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White-Light Emission from Layered Halide Perovskites Matthew D Article Cite This: Acc. Chem. Res. 2018, 51, 619−627 pubs.acs.org/accounts White-Light Emission from Layered Halide Perovskites Matthew D. Smith and Hemamala I. Karunadasa* Department of Chemistry, Stanford University, Stanford, California 94305, United States CONSPECTUS: With nearly 20% of global electricity consumed by lighting, more efficient illumination sources can enable massive energy savings. However, effectively creating the high-quality white light required for indoor illumination remains a challenge. To accurately represent color, the illumination source must provide photons with all the energies visible to our eye. Such a broad emission is difficult to achieve from a single material. In commercial white-light sources, one or more light-emitting diodes, coated by one or more phosphors, yield a combined emission that appears white. However, combining emitters leads to changes in the emission color over time due to the unequal degradation rates of the emitters and efficiency losses due to overlapping absorption and emission energies of the different components. A single material that emits broadband white light (a continuous emission spanning 400−700 nm) would obviate these problems. In 2014, we described broadband white-light emission upon near-UV excitation from three new layered perovskites. To date, nine white-light-emitting perovskites have been reported by us and others, making this a burgeoning field of study. This Account outlines our work on understanding how a bulk material, with no obvious emissive sites, can emit every color of the visible spectrum. Although the initial discoveries were fortuitous, our understanding of the emission mechanism and identification of structural parameters that correlate with the broad emission have now positioned us to design white-light emitters. Layered hybrid halide perovskites feature anionic layers of corner-sharing metal-halide octahedra partitioned by organic cations. The narrow, room-temperature photoluminescence of lead-halide perovskites has been studied for several decades, and attributed to the radiative recombination of free excitons (excited electron−hole pairs). We proposed that the broad white emission we observed primarily stems from exciton self-trapping. Here, the exciton couples strongly to the lattice, creating transient elastic lattice distortions that can be viewed as “excited-state defects”. These deformations stabilize the exciton affording a broad emission with a large Stokes shift. Although material defects very likely contribute to the emission width, our mechanistic studies suggest that the emission mostly arises from the bulk material. Ultrafast spectroscopic measurements support self-trapping, with new, transient, electronic states appearing upon photoexcitation. Importantly, the broad emission appears common to layered Pb−Br and Pb−Cl perovskites, albeit with a strong temperature dependence. Although the emission is attributed to light-induced defects, it still reflects changes in the crystal structure. We find that greater out-of-plane octahedral tilting increases the propensity for the broad emission, enabling synthetic control over the broad emission. Many of these perovskites have color rendering abilities that exceed commercial requirements and mixing halides affords both “warm” and “cold” white light. The most efficient white-light-emitting perovskite has a quantum efficiency of 9%. Improving this value will make these phosphors attractive for solid-state lighting, particularly as large-area coatings that can be deposited inexpensively. The emission mechanism can also be extended to other low-dimensional systems. We hope this Account aids in expanding the phase space of white-light emitters and controlling their exciton dynamics by the synthetic, spectroscopic, theoretical, and engineering communities. 1. INTRODUCTION (PL) use extrinsic dopants1 or surface sites.2 In contrast, intrinsic broadband emitters, or single-phase bulk materials that emit PL 1.1. Generating White Light across the visible spectrum, are rare. Such materials are desirable A fortuitous combination of blackbody radiation, absorption, and for the next generation of solid-state phosphors. Single-source scattering events occurring over astronomical scales yields the pure white light that covers our planet’s surface. Efficiently gener- ating artificial white light indoors, however, remains a challenge. Received: September 1, 2017 Typical inorganic phosphors that emit broad photoluminescence Published: February 20, 2018 © 2018 American Chemical Society 619 DOI: 10.1021/acs.accounts.7b00433 Acc. Chem. Res. 2018, 51, 619−627 Accounts of Chemical Research Article emitters obviate problems associated with current methods of cations reduce the dimensionality of the inorganic sublattice to ff ff I II II II 14 producing white light by mixing multiple di erent phosphors or a ord 2D, or layered, perovskites (A 2B X4 or A B X4). Here, light-emitting diodes (LEDs).1 These include unequal lifetimes the structure of the inorganic layers can be derived from the 3D of different emitters resulting in white light that changes color cubic lattice by slicing along specific crystallographic planes. For over time or self-absorption effects caused by overlapping absorp- example, (001) perovskites contain flat sheets of metal-halide tion and emission energies of the different components.1,3 octahedra (Figure 2B,C), whereas (110) perovskites feature cor- In 2014, we observed intrinsic, broadband white-light emission rugated inorganic sheets (Figure 2A).14 from the two-dimensional (2D) lead-halide perovskites (N- 1 1.3. Halide Perovskites: Optical Properties MEDA)PbBr4 and (EDBE)PbX4 (N-MEDA = N -methylethane- ′ The 3D and 2D lead-halide perovskites are direct-bandgap 1,2-diammonium; EDBE = 2,2 -(ethylenedioxy)bis(ethyl- 14 ammonium); X = Cl and Br; Figures 1 and 2A,B).4,5 Upon semiconductors with similar band-edge orbital compositions. The valence-band maximum is composed primarily of halide p-orbital and lead s-orbital character, while the conduction-band minimum has mostly lead p-orbital character.14 However, the photophysical properties of the 2D and 3D congeners are very different. In 3D perovskites, the polarizable lattice screens the electrostatic attraction between excited electrons and holes. This leads to a small exciton binding energy (Eb, magnitude of the attraction between the electron and hole). In Pb−I perovskites 15 Eb < kBT at room temperature, resulting in free carriers. In contrast, 2D perovskites are excitonic semiconductors16,17 16 with bandgap energies (Eg) typically above 2.5 eV and Eb values 16,18 exceeding 300 meV. These Eb values are comparable to those 19 Figure 1. X-ray crystal structure of the (110) perovskite (EDBE)PbBr4 of organic semiconductors. This large Eb in 2D perovskites (EDBE = 2,2′-(ethylenedioxy)bis(ethylammonium)), and its emission, results from two cooperative effects: quantum and dielectric which spans the entire visible spectrum. Inset: Photographs of an confinement.17 The inorganic layers confine the exciton’s wave (EDBE)PbBr4 crystal. Reprinted with permission from ref 5. Copyright function to two dimensions. This quantum confinement results 2014 American Chemical Society. 20 in a 4-fold enhancement of Eb over a comparable 3D material. The low dielectric constant of the organic layers poorly screens near-ultraviolet (UV) photoexcitation, these crystalline, wide- the attraction between electrons and holes in the inorganic layers, bandgap semiconductors emit photons with energies distributed thereby enhancing E further through dielectric confinement.21,22 across the visible spectrum. We ascribed this broad Stokes-shifted b 4−7 Increasing the dielectric constant of the organic layers can sig- emission to the radiative decay of self-trapped excitons (STEs). fi 23,24 − ni cantly decrease the Eb. Despite their high Eb and small Here, excitons (excited electron hole pairs) cause large elastic Bohr radius,16 excitons in 2D perovskites are classified as Wannier distortions of the deformable inorganic lattices, which can be excitons,25 as a series of excitonic resonances following a Wannier viewed as “excited-state defects”. To date, nine white-light- − progression have been observed in optical absorption experiments. emitting perovskites have been reported4,5,7 12 and several 6−8,13 Absorption. At room temperature, thermal broadening of studies have probed the emission mechanism. Herein, we both the band edge and excitonic absorption features in 2D lead- provide our current understanding of white-light emission from halide perovskites precludes accurate determination of Eb and Eg layered lead-halide perovskites. Although the emission appears to using optical measurements. However, low-temperature optical stem from new states generated upon photoexcitation, intrigu- absorption spectra of 2D lead-halide perovskites reveal both ingly, the emission still varies systematically with parameters E and E (Figure 3A,B). The E onset manifests as a step-like derived from ground-state crystal structures. These relationships b g g feature due to the 2D density of states, and Eb can then be provide synthetic design rules for obtaining white light from this − 26,27 7 estimated as Eg (exciton peak energy). highly tunable family of hybrid phosphors. Emission. The emission from 2D lead-halide perovskites has 1.2. Halide Perovskites: Structure ’ been studied for several decades. The perovskites high Eb and I II 26 ff Three-dimensional (3D) halide perovskites (A B X3; X = halide) large oscillator strength typically
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