Bulletin of the American Physical Society
APS March Meeting 2016
Volume 61, Number 2
Monday–Friday, March 14–18, 2016; Baltimore, Maryland
Session B32: Emerging Nanomaterials for Solar Energy Conversion IFocus
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Sponsoring Units: DCP Chair: Gordana Dukovic, University of Colorado Room: 332 |
Monday, March 14, 2016 11:15AM - 11:51AM |
B32.00001: TBA Invited Speaker: Michael Graetzel |
Monday, March 14, 2016 11:51AM - 12:03PM |
B32.00002: First-principles determination of the structural, vibrational, and thermodynamic properties of Methylammonium Lead Iodide Perovskite Wissam Saidi Intrinsic energy-loss processes in solar cells ultimately increase the operational temperature, which can have profound effect on the power conversion efficiency of solar cells. Here I report investigations on the temperature effects on structural and mechanical properties of CH$_{\mathrm{3}}$NH$_{\mathrm{3}}$PbI$_{\mathrm{3}}$ using well-converged first-principles calculations with van der Waals dispersion corrections. The computed lattice parameters for cubic and tetragonal phases \textit{at finite temperature} are found within 1{\%} of experimentally measured values. Furthermore, the finite-temperature potential energy surface shows how the mechanical properties of the cubic and tetragonal phases of CH$_{\mathrm{3}}$NH$_{\mathrm{3}}$PbI$_{\mathrm{3}}$ evolve with temperature. Finally, I discuss the implications of these calculations on the nature of the tetragonal-to-cubic phase transition, and show that the underpinnings of this transition can be largely attributed to the phonons associated with methylammonium cations. [Preview Abstract] |
Monday, March 14, 2016 12:03PM - 12:15PM |
B32.00003: Rashba Spin-Orbit Coupling Enhanced Carrier Lifetime in CH$_3$NH$_3$PbI$_3$ Fan Zheng, Liang Z. Tan, Shi Liu, Andrew M. Rappe Organometal halide perovskites are promising solar-cell materials for next-generation photovoltaic applications, in particular these materials have long carrier lifetime and diffusion length. Recently, the strong spin-orbit coupling of organometal halide perovskites have attracted the great attention, the consequences of the Rashba effect, driven by this strong spin-orbit coupling, on the photovoltaic properties of these materials are largely unexplored. In this work, taking the electronic structure of methylammonium lead iodide as an example, we propose an intrinsic mechanism for enhanced carrier lifetime in 3D Rashba materials. Based on first-principles calculations and a Rashba spin-orbit model, we demonstrate that the recombination rate is reduced due to the spin-forbidden transition. These results are important for understanding the fundamental physics of organometal halide perovskites and for optimizing and designing the materials with better performance. The proposed mechanism including spin degrees of freedom offers a new paradigm of using 3D Rashba materials for photovoltaic applications. [Preview Abstract] |
Monday, March 14, 2016 12:15PM - 12:27PM |
B32.00004: Rotational Dynamics of Organic Cations in CH$_{\mathrm{3}}$NH$_{\mathrm{3}}$PbI$_{\mathrm{3}}$ Perovskite Tianran Chen, Benjamin Foley, Bahar Ipek, Madhusudan Tyagi, John Copley, Craig Brown, Joshua Choi, Seung-Hun Lee Methylammonium lead iodide (CH$_{\mathrm{3}}$NH$_{\mathrm{3}}$PbI$_{\mathrm{3}})$ based solar cells have shown impressive power conversion efficiencies of above 20{\%}. However, the microscopic mechanism of the high photovoltaic performance is yet to be fully understood. Particularly, the dynamics of CH$_{\mathrm{3}}$NH$_{\mathrm{3}}^{\mathrm{+}}$ cations and their impact on relevant processes are still poorly understood. Using elastic and quasi-elastic neutron scattering techniques and group theoretical analysis, we studied rotational modes of the CH$_{\mathrm{3}}$NH$_{\mathrm{3}}^{\mathrm{+}}$ cation in CH$_{\mathrm{3}}$NH$_{\mathrm{3}}$PbI$_{\mathrm{3}}$.$^{\mathrm{[1]}}$ Our results show that, in the cubic and tetragonal phases, the CH$_{\mathrm{3}}$NH$_{\mathrm{3}}^{\mathrm{+}}$ ions exhibit four-fold rotational symmetry of the C-N axis (C$_{\mathrm{4}})$ along with three-fold rotation around the C-N axis (C$_{\mathrm{3}})$, while in orthorhombic phase only C$_{\mathrm{3}}$ rotation is present. Around room temperature, the characteristic relaxation time for the C$_{\mathrm{4}}$ rotation is found to be 5ps while for the C$_{\mathrm{3}}$ rotation is 1ps. The T-dependent rotational relaxation times were fitted with Arrhenius equations to obtain activation energies. Our data show a close correlation between the C$_{\mathrm{4}}$ rotational mode and the temperature dependent dielectric permittivity. Our findings on the rotational dynamics of CH$_{\mathrm{3}}$NH$_{\mathrm{3}}^{\mathrm{+}}$ and the associated dipole have important implications on understanding the low exciton binding energy and slow charge recombination rate in CH$_{\mathrm{3}}$NH$_{\mathrm{3}}$PbI$_{\mathrm{3}}$ which are directly relevant for the high solar cell performance. [1] T. Chen et al., Phys. Chem. Chem. Phys., 2015, DOI: 10.1039/C5CP05348J. [Preview Abstract] |
Monday, March 14, 2016 12:27PM - 12:39PM |
B32.00005: Imaging the long diffusion lengths of photo-generated carriers in mixed halide perovskite films Shuhao Liu, Lili Wang, Clemens Burda, Xuan Gao Organometal halide perovskite has emerged as a promising photovoltaic material due to its low-cost synthesis process and outstanding performance. Though long diffusion length of photo-generated carriers plays a very important role in its success, a direct measurement of carrier diffusion lengths is still lacking. We fabricated highly crystalline CH$_{3}$NH$_{3}$PbI$_{3-x}$Cl$_{x}$ thin film devices on SiO$_{2}$/Si substrate with either nickel or gold as contact electrodes and parylene as encapsulation layer. By performing spatially scanned photocurrent imaging measurement with a local illumination spot, we show that photo-generated carriers in the prepared perovskite have very long diffusion lengths, with hole diffusion length $L_{h}= 22\pm 7\mu m$ and electron diffusion length $L_{e}= 8\pm 4\mu m$. Our work provides scanning photocurrent microscopy as a powerful tool to directly extract hole and electron diffusion lengths at the same time, and may be further used to elucidate the vastly different carrier diffusion lengths (on order of $100nm$ to $100\mu m$) in organometal halide perovskites prepared by different methods. [Preview Abstract] |
Monday, March 14, 2016 12:39PM - 12:51PM |
B32.00006: Characterizing structural overpotentials for bubble evolution on nanostructured semiconductor-electrocatalyst interfaces Robert H. Coridan Nanostructured electrocatalysts can improve the kinetics of solar-driven photocatalysis at a semiconductor-liquid junction while minimizing the effect on the energetics of that junction. A relevant example is Pt-decorated Si electrodes for hydrogen evolution from water splitting. Nanostructuring can also impair the reaction kinetics by introducing mass transport overpotentials. For reactions that evolve gas, the active surface area can be blocked by bubbles on discrete catalytic sites, possibly halting the reaction entirely. Here, we explore these issues by measuring the high-frequency dynamics of bubbles evolved from nanostructured electrocatalysts at a semiconductor-electrolyte interface. Using transmission x-ray phase contrast microscopy, we can image gas-evolving reactions as a way to directly measure the effects of adhesion, catalyst structure, and buoyancy on the reaction kinetics. From these measurements, we develop a model for electrolytic bubble evolution and transport that considers coalescence on neighboring sites, surface interactions, and the non-equilibrium shape dynamics of bubbles. This model can be used to identify favorable catalyst motifs that promote bubble clearance and mitigate their influence on reaction kinetics for water splitting applications. [Preview Abstract] |
Monday, March 14, 2016 12:51PM - 1:27PM |
B32.00007: TBA Invited Speaker: Gordana Dukovic |
Monday, March 14, 2016 1:27PM - 1:39PM |
B32.00008: Highly stable bimetallic AuIr/TiO2 catalyst: physical origin of the intrinsic stability against sintering. Ernesto Marinero, Chan Wan Han, Paulami Majundar, Antonio Aguilar-Tapia, Rodolfo Zanella, Jeffrey Greeley, Volkan Otarlan It has been a long-lived research topic in the field of heterogeneous catalysis to find a way to stabilizing supported Au catalysts against sintering. Herein, we report highly stable AuIr bimetallic nanoparticles on TiO2 synthesized by sequential deposition-precipitation. To understand the physical origin of the high stability AuIr/TiO2 system, we have used scanning transmission electron microscopy (STEM), STEM-tomography and density functional theory (DFT) calculations. 3D structures of AuIr/TiO2 obtained by STEM-tomography indicate that AuIr nanoparticles on TiO2 have intrinsically lower free energy and less driving force for sintering than Au nanoparticles. DFT calculations on segregation behavior of AuIr slabs on TiO2 showed that the presence of Ir near the TiO2 surface increases the adhesion energy of the bimetallic slabs to the TiO2 and the attractive interactions between Ir and TiO2 lead to higher stability of the AuIr nanoparticles compared to Au nanoparticles. [Preview Abstract] |
Monday, March 14, 2016 1:39PM - 1:51PM |
B32.00009: Surface Proton Hopping and Coupling Pathway of Water Oxidation on Cobalt Oxide Catalyst. Hieu Pham, Mu-jeng Cheng, Heinz Frei, Lin-Wang Wang We propose an oxidation pathway of water splitting on cobalt oxide surface with clear thermodynamic and kinetic details. The density-functional theory studies suggest that the coupled proton-electron transfer is not necessarily sequential and implicit in every elementary step of this mechanistic cycle. Instead, the initial O-O bond could be formed by the landing of water molecule on the surface oxos, which is then followed by the dispatch of protons through the hopping manner and subsequent release of di-oxygen. Our theoretical investigations of intermediates and transition states indicate that all chemical conversions in this pathway, including the proton transfers, are possible with low activation barriers, in addition to their favorable thermodynamics. Our hypothesis is supported by recent experimental observations of surface superoxide that is stabilized by hydrogen bonding to adjacent hydroxyl group, as an intermediate on fast-kinetics catalytic site. [Preview Abstract] |
Monday, March 14, 2016 1:51PM - 2:03PM |
B32.00010: The Dye Sensitized Photoelectrosynthesis Cell (DSPEC) for Solar Water Splitting and CO2 Reduction Thomas Meyer, Leila Alibabaei, Benjamin Sherman, Matthew Sheridan, Dennis Ashford, Alex Lapides, Kyle Brennaman, Animesh Nayak, Subhangi Roy, Kyung-Ryang Wee, Melissa Gish, Jerry Meyer, John Papanikolas The dye-sensitized photoelectrosynthesis cell (DSPEC) integrates molecular level light absorption and catalysis with the bandgap properties of stable oxide materials such as TiO2 and NiO. Excitation of surface-bound chromophores leads to excited state formation and rapid electron or hole injection into the conduction or valence bands of n or p-type oxides. Addition of thin layers of TiO2 or NiO on the surfaces of mesoscopic, nanoparticle films of semiconductor or transparent conducting oxides to give core/shell structures provides a basis for accumulating multiple redox equivalents at catalysts for water oxidation or CO2 reduction. [Preview Abstract] |
Monday, March 14, 2016 2:03PM - 2:15PM |
B32.00011: The Interface Between Chemical and Oxide Materials in the DSPEC. Thomas Meyer, Leila Alibabaei, Benjamin Sherman, Matthew Sheridan, Dennis Ashford, Alex Lapides, Kyle Brennaman, Animesh Nayak, Subhangi Roy Significant challenges exist for both chemical and oxide materials in the Dye Sensitized Photoelectrosynthesis Cell (DSPEC) for water oxidation or CO2 reduction. They arise from light absorption, the energetics of electron or hole injection, the accumulation of multiple redox equivalents at catalysts for water oxidation or water/CO2 reduction in competition with back electron transfer, and sustained, long term performance. These challenges are being met by the use of a variety of chromophores (metal complexes, organic dyes, porphyrins), broad application of nanoparticle mesoscopic oxide films, atomic layer deposition (ALD) to prepare core/shell and stabilizing overlayer structures, and recent advances in the molecular catalysis of water oxidation and CO2 reduction. [Preview Abstract] |
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