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Sulvanite (Cu 3 VS 4 ) 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 b (^) 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
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 2017Available 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 (Cu 3 VS 4 ), also known as sulvanite, has recently emerged as a suitable absorber material for thin film photovoltaics. The
synthesis of Cu 3 VS 4 nanocrystals via a rapid solvothermal route is reported for the first time. The phase purity of the Cu 3 VS 4 nanocrystals has been
confirmed by X-ray powder diffraction (XRD) and Raman spectroscopy, while the nanoparticle size, of about 10 nm, was eval-uated 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 solu-tion processing.
Further annealing of as-synthesized nanocrystals under a sulfur/argon atmosphere at 600 °C, rendered highly crystalline Cu 3 VS 4 powders exhibiting an
impurity that could be potentially mitigated by anneal-ing temperature optimization. Thus, Cu 3 VS 4 , formed solely from Earth-abundant elements, could
provide an inexpensive, reliable approach to fabricating solution processed thin film photovoltaic absorbers.
1. Introduction
Solar photovoltaic (PV) absorber materials have been exten-
sively explored toward harnessing solar energy with high effi-
ciency. 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(InxGa 1 x)S 2 (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 alterna-
tive PV absorbers, composed of Earth-abundant, sustainable, and
preferably non-toxic elements. Isostructural with CIGS, copper zinc
tin sulfide and sulfo-selenides, Cu 2 ZnSnS 4 (CZTS) and Cu 2 ZnSn(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 fabrica-
tion of solution-processed thin-film absorbers from nanocrystal
precursors represent a feasible and economically advantageous
approach. Nevertheless, the stagnation in CZTS/CZTSSe develop-
ment, 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 struc-
tures a new group of p-type semiconductors: Cu 3 MCh 4 (M = V, Nb,
Ta; Ch = S, Se, Te) [14,15], highlighting their potential as photo-
voltaic absorbers given their direct bandgaps in the proximity of
1.5 eV. Among these materials, Cu 3 VS 4 (CVS) stands out for its ele-
mental 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 >10^5 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, high-
efficiency chalcogenide solar cells [5–7,17,18]. Colloidal semicon-
ductor nanocrystals benefit from facile solution synthesis routes,
at moderate temperatures in comparison with their bulk counter-
parts, and allow control of nanoparticle stoichiometry, phase pur-
ity, and particle size [19–21]. Crystalline nanoparticles
⇑ Corresponding author at: Department of Chemistry, Delaware State University,
1200 N. DuPont Highway, Dover, DE 19901, USA.
(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 modifica- tion. 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 micro-
scopy (TEM).
- Materials and methods
2.1. Chemical reagents
Vanadium(III) acetylacetonate (97%), sodium sulfide, and oley- lamine (70%) were purchased from Sigma-Aldrich. Copper(II)
acetylacetonate (98%), sulfur powder (100 mesh, 99.5%), and for- mamide (99%) were purchased from Alfa Aesar. Hexane, toluene,
chloroform (all ACS grade), and ethanol (200 proof), were pur- chased from VWR International. All reagents and solvents were used without further purification.
2.2. Preparation of Cu 3 VS 4 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 suspen-
sion 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
by centrifugation at 7500 rpm for 5 min. The washing step was repeated once, replacing ethanol with isopropanol as an anti- solvent. The final NCs were stored under vacuum.
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.
2.4. Sulfide (S 2 ) ligand exchange of as-synthesized nanocrystals
The ligand exchange method was adapted from previously pub- lished procedures [22,23]. In a typical experiment, as-synthesized nanocrystals (20 mg) were washed with hexane and either iso- propanol or ethanol (hexane to alcohol ratio 1:1), recovered by centrifugation, and dispersed in toluene (5 mL). A sodium sulfide Na 2 S 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.
2.5. Characterization
The crystal structure of bulk sulvanite Cu 3 VS 4 used for reference were obtained from The Database ‘‘The RRUFFTM^ Project” [24] and the simulation of diffraction pattern was performed with the Mer- cury software from the Cambridge Structural Database [25]. X-ray diffraction (XRD) was performed on a Rigaku MiniFlex 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 per- formed on a Discover TGA (RT Instruments). High resolution syn- chrotron powder diffraction data were collected at SSRL beamline 2–1 with 17 keV X-rays (k = 0.729 Å) and Rietveld refinement per- formed using the software TOPAS-Academic [26].
Fig. 1. (a) Crystal structure of sulvanite Cu 3 VS 4 ; (b) Powder XRD patterns of as-synthesized (top) and annealed (middle) Cu 3 VS 4 nanocrystals, and simulated crystal structure of Cu 3 VS 4.
- Conclusion
A solution-based method for Cu 3 VS 4 nanocrystals fabrication is reported. The Cu 3 VS 4 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
modification by ligand exchange of Cu 3 VS 4 nanocrystals demon- strated that the nanocrystals could be solution processed. Syn- chrotron XRD of Cu 3 VS 4 powders upon annealing in a sulfur/ argon atmosphere at 600 °C show a highly crystalline Cu 3 VS 4 sul- vanite phase, accounting for 92.6% of the product along with 7.4% Cu1.8S (digenite) impurity. The reported Cu 3 VS 4 nanocrystal have potential for serving as precursor materials in fabrication of thin- film 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 Light- source, SLAC National Accelerator Laboratory, was supported by the U.S. Department of Energy, Office of Basic Energy Sciences under Contract No. DE-AC02-76SF00515.
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Fig. 4. TEM image of ligand-exchanged Cu 3 VS 4 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 decompo- sition of residual carbon originated in the synthesis of the
nanocrystals, while the loss starting around 640 ° C could be asso- ciated with material decomposition. Synchrotron XRD analysis of the annealed CVS powder enabled
quantification of powder purity, showing with a 92.6% Cu 3 VS 4 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 Cu 3 VS 4 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 sur- face. 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. Cu 3 VS 4 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.
