Hemamala Karunadasa

All Publications

• 3D Lead-Organoselenide-Halide Perovskites and their Mixed-Chalcogenide and Mixed-Halide Alloys. Angewandte Chemie (International ed. in English) Karunadasa, H., Li, J., Wang, Y., Saha, S., Chen, Z., Hofmann, J., Misleh, J., Chapman, K. W., Reimer, J. A., Filip, M. R. 2024: e202408443

  Abstract

  We incorporate Se into the 3D halide perovskite framework using the zwitterionic ligand: SeCYS (+NH3(CH2)2Se-), which occupies both the X- and A+ sites in the prototypical ABX3 perovskite. The new organoselenide-halide perovskites: (SeCYS)PbX2 (X = Cl, Br) expand upon the recently discovered organosulfide-halide perovskites. Single-crystal X-ray diffraction and pair distribution function analysis reveal the average structures of the organoselenide-halide perovskites, whereas the local lead coordination environments and their distributions were probed through solid-state 77Se and 207Pb NMR, complemented by theoretical simulations. Density functional theory calculations illustrate that the band structures of (SeCYS)PbX2 largely resemble those of their S analogs, with similar band dispersion patterns, yet with a considerable bandgap decrease. Optical absorbance measurements indeed show bandgaps of 2.07 and 1.86 eV for (SeCYS)PbX2 with X = Cl and Br, respectively. We further demonstrate routes to alloying the halides (Cl, Br) and chalcogenides (S, Se) continuously tuning the bandgap from 1.86 to 2.31 eV-straddling the ideal range for tandem solar cells or visible-light photocatalysis. The comprehensive description of the average and local structures, and how they can fine-tune the bandgap and potential trap states, respectively, establishes the foundation for understanding this new perovskite family, which combines solid-state and organo-main-group chemistry.

  View details for DOI 10.1002/anie.202408443

  View details for PubMedID 38976771

• Halide Perovskites Breathe Too: The Iodide-Iodine Equilibrium and Self-Doping in Cs2SnI6. ACS central science Vigil, J. A., Wolf, N. R., Slavney, A. H., Matheu, R., Saldivar Valdes, A., Breidenbach, A., Lee, Y. S., Karunadasa, H. I. 2024; 10 (4): 907-919

  Abstract

  The response of an oxide crystal to the atmosphere can be personified as breathing-a dynamic equilibrium between O2 gas and O2- anions in the solid. We characterize the analogous defect reaction in an iodide double-perovskite semiconductor, Cs2SnI6. Here, I2 gas is released from the crystal at room temperature, forming iodine vacancies. The iodine vacancy defect is a shallow electron donor and is therefore ionized at room temperature; thus, the loss of I2 is accompanied by spontaneous n-type self-doping. Conversely, at high I2 pressures, I2 gas is resorbed by the perovskite, consuming excess electrons as I2 is converted to 2I-. Halide mobility and irreversible halide loss or exchange reactions have been studied extensively in halide perovskites. However, the reversible exchange equilibrium between iodide and iodine [2I-(s) ↔ I2(g) + 2e-] described here has often been overlooked in prior studies, though it is likely general to halide perovskites and operative near room temperature, even in the dark. An analysis of the 2I-(s)/I2(g) equilibrium thermodynamics and related transport kinetics in single crystals of Cs2SnI6 therefore provides insight toward achieving stable composition and electronic properties in the large family of iodide perovskite semiconductors.

  View details for DOI 10.1021/acscentsci.4c00056

  View details for PubMedID 38680557

  View details for PubMedCentralID PMC11046464

• Local structure, bonding, and asymmetry of ((NH2)2CH)PbBr3, CsPbBr3, and (CH3NH3)PbBr3 PHYSICAL REVIEW B Bridges, F., Gruzdas, J., MacKeen, C., Mayford, K., Weadock, N. J., Baltazar, V., Rakita, Y., Waquier, L., Vigil, J. A., Karunadasa, H. I., Toney, M. F. 2023; 108 (21)

  View details for DOI 10.1103/PhysRevB.108.214102

  View details for Web of Science ID 001141782800007

• Understanding the evolution of double perovskite band structure upon dimensional reduction. Chemical science Connor, B. A., Su, A. C., Slavney, A. H., Leppert, L., Karunadasa, H. I. 2023; 14 (42): 11858-11871

  Abstract

  Recent investigations into the effects of dimensional reduction on halide double perovskites have revealed an intriguing change in band structure when the three-dimensional (3D) perovskite is reduced to a two-dimensional (2D) perovskite with inorganic sheets of monolayer thickness (n = 1). The indirect bandgap of 3D Cs2AgBiBr6 becomes direct in the n = 1 perovskite whereas the direct bandgap of 3D Cs2AgTlBr6 becomes indirect at the n = 1 limit. Here, we apply a linear combination of atomic orbitals approach to uncover the orbital basis for this bandgap symmetry transition with dimensional reduction. We adapt our previously established method for predicting band structures of 3D double perovskites for application to their 2D congeners, emphasizing new considerations required for the 2D lattice. In particular, we consider the inequivalence of the terminal and bridging halides and the consequences of applying translational symmetry only along two dimensions. The valence and conduction bands of the layered perovskites can be derived from symmetry adapted linear combinations of halide p orbitals propagated across the 2D lattice. The dispersion of each band is then determined by the bonding and antibonding interactions of the metal and halide orbitals, thus affording predictions of the essential features of the band structure. We demonstrate this analysis for 2D Ag-Bi and Ag-Tl perovskites with sheets of mono- and bilayer thickness, establishing a detailed understanding of their band structures, which enables us to identify the key factors that drive the bandgap symmetry transitions observed at the n = 1 limit. Importantly, these insights also allow us to make the general prediction that direct → indirect or indirect → direct bandgap transitions in the monolayer limit are most likely in double perovskite compositions that involve participation of metal d orbitals at the band edges or that have no metal-orbital contributions to the valence band, laying the groundwork for the targeted realization of this phenomenon.

  View details for DOI 10.1039/d3sc03105e

  View details for PubMedID 37920347

  View details for PubMedCentralID PMC10619643

• Stabilizing Au2+ in a mixed-valence 3D halide perovskite NATURE CHEMISTRY Lindquist, K. P., Eghdami, A., Deschene, C. R., Heyer, A. J., Wen, J., Smith, A. G., Solomon, E. I., Lee, Y. S., Neaton, J. B., Ryan, D. H., Karunadasa, H. I. 2023

  View details for DOI 10.1038/s41557-023-01305

  View details for Web of Science ID 001064800000001

• Structural Dynamics of a Novel Pseudohalide Perovskite Cs2Pb(SeCN)(2)Br-2 Investigated with Nonlinear Infrared Spectroscopy JOURNAL OF PHYSICAL CHEMISTRY C Xing, X., Li, J., Breen, J. P., Karunadasa, H. I., Fayer, M. D. 2023

  View details for DOI 10.1021/acs.jpcc.3c02251

  View details for Web of Science ID 001030464100001

• The nature of dynamic local order in CH3NH3PbI3 and CH3NH3PbBr3 JOULE Weadock, N. J., Hemamala, T. C., Vigil, J. A., Gold-Parker, A., Smith, I. C., Ahammed, B., Krogstad, M. J., Ye, F., Voneshen, D., Gehring, P. M., Rappe, A. M., Steinrueck, H., Ertekin, E., Karunadasa, H. I., Reznik, D., Toney, M. F. 2023; 7 (5)

  View details for DOI 10.1016/j.joule.2023.03.017

  View details for Web of Science ID 001001466900001

• Mosaic CuI-CuII-InIII 2D Perovskites: Pressure-Dependence of the Intervalence Charge Transfer and a Mechanochemical Alloying Method. Angewandte Chemie (International ed. in English) Li, J., Matheu, R., Ke, F., Liu, Z., Lin, Y., Karunadasa, H. 2023: e202300957

  Abstract

  The 2D perovskite (BA)4[CuII(CuIInIII)0.5]Cl8 (1BA; BA+ = butylammonium) allows us to study the high-pressure structural, optical, and transport properties of a mixed-valence 2D perovskite. Compressing 1BA reduces the onset energy of CuI/II intervalence charge transfer from 1.2 eV at ambient pressure to 0.2 eV at 21 GPa. The electronic conductivity of 1BA increases by 4 orders of magnitude upon compression to 20 GPa, when the activation energy for conduction decreases to 0.16 eV. In contrast, CuII perovskites achieve similar conductivity at ~50 GPa. The solution-state synthesis of these perovskites is complicated, with more undesirable side products likely from the precursor mixtures containing three different metals. To circumvent this problem, we demonstrate an efficient mechanochemical synthesis to expand this family of halide perovskites with complex composition by simply pulverizing together powders of 2D CuII single perovskites and CuIInIII double perovskites.

  View details for DOI 10.1002/anie.202300957

  View details for PubMedID 36919236

• Quasi-One-Dimensional Metallicity in Compressed CsSnI3. Journal of the American Chemical Society Ke, F., Yan, J., Matheu, R., Niu, S., Wolf, N. R., Yang, H., Yin, K., Wen, J., Lee, Y. S., Karunadasa, H. I., Mao, W. L., Lin, Y. 2022

  Abstract

  Low-dimensional metal halides exhibit strong structural and electronic anisotropies, making them candidates for accessing unusual electronic properties. Here, we demonstrate pressure-induced quasi-one-dimensional (quasi-1D) metallicity in delta-CsSnI3. With the application of pressure up to 40 GPa, the initially insulating delta-CsSnI3 transforms to a metallic state. Synchrotron X-ray diffraction and Raman spectroscopy indicate that the starting 1D chain structure of edge-sharing Sn-I octahedra in delta-CsSnI3 is maintained in the high-pressure metallic phase while the SnI6 octahedral chains are distorted. Our experiments combined with first-principles density functional theory calculations reveal that pressure induces Sn-Sn hybridization and enhances Sn-I coupling within the chain, leading to band gap closure and formation of conductive SnI6 distorted octahedral chains. In contrast, the interchain I...I interactions remain minimal, resulting in a highly anisotropic electronic structure and quasi-1D metallicity. Our study offers a high-pressure approach for achieving diverse electronic platforms in the broad family of low-dimensional metal halides.

  View details for DOI 10.1021/jacs.2c10884

  View details for PubMedID 36534020

• Zwitterions in 3D Perovskites: Organosulfide-Halide Perovskites. Journal of the American Chemical Society Li, J., Chen, Z., Saha, S., Utterback, J. K., Aubrey, M. L., Yuan, R., Weaver, H. L., Ginsberg, N. S., Chapman, K. W., Filip, M. R., Karunadasa, H. I. 2022

  Abstract

  Although sulfide perovskites usually require high-temperature syntheses, we demonstrate that organosulfides can be used in the milder syntheses of halide perovskites. The zwitterionic organosulfide, cysteamine (CYS; +NH3(CH2)2S-), serves as both the X- site and A+ site in the ABX3 halide perovskites, yielding the first examples of 3D organosulfide-halide perovskites: (CYS)PbX2 (X- = Cl- or Br-). Notably, the band structures of (CYS)PbX2 capture the direct bandgaps and dispersive bands of APbX3 perovskites. The sulfur orbitals compose the top of the valence band in (CYS)PbX2, affording unusually small direct bandgaps of 2.31 and 2.16 eV for X- = Cl- and Br-, respectively, falling in the ideal range for the top absorber in a perovskite-based tandem solar cell. Measurements of the carrier dynamics in (CYS)PbCl2 suggest carrier trapping due to defects or lattice distortions. The highly desirable bandgaps, band dispersion, and improved stability of the organosulfide perovskites demonstrated here motivate the continued expansion and exploration of this new family of materials, particularly with respect to extracting photocurrent. Our strategy of combining the A+ and X- sites with zwitterions may offer more members in this family of mixed-anion 3D hybrid perovskites.

  View details for DOI 10.1021/jacs.2c09382

  View details for PubMedID 36416496

• Cesium-mediated electron redistribution and electron-electron interaction in high-pressure metallic CsPbI3. Nature communications Ke, F., Yan, J., Niu, S., Wen, J., Yin, K., Yang, H., Wolf, N. R., Tzeng, Y., Karunadasa, H. I., Lee, Y. S., Mao, W. L., Lin, Y. 2022; 13 (1): 7067

  Abstract

  Electron-phonon coupling was believed to govern the carrier transport in halide perovskites and related phases. Here we demonstrate that electron-electron interaction enhanced by Cs-involved electron redistribution plays a direct and prominent role in the low-temperature electrical transport of compressed CsPbI3 and renders Fermi liquid (FL)-like behavior. By compressing delta-CsPbI3 to 80GPa, an insulator-semimetal-metal transition occurs, concomitant with the completion of a slow structural transition from the one-dimensional Pnma (delta) phase to a three-dimensional Pmn21 (epsilon) phase. Deviation from FL behavior is observed upon CsPbI3 entering the metallic epsilon phase, which progressively evolves into a FL-like state at 186GPa. First-principles density functional theory calculations reveal that the enhanced electron-electron coupling results from the sudden increase of the 5d state occupation in Cs and I atoms. Our study presents a promising strategy of cationic manipulation for tuning the electronic structure and carrier scattering of halide perovskites at high pressure.

  View details for DOI 10.1038/s41467-022-34786-5

  View details for PubMedID 36400789

• Tuning Defects in a Halide Double Perovskite with Pressure. Journal of the American Chemical Society Wolf, N. R., Jaffe, A., Slavney, A. H., Mao, W. L., Leppert, L., Karunadasa, H. I. 2022

  Abstract

  Dopant defects in semiconductors can trap charge carriers or ionize to produce charge carriers─playing a critical role in electronic transport. Halide perovskites are a technologically important semiconductor family with a large pressure response. Yet, to our knowledge, the effect of high pressures on defects in halide perovskites has not been experimentally investigated. Here, we study the structural, optical, and electronic consequences of compressing the small-bandgap double perovskites Cs2AgTlX6 (X = Cl or Br) up to 56 GPa. Mild compression to 1.7 GPa increases the conductivity of Cs2AgTlBr6 by ca. 1 order of magnitude and decreases its bandgap from 0.94 to 0.7 eV. Subsequent compression yields complex optoelectronic behavior: the bandgap varies by 1.2 eV and conductivity ranges by a factor of 104. These conductivity changes cannot be explained by the evolving bandgap. Instead, they can be understood as tuning of the bromine vacancy defect with pressure─varying between a delocalized shallow defect state with a small ionization energy and a localized deep defect state with a large ionization energy. Activation energy measurements reveal that the shallow-to-deep defect transition occurs near 1.5 GPa, well before the cubic-to-tetragonal phase transition. An analysis of the orbital interactions in Cs2AgTlBr6 illustrates how the bromine vacancy weakens the adjacent Tl s-Br p antibonding interaction, driving the shallow-to-deep defect transition. Our orbital analysis leads us to propose that halogen vacancies are most likely to be shallow donors in halide double perovskites that have a conduction band derived from the octahedral metal's s orbitals.

  View details for DOI 10.1021/jacs.2c08607

  View details for PubMedID 36343332

• Reliably obtaining white light from layered halide perovskites at room temperature. Chemical science Crace, E. J., Su, A. C., Karunadasa, H. I. 2022; 13 (34): 9973-9979

  Abstract

  The recent observation of broadband white-light emission from the inorganic sheets of certain layered lead-bromide perovskites has instigated a multitude of studies on this unusual phenomenon. However, the vast majority of layered bromide perovskites have flat (001) inorganic sheets and display a narrow photoluminescence at room temperature. A handful of heavily distorted (001) perovskites display broad emission, but to date, there is no method of predicting which perovskites will produce white light at room temperature prior to screening different organic molecules that can template 2D perovskites and crystallizing and analyzing the material. By studying ten Pb-Cl perovskites, we find that they all exhibit a broad yellow emission, which is strikingly invariant despite different distortions in the inorganic framework seen across the series. We postulate that this broad emission is intrinsic to all layered Pb-Cl perovskites. Although broad, the emission is not white. By adding Br to the Pb-Cl perovskites we obtain both the narrow emission and the broad emission such that the combined emission color smoothly varies from yellow to warm white to cold white as a function of the halide ratio. Thus, alloying Br to Pb-Cl perovskites appears to be a simple and general strategy for reliably obtaining white light at room temperature from (001) perovskites, regardless of the templating effects of the organic molecules, which should greatly expand the number of white-light-emitting layered perovskites.

  View details for DOI 10.1039/d2sc02381d

  View details for PubMedID 36199633

  View details for PubMedCentralID PMC9431451

• Probing Lattice Dynamics in Two-Dimensional InorganicPseudohalide Perovskites with Ultrafast Infrared Spectroscopy br JOURNAL OF PHYSICAL CHEMISTRY C Xing, X., Li, J., Breen, J. P., Nishida, J., Karunadasa, H., Fayer, M. D. 2022; 126 (24): 10145-10158

  View details for DOI 10.1021/acs.jpcc.2c03516

  View details for Web of Science ID 000821434300001

• Charge Reservoirs in an Expanded Halide Perovskite Analog: Enhancing High-Pressure Conductivity through Redox-Active Molecules. Angewandte Chemie (International ed. in English) Matheu, R., Ke, F., Breidenbach, A., Wolf, N., Lee, Y., Liu, Z., Leppert, L., Lin, Y., Karunadasa, H. 2022

  Abstract

  As halide perovskites and their derivatives are being developed for numerous optoelectronic applications, controlling their electronic doping remains a fundamental challenge. Herein, we describe a novel strategy of using redox-active organic molecules as stoichiometric electron acceptors. The cavities in the new expanded perovskite analogs (dmpz)[Sn2X6], (X = Br- ( 1Br ) or I- ( 1I )) are occupied by dmpz2+ (N, N'-dimethylpyrazinium), with the LUMOs lying ca. 1 eV above the valence band maximum (VBM). Compressing the metal-halide framework drives up the VBM in 1I relative to the dmpz LUMO. The electronic conductivity increases by a factor of 105 with pressure, reaching 50(17) S cm-1 at 60 GPa, exceeding the high-pressure conductivities of most halide perovskites. This conductivity enhancement is attributed to an increased hole density created by dmpz2+ reduction. This work elevates the role of organic cations in 3D metal-halides, from templating the structure to serving as charge reservoirs for tuning the carrier concentration.

  View details for DOI 10.1002/anie.202202911

  View details for PubMedID 35421260

• The halogen chemistry of halide perovskites TRENDS IN CHEMISTRY Matheu, R., Vigil, J. A., Crace, E. J., Karunadasa, H. I. 2022; 4 (3): 206-219

  View details for DOI 10.1016/j.trechm.2021.12.002

  View details for Web of Science ID 000759893200006

• Directed assembly of layered perovskite heterostructures as single crystals. Nature Aubrey, M. L., Saldivar Valdes, A., Filip, M. R., Connor, B. A., Lindquist, K. P., Neaton, J. B., Karunadasa, H. I. 2021; 597 (7876): 355-359

  Abstract

  The precise stacking of different two-dimensional (2D) structures such as graphene and MoS2 has reinvigorated the field of 2D materials, revealing exotic phenomena at their interfaces1,2. These unique interfaces are typically constructed using mechanical or deposition-based methods to build a heterostructure one monolayer at a time2,3. By contrast, self-assembly is a scalable technique, where complex materials can selectively form in solution4-6. Here we show a synthetic strategy for the self-assembly of layered perovskite-non-perovskite heterostructures into large single crystals in aqueous solution. Using bifunctional organic molecules as directing groups, we have isolated six layered heterostructures that form as an interleaving of perovskite slabs with a different inorganic lattice, previously unknown to crystallize with perovskites. In many cases, these intergrown lattices are 2D congeners of canonical inorganic structure types. To our knowledge, these compounds are the first layered perovskite heterostructures formed using organic templates and characterized by single-crystal X-ray diffraction. Notably, this interleaving of inorganic structures can markedly transform the band structure. Optical data and first principles calculations show that substantive coupling between perovskite and intergrowth layers leads to new electronic transitions distributed across both sublattices. Given the technological promise of halide perovskites4, this intuitive synthetic route sets a foundation for the directed synthesis of richly structured complex semiconductors that self-assemble in water.

  View details for DOI 10.1038/s41586-021-03810-x

  View details for PubMedID 34526708

• Alloying a single and a double perovskite: a Cu+/2+ mixed-valence layered halide perovskite with strong optical absorption. Chemical science Connor, B. A., Smaha, R. W., Li, J., Gold-Parker, A., Heyer, A. J., Toney, M. F., Lee, Y. S., Karunadasa, H. I. 2021; 12 (25): 8689-8697

  Abstract

  Introducing heterovalent cations at the octahedral sites of halide perovskites can substantially change their optoelectronic properties. Yet, in most cases, only small amounts of such metals can be incorporated as impurities into the three-dimensional lattice. Here, we exploit the greater structural flexibility of the two-dimensional (2D) perovskite framework to place three distinct stoichiometric cations in the octahedral sites. The new layered perovskites AI 4[CuII(CuIInIII)0.5Cl8] (1, A = organic cation) may be derived from a CuI-InIII double perovskite by replacing half of the octahedral metal sites with Cu2+. Electron paramagnetic resonance and X-ray absorption spectroscopy confirm the presence of Cu2+ in 1. Crystallographic studies demonstrate that 1 represents an averaging of the CuI-InIII double perovskite and CuII single perovskite structures. However, whereas the highly insulating CuI-InIII and CuII perovskites are colorless and yellow, respectively, 1 is black, with substantially higher electronic conductivity than that of either endmember. We trace these emergent properties in 1 to intervalence charge transfer between the mixed-valence Cu centers. We further propose a tiling model to describe how the Cu+, Cu2+, and In3+ coordination spheres can pack most favorably into a 2D perovskite lattice, which explains the unusual 1 : 2 : 1 ratio of these cations found in 1. Magnetic susceptibility data of 1 further corroborate this packing model. The emergence of enhanced visible light absorption and electronic conductivity in 1 demonstrates the importance of devising strategies for increasing the compositional complexity of halide perovskites.

  View details for DOI 10.1039/d1sc01159f

  View details for PubMedID 34257867

  View details for PubMedCentralID PMC8246118

• Doubling the Stakes: The Promise of Halide Double Perovskites. Angewandte Chemie (International ed. in English) Wolf, N. R., Connor, B. A., Slavney, A. H., Karunadasa, H. 2021

  Abstract

  When the stakes are doubled in a wager, a player must correctly place two consecutive bets in order to win, but the payout is larger. Similarly, two B sites in combination dictate the properties of A2BB'X6 (A = monocation, X = halide) double perovskites. Correctly picking two B sites is more challenging than picking just one, as in the AIBIIX3 single perovskites. But the options are greater and, we believe, the rewards are higher when the stakes are doubled. In this minireview, we emphasize fundamental aspects of halide double perovskites to provide a foundation for interested readers to explore this extraordinary class of materials. In particular, we highlight the differences and similarities between double and single perovskites and describe how the double perovskite structure potentially offers greater control over photophysical properties.

  View details for DOI 10.1002/anie.202016185

  View details for PubMedID 33621383

• Visualization of dynamic polaronic strain fields in hybrid lead halide perovskites. Nature materials Guzelturk, B., Winkler, T., Van de Goor, T. W., Smith, M. D., Bourelle, S. A., Feldmann, S., Trigo, M., Teitelbaum, S. W., Steinruck, H., de la Pena, G. A., Alonso-Mori, R., Zhu, D., Sato, T., Karunadasa, H. I., Toney, M. F., Deschler, F., Lindenberg, A. M. 2021

  Abstract

  Excitation localization involving dynamic nanoscale distortions is a central aspect of photocatalysis1, quantum materials2 and molecular optoelectronics3. Experimental characterization of such distortions requires techniques sensitive to the formation of point-defect-like local structural rearrangements in real time. Here, we visualize excitation-induced strain fields in a prototypical member of the lead halide perovskites4 via femtosecond resolution diffuse X-ray scattering measurements. This enables momentum-resolved phonon spectroscopy of the locally distorted structure and reveals radially expanding nanometre-scale strain fields associated with the formation and relaxation of polarons in photoexcited perovskites. Quantitative estimates of the magnitude and shape of this polaronic distortion are obtained, providing direct insights into the dynamic structural distortions that occur in these materials5-9. Optical pump-probe reflection spectroscopy corroborates these results and shows how these large polaronic distortions transiently modify the carrier effective mass, providing a unified picture of the coupled structural and electronic dynamics that underlie the optoelectronic functionality of the hybrid perovskites.

  View details for DOI 10.1038/s41563-020-00865-5

  View details for PubMedID 33398119

• Preserving a robust CsPbI3 perovskite phase via pressure-directed octahedral tilt. Nature communications Ke, F. n., Wang, C. n., Jia, C. n., Wolf, N. R., Yan, J. n., Niu, S. n., Devereaux, T. P., Karunadasa, H. I., Mao, W. L., Lin, Y. n. 2021; 12 (1): 461

  Abstract

  Functional CsPbI3 perovskite phases are not stable at ambient conditions and spontaneously convert to a non-perovskite δ phase, limiting their applications as solar cell materials. We demonstrate the preservation of a black CsPbI3 perovskite structure to room temperature by subjecting the δ phase to pressures of 0.1 - 0.6 GPa followed by heating and rapid cooling. Synchrotron X-ray diffraction and Raman spectroscopy indicate that this perovskite phase is consistent with orthorhombic γ-CsPbI3. Once formed, γ-CsPbI3 could be then retained after releasing pressure to ambient conditions and shows substantial stability at 35% relative humidity. First-principles density functional theory calculations indicate that compression directs the out-of-phase and in-phase tilt between the [PbI6]4- octahedra which in turn tune the energy difference between δ- and γ-CsPbI3, leading to the preservation of γ-CsPbI3. Here, we present a high-pressure strategy for manipulating the (meta)stability of halide perovskites for the synthesis of desirable phases with enhanced materials functionality.

  View details for DOI 10.1038/s41467-020-20745-5

  View details for PubMedID 33469021

• Gold-Cage Perovskites: A Three-Dimensional AuIII-X Framework Encasing Isolated MX63- Octahedra (MIII = In, Sb, Bi; X = Cl-, Br-, I-). Journal of the American Chemical Society Lindquist, K. P., Boles, M. A., Mack, S. A., Neaton, J. B., Karunadasa, H. I. 2021

  Abstract

  The Cs8AuIII4MIIIX23 (M = In3+, Sb3+, Bi3+; X = Cl-, Br-, I-) perovskites are composed of corner-sharing Au-X octahedra that trace the edges of a cube containing an isolated M-X octahedron at its body center. This structure, unique within the halide perovskite family, may be derived from the doubled cubic perovskite unit cell by removing the metals at the cube faces. To our knowledge, these are the only halide perovskites where the octahedral sites do not bear an average 2+ charge. Charge compensation in these materials requires a stoichiometric halide vacancy, which is disordered around the Au atom at the unit-cell corner and orders when the crystallization is slowed. Using X-ray crystallography, X-ray absorption spectroscopy, and pair distribution function analysis, we elucidate the structure of this unusual perovskite. Metal-site alloying produces further intricacies in this structure, which our model explains. Compared to other halide perovskites, this class of materials shows unusually low absorption onset energies ranging between ca. 1.0 and 2.4 eV. Partial reduction of Au3+ to Au+ affords an intervalence charge-transfer band, which redshifts the absorption onset of Cs8Au4InCl23 from 2.4 to 1.5 eV. With connected Au-X octahedra and isolated M-X octahedra, this structure type combines zero- and three-dimensional metal-halide sublattices in a single material and stands out among halide perovskites for its ordering of homovalent metals, ordering of halide vacancies, and incorporation of purely trivalent metals at the octahedral sites.

  View details for DOI 10.1021/jacs.1c01624

  View details for PubMedID 33945275

• Revealing Local Disorder in a Silver-Bismuth Halide Perovskite upon Compression. The journal of physical chemistry letters Girdzis, S. P., Lin, Y., Leppert, L., Slavney, A. H., Park, S., Chapman, K. W., Karunadasa, H. I., Mao, W. L. 2020: 532–36

  Abstract

  The halide double perovskite Cs2AgBiBr6 has emerged as a promising nontoxic alternative to the lead halide perovskites APbX3 (A = organic cation or Cs; X = I or Br). Here, we perform high-pressure synchrotron X-ray total scattering on Cs2AgBiBr6 and discover local disorder that is hidden from conventional Bragg analysis. While our powder diffraction data show that the average structure remains cubic up to 2.1 GPa, analysis of the X-ray pair distribution function reveals that the local structure is better described by a monoclinic space group, with significant distortion within the Ag-Br and Bi-Br octahedra and off-centering of the Cs atoms. By tracking the distribution of interatomic Cs-Br distances, we find that the local disorder is enhanced upon compression, and we corroborate these results with molecular dynamics simulations. The observed local disorder affords new understanding of this promising material and potentially offers a new parameter to tune in halide perovskite lattices.

  View details for DOI 10.1021/acs.jpclett.0c03412

  View details for PubMedID 33377386

• Test of the Dynamic-Domain and Critical Scattering Hypotheses in Cubic Methylammonium Lead Triiodide PHYSICAL REVIEW LETTERS Weadock, N. J., Gehring, P. M., Gold-Parker, A., Smith, I. C., Karunadasa, H., Toney, M. F. 2020; 125 (7)

  View details for DOI 10.1103/PhysRevLett.125.075701

  View details for Web of Science ID 000615287000001

• Dimensional reduction of the small-bandgap double perovskite Cs2AgTlBr6 CHEMICAL SCIENCE Connor, B. A., Biega, R., Leppert, L., Karunadasa, H. 2020; 11 (29): 7708–15

  View details for DOI 10.1039/d0sc01580f

  View details for Web of Science ID 000555670200019

• Origins of the Pressure-Induced Phase Transition and Metallization in the Halide Perovskite (CH3NH3)PbI3 ACS ENERGY LETTERS Lee, J., Jaffe, A., Lin, Y., Karunadasa, H., Neaton, J. B. 2020; 5 (7): 2174–81

  View details for DOI 10.1021/acsenergylett.0c00772

  View details for Web of Science ID 000552668000006

• Carrier Diffusion Lengths Exceeding 1 mu m Despite Trap-Limited Transport in Halide Double Perovskites ACS ENERGY LETTERS Delor, M., Slavney, A. H., Wolf, N. R., Filip, M. R., Neaton, J. B., Karunadasa, H., Ginsberg, N. S. 2020; 5 (5): 1337–45

  View details for DOI 10.1021/acsenergylett.0c00414

  View details for Web of Science ID 000535176100001

• Expanded Analogs of Three-Dimensional Lead-Halide Hybrid Perovskites. Angewandte Chemie (International ed. in English) Umeyama, D. n., Leppert, L. n., Connor, B. n., Manumpil, M. A., Neaton, J. n., Karunadasa, H. n. 2020

  Abstract

  Replacing the Pb-X octahedral building unit of A I PbX 3 perovskites (X = halide) with a pair of edge-sharing Pb-X octahedra affords the expanded perovskite analogs: A II Pb 2 X 6 . We report eight members of this new family of materials. In 3D hybrid perovskites, orbitals from the organic molecules do not participate in the band edges. In contrast, the more spacious inorganic sublattice of the expanded analogs accommodates larger pyrizinium-based cations with low-lying π* orbitals that form the conduction band, substantially decreasing the expanded lattice's bandgap. The molecular nature of the conduction band allows us to electronically dope the materials by reducing the organic molecules. By synthesizing derivatives with A II = pyridinium and ammonium, we can isolate the contributions of the pyrazinium-based orbitals in the bandgap transition of A II Pb 2 X 6 . The organic-molecule-based conduction band and the inorganic-ion-based valence band provide an unusual electronic platform with localized states for electrons and more disperse bands for holes upon optical or thermal excitation.

  View details for DOI 10.1002/anie.202005012

  View details for PubMedID 32649785

• Single Ensemble Non-exponential Photoluminescent Population Decays from a Broadband White-Light-Emitting Perovskite. Journal of the American Chemical Society Thomaz, J. E., Lindquist, K. P., Karunadasa, H. I., Fayer, M. D. 2020

  Abstract

  The mechanism of white-light emission from layered Pb-X (X = Cl or Br) perovskites following UV excitation has generated considerable interest. Prior time-dependent studies indicated that the broadband photoluminescence (PL) from (110) perovskites arises from a distribution of self-trapped excitonic sites emitting in different regions of the visible spectrum with different decay dynamics. Here, using time-correlated single photon counting to study single crystals, we show that the white-light emission decay from the (110) perovskite (EDBE)PbBr4 (EDBE = 2,2'-(ethylenedioxy)bis(ethylammonium)) behaves as a single ensemble. Following the rapid decay (0.6 ns) of a small spectral side band, the broad emission line shape is constant to 100 ns. We propose that rapid local structural fluctuations cause the self-trapped excitons (STEs) to experience a wide range of energies, resulting in the very broad PL. The STEs sample fluctuating local environments on time scales fast compared to the PL, which averages the PL decay at all emission wavelengths, yielding single ensemble PL dynamics. Although emission occurs from a very wide, inhomogeneously broadened spectral line with time-averaged single ensemble luminescence dynamics, the decay is tri-exponential. Two heuristic models for the tri-exponential decay involving defects are discussed. Spin-coated films show faster non-exponential decays with the slowest component of the crystal PL absent. Like the crystals, the film PL decays as a single ensemble. These results demonstrate that the broadband emission decay of (EDBE)PbBr4 arises from a time-averaged single ensemble and not from a set of excited states emitting with distinct luminescence decays at different wavelengths.

  View details for DOI 10.1021/jacs.0c05636

  View details for PubMedID 32909430

• A pencil-and-paper method for elucidating halide double perovskite band structures. Chemical science Slavney, A. H., Connor, B. A., Leppert, L., Karunadasa, H. I. 2019; 10 (48): 11041-11053

  Abstract

  Halide double perovskites are an important emerging alternative to lead-halide perovskites in a variety of optoelectronic applications. Compared to ABX3 single perovskites (A = monovalent cation, X = halide), A2BB'X6 double perovskites exhibit a wider array of compositions and electronic structures, promising finer control over physical and electronic properties through synthetic design. However, a clear understanding of how chemical composition dictates the electronic structures of this large family of materials is still lacking. Herein, we develop a qualitative Linear Combination of Atomic Orbitals (LCAO) model that describes the full range of band structures for double perovskites. Our simple model allows for a direct connection between the inherently local bonding between atoms in the double perovskite and the resulting delocalized bands of the solid. In particular, we show how bands in halide double perovskites originate from the molecular orbitals of metal-hexahalide coordination complexes and describe how these molecular orbitals vary within a band. Our results provide both an enhanced understanding of known perovskite compositions and predictive power for identifying new compositions with targeted properties. We present a table, which permits the position of the conduction band minimum and valence band maximum in most double perovskites to be immediately determined from the frontier atomic orbitals of the B-site metals. Using purely qualitative arguments based on orbital symmetries and their relative energies, the direct/indirect nature of the bandgap of almost all halide double perovskites can thus be correctly predicted. We hope that this theory provides an intuitive understanding of halide double perovskite band structures and enables lessons from molecular chemistry to be applied to these extended solids.

  View details for DOI 10.1039/c9sc03219c

  View details for PubMedID 32190254

  View details for PubMedCentralID PMC7066864

• A pencil-and-paper method for elucidating halide double perovskite band structures CHEMICAL SCIENCE Slavney, A. H., Connor, B. A., Leppert, L., Karunadasa, H. 2019; 10 (48): 11041–53

  View details for DOI 10.1039/c9sc03219c

  View details for Web of Science ID 000502319900006

• Tuning the bandgap of Cs2AgBiBr6 through dilute tin alloying CHEMICAL SCIENCE Lindquist, K. P., Mack, S. A., Slavney, A. H., Leppert, L., Gold-Parker, A., Stebbins, J. F., Salleo, A., Toney, M. F., Neaton, J. B., Karunadasa, H. I. 2019; 10 (45): 10620–28

  View details for DOI 10.1039/c9sc02581b

  View details for Web of Science ID 000498611100018

• Halide perovskites under pressure Jaffe, A., Lin, Y., Mao, W., Karunadasa, H. AMER CHEMICAL SOC. 2019

  View details for Web of Science ID 000525061501426

• Halide perovskites and the halogens Slavney, A., Wolf, N., Valdes, A., Karunadasa, H. AMER CHEMICAL SOC. 2019

  View details for Web of Science ID 000525061503637

• Tuning the bandgaps of halide double perovskites Slayney, A., Connor, B., Leppert, L., Neaton, J. B., Karunadasa, H. AMER CHEMICAL SOC. 2019

  View details for Web of Science ID 000525061501502

• Dimensional reduction of halide double perovskites Connor, B., Leppert, L., Smith, M., Neaton, J. B., Karunadasa, H. AMER CHEMICAL SOC. 2019

  View details for Web of Science ID 000525061502012

• Understanding and controlling white-light emission from halide perovskites Smith, M., Connor, B., Crace, E., Lindquist, K., Karunadasa, H. AMER CHEMICAL SOC. 2019

  View details for Web of Science ID 000525061503723

• Molecule-like trap states in halide perovskites: From solar-cell absorbers to white-light emitters Smith, M., Jaffe, A., Lindenberg, A., Karunadasa, H. AMER CHEMICAL SOC. 2019

  View details for Web of Science ID 000478861201710

• Teaching halide double perovskites to absorb sunlight Slavney, A., Connor, B., Leppert, L., Neaton, J. B., Karunadasa, H. AMER CHEMICAL SOC. 2019

  View details for Web of Science ID 000478861201307

• Between the sheets: Post-synthetic transformations in halide perovskites Smith, M., Smith, I., Slavney, A., Valdes, A., Karunadasa, H. AMER CHEMICAL SOC. 2019

  View details for Web of Science ID 000478861201806

• Tuning the Luminescence of Layered Halide Perovskites. Chemical reviews Smith, M. D., Connor, B. A., Karunadasa, H. I. 2019

  Abstract

  Layered halide perovskites offer a versatile platform for manipulating light through synthetic design. Although most layered perovskites absorb strongly in the ultraviolet (UV) or near-UV region, their emission can range from the UV to the infrared region of the electromagnetic spectrum. This emission can be very narrow, displaying high color purity, or it can be extremely broad, spanning the entire visible spectrum and providing high color rendition (or accurately reproducing illuminated colors). The origin of the photoluminescence can vary enormously. Strongly correlated electron-hole pairs, permanent lattice defects, transient light-induced defects, and ligand-field transitions in the inorganic layers and molecular chromophores in the organic layers can be involved in the emission mechanism. In this review, we highlight the different types of photoluminescence that may be attained from layered halide perovskites, with an emphasis on how the emission may be systematically tuned through changes to the bulk crystalline lattice: changes in composition, structure, and dimensionality.

  View details for PubMedID 30689364

• Reactivity of NO2 with Porous and Conductive Copper Azobispyridine Metallopolymers. Inorganic chemistry Clayman, N. E., Manumpil, M. A., Matson, B. D., Wang, S. n., Slavney, A. H., Sarangi, R. n., Karunadasa, H. I., Waymouth, R. M. 2019

  Abstract

  We report the reactivity of copper azobispyridine (abpy) metallopolymers with nitrogen dioxide (NO2). The porous and conductive [Cu(abpy)] n mixed-valence metallopolymers undergo a redox reaction with NO2, resulting in the disproportionation of NO2 gas. Solid- and gas-phase vibrational spectroscopy and X-ray analysis of the reaction products of the NO2-dosed metallopolymer show evidence of nitrate ions and nitric oxide gas. Exposure to NO2 results in complete loss of porosity and a decrease in the room-temperature conductivity of the metallopolymer by four orders of magnitude with the loss of mixed-valence character. Notably, the porous and conductive [Cu(abpy)] n metallopolymers can be reformed by reducing the Cu-nitrate species.

  View details for DOI 10.1021/acs.inorgchem.9b01190

  View details for PubMedID 31364839

• High Compression-Induced Conductivity in a Layered Cu-Br Perovskite. Angewandte Chemie (International ed. in English) Jaffe, A. n., Mack, S. A., Lin, Y. n., Mao, W. n., Neaton, J. B., Karunadasa, H. n. 2019

  Abstract

  We show that the onset pressure for appreciable conductivity in layered copper-halide perovskites can decrease by ca. 50 GPa upon replacement of Cl with Br. Layered Cu-Cl perovskites require pressures >50 GPa to show a conductivity of 10-4 S·cm-1, whereas here a Cu-Br congener, (EA)2CuBr4 (EA = ethylammonium), exhibits conductivity as high as 2 × 10-3 S·cm-1 at only 2.6 GPa, and 0.17 S·cm-1 at 59 GPa. Substitution of higher-energy Br 4p for Cl 3p orbitals lowers the charge-transfer bandgap of the perovskite by 0.9 eV. This 1.7 eV bandgap decreases to 0.3 eV at 65 GPa. High-pressure X-ray diffraction, optical absorption, and transport measurements, and density functional theory calculations allow us to track compression-induced structural and electronic changes. The notable enhancement of the Br perovskite's electronic response to pressure may be attributed to more diffuse Br valence orbitals relative to Cl orbitals. This work brings the compression-induced conductivity of Cu-halide perovskites to more technologically accessible pressures.

  View details for DOI 10.1002/anie.201912575

  View details for PubMedID 31883194

• Acoustic phonon lifetimes limit thermal transport in methylammonium lead iodide. Proceedings of the National Academy of Sciences of the United States of America Gold-Parker, A., Gehring, P. M., Skelton, J. M., Smith, I. C., Parshall, D., Frost, J. M., Karunadasa, H. I., Walsh, A., Toney, M. F. 2018

  Abstract

  Hybrid organic-inorganic perovskites (HOIPs) have become an important class of semiconductors for solar cells and other optoelectronic applications. Electron-phonon coupling plays a critical role in all optoelectronic devices, and although the lattice dynamics and phonon frequencies of HOIPs have been well studied, little attention has been given to phonon lifetimes. We report high-precision momentum-resolved measurements of acoustic phonon lifetimes in the hybrid perovskite methylammonium lead iodide (MAPI), using inelastic neutron spectroscopy to provide high-energy resolution and fully deuterated single crystals to reduce incoherent scattering from hydrogen. Our measurements reveal extremely short lifetimes on the order of picoseconds, corresponding to nanometer mean free paths and demonstrating that acoustic phonons are unable to dissipate heat efficiently. Lattice-dynamics calculations using ab initio third-order perturbation theory indicate that the short lifetimes stem from strong three-phonon interactions and a high density of low-energy optical phonon modes related to the degrees of freedom of the organic cation. Such short lifetimes have significant implications for electron-phonon coupling in MAPI and other HOIPs, with direct impacts on optoelectronic devices both in the cooling of hot carriers and in the transport and recombination of band edge carriers. These findings illustrate a fundamental difference between HOIPs and conventional photovoltaic semiconductors and demonstrate the importance of understanding lattice dynamics in the effort to develop metal halide perovskite optoelectronic devices.

  View details for PubMedID 30401737

• Carving Out Pores in Redox-Active One-Dimensional Coordination Polymers. Angewandte Chemie (International ed. in English) Clayman, N. E., Manumpil, M. A., Umeyama, D., Rudenko, A. E., Karunadasa, H. I., Waymouth, R. M. 2018

  Abstract

  Reduction of the insulating one-dimensional coordination polymer [Cu(abpy)PF6 ]n , 1a(PF6 ), (abpy=2,2'-azobispyridine) yields the conductive, porous polymer [Cu(abpy)]n , 2a. Pressed pellets of neutral 2a exhibit a conductivity of 0.093 Scm-1 at room temperature and a Brunauer-Emmett-Teller (BET) surface area of 56 m2 g-1 . Fine powders of 2a have a BET surface area of 90 m2 g-1 . Cyclic voltammetry shows that the reduction of 1a(PF6 ) to 2a is quasi-reversible, indicative of facile charge transfer through the bulk material. The BET surface area of the reduced polymer 2 can be controlled by changing the size of the counteranion X in the cationic [Cu(abpy)X]n . Reduction of [Cu(abpy)X]n with X=Br (2b) or BArF (2c; BArF =tetrakis(3,5-bis(trifluoromethyl)phenyl)), affords [Cu(abpy)]n polymers with surface areas of 60 and 200 m2 g-1 , respectively.

  View details for PubMedID 30230677

• Structural and electronic correlations in halide perovskites under pressure Jaffe, A., Lin, Y., Mao, W., Karunadasa, H. AMER CHEMICAL SOC. 2018

  View details for Web of Science ID 000447609101166

• Small-Bandgap Halide Double Perovskites. Angewandte Chemie (International ed. in English) Slavney, A. H., Leppert, L., Saldivar Valdes, A., Bartesaghi, D., Savenije, T. J., Neaton, J. B., Karunadasa, H. 2018

  Abstract

  Despite their compositional versatility, most halide double perovskites feature large bandgaps. Herein, we describe a strategy for achieving small bandgaps in this family of materials. The new double perovskites Cs2AgTlX6 (X = Cl (1) and Br (2)) have direct bandgaps of 2.0 and 0.95 eV, respectively, which are ca. 1 eV lower than those of analogous perovskites. To our knowledge, 2 displays the lowest bandgap for any known halide perovskite. Unlike in AIBIIX3 perovskites, the bandgap transition in AI2BB'X6 double perovskites can show substantial metal-to-metal charge-transfer character. We demonstrate how this band-edge orbital composition can be used to achieve small bandgaps through the selection of energetically aligned B- and B'-site metal frontier orbitals. Calculations reveal a shallow, symmetry-forbidden region at the band edges for 1, which results in long (us) microwave conductivity lifetimes. We further describe a facile self-doping reaction in 2 through Br2 loss at ambient conditions.

  View details for PubMedID 30088309

• Dynamically Disordered Lattice in a Layered Pb-I-SCN Perovskite Thin Film Probed by Two-Dimensional Infrared Spectroscopy. Journal of the American Chemical Society Nishida, J., Breen, J. P., Lindquist, K. P., Umeyama, D., Karunadasa, H. I., Fayer, M. D. 2018

  Abstract

  The dynamically flexible lattices in lead halide perovskites may play important roles in extending carrier recombination lifetime in 3D perovskite solar-cell absorbers and in exciton self-trapping in 2D perovskite white-light phosphors. Two-dimensional infrared (2D IR) spectroscopy was applied to study a recently reported Pb-I-SCN layered perovskite. The Pb-I-SCN perovskite was spin-coated on a SiO2 surface as a thin film, with a thickness of 100 nm, where the S12CN- anions were isotopically diluted with the ratio of S12CN:S13CN = 5:95 to avoid vibrational coupling and excitation transfer between adjacent SCN- anions. The 12CN stretch mode of the minor S12CN- component was the principal vibrational probe that reported on the structural evolution through 2D IR spectroscopy. Spectral diffusion was observed with a time constant of 4.1 ± 0.3 ps. Spectral diffusion arises from small structural changes that result in sampling of frequencies within the distribution of frequencies comprising the inhomogeneously broadened infrared absorption band. These transitions among discrete local structures are distinct from oscillatory phonon motions of the lattice. To accurately evaluate the structural dynamics through measurement of spectral diffusion, the vibrational coupling between adjacent SCN- anions had to be carefully treated. Although the inorganic layers of typical 2D perovskites are structurally isolated from each other, the 2D IR data demonstrated that the layers of the Pb-I-SCN perovskite are vibrationally coupled. When both S12CN- and S13CN- were pumped simultaneously, cross-peaks between S12CN and S13CN vibrations and an oscillating 2D band shape of the S12CN- vibration were observed. Both observables demonstrate vibrational coupling between the closest SCN- anions, which reside in different inorganic layers. The thin films and the isotopic dilution produced exceedingly small vibrational echo signal fields; measurements were made possible using the near-Brewster's angle reflection pump-probe geometry.

  View details for PubMedID 30024160

• Layered Halide Double Perovskites: Dimensional Reduction of Cs2AgBiBr6 JOURNAL OF THE AMERICAN CHEMICAL SOCIETY Connor, B. A., Leppert, L., Smith, M. D., Neaton, J. B., Karunadasa, H. I. 2018; 140 (15): 5235–40

  Abstract

  We investigate the consequences of dimensional confinement on halide double perovskites by synthesizing two-dimensional analogues of the recently reported three-dimensional double perovskite Cs2AgBiBr6. The layered perovskites (BA)4AgBiBr8 (1) and (BA)2CsAgBiBr7 (2) (BA = CH3(CH2)3NH3+) feature metal-halide sheets of mono and bilayer thickness, respectively, where the ordered double-perovskite lattice is partitioned by organic cations. Electronic structure calculations indicate that the indirect bandgap of Cs2AgBiBr6 becomes direct when the infinitely thick inorganic lattice is reduced to monolayer thickness. Calculations on model systems allow us to separate the effects of dimensional reduction from those of the accompanying structural distortions in the inorganic sublattice. Detailed optical characterization shows that the photophysical properties of 1 and 2 are markedly different than those of their well-studied lead-halide analogs. Hybrid layered derivatives of double perovskites substantially expand on the substitutional flexibility of halide perovskites to encompass greater compositional and electronic diversity.

  View details for PubMedID 29575889

• Terahertz Emission from Hybrid Perovskites Driven by Ultrafast Charge Separation and Strong Electron-Phonon Coupling ADVANCED MATERIALS Guzelturk, B., Belisle, R. A., Smith, M. D., Bruening, K., Prasanna, R., Yuan, Y., Gopalan, V., Tassone, C. J., Karunadasa, H. I., McGehee, M. D., Lindenberg, A. M. 2018; 30 (11)

  Abstract

  Unusual photophysical properties of organic-inorganic hybrid perovskites have not only enabled exceptional performance in optoelectronic devices, but also led to debates on the nature of charge carriers in these materials. This study makes the first observation of intense terahertz (THz) emission from the hybrid perovskite methylammonium lead iodide (CH3 NH3 PbI3 ) following photoexcitation, enabling an ultrafast probe of charge separation, hot-carrier transport, and carrier-lattice coupling under 1-sun-equivalent illumination conditions. Using this approach, the initial charge separation/transport in the hybrid perovskites is shown to be driven by diffusion and not by surface fields or intrinsic ferroelectricity. Diffusivities of the hot and band-edge carriers along the surface normal direction are calculated by analyzing the emitted THz transients, with direct implications for hot-carrier device applications. Furthermore, photogenerated carriers are found to drive coherent terahertz-frequency lattice distortions, associated with reorganizations of the lead-iodide octahedra as well as coupled vibrations of the organic and inorganic sublattices. This strong and coherent carrier-lattice coupling is resolved on femtosecond timescales and found to be important both for resonant and far-above-gap photoexcitation. This study indicates that ultrafast lattice distortions play a key role in the initial processes associated with charge transport.

  View details for PubMedID 29359820

• Charge Carrier Dynamics in Cs2AgBiBr6 Double Perovskite JOURNAL OF PHYSICAL CHEMISTRY C Bartesaghi, D., Slavney, A. H., Gelvez-Rueda, M. C., Connor, B. A., Grozema, F. C., Karunadasa, H. I., Savenije, T. J. 2018; 122 (9): 4809–16

  Abstract

  Double perovskites, comprising two different cations, are potential nontoxic alternatives to lead halide perovskites. Here, we characterized thin films and crystals of Cs2AgBiBr6 by time-resolved microwave conductance (TRMC), which probes formation and decay of mobile charges upon pulsed irradiation. Optical excitation of films results in the formation of charges with a yield times mobility product, φΣμ > 1 cm2/Vs. On excitation of millimeter-sized crystals, the TRMC signals show, apart from a fast decay, a long-lived tail. Interestingly, this tail is dominant when exciting close to the bandgap, implying the presence of mobile charges with microsecond lifetimes. From the temperature and intensity dependence of the TRMC signals, we deduce a shallow trap state density of around 1016/cm3 in the bulk of the crystal. Despite this high concentration, trap-assisted recombination of charges in the bulk appears to be slow, which is promising for photovoltaic applications.

  View details for PubMedID 29545908

• White-Light Emission from Layered Halide Perovskites ACCOUNTS OF CHEMICAL RESEARCH Smith, M. D., Karunadasa, H. I. 2018; 51 (3): 619–27

  Abstract

  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.

  View details for PubMedID 29461806

• The Diversity of Layered Halide Perovskites ANNUAL REVIEW OF MATERIALS RESEARCH, VOL 48 Smith, M. D., Crace, E. J., Jaffe, A., Karunadasa, H. I., Clarke, D. R. 2018; 48: 111–36

  View details for DOI 10.1146/annurev-matsci-070317-124406

  View details for Web of Science ID 000438011300005

• Electronic Conductivity in a Porous Vanadyl Prussian Blue Analogue upon Air Exposure INORGANIC CHEMISTRY Manumpil, M., Leal-Cervantes, C., Hudson, M. R., Brown, C. M., Karunadasa, H. I. 2017; 56 (21): 12682–86

  Abstract

  Exposure to humid O2 or ambient air affords a 5-order-of-magnitude increase in electronic conductivity of a new Prussian blue analogue incorporating CoII and VIV-oxo units. Oxidation produces a mixed-valence framework, where the O2 exposure time controls the VIV/VV ratio and thereby the material's conductivity. The oxidized framework shows an intervalence charge-transfer band at ca. 4200 cm-1, consistent with mixed valence. The mixed-valence frameworks show semiconducting behavior with conductivity values of 10-5 S·cm-1 at room temperature and 10-4 S·cm-1 at 100 °C and activation energies of ca. 0.3 eV. N2 adsorption measurements at 77 K show that these materials possess permanent porosity before and after oxidation with Brunauer-Emmett-Teller surface areas of 340 and 370 m2·g-1, respectively.

  View details for PubMedID 29058412

• Broadband Emission with a Massive Stokes Shift from Sulfonium Pb-Br Hybrids CHEMISTRY OF MATERIALS Smith, M. D., Watson, B. L., Dauskardt, R. H., Karunadasa, H. I. 2017; 29 (17): 7083–87

  View details for DOI 10.1021/acs.chemmater.7b02594

  View details for Web of Science ID 000410868600006

• Structural origins of broadband emission from layered Pb-Br hybrid perovskites. Chemical science Smith, M. D., Jaffe, A., Dohner, E. R., Lindenberg, A. M., Karunadasa, H. I. 2017; 8 (6): 4497-4504

  Abstract

  Through structural and optical studies of a series of two-dimensional hybrid perovskites, we show that broadband emission upon near-ultraviolet excitation is common to (001) lead-bromide perovskites. Importantly, we find that the relative intensity of the broad emission correlates with increasing out-of-plane distortion of the Pb-(μ-Br)-Pb angle in the inorganic sheets. Temperature- and power-dependent photoluminescence data obtained on a representative (001) perovskite support an intrinsic origin to the broad emission from the bulk material, where photogenerated carriers cause excited-state lattice distortions mediated through electron-lattice coupling. In contrast, most inorganic phosphors contain extrinsic emissive dopants or emissive surface sites. The design rules established here could allow us to systematically optimize white-light emission from layered hybrid perovskites by fine-tuning the bulk crystal structure.

  View details for DOI 10.1039/c7sc01590a

  View details for PubMedID 28970879

  View details for PubMedCentralID PMC5618335

• Pressure-Induced Metallization of the Halide Perovskite (CH3NH3)PbI3 JOURNAL OF THE AMERICAN CHEMICAL SOCIETY Jaffe, A., Lin, Y., Mao, W. L., Karunadasa, H. I. 2017; 139 (12): 4330-4333

  View details for DOI 10.1021/jacs.7b01162

  View details for Web of Science ID 000398247100024

• Between the Sheets: Postsynthetic Transformations in Hybrid Perovskites CHEMISTRY OF MATERIALS Smith, I. C., Smith, M. D., Jaffe, A., Lin, Y., Karunadasa, H. I. 2017; 29 (5): 1868-1884

  View details for DOI 10.1021/acs.chemmater.6b05395

  View details for Web of Science ID 000396639400002

• Chemical Approaches to Addressing the Instability and Toxicity of Lead-Halide Perovskite Absorbers INORGANIC CHEMISTRY Slayney, A. H., Smaha, R. W., Smith, I. C., Jaffe, A., Umeyama, D., Karunadasa, H. I. 2017; 56 (1): 46-55

  Abstract

  The impressive rise in efficiencies of solar cells employing the three-dimensional (3D) lead-iodide perovskite absorbers APbI3 (A = monovalent cation) has generated intense excitement. Although these perovskites have remarkable properties as solar-cell absorbers, their potential commercialization now requires a greater focus on the materials' inherent shortcomings and environmental impact. This creates a challenge and an opportunity for synthetic chemists to address these issues through the design of new materials. Synthetic chemistry offers powerful tools for manipulating the magnificent flexibility of the perovskite lattice to expand the number of functional analogues to APbI3. To highlight improvements that should be targeted in new materials, here we discuss the intrinsic instability and toxicity of 3D lead-halide perovskites. We consider possible sources of these instabilities and propose methods to overcome them through synthetic design. We also discuss new materials developed for realizing the exceptional photophysical properties of lead-halide perovskites in more environmentally benign materials. In this Forum Article, we provide a brief overview of the field with a focus on our group's contributions to identifying and addressing problems inherent to 3D lead-halide perovskites.

  View details for DOI 10.1021/acs.inorgchem.6b01336

  View details for Web of Science ID 000391248900007

  View details for PubMedID 27494338

• Defect-Induced Band-Edge Reconstruction of a Bismuth-Halide Double Perovskite for Visible-Light Absorption Journal of the American Chemical Society Slavney, A. H., Leppart, L., Bartesaghi, D., Gold-Parker, A., Toney, M. F., Savenije, T. J., Neaton, J. B., Karunadasa, H. I. 2017; 139: 5015

  View details for DOI 10.1021/jacs.7b01629

• Light-induced picosecond rotational disordering of the inorganic sublattice in hybrid perovskites. Science advances Wu, X. n., Tan, L. Z., Shen, X. n., Hu, T. n., Miyata, K. n., Trinh, M. T., Li, R. n., Coffee, R. n., Liu, S. n., Egger, D. A., Makasyuk, I. n., Zheng, Q. n., Fry, A. n., Robinson, J. S., Smith, M. D., Guzelturk, B. n., Karunadasa, H. I., Wang, X. n., Zhu, X. n., Kronik, L. n., Rappe, A. M., Lindenberg, A. M. 2017; 3 (7): e1602388

  Abstract

  Femtosecond resolution electron scattering techniques are applied to resolve the first atomic-scale steps following absorption of a photon in the prototypical hybrid perovskite methylammonium lead iodide. Following above-gap photoexcitation, we directly resolve the transfer of energy from hot carriers to the lattice by recording changes in the mean square atomic displacements on 10-ps time scales. Measurements of the time-dependent pair distribution function show an unexpected broadening of the iodine-iodine correlation function while preserving the Pb-I distance. This indicates the formation of a rotationally disordered halide octahedral structure developing on picosecond time scales. This work shows the important role of light-induced structural deformations within the inorganic sublattice in elucidating the unique optoelectronic functionality exhibited by hybrid perovskites and provides new understanding of hot carrier-lattice interactions, which fundamentally determine solar cell efficiencies.

  View details for PubMedID 28782016

• Decreasing the electronic confinement in layered perovskites through intercalation CHEMICAL SCIENCE Smith, M. D., Pedesseau, L., Kepenekian, M., Smith, I. C., Katan, C., Even, J., Karunadasa, H. I. 2017; 8 (3): 1960-1968

  Abstract

  We show that post-synthetic small-molecule intercalation can significantly reduce the electronic confinement of 2D hybrid perovskites. Using a combined experimental and theoretical approach, we explain structural, optical, and electronic effects of intercalating highly polarizable molecules in layered perovskites designed to stabilize the intercalants. Polarizable molecules in the organic layers substantially alter the optical and electronic properties of the inorganic layers. By calculating the spatially resolved dielectric profiles of the organic and inorganic layers within the hybrid structure, we show that the intercalants afford organic layers that are more polarizable than the inorganic layers. This strategy reduces the confinement of excitons generated in the inorganic layers and affords the lowest exciton binding energy for an n = 1 perovskite of which we are aware. We also demonstrate a method for computationally evaluating the exciton's binding energy by solving the Bethe-Salpeter equation for the exciton, which includes an ab initio determination of the material's dielectric profile across organic and inorganic layers. This new semi-empirical method goes beyond the imprecise phenomenological approximation of abrupt dielectric-constant changes at the organic-inorganic interfaces. This work shows that incorporation of polarizable molecules in the organic layers, through intercalation or covalent attachment, is a viable strategy for tuning 2D perovskites towards mimicking the reduced electronic confinement and isotropic light absorption of 3D perovskites while maintaining the greater synthetic tunability of the layered architecture.

  View details for DOI 10.1039/c6sc02848a

  View details for Web of Science ID 000395906900032

  View details for PubMedID 28451311

• Between the sheets: Post-synthetic transformations in hybrid perovskites Chemistry of Materials Smith, I. C., Smith, M. D., Jaffe, A., Lin, Y., Karunadasa, H. I. 2017; 29: 1868

  View details for DOI 10.1021/acs.chemmater.6b05395

• Structural origins of broadband emission from layered Pb–Br hybrid perovskites Chemical Science Smith, M. D., Jaffe, A., Dohner, E. R., Lindenberg, A. M., Karunadasa, H. I. 2017: 4497-4504

  Abstract

  Through structural and optical studies of a series of two-dimensional hybrid perovskites, we show that broadband emission upon near-ultraviolet excitation is common to (001) lead-bromide perovskites. Importantly, we find that the relative intensity of the broad emission correlates with increasing out-of-plane distortion of the Pb-(μ-Br)-Pb angle in the inorganic sheets. Temperature- and power-dependent photoluminescence data obtained on a representative (001) perovskite support an intrinsic origin to the broad emission from the bulk material, where photogenerated carriers cause excited-state lattice distortions mediated through electron-lattice coupling. In contrast, most inorganic phosphors contain extrinsic emissive dopants or emissive surface sites. The design rules established here could allow us to systematically optimize white-light emission from layered hybrid perovskites by fine-tuning the bulk crystal structure.

  View details for DOI 10.1039/C7SC01590A

  View details for PubMedCentralID PMC5618335

• Pressure-Induced Metallization of the Halide Perovskite (CH3NH3)PbI3 Journal of the American Chemical Society Jaffe, A., Lin, Y., Mao, W. L., Karunadasa, H. I. 2017; 139: 4330

  View details for DOI 10.1021/jacs.7b01162

• Light-Induced Phase Segregation in Halide-Perovskite Absorbers ACS ENERGY LETTERS Slotcavage, D. J., Karunadasa, H. I., McGehee, M. D. 2016; 1 (6): 1199-1205

  View details for DOI 10.1021/acsenergylett.6b00495

  View details for Web of Science ID 000390086400021

• Mechanism for Broadband White-Light Emission from Two-Dimensional (110) Hybrid Perovskites JOURNAL OF PHYSICAL CHEMISTRY LETTERS Hu, T., Smith, M. D., Dohner, E. R., Sher, M., Wu, X., Tuan Trinh, M., Fisher, A., Corbett, J., Zhu, X., Karunadasa, H. I., Lindenberg, A. M. 2016; 7 (12): 2258-2263

  Abstract

  The recently discovered phenomenon of broadband white-light emission at room temperature in the (110) two-dimensional organic-inorganic perovskite (N-MEDA)[PbBr4] (N-MEDA = N(1)-methylethane-1,2-diammonium) is promising for applications in solid-state lighting. However, the spectral broadening mechanism and, in particular, the processes and dynamics associated with the emissive species are still unclear. Herein, we apply a suite of ultrafast spectroscopic probes to measure the primary events directly following photoexcitation, which allows us to resolve the evolution of light-induced emissive states associated with white-light emission at femtosecond resolution. Terahertz spectra show fast free carrier trapping and transient absorption spectra show the formation of self-trapped excitons on femtosecond time-scales. Emission-wavelength-dependent dynamics of the self-trapped exciton luminescence are observed, indicative of an energy distribution of photogenerated emissive states in the perovskite. Our results are consistent with photogenerated carriers self-trapped in a deformable lattice due to strong electron-phonon coupling, where permanent lattice defects and correlated self-trapped states lend further inhomogeneity to the excited-state potential energy surface.

  View details for DOI 10.1021/acs.jpclett.6b00793

  View details for Web of Science ID 000378196000017

  View details for PubMedID 27246299

• Red-to-Black Piezochromism in a Compressible Pb-l-SCN Layered Perovskite CHEMISTRY OF MATERIALS Umeyama, D., Lin, Y., Karunadasa, H. I. 2016; 28 (10): 3241-3244

  View details for DOI 10.1021/acs.chemmater.6b01147

  View details for Web of Science ID 000376825700003

• A Bismuth-Halide Double Perovskite with Long Carrier Recombination Lifetime for Photovoltaic Applications. Journal of the American Chemical Society Slavney, A. H., Hu, T., Lindenberg, A. M., Karunadasa, H. I. 2016; 138 (7): 2138-2141

  Abstract

  Despite the remarkable rise in efficiencies of solar cells containing the lead-halide perovskite absorbers RPbX3 (R = organic cation; X = Br(-) or I(-)), the toxicity of lead remains a concern for the large-scale implementation of this technology. This has spurred the search for lead-free materials with similar optoelectronic properties. Here, we use the double-perovskite structure to incorporate nontoxic Bi(3+) into the perovskite lattice in Cs2AgBiBr6 (1). The solid shows a long room-temperature fundamental photoluminescence (PL) lifetime of ca. 660 ns, which is very encouraging for photovoltaic applications. Comparison between single-crystal and powder PL decay curves of 1 suggests inherently high defect tolerance. The material has an indirect bandgap of 1.95 eV, suited for a tandem solar cell. Furthermore, 1 is significantly more heat and moisture stable compared to (MA)PbI3. The extremely promising optical and physical properties of 1 shown here motivate further exploration of both inorganic and hybrid halide double perovskites for photovoltaics and other optoelectronics.

  View details for DOI 10.1021/jacs.5b13294

  View details for PubMedID 26853379

• Chemical approaches to addressing the instability and toxicity of lead-halide perovskite absorbers Inorganic Chemistry Slavney, A. H., Smaha, R. W., Smith, I. C., Jaffe, A., Umeyama, D., Karunadasa, H. I. 2016

  View details for DOI 10.1021/acs.inorgchem.6b01336

• High-pressure single-crystal structures of 3D lead-halide hybrid perovskites and pressure effects on their electronic and optical properties ACS Cent. Sci Jaffe, A., Lin, Y., Beavers, C. M., Voss, J., Mao, W. L., Karunadasa, H. I. 2016; 2: 201

  Abstract

  We report the first high-pressure single-crystal structures of hybrid perovskites. The crystalline semiconductors (MA)PbX3 (MA = CH3NH3 (+), X = Br(-) or I(-)) afford us the rare opportunity of understanding how compression modulates their structures and thereby their optoelectronic properties. Using atomic coordinates obtained from high-pressure single-crystal X-ray diffraction we track the perovskites' precise structural evolution upon compression. These structural changes correlate well with pressure-dependent single-crystal photoluminescence (PL) spectra and high-pressure bandgaps derived from density functional theory. We further observe dramatic piezochromism where the solids become lighter in color and then transition to opaque black with compression. Indeed, electronic conductivity measurements of (MA)PbI3 obtained within a diamond-anvil cell show that the material's resistivity decreases by 3 orders of magnitude between 0 and 51 GPa. The activation energy for conduction at 51 GPa is only 13.2(3) meV, suggesting that the perovskite is approaching a metallic state. Furthermore, the pressure response of mixed-halide perovskites shows new luminescent states that emerge at elevated pressures. We recently reported that the perovskites (MA)Pb(Br x I1-x )3 (0.2 < x < 1) reversibly form light-induced trap states, which pin their PL to a low energy. This may explain the low voltages obtained from solar cells employing these absorbers. Our high-pressure PL data indicate that compression can mitigate this PL redshift and may afford higher steady-state voltages from these absorbers. These studies show that pressure can significantly alter the transport and thermodynamic properties of these technologically important semiconductors.

  View details for DOI 10.1021/acscentsci.6b00055

  View details for PubMedCentralID PMC4850512

• Quinone-Functionalized Carbon Black Cathodes for Lithium Batteries with High Power Densities CHEMISTRY OF MATERIALS Jaffe, A., Valdes, A. S., Karunadasa, H. I. 2015; 27 (10): 3568-3571

  View details for DOI 10.1021/acs.chemmater.5b00990

  View details for Web of Science ID 000355382700005

• Pressure-Induced Conductivity and Yellow-to-Black Piezochromism in a Layered Cu-Cl Hybrid Perovskite. Journal of the American Chemical Society Jaffe, A., Lin, Y., Mao, W. L., Karunadasa, H. I. 2015; 137 (4): 1673-1678

  Abstract

  Pressure-induced changes in the electronic structure of two-dimensional Cu-based materials have been a subject of intense study. In particular, the possibility of suppressing the Jahn-Teller distortion of d(9) Cu centers with applied pressure has been debated over a number of decades. We studied the structural and electronic changes resulting from the application of pressures up to ca. 60 GPa on a two-dimensional copper(II)-chloride perovskite using diamond anvil cells (DACs), through a combination of in situ powder X-ray diffraction, electronic absorption and vibrational spectroscopy, dc resistivity measurements, and optical observations. Our measurements show that compression of this charge-transfer insulator initially yields a first-order structural phase transition at ca. 4 GPa similar to previous reports on other Cu(II)-Cl perovskites, during which the originally translucent yellow solid turns red. Further compression induces a previously unreported phase transition at ca. 8 GPa and dramatic piezochromism from translucent red-orange to opaque black. Two-probe dc resistivity measurements conducted within the DAC show the first instance of appreciable conductivity in Cu(II)-Cl perovskites. The conductivity increases by 5 orders of magnitude between 7 and 50 GPa, with a maximum measured conductivity of 2.9 × 10(-4) S·cm(-1) at 51.4 GPa. Electronic absorption spectroscopy and variable-temperature conductivity measurements indicate that the perovskite behaves as a 1.0 eV band-gap semiconductor at 39.7 GPa and has an activation energy for electronic conduction of 0.232(1) eV at 40.2 GPa. Remarkably, all these changes are reversible: the material reverts to a translucent yellow solid upon decompression, and ambient pressure powder X-ray diffraction data taken before and after compression up to 60 GPa show that the original structure is maintained with minimal hysteresis.

  View details for DOI 10.1021/ja512396m

  View details for PubMedID 25580620

• Reversible photo-induced trap formation in mixed-halide hybrid perovskites for photovoltaics CHEMICAL SCIENCE Hoke, E. T., Slotcavage, D. J., Dohner, E. R., Bowring, A. R., Karunadasa, H. I., McGehee, M. D. 2015; 6 (1): 613-617

  View details for DOI 10.1039/c4sc03141e

  View details for Web of Science ID 000345901600072

• Reversible photo-induced trap formation in mixed-halide hybrid perovskites for photovoltaics. Chemical science Hoke, E. T., Slotcavage, D. J., Dohner, E. R., Bowring, A. R., Karunadasa, H. I., McGehee, M. D. 2015; 6 (1): 613-617

  Abstract

  We report on reversible, light-induced transformations in (CH3NH3)Pb(Br x I1-x )3. Photoluminescence (PL) spectra of these perovskites develop a new, red-shifted peak at 1.68 eV that grows in intensity under constant, 1-sun illumination in less than a minute. This is accompanied by an increase in sub-bandgap absorption at ∼1.7 eV, indicating the formation of luminescent trap states. Light soaking causes a splitting of X-ray diffraction (XRD) peaks, suggesting segregation into two crystalline phases. Surprisingly, these photo-induced changes are fully reversible; the XRD patterns and the PL and absorption spectra revert to their initial states after the materials are left for a few minutes in the dark. We speculate that photoexcitation may cause halide segregation into iodide-rich minority and bromide-enriched majority domains, the former acting as a recombination center trap. This instability may limit achievable voltages from some mixed-halide perovskite solar cells and could have implications for the photostability of halide perovskites used in optoelectronics.

  View details for DOI 10.1039/c4sc03141e

  View details for PubMedID 28706629

  View details for PubMedCentralID PMC5491962

• CH3NH3PbI3 perovskite single crystals: surface photophysics and their interaction with the environment CHEMICAL SCIENCE Grancini, G., D'Innocenzo, V., DOHNER, E. R., Martino, N., Kandada, A. R., Mosconi, E., De Angelis, F., Karunadasa, H. I., Hoke, E. T., Petrozza, A. 2015; 6 (12): 7305-7310

  View details for DOI 10.1039/c5sc02542g

  View details for Web of Science ID 000365225300074

• Post-synthetic halide conversion and selective halogen capture in hybrid perovskites CHEMICAL SCIENCE Solis-Ibarra, D., Smith, I. C., Karunadasa, H. I. 2015; 6 (7): 4054-4059

  View details for DOI 10.1039/c5sc01135c

  View details for Web of Science ID 000356176200048

• A Layered Hybrid Perovskite Solar-Cell Absorber with Enhanced Moisture Stability ANGEWANDTE CHEMIE-INTERNATIONAL EDITION Smith, I. C., Hoke, E. T., Solis-Ibarra, D., McGehee, M. D., Karunadasa, H. I. 2014; 53 (42): 11232-11235

  View details for DOI 10.1002/anie.201406466

  View details for Web of Science ID 000343751100020

• A layered hybrid perovskite solar-cell absorber with enhanced moisture stability. Angewandte Chemie (International ed. in English) Smith, I. C., Hoke, E. T., Solis-Ibarra, D., McGehee, M. D., Karunadasa, H. I. 2014; 53 (42): 11232-11235

  Abstract

  Two-dimensional hybrid perovskites are used as absorbers in solar cells. Our first-generation devices containing (PEA)2(MA)2[Pb3I10] (1; PEA=C6H5(CH2)2NH3(+), MA=CH3NH3(+)) show an open-circuit voltage of 1.18 V and a power conversion efficiency of 4.73%. The layered structure allows for high-quality films to be deposited through spin coating and high-temperature annealing is not required for device fabrication. The 3D perovskite (MA)[PbI3] (2) has recently been identified as a promising absorber for solar cells. However, its instability to moisture requires anhydrous processing and operating conditions. Films of 1 are more moisture resistant than films of 2 and devices containing 1 can be fabricated under ambient humidity levels. The larger bandgap of the 2D structure is also suitable as the higher bandgap absorber in a dual-absorber tandem device. Compared to 2, the layered perovskite structure may offer greater tunability at the molecular level for material optimization.

  View details for DOI 10.1002/anie.201406466

  View details for PubMedID 25196933

• Intrinsic White-Light Emission from Layered Hybrid Perovskites JOURNAL OF THE AMERICAN CHEMICAL SOCIETY Dohner, E. R., Jaffe, A., Bradshaw, L. R., Karunadasa, H. I. 2014; 136 (38): 13154-13157

  View details for DOI 10.1021/ja507086b

  View details for Web of Science ID 000342328200021

• Intrinsic white-light emission from layered hybrid perovskites. Journal of the American Chemical Society Dohner, E. R., Jaffe, A., Bradshaw, L. R., Karunadasa, H. I. 2014; 136 (38): 13154-13157

  Abstract

  We report on the second family of layered perovskite white-light emitters with improved photoluminescence quantum efficiencies (PLQEs). Upon near-ultraviolet excitation, two new Pb-Cl and Pb-Br perovskites emit broadband "cold" and "warm" white light, respectively, with high color rendition. Emission from large, single crystals indicates an origin from the bulk material and not surface defect sites. The Pb-Br perovskite has a PLQE of 9%, which is undiminished after 3 months of continuous irradiation. Our mechanistic studies indicate that the emission has contributions from strong electron-phonon coupling in a deformable lattice and from a distribution of intrinsic trap states. These hybrids provide a tunable platform for combining the facile processability of organic materials with the structural definition of crystalline, inorganic solids.

  View details for DOI 10.1021/ja507086b

  View details for PubMedID 25162937

• Lithium cycling in a self-assembled copper chloride-polyether hybrid electrode. Inorganic chemistry Jaffe, A., Karunadasa, H. I. 2014; 53 (13): 6494-6496

  Abstract

  Atomic-scale integration of polyether molecules and copper(II) chloride layers in a two-dimensional perovskite affords, to the best of our knowledge, the first example of extended Li(+) cycling in a metal chloride electrode. The hybrid can cycle over 200 times as a cathode in a lithium battery with an open-circuit voltage of 3.2 V. In contrast, CuCl2 alone or the precursors to the hybrid cannot be cycled in a lithium battery, demonstrating the importance of the layered, organic-inorganic architecture. This work shows that appropriate organic groups can enable Li(+) cycling in inexpensive, nontoxic, metal halide electrodes, which is promising for large-scale applications.

  View details for DOI 10.1021/ic500860t

  View details for PubMedID 24917248

• Self-Assembly of Broadband White-Light Emitters JOURNAL OF THE AMERICAN CHEMICAL SOCIETY Dohner, E. R., Hoke, E. T., Karunadasa, H. I. 2014; 136 (5): 1718-1721

  Abstract

  We use organic cations to template the solution-state assembly of corrugated lead halide layers in bulk crystalline materials. These layered hybrids emit radiation across the entire visible spectrum upon ultraviolet excitation. They are promising as single-source white-light phosphors for use with ultraviolet light-emitting diodes in solid-state lighting devices. The broadband emission provides high color rendition and the chromaticity coordinates of the emission can be tuned through halide substitution. We have isolated materials that emit the "warm" white light sought for many indoor lighting applications as well as "cold" white light that approximates the visible region of the solar spectrum. Material syntheses are inexpensive and scalable and binding agents are not required for film deposition, eliminating problems of binder photodegradation. These well-defined and tunable structures provide a flexible platform for studying the rare phenomenon of intrinsic broadband emission from bulk materials.

  View details for DOI 10.1021/ja411045r

  View details for Web of Science ID 000331493700010

  View details for PubMedID 24422494

• Reversible and Irreversible Chemisorption in Nonporous-Crystalline Hybrids ANGEWANDTE CHEMIE-INTERNATIONAL EDITION Solis-Ibarra, D., Karunadasa, H. I. 2014; 53 (4): 1039-1042

  Abstract

  The tools of synthetic chemistry allow us to fine-tune the reactivity of molecules at a level of precision not yet accessible with inorganic solids. We have investigated hybrids that couple molecules to the superior thermal and mechanical properties of solids. Herein we present, to the best of our knowledge, the first demonstration of reactivity between hybrid perovskites and substrates. Reaction with iodine vapor results in a remarkable expansion of these materials (up to 36 % in volume) where new covalent CI bonds are formed with retention of crystallinity. These hybrids also show unusual examples of reversible chemisorption. Here, solid-state interactions extend the lifetime of molecules that cannot be isolated in solution. We have tuned the half-lives of the iodinated structures from 3 h to 3 days. These nonporous hybrids drive substrate capture and controlled release through chemical reactivity. We illustrate the strengths of the hybrid by considering radioactive iodine capture.

  View details for DOI 10.1002/anie.201309786

  View details for Web of Science ID 000329879500022

  View details for PubMedID 24311056

• Low-Spin Hexacoordinate Mn(III): Synthesis and Spectroscopic Investigation of Homoleptic Tris(pyrazolyl)borate and Tris(carbene)borate Complexes INORGANIC CHEMISTRY Forshaw, A. P., Smith, J. M., Ozarowski, A., Krzystek, J., Smirnov, D., Zvyagin, S. A., Harris, T. D., Karunadasa, H. I., Zadrozny, J. M., Schnegg, A., Holldack, K., Jackson, T. A., Alamiri, A., Barnes, D. M., Telser, J. 2013; 52 (1): 144-159

  Abstract

  Three complexes of Mn(III) with "scorpionate" type ligands have been investigated by a variety of physical techniques. The complexes are [Tp(2)Mn]SbF(6) (1), [Tp(2)*Mn]SbF(6) (2), and [{PhB(MeIm)(3)}(2)Mn](CF(3)SO(3)) (3a), where Tp(-) = hydrotris(pyrazolyl)borate anion, Tp*(-) = hydrotris(3,5-dimethylpyrazolyl)borate anion, and PhB(MeIm)(3)(-) = phenyltris(3-methylimidazol-2-yl)borate anion. The crystal structure of 3a is reported; the structures of 1 and 2 have been previously reported, but were reconfirmed in this work. The synthesis and characterization of [{PhB(MeIm)(3)}(2)Mn]Cl (3b) are also described. These complexes are of interest in that, in contrast to many hexacoordinate (pseudo-octahedral) complexes of Mn(III), they exhibit a low-spin (triplet) ground state, rather than the high-spin (quintet) ground state. Solid-state electronic absorption spectroscopy, SQUID magnetometry, and high-frequency and -field electron paramagnetic resonance (HFEPR) spectroscopy were applied. HFEPR, in particular, was useful in characterizing the S = 1 spin Hamiltonian parameters for complex 1, D = +19.97(1), E = 0.42(2) cm(-1), and for 2, D = +15.89(2), E = 0.04(1) cm(-1). In addition, frequency domain Fourier-transform THz-EPR spectroscopy, using coherent synchrotron radiation, was applied to 1 only and gave results in good agreement with HFEPR. Variable-temperature dc magnetic susceptibility measurements of 1 and 2 were also in good agreement with the HFEPR results. This magnitude of zero-field splitting (zfs) is over 4 times larger than that in comparable hexacoordinate Mn(III) systems with S = 2 ground states. Complexes 3a and 3b (i.e., regardless of counteranion) have a yet much larger magnitude zfs, which may be the result of unquenched orbital angular momentum so that the spin Hamiltonian model is not appropriate. The triplet ground state is rationalized in each complex by ligand-field theory (LFT) and by quantum chemistry theory, both density functional theory and unrestricted Hartree-Fock methods. This analysis also shows that spin-crossover behavior is not thermally accessible for these complexes as solids. The donor properties of the three different scorpionate ligands were further characterized using the LFT model that suggests that the tris(carbene)borate is a strong σ-donor with little or no π-bonding.

  View details for DOI 10.1021/ic301630d

  View details for Web of Science ID 000313220500019

  View details for PubMedID 23259486

• Electrochemical generation of hydrogen from acetic acid using a molecular molybdenum-oxo catalyst ENERGY & ENVIRONMENTAL SCIENCE Thoi, V. S., Karunadasa, H. I., Surendranath, Y., Long, J. R., Chang, C. J. 2012; 5 (7): 7762-7770

  View details for DOI 10.1039/c2ee21519e

  View details for Web of Science ID 000305530900010

• A molecular MoS2 edge site for catalytic hydrogen production Science Karunadasa, H. I., Montalvo, E., Sun, Y., Majda, M., Majda, J. R., Chang, C. J. 2012; 335 (698)
• A computational and experimental study of the mechanism of hydrogen generation from water by a molecular molybdenum-oxo electro catalyst J. Am. Chem. Soc Sundstrom, E. J., Yang, X., Thoi, V. S., Karunadasa, H. I., Chang, C. J., Long, J. R., Head-Gordon, M. 2012; 134 (5233)
• A molecular molybdenum-oxo catalyst for generating hydrogen from water NATURE Karunadasa, H. I., Chang, C. J., Long, J. R. 2010; 464 (7293): 1329-1333

  Abstract

  A growing awareness of issues related to anthropogenic climate change and an increase in global energy demand have made the search for viable carbon-neutral sources of renewable energy one of the most important challenges in science today. The chemical community is therefore seeking efficient and inexpensive catalysts that can produce large quantities of hydrogen gas from water. Here we identify a molybdenum-oxo complex that can catalytically generate gaseous hydrogen either from water at neutral pH or from sea water. This work shows that high-valency metal-oxo species can be used to create reduction catalysts that are robust and functional in water, a concept that has broad implications for the design of 'green' and sustainable chemistry cycles.

  View details for DOI 10.1038/nature08969

  View details for Web of Science ID 000277149000042

  View details for PubMedID 20428167

• Magnetic properties of Ba2HoSbO6 with a frustrated lattice geometry PHYSICAL REVIEW B Calder, S., Ke, X., Bert, F., Amato, A., Baines, C., Carboni, C., Cava, R. J., Daoud-Aladine, A., Deen, P., Fennell, T., Hillier, A. D., Karunadasa, H., Taylor, J. W., Mendels, P., Schiffer, P., Bramwell, S. T. 2010; 81 (6)

  View details for DOI 10.1103/PhysRevB.81.064425

  View details for Web of Science ID 000274998100076

• Enhancing the magnetic anisotropy of cyano-ligated Cr(II) and Cr(III) complexes via heavy-halide ligand effects Inorg. Chem. Karudanasa, H. I., Arquero, K. D., Berben, L. A., Long, J. R. 2010; 49 (4738)
• Synthesis and Redox-Induced Structural Isomerization of the Pentagonal Bipyramidal Complexes [W(CN)(5)(CO)(2)](3-) and [W(CN)(5)(CO)(2)](2-) ANGEWANDTE CHEMIE-INTERNATIONAL EDITION Karunadasa, H. I., Long, J. R. 2009; 48 (4): 738-741

  View details for DOI 10.1002/anie.200804199

  View details for Web of Science ID 000262676000010

  View details for PubMedID 19072955

• Honeycombs of triangles and magnetic frustration in SrLn2O4 (Ln = Gd, Dy, Ho, Er, Tm, and Yb) Phys. Rev. B Karudanasa, H. I., Huang, Q., Ueland, B. G., Schiffer, P., Cava, R. J. 2005; 71 (144414)
• Quantum and thermal spin relaxation in the diluted spin ice Dy2-xMxTi2O7 (M=Lu,Y) PHYSICAL REVIEW B Snyder, J., Ueland, B. G., Mizel, A., Slusky, J. S., Karunadasa, H., Cava, R. J., Schiffer, P. 2004; 70 (18)

  View details for DOI 10.1103/PhysRevB.70.184431

  View details for Web of Science ID 000225477300083

• 2,2 '-dibromo-3,3 ',4,4 ',5,5 ',6,6 '-octamethyl-1,1 '-biphenyl ACTA CRYSTALLOGRAPHICA SECTION E-STRUCTURE REPORTS ONLINE Karunadasa, H., Leggett, C., Wong, S. 2004; 60: O1499-O1500

  View details for DOI 10.1107/S160053680401709X

  View details for Web of Science ID 000223624500082

• Low temperature spin freezing in Dy2Ti2O7 spin ice Phys. Rev. B Snyder, J., Ueland, B. G., Slusky, J. S., Karunadasa, H. I., Cava, R. J., Schiffer, P. 2004; 69 (064414)
• Quantum-classical reentrant relaxation crossover in DY2Ti2O7 spin ice PHYSICAL REVIEW LETTERS Snyder, J., Ueland, B. G., Slusky, J. S., Karunadasa, H., Cava, R. J., Mizel, A., Schiffer, P. 2003; 91 (10)

  Abstract

  We have studied spin relaxation in the spin ice compound Dy2Ti2O7 through measurements of the ac magnetic susceptibility. While the characteristic spin-relaxation time (tau) is thermally activated at high temperatures, it becomes almost temperature independent below T(cross) approximately 13 K. This behavior, combined with nonmonotonic magnetic field dependence of tau, indicates that quantum tunneling dominates the relaxational process below that temperature. As the low-entropy spin ice state develops below T(ice) approximately 4 K, tau increases sharply with decreasing temperature, suggesting the emergence of a collective degree of freedom for which thermal relaxation processes again become important as the spins become strongly correlated.

  View details for DOI 10.1103/PhysRevLett.91.107201

  View details for Web of Science ID 000185485700035

  View details for PubMedID 14525500

• Ba2LnSbO6 and Sr2LnSbO6 (Ln = Dy, Ho, Gd) double perovskites: lanthanides in the geometrically frustrating fcc lattice Proc. Natl. Acad. Sci. Karunadasa, H. I., Huang, Q., Ueland, B. G., Schiffer, P., Cava, R. J., 2000; 100: 8097