Bulletin of the American Physical Society
APS March Meeting 2017
Volume 62, Number 4
Monday–Friday, March 13–17, 2017; New Orleans, Louisiana
Session A34: Hybrid Organic-Inorganic Halide Perovskites IIFocus
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Sponsoring Units: DMP GERA Chair: Feliciano Giustino, University of Oxford Room: 297 |
Monday, March 13, 2017 8:00AM - 8:36AM |
A34.00001: Hybrid Organic-Inorganic Perovskites: Structural Diversity and Opportunities for Semiconductor Design Invited Speaker: David Mitzi Photovoltaic (PV) devices based on three-dimensional perovskites, (Cs, MA, FA)Pb(I, Br)$_{\mathrm{3}}$ (MA$=$methylammonium, FA$=$formamidinium), have attracted substantial recent interest, because of the unprecedented rise in power conversion efficiency to values above 20{\%}, which in turn is made possible by the near ideal band gap, strong optical absorption, high carrier mobilities, long minority carrier lifetimes, and relatively benign defects and grain boundaries for the absorbers. Some of the same properties that render these materials near-ideal for PV, also make them attractive for LED and other optoelectronic applications. Despite the high levels of device performance, the incorporation of the heavy metal lead, coupled with issues of device stability and electrical hysteresis pose challenges for commercializing these exciting technologies. This talk will provide a perspective on and discuss recent advances related to the broader perovskite family, focusing on the extraordinary structural/chemical diversity, including ability to control structural/electronic dimensionality, substitute on the organic cation, metal or halogen sites, and prospects of multi-functionality arising from separately engineered organic/inorganic structural components (e.g., see [1]). Further exploration within this perovskite structural and chemical space offers exciting opportunities for future energy and electronic materials design. [1] B. Saparov and D. B. Mitzi, Chemical Reviews 116, 4558-4596 (2016). [Preview Abstract] |
Monday, March 13, 2017 8:36AM - 8:48AM |
A34.00002: Temperature-Dependent Polarization Effects in Methylammonium Lead Iodide Field Effect Transistors John Labram, Douglas Fabini, Erin Perry, Hayden Evans, Ram Seshadri, Michael Chabinyc The recent progress in organo-metallic hybrid perovskite solar cells can be viewed as a highly significant historic event. The peak reported power conversion efficiency (PCE) has increased at an unprecedented rate, to a value now in excess of 22{\%},$^{\mathrm{1}}$ However many aspects of device operation remain poorly understood. Despite high reported carrier mobilities,$^{\mathrm{\thinspace }}$easily-accessible conduction and valence band energies and previous reports employing other organo-metallic hybrid perovkistes,$^{\mathrm{2}}$ field-effect transistors (FETs) based on methylammonium lead iodide (MAPbI$_{\mathrm{3}})$ have been scarcely studied. Using various electronic measurements, we here present a body of experimental evidence consistent with the existence of a mobile ionic species within the MAPbI$_{\mathrm{3}}$ perovskite. Temperature-dependent FET measurements reveal operating devices only below 210K. This is attributed to ionic screening of the semiconductor-dielectric interface. Temperature-dependent pulsed gate experiments, reveal a time-dependent source-drain current behavior consistent with this interpretation. Capacitors exhibit a decreasing low-frequency capacitance with temperature and a temperature-independent capacitance at higher frequencies. [1]www.nrel.gov/ncpv/images/efficiency\textunderscore chart.jpg [2]C. R. Kagan, et. al. Science 1999, \textbf{286}, 945. [Preview Abstract] |
Monday, March 13, 2017 8:48AM - 9:00AM |
A34.00003: Direct Observation of Electron--Phonon Coupling and Slow Vibrational Relaxation in Organic--Inorganic Hybrid Perovskites Sebastian Hurtado Parra, Daniel Straus, Natasha Iotov, Bryan Fichera, Julian Gebhardt, Andrew Rappe, Joseph Subotnik, James Kikkawa, Cherie Kagan Quantum and dielectric confinement effects in Ruddlesden-Popper 2D hybrid perovskites create excitons with a binding energy exceeding 150 meV. We exploit the large exciton binding energy to study exciton and carrier dynamics as well as electron--phonon coupling (EPC) in hybrid perovskites using absorption and photoluminescence (PL) spectroscopies. At temperatures \textless 75 K, we resolve splitting of the excitonic absorption and PL into multiple regularly spaced resonances every 40--46 meV, consistent with EPC to phonons located on the organic cation. We also resolve resonances with a 14 meV spacing, in accord with coupling to phonons with mixed organic and inorganic character. These assignments are supported by density-functional theory calculations. Hot exciton PL and time-resolved PL measurements show that vibrational relaxation occurs on a picosecond time scale competitive with that for PL. At temperatures \textgreater 75 K, excitonic absorption and PL exhibit homogeneous broadening. While absorption remains homogeneous, PL becomes inhomogeneous at temperatures \textless 75K, which we speculate is caused by the formation and subsequent dynamics of a polaronic exciton. (JACS 138, 13798 (2016)) [Preview Abstract] |
Monday, March 13, 2017 9:00AM - 9:12AM |
A34.00004: Studies of magnetoresistance in organometal halide perovskite based spin-valves* Jingying Wang, Dali Sun, Chuang Zhang, Z. Valy Vardeny Lead-based organometal halide perovskites (OHP) have recently attracted substantial research interest due to their potential photovoltaic and optoelectronic applications as well as non-traditional semiconductor physics. The carrier spin characteristic properties in these compounds are influenced by the large spin-orbit coupling of the Pb and halogen atoms, and consequently offer a new class of semiconductors for spintronic applications. We have studied magnetoresistance (MR) in OHP-based spin-valves using LSMO/OHP/Co trilayer devices. We report sizable `giant-MR' and `tunneling-MR' in spin-valves based on several 3D and 2D OHP active layers. The two types of MR responses were studied and compared as a function of the applied bias voltage and temperature, respectively. *Supported by the DOE grant DE-SC0014579 and NSF-MRSEC (DMR-1121252). [Preview Abstract] |
Monday, March 13, 2017 9:12AM - 9:24AM |
A34.00005: High-yield, high-mobility mixed halide perovskite field-effect transistors operating at room temperature Andrew Zeidell, Colin Tyznik, Oana Jurchescu Hybrid halide perovskites combine the ease of processing inherent to organic compounds with the high performance of inorganic materials. Charge transport in perovskite field effect transistors (FETs) is strongly dependent on the quality of the perovskite film and that of the semiconductor/dielectric interface. Si/SiO$_{\mathrm{2\thinspace }}$substrates are widely used as gate electrode/dielectric platforms in thin-film transistors. This architecture, however, was previously unsuccessful for perovskite transistors at room temperature due to a combination of high interfacial trap densities, defect states in perovskite films, and possibly ionic transport. In this study, we incorporated the hybrid halide perovskite CH$_{\mathrm{3}}$NH$_{\mathrm{3}}$PbI$_{\mathrm{3-x}}$Cl$_{\mathrm{x}}$ in FETs with Si bottom-gate electrode and SiO$_{\mathrm{2}}$ dielectric. We obtained working devices at room-temperature by drastically improving the quality of the perovskite layer using film processing techniques such as solvent annealing to increase the uniformity and coverage, and by encapsulation in hydrophobic polymers to protect from moisture. The resulting films provided a high yield of ambipolar FETs with uniform operation, achieving reproducible room temperature hole mobilities on the order of 1 cm$^{\mathrm{2}}$/Vs, and electron mobilities on the order of 0.1 cm$^{\mathrm{2}}$/Vs. We show that this is due to lowering the trap-density in the transistor channel by increasing the grain size, thus reducing the density of grain boundaries, and minimizing ionic migration, which occurs predominantly at grain boundaries. [Preview Abstract] |
Monday, March 13, 2017 9:24AM - 9:36AM |
A34.00006: Light Induced Surface Degradation of CH$_{\mathrm{3}}$NH$_{\mathrm{3}}$PbBr$_{\mathrm{3}}$ Single Crystals Benjamin Ecker, Congcong Wang, Haotong Wei, Jinsong Huang, Yongli Gao Organometallic trihalide perovskites have shown great potential for solar cell device applications, and numerous studies have investigated their long term stability in various environments. In this study we investigated the light induced surface degradation on CH$_{\mathrm{3}}$NH$_{\mathrm{3}}$PbBr$_{\mathrm{3}}$ crystals by using x-ray photoelectron spectroscopy (XPS) and scanning electron microscopy (SEM). Substantial surface degradation became apparent during the exposure as observed in the elemental core level spectrums. There were significant losses in the surface bromine and nitrogen concentrations. The most interesting core level change however was the production of a new form of lead on the surface, which is most like in the form of metallic lead. The new metallic lead component saturated and was approximately two times the value of the perovskite lead component. SEM images were also taken to further confirm the surface degradation. [Preview Abstract] |
Monday, March 13, 2017 9:36AM - 9:48AM |
A34.00007: Screening in crystalline liquids protects energetic carriers in hybrid perovskites Haiming Zhu, Kiyoshi Miyata, Yongping Fu, Jue Wang, Prakriti Joshi, Daniel Niesner, Kristopher Williams, Song Jin, Xiaoyang Zhu Hybrid lead halide perovskites exhibit carrier properties that resemble those of pristine nonpolar semiconductors despite static and dynamic disorder, but how carriers are protected from efficient scattering with charged defects and optical phonons is unknown. Here, we reveal the carrier protection mechanism by comparing three single-crystal lead bromide perovskites: CH$_{3}$NH$_{3}$PbBr$_{3}$, CH(NH$_{2}$)$_{2}$PbBr$_{3}$, and CsPbBr$_{3}$. We observed hot fluorescence emission from energetic carriers with $\sim$ 10$^{2}$ picosecond lifetimes in CH$_{3}$NH$_{3}$PbBr$_{3}$ or CH(NH$_{2}$)$_{2}$PbBr$_{3}$, but not in CsPbBr$_{3}$. The hot fluorescence is correlated with liquid-like molecular reorientational motions, suggesting that dynamic screening protects energetic carriers via solvation or large polaron formation on time scales competitive with that of ultrafast cooling. Similar protections likely exist for band-edge carriers. The long-lived energetic carriers may enable hot-carrier solar cells with efficiencies exceeding the Shockley-Queisser limit. (Science 2016, 353, 1409.) [Preview Abstract] |
Monday, March 13, 2017 9:48AM - 10:00AM |
A34.00008: Understanding exciton dynamics in layered 2D organic-inorganic hybrid perovskites Eric Amerling, Sangita` Baniya, Evan Lafalce, Charlie Zhang, Zeev Valy Vardeny, Luisa Whittaker-Brooks 2D organic-inorganic hybrid perovskite multiple quantum wells which consist of multilayers of alternate organic and inorganic layers exhibit large exciton binding energies ($\approx $ 0.3 eV) due to the dielectric confinement between the inorganic and organic layers, as well as multiexciton resonances. Such large exciton binding energies lead to huge exciton oscillation strength with Rabi frequency of the order of 50 meV. We have investigated the exciton dynamics of 2D butylammonium lead iodide, (CH$_{\mathrm{3}}$(CH$_{\mathrm{2}})_{\mathrm{3}}$NH$_{\mathrm{3}})_{\mathrm{2}}$PbI$_{\mathrm{4}}$, via photoluminescense (PL) in the temperature range of 300 K to 10 K and electroabsorption (EA) spectroscopy at RT and 40K. A blue shift of the PL emission spectrum and the evolution of an additional emission peak suggest that$_{\mathrm{\thinspace }}$this compound$_{\mathrm{\thinspace }}$undergoes a phase transition at $\approx $160 K. The EA spectroscopy has allowed us to determine the exciton transitions and binding energies in the two structural phases more precisely than from the PL and absorption spectra. [Preview Abstract] |
Monday, March 13, 2017 10:00AM - 10:12AM |
A34.00009: Time Domain View of Liquid-like Screening and Large Polaron Formation in Lead Halide Perovskites Prakriti Pradhan Joshi, Kiyoshi Miyata, M. Tuan Trinh, Xiaoyang Zhu The structural softness and dynamic disorder of lead halide perovskites contributes to their remarkable optoelectronic properties through efficient charge screening and large polaron formation. Here we provide a direct time-domain view of the liquid-like structural dynamics and polaron formation in single crystal CH$_{3}$NH$_{3}$PbBr$_{3}$ and CsPbBr$_{3}$ using femtosecond optical Kerr effect spectroscopy in conjunction with transient reflectance spectroscopy. We investigate structural dynamics as function of pump energy, which enables us to examine the dynamics in the absence and presence of charge carriers. In the absence of charge carriers, structural dynamics are dominated by over-damped picosecond motions of the inorganic PbBr$_{3}$$^{-}$ sub-lattice and these motions are strongly coupled to band-gap electronic transitions. Carrier injection from across-gap optical excitation triggers additional $\sim$0.26 ps dynamics in CH$_{3}$NH$_{3}$PbBr$_{3}$ that can be attributed to the formation of large polarons. In comparison, large polaron formation is slower in CsPbBr$_{3}$ with a time constant of $\sim$0.6 ps. We discuss how such dynamic screening protects charge carriers in lead halide perovskites. [Preview Abstract] |
Monday, March 13, 2017 10:12AM - 10:24AM |
A34.00010: A Combined Theoretical and Experimental View on Valence and Conduction Band Densities of States of Lead Halide Perovskites Leeor Kronik, James Endres, David A. Egger, Michael Kulbak, Ross A. Kerner, Lianfeng Zhao, Scott H. Silver, Gary Hodes, Barry P. Rand, David Cahen, Antoine Kahn We present results for the valence and conduction band density of states (DOS), measured via ultraviolet and inverse photoemission spectroscopies for three lead halide perovskites. Specifically, the DOS of MAPbI$_3$, MAPbBr$_3$, and CsPbBr$_3$, grown on different substrates, are compared. Theoretical DOS, calculated via hybrid density functional theory and including spin-orbit coupling, are compared to experimental data. The agreement between experiment and theory, obtained after correcting the latter for quantitative discrepancies, leads to the identification of valence and conduction band spectral features. In particular, this comparison allows for precise determination of the energy position of the band edges, namely ionization energies and electron affinities of these materials. We find an unusually low DOS at the valence band maximum (VBM) of these systems, which confirms and generalizes previous findings of strong band dispersion and low DOS at the VBM of MAPbI$_3$. This calls for special attention when using electron spectroscopy to determine the frontier electronic states of lead halide perovskites. [1] Endres et al., J. Phys. Chem. Lett. 7, 2729 (2016) [Preview Abstract] |
Monday, March 13, 2017 10:24AM - 10:36AM |
A34.00011: Charge Transport in Two-Dimensional Hybrid Halide Perovskites Naveen Venkatesan, John Labram, Christopher Takacs, Hayden Evans, Erin Perry, Fred Wudl, Michael Chabinyc Hybrid-halide perovskite materials have garnered attention because they are earth-abundant, solution processable materials for photovoltaic cells. In this study, two methods were used to create two-dimensional, layered perovskites: replacement of halide ions by the pseudohalide thiocyanate (SCN$^{\mathrm{-}})$, and the introduction of a large cationic spacer to form layered crystals with Ruddlesden-Popper structures. Films with large, well-oriented grains of (MA)$_{\mathrm{2}}$Pb(SCN)$_{\mathrm{2}}$I$_{\mathrm{2}}$ formed during growth by spin coating. Using time-resolved microwave conductivity (TRMC) experiments, the carrier mobility in-plane was found to be comparable to that of methylammonium lead iodide (MAPbI$_{\mathrm{3}})$, with carrier lifetimes on the order of 100 ns. Results as a function of dimensionality in R-P series will be presented. This charge transport data, along with increased stability that has been recently found in lower-dimensional perovskite systems, leads us to conclude that a three-dimensional structure is not a prerequisite for long carrier lifetime and carrier mobility. [Preview Abstract] |
Monday, March 13, 2017 10:36AM - 10:48AM |
A34.00012: Higher-order effects in the temperature dependence of the energy levels in Methylammonium Lead Iodide Perovskite Wissam A. Saidi, Samuel Ponce, Bartomeu Monserrat We determine temperature effects on the bandgap and band edges of CH$_3$NH$_3$PbI$_3$ from first principles by accounting for electron-phonon coupling and thermal expansion. From $290$ to $380$K, the computed bandgap change of $40$ meV coincides with the experimental change of $30$-$40$meV. We show that the calculation of electron-phonon coupling in CH$_3$NH$_3$PbI$_3$ is particularly intricate, as the commonly used Allen-Heine-Cardona (AHC) theory significantly overestimates the bandgap change, and excellent agreement with experiment is only obtained when high-order terms in the electron-phonon coupling are included in conjunction with spin-orbit corrections. In contrast, we find that the inclusion of nonlocal correlations using hybrid functionals has little effect. We show that the low-energy phonon modes contribute the most to the energy levels renormalization due to temperature, which suggests that the AHC failure is likely common to metal-halide perovskites and not only to hybrid ones. We have verified this on CsPbI$_3$. Our results unambiguously confirm for the first time the importance of higher-order terms in the electron-phonon coupling by directly comparing with experimental results [Preview Abstract] |
Monday, March 13, 2017 10:48AM - 11:00AM |
A34.00013: Tunable exciton-polariton lasing in perovskite microcrystal cavities Andrew Schlaus, Yongping Fu, Song Jin, Louis Brus, Tyler Evans, Xiaoyang Zhu Strong light-matter interactions have recently been discovered in lead halide perovskites. The resulting polariton quasiparticles undergo spontaneous condensation, leading to the emission of coherent light. Here we demonstrate ultra-low-power, wavelength-tunable coherent light emission from polariton condensates in perovskite nanowires and microplates. Through halide alloying and optical cavity engineering, we control the polariton dispersion and the color of broadly tunable coherent light emission. These results reveal yet another surprise in the excellent optoelectronic properties of lead halide perovskites and suggest that this material system may be excellent models for exciton-polariton physics and devices. [Preview Abstract] |
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