Bulletin of the American Physical Society
APS March Meeting 2018
Volume 63, Number 1
Monday–Friday, March 5–9, 2018; Los Angeles, California
Session B11: Organometal Halide Perovskites IIFocus
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Sponsoring Units: DMP Chair: Dong Yu, University of California, Davis Room: LACC 303A |
Monday, March 5, 2018 11:15AM - 11:51AM |
B11.00001: Unveiling the Operation Mechanism of “Two-dimensional” Perovskite Solar Cells Invited Speaker: Jinsong Huang Two dimensional (2D) perovskites have been shown to improve the stability of perovskite solar cells while its operation mechanism remains unclear. Here we investigate the process for the conversion of light to electrical current in high performance 2D perovskite solar cells by examining the real morphology at multiple scales and anisotropic carrier transport properties. A phase separation at micrometer scale and nanometer scale are identified in both vertical and lateral directions for the 2D perovskite films prepared by the hot-casting method. High resolution transmission electron microscopy revealed that the 2D layer structures with the size much smaller than the film thickness are surrounded by three dimensional (3D) perovskites. In the addition, no preferred orientation of the 2D layer structures was observed in our samples. Based on the morphology scenario, a model of energy transfer from 2D perovskites to 3D perovskite followed by exciton dissociation and free- charge transport via 3D perovskite is proposed to explain the exciton dissociation and charge collection in 2D perovskite solar cells. The impact of the 2D perovskite on efficiency and stability of perovskite solar cells are also discussed to provide guidelines for the further improvement of the “2D perovskite” thin film solar cells. |
Monday, March 5, 2018 11:51AM - 12:03PM |
B11.00002: Structural and Electronic Defects in Layered Hybrid Perovskites Naveen Venkatesan, John Labram, Erin Perry, Michael Chabinyc Solution-processable, hybrid halide Ruddlesden-Popper phases have favorable electronic properties with increased stability relative to their 3D counterparts (CH3NH3PbI3). Although layer confinement leads to anisotropic charge transport, our previous work shows that the in-plane mobility of single-layer Pb-I perovskite sheets is comparable to CH3NH3PbI3,1 making control over thin-film structure essential to device performance. Here we study R-P compounds with an n-butylammonium cation, (BA)2(MA)n-1PbnI3n+1, with varying thickness of Pb-I perovskite sheets. Specular diffraction shows a gradual change in thin-film texture with increasing n, with the Pb-I sheets changing from parallel to perpendicular with respect to the substrate, however; disorder is apparent from grazing-incidence diffraction. Time-resolved microwave conductivity measurements show carrier lifetimes smaller than in MAPbI3. Most interestingly, external quantum efficiency measurements of solar cells formed with these materials show sharp absorption edges despite structural disorder. From this, we conclude that these R-P phases have a high electronic defect tolerance and their performance is related to the alignment of the layers relative to the direction of transport. |
Monday, March 5, 2018 12:03PM - 12:15PM |
B11.00003: Identifying the Bandgap of Two-dimensional Perovskite CsPb2Br5 Chong Wang, Yanan Wang, Yizhou Ni, Viktor Hajdev, Xinghua Su, Shuo Chen, Zhiming Wang, Zhifeng Ren, Jiming Bao CsPb2Br5 is a 2D perovskite where single layer Cs acts as a spacer, monolayer PbBr6 octahedrons are edge shared. Although CsPb2Br5 was synthesized more than 10 years ago, only recently it began to draw a lot of attention. Despite extensive research over past two years, its fundamental bandgap and basic electronic and optical properties still remain controversial: is this a wide bandgap indirect semiconductor or a direct bandgap material with strong green luminescence at ~520 nm? In this talk, I will discuss the reason for different observations and present our result of the bandgap investigation of CsPb2Br5. |
Monday, March 5, 2018 12:15PM - 12:27PM |
B11.00004: Two-photon absorption spectroscopy of 2D and 3D hybrid perovskites Evan Lafalce, Sangita Baniya, Qingji Zeng, Zeev Vardeny We provide a unique perspective on the dependence of the electronic structure of various phases of organic lead-halide perovskites through spectrally resolved non-linear optical coefficient. In particular, we compare crystals of MAPbX3 (X = Br and I) at different temperatures and macroscopic crystals of the layered, 2D lead-halide phase trough two-photon photoluminescence excitation spectroscopy in combination with other methods for determining the two-photon absorption (TPA) coefficient. The assignment of features is aided through electroabsorption spectroscopy and calculation of the TPA spectrum from the band structure. Comparison of photoluminescence under one-photon and two-photon excitations allows separation of surface and bulk properties. These interesting features are thus extremely useful for understanding the bulk electronic properties of many hybrid perovskites and may guide their further development for non-linear optics and optoelectronic technology. |
Monday, March 5, 2018 12:27PM - 12:39PM |
B11.00005: Optical studies of 2D Hybrid Lead Iodide Perovskites Sangita Baniya, Chuan-Xiang Sheng, Chuang Zhang, Yaxin Zhai, Zeev Valy Vardeny 2D organic-inorganic hybrid perovskites have recently attracted substantive interest because of their outstanding optoelectronic characteristics and stability under ambient conditions. These compounds consist of alternating organic and inorganic layers that form natural multiple quantum wells. We have investigated the photoexcitations in (C6H5C2H2NH3)2PbI4 films using photoinduced absorption (PA) and electroabsorption (EA) spectroscopies. We found a strong PA band at 0.15 eV that is due to long-lived free carrier absorption that is caused by the Rashba-splitting in this material. From the EA spectrum we found that the exciton binding energy is about 200 meV, whereas the continuum shows Franz-Keldysh oscillation that unambiguously reveals the band-edge energy. |
Monday, March 5, 2018 12:39PM - 12:51PM |
B11.00006: Color Selective Control of Terahertz Radiation Using Two-Dimensional Hybrid Organic Inorganic Lead-Trihalide Perovskites Yaxin Zhai, Ashish Chanana, Sangita Baniya, Chuang Zhang, Ajay Nahata, Zeev Valy Vardeny Controlling and modulating terahertz signals is of fundamental importance to allow systems level applications. We demonstrate an innovative approach for controlling the propagation properties of terahertz (THz) radiation, through use of both the excitation optical wavelength and intensity. We accomplish this using two-dimensional (2D) layered hybrid trihalide perovskites that are deposited onto silicon substrates. Optical absorption in 2D perovskites occurs over a broad spectral range above the bandgap, resulting in free carrier generation, as well as over a narrow spectral range near the band-edge due to exciton formation. We find that only the latter contribution gives rise to photoinduced THz absorption. By patterning multiple 2D perovskites with different optical absorption properties onto a single device, we demonstrate both color selective modulation and focusing of THz radiation. These findings open new directions for creating active THz devices. |
Monday, March 5, 2018 12:51PM - 1:03PM |
B11.00007: Bose–Einstein condensation of exciton polariton in a perovskite cavity Wei Bao, Xiaoze Liu, Fan Zheng, Yang Xia, Mervin Zhao, Jeongmin Kim, Sui Yang, YING WANG, Yuan Wang, Lin-Wang Wang, Xiang Zhang The formation of half-light half-matter quasiparticle exciton polariton and its condensation in semiconductor optical cavities are striking phenomena for the macroscopic quantum coherence at elevated temperature. The matter constituent of exciton polariton dictates the interacting behaviors of these bosonic particles primarily via exciton-exciton interactions. However, these interactions are all limited to the ground state exciton, although they can be orders of magnitude stronger at Rydberg states with higher principal numbers. Here, for the first time, we observe the spontaneous formed Rydberg exciton polaritons (REPs) in a high quality Fabry-Perot cavity embedded with single crystal inorganic perovskite, CsPbBr3. Such REPs exhibit strong nonlinear behavior, enabling an anomalous dynamic process that leads to a coherent polariton condensate with prominent blue shift. Furthermore, we discover the first anisotropic REP with large extinction ratio over 50, arising from the perovskite’s asymmetric orthorhombic crystal structure. Our observation not only sheds light on the importance of coherent many-body Rydberg interactions in polariton systems, but also paves the way for exploring these coherent interactions for solid state quantum optical information processing. |
Monday, March 5, 2018 1:03PM - 1:15PM |
B11.00008: Optical Properties of Inkjet Printed CsPbBr3 Quantum Dot Perovskite Films Shashank ram Nandyala, Dinesh Baral, Mikhail Shekhirev, Carolina Ilie, Sharmin Sikich, Axel Enders, Peter Dowben, Alexander Sinitskii, Te-Yu Chien, Andrew Yost, Jon Pikal Inkjet printing of quantum dots(QDs) on to a substrate has several advantages like mass production, tailored patterning, and substrate flexibility. We used a Canon 7220 inkjet printer to print solution based CsPbBr3 QDs onto glass substrates. We printed one layer and two layer samples of QDs and compared the optical properties to those of a dropcast sample. The crystal structure was characterized by X-Ray diffraction which indicated that the original crystal structure was retained after the inkjet printing process. Absorption spectroscopy measurements showed nearly identical characteristics before and after printing with only a slight red-shift of the absorption edge for the printed samples. The Photoluminescence spectra all show a strong PL peak around 500nm and a much weaker PL peak near 455nm. The carrier dynamics of the two PL peaks were measured using Time Correlated Single Photon Counting, which showed overall faster decay of the 455nm peak than the main 500nm peak. We will also present ultrafast Transient Absorption data where we observe the charge transfer dynamics of CsPbBr3 QDs printed on ITO substrates. |
Monday, March 5, 2018 1:15PM - 1:27PM |
B11.00009: Understanding luminescent properties in zero-dimensional halide perovskites Maohua Du, Hongliang Shi, Wenmei Ming, Biwu Ma, Bayrammurad Saparov Zero-dimensional (0D) halide perovskites exhibit strong exciton self-trapping, which may lead to efficient exciton emission. First-principles calculations on hybrid halide perovskites (C4N2H14X)4SnX6 (X = Br, I) and inorganic Cs4PbBr6 show large exciton binding energies and exothermic exciton trapping at deep halogen vacancy levels. The calculated excitation and emission energies of excitons are in good agreement with experimental values. Suppressing exciton migration and the subsequent energy loss at defects are critical for efficient luminescence. Although the thermally-activated exciton hopping should be limited, fast exciton migration may occur through the resonant exchange of the excitation energy. To prevent this, large molecular cations may be incorporated in 0D halide perovskites; the key is to suppress the wavefunction overlap between luminescent centers and to increase the Stokes shift to prevent the spectral overlap between excitation and emission. Our results explain the high photoluminescence quantum efficiency observed in (C4N2H14X)4SnX6 and the severe thermal quenching of luminescence in Cs4PbBr6. The green luminescence in Cs4PbBr6 is likely the result of exciton emission from CsPbBr3 inclusions within the bulk of Cs4PbBr6. |
Monday, March 5, 2018 1:27PM - 1:39PM |
B11.00010: Distortion modes in inorganic halide perovskites: to twist or to stretch Santosh kumar Radha, Walter Lambrecht While the hybrid organic metal halide perovskites have shown promise for solar cell and other applications, an issue in this family of materials is structural stability and the distortions of the structure. In this work, the structural stability of CsBX3 compounds with B from group IV elements and X=I,Br,Cl are studied using first-principles calculations with the full-potential linearized muffin-tin orbital method. It is shown that CsBX3 halides with B=Sn or Pb undergo octahedral rotation distortions, while B=Ge or Si undergo a ferro-electric off-centering distortion along the [111] cubic axis along with rhombohedral stretching of the lattice. Specifically, we study the rotation about one cubic axis leading to the cubic to tetragonal phase transition. It is also shown that the preference for rotation or ferro-electric distrortion are mutually exclusive at their equilibrium volume. In other words, Ge and Si do not lead to rotations while Sn and Pb do not lower their energy under ferroelectric distortions. However the rotating structures, under thermal expansion supresses the rotation and encourages ferroelectric distortion. This behavior has been observed before in CsSnBr3 but is here extended to the other compounds and related to lone-pair induced anisotropic bonding. |
Monday, March 5, 2018 1:39PM - 1:51PM |
B11.00011: First-Principles Studies of Band Gap Reductions via Substitutional Defects in Cs2AgBiBr6 Stephanie Mack, Linn Leppert, Kurt Lindquist, Hemamala Karunadasa, Jeffrey Neaton Pb-based halide perovskites have emerged as a promising class of materials for photovoltaic applications, but are undesirable due to their toxicity and instability towards water. Halide double perovskites afford the opportunity for heterovalent substitutions on the metal site and in particular Cs2AgBiBr6 has been found to host a comparably small, if indirect, band gap [1]. Recent studies show that dilute amounts of Tl can further decrease the band gap and moreover, become direct due to the symmetry of the bands [2]. Here we use first-principles calculations to study how substitution of other, non-toxic heavy elements affects the band structure of Cs2AgBiBr6 with the aim of achieving a smaller, direct band gap. Accurately capturing the effect of including heavy elements in a complex, quaternary system requires using large supercells, examining the local structure of substitutional defects, and particular attention must be paid on how to interpret band gap trends calculated with DFT. We explore the possibility of using a semi-empirical approach based on GW quasiparticle corrections to predict band gap trends for dilute defect concentrations. [1] A.H. Slavney et al, J. Am. Chem. Soc., 2016, 138 (7), 2138–2141 [2] A.H. Slavney et al, J. Am. Chem. Soc., 2017, 139 (14), 5015–5018 |
Monday, March 5, 2018 1:51PM - 2:03PM |
B11.00012: Prediction of Thermodynamic Stability of Double-Perovskite Halides: Density Functional Theory Calculations and the Role of Different Exchange-Correlation Functionals Tao Zhang, Dan Han, Menglin Huang, Deyan Sun, Maohua Du, Shiyou Chen Recently, a set of double-perovskite halide compounds have attracted intensive interests as promising alternatives to CH3NH3PbI3 because they are Pb-free and may exhibit enhanced stability. The thermodynamic stability of lots of double-perovskite halides has been predicted based on density functional theory calculations of compound formation energies. We found that the stability prediction can be sensitive to the exchange-correlation functionals, e.g., the widely-used PBE functional predict that Cs2AgBiBr6 is thermodynamically unstable, obviously inconsistent with the good stability observed experimentally. The incorrect prediction by the PBE functional results from its failure to predict the correct ground-state structures of AgBr, AgCl and CsCl. In contrast, the DFT calculations based on LDA, optB86b-vdW and optB88-vdW functionals can correctly predict the ground-state structures of these binary halides. Furthermore, the optB88-vdW functional is found to give the most accurate description of the lattice constants of the double-perovskite halides and their competing phases. Hence, we suggest that the optB88-vdW functional should be used for predicting thermodynamic stability for new double-perovskite halides. |
Monday, March 5, 2018 2:03PM - 2:15PM |
B11.00013: Design of Inorganic Pb-free Double-perovskite Halides for Optoelectronic Applications Lijun Zhang, Alex Zunger, Suhuai Wei, Ji-Hui Yang, Liping Yu Pb-based halide perovskites AMX3 (prototype CH3NH3PbI3) have recently attracted much interest for optoelectronic applications, yet two key impediments need to be resolved: the intrinsic material instability (e.g., towards decomposition) and the toxicity due to water soluble Pb2+. We address these issues by exploiting the idea of “cation-transmutation” in tetrahedral semiconductors to design stable inorganic Pb-free halide perovskites with appropriate direct band gaps. The idea is to convert two divalent Pb2+ ions in A2Pb2X6 into one monovalent M+ and one trivalent M3+ ions, thus forming a rich class of quaternary halide double-perovskites A2[M+M3+]X6 structures [JACS 139, 2630 (2017), JACS 139, 6718 (2017)]. With optoelectronic-functionality-directed materials screening, we identify a series of optimal materials with intrinsic thermodynamic stability, and favorable optoelectronic properties (e.g., band gaps, carrier effective masses, and excitons binding energies) as promising candidates to replace Pb-based halide perovskites. Several of our proposed compounds have been verified by experimental synthesis. |
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