Bulletin of the American Physical Society
APS March Meeting 2018
Volume 63, Number 1
Monday–Friday, March 5–9, 2018; Los Angeles, California
Session A11: Organometal Halide Perovskites IFocus
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Sponsoring Units: DMP Chair: Hanwei Gao, Florida State University Room: LACC 303A |
Monday, March 5, 2018 8:00AM - 8:36AM |
A11.00001: Developing metal-halide perovskites with optimal band gaps, slow recombination and high stability for tandem solar cells Invited Speaker: Michael McGehee We have made perovskite on silicon tandem solar cells with efficiency greater than 25 %. We have also made all-perovskite tandems using an ABX3 perovskite composition containing a mixture of tin and lead on the B site that have greater than 20% efficiency. We will show how the composition of the perovskite can be used to adjust the bandgap by controlling both the lattice parameter and octahedral tilting. We have measured the photovoltaic energy conversion efficiency and photoluminescence lifetime for a wide variety of perovskite compositions and observed interesting trends. We will present our progress in understanding why light -induced phase separation occurs in compounds with large amounts of bromine. |
Monday, March 5, 2018 8:36AM - 8:48AM |
A11.00002: Understanding the Role of Solvent-Solute Interactions on Hybrid Organic-Inorganic Perovskite Formation J. Hamill, Jr., Jeffrey Schwartz, Lynn Loo Hybrid organic-inorganic perovskites (HOIPs) formed from organoammonium iodide and lead iodide precursor solutions are promising materials for photovoltaic applications. While lead polyiodide and lead-solvent complexes formed in solution are intermediates for HOIP crystallization, the influence of solvent choice upon the formation of such intermediates is not well understood. We show that the formation of lead polyiodides in the precursor solutions is correlated with the basicity of the processing solvent (quantified by Gutmann’s donor number, DN). Solvents with low DN exhibit a strong propensity to allow the formation of lead polyhalides. We infer that such solvents interact weakly with lead iodide, which favors the precipitation of HOIP single crystals from solution. Conversely, high-DN solvents suppress the formation of lead polyiodides, indicative of strong lead-solvent coordination. Such solvents support the formation of stable precursor solutions for HOIP thin-film processing and may be added in fractional quantities to tune the basicity of the processing solvent. The tunabilty introduced by high-DN additives provides finer control over perovskite crystallization, post-deposition processability, and the morphology of HOIP active layers for photovoltaic applications. |
Monday, March 5, 2018 8:48AM - 9:00AM |
A11.00003: In Situ Dynamic Observations of Perovskite Crystallization and Morphology Evolution Qin Hu, Lichen Zhao, Feng Liu, Thomas Russell, Rui Zhu, Qihuang Gong Hybrid lead halide perovskites have emerged as high-performance photovoltaic materials with their extraordinary optoelectronic properties. In particular, the remarkable device efficiency is strongly influenced by the perovskite crystallinity and the film morphology. Here, we investigate the perovskites crystallization kinetics and growth mechanism in real time from liquid precursor continually to the final uniform film. We utilize some advanced in situ characterization techniques including synchrotron-based grazing incident X-ray diffraction to observe crystal structure and chemical transition of perovskites. The nano-assemble model from perovskite intermediated [PbI6]4- cage nanoparticles to bulk polycrystals is proposed to understand perovskites formation at a molecular- or nano-level. A crystallization-depletion mechanism is developed to elucidate the periodic crystallization and the kinetically trapped morphology at a mesoscopic level. Based on these in situ dynamics studies, the whole process of the perovskites formation and transformation from the molecular to the microstructure over relevant temperature and time scales is successfully demonstrated. |
Monday, March 5, 2018 9:00AM - 9:12AM |
A11.00004: Unlocking the Single-Domain Epitaxy of Halide Perovskites Lili Wang, Pei Chen, Non Thongprong, Margaret Young, Padmanaban Kuttipillai, Chuanpeng Jiang, Pengpeng Zhang, Kai Sun, Phillip Duxbury, Richard Lunt While the recent emergence of halide perovskites offer exciting opportunities for a range of thin-film electronics, current research on optoelectronic applications of inorganic lead-free halide perovskites has, to date, been less encouraging. These systems have likely been limited by the low degree of crystalline ordering and high defect density which has been linked in many semiconductors to impact carrier transport and recombination rate. The growth of epitaxial semiconductors and oxide perovskites has long since revolutionized the electronic and optic fields and continues to be exploited to uncover new quantum physics. The principles of true epitaxy have yet to be applied to halide perovskites. In this work we demonstrate the single-domain epitaxy of halide perovskites that is enabled by reactive vapor deposition. We uncover several new epitaxial phases via stoichiometry control that are stabilized with vastly differing lattice constants and accommodated via epitaxial rotation. We utilize precise epitaxial growth to fabricate quantum wells and extract the excited state Bohr radius. Moreover, we demonstrate the epitaxial lift-off and regrowth for further application. This work ultimately provides a new route to uncover the full potential of halide perovskites. |
Monday, March 5, 2018 9:12AM - 9:24AM |
A11.00005: Intermediate Bands in Hybrid Perovskites Arun Mannodi-Kanakkithodi, Ji-Sang Park, Duyen Cao, Nari Jeon, Alex Martinson, Maria Chan To further improve energy conversion efficiencies, we studied substitutions in MAPbX3 perovskites (where MA = methylammonium and X = Br/Cl) to obtain mid-gap states which can act as intermediate bands (IBs) for additional absorption of sub-gap photons. Density functional theory (DFT) calculations revealed that substitution of Pb by Co in MAPbBryCl3-y perovskites creates mid-gap energy states, which was experimentally confirmed via absorption and photoluminescence spectroscopy.1 The lack of a second sub-gap absorption feature indicated that the mid-gap states may be unfilled. We report the electronic structure changes upon substitution of Pb by other metals, specifically noting trends in parent band gap and substituent energy states within the band gap. We further determined the thermodynamic equilibrium growth conditions necessary for creating a stable external substitution that compensates for dominant intrinsic defects. Based on this work, promising candidates were identified as Pb-substituents in lead-based hybrid perovskites to create IB photovoltaic materials. |
Monday, March 5, 2018 9:24AM - 9:36AM |
A11.00006: Grain Boundaries Are Strong Sinks for Native defects in Methylammonium Lead Iodide Perovskite Weitao Shan, Wissam Saidi The organometal halide perovskites (OMHPs), and in particular methylammonium lead triiodide perovskite (MAPbI3, MA=CH3NH3), are attracting intense interest due to their unprecedented rapid increase in the solar efficiency in the past few years. We use density functional theory (DFT) with corrections due to van der Waals interactions and spin-orbit coupling, in conjunction with a thermodynamic approach to determine the stability and electronic properties of all native point defects, and their interplays with Σ5-(210) grain boundary (GB) in MAPbI3. The transition levels of charged defects are investigated with the inclusion of electrostatic charge corrections due to fictitious long-range interactions. We find that the GB region is a sink for most of the native defects under different synthesis conditions. For the crystalline MAPbI3 and the Σ5-(210) GB, we find respectively that only the p-type antisite defects MAI and PbI, where I replaces MA and Pb, introduce midgap transition levels and both are relatively stable under I-rich conditions. Hence, I-poor conditions are more preferable for synthesizing polycrystalline MAPbI3 layers with defects that have electronically benign character. |
Monday, March 5, 2018 9:36AM - 9:48AM |
A11.00007: Defect-Assisted Hole and Electron Recombination in Methylammonium Lead Iodide Perovskite Weibin Chu, Weitao Shan, Jin Zhao, Wissam Saidi We investigate the nonradiative electron and hole recombination facilitated by native point and extended grain boundary defects in methylammonium lead iodide perovskite (MAPbI3) using an ab initio nonadiabatic molecular dynamics (NAMD) approach in conjunction with density functional theory. The recombination rates are due to elastic and inelastic temperature-dependent electron-vibrational interactions, and are the main energy loss mechanisms in the solar sensitizer. We focus our study on the most stable defects under different synthesis conditions, which include shallow defects under moderate and poor iodine conditions, and defects with deep-level transition levels under iodine rich conditions. This investigation using NAMD combining with time-domain density functional theory will determine the lifetimes, decay mechanism, and the extent these defects can affect the solar efficiency. |
Monday, March 5, 2018 9:48AM - 10:00AM |
A11.00008: Suppression of degradation and polymorph transitions in nanoconfined MAPbI3 Xiaoqing Kong, Kamran Shayan, Sangchul Lee, Christian Ribeiro, Stefan Strauf, Stephanie Lee We present the effect of various extents of nanoconfinement on methylammonium lead iodide (MAPbI3) crystallization, polymorphism, and stability. Specifically, MAPbI3 precursors were spin coated from co-solutions onto flat SiO2/Si substrates, vertically oriented AAO nanorods and nanoporous AAO templates. Two-dimensional X-ray diffraction (XRD) patterns of MAPbI3 films revealed formation of the tetragonal phase on SiO2/Si substrates and AAO nanorods at room temperature. Surprisingly, the cubic phase of MAPbI3, typically observed only at temperatures above 60oC, was present within AAO nanopores. Temperature-dependent XRD analysis and micro photoluminescence revealed that the confinement of MAPbI3 crystals retarded polymorph phase transitions. Nanoconfinement was further observed to improve the stability of MAPbI3 crystals exposed to air. MAPbI3 deposited on SiO2/Si degraded to PbI2 completely after only 21 days. Nanoconfined MAPbI3 crystals formed on nanorod-coated SiO2 and in nanoporous AAO templates remained stable for more than ten times longer. Our current findings suggest nanoconfinement as a strategy to improve the stability of metal-halide perovskites for solar cell applications. |
Monday, March 5, 2018 10:00AM - 10:12AM |
A11.00009: Elucidating the Origin of Perovskites Degradation through Nanoimaging Elizabeth Tennyson, John Howard, Marina Leite The primary barrier for perovskite solar cells towards commercialization is related to the fact that they degrade when exposed/submitted to ambient conditions. Thus, elucidating the driving forces for materials’ instabilities is imperative. Because the perovskite absorbing layer is often composed by mesoscale constructs, we resolve their performance with nanoscale spatial resolution, as a function of humidity, illumination, and temperature. For that, we realize and implement a scanning microscopy platform to quantify and spatially resolve the perovskites local electrical response. We find local Voc variations >300 mV, and a dynamic behavior upon illumination due to ion migration. Further, we identify a fully reversible voltage response within grains for Cs-triple cations perovskites. We anticipate the measurements of perovskites under controlled environment to advance the knowledge of the transient physical behaviors observed for certain compositions, including lead-free options and alternatives for dual junction designs. |
Monday, March 5, 2018 10:12AM - 10:24AM |
A11.00010: Light Induced Degradation at the Interface between CH3NH3PbI3 and LiF Ben Ecker, John Kauppi, Congcong Wang, Yongli Gao Organometallic trihalide perovskites have shown to be remarkably efficient as a light absorbing layer in a solar cell, which has prompted a tremendous surge in research interests. Several of their environmental and long term operational instabilities preventing widespread commercialization, are still actively being investigated by the community. One of their more puzzling instabilities, is the influence of light exposure to the active perovskite layer. Previous reports have shown substantial chemical and structural degradation to the perovskite material as a result of exposure. In an effort to better understand the influence of light exposure on a full device, we investigated the impact light exposure had on the interface between CH3NH3PbI3 and LiF with xray photoemission spectroscopy measurements. An interface with LiF was chosen, as LiF is chemically stable under most conditions and has commonly been used as an insulating buffer layer in OLEDs. While the chemical degradation was similar to previous reports, the binding energy shifts of the interface with LiF differed with the pure perovskite degradation. Surface morphology changes were also recorded with a scanning electron microscope. |
Monday, March 5, 2018 10:24AM - 10:36AM |
A11.00011: Study of Structure and Electronic Properties of Hetero-Interfaces for Photovoltaic Applications Rabi Khanal, Nicholas Ayers, Soumik Banerjee, Samrat Choudhury The photovoltaic performance of hybrid halide perovskite is linked to the atomic and electronic structure at the interface between carrier generating perovskite layer and the electron transport layer (ETL). With the help of ab-initio molecular dynamics simulations and density functional theory calculations, we have determined the structure, electronic, and transport properties at the perovskite/ETL interface. As a model system, we used CH3NH3PbI3 hybrid halide perovskite and phenyl-C61-butyric acid methyl ester (PCBM) as the ETL. Our results reveal that PCBMs prefer to attach to the perovskite surface via the ester moiety. However, bonding at the interface is sensitive to the chemical composition at the surface termination (CH3NH3I vs. PbI2) of the perovskite. For PbI2 terminations, the carbonyl O atom of PCBM forms a bond with Pb atom on perovskite surface, whereas for CH3NH3I terminations methyl H on PCBM site forms bonds with I atom on the perovskite surface. Different preferences in bonding at the interface leads to distinct electronic and transport properties across two chemical terminations. |
Monday, March 5, 2018 10:36AM - 10:48AM |
A11.00012: High-performance, hysteresis free, ambipolar hybrid perovskite based field-effect transistors Noelia Devesa Canicoba, Kasun Fernando, Jean-Christophe Blancon, Fangze Liu, Laurent Le Brizoual, Regis Rogel, Jacky Even, Bruce W. Alphenaar, Wanyi Nie, Aditya Mohite Hybrid perovskites are solution processed crystalline materials with excellent electronic and optical properties, which enables high-efficiency optoelectronic devices. However, hybrid perovskites-based field effect transistor operation at room temperature has remained elusive. This is due to the non-reproducibility induced by polar nature of the structure coupled with ionic motions, which screens the capacitively coupled gate voltage. In this study, we report high-performance, hysteresis-free ambipolar FETs using highly crystalline hybrid perovskites thin films, operating at room temperature. We systematically improved the film quality, the effect of high-K dielectrics between the perovskites and gate. As a result, we obtained FETs with high trans-conductance with low subthreshold slopes leading to an on/off ration >104. Moreover, we achieve ambipolar transport at room temperature that strongly correlates to the choice of the gate-dielectric, that allow to tune the Fermi energy of perovskites for electrons and holes injections. We anticipate these results will open up the systematic investigation on the electronic properties in hybrid perovskites materials, for the opportunities to discover novel devices functionalities such as ultrasensitive photo-transistors and spin FETs. |
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