Generalized Water-Processed Metal Chalcogenide Complexes

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Generalized Water-Processed Metal Chalcogenide Complexes: Synthesis and Applications Zhe Xia, Jie Zhong, Meiying Leng, Long Hu, Ding-Jiang Xue, Bo Yang, Ying Zhou, Xinsheng Liu, Sikai Qin, and Jiang Tang Chem. Mater., Just Accepted Manuscript • DOI: 10.1021/acs.chemmater.5b03614 • Publication Date (Web): 06 Nov 2015 Downloaded from http://pubs.acs.org on November 13, 2015

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Chemistry of Materials

Generalized Water-Processed Metal Chalcogenide Complexes: Synthesis and Applications Zhe Xia, Jie Zhong, Meiying Leng, Long Hu, Ding-Jiang Xue,* Bo Yang, Ying Zhou, Xinsheng Liu, Sikai Qin, and Jiang Tang* Wuhan National Laboratory for Optoelectronics (WNLO) and School of Optical and Electronic Information, Huazhong University of Science and Technology (HUST), Wuhan, 430074, China ABSTRACT: Metal chalcogenide complexes (MCCs) have attracted considerable attention recently due to their high solubility in polar solvents, low-temperature decomposition to metal chalcogenide films and service as ligands for colloidal nanocrystals (NCs). However, most of the MCCs were typically synthesized in the highly toxic and explosive hydrazine (denoted as hydrazine-MCCs), severely restricting the wide applications of MCCs. Here we present a versatile and environmentally benign water-based solution method for the preparation of various MCCs (denoted as water-MCCs) through directly dissolving a series of bulk V2VI3 chalcogenides (V= Sb, As and VI= S, Se, Te) in water with the presence of (NH4)2S at room temperature and ambient atmosphere. We further show that such water-MCCs can be readily processed into corresponding semiconducting thin films upon mild thermal treatment, and then be extended to fabricate compositionally controlled semiconductor alloys with tunable band gaps (i.e. Sb2(S1-x,Sex)3, 0≤x≤1) through simple control of substrate temperature. Furthermore, we present that our water-MCCs, especially for Sb4S72-, can be utilized to serve as ligands for in-situ synthesized water-based PbS quantum dots (QDs), achieving a homogeneous and stable aqueous QDs solution without needing further conventional secondary ligand exchange. Our study provides a general strategy for the synthesis of various MCCs using water as safer and more environmentally friendly solvent alternative to hydrazine, thus greatly enhancing the wide applications of MCCs in solution-processed inorganic semiconductors.

1.

lecular MCCs precursors for high quality thin semiconducting films.

INTRODUCTION

Solution processing of metal chalcogenide thin films holds great potential for wide applications in solar cells, transistors, and thermoelectrics due to its low manufacturing cost and good compatibility with high-throughput deposition techniques such as spray and printing.1-9 However, the covalent nature of metal chalcogenides, despite providing their outstanding electronic properties, makes it challenging for direct dissolution of these materials. Fortunately, the utility of hydrazine as solvent opens the door for dissolution of most metal chalcogenides through the formation of highly soluble precursors: metal chalcogenide complexes (MCCs), denoted as hydrazine-MCCs, thus providing true solutions down to molecular level.10-11 More importantly, MCCs can be readily decomposed to the starting metal chalcogenides upon mild thermal treatment due to the weak incorporation of hydrazinium cations.1, 10-13 Therefore, the feature of high solubility, combined with low-temperature decomposition, makes MCCs ideal for solution-processed thin-film semiconductors. The most successful example of this MCCs-based approach is Cu(In,Ga)(S,Se)2 (CIGS) and Cu2ZnSn(S,Se)4 (CZTSSe) thin-film solar cells developed by Mitzi et al., demonstrating remarkable efficiencies of 15.2% and 12.7% respectively,14-15 thereby leading to a breakthrough in mo-

Another attractive feature of MCCs is serving as ligands for colloidal nanocrystals (NCs) due to their high affinity of terminal chalcogen atoms toward binding undercoordinated metal atoms on NC surface based on Pearson’s hard and soft acids and bases (HSAB) principle, as well as their small size such as thiostannate ligands (Sn2S76−, SnS44−) formed in different solvents have less than 1 nm size, much smaller than traditional long-chain organic ligands with a length of ~ 2 nm.16-19 The above characteristics enable MCCs directly replacing organic ligands and attaching firmly to the surface of NCs, thus not only providing colloidal stabilization in solution, but also effectively minimizing interparticle spacing and facilitating the charge transport between individual NCs when processed into films. This concept of MCCs-ligands was first introduced by Talapin et al. in 2009,16 demonstrating the dramatic impact of various MCCs on coupled Au and CdSe NCs in thin films to increase conductivity by several orders of magnitude respectively. Very recently, Kovalenko et al. reported a detailed experimental and theoretical study of the inorganic surface functionalization of CdSe NCs by MCCs (Sn2S64-, Sn2S76- and SnS44-),18 illustrating the atomistic details of the organic-to-inorganic ligand exchange and binding motifs at the NC surface, greatly

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promoting the full understanding of MCCs. In addition, MCCs can also be used as “solders” for semiconductors widely used in photovoltaics and thermoelectrics, thus opening new prospects for semiconductor technologies.20 Whereas MCCs hold great promise for solution processing of thin-film semiconductors and ligands for NCs, it is unfortunate that most of the MCCs were typically synthesized in the highly toxic and explosive hydrazine, such that the synthesis process must be performed with great caution under inert environment, thus severely restricting the wide applications of MCCs. Consequently, it is highly desirable to develop a green, general and lowcost approach free of hydrazine to synthesize MCCs. In the search for alternative solvent, Brutchey et al. have recently made a series of impressive progress through using a simple thiol-amine solvent mixture.4, 21-23 We take the view that water can undoubtedly be considered the cleanest and cheapest solvent available. Inspired by the synthesis of (NH4)4Sn2S6 reported by Talapin et al.,24 we propose a water-based strategy to synthesize As and Sb based MCCs in the presence of (NH4)2S, denoted as water-MCCs, based on the following design principle: Aqueous (NH4)2S solution can offer a combination of two essential functional groups for the formation of MCCs via the well-known dimensional reduction process: S2- and NH4+ (serving as counterion), analogous to the excess reduced chalcogen and N2H5+ in hydrazine.10 More importantly, especially for the following thin-film deposition, water as the only solvent can be easily evaporated upon moderate thermal treatment, while (NH4)2S can be readily eliminated completely via H2S and NH3 gas. Combined with no carbon source in our aqueous system, O, N and C-free samples would be obtained through our proposed water-MCCs method.

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In this work, we present a general approach to synthesize various MCCs using water as safer and more environmentally friendly alternative to hydrazine. Raman spectroscopy was applied to thoroughly study the synthesis mechanism of water-MCCs. As shown in Figure 1, the resulting water-MCCs were directly solution-processed into corresponding metal chalcogenides or alloys films by spray pyrolysis, and a complete Sb2(S1-x,Sex)3 thin-film solar cell was built achieving 1.43% solar energy conversion efficiency. In addition to thin-film solar cells, such water-MCCs solutions may be further extended to fabricate chalcogenides-sensitized solar cells, another important structure of photovoltaic devices, due to their true solution characteristic. Furthermore, our waterMCCs were applied for capping in-situ synthesized PbS quantum dots (QDs), achieving a homogeneous and stable aqueous QDs solution. Compared with conventional hydrazine-based approach, our water-based synthesis of MCCs greatly enhance the potential application of MCCs in a wide range of photovoltaic, electronic, and thermoelectric devices with high throughput, low cost, and improved safety. 2.

EXPERIMENTAL SECTION

2.1 Chemicals. Antimony sulfide (Sb2S3, powder, 99.999%), antimony selenide (Sb2Se3, powder, 99.999%), antimony telluride (Sb2Te3, powder, 99.96%), arsenic(III) sulfide (As2S3, powder, 99.9%), arsenic(III) selenide (As2Se3, powder, 99.999%), arsenic(III) telluride (As2Te3, powder, 99%), and selenium (Se, amorphous, 99.999%) were all purchased from Alfa Assar. Ammonium sulfide (40-48 wt. % in water) and tellurium (Te, 99.99%) were purchased from Aladdin. Lead (II) nitrate (Pb(NO3)2, analytical reagent grade) and sodium hydroxide (NaOH, analytical reagent grade) were purchased from Sinopharm Chemical Reagent Co., Ltd. NOTE: X-ray diffraction demonstrated that the elemental Se and Te were amorphous and hexagonal tellurium, respectively. All chemicals were used as received without any further purification. 2.2 Synthesis of Metal Chalcogenide Complexes Solutions. All the synthesis were performed at room temperature in a well-ventilated fume hood. To synthesize a series of V2VI3 chalcogenides (V= Sb, As and VI= S, Se, Te) based water-MCCs, Sb2S3 (1 mmol), Sb2Se3 (1 mmol), Sb2Te3 (0.1 mmol), As2S3 (1 mmol), As2Se3 (1 mmol), and As2Te3 (0.1 mmol) were respectively mixed with 10 mL deionized water and 10 mL of ammonium sulfide solution (40-48 wt.% in water) in a 25 mL conical flask at room temperature. The mixtures were fully dissolved within 7 days of magnetic stirring at room temperature.

Figure 1. Schematic illustration for the synthesis and applications of water-MCCs.

2.3 Synthesis of PbS Quantum Dots with in-situ Water-MCCs Capping. 2 mL of Pb(NO3)2 aqueous solution (0.1 mol/L) and 14 mL of NaOH aqueous solution (0.1 mol/L) were mixed in a 25 mL conical flask with magnetic stirring until a transparent colorless solution was formed. Next, once adding the above solution drop by drop into


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the vigorously stirred (NH4)2Sb4S7 aqueous solution, a dark brown colloidal solution was formed immediately, indicating the formation of PbS QDs. Note that the (NH4)2Sb4S7 aqueous solution used for the synthesis of PbS QDs was purified in the following steps: (i) The resulting Sb2S3-MCCs solution was heated at 65 oC in a N2 filled glovebox for several hours to remove water and (NH4)2S. (ii) The obtained yellow solid was redissolved in water only upon addition of small quantities of (NH4)2S, and then filtered to remove excessive sulfur as a precipitate. Finally, the resultant dark brown PbS QDs were separated out via centrifugation and re-dispersed in water for the following measurement. All the synthesis were performed at room temperature in a fume hood. 2.4 Film Deposition and Device Fabrication. Sb2S3 films were deposited on FTO glass at the temperature of 350 oC by spray pyrolysis using the Sb2S3-MCCs solution. The Sb2(S1-x,Sex)3 alloy thin films were obtained using the Sb2Se3-MCCs solution through the similar spray pyrolysis technique except using different substrate temperatures. Sb2Se3 films were prepared by an additional selenization of Sb2(S0.44,Se0.56)3 films in a covered hot-plate under Se atmospheres at 380 oC. All the above experiments were done in a N2 filled glovebox. Solar cells were fabricated with a conventional planar device structure (glass/FTO/TiO2/ Sb2(S0.44,Se0.56)3/Au). First, TiO2 electrodes were fabricated by depositing Ti-sols on FTO substrates using spin-coating with following heat treatment. The preparation of Ti-sols was carried out in air ambient at room temperature. Titanium (IV) n-butoxide (4.25 mL) was mixed with triethanolamine (2 mL) and anhydrous ethyl alcohol (25 mL) in a flask under continuous magnetic for 2 h. Acetic acid (5 mL) and deionized water (5 mL) were then added into the mixture with continuous magnetic agitation for 24 h. The mixture was then stored in a beaker and placed inside fumehood to allow condensation reactions until the volume reached 15 mL. Ti-sols were deposited on the FTO substrates by spin-coating at 2500 RPM for 15 s. One edge of the as-deposited film was then wiped free of the sols with a swab soaked in anhydrous ethyl alcohol to expose a region of clean FTO for electrical contacting. This wipe was immediately followed by heat treatment in air on a hotplate at 520 ºC for 1 hour. Sb2(S0.44,Se0.56)3 absorber layers were then deposited onto the TiO2 electrode at the temperature of 380 oC by spray pyrolysis using the Sb2Se3-MCCs solution. Finally, gold contacts (50 nm thickness) were deposited by thermal evaporation. Each device had a total area of approximately 0.09 cm2 (3 mm× 3 mm) fixed by the mask pattern. 2.5 Materials and Device Characterization. Raman analysis (Horiba JobinYvon, LabRAM HR800, 532 nm excitation) was performed on the V2VI3 chalcogenides (V= Sb, As and VI= S, Se, Te) solutions in a backscattering confocal configuration at room temperature using 2 ml glass bottles as the solution containers and focusing the laser beam in the liquid. Thermogravimetric analysis (TGA, PerkinElmer Instruments, Diamond TG/DTA6300) was performed in a flowing N2 atmosphere at 10 °C/min.

The crystal structures of the products were characterized by X-ray diffraction (XRD, Philips, X pert pro MRD, with Cu Kα radiation, λ = 1.54178 Å). The absorption was recorded by UV-vis-IR spectrophotometer (PerkinElmer Instruments, Lambda 950 using integrating sphere). The morphology of V2VI3 chalcogenides films was tested by scanning electron microscopy (SEM, FEI Nova NanoSEM450, without Pt coating). The compositions of thin films were obtained through energy dispersive spectroscopy (FEI Quanta 600 scanning electron microscope, 20 kV). Transmission electron microscopy (TEM, Tecnai G2 20U-TWIN) was used to characterize the as-synthesized PbS QDs. The photoluminescence of the sample was excited by a Ti:Sapphire (Mira 900) laser with the excitation wavelength of 488 nm, and recorded by an liquid N2 cooled InGaAs CCD spectrometer (Princeton, OMV5). The dynamic light scanning (DLS) and zeta-potential results were obtained by a Malvern NanoZS ZEN3600 analyzer. Fourier transform infrared spectroscopy (FTIR) tests were carried out using a Bruker VERTEX 70 instrument. Current density–voltage characteristics in the dark and under light were measured by a Keithley 2420 sourcemeter under ambient conditions. Light source is standard AM1.5G (100 mW/cm2) made by the Newport Sol3A Class AAA solar simulator (Oriel, model 94023 A). No intentional temperature control or aperture was used for the efficiency measurement. 3. RESULTS AND DISCUSSION

3.1 Synthesis and Characterization of Water-MCCs.

Figure 2. (Top) Photograph of a series of solutions by dissolving six V2VI3 chalcogenides in aqueous (NH4)2S solution, respectively. (Bottom) Photograph of scale-up dissolution of As2Se3 and Sb2S3 in aqueous (NH4)2S solution.



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Figure 3. Raman spectra of (a) As2S3, (b) As2Se3, (c) As2Te3, (d) Sb2S3, (e) Sb2Se3, and (f) Sb2Te3 precursor solutions.

As shown in the top of Figure 2, a series of six bulk V2VI3 chalcogenides (V= As, Sb; VI= S, Se, Te) can be readily dissolved in aqueous (NH4)2S solution under magnetic stirring at room temperature and ambient pressure. The resulting solutions were free of visible scattering, indicating a complete dissolution to form true solutions rather than nanoparticulate dispersions. Moreover, the prepared solutions were stable when stored in ambient conditions for over 6 months, and no precipitation was observed. More importantly, this dissolution process could be easily scalable, which was demonstrated on the scale-up dissolution of As2Se3 and Sb2S3 (bottom of Figure 2), highlighting the great potential of this water-based dissolution process for mass production. We applied Raman spectroscopy to characterize our chalcogenides solutions to investigate the dissolution mechanism. Figure 3 showed typical Raman spectra of all six V2VI3 chalcogenides solutions. In the case of As2S3 and Sb2S3, the distinct peak located at 2560 cm-1 can be assigned to the S-H vibration in HS- species formed upon dissolution of (NH4)2S in water and subsequent hydrolysis of S2-,25 while the peak located at 394 cm-1 (Figure 3a) and 363 cm-1 (Figure 3d) could presumably be attributed to the As-S and Sb-S stretching mode of As and Sb chalcogenide complexes: AsS33- and Sb4S72-, respectively.26-28 Therefore, we proposed that the dissolution mechanism of As2S3 and Sb2S3 in aqueous (NH4)2S solution may be processed by forming highly soluble MCCs through the following reactions:

As2S3 + 3(NH4)2S → 6NH4+ + 2AsS33-

2Sb2S3 + (NH4)2S → 2NH4+ + Sb4S72-

(1) (2)

With regard to Sb2Se3, besides the predictable Sb-Se peak located at 216 cm-1, a Sb-S peak at 363 cm-1 was observed, as shown in Figure 3e. Meanwhile, the Se chain (235 cm-1) and Se8 ring (260 cm-1) peaks were also detected, despite no elemental Se added to the solution. Moreover, as shown in Figure S1 in the Supporting Information, there was a certain amount of (NH4)2Sx present in aqueous (NH4)2S solution, which could be easily decomposed into elemental S.29 Based on the above information, we therefore proposed that the dissolution process of Sb2Se3 could be attributed to substitution of Se with S, thus forming Sb4(S1-x,Sex)72− while leaving Se in the solution through the following overall chemical equation: 2Sb2Se3 +(6-7x)S + (NH4)2S

→ 2NH4+ + Sb4(S1-x,Sex)72- + (6-7x)Se

(3)

Such substitution reaction was expected considering the very similar chemistry between S and Se and the unchanged complex configuration after the replacement. This result was further strengthened by the control experiment in directly dissolving the elemental Se in aqueous (NH4)2S solution (Figure S2), similar to the dissolution of Se and Te in a thiol-amine solvent mixture.30 Furthermore, the analogous substitution reaction was also observed in


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the dissolution process of As2Se3 due to the presence of Se chain, Se8 ring, and As-S peaks (Figure 3b). However, the frequency of As-Se stretching mode was not available in the literature. Here we calculated As-Se vibration frequency based on the known As-S vibration frequency through the harmonic oscillator approximation for isolated binary vibrational modes:31 v(As-Se)= v(As-S)

M M M

4

M M M

The v(As-Se) calculated from this expression was approximately 297 cm-1, agreeing well with our experimentally observed Raman peak located at 291 cm-1. We therefore proposed that this new Raman peak resulted from an AsSe vibrational mode, thus confirming the formation of As(S1-x,Sex)33-. As analogous to the As2Se3 and Sb2Se3, the As2Te3 and Sb2Te3 were dissolved in aqueous (NH4)2S solution through the similar substitution reaction of S exchanging Te to form corresponding As(S1-x,Tex)33- and Sb4(S1-x,Tex)72-, as shown in Figure 3c and 3f. In brief, on the basis of the design principle, we successfully synthesized various MCCs through directly dissolving a series of bulk V2VI3 chalcogenides (V= Sb, As and VI= S, Se, Te) in water with the presence of (NH4)2S. 3.2 Thermal Analysis of As-prepared Water-MCCs.

Figure 4. (a) TGA of Sb2S3-MCCs (blue curve) and As2Te3MCCs (pink curve; run at 10 °C min-1 in a N2 flowing environment). (b) FTIR spectra of dried (black curve) and annealed (300 °C; red curve) Sb2S3-MCCs. (c) XRD pattern of Sb2S3-MCCs annealed at 300 °C. (d) XRD pattern of As2Te3-MCCs annealed at 300 °C. Temperature-dependent saturated vapor pressure of (e) Sb2S3 and S and (f) As2Te3 and Te in the temperature range from 300 °C to 500 °C.

With the formation of water-MCCs solutions, we are now exploring their application for solution deposition of corresponding chalcogenides. Decomposition temperature of MCCs is the most fundamental parameter for the deposition; therefore, thermogravimetric analysis (TGA) of dried water-MCCs solutions was first measured to determine the temperature at which water-MCCs decomposed. As shown in Figure 4a (blue curve), decomposition of (NH4)2Sb4S7 (denoted as Sb2S3-MCCs) occurred in essentially a single step and was completed at ~ 200 °C. The observed weight loss may correspond to the dissociation and loss of volatile decomposition products through the following reaction: (NH4)2Sb4S7 = 2Sb2S3 + 2NH3↑+H2S↑

(5)

Corroboration of the decomposition reaction was obtained by X-ray diffraction (XRD) using Sb2S3-MCCs solutions annealed at 300 °C. As shown in Figure 4c, all of the diffraction peaks matched well with orthorhombic Sb2S3 (JCPDS 06-0474), indicating the decomposition product was phase-pure Sb2S3 without any impurities or secondary phase. Fourier transform infrared spectroscopy (FTIR) further verified the thermal decomposition of (NH4)2Sb4S7. Figure 4b showed strong ν (N−H) and ν (O−H) stretching bands at 2500~3500 cm-1 for the dried Sb2S3-MCCs solution corresponding to the NH4+ and water, which disappeared after annealing at 300 oC for 10 min, in accordance with TGA result. Similarly, the Sb2Se3-MCCs, As2S3-MCCs and As2Se3-MCCs could also be decomposed into corresponding chalcogenides, as shown in Figure S5 and S6. X-ray photoelectron spectroscopy (XPS) was further performed to investigate the chemistry state of asprepared Sb2S3 and check the presence of oxygen impurity. As shown in Figure S4, magnified XPS spectra of Sb (3d), S (2p) demonstrated that the Sb and S were in the expected valence states (Sb23+S32-), and no detectable oxygen ( 104 cm-1), and non-toxic, low-cost, and earth abundant nature.26, 38-41 Importantly, the true solution characteristic of our water-MCCs solution allowed us to choose the simple and inexpensive spray pyrolysis technique to deposit Sb2(S1-x,Sex)3 alloy thin films, which can also be adapted easily for large-scale manufacturing.

7

Constants involved for calculation are shown in Table 1,36and the temperature-dependent equilibrium pressure are drawn in Figure 4e and 4f. As shown in Figure 4a (pink curve), the low-temperature transition (200 ~ 300 o C) indicated the loss of volatile decomposition products such as NH3 and H2S. Based on the calculated temperature-dependent vapor pressure of Te and As2Te3, the transition (300 ~ 380 oC) probably represented the loss of As2Te3, due to the higher vapor pressure of As2Te3 versus Te; the high-temperature transition (> 380 oC) may correspond to the evaporation of both As2Te3 and Te. As shown in Figure 4d, XRD study of As2Te3 precursor solutions annealed at 300 °C further confirmed the formation of As2Te3 and elemental Te, analogous to the recovery of dissolved bulk As2Te3 in a thiol–amine solvent mixture via low-temperature annealing.4 Note that the excess Te, inevitably remaining in the sample after the lowtemperature stage of decomposition due to the lower volatility of Te versus S, proves much more difficult to be removed from the sample than for Sb2S3. With regard to Sb2Te3-MCCs, the decomposition products were analogous to that of As2Te3-MCCs, as shown in Figure S6d. Briefly, the above characterizations gathered from TGA, XRD and FTIR showed that our water-MCCs were well suited for the solution deposition of corresponding chalcogenides upon mild thermal treatment. 37

Table 1. Constants Involved for Calculation Material

eq

A

B

C

S

6

6.84

2500.12

186.30

Te

6

7.301

5370.6

221

Sb2S3

7

13.96

10490

As2Te3

7

10.45

8185

3.3 Fabrication of Sb2(S1-x,Sex)3 Alloy Thin Films and Their Application in Solar Cells. To further demonstrate the utility of our water-MCCs for solution processing of thin-film chalcogenides, we sought to use the Sb2S3-MCCs and Sb2Se3-MCCs solutions as an example to prepare high quality Sb2(S1-x,Sex)3 alloy thin films across the entire compositional range from x = 0 to 1 through simply controlling the annealing conditions. Very recently, our group as well as others have demonstrated the great potential of this Sb2(S1-x,Sex)3 alloy as absorber material for

Figure 5. (a) XRD patterns of Sb2(S1-xSex)3 alloy films deposited by spray pyrolysis at different substrate temperatures. (b) Magnified view of (020) and (120) diffraction peaks in the region indicated by the light blue. (c) Lattice constants a (red), b (blue), and c (green), derived from XRD diffraction peaks, plotted as functions of Se concentration x in the Sb2(S1-xSex)3 alloy films. (d) Transmittance spectra of Sb2(S1-xSex)3 alloy films. Figure 1 shows the schematic illustration of the deposition process for Sb2(S1-x,Sex)3 alloy thin films. Binary Sb2S3 films were deposited on FTO substrates at the temperature of 300 oC by spray pyrolysis using the Sb2S3-MCCs solution, while the Sb2(S1-x,Sex)3 alloy thin films were prepared using the Sb2Se3-MCCs solution through spray pyrolysis at different substrate temperatures. XRD measurements were first employed to characterize the crystalline structure of as-prepared films. As shown in Figure 5a, all of the films crystallized into orthorhombic structure with no additional and split diffraction peaks observed, indicating the formation of phase-pure Sb2(S1-x,Sex)3 alloys without any impurities or secondary phase. As expected with the smaller atom radius of S, a gradual shift to larger 2θ angle was clearly visible in the magnified view of (020) and (120) peaks shown in Figure 5b with increasing S concentration. Figure 5c showed the lattice constants a, b and c derived from the position of (200), (020) and (221) diffraction peaks respectively. The Se concentration x was then obtained from Vegard’s law using the calculated lat-


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tice parameters deduced from the XRD data, which can be written as:26, 40 m(x)= x × m(Sb2Se3) + (1-x) × m(Sb2S3)

(8)

where x, m(x), m(Sb2Se3) and m(Sb2S3) are the respective Se concentration and lattice parameters a, b, and c of these orthorhombic structured samples. To further verify the composition more precisely, we applied energy dispersive spectroscopy (EDS) to determine the Se concentration. As shown in Table S2, the Se concentration measured directly from EDS is very close to the results obtained from XRD patterns, further confirming the composition of the as-prepared films as Sb2(S1-x,Sex)3, respectively. The linear dependence of lattice parameters on alloy composition x further confirmed the compositional homogeneity of our Sb2(S1-x,Sex)3 alloy films. Based on XRD and EDS results, we therefore concluded that as the substrate temperature was increased, sulfur could be more prone to volatilize than selenium due to the higher vapor pressure of sulfur versus selenium (Figure S7) and then were substituted by excess selenium from the solution, thus leading to the gradually increased selenium content from x= 0.44 to 0.76 in the final Sb2(S1-x,Sex)3 alloy films. This chalcogen exchange during annealing has already been widely used in the fabrication of CIGS and CZTSSe films.10 It should also be noted that even at high substrate temperature of 400 o C, we still failed to get the pure Sb2Se3 film. Therefore, we applied a post-selenization step for our alloy films to obtain the pure Sb2Se3 films; please read more details in the Experimental Section.

Scanning electron microscopy (SEM) was utilized to characterize the morphologies of Sb2(S1-x,Sex)3 alloy films with increasing Se composition from 0 to 1. As shown in Figure 6, almost all of the alloy films showed a smooth and compact surface morphology except the Sb2(S0.24,Se0.76)3 film (Figure 6e) with apparent pinholes and cracks, which may arise from the high annealing temperature of 400 oC, thus leading to easier volatilization and larger volume shrinkage. It was notable that the grain size of Sb2Se3 was quite large close to one micrometer, which could be desirable for high-performance optoelectronic devices. In brief, the above material and optical characterizations demonstrated that our water-MCCs solution could be readily utilized to deposit chalcogenide thin films via spray pyrolysis, and even precisely control composition of Sb2(S1-xSex)3 alloy films and then conveniently tune the resulting band gaps through simply regulating the substrate temperatures.

Transmission spectroscopy was then applied to investigate the optical properties of our Sb2(S1-x,Sex)3 alloy films. As shown in Figure 5d, there was an obvious systematic red shift in transmission spectra of Sb2(S1-x,Sex)3 alloy films with increase of Se composition, due to a narrower band gap of Sb2Se3 than that of Sb2S3.39 Furthermore, band gaps of our six samples were estimated by plotting (αhν)1/2 versus (hν) and found to be 1.61, 1.33, 1.24, 1.20, 1.16 and 1.10 eV for x= 0, 0.44, 0.48, 0.56, 0.76 and 1, respectively, covering the entire range of bulk band gaps previously reported for Sb2S3 and Sb2Se3.33, 42

Figure 7. (a) Schematic configuration of superstrate TiO2/Sb2(S0.44,Se0.56)3 heterojunction solar cell. (b) Crosssectional SEM image of Sb2(S0.44,Se0.56)3 film. (c) Photosensitivity of Sb2(S0.44,Se0.56)3 film, illuminated under 650 nm LED, -2 whose power density is 430 μW cm . Current was tested using 40 V driving voltage. (d) J-V characteristics of Sb2(S0.44,Se0.56)3 device performance in the dark and under −2 100 mW cm simulated AM1.5G irradiation, respectively.

Figure 6. SEM images of Sb2(S1-xSex)3 alloy films with Se composition of (a) x= 0, (b) x = 0.44, (c) x = 0.48, (d) x = 0.56, (e) x = 0.76, and (f) x = 1, respectively.

To explore the suitability of the resulting alloy films for solar cell applications, we chose the Sb2(S0.44,Se0.56)3 film as an example to fabricate heterojunction solar cells with the structure of glass/FTO/TiO2/ Sb2(S0.44,Se0.56)3/Au, as shown in Figure 7a. The thickness of the alloy film was directly measured to be around 500 nm from Figure 7b, which was thick enough for absorbing much of incident sunlight. We first investigated the photoresponse of our Sb2(S0.44,Se0.56)3 thin film through evaporating Au electrodes onto this alloy film to build a photoconductive photodetector. Current-time (I-t) characteristic of the device was recorded with a light source (650 nm wavelength, 430 μW cm-2) generated by a functional generator controlled light-emitting diode. As shown in Figure 7c, under an external bias of 40 V, the dark and photocurrent were about 15 nA and 45 nA respectively, corresponding



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to a photo-to-dark current ratio of 3. The observed strong and reversible photoresponse suggested the good optoelectronic properties of our alloy film. Figure 7d showed the current density versus voltage (J –V) characteristics of the device measured in the dark and under 100 mW cm-2 simulated AM1.5G irradiation. The best device exhibited an open circuit voltage (Voc) of 0.49 V, a short-circuit current density (Jsc) of 6.6 mA/cm2, and fill factor (FF) of 44.2%, corresponding to a power conversion efficiency of 1.43%. As shown in Table S3, our device efficiency was quite low, mainly limited by the high defect density at the heterojunction interface as evidenced by the high value of saturation current density (J0= 2.0 × 10-2 mA cm-2) and diode ideality factor (A= 2.46) calculated according to the Sites’s method,43-45 which should be further improved by passivating interfacial defects. However, this preliminary 1.43% device efficiency is still encouraging, and fully demonstrates the great potential of our water-MCCs for thin-film solar cell applications, considering the simple aqueous solution process as well as the very limited optimization work done. 3.4 Aqueous Synthesis of PbS Quantum Dots with In-situ Water-MCCs Passivation. PbS colloidal quantum dots (CQDs) are attractive materials for application in photovoltaic devices due to their convenient solution processing and quantum size effect bandgap tunability.4650 To date, traditional synthesis method for PbS CQDs is always based on the use of long hydrocarbon ligands (e.g., oleic acid) to ensure their solution processability, subsequently replacing them with short thiols or halide anions during the layer-by-layer spin-coating process, thus providing high carrier transport and low defect density to reduce recombination loss. We took the view that introducing our water-MCCs during the growth of the PbS QDs could stabilize in-situ formed PbS QDs during onepot mixing of Pb2+ and S2− due to the strong binding affinity of our MCCs to the QDs, thus providing an effective surface passivation while achieving a simplified device fabrication procedure without further solid state ligand exchange.

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05-0592). (b) TEM image of as-prepared PbS QDs. Inset shows a magnified TEM image of PbS QDs. (c) Size histogram obtained from dynamic light scattering for aqueous PbS QDs solution: 5 ± 0.8 nm. (d) Absorption and photoluminescence spectra of aqueous PbS QDs solution.

Our aqueous synthesis of PbS QDs with in-situ waterMCCs passivation was depicted in Figure 1. The PbS QDs were prepared in water at room temperature using ionic starting materials Pb(NO3)2 and (NH4)2S in the presence of (NH4)2Sb4S7. Moreover, NaOH as the chelating agent was also added to control the hydrolysis of Pb2+ through the formation of Pb(OH)3-, considering the high stability constant of Pb(OH)3- complex β3= 3.8×1014. Once Pb(OH)3aqueous solution was directly added to the previously prepared (NH4)2Sb4S7 solution, the dark PbS QDs would be immediately generated with negligible formation of Pb(OH)2 due to the very low Ksp value of PbS (1.0×10-28) compared with that of Pb(OH)2 (1.2×10-15), and Sb4S72could nearly simultaneously stabilize in-situ formed PbS QDs to prevent them from growing into large chunks and further increase ink stability, thus leading to a selfstabilized aqueous PbS QDs solution. XRD was applied to investigate the crystal structure of as-synthesized QDs. As shown in Figure 8a, all of the diffraction peaks were in good agreement with cubic PbS (JCPDS 05-0592), indicating the as-synthesized QDs were phase-pure PbS. Moreover, the broadness of the diffraction peaks was attributed to the small size of PbS QDs. The average crystal grain size estimated from Rietveld refinement of XRD data by the Scherrer equation was about 5 nm. Figure 8b showed a typical transmission electron microscopy (TEM) image of PbS QDs, which clearly indicated the size of PbS QDs to be 5 ± 0.5 nm with no agglomeration, consistent with the calculated size from XRD characterization. Dynamic light scattering (DLS) measurements also confirmed single-particle populations in solutions with similar average diameter (Figure 8c). We applied absorption and photoluminescence spectroscopy to investigate the optical properties of PbS QDs in aqueous solution. As seen in Figure 8d (red curve), our as-synthesized PbS QDs exhibited a pronounced excitonic absorption peak centered at 1170 nm, corresponding to a quantum confinement induced band gap E0 of 1.06 eV. The small full width at half maximum (FWHM) of 70 nm reflected the narrow size dispersions. We calculated the average diameter of as-synthesized PbS QDs as 4.8 nm through the equation proposed by Hens:51 E0 = 0.41 + (0.0252d2 + 0.283d)-1, in good agreement with the TEM observation shown in Figure 8b. In addition, as shown in Figure 8d (black curve), our PbS QDs showed band-edge photoluminescence with its peak located at 0.95 eV, implying good passivation of nonradiative surface defects.

Figure 8. (a) XRD pattern of as-prepared PbS QDs. The bottom shows the standard diffraction pattern of PbS (JCPDS


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Chemistry of Materials o

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Sb2S3-MCCs at 300 C; TGA of As2S3, As2Se3, As2Te3 ,Sb2S3, Sb2Se3, and Sb2Te3 precursors; Results summary for the composition of Sb2(S1-xSex)3 films measured by EDS and XRD; Device performance parameters of the TiO2/Sb2(S0.44Se0.56)3 thin film solar cells. This material is available free of charge via the Internet at http://pubs.acs.org.

AUTHOR INFORMATION Corresponding Author Figure 9. (a) Zeta-potential curve of aqueous PbS QDs solution associated with MCCs capping. (b) FTIR spectra for PbS QDs capped with (NH4)2Sb4S7 ligands before (black line) and after (red line) annealing at 150 °C.

To investigate the stabilization mechanism of our PbS QDs, we carried out zeta-potential measurement on aqueous PbS QDs solution. As shown in Figure 9a, the measured zeta-potential of -35.2 mV indicated the binding of negatively charged Sb4S72- MCCs to the surface of PbS QDs, thereby providing the strong electrostatic repulsion between QDs needed to achieve a stable colloidal dispersion, as widely observed in other examples of colloidal NCs in polar solvents.34, 52 We further studied the efficacy of Sb4S72- MCCs ligands through FTIR. Figure 9b showed the FTIR spectra of the precipitates from centrifugation of aqueous PbS QDs solution before and after annealing at 150 oC. The absorption bands at 1700−1400 cm−1 indicated Sb-S vibrations of Sb4S72- MCCs, while the bands at 3000 cm-1 arising from characteristic N−H stretching which could originate from NH4+ counterions in close proximity to negatively charged MCC ligands, suggesting the attachment of negatively charged MCC ligands to the surface of PbS QDs. To conclude, on the basis of our novel in-situ water-MCCs capping strategy, we successfully obtained high-quality and stable aqueous PbS QDs solution readily processed in ambient at room temperature, thereby showing great promise of our waterMCCs serving as surface ligands for colloidal NCs. 4.

CONCLUSION

In conclusion, we demonstrated a general and environmentally benign water-based solution approach to synthesize a series of MCCs through directly dissolving bulk V2VI3 chalcogenides (V= Sb, As and VI= S, Se, Te) in water with the presence of (NH4)2S at room temperature and ambient atmosphere. Our synthesized water-MCCs were particularly attractive for solution deposition of corresponding chalcogenides thin films, as well as serving as in-situ capping ligands for colloidal NCs without further ligand exchange. Overall, the simple and environmentally benign synthetic method, combined with the versatility, would greatly broaden the application of MCCs in solution-processed inorganic semiconductors.

ASSOCIATED CONTENT Supporting Information. Raman spectra of (NH4)2S solution; Raman spectra of solution of Se in (NH4)2S and solution of Te in (NH4)2S; XRD patterns of as-bought Se and Te; XPS spectra of Sb2S3 prepared through thermal decomposition of

* [email protected]; [email protected]

ACKNOWLEDGMENT This work was financially supported by the “National 1000 Young Talents” project, the National Natural Science Foundation of China (91433105, 61322401, 51402115 and 21403078), and the director fund of Wuhan National Laboratory for Optoelectronics. The authors thank Beijing Technol Science Co. Ltd. for thermal evaporator technical assistance and thank the Analytical and Testing Center of HUST and the facility support of the Center for Nanoscale Characterization and Devices, Wuhan National Laboratory for Optoelectronics (WNLO).

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Generalized Water-Processed Metal Chalcogenide Complexes

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