Sulvanite (Cu3VS4) nanocrystals for printable thin film photovoltaics

Materials Letters 211 (2018) 179–182

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Sulvanite (Cu3VS4) nanocrystals for printable thin film photovoltaics Ching-Chin Chen a, Kevin H. Stone b, Cheng-Yu Lai a, Kevin D. Dobson c, Daniela Radu a,d,⇑ a

Department of Chemistry, Delaware State University, 1200 N. DuPont Highway, Dover, DE 19901, USA SLAC National Accelerator Laboratory, Menlo Park, CA 94025, USA c Institute of Energy Conversion, University of Delaware, Newark, DE 19716, USA d Department of Material Science and Engineering, University of Delaware, Newark, DE 19716, USA b

a r t i c l e

i n f o

Article history: Received 19 July 2017 Received in revised form 3 September 2017 Accepted 17 September 2017 Available online 21 September 2017 Keywords: Nanocrystalline materials Nanoparticles Colloidal processing Thin films Solar energy materials Electronic materials

a b s t r a c t Copper Vanadium Sulfide (Cu3VS4), also known as sulvanite, has recently emerged as a suitable absorber material for thin film photovoltaics. The synthesis of Cu3VS4 nanocrystals via a rapid solvothermal route is reported for the first time. The phase purity of the Cu3VS4 nanocrystals has been confirmed by X-ray powder diffraction (XRD) and Raman spectroscopy, while the nanoparticle size, of about 10 nm, was evaluated by transmission electron microscopy (TEM). Successful ligand exchange with sulfide, an inorganic ligand, demonstrated that the nanoparticles are amenable to surface modifications, key element in solution processing. Further annealing of as-synthesized nanocrystals under a sulfur/argon atmosphere at 600 °C, rendered highly crystalline Cu3VS4 powders exhibiting an impurity that could be potentially mitigated by annealing temperature optimization. Thus, Cu3VS4, formed solely from Earth-abundant elements, could provide an inexpensive, reliable approach to fabricating solution processed thin film photovoltaic absorbers. Ó 2017 Elsevier B.V. All rights reserved.

1. Introduction Solar photovoltaic (PV) absorber materials have been extensively explored toward harnessing solar energy with high efficiency. As part of second generation PV, thin film technologies have been regarded as inexpensive alternatives to Gen1 silicon PV, and have recently reached comparable power conversion efficiencies [1,2]. The commercial thin film PV is championed by two absorber materials: copper indium gallium diselenide Cu(InxGa1 x)S2 (CIGS) and cadmium telluride (CdTe). Nevertheless, indium and tellurium scarcity and their price escalation impact tremendously CdTe and CIGS module production costs [3]; along with cadmium toxicity, these concerns drove attention to alternative PV absorbers, composed of Earth-abundant, sustainable, and preferably non-toxic elements. Isostructural with CIGS, copper zinc tin sulfide and sulfo-selenides, Cu2ZnSnS4 (CZTS) and Cu2ZnSn(S, Se)4 (CZTSSe), gained exponential interest as solar absorbers in the past two decades. Since the first CZTS report in 1988 [4], increases in power conversion efficiency reached a 12.6% record [5–11]. Importantly, CZTS research also demonstrated that fabrication of solution-processed thin-film absorbers from nanocrystal precursors represent a feasible and economically advantageous ⇑ Corresponding author at: Department of Chemistry, Delaware State University, 1200 N. DuPont Highway, Dover, DE 19901, USA. E-mail address: [email protected] (D. Radu). https://doi.org/10.1016/j.matlet.2017.09.063 0167-577X/Ó 2017 Elsevier B.V. All rights reserved.

approach. Nevertheless, the stagnation in CZTS/CZTSSe development, primarily due to open-circuit voltage (Voc) deficiencies and the inability to eliminate structural defects [12,13], has further fueled the quest for alternative materials. Recently, Kehoe et al. have reported the calculated band structures a new group of p-type semiconductors: Cu3MCh4 (M = V, Nb, Ta; Ch = S, Se, Te) [14,15], highlighting their potential as photovoltaic absorbers given their direct bandgaps in the proximity of 1.5 eV. Among these materials, Cu3VS4 (CVS) stands out for its elemental composition, which includes only Earth-abundant elements. Bulk CVS powder was previously synthesized through a solid state method, and the energy bandgap was estimated to be 1.3 eV [16]. Recently, nano-CVS thin films were prepared by pulsed laser deposition (PLD). The reported bandgap of 1.35 eV and absorption coefficient >105 cm 1 have demonstrated the potential of CVS as a thin film PV absorber. Success in solution processed CIGS and CZTSSe encouraged investigation of nanoscale precursor routes in fabricating thin film absorbers. Sintering of colloidal nanocrystals led to low-cost, highefficiency chalcogenide solar cells [5–7,17,18]. Colloidal semiconductor nanocrystals benefit from facile solution synthesis routes, at moderate temperatures in comparison with their bulk counterparts, and allow control of nanoparticle stoichiometry, phase purity, and particle size [19–21]. Crystalline nanoparticles

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(nanocrystals) dispersed in adequate solvents (‘‘inks”) could be readily deposited onto substrates through simple coating methods (spin-, dip-, spray-coating), as a preamble to the ultimately desired roll-to-roll processing technologies on flexible substrates, toward enabling low-cost thin film PV. In this report, we demonstrate the first synthesis of colloidal CVS nanocrystals (NCs) and their amenability to surface modification. The CVS NCs were characterized by powder X-ray diffraction (XRD) and Raman spectroscopy to validate phase purity. Their morphology was characterized by transmission electron microscopy (TEM).

by centrifugation at 7500 rpm for 5 min. The washing step was repeated once, replacing ethanol with isopropanol as an antisolvent. The final NCs were stored under vacuum.

2. Materials and methods

2.4. Sulfide (S2 ) ligand exchange of as-synthesized nanocrystals

2.1. Chemical reagents

The ligand exchange method was adapted from previously published procedures [22,23]. In a typical experiment, as-synthesized nanocrystals (20 mg) were washed with hexane and either isopropanol or ethanol (hexane to alcohol ratio 1:1), recovered by centrifugation, and dispersed in toluene (5 mL). A sodium sulfide Na2S solution in formamide (FA) (5 mL, 0.18 M) was added to the nanocrystals dispersion and the final dispersion was vigorously stirred overnight. Upon transfer of CVS NCs from the toluene to the FA layer, the toluene layer was removed and the NCs in the FA layer were washed three times with toluene (3 mL each time). Ethanol (5 mL) was added to the FA dispersion to precipitate the CVS NCs, which were further recovered by centrifugation at 7500 rpm for 5 min.

Vanadium(III) acetylacetonate (97%), sodium sulfide, and oleylamine (70%) were purchased from Sigma-Aldrich. Copper(II) acetylacetonate (98%), sulfur powder ( 100 mesh, 99.5%), and formamide (99%) were purchased from Alfa Aesar. Hexane, toluene, chloroform (all ACS grade), and ethanol (200 proof), were purchased from VWR International. All reagents and solvents were used without further purification. 2.2. Preparation of Cu3VS4 nanocrystals All solution syntheses were conducted in an argon atmosphere and all steps of the nanocrystals purification were conducted in air. In a typical experiment, vanadium(III) acetylacetonate (870.7 mg, 2.5 mmol) and copper(II) acetylacetonate (1570.6 mg, 6 mmol) were dissolved in oleylamine (30 mL) at 120 °C and the mixture was further degassed under vacuum at 120 °C for 30 min. Sulfur (256.5 mg, 8 mmol) was separately dissolved in oleylamine (10 mL) and heated at 150 °C. The temperature of the Cu-V solution was increased to 230 °C over a 15-min period and held at 230 °C for another 30 min, followed by dropwise injection of the sulfur solution. The final reaction mixture was stirred at 230 °C for 30 min. After cooling to room temperature by removing the heating source, 10 mL of hexane were added and the suspension was separated into two 50 mL centrifuge tubes. Further, 20 mL of ethanol were added to each tube and the CVS NCs were collected by centrifugation at 7500 rpm for 5 min. Next, CVS NCs were washed by redispersion in a mixture of 10 mL hexane and 5 mL chloroform, and precipitated in 25 mL of ethanol followed

2.3. Annealing of as-synthesized nanocrystals An amount of 700 mg CVS NCs was placed in a graphite crucible and annealed under a sulfur and argon atmosphere at 600 °C. The sulfur atmosphere was obtained by placing sulfur powder (1 g) in another crucible in the 350 °C zone of the tube furnace, preceding the graphite crucible in the argon flow.

2.5. Characterization The crystal structure of bulk sulvanite Cu3VS4 used for reference were obtained from The Database ‘‘The RRUFFTM Project” [24] and the simulation of diffraction pattern was performed with the Mercury software from the Cambridge Structural Database [25]. X-ray diffraction (XRD) was performed on a Rigaku MiniFlex600 system equipped with a Cu Ka radiation source (k = 1.5405 Å) and operated at 30 mV and 10 mA. Raman spectroscopy analysis was carried out on Horiba Scientific XploRA PLUS equipped with an Ar-laser source (k = 532 nm). TEM images were recorded on a Zeiss LIBRA 120 instrument. Thermogravimetric Analysis (TGA) was performed on a Discover TGA (RT Instruments). High resolution synchrotron powder diffraction data were collected at SSRL beamline 2–1 with 17 keV X-rays (k = 0.729 Å) and Rietveld refinement performed using the software TOPAS-Academic [26].

Fig. 1. (a) Crystal structure of sulvanite Cu3VS4; (b) Powder XRD patterns of as-synthesized (top) and annealed (middle) Cu3VS4 nanocrystals, and simulated crystal structure of Cu3VS4.

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3. Results and discussion 3.1. Characterization of phase purity Crystal structure information of sulvanite Cu3VS4 from the American Mineralogist Crystal Structure Database (AMCSD) [27], based a report from Pauling et al. [28] were used to simulate the crystal structure (Fig. 1(a)) and the powder X-ray diffraction pattern (Mercury software). The crystallographic parameters of CVS are listed in Table 1. The simulated pattern is showed in Fig. 1(b), along with the X-ray diffraction pattern of as-synthesized CVS powder and the

annealed powder, both in good agreement with the simulated pattern. The identity of CVS NCs was further demonstrated by Raman spectroscopy. Fig. 2 shows Raman spectra of both as-synthesized and annealed Cu3VS4 nanocrystals. A spectrum of bulk CVS is shown for comparison (Fig. 2 inset). The Raman pattern of the synthesized Cu3VS4 nanocrystals is in agreement with literature reported data, showing an enhanced peaks intensity and signal-to-noise ratio after annealing [16]. Upon annealing in sulfur/argon atmosphere at 600 °C for 1 h, TGA data were collected for the annealed Cu3VS4 from ambient temperature (25 °C) to 900 °C, at a ramping temperature of

Table 1 Crystallographic parameters of sulvanite Cu3VS4 structure. Space group: P-43 m, a = b = c = 5.3921(1) Å, a = b = c = 90° Atom

x

y

z

Occupancy

Cu V S

0.500 0.000 0.2391(2)

0.000 0.000 0.2391(2)

0.000 0.000 0.2391(2)

1 1 1

Fig. 2. (a) Raman spectra of as-synthesized (blue) and annealed (red) Cu3VS4 nanocrystals. Inset: Raman spectrum of bulk Cu3VS4 sample (Courtesy of The Database ‘‘The RRUFFTM Project”, reference [24]); (b) TGA analysis (in N2 atmosphere) of the annealed Cu3VS4 powder.

Fig. 3. (a) Synchrotron XRD pattern and (b) refinement of Cu3VS4 annealed powder.

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modification by ligand exchange of Cu3VS4 nanocrystals demonstrated that the nanocrystals could be solution processed. Synchrotron XRD of Cu3VS4 powders upon annealing in a sulfur/ argon atmosphere at 600 °C show a highly crystalline Cu3VS4 sulvanite phase, accounting for 92.6% of the product along with 7.4% Cu1.8S (digenite) impurity. The reported Cu3VS4 nanocrystal have potential for serving as precursor materials in fabrication of thinfilm solar photovoltaics. Acknowledgements This material is based upon work supported in part by the National Science Foundation under grants No. 1435716 and No. 1535876, and by the U.S. DOE Sunshot Initiative, Award No. DE-EE0006322. Use of the Stanford Synchrotron Radiation Lightsource, SLAC National Accelerator Laboratory, was supported by the U.S. Department of Energy, Office of Basic Energy Sciences under Contract No. DE-AC02-76SF00515. References

Fig. 4. TEM image of ligand-exchanged Cu3VS4 nanocrystals (inset shows ligand exchange experiment).

10 °C/minute under nitrogen atmosphere. The weight loss starting around 390 °C (inset 1) could be correlated with further decomposition of residual carbon originated in the synthesis of the nanocrystals, while the loss starting around 640 °C could be associated with material decomposition. Synchrotron XRD analysis of the annealed CVS powder enabled quantification of powder purity, showing with a 92.6% Cu3VS4 sulvanite content (Rietveld refinement), while Cu1.8S (digenite) accounted for the other 7.4% of the product (Fig. 3). FWHM of X-ray diffraction peaks decreased in comparison with the Cu3VS4 NCs spectrum, suggesting an increase in primary crystallite size. 3.2. Ligand exchange results Nanocrystals synthesized in long-alkyl chain solvents retain carbon on their surfaces, which could be detrimental in electronic applications, given the layer created around the nanocrystals [29]. The inset in Fig. 4 shows the results of the ligand exchange of CVS NCs, which transferred from the non-polar (toluene) into the polar (formamide) layer, indicating polarity change of the NCs surface. This experiment validates the capability of CVS NCs to undergo surface modifications, thus opening the opportunity to utilize them in thin film solar cells. 3.3. Cu3VS4 nanocrystals size TEM imaging of as-synthesized CVS NCs was performed after ligand exchange. As showed in the TEM micrograph (Fig. 4), CVS NCs present a narrow particle size distribution, with the average particle size of 10 nm. 4. Conclusion A solution-based method for Cu3VS4 nanocrystals fabrication is reported. The Cu3VS4 nanocrystals showed phase purity by XRD and Raman spectroscopy. A narrow particle size distribution, with particle size of 10 nm was shown by TEM. Successful surface

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