Article pubs.acs.org/cm
Ligand-Free, Quantum-Confined Cs2SnI6 Perovskite Nanocrystals Dmitriy S. Dolzhnikov,†,‡ Chen Wang,† Yadong Xu,† Mercouri G. Kanatzidis,†,‡ and Emily A. Weiss*,†,‡ †
Department of Chemistry, Northwestern University, 2145 Sheridan Road, Evanston, Illinois 60208-3113, United States Argonne-Northwestern Solar Energy Research Center, 2145 Sheridan Road, M490, Evanston, Illinois 60208, United States
‡
S Supporting Information *
ABSTRACT: Tin-halide perovskite nanocrystals are a viable precursor for lead-free, high-efficiency active layers for photovoltaic cells. We describe a new synthetic procedure for quantum-confined Cs2SnI6 nanocrystals with size-dependent band gaps in the long-visible to nearinfrared (1.38−1.47 eV). Hot injection synthesis produces particles with no organic capping ligands, with average diameters that increase from 12 ± 2.8 nm to 38 ± 4.1 nm with increasing reaction temperature. The band gap, energies of the first excitonic peak, ground-state bleach peak (in the transient absorption spectrum), and photoluminescence peak depend linearly on the inverse square of diameter, consistent with quantumconfined excitons with an effective mass of (0.12 ± 0.02)m0, where m0 is the mass of an electron, a factor of 4.6 smaller than that in the bulk material. Transient absorption measurements show that approximately 90% of the bleach amplitude decays within 30 ps, probably because of carrier trapping on unpassivated surface sites. The films made by simple drop-casting of Cs2SnI6 nanocrystal solutions, with no postsynthetic ligand exchange or removal, are smooth and uniform, resist delamination, and have no visible gaps at the film− substrate interface.
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INTRODUCTION This manuscript describes a new procedure for synthesizing allinorganic quantum-confined Cs2SnI6 perovskite nanocrystals (NCs), where we control the average size of the particles through the reaction temperature, the optical and structural characterization of these NCs, and their deposition into highquality thin films. All-inorganic and organic−inorganic metal halide perovskite compounds have proven to be effective active materials within photovoltaic devices.1−7 Devices made with one of the most promising materials, CH3NH3PbI3, have a power conversion efficiency of up to 20.1%.8 The high toxicity of lead, however, limits the large-scale applicability of Pb-based perovskite materials.9 Substitution of Pb with Sn in the lattice reduces toxicity but compromises the air stability of these materials. One approach to this challenge is to use compounds with tetravalent rather than divalent tin. The deficient perovskite Cs2SnI6 consists of air-stable tetravalent Sn4+ and has a similar crystal structure to CH3NH3PbI3. The main difference between the two is that, in Cs2SnI6, Sn4+ cations only occupy half of the perovskite heavy-metal centers, whereas in CH3NH3PbI3, every heavy-metal center is occupied creating a 3D network of vortex-sharing [PbI6]4− octahedra. The latter leads to formation of isolated [SnI6]2− octahedra in the crystals of Cs2SnI6.10,11 Nonetheless, this material has shown promise as a hole conductor in dye-sensitized solar cells and as a light absorber in thin-film solar cells, with top efficiencies of 7.8% and 1.47%, respectively.11−15 © 2017 American Chemical Society
The best-performing Cs2SnI6 solar cells are made by iteratively electrospraying solutions of the molecular precursors CsI and SnI4 in high-boiling point solvents like N,Ndimethylformamide.11 Electrospray is a time- and energyintensive process and requires extensive optimization of deposition conditions to produce smooth, uniform films of Cs2SnI6, because Cs2SnI6 tends to form large octahedral crystals (∼10 μm in diameter). One alternative to electrospray is to deposit active layer films from dispersions of semiconductor nanocrystals (NCs) of Cs2SnI6. Quantum-confined NCs have the advantage of size-tunable band gaps, narrow emission lines, and higher absorption cross sections than the corresponding bulk materials.16−21 Lead halide perovskite nanocrystals have been synthesized and have shown highly tunable optical properties.22,23 The synthesis of CsSnX3 NCs24,25 has proven more challenging; there is only one report of Cs 2SnI6 nanoparticle synthesis, and it focused on the structural properties of Cs2SnI6 nanowires.26 Here, we use hot injection of cesium oleate (CsOA) into tin tetraiodide (SnI4) to synthesize quantum-confined Cs2SnI6 NCs, to demonstrate the dependence of their band gap on their size, and to measure their exciton lifetimes using transient absorption spectroscopy. Importantly, our nanocrystals can be Received: July 5, 2017 Revised: August 22, 2017 Published: August 27, 2017 7901
DOI: 10.1021/acs.chemmater.7b02803 Chem. Mater. 2017, 29, 7901−7907
Article
Chemistry of Materials conveniently deposited into high-quality thin films using a simple drop-cast procedure and have no surface ligands, so in fabricating an NC-based device, no postsynthetic removal of organic material is needed to optimize interparticle electronic coupling in the film (an important parameter in determining charge-carrier mobilities) or to minimize the contact resistance at the NC−electrode interface.19,27−31 For perovskite NCs, these postsynthetic procedures, which usually involve washing of the particles using nonsolvents and repeated centrifugation cycles, are especially problematic because they cause decomposition.32 Our synthetic procedure eliminates these steps and allows film deposition from as-synthesized NCs.
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RESULTS AND DISCUSSION Synthesis of Cs2SnI6 Nanocrystals. We synthesized Cs2SnI6 nanocrystals (NCs) using a method modified from the synthesis of CsPbX3 nanocrystals, on the basis of the reaction in eq 1.22 In 4CsOA + 2SnI4 → Cs 2SnI6 + 2CsI + Sn(OA)4
(1)
a typical synthesis, 117 mg (0.187 mmol) of SnI4 was added to 10 mL of octadecene (ODE) in a 25 mL three-neck roundbottom flask. We heated the mixture to 80 °C under vacuum (
