Colloidal Synthesis of Wurtzite Cu₂ZnSnS₄ Nanorods and Their Perpendicular Assembly

Abstract

ABSTRACT:The quaternary copper chalcogenide, Cu 2 ZnSnS 4, is an important emerging material for the development of low cost and sustainable solar cells. Here we report a facile solution synthesis of stoichiometric Cu 2 ZnSnS 4 in size controlled nanorod form (11 ×35 nm). The monodisperse nanorods have a band gap of 1.43 eV and can be assembled into perpendicularly aligned arrays by controlled evaporation from solution.Colloidal semiconductor nanocrystals are a remarkable material set, that can be synthesized and processed as a 'chemical' in high yield while exhibiting optical and electronic properties that are size dependent.1 Applications ranging from bio-labeling to photocatalysis and photovoltaics have emerged exploiting either the discrete or collective properties of these size controlled crystals. 2 The synthesis of the archetypal binary (II-VI) nanocrystals has progressed to the point where precise control over their size, shape, composition and crystal phase is routine thereby rapidly accelerating the advances that utilize these as building blocks.1b,3 Extension of colloidal nanocrystal synthesis to ternary and quaternary semiconductors has the capacity to greatly expand this research platform. 4,5 In particular, copper based ternary and quaternary semiconductors such as CuInS(Se) 2 (CIS), CuIn x Ga 1-x S(Se) 2 (CIGS) and Cu 2 ZnSnS(Se) 4 (CZTS) are of interest due to their high absorption coefficients, low toxicity and suitable band gap for solar energy conversion. [4][5][6] Advances in the colloidal synthesis and shape control of nanocrystal CIS and CIGS have been demonstrated although similar reports with CZTS remain elusive. 4c-4e CZTS is flagged as the material most likely to allow unrestricted PV application on a global scale, given the relatively abundant nature of Zn and Sn in comparison to In and Ga and the promising efficiencies of 9.7%. 6,7 Generating CZTS in nanocrystal form allows absorber layer production by simple solution processes (spin-casting, spraying or printing methods) dramatically offsetting the cost of expensive vacuum processes. 6b,7,8b While synthesis of 0D CZTS nanocrystals in the tetragonal crystal structure, 5a-5e has been achieved, their formation in the more attractive rod geometry remains elusive. In nanorods, maximization of total absorption and directional charge transfer is possible by controlling the length while retaining the diameter dependent properties such as band gap. Moreover, control of orientation and positioning such that each nanorod is vertically aligned and close packed allows their collective properties to be harnessed at a device scale. Herein we describe a colloidal synthesis of monodisperse stoichiometric Cu 2 ZnSnS 4 nanorods in high yield. The quaternary semiconductor nanorods occur in the wurtzite crystal structure with elongation occurring along the [002] direction and exhibit a band gap of 1.43 eV. This crystal phase is attractive not just for shape control but is also known to allow wide range tuning of the band gap due to random distribu...

Phase-transition-driven growth of compound semiconductor crystals from ordered metastable nanorods

“…The observed Raman peaks at 150 cm -1 , 260 cm -1 , 288 cm -1 , 338 cm -1 (A-mode), 366 cm -1 and 376 cm -1 (vertical lines in Fig. 5a) are in good agreement with peak positions earlier found for kesterite CZTS [43], while dominant modes at 334 cm -1 and 377cm -1 (arrows) of CZTS nanorods have been also reported for wurtzite CZTS [10,20]. The strongest A-mode symmetry of kesterite CZTS annealed samples shifts to lower frequency region and significantly broadens with decreasing heating rate as shown in the inset of Fig.…”

“…X-ray diffraction (XRD) confirms a hexagonal wurtzite structure ( Fig. 1e) with space group P63mc, which was already reported before [20,26] (see Supplementary Fig. 1a for a larger angle range).…”

“…The signals of the annealed films at 150 cm -1 , 260 cm -1 , 288 cm -1 , 338 cm -1 (A-mode), 366 cm -1 and 376 cm -1 (vertical lines) can be attributed to kesterite [43]. The arrows at 334 cm -1 and 377cm -1 mark the positions of the wurtzite signals [10,20]. The inset shows peak position and full width at half maximum (FWHM) of the main kesterite peak (A-mode) as function of heating rate.…”

“…CZTS nanorods were synthesized by a hot-injection method adapted from our previous report [20] (see Methods for details). These nanorods show good monodispersity with a diameter of 9±0.5 nm and a length of 35±1 nm (Fig.…”

“…Cu2ZnSnS4 (CZTS) nanorods were synthesized by using a modification of our previously published procedure [20]. Cu(acac)2 (1.332 mmol), Zinc acetate (0.915 mmol), Tin(IV) acetate (0.75 mmol) and TOPO (4 mmol) were mixed with 10 mL of 1-octadecene in a 25 mL three-neck flask and evacuated at room temperature for 30-45 minutes.…”