Advances in the colloidal synthesis of high quality nanocrystals have opened up new possible routes to address the challenge of fabricating low-cost, high-efficiency solar cells. In particular, photovoltaic devices based on several semiconductor nanocrystals have recently been demonstrated, including Cu 2 S, 2 CdTe, 3 PbSe, 4 Pb(S x ,Se 1-x ), 5 CuInSe 2 , Cu(In,Ga)Se 2 , and Cu(In 1-x ,Ga x )S 2 . In one approach, the semiconductor nanocrystals can be used in the form of a nanocrystal ink that is coated on a substrate and sintered into bulk material to yield a low-cost solar cell fabrication process.Among the various semiconductor nanocrystals, one of the most promising candidates for low cost thin film photovoltaics is the I-III-VI 2 family of chalcogenide nanocrystals. However, due to the limited supply and increasing price of rare metals, such as indium and gallium, there is a need to find alternative materials with high abundance and low cost. Recently, there has been an effort to investigate direct band gap Cu 2 ZnSnS 4 10 (CZTS) and Cu 2 ZnSnSe 4 (CZTSe) thin films for photovoltaic applications. CZTS and CZTSe are particularly attractive because Sn and Zn are naturally abundant in the Earth's crust and have relatively low toxicity. Solar cells based on CZTS have achieved power conversion efficiencies as high as 6.77% under AM1.5G illumination. 11 However, Shockley-Queisser photon balance calculations show that the theoretical limit for CZTS is 32.2%. Various high-vacuum and nonvacuum based techniques similar to those explored for Cu(In,Ga)Se 2 absorbers have been investigated for the deposition of CZTS thin films including coevaporation and selenization of various precursor layers. Here, we report the first synthesis of I 2 -II-IV-VI 4 nanocrystals of Cu 2 ZnSnS 4 and demonstrate their use in the fabrication of solar cells.CZTS nanocrystals are synthesized by hot injection of a solution of elemental sulfur in oleylamine into an oleylamine solution containing 1.5 mmol of copper(II) acetylacetonate, 0.75 mmol of zinc acetylacetonate, and 0.75 mmol of tin(IV) bis(acetylacetonate) dibromide at 225 ยฐC. Please refer to the Supporting Information for experimental details. Figure shows the PXRD pattern of the as-synthesized CZTS nanocrystals. CZTS is a tetrahedrally coordinated semiconductor where each sulfur anion is bonded to four cations and each cation is bonded to four sulfur anions. The ordering of the cations in the cation sublattice may occur in at least two variations. In one (the stannite structure), cation layers alternate with sulfur anion layers along the crystallographic c-direction as CuCu/SS/ZnSn/SS. In another (the kesterite structure), the cation and anion layers alternate as CuZn/SS/CuSn/SS. Both variations are described by tetragonal unit cells. Note these are both similar to the chalcopyrite structure such as CuInS 2 where cation and anion layers alternate as CuIn/SS/CuIn/SS. The kesterite and stannite powder X-ray diffraction (PXRD) patterns differ only slightly in the splitting of high order peaks, such as (220)/( ) and ( )/ (312) due to a slightly different tetragonal distortion (c/2a). However, here the structure cannot be distinguished between kesterite and stannite using PXRD due to peak broadening, and the PXRD pattern of the as-synthesized CZTS nanocrystals (Figure