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 a ord strong PL even at room form an anionic sublattice of corner-sharing B−X octahedra, with temperature.28 Here, radiative recombination of free exci- small A-site cations residing in the lattice cavities. Larger A-site tons (FEs) leads to narrow green or blue PL with a fwhm of

Figure 2. Crystal structures of selected white-light-emitting perovskites (A) N-MEDA = N1-methylethane-1,2-diammonium,4 (B) EDBE = 2, 2′-(ethylenedioxy)bis(ethylammonium),5 (C) CyA = cyclohexylammonium,8 and (D) α-DMEN = N1,N1-dimethylethane-1,2-diammonium.9

620 DOI: 10.1021/acs.accounts.7b00433 Acc. Chem. Res. 2018, 51, 619−627 Accounts of Chemical Research Article

Figure 3. (A) Optical absorption spectrum of a 2D Pb−I perovskite24 (Eg = bandgap energy; Eb = exciton binding energy). (B) Energy-level diagram of the typical excitonic and band-to-band transitions of 2D lead- halide perovskites (FC = free carriers; FE = free excitons; GS = ground Figure 4. (A) Schematic of large polaron formation upon adding an state). Colored arrows indicate absorption or photoluminescence, and electron to an ionic lattice, featuring long-range distortions of the lattice. the black arrow represents nonradiative relaxation. (B) Fragment of the PbBr2 crystal structure. Green and brown spheres − represent Pb and Br atoms, respectively. (C) Schematic of the Br2 3+ ca. 100 meV and a minimal Stokes shift (ca. 10 meV).29 Several (orange) and Pb2 (turquoise) dimers formed by the self-trapped hole reports have characterized the excited-state landscape of these and self-trapped electron, respectively, following photoexcitation. materials using low-temperature or field-dependent spectros- copies.18,30,31 These studies were primarily concerned with the spectral region of the narrow (primarily FE) PL. A weak, broad, red PL in 3D Pb−Cl perovskites, occurring below 175 K, has been attributed to STEs.32 The broad yellow PL from the (110) perovskite (1-(3-ammoniopropyl)-1H-imidazol-3-ium)PbBr4 was attributed to the π−π* transition of the imidazolium cations.33 However, given its similarities to the white-light-emitting perov- skites described here, we believe that the emission arises from the inorganic lattice.4 We observed broad emission from all (110) Pb−Br perov- skites we studied and a combination of narrow and broad emis- sion at low temperatures in a series of (001) Pb−Br perovskites, allowing us to postulate that self-trapping is common to these materials. The similarity of the broad emission’s properties in (110) and (001) perovskites reinforces the expected common- ality of photophysical processes in 2D Pb−Br (and by analogy Figure 5. (A) Self-trapping, (B) trapping at permanent defects, and Pb−Cl) perovskites. (C) self-trapping influenced by permanent defects represented by a ball interacting with a rubber sheet. 2. EXCITON SELF-TRAPPING Furthermore, if the ball contacts the sheet near an indentation, 2.1. Self-Trapping in a Deformable Lattice it will sink to a different depth compared to the case for an Charge carriers can couple to a soft lattice, inducing elastic indentation-free sheet. This is extrinsic self-trapping, where self- structural distortions that lower the system’s energy. This relaxed trapping is influenced by the local heterogeneity of the lattice charge carrier, or polaron, is classified according to the dominant (Figure 5C). For example, extrinsic self-trapping occurs in 34 37 interaction governing its potential well depth and size. Large TlCl0.98Br0.02, where the bromides act as defects. Distinguishing polarons show primarily a long-range Coulombic interaction between intrinsic and extrinsic self-trapping using optical spectra between the carrier and the lattice and can be delocalized across can be difficult, and intrinsic self-trapping can precede the for- several unit cells (Figure 4A).35,36 Changes to the local bonding mation of permanent defects.35 For example, STEs in NaCl relax owing to the localized carrier’s charge provides shorter-range nonradiatively to generate color centers (an anion vacancy filled lattice distortions, resulting in the small polaron.34 Although not by an electron) and halogen interstitials within 3 ps.35,38 charge carriers, STEs are similar to small polarons, featuring small The structures of the self-trapped electron and hole have been 35 exciton Bohr radii and large associated lattice deformations. elucidated in PbBr2 through single-crystal electron spin reso- Intrinsic self-trapping causes transient lattice distortions and nance experiments.39 Owing to the similarity in band-edge com- does not require permanent material defects. This can be exem- position between lead(II) halides and lead(II)-halide perov- plified by a hard ball (electron/hole/exciton) dropping on a skites,14,40 this is an instructive example of STE formation.5 The pliable rubber sheet (a deformable lattice). The sheet distorts self-trapped electron localizes on a Pb2+, which combines with only due to the presence of the ball, which sinks into a potential another Pb2+ to form a Pb 3+ dimer. Similarly, the hole localizes − 2 − − minimum it created (Figure 5A). Removing the ball returns the on a Br , which dimerizes with an adjacent Br to form Br2 sheet to its original state. This differs from the ball dropping into (Figure 4B,C). Capturing the electronic excited states of large an indentation in the sheet (a permanent defect), where the systems such as 2D perovskites is computationally difficult. distortion existed prior to contact with the ball (Figure 5B). Despite progress in recent work13,41 an accurate theoretical

621 DOI: 10.1021/acs.accounts.7b00433 Acc. Chem. Res. 2018, 51, 619−627 Accounts of Chemical Research Article description of the self-trapped exciton in these materials remains PL. Upon near-UV photoexcitation, they emit across the entire an open challenge. visible spectrum, affording broadband white light. This was first 4 2.2. Luminescence from Self-Trapped Carriers seen in (N-MEDA)PbBr4 (Figure 7B). Here, the broad PL consists of a wide emission with a maximum at ca. 560 nm, and a Exciton self-trapping occurs in a wide variety of materials, including 35 35 42 higher-energy shoulder at ca. 420 nm attributed to FEs. condensed rare gases, alkali halides, lead(II) halides, and 43 Halide substitution in (N-MEDA)PbBr − Cl tunes the color organic semiconductors such as pyrene. Unlike FE PL, STE 4 x x of the broad PL, affording both “cold” and “warm” white light. luminescence is typically broad, and significantly Stokes shifted 35 Changing the halide speciation primarily alters the composition from the FE absorption. Here, the distortion of the self-trapped of the valence-band maximum. The more electronegative halides state with respect to the ground state (given by the Huang−Rhys 29,45 44 increase E , as reflected in the blueshift of the optical absorp- parameter, S) broadens the emission. Both S and the stabi- g tion features in (N-MEDA)PbBr4−xClx with increasing x. Tuning lization of the self-trapped state with respect to the untrapped ’ state (self-trapping depth) contributes to the Stokes shift (Figure 6). x improves the emission s color-rendering index (CRI) from 82 to 85. The CRI value is a quantitative measure of a light source’s similarity to a blackbody radiation light source, or the ability of the light source to accurately reflect the true color of an illuminated sample. Commercial phosphors for indoor illumination require CRI values greater than 80.3 Partial chloride substitution also moves the chromaticity coordinates (CIE),1 or the color of the broad emis- sion, closer to pure white light, approximating sunlight (Figure 8).

Figure 6. Nuclear coordinate diagram for exciton self-trapping (blue) and detrapping (red) in a 2D perovskite (GS = ground state, FE = free exciton state, STE = self-trapped exciton state, Ea,trap = activation energy − for self-trapping, Ea,detrap = activation energy for detrapping, S = Huang Rhys parameter). Pink and orange arrows depict FE and STE photoluminescence, respectively. Adapted from ref 7 with permission from the Royal Society of Chemistry. Figure 8. Chromaticity coordinate diagram1 for some white-light emitters. Inset: Solar spectrum (orange) with the visible region shaded in yellow and the emission spectra of (N-MEDA)PbBr4 (red) and 3. REALIZING BROAD EMISSION (N-MEDA)PbBr3.5Cl0.5 (black). Adapted with permission from ref 4. Copyright 2014 American Chemical Society. 3.1. (110) Perovskites

The vast majority of 2D lead-halide perovskites have (001) layers Although (N-MEDA)PbBr4−xClx exhibits a low photo- and strong, sharp FE absorption and PL features at room luminescence quantum efficiency (PLQE) of 0.5−1.5%,4 our 14,29 temperature (Figure 7A). However, despite the similarity of second (110) white-light-emitting perovskite (EDBE)PbBr4 the absorption spectra to those of (001) perovskites, the (110) showed a substantially improved PLQE of 9% for warm white Pb−Br perovskites we have studied show dramatically different light with a CRI of 84 (Table 1).5 Kanatzidis and co-workers α recently reported white-light emission from -(DMEN)PbBr4 (DMEN = N1,N1-dimethylethane-1,2-diammonium), where the corrugation in the inorganic sheets occurs over longer lengths (Figure 2D). This material emits cold white light with a CRI of 73.9 3.2. (001) Perovskites

The (001) perovskite (EDBE)PbCl4 displays very similar optical 5 properties to the (110) perovskite (EDBE)PbBr4. UV excitation at 310 nm results in white PL that peaks at ca. 540 nm, with a fwhm of 208 nm and a high-energy shoulder at 358 nm. Although fi the PLQE of (EDBE)PbCl4 is only 2%, this was the rst example ff Figure 7. Absorption and photoluminescence (PL) spectra for (A) of a (001) white-light-emitting perovskite, a ording cold white light with a CRI of 81.5 the (001) perovskite (N-MPDA)PbBr4 and (B) the (110) perovskite (N-MEDA)PbBr . Insets: photos of perovskite powders under near-UV Boukheddaden and co-workers subsequently reported white- 4 8 photoexcitation. Adapted with permission from refs 4 and 6. Copyright light emission from (C6H11NH3)2PbBr4 (Figure 2C). They also 2014 and 2016, respectively American Chemical Society. found that the broad PL could be quenched by increasing iodide

622 DOI: 10.1021/acs.accounts.7b00433 Acc. Chem. Res. 2018, 51, 619−627 Accounts of Chemical Research Article

Table 1. Reported Room-Temperature Photoluminescence powders indicated that the emission did not arise from particle Quantum Efficiencies (PLQE), Color Rendering Indices edge/surface sites.5 Furthermore, the integrated intensity of the (CRI),1 and CIE Chromaticity Coordinates1 for White-Light- broad PL depended linearly on excitation intensity over 3 orders Emitting Perovskites of magnitude.5,6 Excitonic emission exhibits linear or slightly super- linear behavior.47 Emission involving defects, in contrast, should compd ref PLQE (%) CRI CIE (x, y) show sublinear behavior if the limited number of defect states 47 (N-MEDA)PbBr4 4 0.5 82 (0.36, 0.41) become saturated. The broad emission from (001) perovskites 7,8 (N-MEDA)PbCl0.5Br3.5 4 85 (0.31, 0.36) was also shown to depend linearly on excitation intensity. (N-MEDA)PbCl1.2Br2.8 4 1.5 Photoluminescence excitation (PLE) spectra collected at dif- (EDBE)PbBr4 5 9 84 (0.39, 0.42) ferent wavelengths across the broad PL have similar shapes, (EDBE)PbCl4 5 2 81 (0.33, 0.39) suggesting that the same initial excited state is responsible for the α -(DMEN)2PbBr4 9 73 (0.28, 0.36) emission. Additionally, PLE spectra did not show below-exciton (AEA)PbBr4 7 87 (0.29, 0.34) or subgap contributions to the PL that typically arise from per- (PEA)2PbCl4 10 <1 84 (0.37, 0.42) manent defects. The broad emission in (EDBE)PbBr4 becomes (EA)4Pb3Cl10 11 66 (0.27, 0.39) much narrower with decreasing temperature, indicating that vibra- (EA)4Pb3Cl9.5Br0.5 11 83 (0.30, 0.35) tional coupling contributes significantly to the emission width. (CyBMA)PbBr4 12 1.5 (0.23, 0.29) This temperature dependence allowed us to estimate the effective frequency of the vibrational modes that couple to the electronic 46 5 − substitution, similar to the emission in (EDBE)PbBr4−xIx. We transition as 97 cm 1, comparable to the energy of the material’s found the perovskite (AEA)PbBr4 (AEA = H3NC6H4(CH2)2NH3) Pb−Br modes observed in Raman spectra.5 Although this analysis to emit cool white light with a high CRI of 87.7 Mathews and co- + used a rough model, it further supported carrier trapping in the workers reported that (PEA)2PbCl4 (PEA = C6H5(CH2)2NH3 ) inorganic layers.5 The emission also becomes much more intense emits white light with a CRI of 84 and (CyBMA)PbBr4 (CyBMA = as nonradiative decay pathways shut down at lower temperatures. cis-1,3-bis(ammoniomethyl)cyclohexane) emits bluish-white 10,12 For example, the PLQE for (EDBE)PbBr4 increases from 9% at light. Kanatzidis and co-workers reported white-light emis- 5 + 300 K to 85% at 105 K. These early experiments, however, did sion from (EA)4Pb3BrxCl10−x (EA = CH3CH2NH3 ) featuring not conclusively show self-trapping. We therefore sought to thicker (001) inorganic sheets, with a maximum CRI of 83 for 11 probe the excited-state energy landscape and carrier dynamics of x = 0.5. these materials using ultrafast spectroscopic probes.6 4. UNDERSTANDING BROAD EMISSION Excited-State Dynamics. To follow the path of excited carriers, we probed the (110) perovskite (N-MEDA)PbBr4 Our initial reports proposed exciton self-trapping as the primary immediately following photoexcitation.6 Optical pump-terahertz source of the broad Stokes-shifted PL in 2D perovskites, but 4,5 5−7 (THz) probe measurements revealed that resonantly exciting the without direct evidence for it. Subsequent work by our lab exciton absorption peak produces no free carriers, consistent and by others8,13,41,46 has sought to clarify the origins and mech- with its large Eb. Above-band-gap excitation, however, produces anistic underpinnings of the broad emission through structural, extremely short-lived free carriers with subpicosecond lifetimes. spectroscopic, and theoretical studies. So far, these studies have This measurement, however, does not distinguish between car- supported the original proposal. We hope more studies will probe riers that form FEs and STEs. The most direct evidence for this mechanism and reveal routes to controlling and optimizing the exciton self-trapping comes from transient absorption data. With emission. fi near-UV photoexcitation, thin lms of (N-MEDA)PbBr4 exhibit 4.1. Mechanistic Insights a broad induced absorption at energies below the exciton that Early Indicators of Self-Trapping. The shape of the broad extends across the visible spectrum (Figure 9C), with an onset 6 PL from the (110) perovskites (N-MEDA)PbBr4 and (EDBE)- time of ca. 400 fs, consistent with the formation of transient, PbBr4 was essentially insensitive to morphology, with polycrystal- light-induced, trap states. In contrast, 2D perovskites with nar- line thin films, ball-milled powders, and large single crystals exhibiting row PL exhibit below-exciton bleaching features owing to filling similar PL.4,5 Similar PLQE from single crystals and μm-scale of permanent trap states.48 The subpicosecond decay time of free

Figure 9. (A) Wavelength-dependent photoluminescence (PL) rise and (B) decay times for (N-MEDA)PbBr, with the instrument time resolution − shown in gray. (C) Transient absorbance of (N-MEDA)PbBr4 upon excitation at 345 nm. Induced absorption is shown in red. The data at ca. 650 700 nm are masked owing to the laser excitation line. (D) Schematic of exciton self-trapping from the free carrier (FC) or free-exciton (FE) state to a distribution of self-trapped states (STEs). PL is shown with dotted arrows. Adapted with permission from ref 6. Copyright 2016 American Chemical Society.

623 DOI: 10.1021/acs.accounts.7b00433 Acc. Chem. Res. 2018, 51, 619−627 Accounts of Chemical Research Article carriers from THz measurements means that free-carrier absorp- in materials such as pyrene.43 A similar model of thermally acti- tion cannot account for this feature. A similar broad induced vated trapping and detrapping captures the behavior of 2D absorption was later reported for (EDBE)PbBr4 and (EDBE)- perovskites well (Figure 6). When cooling from room temper- 13 10 PbCl4, and for (PEA)2PbCl4. ature, the STEs become decreasingly able to detrap back to the ’ Kerr-gate measurements show that the broad PL in (N- FE state (kBT < Ea,detrap), increasing the broad PL s intensity. − 6 MEDA)PbBr4 turns on in 1 3 ps, more slowly than the FE PL. Then, at very low temperatures (below 80 K), the carriers are less Additionally, the broad emission’s onset time is wavelength- able to surmount the activation barrier from the FE to the STE dependent (Figure 9A), with lower-energy components of the PL states (kBT < Ea,trap) leading to a relative increase in narrow, FE rising more slowly than the higher-energy components.6 Time- PL. At sufficiently low temperatures, tunneling from free to self- 8 43 − resolved PL traces of the (001) perovskite (C6H11NH3)2PbBr4 trapped states, singlet triplet exciton interconversion, FEs bound 6 26,50 30 and of (N-MEDA)PbBr4 exhibit a spread in PL lifetimes to defects, or biexcitons can also be observed, complicating (Figure 9B), with increasing, subnanosecond lifetimes for the this relationship. Eight (001) Pb−Br perovskites showed similar −1 lower-energy PL. Similar wavelength-dependent kinetics are temperature-dependent PL. Importantly, the ln(IBEINE ) ratio found in other materials known for exciton self-trapping such as at a given temperature is proportional to the self-trapping depth 49 Δ − Si nanocrystals. ( Gself‑trap = Ea,trap Ea,detrap < 0): Taken together, these ultrafast spectroscopic probes suggest I k ΔGself‐ trap the following mechanism for the broad emission. Following pho- lnlnBE ∝−r,s toexcitation, free carriers form excitons in less than 600 fs. These INE kr,f kTB excitons self-trap within ca. 400 fs and then begin to luminesce. The lattice distortion (defined by S; Figure 6) provides for homo- where kr,f and kr,s are the radiative recombination rates of the FEs 5,35,44 and STEs, respectively. We can thereby relate the propensity for geneous emission broadening. Additionally, the presence of 7 multiple STE states can inhomogeneously broaden the emission.6 white-light emission to the thermodynamics of self-trapping. Here, the more deeply self-trapped states produce lower-energy 4.2. Structural Origins PL and take longer to emit (Figure 9D), consistent with greater Self-trapping depends on the deformability of the lattice and lattice distortions required for lower-energy PL. A cascade mech- electron−phonon coupling. Therefore, although the emission is anism, where shallowly trapped excitons shuttle to more deeply attributed to arise from “excited-state defects”, it should reflect trapped states, is also possible, aided by the local heterogeneity of changes to the bulk crystal structure. To articulate design rules the excited-state potential-energy surface. This heterogeneity for realizing white-light from perovskites, we searched for corre- may arise from permanent lattice defects (extrinsic self-trapping) lations between the broad PL and structural parameters obtained or from correlated self-trapped states where one self-trapped from the crystal structures of eight (001) perovskites.7 − μ − θ state serves as a nucleating site for subsequent self-trapping events. The metal ( -halide) metal angle ( tilt) can be decomposed Generality of the Broad Emission. The PL from both (110) into in-plane and out-of-plane projections onto the plane of Pb − fi θ θ and (001) Pb Br perovskites exhibits two distinct temperature atoms, de ned as in and out, respectively (Figure 11A,C). These fi −1 regimes, de ned by the changing behavior of the (IBEINE ) ratio, angles have previously been correlated with the absorption 51 where IBE and INE are the integrated intensities of the broad spectra of 2D perovskites. In the series of (001) Pb−Br perov- and narrow PL, respectively. Upon cooling the (110) perovskite skites, we observed a linear correlation between the largest mea- ° − θ −1 (N-MEDA)PbBr4, IBE increases dramatically relative to INE until sured out-of-plane distortion Dout (180 out) and IBEINE at a ca. 40 K, below which this ratio remains essentially constant.6 given temperature (Figure 10C). In contrast, the correlations are The (001) perovskite (HIS)PbBr4 shows similar behavior, where much weaker or nonexistent for over 50 other structural param- fi −1 ° − θ ° − θ cooling signi cantly increases IBEINE until ca. 80 K, and then eters we considered, including Dtilt (180 tilt), Din (180 in), decreases below this threshold (Figure 10A,B).7 The thermal and deviations of the Pb−Br coordination sphere from octa- 7 equilibrium between FEs and STEs has been previously studied hedral symmetry. For example, (HIS)PbBr4 and (BA)2PbBr4

Figure 10. (A) Photoluminescence (PL) of (HIS)PbBr4 (HIS = histammonium) showing both narrow (NE) and broad emission (BE). −1 (B) Temperature-dependent ratio of integrated BE:NE intensity, ln(IBEINE ), for a single crystal of (HIS)PbBr4. The colored symbols correspond to −1 − the PL spectra of the same colors in (A). (C) Correlation between ln(IBEINE ) at 80 K and out-of-plane distortion (Dout) for a series of (001) Pb Br perovskites. Inset: A 2D perovskite fragment showing the plane of layer propagation. Adapted from ref 7 with permission from the Royal Society of Chemistry.

624 DOI: 10.1021/acs.accounts.7b00433 Acc. Chem. Res. 2018, 51, 619−627 Accounts of Chemical Research Article

proposal of exciton self-trapping as the origin of the broad PL, with material defects further widening the emission. Although only a few white-light-emitting perovskites are still known, we find that self-trapping is common to 2D Pb−Br and Pb−Cl perovskites, albeit with a large dependence on temperature. With recently developed mechanistic understanding and design prin- ciples, we are now primed to transition from fortuitous discovery to rational synthesis. Future studies should continue to probe the emission mechanism, elucidate the structure of the self-trapped state(s), and understand how defect chemistry affects the broad PL. Broadband emission from halide perovskites can be gener- alized to other low-dimensional systems. Indeed, several non- perovskite hybrids have been recently reported to display broad Stokes-shifted emission, which has been attributed to self-trapping by analogy to the perovskite white-light emitters.54,55 This field is young, and offers plentiful opportunities for manipulating excitons in low-dimensional metal halides. Figure 11. Distortions in the inorganic layers and variable temperature ■ AUTHOR INFORMATION photoluminescence spectra for (A,B) (BA)2PbBr4(BA = butylammo- nium) and (C,D) (HIS)PbBr4 (HIS = histammonium). Adapted from Corresponding Author ref 7 with permission from the Royal Society of Chemistry. *E-mail: [email protected]. (HIS = histammonium, 4-(2-ammonioethyl)-1H-imidazol-3- ORCID ium; BA = n-butylammonium) have similar values of Dtilt Matthew D. Smith: 0000-0002-4197-5176 ° ° (26.1(2) and 25.2(1.3) , respectively). However, (HIS)PbBr4 Hemamala I. Karunadasa: 0000-0003-4949-8068 ° displays a much larger Dout of 22.8(2) compared to (BA)2PbBr4 ° Funding (Dout = 3(3) )(Figure 11A,C). Variable-temperature PL spectra of (HIS)PbBr4 show a major contribution from the broad This research was supported by Stanford University, the Stanford emission upon cooling, while (BA)2PbBr4 exhibits primarily Precourt Institute, and the Alfred P. Sloan Fellowship. M.D.S. is narrow emission (Figure 11B,D). Indeed, (AEA)PbBr4 has a supported by Graduate Fellowships from the National Science ° very large Dout (22.4(1.9) ) and emits white light at room Foundation (DGE-114747) and the Center for Molecular Anal- temperature with excellent color rendition (Table 1). This large ysis and Design at Stanford University. structural dependence suggests a strong intrinsic component to the Notes broad emission. The correlation between average Dout and fi −1 The authors declare no competing nancial interest. ln(IBEINE ) is much weaker compared to the correlation for the largest Dout, consistent with the localized nature of self-trapping. Biographies These design rules may lead to the rational synthesis of white-light- Matthew D. Smith received his B.S. in Chemistry from Haverford emitting perovskites. College working with Alexander Norquist and Joshua Schrier, and is 5. CONCLUSIONS AND OUTLOOK currently a Ph.D. student at Stanford University with Hemamala Karunadasa. His research focuses on excitons in dimensionally confined systems. Intrinsic, bulk white-light emitters are rare. These include layered cadmium chalcogenides52 and gold(I)-containing molecular aggre- Hemamala I. Karunadasa studied solid-state chemistry with Robert gates.53 Commercial white-light sources use blue LEDs coated with Cava at Princeton University for her A.B. and molecular inorganic chem- ff yellow phosphors (such as cerium-doped yttrium aluminum istry at UC Berkeley for her Ph.D. with Je rey Long. Her postdoctoral ff garnet)1,3 to approximate white light. However, these devices are research with Christopher Chang and Je rey Long at UC Berkeley and often combined with red LEDs or phosphors to improve their with Harry Gray at Caltech focused on molecular catalysis. She joined CRI values.1 Single-component broadband white-light emitters the Stanford Chemistry faculty in 2012. Her group synthesizes materials can improve on current phosphor technology. Commercial inor- that harness the advantages of extended solids and discrete molecules. ganic phosphors1 easily surpass halide perovskites with respect to PLQE. 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