The Phosphine Oxide Route toward Lead Halide Perovskite Nanocrystals.

Guilherme Almeida,Daniela Maggioni,Ivan Infante,Nimai Mishra,Quinten A Akkerman,Olivia J Ashton,Henry J Snaith,Urko Petralanda,Liberato Manna,Luca Goldoni

doi:10.1021/jacs.8b08978

Abstract

We report an amine-free synthesis of lead halide perovskite (LHP) nanocrystals, using trioctylphosphine oxide (TOPO) instead of aliphatic amines, in combination with a protic acid (e.g., oleic acid). The overall synthesis scheme bears many similarities to the chemistry behind the preparation of LHP thin films and single crystals, in terms of ligand coordination to the chemical precursors. The acidity of the environment and hence the extent of protonation of the TOPO molecules tune the reactivity of the PbX2 precursor, regulating the size of the nanocrystals. On the other hand, TOPO molecules are virtually absent from the surface of our nanocrystals, which are simply passivated by one type of ligand (e.g., Cs-oleate). Furthermore, our studies reveal that Cs-oleate is dynamically bound to the surface of the nanocrystals and that an optimal surface coverage is critical for achieving high photoluminescence quantum yield. Our scheme delivers NCs with a controlled size and shape: only cubes are formed, with no contamination with platelets, regardless of the reaction conditions that were tested. We attribute such a shape homogeneity to the absence of primary aliphatic amines in our reaction environment, since these are known to promote the formation of nanocrystals with sheet/platelet morphologies or layered phases under certain reaction conditions. The TOPO route is particularly appealing with regard to synthesizing LHP nanocrystals for large-scale manufacturing, as the yield in terms of material produced is close to the theoretical limit: i.e., almost all precursors employed in the synthesis are converted into nanocrystals.

Highlights

We report an amine-free synthesis of lead halide perovskite (LHP) nanocrystals, using trioctylphosphine oxide (TOPO) instead of aliphatic amines, in combination with a protic acid
APbX3 (A = CH3NH3, Cs, etc.; X = Cl, Br, I), are a class of gap-tunable semiconductors (1.4 ≤ Eg ≤ 2.9 eV) that has been intensively investigated over the past decade and holds great promise for applications in solution-processed photonic and optoelectronic technologies.[1−5] As LHP thin film solar cells are attaining record power conversion efficiencies and the performance of light-emitting diodes based on nanocrystalline LHP films is approaching commercial values, considerable work has been devoted to improving and optimizing the fabrication and stability of materials[6−13] and devices.[2,14−18] Large-grain (>100 nm) LHP crystals and thin films are typically fabricated using polar solvents, such as dimethyl sulfoxide (DMSO), dimethylformamide (DMF), and γbutyrolactone (GBL), all of which have carbonyl or thionyl (RX O; X = C, S) groups.[1,6,10]
We study the synthesis of CsPbBr3 NCs using trioctylphosphine oxide ((C8H17)3P O, TOPO) and oleic acid (OA), and we avoid the use of aliphatic amines

Summary

■ INTRODUCTION

Under the usually strong acidic conditions at which our syntheses are carried out, all TOPO molecules are hydrogen-bonded; they are unable to stabilize any PbBr2 species that would most likely be extracted from the NCs in the case of Ostwald ripening This is different from the standard syntheses of nanocubes, in the presence of primary amines, in which strongly acidic environments are carefully avoided, forcing the synthesis to be conducted in an acid−base regime (low [acid], high [primary amine]) propitious to the extraction of PbBr2.12 On the other hand, when strongly acidic conditions are employed in the amine route, they lead to nanosheets/nanoplatelets with narrow thickness distributions, which might again suggest that Ostwald ripening does not occur in this case

■ CONCLUSION

■ ACKNOWLEDGMENTS

■ REFERENCES