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  1. Influence of Annealing and Composition on the Crystal Structure of Mixed-Halide, Ruddlesden–Popper Perovskites

    Wright, Niara E. ; Qin, Xixi ; Xu, Junwei ; ... - Chemistry of Materials
    Mixed-halide two-dimensional (2D) hybrid organic–inorganic perovskites offer an important opportunity to control the band gap for applications in optoelectronic devices. In this study, we focus on phenethylammonium lead halide [(PEA)2Pb(I1–xBrx)4] films, the pure iodide form of which is one of the most widely studied optically active 2D perovskite systems. Resonant infrared, matrix-assisted pulsed laser evaporation is used to grow films to explore the effects of post-growth annealing and mixed-halide composition. The composition, crystal structure, and optical properties are studied for as-grown and annealed films of (PEA)2Pb(I1–xBrx)4 for x = 0, 0.25, 0.5, 0.75, and 1. First-principles calculations are used inmore » conjunction with the experimental data to explain the mixed-halide behavior that does not trend monotonically with the bromide content. Important results of this work are as follows: (i) X-ray diffraction reveals evidence for halide phase separation around x = 0.25, consistent with first-principles calculations, whereas no phase separation is observed for x = 0.5 and above and (ii) a unique photoluminescence (PL) peak splitting is observed for x = 0.75, a composition for which no compositional phase separation is observed. The PL splitting is tentatively explained by the coexistence of two distinct types of halide short-range ordering at x = 0.75. Overall, this study demonstrates that mixed-halide n = 1 Ruddlesden–Popper perovskites are not simple random alloys but that instead, they display distinct sites and ordering preferences of the different halide anions. These preferences are critical to understand and rationally tune the properties of the materials.« less

  2. Understanding Magnetic Exchange Interactions by the Pressure Dependent Curie Temperature in FeCoNiCuMn High Entropy Alloys

    Perrin, Alice ; McCall, Scott ; McElfresh, Michael ; ... - Journal of Phase Equilibria and Diffusion
    Here, we report the pressure (P) dependent Curie temperature, Tc (P) in a FeCoNiCuMn high entropy alloy (HEA). We analyze Tc (P) in terms of d-orbital contraction to explain changes in magnetic exchange interactions (Jex). Considerations of the d-radius contraction inferred from the composition dependence of Tc in γ-Fe-Ni are combined with experimental data for P-dependent lattice constants and magnetic measurements of Tc (P), to calculate contributions of atomic spacing and d-orbital radii to Jex. We show the d-orbital contraction with P captures most of the Tc variation in this alloy.

  3. Arylammonium-Assisted Reduction of the Open-Circuit Voltage Deficit in Wide-Bandgap Perovskite Solar Cells: The Role of Suppressed Ion Migration

    Chen, Cong ; Song, Zhaoning ; Xiao, Chuanxiao ; ... - ACS Energy Letters
    Surface treatment using large alkyl/aryl ammonium cations has demonstrated reduced open-circuit voltage (VOC) deficits in perovskite solar cells (PSCs), but the origin of the improvements has been vaguely attributed to defect passivation. Here, we combine the microscopic probing of the local electrical properties, thermal admittance spectroscopic analysis, and first principles calculations to elucidate the critical role of arylammonium interface layers on suppressing the ion migration in wide-bandgap (WBG) PSCs. Our results reveal that arylammonium surface treatment using phenethylammonium iodide increases the activation energy barrier for ion migration on the surface, which suppresses the accumulation of charge defects at surface andmore » grain boundaries (GBs), leading to reduced dark saturation current density in WBG PSCs. Furthermore, with device optimization, our champion 1.73-eV PSC delivers a power conversion efficiency of 19.07% with a VOC of 1.25 V, achieving a VOC-deficit of 0.48 V.« less

  4. A Measurement of the Cosmic Microwave Background Lensing Potential and Power Spectrum from 500 deg2 of SPTpol Temperature and Polarization Data

    Wu, W. L. K. ; Mocanu, L. M. ; Ade, P. A. R. ; ... - The Astrophysical Journal (Online)
    In this work, we present a measurement of the cosmic microwave background (CMB) lensing potential using 500 deg$^2$ of 150 GHz data from the SPTpol receiver on the South Pole Telescope. The lensing potential is reconstructed with signal-to-noise per mode greater than unity at lensing multipoles $$L \lesssim 250$$, using a quadratic estimator on a combination of CMB temperature and polarization maps. We report measurements of the lensing potential power spectrum in the multipole range of $100< L < 2000$ from sets of temperature-only, polarization-only, and minimum-variance estimators. We measure the lensing amplitude by taking the ratio of the measuredmore » spectrum to the expected spectrum from the best-fit $$\Lambda$$CDM model to the $$\textit{Planck}$$ 2015 TT+lowP+lensing dataset. For the minimum-variance estimator, we find $$A_{\rm{MV}} = 0.944 \pm 0.058{\rm (Stat.)}\pm0.025{\rm (Sys.)}$$; restricting to only polarization data, we find $$A_{\rm{POL}} = 0.906 \pm 0.090 {\rm (Stat.)} \pm 0.040 {\rm (Sys.)}$$. Considering statistical uncertainties alone, this is the most precise polarization-only lensing amplitude constraint to date (10.1 $$\sigma$$), and is more precise than our temperature-only constraint. We perform null tests and consistency checks and find no evidence for significant contamination.« less

  5. Implementation of High-Order Multireference Coupled-Cluster Methods on Intel Many Integrated Core Architecture

    Aprà, Edoardo ; Kowalski, Karol - Journal of Chemical Theory and Computation
    Here we discuss the implementation of multireference coupled-cluster formalism with singles, doubles, and noniterative triples (MRCCSD(T)), which is capable of taking advantage of the processing power of the Intel Xeon Phi coprocessor. We discuss the integration of two levels of parallelism underlying the MRCCSD(T) implementation with computational kernels designed to offload the computationally intensive parts of the MRCCSD(T) formalism to Intel Xeon Phi coprocessors. Special attention is given to the enhancement of the parallel performance by task reordering that has improved load balancing in the noniterative part of the MRCCSD(T) calculations. We also discuss aspects regarding efficient optimization and vectorizationmore » strategies.« less
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