Bulletin of the American Physical Society
APS March Meeting 2019
Volume 64, Number 2
Monday–Friday, March 4–8, 2019; Boston, Massachusetts
Session V20: Hybrid Perovskites -- Optical, Structural Properties and more |
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Sponsoring Units: DMP Chair: Laura Herz, University of Oxford Room: BCEC 157A |
Thursday, March 7, 2019 2:30PM - 2:42PM |
V20.00001: First-principles evidence of strong radiative recombination in hybrid perovskites Xie Zhang, Jimmy Shen, Wennie Wang, Chris Van de Walle Understanding the origin of the high solar conversion efficiency of hybrid perovskites is one key research focus in the field. A number of research groups attributed the high efficiency to low radiative recombination due to strong Rashba spin-orbit coupling. In this work, we perform first-principles calculations to explicitly compute the radiative recombination coefficient in the prototypical hybrid perovskite, CH3NH3PbI3. We demonstrate that the radiative recombination in hybrid perovskites is actually strong, and that spin-orbit coupling has only a minor impact on radiative recombination. The computed radiative recombination coefficient is around 10-10 cm3s-1, which is as high as in typical direct-gap semiconductors. The demonstrated high radiative recombination coefficient thus enables promising applications in light-emitting diodes. However, our first-principles calculations of nonradiative rates show that strong Auger recombination may suppress efficiency. Fortunately, our insights into the origins of the strong Auger recombination indicates potential avenues for engineering the Auger coefficient. |
Thursday, March 7, 2019 2:42PM - 2:54PM |
V20.00002: Nonlinear optical properties of organic-inorganic hybrid perovskite Wenshen Song, Su Huang, Guang-Yu Guo, Lan Yang, Li Yang Nonlinear optical properties of materials play important roles in lasers, frequency conversion, electro-optic modulators, etc. Here we explore the nonlinear optical properties of hybrid CH3NH3MX3 perovskites (M= Ge, Sn, Pb; X=Cl, Br, I), which are widely studied in photovoltaic areas. Using first-principles approaches, different phases of hybrid perovskites have been studied, and we find considerably large second harmonic generation (SHG) in some of them. Interestingly, large non-diagonal SHG tensors appear in certain structures, which are fairly useful for real-world applications, such as tuning the directions of lasers. All calculations are implemented in our self-developed package aiming for large-scale parallel calculations of nonlinear optical properties. |
Thursday, March 7, 2019 2:54PM - 3:06PM |
V20.00003: Second Order Nonlinear Optical Study on The Polar Order in MAPbI3 Xiangpeng Luo, Wencan Jin, Suneel Joglekar, Lingjie Jay Guo, Liuyan Zhao Hybrid organic-inorganic methylammonium (MA) lead-halide perovskite MAPbI3 has shown record-breaking photovoltaic conversion efficiency and demonstrated great potential in solar cell applications. Its unusual long carrier lifetime has drawn much research interest, yet its origin remains a debated subject. Whether or not the tetragonal phase of this material is polar is key to resolve the ongoing debates. In this talk, we present clarifying evidence to show the absence of spatial inversion symmetry in single-crystalline MAPbI3 bulk over a wide temperature range starting from 300 K down to 80K, using rotational anisotropy (RA) second harmonic generation (SHG) spectroscopy. We will further discuss the tetragonal to orthorhombic structural phase transition, by analyzing the temperature-dependence of SHG susceptibility tensors. Finally, we will discuss how this non-centrosymmetric phase response to ultrafast optical excitations, from the time-resolved RA SHG measurement results. |
Thursday, March 7, 2019 3:06PM - 3:18PM |
V20.00004: Ultrafast Amplified Spontaneous Emission Dynamics in Hybrid Perovskites Kenan Gundogdu Methylammonium lead iodide (MAPI) has received significant interest due to high performance of this material in photovoltaic applications. MAPI is also now known to be an efficient light emitter, due in part to its high mobility and long carrier diusion lengths, which make it a good solar cell. Specifically at reduced temperatures, the light emission process is quite interesting. Specifically at low temperatures due to coexistence of multiple crystal phases, energy and charge transfer dynamics play a significant role in light emission process. At room temperature the crystal structure is tetrogonal, whereas at low temperatures material goes through a phase change to orthorombic crystal structure. However even below the phase transition temperatures, the thin films exhibit tetragonal domain inclusions. Since the tetragonal phase has lower bandgap, most excitations diffuse to these low eenrgy sites and recombine. In this work we studied exciton and charge relaxation dynamics that lead to amplified spontaneous emission (ASE) in MAPI. Our results indicate that reveal the duration of the ASE process and the dynamics that limits ASE in MAPI thin films. |
Thursday, March 7, 2019 3:18PM - 3:30PM |
V20.00005: Waterproof perovskite-hexagonal boron nitride hybrid nanolasers with low lasing thresholds and high operating temperature Haoran Yu, Yu Ye, Lun Dai Solid-state perovskites have recently emerged as promising coherent light sources. Realization of an electrical perovskite-based laser diode still remains challenging due to the heat management and intrinsic instability of the perovskites. Here, we demonstrate waterproof perovskite-hexagonal boron nitride (hBN) hybrid nanolasers with low lasing thresholds and high operating temperature. After capping with the hBN flake which possesses superb and anisotropic thermal conductivity, heat dissipation of the hybrid nanolaser is accelerated, resulting in the significant reduction of lasing thresholds, and clear lasing behavior under a temperature as high as 75.6 °C. Moreover, hBN with high environmental stability can effectively protect the perovskite from the polar solvents. The hBN encapsulated CsPbI3 nanolasers can incessantly lase in water for an hour, and the lasing behavior can be retained even after 24-hour immersion in water. The reduction of lasing threshold, improved heat removal, and higher temperature tolerance of the hybrid nanolaser marks a major step towards CW-pumped perovskite laser at room temperature, while also allowing perovskites to be integrated into high power density optoelectronic devices and future electrically driven lasers. |
Thursday, March 7, 2019 3:30PM - 3:42PM |
V20.00006: Shifting the Thermodynamics of Polymorph Transitions in Metal-Halide Perovskites Xiaoqing Kong, Kamran Shayan, Stefan Strauf, Stephanie Lee We demonstrate nanoconfinement as a strategy to stabilize high-temperature cubic phases of metal-halide perovskites and prevent humidity-induced degradation. A combination of temperature-dependent x-ray diffraction and photoluminescence experiments were employed to track polymorph transitions in the range of 4 – 373 K. For methylammonium lead triiodide (MAPbI3) confined within the nanopores of anodized aluminum oxide templates, the tetragonal-to-cubic phase transition shifted from T = 300 K to T = 170 K. These crystals were stable for a period of at least two years of storage in air, compared to unconfined crystals that degrade into perovskite precursors within two weeks of air exposure. For nanoconfined cesium lead triodide (CsPbI3), the tetragonal-to-cubic phase transition, which typically occurs at 583 K, was absent to temperatures as low as 4 K. We hypothesize that nanoconfinement introduces lattice strain into the crystals, shifting the relative free energies of the respective polymorphs and increasing energy barrier to polymorph transitions. In both systems, the cubic phase represents the smallest band gap polymorph, with important implications for solar cell device operation and efficiency. |
Thursday, March 7, 2019 3:42PM - 3:54PM |
V20.00007: Anisotropic heat conduction in the metal-organic framework perovskites Dharmendra Shukla, Narayan Prasai, Thomas M. Carlino, Mercedes M. A. Mazza, Amy M. Scott, Joshua Cohn We report thermal conductivity (κ) measurements on single crystals of the metal-organic framework perovskite compounds [C(NH2)3]X(HCOO)3 (X=Cu, Zn) in the temperature range 5K ≤ T ≤ 300K. The directionality of N-H---O bonds and their stretching due to the Jahn-Teller distortion in the Cu compound are identified as mechanisms underlying differences in κ(T) behavior for the two compounds and its anisotropy. For heat flow along a direction preferentially transverse to the N-H---O bonds, κ≈ 0.6 W/mK near 300 K, approaching the theoretical minimum value. A possible magnetic contribution to the heat flow in the Cu compound will also be discussed. |
Thursday, March 7, 2019 3:54PM - 4:06PM |
V20.00008: Halide perovskite thin films for thermoelectrics Tianjun Liu, Oliver Fenwick Halide perovskites have emerged as promising candidates for photovoltaics and light-emitting diodes. Recently, promising thermoelectric performance has been reported for single nanocrystals of a halide perovskite, but there is not yet a good understanding of how thermoelectric performance can be optimised in these materials, especially in thin films where a diverse range of structures and morphologies are accessible. In this presentation I will report a record thermoelectric figure of merit (ZT) for halide perovskites, using the example of CsSnI3 thin films. This result is in part due to the ultralow thermal conductivity of our films (0.38 W/mK at room temperature), as well as high electrical conductivity enabled by self-doping of the films through controlled Sn oxidation. I will also discuss the potential role of mixed-halide films in developing these materials further. |
Thursday, March 7, 2019 4:06PM - 4:18PM |
V20.00009: Origin of Pressure-Induced Phase Transition and Metallization in Hybrid Halide Perovskites Jung-Hoon Lee, Jeffrey B Neaton Hybrid halide perovskites are promising optoelectronic materials due to their favorable electronic properties and low cost. Here we investigate the effects of high pressure on the structural and electronic properties of (MA)PbI3 (MA = CH3NH3+) using first-principles density functional theory (DFT) calculations. Our calculations predict that with increasing external pressure, the cubic high-pressure Im-3 phase becomes more energetically favorable for the MA cations than the orthorhombic low-pressure Fmmm phase, leading to a predicted phase transition at 0.22 GPa, in good agreement with the experiment (~0.3 GPa). With increasing pressure we also find severe octahedral tilting occurs at 6 GPa, introducing I 5p – I 5p* antibonding and Pb 6p – Pb 6p bonding character into the valence band maximum (VBM) and the conduction band minimum (CBM) states, respectively. This leads to a different trend in the VBM and CBM energies under compression compared to their behavior below 6 GPa. Our DFT calculations show that this trend eventually leads to metallization at significantly higher pressures. |
Thursday, March 7, 2019 4:18PM - 4:30PM |
V20.00010: Pb-free halide double perovskites: Computational materials design and surface properties George Volonakis, Feliciano Giustino Halide double perovskites have been investigated extensively as lead-free alternatives to lead halide perovskites. Since our theoretical predictions, four crystals have successfully been synthesized: Cs2BiAgCl6, Cs2BiAgBr6, Cs2SbAgCl6, and Cs2InAgCl6. These compounds exhibit low carrier effective masses, and Cs2BiAgBr6 has a band gap of 1.9 eV, well within the visible range. Hence, these are candidates for opto-electronic applications such as photovoltaics, sensors, and photo-catalysts. In this talk, we will briefly present the computational design principles that lead to the synthesis of these compounds. In addition, we will focus on the structural and electronic properties of the four synthesised compounds and their surfaces. We will show that based on first-principles calculations, double perovskites Cs2BiAgCl6 and Cs2BiAgBr6 might be promising compounds for photo-catalytic water splitting. For all compounds it is shown that their energy levels, hence their photocatalytic properties, strongly depend on the surface termination. Our computational findings expand the range of potential applications of Pb-free double perovskites. |
Thursday, March 7, 2019 4:30PM - 4:42PM |
V20.00011: Structural relationships and relative stability of different ABX3 phases of inorganic halide perovskites with B=Ge Santosh kumar Radha, Walter RL Lambrecht
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Thursday, March 7, 2019 4:42PM - 4:54PM |
V20.00012: Effect of water on structural, optical and hot carrier cooling properties of MASnI3 perovskite Mohamed El-Amine Madjet, Ali Kachmar, Gilibjon Berdiyorov Understanding the stability, carrier transport and relaxation of Sn based perovskites CH3NH3SnI3 are of high interest to develop lead free perovskite solar cells. In this study, we performed first principles and non-adiabatic molecular dynamics simulations to address the electronic, optical, and hot carrier relaxation dynamics of the pristine CH3NH3SnI3 and its monohydrated phase CH3NH3SnI3.H2O. Our results show that the water molecule interacts strongly with the organic cation by forming hydrogen bonding, while having also an interaction with the iodide ions. Our study also indicates that the water molecule has in general a negative impact on the optical properties of perovskite materials. In fact, the water molecule reduces the absorption of the system mostly in the visible range of the solar spectrum, and it leads to a faster hot-carrier cooling and to an increase in the polarization of the material. |
Thursday, March 7, 2019 4:54PM - 5:06PM |
V20.00013: Investigation of charge dynamics and photoinduced charge transfer in metal halide perovskite/carbon nanotube composites by vibrational spectroscopy Katalin Kamaras, Leila Badeeb, Mehmet Derya Özeren, Aron Pekker, Seham Kamal Abdel-Aal, Ahmed Sabry Abdel-Rahman, Pavao Andricevic, Laszlo Forro, Endre Horvath Composites formed from metal halide perovskites and semiconducting carbon nanotubes show a fast photoinduced charge diffusion and a slow structural rearrangement due to ionic movement. To understand this latter, phonon-related mechanism, we performed a study of vibrational spectra on various perovskites and composites, changing the central ion of (Pb, Sn, Co and Cu) and the halide (I, Br and Cl). We measured the temperature dependence, followed the phase transitions, and investigated the photoinduced changes by both infrared and Raman spectroscopy. As the structural rearrangement is accompanied by Fermi level alignment and thus modifies the optical gap, our results are useful for fine-tuning the optical properties in solar cell applications of perovskites and perovskite/carbon nanostructure composites. |
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