Advances in Quantum-Confined Perovskite Nanocrystals for Optoelectronics

Transcription

1 RESEARCH NEWS Optoelectronics Advances in Quantum-Confined Perovskite Nanocrystals for Optoelectronics Lakshminarayana Polavarapu,* Bert Nickel, Jochen Feldmann, and Alexander S. Urban* Metal halide perovskites have emerged as a promising new class of layered semiconductor material for light-emitting and photovoltaic applications owing to their outstanding optical and optoelectronic properties. In nanocrystalline form, these layered perovskites exhibit extremely high photoluminescence quantum yields (PLQYs) and show quantum confinement effects analogous to conventional semiconductors when their dimensions are reduced to sizes comparable to their respective exciton Bohr radii. The reduction in size leads to strongly blueshifted photoluminescence and large exciton binding energies up to several hundreds of mev. This not only makes them interesting for optoelectronic devices, but also enables complex architectures based on cascaded energy transfer. Here, an overview of the current state-of-the-art of quantum confinement effects in perovskite nanocrystals is provided, with a focus on synthetic strategies and resulting optical properties, characterization methods, and emerging applications. 1. Introduction The remarkable optical and electronic properties of metal halide perovskites have generated a great excitement in various fields of science and engineering, especially in photovoltaics and light-emitting devices. [1 4] Although perovskites have been known for decades, it was the realization of a highly efficient solar cell in 2012 that brought them into the limelight, fueling solar cell research. [5] Although most of the initial studies were mainly focused on thin films of perovskite for solar cells, [6,7] recently there has been growing interest in highly luminescent nanocrystalline perovskites for many optoelectronic applications. [2,8 12] Perovskites generally possess an ABX 3 type crystal structure, where B is a Dr. L. Polavarapu, Prof. Dr. J. Feldmann, Dr. A. S. Urban Chair for Photonics and Optoelectronics Department of Physics and Center for Nanoscience (CeNS) Ludwig-Maximilians-Universität München Amalienstraße 54, Munich, Germany l.polavarapu@physik.uni-muenchen.de; urban@lmu.de Dr. B. Nickel Soft Condensed Matter Group Department of Physics and Center for Nanoscience (CeNS) Ludwig-Maximilians-Universität München Geschwister-Scholl-Platz 1, Munich, Germany Dr. L. Polavarapu, Dr. B. Nickel, Prof. Dr. J. Feldmann, Dr. A. S. Urban Nanosystems Initiative Munich (NIM) Schellingstraße 4, Munich, Germany DOI: /aenm bivalent cation (Pb 2+, Sn 2+ or Ge 2+ ) enveloped by an octahedron comprising six halide ions (X Cl, Br or I ) with A being a monovalent cation (e.g., CH 3 NH 3 +, NH 2 CH NH 2 +, or Cs + ) residing in between these corner sharing octahedra. [13,14] These components not only dictate the crystal structure, but also control their optical and electronic properties. [2,15] Exemplary is the optical bandgap, which can be tuned throughout the visible range by controlling the halide content. Halide perovskites can form two-dimensional (2D) or quasi-2d layered structures with thickness of one or a few octahedral layers. [16] The reduced thickness of these layers leads to quantum confinement effects, which changes their optical properties significantly in comparison with their three-dimensional (3D) counterparts, as studied since the 80 s on thin film perovskites. [17] Recent studies have shown that nanocrystalline perovskites exhibit enhanced PLQYs and offer tunable optical properties not only through their constituent ions but also through their size. [3,8 11,15,18,19] Quantum size effects, as illustrated in Figure 1a, have been extensively investigated in conventional semiconductor nanocrystals such as metal chalcogenides and utilized for a wide range of applications during the last two decades. [20] As the size of the nanocrystals approaches the exciton Bohr radius of the material, quantum confinement effects start to influence the excitonic wave function and the energetic states of the exciton, leading to blueshifted photoluminescence (Figure 1a). Precise control of colloidal semiconductor nanocrystal size has enabled the tuning of the PL emission over wide wavelength ranges. Similarly, perovskite nanocrystals of reduced dimensionality exhibit quantum confinement effects, as shown for the case of nanoplatelets in [8 10,21,22] Despite recent advances, an accurate understanding of thickness- and dimensionality-dependent optical properties of perovskite nanocrystals still proves elusive due to difficulties in the controlled syntheses and characterization of their dimensions. Additionally, while many recent publications present perovskite nanocrystals, most of these show negligible or no quantum confinement. [23,24] Here we will provide a concise overview of quantum confinement effects in perovskite nanocrystals, highlighting synthetic methods and resulting properties, characterization methods and finally applications for these enticing nanostructures (1 of 9)

2 Figure 1. a) Schematic illustration of the quantum confinement effect on the energy levels of semiconductor nanocrystals. The emission of semiconductor nanocrystals can be tuned across the visible range by controlling their size. Reproduced with permission. [25] Copyright 2011, Royal Society of Chemistry. b) Absorption and photoluminescence spectra of MAPbBr 3 nanoplatelets of different thickness prepared by varying OA to MA concentration during the synthesis. Reproduced with permission. [8] Copyright 2015, American Chemical Society. c) Photograph of colloidal solutions of MAPbI 3 nanoplatelets (under UV illumination) of different thicknesses (n = 1, 2, 3, 3, and ) prepared by ligand-assisted ultrasonication. Reproduced with permission. [9] Copyright 2016, John Wiley & Sons. d) Absorption and photoluminescence spectra of CsPbBr 3 nanoplatelets of different thicknesses (n = 1, 2, 3, 4, 5, and ) prepared by lowering the temperature during the hot injection synthesis method. Reproduced with permission. [26] Copyright 2015, American Chemical Society. e) TEM image of quantum-confined CsPbBr 3 nanoplatelets of lateral size 1 µm prepared by varying the ratio of shorter to longer ligands. Reproduced with permission. [27] Copyright 2016, American Chemical Society. f) TEM image of ultrathin CsPbBr 3 nanowires obtained using short chain acid ligands (octanoic acid or hexanoic acid). Reproduced with permission. [28] Copyright 2016, American Chemical Society. 2. State-of-the-Art in Quantum Confinement in Perovskite Nanocrystals 2.1. Ligand-Guided Synthesis The first report on the fabrication of colloidal perovskite nanocrystals came from the group of Pérez-Prieto, who used a mixture of long and short alkyl chain ammonium cations in the so-called reprecipitation technique to obtain methylammonium (CH 3 NH 3 +, or MA) lead bromide NCs (CH 3 NH 3 PbBr 3 ). [18] The purpose of the long alkyl ammonium cation in this bottomup synthetic approach is twofold. Firstly, due to their large size, they cannot be integrated into the perovskite crystal structure, in contrast to the MA cation, and so lead to a self-termination of the nanocrystal growth. Secondly, they passivate and protect the newly-formed crystal facets. [18] However, instead of leading to a self-termination in all three-dimensions, the resulting nanocrystals tend to be two-dimensional, forming so-called nanoplatelets, with thicknesses below 20 nm and lateral dimensions of several hundreds of nanometers, as we demonstrated by systematically varying the ratio of longer and shorter chain alkyl cations (octylammonium (OA) and MA) (Figure 1b). [8] The increase of OA to MA ratio leads to a decrease of nanoplatelet thickness that results in a strong blueshift in the photoluminescence of the MAPbBr 3 nanocrystals from green to blue color due to the quantum confinement effect. In the extreme case of using only OA, the formed nanoplatelets consist of a single monolayer of corner-sharing PbBr 6 octahedra, encased in an organic layer (Figure 1b). Accordingly, these nano platelets can be considered to comprise n = 1 perovskite layers (Figure 1c, 3a). Based on these results, we were the first to assign the observed PL maxima, obtained for different OA to MA ratios, to nanoplatelets of increasing thickness up to n = 5 layers. We were able to confirm this through theoretical calculations, with further corroboration coming through similar observations obtained by the Tisdale group. [21] The thickness of nanoplatelets obtained in the reprecipitation method can also be controlled by varying the ratio of oleylamine and oleic acid ligands used, reported by Levchuk et al. [29] In addition to the bottom-up reprecipitation method, perovskite nanocrystals can also be fabricated in a top-down fashion, as we recently demonstrated in the ligand-assisted transformation of bulk perovskites into nanoplatelets of various compositions and thicknesses by means of ultrasonication. [9] Figure 1c shows the photograph of solutions of colloidal MAPbI 3 nanoplatelets (n = 1,2,3, 3 and ) illuminated (2 of 9)

3 with UV light, which were obtained through a subsequent centrifugation of the solutions after the ultrasonication step. The photoluminescence of thicker MAPbI 3 platelets is around 760 nm, corresponding to bulk-like emission. For thinner nanoplatelets the emission maximum gradually blueshifts with decreasing thickness down to 539 nm for the n = 1 nanoplatelets (Figure 1c). Not only do these nanoplatelets show a strong blueshift, but also significantly increased exciton binding energies up to several hundred mev and increased radiative decay rates, which likely stem from the increased binding energies. [9,11] While the perovskite nanocrystals tend to be very stable in solution in presence of excess ligands, when diluted strongly, an interesting effect can occur. The solvent penetrates the perovskite crystal, fragmenting it in the process. The newly-formed surfaces are then passivated by previously unbound ligands. This can be used to fabricate perovskite nanoplatelets with thicknesses of only a few monolayers and with strongly reduced lateral dimensions of nm, as we found in a recent study. [11] In addition to colloidal synthesis, perovskite nanoplatelets of single- and few-unit-cell thickness can also be prepared directly on substrate using diluted precursor solutions in the presence of long chain alkylamine ligands, [22] similar to the preparation of layered perovskite thin films reported in 80 s and 90 s. [17,30] These 2D perovskites tend to form larger stacks as the long chain alkyl ammonium cations intercalate, leading to thicker crystals. Although they appear to possess bulk-like nanocrystal morphology they still exhibit quantum-confined optical properties regardless of the number of interdigitated layers in the platelet. [22,31] The optical properties of nanoplatelets can be significantly tuned not only by their thickness but also by varying the composition. While the bivalent cation (Pb) and halide (X) strongly affect the optical properties, the variation of the monovalent cation (MA) only leads to minimal changes. [9,15] However, the type of monovalent cation can greatly influence the stability and PLQY of perovskite nanocrystals. [15] It has been found that all-inorganic perovskite nanocrystals, which generally contain Cs + as the A-site cation, replacing the MA +, and are fabricated using the hot-injection method, exhibit higher stability as well as higher PLQYs than organic/inorganic hybrid perovskites. [3,10,32] The groups of Manna and Alivisatos extended this approach to 2D nanocrystals, simultaneously reporting CsPbX 3 nanoplatelets. [26,33] The formation of these was achieved by decreasing the reaction temperature in the hot injection method, leading to the formation of nanoplatelets with different thicknesses (n = 1, 2,3,4,5, and ) (Figure 1d). [26] Interestingly, most of the syntheses of Cs-based perovskite nanocrystals are based on a two-step process of fabrication of the bromide-based variant with a subsequent halide ion replacement step. [23,34 36] This is due to the fact that the direct synthesis of iodide and chloride perovskites, has proven elusive. One method that could permit this direct synthesis is a top-down ultrasonication approach, similar to the one reported for the hybrid perovskites. [10] We recently found that the morphology of CsPbX 3 nanocrystals could also be tuned from 3D to 2D by varying the ratio between Cs and Pb precursors in this approach. [9,10] Currently unique to all-inorganic nanocrystals, the lateral size of CsPbBr 3 nanoplatelets can also be tuned up to the micrometer range by varying the ratio of shorter (octanoic acid and octylamine) to longer ligands (oleic acid and oleylamine) (Figure 1e). [27] Although the growth mechanism of perovskite nanocrystals is not well understood, based on recent studies it is clear that the type and size of ligands used (alkyl amines and acids) play a crucial role in determining the final morphology of nanocrystals obtained. [8,23,26 29] For example, the replacement of a long chain acid (oleic acid) with short chain acid (octanoic acid or hexanoic acid) in the synthesis of nanoplatelets can lead to a reduction in dimension form 2D to 1D to obtain quantumconfined ultrathin nanowires (Figure 1f). The thickness of these nanowires could be tuned down to a single monolayer by varying the ratio of short chain to long chain amines. [28] In contrast to the 2D and 1D analogues, strongly-confined 0D perovskite nanocrystals or quantum dots have so far not been reported. Weekly-confined nanocrystals with dimensions of around 10 nm have been reported, but they generally exhibit more bulk-like optical properties. [24,37] These studies clearly suggest that, while an excellent shape of 2D and 1D quantum-confined perovskite nanocrystals has been achieved in a relatively short period of time, there is still more work to do to obtain a 0D variant and also acquire more control over the other dimensions and the overall performance of such nanocrystals. Nevertheless, these structures enable a detailed study of size- and dimensionalitydependent optical properties of perovskite nanocrystals Template-Assisted Fabrication As shown previously, control over size and shape of colloidal perovskite nanocrystals has proved complicated, especially for nanocrystals showing quantum-confinement. One approach that has recently been applied to perovskites is the use of solidstate matrices for templated growth of perovskite nanocrystals. An additional advantage of this approach could be an improved energy and charge transport due to the lack of surface ligands on the perovskite nanocrystals. This strategy has been explored previously for fabrication of metals, oxides or even II VI or III V semiconductor nanocrystals. However, it has proven ineffective for fluorescent semiconductor nanocrystals, as due to the lack of surface-passivation the remaining surface traps with energy levels in the bandgap present non-radiative decay channels, leading to a strong quenching of the fluorescence. For perovskites, this could be less of an issue, as surface defects generally do not produce strong quenching states within the bandgap, potentially retaining their high PLQYs. This approach was taken by three groups simultaneously, albeit with different foci. Malgras et al. prepared mesoporous silica powders with pore sizes varying between 7.1 nm and 3.3 nm and infused these with perovskite precursor solutions, drying them subsequently yielding the silica/perovskite powders. [38] Using MA and Br x I 3-x halide compositions, with 0 x 1, the powders they obtained showed distinct color variations in ambient light, both with respect to halide composition and pore size of the silica host (Figure 2a). The authors confirmed the existence of the perovskite using X-ray diffraction (XRD), and the filling of the silica pores with perovskite could be nicely imaged with high-resolution transmission electron microscopy (HR-TEM) (Figure 2b,c). These images, combined with powder XRD measurements show only a partial filling with nanocrystal sizes slightly smaller than the pores. The effect of quantum confinement can be seen clearly in UV-Vis spectra acquired from the powders (Figure 2d), which (3 of 9)

4 Figure 2. Template-assisted fabrication of nanocrystals exhibiting quantum confinement. a d): The authors used mesoporous silica with varying pore sizes infused with perovskite precursors to produce nanocrystal powders exhibiting quantum confinement. Reproduced with permission. [38] Copyright 2016, American Chemical Society. e h): The versatility of this method was shown in a similar approach in which hybrid and all-inorganic perovskite nanocrystal powders with varying halide composition were demonstrated. Reproduced with permission. [39] Copyright 2016, American Chemical Society. i l) Integration into a working device was achieved by Demchyshyn et al., who used nanoporous alumina as a template for perovskite nanocrystal growth to produce a working LED. Reproduced with permission. [40] Copyright 2016, arxiv. show a progressive blueshift and concomitant increase of the excitonic absorption peak with decreasing pore size. PLQYs for the confined perovskites were below 6%, and combined with PL decay times, the authors found that both the radiative and non-radiative decay rates increased with decreasing nanocrystallite size, with the latter decay channel clearly dominant. Dirin et al. applied a similar approach to the Malgras group, working under the premise that surface defects in perovskites hardly play a role, rendering the passivation with ligands superfluous. [39] Using mesoporous silica matrixes infused with highly concentrated precursor salt solutions, perovskites formed after a subsequent drying of the samples (Figure 2e), yielding brightly fluorescing powders with distinct colors depending on precursor material and pore size (Figure 2f,g). The authors not only varied the halide composition between iodide and bromide, but also showed mixed bromide/chloride samples and replaced the organic cation, MA for formamidinium (FA) and for Cs, extending the applicability to all-inorganic perovskites. For matrixes with pore sizes below 10 nm, an increasing blueshift of the PL peaks for decreasing pore size could be observed for all perovskites (Figure 2h). Through the size tuning and a compositional variation, the PL emission could be tuned from 470 to 775 nm with a gap between 540 and 600 nm. PLQYs were slightly higher, with bromide and chloride perovskites exhibiting roughly 20 25% and iodide perovskites around 10%. This is significantly below those of perovskite NCs in solution, showing significant room for improvement in this method. Nevertheless, the powders proved to be highly stable to illumination with UV- and laser-light and could be merged with polymers, forming brightly luminescing uniform films. The focus of the publication by Demchyshyn et al. was slightly different, as the group wanted to show LEDs with color tunability enabled through quantum-confined perovskite NCs. [40] Employing nanoporous alumina (npaao) matrixes, the perovskite nanocrystals were formed in a manner similar to the other two approaches (Figure 2i). Pores were only partially filled, as visualized with scanning TEM (STEM), with sphericallike perovskite NCs clearly visible, even in the rod-like pores of the npaao (Figure 2j,k). The authors were able to show blueshifts of up to 0.37 ev, 0.25 ev and 0.11 ev for the iodide, bromide, and chloride perovskites, respectively (Figure 2l). Despite the AAO being an electrical insulator, the perovskiteinfused npaao proved to be electrically conductive and was consequently integrated into a device structure enabling low voltage electroluminescent devices. The method of template-assisted fabrication provides an interesting way of obtaining perovskite nanocrystals exhibiting quantum confinement. The average size of the nanocrystallites is nicely tunable through the pore size of the matrix; and various perovskite compositions and matrix materials can be used. Nevertheless, the PLQYs remain relatively low, and issues concerning stability and integrability into functioning devices have not been fully resolved Characterization Methods The characterization of morphology (shape, size and thickness) and crystal structure of nanocrystals has always been crucial for understanding their morphology- and structure-dependent (4 of 9)

5 properties. Significant advances in scientific instrumentation over the years have made the characterization of nanomaterials relatively easy and accurate. However, the morphological characterization of perovskite nanocrystals, especially atomically thin nanoplatelets, is still challenging. [8,9,41,42] In this section, we provide a tutorial overview for an accurate characterization of perovskite nanocrystals through a combination of electron microscopy, atomic force microscopy (AFM) and XRD techniques. Transmission and scanning electron microscopy (SEM) are often successfully used for the characterization of nanomaterials. However, we and others have found that perovskite nanocrystals, especially nanoplatelets, are quite sensitive to electron beam irradiation (Figure 3c). [8,10] Until only very recently, the electron beam-induced degradation mechanism of perovskites was not well understood. However, as postulated by us and other groups and now confirmed by Manna and co-workers, the newly formed spherical particles turn out to be metallic lead (Pb), as determined by EDS and careful HRTEM measurements. [8,42] These spherical particles were previously often claimed to have been perovskite quantum dots, although the size of these stood in conflict with the observed optical properties of the nanoparticles, which tended to be bulklike. [18,24] As such, it would make sense to reinvestigate some of the synthetic procedures and reassess the obtained materials. Recent studies show that all-inorganic perovskite nanocrystals are more stable than hybrid perovskites to electron beam illumination and that thinner platelets degrade much faster than bulk-like nanocrystals. We find that high angle annular dark field scanning transmission electron microscopy (HAADF- STEM) at low magnification with low electron beam intensities provides a relatively accurate determination of the morphology of perovskites with good contrast, especially in the case of thin platelets. Additionally, a lowering of the substrate temperature reduces the migration of ions and atoms, retarding the degradation mechanisms. [42] Nevertheless, conventional TEM cannot provide nanoplatelet thickness, which is vital information needed to understand the quantum confinement effects. One method with which to obtain the thickness of single nanoplatelets lying flat on substrates is atomic force microscopy (AFM). However, as routine AFM measurements are generally performed under ambient conditions, for perovskites the measurements have to be performed immediately after sample preparation to avoid moisture-induced degradation of the nanocrystals. We and others have shown that the thickness of individual nanoplatelets can be determined by AFM (Figure 3b), however this becomes impossible in the case that the nanoplatelets form stacks. [9,11,22,27] In x-ray scattering experiments, the stacking of nanoplatelets gives rise to a new series of Bragg peaks, enabling the Figure 3. a) (top) Bragg series in the low angle x-ray signal of lead bromide nanoplatelets. The diamonds indicate Bragg peaks for stacks, while the dots indicate peaks that stem from the bulk unit cell. (bottom) Structure model of the nanoplatelet stacking in agreement with the x-ray data. Reproduced with permission. [15] Copyright 2016, American Chemical Society. b) AFM image of colloidal CsPbBr 3 nanoplatelets; scale bar: 1 µm. Reproduced with permission. [27] Copyright 2016, American Chemical Society. c) STEM image of MAPbBr 3 nanoplatelets. Electron beam-induced degradation leads to the formation of spherical, metallic Pb particles on the platelets. Reproduced with permission. [9] Copyright 2016, John Wiley & Sons (5 of 9)

6 determination of the thickness of the nanoplatelets. Such Bragg peaks were observed several times, e.g., by Weidman et al. (Figure 3a). [15] Here, the stacking periodicity d could be obtained through Bragg s law, and was found to be 1.7 nm and 2.3 nm for L 2 PbBr 4 and L 2 (FAPbBr 3 )PbBr 4, respectively, leading to a value of the thickness of a halide octahedral layer (FAPbBr 3 ) of 0.6 nm. Similar Bragg signals have been observed for stacked platelets of cesium lead perovskites. [33] Note that the determination of size and shape of dispersed nanocrystals, i.e., separate nanocrystals without stacking, is also possible in absence of Bragg signals in the so-called small angle region. [43] Several small-angle X-ray scattering (SAXS) methods exist which enable the determination of the diameter of dispersed perovskite nanocrystals with an average size of nm, e.g., evaluation of the pair distance distribution function, which is obtained through modified Fourier transformations of the SAXS data. [44] Most of the reported x-ray studies are however classical crystallography experiments with the goal of determining the details of crystal structure. Powder XRD enables a distinction between crystal structures typical for perovskites (cubic, tetragonal, and orthorhombic). The lattice parameters and structure can be rationalized from geometric considerations of sphere packing using the ionic radii, for this we refer to an excellent review of perovskite bulk properties. [45] Preferential orientation of the nanocrystals on surfaces can be determined through observation of the absence of certain peaks in the scattering spectra. [46] In view of quantum confinement effects, it is important to note that these experiments allow also the determination of particle sizes by means of the Scherrer equation, which relates the width of a Bragg diffraction peak (2θ) to the crystallite size L according to the relation Δ(2θ) = (K λ)/(l cos[θ]). For highly symmetric nanocrystals such as cubes or spheres, the Scherrer equation is well suited to determine crystal size, especially if this is small, i.e., comparable to the exciton Bohr radius. Indeed, peak broadening has been observed for 8.4 nm cubic perovskite nanocrystals or in perovskite nanocrystallites synthesized in the confined geometry of porous Si wafers. [33,40] For anisotropic particles, a more careful analysis is needed. For example, several groups have successfully synthesized nanoplatelets in the cubic phase. [8,21,33] Here, the crystal dimension is reduced in one dimension only (thickness), while the lateral size remains large compared to the exciton Bohr radius. In this case, the powder XRD pattern shows no broadening because the sharper peaks from the larger lateral directions dominate in the averaging of equivalent crystallographic directions. Therefore, cubic nanoplatelets can show resolution-limited Bragg peak linewidths; and it can happen that the Bragg peak width increases once platelets transform into thicker but laterally shorter cubes. [8,33] 2.4. Applications Lead halide perovskite nanocrystals have rapidly emerged as a unique class of materials with a promising future of low-cost and solid-state bright light-emitting diodes (LEDs) owing to their high PLQYs, tunable light emission and solution processability. The history of layered perovskite-based LEDs dates back to the 90 s when Saito and co-workers reported the fabrication of green-emitting electroluminescent devices based on a monolayer perovskite (C 6 H 5 C 2 H 4 NH 3 ) 2 PbI 4 ). [47] However, it did not gain much attention as the device only worked at liquid nitrogen temperature, and as significant progress was made in organic and inorganic semiconductor LEDs during the last two decades. The report on highly luminescent CH 3 NH 3 PbBr 3 perovskite dots in porous alumina matrix by Kojima et al. ignited the research on perovskites for both light-emission and solar cells. [48] Friend and co-workers demonstrated the fabrication of bright LEDs using 3D halide perovskites in the year [49] Since then a large number of research and review articles have been published on perovskite-based LEDs. [13,19,50] The efficiency of these LEDs has been significantly improved by replacing bulk 3D perovskites with corresponding nanocrystals that exhibit weak quantum confinement effects (emission wavelength close to 3D perovskites, but significantly enhanced PLQYs approaching 100%). [37,51,52] The fabrication of bright LEDs covering the entire visible spectrum using both organic/ inorganic hybrid and all-inorganic perovskite nanocrystals has been reported. [13,37,50 52] Studies show that all-inorganic perovskite nanocrystals offer higher stability and device efficiency than hybrid halide perovskite nanocrystals. [37,51] However, two things currently limit the applicability of perovskites and the realization of blue-emitting and all-perovskite white light LEDs. Firstly, due to the low PLQY of chloride-based perovskite nanocrystals (all-inorganic and hybrid) a true blue perovskite emitter is still lacking. Secondly, spontaneous halide ion exchange among perovskite nanocrystals comprising different halide compositions hinders energy gradient structures and makes device fabrication more complicated. Quantum-confined perovskite nanocrystals offer solutions for both problems. As discussed before, blue emitting bromide-based perovskites can be prepared with higher PLQYs and stronger exciton binding energies than the bulk-like chloride variants, enabling more efficient blue as well as white light-emitting LEDs. [11,53 55] For instance, Kumar et al. have demonstrated the fabrication of blue LEDs using atomically thin halide perovskite nanoplatelets; and the electroluminescence can be tuned across the blue-green region by using hybrid perovskite nanoplatelets of different thickness (Figure 4a d). [53] In this work, nanoplatelets were dispersed in a low dielectric constant (low-k) and wide bandgap organic semiconducting matrix to increase the exciton binding energy through the dielectric confinement effect (Figure 4b). Moreover, the organic matrix facilitates the radiative recombination in nanoplatelets through Förster resonance energy transfer and thus enhances the efficiency of the LED. The same energy transfer mechanism can be used to funnel energy from nanoplatelets with large bandgaps to those with smaller ones enhancing the efficiency of photoluminescence (Figure 4e,f). Such cascaded energy transfer processes have been used to increase the photoluminescence of low band gap regions in structures comprising layer by layer assembled conventional semiconductor quantum dots. [57] Very recently, Yuan et al. applied this approach for the fabrication of highly-efficient near-infrared LEDs with an external quantum efficiency of 8.8% by using a mixture of quantum-confined 2D perovskite platelets of different thickness (Figure 4e,f). [56] In addition, Manna and co-workers have shown the possibility of white LEDs that (6 of 9)

7 Figure 4. a) Molecular model of a two-dimensional CH 3 NH 3 PbBr 3 perovskite (n = 1, defined as one unit cell in this case) protected by organic ligands (carbon: brown, hydrogen: pink, bromide: green, and nitrogen: sky blue color). b) LED device architecture, materials used and corresponding band diagram. c) Electroluminescence spectra of LED devices fabricated using colloidal solutions of CH 3 NH 3 PbBr 3 perovskite nanoplatelets of different thicknesses (n = 7 10, 5, 3, and 1). d) Plot showing current density vs applied voltage and a photograph of a blue LED. Figures a d) reproduced with permission. [53] Copyright 2016, American Chemical Society. e) Perovskite structures with different thicknesses (n = number of layers), showing the evolution of 2D (n = 1) to 3D (n = ). f) Schematic illustration of carrier transfer in n = 3 and n = 5 perovskites in a thin film made of mixed (n = 1 to ) structures and a scheme showing the energy channelling to a lowest band gap emitter. Reproduced with permission. [56] Copyright 2016, Nature Publishing Group. covers a broad spectrum of nm using the combination of 3D and 2D perovskites using iodide and bromide. [54] They have shown that an X-ray irradiation step applied between successive coatings of perovskite nanocrystal layers can be used to prevent halide ion exchange. Besides LEDs, there is an ever increasing interest in the fabrication of efficient and stable solar cells using 2D perovskites in the form of thin films. [16,58] Although these 2D perovskites in the form of thin films exhibit similar properties to nanoplatelets, they are generally prepared directly on substrate using long chain ligands. Cao et al. showed the fabrication of thin films of 2D perovskites on substrate with different numbers of layers in the perovskite structure. [16] While the initial power conversion efficiency of 4.02% was rather low, it has recently been improved to a very respectable 12.52%. [16,58] More importantly, the stability of solar cells has been greatly improved with 2D perovskites through organic layer protection from the environment. Despite multiple reports of amplified spontaneous emission (ASE) and lasing in individual perovskite nanowires and films comprising perovskite crystals, so far there has not been a report showing similar features in perovskite nanocrystals exhibiting quantum-confinement. This is potentially due to the significantly lower PLQYs of such structures as well as difficulties in creating smooth films comprising such nanocrystals. Future work should focus on creating structures using the quantum confinement effect to efficiently guide and concentrate light to enable lasing. 3. Summary and Outlook In summary, quantum size effects offer an inherent way to tune the optical and electrical properties of perovskite nanocrystals, analogous to conventional semiconductor nanocrystals. Recent (7 of 9)

8 advances in the fabrication of colloidal perovskite nanoplatelets have enabled a strong tuning of the photoluminescence emission by controlling the nanoplatelet thickness. These have also led to a better understanding of their quantum confinement effects and consequently their thickness-dependent optical properties. Porous template-assisted approaches facilitate the thickness control of nanocrystals by their pore size, while also enhancing the stability of perovskites. However, the crystallite sizes still vary over a certain range and PLQYs are significantly lower than for ligand-assisted syntheses. In spite of the advances in synthesis, the characterization of nanoplatelet dimensions is still challenging due to electron beam-induced degradation, moisture sensitivity, and the spontaneous formation of stacks on substrates. Small-angle X-ray diffraction techniques and careful control over the measurement parameters in electron microscopy can enable a precise determination of morphology, size and composition. The radiative decay rate of perovskite nanoplatelets increases with decreasing nanoplatelet thickness due to an increase of the exciton binding energy, making them promising for LEDs. In future, synthetic strategies will need to focus on reducing the non-radiative decay rates, maximizing the potential of these nanocrystals. These could then serve as an alternative, more efficient source for all-perovskite white LEDs, as the chloride-containing perovskites yield only poor PLQYs. In addition, mixtures of quantum-confined perovskites of identical halide compositions, but different thicknesses can boost the efficiency of LEDs and solar cells through energy cascades; and 2D thin films have been shown to have greater stability than their 3D counterparts. More work will then need to focus on the organic ligands to find the optimum between stability and charge/energy transfer capabilities. In summary, we believe that perovskite optoelectronics have a bright future, with the demonstration of impressive LEDs, lasing and solar cells based on quantum-confined nanocrystals only a question of time. Acknowledgements This work is supported by the Bavarian State Ministry of Science, Research and Arts through the grant Solar Technologies go Hybrid (SolTech). L.P. acknowledges the financial support from the Alexander von Humboldt-Stiftung. The authors declare no conflict of interest. Keywords nanocrystals, perovskite, photoluminescence, quantum confinement Received: January 27, 2017 Revised: February 23, 2017 Published online: April 10, 2017 [1] M. Liu, M. B. Johnston, H. J. Snaith, Nature 2013, 501, 395. [2] J. S. Manser, J. A. Christians, P. V. Kamat, Chem. Rev. 2016, 116, [3] L. Protesescu, S. Yakunin, M. I. Bodnarchuk, F. Krieg, R. Caputo, C. 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