalline selected area electron diffraction (SAED) patterns of the NCs show clearly diffraction rings that match well with the XRD patterns of the corresponding Ag8Ge(S6 y,Sey) NCs (shown in Figure 1). High-resolution transmission electron microscopy (HRTEM) images show that all Chem. Eur. J. 2014, 20, 12426 – 12431
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Figure 2. TEM analysis of the Ag8Ge(S6 y,Sey) (0 y 6) NCs. a) Ag8GeS6, b) Ag8Ge(S4,Se2), c) Ag8Ge(S2,Se4), and d) Ag8GeSe6 NCs; 1, 2, 3, and 4 represent the corresponding low magnification TEM images, HRTEM images, SAED patterns, and EDS spectra for these samples. The inset scale bar is 5 nm for HRTEM images.
Ag8Ge(S6 y,Sey) NCs are highly crystalline with continuous lattice fringes. The elemental composition of the as-prepared Ag8Ge(S6 y,Sey) NCs was analyzed by energy-dispersive X-ray spectroscopy (EDS) (see Table S1 in the Supporting Information), which showed that all samples have a similar Ag/Ge molar ratio of approximately 8/1, and that increasing the Se
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Communication content was accompanied by a decrease of the S content in the ranges of 0 y 6.The elemental line scan results (see Figure S1) confirmed the homogeneous distribution for all of these elements in the nanocrystals. These results indicate the successful formation of well-crystalline Ag8Ge(S6 y,Sey) NCs, displaying a uniform size distribution (Figure S2) with varied S/Se ratios across the whole compositional range (0 y 6). On the basis of Vegard’s law,[20] semiconducting alloys have tunable band gaps that change upon varying their compositions with a nearly linear relationship. UV-Vis-NIR absorption spectra of Ag8Ge(S6 y,Sey) NCs (0 y 6) were therefore measured to study their optical properties and to determine their band gaps. A clear solution of each Ag8Ge(S6 y,Sey) NC sample (0 y 6) showed a continuous absorption spectrum spanning the whole visible spectrum to the near IR region (see Figure 3 a), resulting in the black color of the Ag8Ge(S6 y,Sey) NCs.
Figure 3. a) UV-Vis-NIR absorption spectra for the Ag8Ge(S6 y,Sey) NCs with various S/Se ratios (0 y 6); b) relationship between the band gaps of the Ag8Ge(S6 y,Sey) NCs and the contents of Se/S in the products.
Tauc plots (see Figure S3) were performed to determine the optical band gaps with a relationship of the [ahn]1/2 versus the energy, revealing that Ag8GeS6 NCs exhibit an indirect band gap of 1.45 eV, which covers most of the sunlight spectrum from the visible to the near infrared ranges. Moreover, the band gap energies are progressively reduced upon increasing the Se content in the Ag8Ge(S6 y,Sey) NCs (0 y 6), presenting a nearly linear relationship between the band gaps and the S/Se content (Figure 3 b). These results further demonstrate the successful formation of homogeneous alloyed Ag8Ge(S6 y,Sey) NCs (0 y 6) in the present experiments. To further exploit the suitability of the present synthesis of Ag8Ge(S6 y,Sey) NCs for other Ag-based I–IV–VI materials, we have expanded the present synthetic strategy to the preparation of Ag8(Ge1 x,Snx)(S6 y,Sey) NCs (0 x 1), where the S/Se molar ratio was kept at 2/4 (i.e., y = 4) for simplicity. Experimental results indicate again that the Ge/Sn molar ratio can be tuned across the entire compositional range for the Ag8(Ge1 x,Snx)(S2,Se4) NCs (0 x 1). XRD (Figure S4), TEM (Figures S5–S9), and UV-Vis-NIR absorption spectra (Figures S10 and S11) all confirm the formation of the homogeneous alloyed Ag8(Ge1 x,Snx)(S2,Se4) NCs (0 x 1) with a narrow size distribution. Moreover, growth of the Ag8Sn(S6 y,Sey) (0 y 6) NCs has also been examined by this synthetic approach, and Chem. Eur. J. 2014, 20, 12426 – 12431
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monodisperse NCs were once again obtained in the entire composition ranges (0 y 6). Therefore, we have presented a facile solution approach to achieve the synthesis of Ag8(Ge1 x,Snx)(S6 y,Sey) NCs in a highly controlled way, and achieved precise control over their chemical compositions, optical properties, and band gaps across the entire compositional ranges (0 x 1, 0 y 6). Additionally, we tested the possibility of up-scaling the synthesis of the Ag8Ge(S2,Se4) NCs using eight-times larger amounts of each precursor in the reaction; this resulted in over 1 g of product per run with a narrow size distribution and pure phase (Figure S12), and demonstrated the feasibility of the present synthesis method to the Ag-based I–VI–VII NCs. The 1.45 eV band gap of the Ag8GeS6 NCs makes them very promising as a light absorber for single-junction solar cells, the Ag8GeS6 NCs were hence chosen as the model materials to assess the photovoltaic applications. It is beneficial to understand the conducting behavior of the semiconductor NCs prior to exploiting their device application. The transient photocurrents of the Ag8GeS6 NC films were then recorded in a photoelectrochemical cell. The electrode of the Ag8GeS6 nanocrystal film generates a cathodic photocurrent that increases gradually with increasing negative bias (Figure 4 a) upon illumination (100 mW cm 2), which is typical of p-type semiconducting behavior.[21] Under a constant bias of 0.2 V, the photocurrents of the Ag8GeS6 NC film increased very quickly upon turning on the light, and drop to their pre-illumination values without apparent degradation over many cycles upon turning on and off the illumination. (Figure 4 a inset). These results disclose that the Ag8GeS6 NCs are sensitive to light illumination and are stable under the experimental conditions, which would be favorable for further exploring their device application. The band energy levels of the Ag8GeS6 NCs were then estimated by cyclic voltammetry (CV) measurements of the NC film along with UV-Vis-NIR spectra. CV results (Figure S13) reveal that the conduction band minimum (CBM) of the Ag8GeS6 NCs was at approximately 3.7 eV relative to the vacuum level, and the valence band maximum (VBM) was hence estimated to be approximately 5.1 eV on the basis of the 1.45 eV optical band gap. The CBM of Ag8GeS6 NCs therefore matches well with that of CdS (CBM ~ 4.0 eV). Consequently, photovoltaic devices were fabricated in the configuration of a FTO/TiO2 compact layer (c-TiO2)/CdS nanorods (NRs)/Ag8GeS6 NCs/Spiro-MeTAD/ Au, and the relative band positions of the materials employed in the devices is given in Figure 4 b. The CdS NRs prepared on FTO coated with a compact layer of TiO2, by a hydrothermal approach,[22] were employed as the n-type electron transporter, and the Ag8GeS6 NCs, as the light harvester, were deposited onto them by spin coating of a nanocrystal solution by using a layer-by-layer approach. A Spiro-MeOTAD interfacial layer was then deposited onto them because it favors hole extraction and eases the formation of the ohmic contact for the back electrode, resulting in a relatively high device performance for solar cells.[23] The Ag8GeS6 NC film thickness must be slightly greater than the height of the CdS NRs to avoid direct contact between the Spiro-MeOTAD layer and the CdS NRs. A typical cross-sectional SEM image (Figure S14) indicates
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Communication structure. The band gaps of the NCs could be effectively tuned in the range of 0.88–1.45 eV by varying their chemical compositions. Preliminary application in photovoltaic devices has demonstrated the usefulness of the Ag8GeS6 NCs as a light absorber in fully solution-processing solar cells, generating efficiency of 0.28 % under AM1.5 illumination. These results should be beneficial for further investigating the synthesis, properties, and potential applications of these systems in the future.
Experimental Section All experiments were performed under nitrogen atmospheres using Schlenk techniques. AgNO3, S and Se stock solutions: A 0.1 m OLA-AgNO3 stock solution was made by dissolving AgNO3 (1.702 g, 10 mmol) into OLA (100 mL) under stirring overnight in a glove box, and a 1 m OLA-S stock solution was prepared by the same process. A 0.5 m OLA-Se stock solution was made by dissolving selenium powder (0.790 g, 10 mmol) into OLA (20 mL) under stirring at 270 8C for 4 h. Synthesis of Ag8GeS6 NCs and Ag8SnS6 NCs: In the synthesis of Ag8GeS6 NCs, 0.1 m OLA-AgNO3 stock solution (8 mL), GeI4 (0.116 g, 0.2 mmol), TMS (128 mL, 0.6 mmol) and 1 m OLA-S stock solution (1.5 mL) were loaded sequentially into a Schlenk reaction tube inside a glovebox. The reaction tube was then inserted into a preheated oil bath at 220 8C with constant stirring, and maintained at this temperature for 15 min to produce a black solution. Afterward, the colloidal solution was rapidly cooled to room temperature with a water bath, then precipitated with ethanol (20 mL) and centrifuged at 8000 rpm for 5 min. The upper solution was discarded and the nanoparticles were additionally purified twice by re-dispersing them in hexane and precipitating with ethanol. The nanoparticles were re-dispersed in hexane and stored in glove box until further use. The synthesis of Ag8SnS6 NCs was carried out under otherwise identical conditions to Ag8GeS6 NCs, using SnI4 as the Sn precursor. 2
Figure 4. a) Transient photocurrent response of the Ag8GeS6 NCs film at an illumination intensity of 100 mW cm , and the active device area was 0.64 cm2. Inset represents the photocurrents obtained under constant bias of 0.2 V. b) Scheme of the energy levels of the materials employed in the devices; c) J–V curves and d) EQE spectra of the solar cells with the configuration of FTO/c-TiO2/CdS NRs/Ag8GeS6 NCs/Spiro-MeOTAD/Au.
the resulting Ag8GeS6 NC film has a thickness of approximately 700 nm, which is slightly greater than the height of the CdS NRs (~ 600 nm). Thus, a thin layer of Ag8GeS6 NC film separates the tops of the CdS NRs from the Spiro-MeOTAD/Au contacts. The corresponding device performance was determined by current density–voltage (J–V) measurement under simulated AM 1.5G (100 mW cm 2) solar irradiation in air. Figure 4 c presents the photovoltaic characteristics, showing an open-circuit voltage (Voc) of 0.57 V, short-circuit current density (Jsc) of 0.79 mA cm 2, and fill factor (FF) of 0.63, resulting in a power conversion efficiency (h) of 0.28 %. A control cell device without the Ag8GeS6 NCs was also fabricated under otherwise identical conditions to the Ag8GeS6 NC cell. However, its performance (Figure S15) is quite poor, with an obtained PCE of only 0.02 %. Figure 4 d shows the external quantum efficiency (EQE) spectrum for the Ag8GeS6 NC-based cell. Photocurrent generation starts at approximately 870 nm, in agreement with the band gap of the Ag8GeS6 NCs, while the sharp decay at approximately 520 nm is due to the absorption of CdS, which has no contribution to the net photocurrent generation. These results demonstrate the key role of the Ag8GeS6 NCs as the ptype light harvester. It is expected that the device performance can be improved significantly for these MC-NC-based photovoltaic cells by optimizing the device fabrication, for example by displacing large organic ligands by shorter organic groups or inorganic ligands at the NC surface to improve the charge carrier transfer and transport through the NCs. In summary, we have presented a facile solution approach to realize the synthesis of Ag8(Ge1 x,Snx)(S6 y,Sey) NCs in a highly controllable way across the entire compositional range (0 x 1, 0 y 6). The Ag8(Ge1 x,Snx)(S6 y,Sey) NCs displayed uniform size distributions along with a highly crystalline Chem. Eur. J. 2014, 20, 12426 – 12431
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Synthesis of Ag8GeSe6 NCs and Ag8SnSe6 NCs: In the synthesis of Ag8GeSe6 NCs, 0.1 m OLA-AgNO3 stock solution (8 mL), Ge[N(SiMe3)2]2 (39 mL, 0.1 mmol), TMSe (150 mL, 0.6 mmol) and 0.5 m OLA-Se stock solution (3 mL) were loaded sequentially into a Schlenk reaction tube inside a glove box, while keeping all other steps the same as the synthesis of Ag8GeS6 NCs. The resulting colloidal solution was rapidly cooled to room temperature, and precipitated with ethanol (20 mL) and centrifuged at 8000 rpm for 5 min. The resulting nanoparticles were re-dispersed in chlorobenzene and stored in glove box prior to their characterization. The synthesis of Ag8SnSe6 NCs was performed with the same procedure by using Sn[N(SiMe3)2]2 instead of Ge[N(SiMe3)2]2 as the Sn precursor. Synthesis of Ag8(Ge1 x,Snx)(S6 y,Sey) (0 x 1, 0 < y < 6) alloyed NCs: The Ag8(Ge1 x,Snx)(S6 y,Sey) NCs were synthesized following
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Communication similar procedures to those used for the Ag8GeSe6 NCs by replacing Ge[N(SiMe3)2]2 with Sn[N(SiMe3)2]2, and TMS with TMSe partly or wholly, respectively, where the overall ratio Ag/(Ge+Sn)/(TMS+TMSe) remained stoichiometric. For example, for the synthesis of Ag8(Ge0.5,Sn0.5)(S2,Se4) NCs, 0.1 m OLA-AgNO3 stock solution (8 mL), Ge[N(SiMe3)2]2 (19.5 mL, 0.05 mmol), Sn[N(SiMe3)2]2 (19.5 mL, 0.05 mmol), TMSe (100 mL, 0.4 mmol), TMS (43 mL, 0.2 mmol) and 1 m OLA-S (1.5 mL) were used while keeping all other steps the same as for the synthesis of Ag8GeSe6 NCs. Gram-scale synthesis of Ag8Ge(S2,Se4) NCs: For a gram-scale synthesis of Ag8Ge(S2,Se4) NCs, 0.4 m OLA-AgNO3 stock solution (16 mL), Ge[N(SiMe3)2]2 (312 mL, 0.8 mmol), TMSe (801.6 mL, 3.2 mmol), TMS (342.4 mL, 1.6 mmol) and 2 m OLA-S stock solution (6 mL) were loaded sequentially into a Schlenk reaction tube inside a glove box. The reaction tube was then inserted into a preheated oil bath at 220 8C with constant stirring, and maintained at this temperature for 15 min to produce a black solution, as described before. Afterward, the colloidal solution was rapidly cooled to room temperature with a water bath and cleaned twice using the same procedure as before, and finally the NCs were dispersed in toluene. Fabrication of photovoltaic devices: Prior to the growth of CdS nanorods, a dense TiO2 layer was first deposited on a cleaned FTO glass substrate by dip-coating with a TiO2 organic solution (made by using a titanium butoxide/diethanolamine/absolute ethanol solution), followed by a sintering at 500 8C for 30 min. CdS nanorod arrays on TiO2-coated FTO were then prepared by using a literature method with incorporated modifications.[22] Briefly, 0.05 m Cd(NO3)2 (20 mL), 0.05 m thiourea solution (20 mL), and 0.03 m glutathione solution (20 mL) were mixed and loaded into a 100 mL autoclave containing a piece of TiO2-coated FTO substrate. This mixture was then subjected to hydrothermal treatment at 200 8C for 3 h to give 500–600 nm CdS nanorod arrays on TiO2-coated FTO. These CdS nanorod arrays are employed as the n-type electron conducting materials of solar cells with Ag8GeS6 NCs as the p-type light harvester. In a typical fabrication of a nanocrystal solar cell, purified Ag8GeS6 NCs (0.4 mmol) were dispersed in CB (4 mL), then mixed with 1-butylamine (4 mL), MPA (1 mL) in a 20 mL vial for ligand exchange. The vial was sealed and sonicated for 45 min, then the Ag8GeS6 NCs were precipitated with methanol, and re-dispersed in a mixed solvent of CB (4 mL), 1-butylamine (690 mL), and MPA (600 mL). The solution was centrifuged at 8000 rpm for 5 min to precipitate any aggregations. The upper solution was filtered through a PTFE filter (pore size 200 nm). An Ag8GeS6 NC film was then deposited onto CdS nanorod arrays using a layer-by-layer spin-coating method in an Ar-filled glove box. First 80 mL of Ag8GeS6 NC solution was spincoated at 2500 rpm for 15 s, then placed on a hotplate preheated to 150 8C for 3 min. This process was repeated three times, resulting in a film with CdS NRs covered by Ag8GeS6 NCs, onto which HTM solution (30 mL) (spiro-MeOTAD/chlorobenzene (90 mg/1 mL) solution was used with added LiTFSI/acetonitrile (520 mg/1 mL) (35.8 mL) and TBP (21.8 mL)) was finally spin-coated at 4000 rpm for 50 s. Finally 120 nm of Au films were deposited by thermal evaporation under 6 10 4 Pa pressure. The active area of the device was 0.1134 cm2. Photovoltaic measurement was recorded with an AM 1.5G Oriel solar simulator (model 92250A-1000) at an illumination intensity of 100 mW cm 2. The current–voltage characteristics of each cell were recorded with a Keithley 2400 source meter. The corresponding external quantum efficiency was characterized on a QTest Station 2000ADI system (Crowntech Inc.). Chem. Eur. J. 2014, 20, 12426 – 12431
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Acknowledgements We thank Dr. J. Ning for fruitful discussions on nanocrystal synthesis, and gratefully acknowledge financial support from the “Hundred Talents Program” of the Chinese Academy of Sciences, the National Science Foundation of China (nos. 20873141). Keywords: colloidal synthesis semiconductors · silver · solar cells
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[1] a) Y. He, Y. Zhong, Y. Su, Y. Lu, Z. Jiang, F. Peng, T. Xu, S. Su, Q. Huang, C. Fan, S.-T. Lee, Angew. Chem. 2011, 123, 5813 – 5816; Angew. Chem. Int. Ed. 2011, 50, 5695 – 5698; b) W. Li, R. Zamani, P. Rivera Gil, B. Pelaz, M. IbÇez, D. Cadavid, A. Shavel, R. A. Alvarez-Puebla, W. J. Parak, J. Arbiol, A. Cabot, J. Am. Chem. Soc. 2013, 135, 7098 – 7101. [2] a) V. L. Colvin, M. C. Schlamp, A. P. Alivisatos, Nature 1994, 370, 354 – 357; b) A. L. Rogach, N. Gaponik, J. M. Lupton, C. Bertoni, D. E. Gallardo, S. Dunn, N. Li Pira, M. Paderi, P. Repetto, S. G. Romanov, C. O’Dwyer, C. M. Sotomayor Torres, A. Eychmller, Angew. Chem. 2008, 120, 6638 – 6650; Angew. Chem. Int. Ed. 2008, 47, 6538 – 6549; c) S. K. Panda, S. G. Hickey, H. V. Demir, A. Eychmller, Angew. Chem. 2011, 123, 4524 – 4528; Angew. Chem. Int. Ed. 2011, 50, 4432 – 4436. [3] a) A. Carrete, A. Shavel, X. Fontan, J. Montserrat, J. Fan, M. IbÇez, E. Saucedo, A. Prez-Rodrguez, A. Cabot, J. Am. Chem. Soc. 2013, 135, 15982 – 15985; b) J. Tang, K. W. Kemp, S. Hoogland, K. S. Jeong, H. Liu, L. Levina, M. Furukawa, X. Wang, R. Debnath, D. Cha, K. W. Chou, A. Fischer, A. Amassian, J. B. Asbury, E. H. Sargent, Nat. Mater. 2011, 10, 765 – 771. [4] a) F.-J. Fan, Y.-X. Wang, X.-J. Liu, L. Wu, S.-H. Yu, Adv. Mater. 2012, 24, 6158 – 6163; b) M. Scheele, N. Oeschler, K. Meier, A. Kornowski, C. Klinke, H. Weller, Adv. Funct. Mater. 2009, 19, 3476 – 3483; c) M. IbÇez, R. Zamani, A. LaLonde, D. Cadavid, W. Li, A. Shavel, J. Arbiol, J. R. Morante, S. Gorsse, G. J. Snyder, A. Cabot, J. Am. Chem. Soc. 2012, 134, 4060 – 4063; d) C. Xiao, X. Qin, J. Zhang, R. An, J. Xu, K. Li, B. Cao, J. Yang, B. Ye, Y. Xie, J. Am. Chem. Soc. 2012, 134, 18460 – 18466. [5] a) W. W. Yu, X. Peng, Angew. Chem. 2002, 114, 2474 – 2477; Angew. Chem. Int. Ed. 2002, 41, 2368 – 2371; b) W. P. Lim, Z. Zhang, H. Y. Low, W. S. Chin, Angew. Chem. 2004, 116, 5803 – 5807; Angew. Chem. Int. Ed. 2004, 43, 5685 – 5689. [6] a) B. Xu, P. He, H. Liu, P. Wang, G. Zhou, X. Wang, Angew. Chem. 2014, 126, 2371 – 2375; Angew. Chem. Int. Ed. 2014, 53, 2339 – 2343; b) S.-K. Han, M. Gong, H.-B. Yao, Z.-M. Wang, S.-H. Yu, Angew. Chem. 2012, 124, 6471 – 6474; Angew. Chem. Int. Ed. 2012, 51, 6365 – 6368; c) S. Shen, Y. Zhang, L. Peng, Y. Du, Q. Wang, Angew. Chem. 2011, 123, 7253 – 7256; Angew. Chem. Int. Ed. 2011, 50, 7115 – 7118. [7] a) L. Liu, Z. Zhuang, T. Xie, Y.-G. Wang, J. Li, Q. Peng, Y. Li, J. Am. Chem. Soc. 2009, 131, 16423 – 16429; b) J.-j. Wang, P. Liu, C. C. Seaton, K. M. Ryan, J. Am. Chem. Soc. 2014, 136, 7954 – 7960; c) W. Li, R. Zamani, M. IbÇez, D. Cadavid, A. Shavel, J. R. Morante, J. Arbiol, A. Cabot, J. Am. Chem. Soc. 2013, 135, 4664 – 4667; d) J. Wang, A. Singh, P. Liu, S. Singh, C. Coughlan, Y. Guo, K. M. Ryan, J. Am. Chem. Soc. 2013, 135, 7835 – 7838. [8] J. J. Wang, D. J. Xue, Y. G. Guo, J. S. Hu, L. J. Wan, J. Am. Chem. Soc. 2011, 133, 18558 – 18561. [9] J. Akhtar, M. Afzaal, M. Banski, A. Podhorodecki, M. Syperek, J. Misiewicz, U. Bangert, S. J. O. Hardman, D. M. Graham, W. R. Flavell, D. J. Binks, S. Gardonio, P. O’Brien, J. Am. Chem. Soc. 2011, 133, 5602 – 5609. [10] a) X. Zhong, Y. Feng, W. Knoll, M. Han, J. Am. Chem. Soc. 2003, 125, 13559 – 13563; b) L. A. Swafford, L. A. Weigand, M. J. Bowers, J. R. McBride, J. L. Rapaport, T. L. Watt, S. K. Dixit, L. C. Feldman, S. J. Rosenthal, J. Am. Chem. Soc. 2006, 128, 12299 – 12306; c) T. Aubert, M. Cirillo, S. Flamee, R. Van Deun, H. Lange, C. Thomsen, Z. Hens, Chem. Mater. 2013, 25, 2388 – 2390; d) Z. Deng, H. Yan, Y. Liu, J. Am. Chem. Soc. 2009, 131, 17744 – 17745; e) S. Singh, A. Singh, K. Palaniappan, K. M. Ryan, Chem. Commun. 2013, 49, 10293 – 10295. [11] C. Yang, B. Zhou, S. Miao, C. Yang, B. Cai, W.-H. Zhang, X. Xu, J. Am. Chem. Soc. 2013, 135, 5958 – 5961.
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Communication [12] a) A. Singh, S. Singh, S. Levcenko, T. Unold, F. Laffir, K. M. Ryan, Angew. Chem. 2013, 125, 9290 – 9294; Angew. Chem. Int. Ed. 2013, 52, 9120 – 9124; b) S. C. Riha, B. A. Parkinson, A. L. Prieto, J. Am. Chem. Soc. 2011, 133, 15272 – 15275; c) J. van Embden, A. S. R. Chesman, E. Della Gaspera, N. W. Duffy, S. E. Watkins, J. J. Jasieniak, J. Am. Chem. Soc. 2014, 136, 5237 – 5240. [13] G. M. Ford, Q. Guo, R. Agrawal, H. W. Hillhouse, Chem. Mater. 2011, 23, 2626 – 2629. [14] a) E. Dilena, Y. Xie, R. Brescia, M. Prato, L. Maserati, R. Krahne, A. Paolella, G. Bertoni, M. Povia, I. Moreels, L. Manna, Chem. Mater. 2013, 25, 3180 – 3187; b) Y.-H. A. Wang, X. Zhang, N. Bao, B. Lin, A. Gupta, J. Am. Chem. Soc. 2011, 133, 11072 – 11075; c) A. Singh, C. Coughlan, F. Laffir, K. M. Ryan, Acs Nano 2012, 6, 6977 – 6983; d) Q. Guo, G. M. Ford, H. W. Hillhouse, R. Agrawal, Nano Lett. 2009, 9, 3060 – 3065; e) M. G. Panthani, V. Akhavan, B. Goodfellow, J. P. Schmidtke, L. Dunn, A. Dodabalapur, P. F. Barbara, B. A. Korgel, J. Am. Chem. Soc. 2008, 130, 16770 – 16777. [15] Y. Cao, M. S. Denny, J. V. Caspar, W. E. Farneth, Q. Guo, A. S. Ionkin, L. K. Johnson, M. Lu, I. Malajovich, D. Radu, H. D. Rosenfeld, K. R. Choudhury, W. Wu, J. Am. Chem. Soc. 2012, 134, 15644 – 15647. [16] M. IbÇez, R. Zamani, W. Li, D. Cadavid, S. Gorsse, N. A. Katcho, A. Shavel, A. M. L pez, J. R. Morante, J. Arbiol, A. Cabot, Chem. Mater. 2012, 24, 4615 – 4622.
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Received: July 2, 2014 Published online on August 12, 2014
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& Nanoparticles
Synthesis and Characterization of Ag8(Ge1 x,Snx)(S6 y,Sey) Colloidal Nanocrystals Bin Zhou, Yedi Xing, Shu Miao, Mingrun Li, Wen-Hua Zhang,* and Can Li*[a] Abstract: A facile colloidal approach to synthesize Ag8(Ge1 x,Snx)(S6 y,Sey) nanocrystals (NCs) in a highly controlled way across the entire compositional ranges (0 x 1, 0 y 6) has been developed. The NCs exhibit a uniform size distribution, highly crystalline structure, over 1 g scalable synthesis, and tunable band gaps in the range of 0.88–1.45 eV by varying their chemical compositions. The Ag8GeS6 NCs with a band gap of approximately 1.45 eV were employed as a model light harvester to assess their applicability in solar cells by a full solution-processing device, yielding an efficiency of 0.28 % under AM1.5 illumination, demonstrating their application potential in solar energy utilization.
Colloidal semiconductor nanocrystals (NCs) have attracted remarkable attention due to their novel properties that are distinct from their bulk counterparts and offer great application potential, for example, in bioimaging,[1] light-emitting devices,[2] photovoltaics,[3] and thermoelectrics.[4] They exhibit interesting properties that can be controlled by varying their size,[5] structure,[6] or shape,[1b, 7] which are difficult to achieve in their bulk analogues. The field of semiconductor NC synthesis has been dominated by binary systems, in which precise control over their dimensions and crystal phase has been well developed. It is currently highly desirable to exploit more complex multicomponent nanocrystals (MC-NCs).[8–10] MC-NCs have the ability to tune their optical and electronic properties by controlling chemical compositions, which may affect significantly the performance of devices made from them. Moreover, the band gaps of the MC-NCs can be consecutively tailored by alloying the constituents in a wide range, providing an alternative to the quantum confinement effect. Therefore, it is currently very appealing to study systematically the synthesis, properties, and application of MC-NCs. For instance, successes have been achieved with a number of chalcogenide MC-NCs by alloying two constituents to show consecutively tunable properties, for example, Cu2(S,Se),[8] Pb(S,Se),[9] (Zn,Cd)(S,Se),[10] [a] B. Zhou, Y. Xing, Prof. S. Miao, Prof. M. Li, Prof. W.-H. Zhang, Prof. C. Li State Key Laboratory of Catalysis Dalian Institute of Chemical Physics, Chinese Academy of Sciences Dalian National Laboratory for Clean Energy, Dalian 116023 (China) E-mail: [email protected] [email protected] Supporting information for this article is available on the WWW under http://dx.doi.org/10.1002/chem.201404220. Chem. Eur. J. 2014, 20, 12426 – 12431
Cu2Ge(S,Se)3,[11] Cu2ZnSn(S,Se)4,[12] Cu2Zn(Sn,Ge)Se4,[13] and Cu(In,Ga)(S,Se)2.[14] Nevertheless, in comparison with the binary systems, there is still the major challenge of balancing the reactivity of each precursor in a multicomponent system, and thus it remains a challenge to achieve the controlled synthesis of targeted MC-NCs. The ever increasing demand for clean and sustainable energy necessitates the development of solar energy utilization. The colloidal NCs offer scalable synthesis, wet-chemical processing, and tunable photoelectric properties, thereby providing a possible strategy to address the challenge for developing cost-effective optoelectrical devices, for example, the solution-processing solar cells. In this regard, I–III–VI NCs [e.g., CuInS2/CuInSe2 and Cu(In,Ga)(S,Se)2] and I–II–IV–VI NCs [such as CuZnSn(S,Se)4] have shown significant promise; hence, extensive research has been conducted to understand their synthesis and their application in solution-processed NC solar cells with impressive performance,[15] demonstrating the potential in photovoltaic devices derived from semiconductor NCs. Along the above lines, I–IV–VI semiconductors are potentially interesting, yet not well studied for solar energy utilization. They exhibit band gaps suitable for sunlight harvesting, high absorption coefficients, and carrier mobility in bulk materials. The current studies of I–IV–VI materials were mainly focused on Cu2GeSe3,[16] Cu2SnS3,[17] and Cu2SnSe3,[7b, 18] whereas Agbased I–IV–VI NCs have been largely overlooked. Qian and coworkers reported the synthesis of Ag8SnS6 and Ag8SnSe6 nanomaterials using a solvothermal process, which resulted in products exhibiting agglomeration and broad size distributions.[19] To the best of our knowledge, the synthesis of Ag8(Ge1 x,Snx)(S6 y,Sey) (0 x 1, and 0 y 6) NCs in a controlled way has not been reported to date, hampering the studies of their properties and potential applications. Herein we demonstrate a facile solution approach to the synthesis of Ag-based I–IV–VI NCs with the nominal formula of Ag8(Ge1 x,Snx)(S6 y,Sey) (0 x 1, 0 y 6), and presented their fundamental properties. The chemical compositions of the resulting NCs are highly tunable in the entire compositional ranges (0 x 1, 0 y 6), and the band gaps of the NCs were tuned in the range of 0.88–1.45 eV, and the latter one is ideal for solar light absorption. Therefore, we have assessed their applicability as photovoltaic materials by fabricating solar cells using Ag8GeS6 NCs (with a band gap of 1.45 eV) as a light harvester by a low-cost solution process. The synthesis of Ag8(Ge1 x,Snx)(S6 y,Sey) colloidal NCs was performed in oleylamine (OLA) solution by a simple non-injection synthetic approach. In a typical synthesis, the OLA-AgNO3
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Communication solution, OLA-S solution, Ge[N(SiMe3)2]2, Sn[N(SiMe3)2]2, bis(trimethyl- silyl)selenide (TMSe), and bis(trimethylsilyl)sulfide (TMS) with specific ratios were loaded into a Schlenk tube under an inert atmosphere. The reaction tube was then inserted into a preheated oil bath at 220 8C and held at this temperature for 15 min to produce Ag8(Ge1 x,Snx)(S6 y,Sey) NCs. We additionally demonstrated the possibility of up-scaling this method by performing the gram-scale synthesis of Ag8Ge(S2,Se4) NCs. Full experimental details can be found in Experimental Section. Figure 1 presents powder X-ray diffraction (XRD) patterns of the as-synthesized Ag8Ge(S6 y,Sey) NCs with 0 y 6. The
Figure 1. XRD patterns of the Ag8Ge(S6 y,Sey) NCs with various S/Se ratios (0 y 6). a) Ag8GeS6, b) Ag8Ge(S4,Se2), c) Ag8Ge(S2,Se4), and d) Ag8GeSe6
broadening of the reflection peaks on the background occurs, which is typical of small NCs. All the diffraction peaks of the Ag8GeS6 NCs and Ag8GeSe6 NCs match well with the major peaks of Canfieldite-structured Ag8GeS6 (JCPDS No. 44-1416) and Ag8GeSe6 (JCPDS No. 71-1690), respectively. Additionally, as the Se content increases, the major reflection peaks systematically shift toward lower angles, which is attributed to the replacement of the smaller S atoms (1.84 ) by the larger Se atoms (1.98 ) in the lattices of the resulting NCs. Very importantly, no additional peak or peak splitting can be detected by XRD, thus precluding the possibility of phase separation that often takes place in the preparation of MC-NCs. Therefore, pure phase Ag8Ge(S6 y,Sey) NCs were successfully obtained, with the characteristics that the chemical compositions can be consecutively tailored across the entire compositional range (0 y 6). This is in line with Vegard’s law,[20] demonstrating the formation of alloyed Ag8Ge(S6 y,Sey) NCs in a highly controlled way. We next applied transmission electron microscopy (TEM) to gain more insight into the microstructures of the Ag8Ge(S6 y,Sey) NCs (0 y 6) (see Figure 2). The low-magnification TEM images reveal a uniform size distribution of the Ag8Ge(S6 y,Sey) NCs. The polycrystalline selected area electron diffraction (SAED) patterns of the NCs show clearly diffraction rings that match well with the XRD patterns of the corresponding Ag8Ge(S6 y,Sey) NCs (shown in Figure 1). High-resolution transmission electron microscopy (HRTEM) images show that all Chem. Eur. J. 2014, 20, 12426 – 12431
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Figure 2. TEM analysis of the Ag8Ge(S6 y,Sey) (0 y 6) NCs. a) Ag8GeS6, b) Ag8Ge(S4,Se2), c) Ag8Ge(S2,Se4), and d) Ag8GeSe6 NCs; 1, 2, 3, and 4 represent the corresponding low magnification TEM images, HRTEM images, SAED patterns, and EDS spectra for these samples. The inset scale bar is 5 nm for HRTEM images.
Ag8Ge(S6 y,Sey) NCs are highly crystalline with continuous lattice fringes. The elemental composition of the as-prepared Ag8Ge(S6 y,Sey) NCs was analyzed by energy-dispersive X-ray spectroscopy (EDS) (see Table S1 in the Supporting Information), which showed that all samples have a similar Ag/Ge molar ratio of approximately 8/1, and that increasing the Se
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Communication content was accompanied by a decrease of the S content in the ranges of 0 y 6.The elemental line scan results (see Figure S1) confirmed the homogeneous distribution for all of these elements in the nanocrystals. These results indicate the successful formation of well-crystalline Ag8Ge(S6 y,Sey) NCs, displaying a uniform size distribution (Figure S2) with varied S/Se ratios across the whole compositional range (0 y 6). On the basis of Vegard’s law,[20] semiconducting alloys have tunable band gaps that change upon varying their compositions with a nearly linear relationship. UV-Vis-NIR absorption spectra of Ag8Ge(S6 y,Sey) NCs (0 y 6) were therefore measured to study their optical properties and to determine their band gaps. A clear solution of each Ag8Ge(S6 y,Sey) NC sample (0 y 6) showed a continuous absorption spectrum spanning the whole visible spectrum to the near IR region (see Figure 3 a), resulting in the black color of the Ag8Ge(S6 y,Sey) NCs.
Figure 3. a) UV-Vis-NIR absorption spectra for the Ag8Ge(S6 y,Sey) NCs with various S/Se ratios (0 y 6); b) relationship between the band gaps of the Ag8Ge(S6 y,Sey) NCs and the contents of Se/S in the products.
Tauc plots (see Figure S3) were performed to determine the optical band gaps with a relationship of the [ahn]1/2 versus the energy, revealing that Ag8GeS6 NCs exhibit an indirect band gap of 1.45 eV, which covers most of the sunlight spectrum from the visible to the near infrared ranges. Moreover, the band gap energies are progressively reduced upon increasing the Se content in the Ag8Ge(S6 y,Sey) NCs (0 y 6), presenting a nearly linear relationship between the band gaps and the S/Se content (Figure 3 b). These results further demonstrate the successful formation of homogeneous alloyed Ag8Ge(S6 y,Sey) NCs (0 y 6) in the present experiments. To further exploit the suitability of the present synthesis of Ag8Ge(S6 y,Sey) NCs for other Ag-based I–IV–VI materials, we have expanded the present synthetic strategy to the preparation of Ag8(Ge1 x,Snx)(S6 y,Sey) NCs (0 x 1), where the S/Se molar ratio was kept at 2/4 (i.e., y = 4) for simplicity. Experimental results indicate again that the Ge/Sn molar ratio can be tuned across the entire compositional range for the Ag8(Ge1 x,Snx)(S2,Se4) NCs (0 x 1). XRD (Figure S4), TEM (Figures S5–S9), and UV-Vis-NIR absorption spectra (Figures S10 and S11) all confirm the formation of the homogeneous alloyed Ag8(Ge1 x,Snx)(S2,Se4) NCs (0 x 1) with a narrow size distribution. Moreover, growth of the Ag8Sn(S6 y,Sey) (0 y 6) NCs has also been examined by this synthetic approach, and Chem. Eur. J. 2014, 20, 12426 – 12431
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monodisperse NCs were once again obtained in the entire composition ranges (0 y 6). Therefore, we have presented a facile solution approach to achieve the synthesis of Ag8(Ge1 x,Snx)(S6 y,Sey) NCs in a highly controlled way, and achieved precise control over their chemical compositions, optical properties, and band gaps across the entire compositional ranges (0 x 1, 0 y 6). Additionally, we tested the possibility of up-scaling the synthesis of the Ag8Ge(S2,Se4) NCs using eight-times larger amounts of each precursor in the reaction; this resulted in over 1 g of product per run with a narrow size distribution and pure phase (Figure S12), and demonstrated the feasibility of the present synthesis method to the Ag-based I–VI–VII NCs. The 1.45 eV band gap of the Ag8GeS6 NCs makes them very promising as a light absorber for single-junction solar cells, the Ag8GeS6 NCs were hence chosen as the model materials to assess the photovoltaic applications. It is beneficial to understand the conducting behavior of the semiconductor NCs prior to exploiting their device application. The transient photocurrents of the Ag8GeS6 NC films were then recorded in a photoelectrochemical cell. The electrode of the Ag8GeS6 nanocrystal film generates a cathodic photocurrent that increases gradually with increasing negative bias (Figure 4 a) upon illumination (100 mW cm 2), which is typical of p-type semiconducting behavior.[21] Under a constant bias of 0.2 V, the photocurrents of the Ag8GeS6 NC film increased very quickly upon turning on the light, and drop to their pre-illumination values without apparent degradation over many cycles upon turning on and off the illumination. (Figure 4 a inset). These results disclose that the Ag8GeS6 NCs are sensitive to light illumination and are stable under the experimental conditions, which would be favorable for further exploring their device application. The band energy levels of the Ag8GeS6 NCs were then estimated by cyclic voltammetry (CV) measurements of the NC film along with UV-Vis-NIR spectra. CV results (Figure S13) reveal that the conduction band minimum (CBM) of the Ag8GeS6 NCs was at approximately 3.7 eV relative to the vacuum level, and the valence band maximum (VBM) was hence estimated to be approximately 5.1 eV on the basis of the 1.45 eV optical band gap. The CBM of Ag8GeS6 NCs therefore matches well with that of CdS (CBM ~ 4.0 eV). Consequently, photovoltaic devices were fabricated in the configuration of a FTO/TiO2 compact layer (c-TiO2)/CdS nanorods (NRs)/Ag8GeS6 NCs/Spiro-MeTAD/ Au, and the relative band positions of the materials employed in the devices is given in Figure 4 b. The CdS NRs prepared on FTO coated with a compact layer of TiO2, by a hydrothermal approach,[22] were employed as the n-type electron transporter, and the Ag8GeS6 NCs, as the light harvester, were deposited onto them by spin coating of a nanocrystal solution by using a layer-by-layer approach. A Spiro-MeOTAD interfacial layer was then deposited onto them because it favors hole extraction and eases the formation of the ohmic contact for the back electrode, resulting in a relatively high device performance for solar cells.[23] The Ag8GeS6 NC film thickness must be slightly greater than the height of the CdS NRs to avoid direct contact between the Spiro-MeOTAD layer and the CdS NRs. A typical cross-sectional SEM image (Figure S14) indicates
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Communication structure. The band gaps of the NCs could be effectively tuned in the range of 0.88–1.45 eV by varying their chemical compositions. Preliminary application in photovoltaic devices has demonstrated the usefulness of the Ag8GeS6 NCs as a light absorber in fully solution-processing solar cells, generating efficiency of 0.28 % under AM1.5 illumination. These results should be beneficial for further investigating the synthesis, properties, and potential applications of these systems in the future.
Experimental Section All experiments were performed under nitrogen atmospheres using Schlenk techniques. AgNO3, S and Se stock solutions: A 0.1 m OLA-AgNO3 stock solution was made by dissolving AgNO3 (1.702 g, 10 mmol) into OLA (100 mL) under stirring overnight in a glove box, and a 1 m OLA-S stock solution was prepared by the same process. A 0.5 m OLA-Se stock solution was made by dissolving selenium powder (0.790 g, 10 mmol) into OLA (20 mL) under stirring at 270 8C for 4 h. Synthesis of Ag8GeS6 NCs and Ag8SnS6 NCs: In the synthesis of Ag8GeS6 NCs, 0.1 m OLA-AgNO3 stock solution (8 mL), GeI4 (0.116 g, 0.2 mmol), TMS (128 mL, 0.6 mmol) and 1 m OLA-S stock solution (1.5 mL) were loaded sequentially into a Schlenk reaction tube inside a glovebox. The reaction tube was then inserted into a preheated oil bath at 220 8C with constant stirring, and maintained at this temperature for 15 min to produce a black solution. Afterward, the colloidal solution was rapidly cooled to room temperature with a water bath, then precipitated with ethanol (20 mL) and centrifuged at 8000 rpm for 5 min. The upper solution was discarded and the nanoparticles were additionally purified twice by re-dispersing them in hexane and precipitating with ethanol. The nanoparticles were re-dispersed in hexane and stored in glove box until further use. The synthesis of Ag8SnS6 NCs was carried out under otherwise identical conditions to Ag8GeS6 NCs, using SnI4 as the Sn precursor. 2
Figure 4. a) Transient photocurrent response of the Ag8GeS6 NCs film at an illumination intensity of 100 mW cm , and the active device area was 0.64 cm2. Inset represents the photocurrents obtained under constant bias of 0.2 V. b) Scheme of the energy levels of the materials employed in the devices; c) J–V curves and d) EQE spectra of the solar cells with the configuration of FTO/c-TiO2/CdS NRs/Ag8GeS6 NCs/Spiro-MeOTAD/Au.
the resulting Ag8GeS6 NC film has a thickness of approximately 700 nm, which is slightly greater than the height of the CdS NRs (~ 600 nm). Thus, a thin layer of Ag8GeS6 NC film separates the tops of the CdS NRs from the Spiro-MeOTAD/Au contacts. The corresponding device performance was determined by current density–voltage (J–V) measurement under simulated AM 1.5G (100 mW cm 2) solar irradiation in air. Figure 4 c presents the photovoltaic characteristics, showing an open-circuit voltage (Voc) of 0.57 V, short-circuit current density (Jsc) of 0.79 mA cm 2, and fill factor (FF) of 0.63, resulting in a power conversion efficiency (h) of 0.28 %. A control cell device without the Ag8GeS6 NCs was also fabricated under otherwise identical conditions to the Ag8GeS6 NC cell. However, its performance (Figure S15) is quite poor, with an obtained PCE of only 0.02 %. Figure 4 d shows the external quantum efficiency (EQE) spectrum for the Ag8GeS6 NC-based cell. Photocurrent generation starts at approximately 870 nm, in agreement with the band gap of the Ag8GeS6 NCs, while the sharp decay at approximately 520 nm is due to the absorption of CdS, which has no contribution to the net photocurrent generation. These results demonstrate the key role of the Ag8GeS6 NCs as the ptype light harvester. It is expected that the device performance can be improved significantly for these MC-NC-based photovoltaic cells by optimizing the device fabrication, for example by displacing large organic ligands by shorter organic groups or inorganic ligands at the NC surface to improve the charge carrier transfer and transport through the NCs. In summary, we have presented a facile solution approach to realize the synthesis of Ag8(Ge1 x,Snx)(S6 y,Sey) NCs in a highly controllable way across the entire compositional range (0 x 1, 0 y 6). The Ag8(Ge1 x,Snx)(S6 y,Sey) NCs displayed uniform size distributions along with a highly crystalline Chem. Eur. J. 2014, 20, 12426 – 12431
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Synthesis of Ag8GeSe6 NCs and Ag8SnSe6 NCs: In the synthesis of Ag8GeSe6 NCs, 0.1 m OLA-AgNO3 stock solution (8 mL), Ge[N(SiMe3)2]2 (39 mL, 0.1 mmol), TMSe (150 mL, 0.6 mmol) and 0.5 m OLA-Se stock solution (3 mL) were loaded sequentially into a Schlenk reaction tube inside a glove box, while keeping all other steps the same as the synthesis of Ag8GeS6 NCs. The resulting colloidal solution was rapidly cooled to room temperature, and precipitated with ethanol (20 mL) and centrifuged at 8000 rpm for 5 min. The resulting nanoparticles were re-dispersed in chlorobenzene and stored in glove box prior to their characterization. The synthesis of Ag8SnSe6 NCs was performed with the same procedure by using Sn[N(SiMe3)2]2 instead of Ge[N(SiMe3)2]2 as the Sn precursor. Synthesis of Ag8(Ge1 x,Snx)(S6 y,Sey) (0 x 1, 0 < y < 6) alloyed NCs: The Ag8(Ge1 x,Snx)(S6 y,Sey) NCs were synthesized following
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Communication similar procedures to those used for the Ag8GeSe6 NCs by replacing Ge[N(SiMe3)2]2 with Sn[N(SiMe3)2]2, and TMS with TMSe partly or wholly, respectively, where the overall ratio Ag/(Ge+Sn)/(TMS+TMSe) remained stoichiometric. For example, for the synthesis of Ag8(Ge0.5,Sn0.5)(S2,Se4) NCs, 0.1 m OLA-AgNO3 stock solution (8 mL), Ge[N(SiMe3)2]2 (19.5 mL, 0.05 mmol), Sn[N(SiMe3)2]2 (19.5 mL, 0.05 mmol), TMSe (100 mL, 0.4 mmol), TMS (43 mL, 0.2 mmol) and 1 m OLA-S (1.5 mL) were used while keeping all other steps the same as for the synthesis of Ag8GeSe6 NCs. Gram-scale synthesis of Ag8Ge(S2,Se4) NCs: For a gram-scale synthesis of Ag8Ge(S2,Se4) NCs, 0.4 m OLA-AgNO3 stock solution (16 mL), Ge[N(SiMe3)2]2 (312 mL, 0.8 mmol), TMSe (801.6 mL, 3.2 mmol), TMS (342.4 mL, 1.6 mmol) and 2 m OLA-S stock solution (6 mL) were loaded sequentially into a Schlenk reaction tube inside a glove box. The reaction tube was then inserted into a preheated oil bath at 220 8C with constant stirring, and maintained at this temperature for 15 min to produce a black solution, as described before. Afterward, the colloidal solution was rapidly cooled to room temperature with a water bath and cleaned twice using the same procedure as before, and finally the NCs were dispersed in toluene. Fabrication of photovoltaic devices: Prior to the growth of CdS nanorods, a dense TiO2 layer was first deposited on a cleaned FTO glass substrate by dip-coating with a TiO2 organic solution (made by using a titanium butoxide/diethanolamine/absolute ethanol solution), followed by a sintering at 500 8C for 30 min. CdS nanorod arrays on TiO2-coated FTO were then prepared by using a literature method with incorporated modifications.[22] Briefly, 0.05 m Cd(NO3)2 (20 mL), 0.05 m thiourea solution (20 mL), and 0.03 m glutathione solution (20 mL) were mixed and loaded into a 100 mL autoclave containing a piece of TiO2-coated FTO substrate. This mixture was then subjected to hydrothermal treatment at 200 8C for 3 h to give 500–600 nm CdS nanorod arrays on TiO2-coated FTO. These CdS nanorod arrays are employed as the n-type electron conducting materials of solar cells with Ag8GeS6 NCs as the p-type light harvester. In a typical fabrication of a nanocrystal solar cell, purified Ag8GeS6 NCs (0.4 mmol) were dispersed in CB (4 mL), then mixed with 1-butylamine (4 mL), MPA (1 mL) in a 20 mL vial for ligand exchange. The vial was sealed and sonicated for 45 min, then the Ag8GeS6 NCs were precipitated with methanol, and re-dispersed in a mixed solvent of CB (4 mL), 1-butylamine (690 mL), and MPA (600 mL). The solution was centrifuged at 8000 rpm for 5 min to precipitate any aggregations. The upper solution was filtered through a PTFE filter (pore size 200 nm). An Ag8GeS6 NC film was then deposited onto CdS nanorod arrays using a layer-by-layer spin-coating method in an Ar-filled glove box. First 80 mL of Ag8GeS6 NC solution was spincoated at 2500 rpm for 15 s, then placed on a hotplate preheated to 150 8C for 3 min. This process was repeated three times, resulting in a film with CdS NRs covered by Ag8GeS6 NCs, onto which HTM solution (30 mL) (spiro-MeOTAD/chlorobenzene (90 mg/1 mL) solution was used with added LiTFSI/acetonitrile (520 mg/1 mL) (35.8 mL) and TBP (21.8 mL)) was finally spin-coated at 4000 rpm for 50 s. Finally 120 nm of Au films were deposited by thermal evaporation under 6 10 4 Pa pressure. The active area of the device was 0.1134 cm2. Photovoltaic measurement was recorded with an AM 1.5G Oriel solar simulator (model 92250A-1000) at an illumination intensity of 100 mW cm 2. The current–voltage characteristics of each cell were recorded with a Keithley 2400 source meter. The corresponding external quantum efficiency was characterized on a QTest Station 2000ADI system (Crowntech Inc.). Chem. Eur. J. 2014, 20, 12426 – 12431
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Acknowledgements We thank Dr. J. Ning for fruitful discussions on nanocrystal synthesis, and gratefully acknowledge financial support from the “Hundred Talents Program” of the Chinese Academy of Sciences, the National Science Foundation of China (nos. 20873141). Keywords: colloidal synthesis semiconductors · silver · solar cells
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Received: July 2, 2014 Published online on August 12, 2014
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