Lead Halide Perovskite Nanocrystals in the Research Spotlight: Stability and Defect Tolerance

Abstract

This Perspective outlines basic structural and optical properties of lead halide perovskite colloidal nanocrystals, highlighting differences and similarities between them and conventional II-VI and III-V semiconductor quantum dots. A detailed insight into two important issues inherent to lead halide perovskite nanocrystals then follows, namely, the advantages of defect tolerance and the necessity to improve their stability in environmental conditions. The defect tolerance of lead halide perovskites offers an impetus to search for similar attributes in other related heavy metal-free compounds. We discuss the origins of the significantly blue-shifted emission from CsPbBr₃ nanocrystals and the synthetic strategies toward fabrication of stable perovskite nanocrystal materials with emission in the red and infrared parts of the optical spectrum, which are related to fabrication of mixed cation compounds guided by Goldschmidt tolerance factor considerations. We conclude with the view on perspectives of use of the colloidal perovskite nanocrystals for applications in backlighting of liquid-crystal TV displays.

Figure 1

Colloidal synthesis of CsPbX₃ NCs and (a) photographs of resulting colloidal solutions of CsPbX₃ NCs; (b) HAADF-STEM image of a single CsPbBr₃ NC; (c) idealized perovskite crystal structure with 3D interconnection of PbX₆ octahedra. In reality, all APbX₃ perovskites not only adopt the cubic polymorph but are commonly observed in lattices with lower symmetry, for example, orthorhombic and tetragonal, due to distortions along one or more Pb–X–Pb axes. Adapted from refs (35) and (112).

Figure 2

Colloidal synthesis of MAPbBr₃ NCs and (a) schematic diagram of the LARP reaction system; (b) colloidal solutions of MAPbBr₃ NCs obtained by using different temperatures of toluene solvent; (c) HRTEM image of a single MAPbBr₃ NC. Adapted from ref (40). Copyright 2015, Wiley Online Library.

Figure 3

Illustration of the “perovskite red wall”: 3D phases of CsPbI₃ (orthorhombic) and FAPbI₃ (pseudocubic) materials, with bandgap energies at ca. 710 and 840 nm, respectively, easily convert into wide-bandgap 1D polymorphs. The transition is typically observed as a change of the color from dark-red/black to yellow.

Figure 4

(a) Schematics of two limiting cases of a band-structure in semiconductors: defect-intolerant (conventional, left) and ideal hypothetical defect-tolerant (right). Bonding and antibonding orbitals are denoted as σ and σ*, respectively. (b) Simplified depiction of the bonding in APbI₃ (adapted from ref (96)). The VB exhibits the desired antibonding character at its maximum, as in the ideal defect-tolerant case in (a). Copyright 2015, Materials Research Society.

Figure 5

Energy levels associated with the defect states corresponding to neutral and charged vacancies (V_Pb, V_I, V_MA), neutral and charged interstitials (Pb_i, I_i, MA_i), and neutral and charged states associated with antisites (Pb_I and I_Pb) in MAPbI₃. Adapted from ref (91). Copyright 2015, American Chemical Society.

Figure 6

(a) Molecular structure of APTES and NH₂-POSS and photograph of MAPbBr₃ NCs (different capping ligands) dispersed in ethanol under UV light. Adapted from ref (97). (b) PL of MAPbBr₃ NCs P_AD–CB (in black) and P_AD (in green) dispersed in toluene and in contact with water as a function of the irradiation time. (Right side) Images of colloidal dispersions immediately after addition (left) of water and 120 min afterward (right); the inset shows the molecular structures of cucurbit[7]uril (CB) and 2-adamantylammonium (AD) ligands. Adapted from ref (44). Copyright 2016 and 2016, Wiley Online Library. (c) Crystal structures of the CsPbI₃ (i) cubic and (ii) orthorhombic perovskite. PL spectra for CsPbI₃–OA (iii) and CsPbI₃–TMPPA (iv). Insets of (iii and iv): Solutions of the respective washed NCs under UV light at different times following synthesis. Adapted from ref (98). (d) Schematics showing the core–shell type of octylammonium lead bromide nanomaterials over MAPbBr₃ NPs. Adapted from ref (99). Copyright 2016 and 2016, The Royal Society of Chemistry. (e) Optical stability of CsPbBr_3–xI_x and CsPbBr_3–xI_x/ZnS. Adapted from ref (100). Copyright 2017, Wiley Online Library. (f) PL intensity as a function of time after storing the self-passivation layer formation on mixed-halide perovskite NCs after acetone etching CsPb(Br_xI_1–x)₃ NCs in cyclohexane under ambient conditions. Adapted from ref (101). Copyright 2017, The Royal Society of Chemistry.

Figure 7

(a) Evaluation of photostability of MAPbBr₃(MAPB)-QD and MAPB-QDs/SiO₂ powders in relative humidities (RHs) of 60 and 80% under 470 nm LED light irradiation. Optical images of the as-prepared colloidal MAPB-QD solutions with TEOS and TMOS and storage after 4 days. Adapted from ref (102). Copyright 2016, American Chemical Society. (b) Thiol-functionalized POSS structure: a schematic diagram illustrating the POSS coating process to obtain perovskite NC powders. Adapted from ref (103). Copyright 2016, The Royal Society of Chemistry. (c) Photostability test of MP-CsPbBr₃ and CsPbBr₃. Adapted from ref (104). Copyright 2016, Wiley Online Library. (d) Schematic of the silicone resin (SR) coating process for the preparation of SR/PVP-CsPbX₃. Thermal stability test of SR/PVP-CsPbBr₃ and CsPbBr₃ QDs. Photostability test of SR/PVP-CsPbBr₃ and CsPbBr₃ QDs under continuous UV light irradiation. Adapted from ref (105). Copyright 2017, The Royal Society of Chemistry.

Figure 8

(a) Normalized integrals of the emission peaks between 460 and 600 nm of CsPbBr₃ with/without PMA over 12 h of constant irradiation. The inset shows the structure of PMA. Adapted from ref (107). (b) Stability of CsPbBr₃ perovskite colloid and film. Perovskite colloids mixed with different solvents (ethanol, IPA). Time evolution of fluorescence intensity after adding 1:1 v:v solvents into perovskite colloids. Time evolution of fluorescence intensity after immersing perovskite/polymer hybrid films in different solvents. Adapted from ref (108). (c) (i) Relative PL QYs of as-synthesized CsPbBr₃ NCs in water after different times. Inset pictures show as-synthesized samples before and after 60 min of water soaking under a UV lamp. (ii) Relative and absolute PL QYs of 150 μm thick nanocube polymer composite films after over 4 months of water-soaking. Thin 3 μm composite films also showed enhanced water stability (see inset pictures). Adapted from ref (109). Copyright 2016, 2017, and 2016, respectively, American Chemical Society.