A general route to the synthesis of surfactant-free CuInS2 (CIS), Cu2CoSnS4 (CCTS) and Cu2ZnSnS4 (CZTS) nanocrystals dispersible in low boiling point solvents is proposed. These nanocrystal inks should be of great interest to the fabrication of thin film absorbers of chalcogenide solar cells

Chane-Ching, Jean-Yves

Durand, Bernard

Gillorin, Arnaud

Zaberca, Oana

English

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                         To link to this article: DOI: 10.1039/C1JM10894H          URL: http://dx.doi.org/10.1039/C1JM10894H            This is an author-deposited version published in: http://oatao.univ-toulouse.fr/ Eprints ID: 5560 To cite this version:  Zaberca, O. and Gillorin, A. and Durand, Bernard and Chane-Ching, Jean-Yves A general route to the synthesis of surfactant-free, solvent-dispersible ternary and quaternary chalcogenide nanocrystals. (2011) Journal of Materials Chemistry, vol. 21 (n° 18). pp.6483-6486. ISSN 0959-9428 Open Archive Toulouse Archive Ouverte (OATAO)  OATAO is an open access repository that collects the work of Toulouse researchers and makes it freely available over the web where possible.   Any correspondence concerning this service should be sent to the repository administrator: staff-oatao@listes.diff.inp-toulouse.fr A general route to the synthesis of surfactant-free, solvent-dispersible ternaryand quaternary chalcogenide nanocrystals†O. Zaberca, A. Gillorin, B. Durand and J. Y. Chane-Ching*DOI: 10.1039/c1jm10894hA general route to the synthesis of surfactant-free CuInS2 (CIS), S2 such as thiourea, thio-acetamide.Here we propose a general routeCu2CoSnS4 (CCTS) and Cu2ZnSnS4 (CZTS) nanocrystalsdispersible in low boiling point solvents is proposed. These nano-to the fabrication in various solvents of ternary (CuInS2) andquaternary (Cu2CoSnS4, Cu2ZnSnS4) chalcogenide dispersed nano-crystal inks should be of great interest to the fabrication of thin filmabsorbers of chalcogenide solar cells.Advances in high quality semiconducting nanocrystal synthesis1,2 andnew solution-based processing techniques3,4 have opened up newroutes to address the challenge of fabricating low cost, high efficiencyFig. 1 (a) CIS, (b) CCTS and (c) CZTS formation domains determinedin alcohol (top) and in ethylene glycol (bottom) under autogenouspressures at 200 C—16 h except for CIS in ethanol (150 C—16 h).Metallic cations were incorporated in stoichiometric ratios in reactionmixtures. Full circles depict ternary or quaternary chalcogenide singlephase formation evidenced by X-ray diffraction, Raman spectroscopysolar cells. For example, continuous roll to roll processing5 has beenrecently developed for thin film solar cells combining high speed andnon-vacuum deposition and involves the use of nanocrystal disper-sions. These solar paints can also be printed, sprayed onto large areasubstrates of thin film solar cells. The most successful synthetic routesdeveloped for producing colloidal semi-conducting nanocrystalsinvolve the use of coordinating solvents6 or surface ligands7–9 dis-playing long hydrocarbon chains to inhibit particle growth. Prom-ising material candidates are CuInSe2 ternary chalcogenides10,11 sincethey possess optimum band gap, high absorption coefficient andradiation stability.12 More recently, quaternary chalcogenidesCu2ZnSnS4 have attracted much interest13,14 because Sn and Zn arenaturally abundant in the Earth’s crust and have relatively lowtoxicity. Few reports15,16 describe the synthesis of the quaternarychalcogenide nanocrystals and to the best of our knowledge, the onlyworks published deal with the arrested precipitation process.Nevertheless, first solar cells fabricated using these building blocksyield low device efficiencies, likely due to the presence of the highlyinsulating barriers around the nanocrystals. Although much efforthave been devoted to remove the growth inhibiting reagents includingseveral ligand exchange stages bymild chemical treatment,17 completeremoval of coordinating solvent or surface ligands has proven to bedifficult. A need therefore exists for the fabrication of free-surfactant,ternary or quaternary chalcogenide nanocrystals redispersible in lowboiling point solvent required for the subsequent film forming stage.Indeed, the size and surface charge control is extremely difficult aslong as conventional reactions18–22 are employed such as the reactionbetween metallic cations (Cu+, Sn4+, Zn2+) and various sources ofCIRIMAT, CNRS, INPT, Universite de Toulouse, 118, Route deNarbonne, 31062 Toulouse Cedex 9, France† Electronic supplementary information (ESI) available. See DOI:10.1039/c1jm10894hparticles involving the high-temperature polycondensation of tailoredoxy-hydroxy-thiourea-Mn+ (Mn+ ¼ metallic cation) complex precur-sors. These complex precursors formed in various solvents can bedescribed as ionic, colloidal or slightly aggregated dispersions,depending mainly on the thiourea additions and the OH partialneutralization rate of the metallic cations. Thus, by fine tuning of theprecursors composition achieved in a very limited range of OH,loosely packed aggregates formed from surface charged ultrafineprimary crystallites were produced yielding a family of solvent-dispersible pure chalcogenide nanoparticles.Formation domains of CuInS2 (CIS), Cu2CoSnS4 (CCTS),Cu2ZnSnS4 (CZTS) metal chalcogenides by polycondensation reac-tions at 200 C were determined in various solvents such as ethyleneglycol, ethanol or isopropanol using CuCl2$2H2O, SnCl4$5H2O,MClx$yH2O (M ¼ Zn(II), Co(II), In(III)) as metallic salts andrespectively thiourea and tetramethylammonium hydroxideand chemical analysis. For syntheses in alcohols, diagrams are givenrespectively in ethanol for CIS and CCTS, and in isopropanol for CZTS.Table 1 Nanocrystals chemical composition determined from chemical elemental analysis (1) or EDS (2) showing nearly stoichiometric composition ofCIS and CZTS samples synthesized in ethylene glycol. In contrast, incorporation of Zn is difficult for CZTS syntheses performed in ethanol. The excessof S content is attributed to the presence of thiourea surface speciesCompound SolventReactionmixture composition Nanocrystal compositionCuInS2 Ethylene glycol (Cu : In) ¼ (1.00 : 1.20) CuIn1.22S2.13(1)CuCoSnS4 Ethanol (Cu : Co : Sn) ¼ (2.00 : 1.00 : 100) Cu2.0Co0.77Sn0.96S 3.91(1)Ethylene glycol (Cu : Co : Sn) ¼ (2.00 : 1.00 : 1.00) Cu2.0Co0.85Sn0.96S 4.94(1)Cu2ZnSnS4 Ethanol (Cu : Zn : Sn) ¼ (2.00 : 1.25 : 1.00) Cu2Zn0.25Sn1.02S3.2(1)Isopropanol (Cu : Zn : Sn) ¼ (2.00 : 1.00 : 1.00) Cu2.00Zn0.66 Sn1.16S4.42(2)Isopropanol (Cu : Zn : Sn) ¼ (2.00 : 1.25 : 1.00) Cu2.00Zn0.75Sn1.19S4.50(2)Isopropanol (Cu : Zn : Sn) ¼ (2.00 : 1.50 : 1.00) Cu2.00 Zn0.84 Sn1.13S4.52(2)Ethylene glycol (Cu : Zn : Sn) ¼ (2.00 : 1.00 : 1.00) Cu2Zn0.84Sn0.97S4.55(1)Ethylene glycol (Cu : Zn : Sn) ¼ (2.00 : 1.30 : 1.10) Cu2Zn1.07Sn1.05S4.46(1)(TMaOH) as sulfide and OH sources. Because Cu(II) salt wasemployed in order to improve the chemical homogeneity of theprecursors, production of small crystallite size was promoted byachieving high supersaturation conditions through ascorbic acidaddition yielding an increase in [Cu+] when employing non-reductivesolvents such as ethanol or isopropanol. Crucial parametersdescribing the complex precursors are sulfide source (S2, HS,thiourea) and OH compositions. Thus, formation diagrams wereestablished with various [OH]/[Cu] and [S]/[Cu] ratios, where [OH]denotes the [OH] concentration, [S] the total sulfur source content, [S]¼ ([S2] + [HS] + [CS(NH2)2]) and [Cu] the total Cu content, [Cu]¼([Cu+] + [Cu2+]). CIS, CCTS and CZTS single phase formationdomains (Fig. 1) in ethylene glycol and alcohols were determined for[S]/[Cu] < 7 and [OH]/[Cu] < 5. X-Ray investigation coupled toRaman spectroscopy indicates largermetallic chalcogenide formationFig. 2 (a–c) TEM images of CIS, CCTS, and CZTS samples respectively sa loose packing of primary crystallites. (d) FTIR spectra recorded on Cu2Cthiourea. (e) z Potential of CZTS nanocrystals dispersed in EtOH showing thdomains in alcohols compared with ethylene glycol. Interestingly,significant differences were observed between formation conditions ofCZTS and CCTS. Whereas CCTS formation was observed both inethanol and in ethylene glycol, chemical analysis has clearlydemonstrated formation of Cu2SnS3 rather than the desired CZTSstructure in ethanol (Table 1). In contrast, formation of CZTS wassuccessfully achieved in a slightly different alcohol such as iso-propanol. This illustrates the crucial effect of the solvent coordinationof the Zn2+ cation and probably the nature of the zinc–sulfide inter-mediate clusters on the CZTS structure formation. Moreover,although in some samples the X-ray pattern and the Raman spec-trum indicate single-phase formation, non-stoichiometric chemicalcomposition revealed by chemical analysis suggests the presence ofsome amorphous phase in these samples. Indeed, stoichiometricquaternary chalcogenide compositions such as CZTS were achievedynthesized from precursors prepared with OH incorporation showingoSnS4 and Cu2ZnSnS4 samples showing peak vibrations attributed toe presence of low surface charge.Fig. 3 Low magnification TEM micrographs showing the presence of a large number of nanocrystals dispersed in ethanol. (a) CIS nanocrystalssynthesized in ethylene glycol [OH]/[Cu] ¼ 5, [S]/[Cu] ¼ 5, (b) CCTS nanocrystals synthesized in ethylene glycol [OH]/[Cu] ¼ 1, [S]/[Cu] ¼ 5, (c) CCTSnanocrystals synthesized in ethanol [OH]/[Cu] ¼ 1, [S]/[Cu] ¼ 5, (d) CZTS nanocrystals synthesized in ethylene glycol, [OH]/[Cu] ¼ 1, [S]/[Cu]¼ 5. Scalebar: 2 mm. (e) Size distribution of CCTS and CZTS nanocrystals.with Zn excess (Cu : Zn : Sn : S) ¼ (2.0 : 1.3 : 1.1 : 10) mole in thereaction mixture. In comparison with the conventional approachinvolving reactions between metallic cations and a sulfide source, fineinspection of formation diagrams indicates a significant extension ofmetal chalcogenide formation domains achieved with OH addition.Corresponding XRD patterns and Raman spectra confirmed thatthese samples could be fully identified as single phase ternary orquaternary chalcogenides. As expected, small primary crystallite sizeswere determined of around 4–8 nm, as calculated from the Scherrerequation using the full-width at the half-maximumof the chalcopyrite(112) peak. For CIS or CCTS samples, these primary crystallitescorrespond to crystallites observed by HRTEM. Interestingly, TEMobservations reveal a loose aggregation of the ternary and quaternarychalcopyrite primary crystallites when samples are synthesized fromprecursors with OH addition regardless of the solvent used. TheTEM observations are consistent with the N2 absorption–desorptionBET curves showing the presence of pores of larger size (40 nm) andlarger volume (73 cm3 g1) for these samples prepared with OHaddition. We take advantage of this loose aggregation to produce byball-milling discrete ternary and quaternary chalcogenide nano-crystals in the size range 150–250 nm. Redispersion of these ball-milled nanocrystals was achieved in polar solvents arising probablyfrom the presence of surface species such as thiourea and evidencedboth from S and C chemical analysis and FTIR spectra recorded onthese samples. Peaks were observed at 1626 cm1, 1410 cm1 and at1110 cm1 and attributed respectively to NH2 bending deformation,C]S stretching vibration and NH2 rocking vibration23 (Fig. 2d).Indeed, the presence of thiourea surface species probably causes thepositive surface charge24 as determined from the z potential curves(Fig. 2e) indicating slightly positive CIS, CCTS, and CZTS nano-crystal charged surfaces in polar solvents such as ethanol or aceto-nitrile. Because the nanocrystals surface charge was relatively low,solvent-redispersibilty of the ternary and quaternary chalcogenidenanocrystals was only achieved in dispersions displaying low ionicstrength. Typical conditions of preparation of pure ternary andquaternary chalcogenide solvent-redispersible nanocrystals via thepolycondensation of tailored oxy-hydroxy-thiourea-Mn+ complexprecursors were thus determined from a rationale study performed invarious solvents. In a typical experiment, Cu2ZnSnS4 nanocrystalsin ethylene glycol were synthesized using the following molarratios: 2.0CuCl2$2H2O : 1.3ZnCl2$6H2O : 1.1SnCl4$5H2O : 10CS(NH2)2 : 2.0 TMaOH. The resulting mixture was transferred intoa Teflon-line stainless steel autoclave for 16 h at 200 C. Afterwashing, the precipitated solid was ball-milled in EtOHusing 0.3 mmZrO2–Y2O3 balls. Decrease in ionic strength of the suspensions wasachieved through EtOH washing and CZTS nanocrystals dispersionwas collected after size selection performed by centrifugation at3000 rpm. Fig. 3 shows TEM images of discrete CIS, CCTS andCZTS nanocrystals of mean size ranging from 150 nm (CCTS) to 250nm (CZTS) and dispersed in ethanol (Fig. 3e). These nanocrystalswhich consist of aggregates formed from 5–8 nm primary crystallitesdisplay substantial stability up to 5 days, allowing formation ofternary and quaternary chalcogenides thin films of solar cells.In conclusion, a novel approach for the synthesis of surfactant-freesolvent-redispersible metallic chalcogenide nanocrystals is proposed.Ternary and quaternary chalcogenide nanocrystals displaying surfacecharge were obtained by high-temperature polycondensation ofcomplex precursors designed in a narrow range of OH additioncorresponding to the partial neutralization of the cations. The abilityof generating surfactant-free solvent-redispersible nanocrystalsrenders this approach appealing, particularly for the ink-basedfabrication of CZTS solar cells.AcknowledgementsThis work was supported by Ecole des Beaux Arts, Region Reunion,France. The authors thank Alain Seraphine for helpful discussions.References1 C. B. Murray, D. J. Norris and M. G. Bawendi, J. Am. Chem. 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A general route to the synthesis of surfactant-free, solvent-dispersible ternary and quaternary chalcogenide nanocrystals

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