Image resonance in the many-body density of states at a metal surface

The electronic properties of a semi-infinite metal surface without a bulk gap are studied by a formalism that is able to account for the continuous spectrum of the system. The density of states at the surface is calculated within the GW approximation of many-body perturbation theory. We demonstrate the presence of an unoccupied surface resonance peaked at the position of the first image state. The resonance encompasses the whole Rydberg series of image states and cannot be resolved into individual peaks. Its origin is the shift in spectral weight when many-body correlation effects are taken into account

Metal Halide Perovskite Polycrystalline Films Exhibiting Properties of Single Crystals

Metal halide perovskites are generating enormous excitement for use in solar cells and light-emission applications, but devices still show substantial non-radiative losses. Here, we show that by combining light and atmospheric treatments, we can increase the internal luminescence quantum efficiencies of polycrystalline perovskite films from 1% to 89%, with carrier lifetimes of 32 μs and diffusion lengths of 77 μm, comparable with perovskite single crystals. Remarkably, the surface recombination velocity of holes in the treated films is 0.4 cm/s, approaching the values for fully passivated crystalline silicon, which has the lowest values for any semiconductor to date. The enhancements translate to solar cell power-conversion efficiencies of 19.2%, with a near-instant rise to stabilized power output, consistent with suppression of ion migration. We propose a mechanism in which light creates superoxide species from oxygen that remove shallow surface states. The work reveals an industrially scalable post-treatment capable of producing state-of-the-art semiconducting films.S.D.S. has received funding from the European Union's Seventh Framework Program (Marie Curie Actions) under REA grant number PIOF-GA-2013-622630. This work made use of the Shared Experimental Facilities supported in part by the MRSEC Program of the National Science Foundation (NSF) under award number MDR – 1419807. R.B. acknowledges support from the MIT Undergraduate Research Opportunities Program (UROP). A.O. acknowledges support from the NSF under grant no. 1605406 (EP/L000202). D.G. acknowledges the China Scholarship Council for funding, file no. 201504910812. The authors acknowledge funding from the Engineering and Physical Sciences Research Council (EPSRC) under EP/P02484X/1 and the Programme Grant EP/M005143/1. M.S.I. and C.E. acknowledge support from the EPSRC Program grant on Energy Materials (EP/KO16288) and the Archer HPC/MCC Consortium (EP/L000202). E.M.H. gratefully acknowledges the Netherlands Organization for Scientific Research (NWO) Echo number 712.014.007 for funding. The work was also partially supported by Eni S.p.A. via the Eni-MIT Solar Frontiers Center. The authors thank Mengfei Wu and Marc Baldo for access to an integrating sphere, Jay Patel and Michael Johnston for EQE verifications, and Eli Yablonovitch and Luis Pazos-Outón for helpful discussion

Direct-indirect character of the bandgap in methylammonium lead iodide perovskite.

Metal halide perovskites such as methylammonium lead iodide (CH3NH3PbI3) are generating great excitement due to their outstanding optoelectronic properties, which lend them to application in high-efficiency solar cells and light-emission devices. However, there is currently debate over what drives the second-order electron-hole recombination in these materials. Here, we propose that the bandgap in CH3NH3PbI3 has a direct-indirect character. Time-resolved photo-conductance measurements show that generation of free mobile charges is maximized for excitation energies just above the indirect bandgap. Furthermore, we find that second-order electron-hole recombination of photo-excited charges is retarded at lower temperature. These observations are consistent with a slow phonon-assisted recombination pathway via the indirect bandgap. Interestingly, in the low-temperature orthorhombic phase, fast quenching of mobile charges occurs independent of the temperature and photon excitation energy. Our work provides a new framework to understand the optoelectronic properties of metal halide perovskites and analyse spectroscopic data

Lattice strain causes non-radiative losses in halide perovskites

Halide perovskites are found to exhibit strain patterns over large areas, which influences the lifetimes of charge carriers.EPSR

Optical nanofibers and spectroscopy

We review our recent progress in the production and characterization of tapered optical fibers with a sub-wavelength diameter waist. Such fibers exhibit a pronounced evanescent field and are therefore a useful tool for highly sensitive evanescent wave spectroscopy of adsorbates on the fiber waist or of the medium surrounding. We use a carefully designed flame pulling process that allows us to realize preset fiber diameter profiles. In order to determine the waist diameter and to verify the fiber profile, we employ scanning electron microscope measurements and a novel accurate in situ optical method based on harmonic generation. We use our fibers for linear and non-linear absorption and fluorescence spectroscopy of surface-adsorbed organic molecules and investigate their agglomeration dynamics. Furthermore, we apply our spectroscopic method to quantum dots on the surface of the fiber waist and to caesium vapor surrounding the fiber. Finally, towards dispersive measurements, we present our first results on building and testing a single-fiber bi-modal interferometer.Comment: 13 pages, 18 figures. Accepted for publication in Applied Physics B. Changes according to referee suggestions: changed title, clarification of some points in the text, added references, replacement of Figure 13

Impact of microstructure on the electron-hole interaction in lead halide perovskites

Despite the remarkable progress in the performance of devices based on the lead halide perovskite semiconductor family, there is still a lack of consensus on their fundamental photophysical properties. Here, using magneto-optical transmission spectroscopy we elucidate the impact of the microstructure on the Coulomb interaction between photo-created electron-hole pairs in methylammonium lead triiodide (MAPbI₃) and the triple-cation lead mixed-halide composition, Cs₀.₀₅(MA₀.₁₇ FA₀.₈₃)₀.₉₅Pb(I₀.₈₃Br₀.₁₇)₃ (Cs: cesium, MA: methylammonium, FA: formamidinium) by investigating thin films with a wide range of grain sizes from tens of nanometers to microns. At low temperatures, in which thermal fluctuations of the interactions are frozen and the rotational disorder of the organic cation is negligible, the exciton binding energy and reduced effective mass of carriers remain effectively unchanged with grain size. We conclude that the microstructure plays a negligible role in the Coulomb interaction of the photo-created electron-hole pairs, in contrast to previous reports. This renewed understanding of the relationship between these fundamental electronic properties and the microstructure is critical for future fundamental studies and improving device design.The authors acknowledge support from the Australian Government through the Australian Renewable Energy Agency (ARENA) and the Australian Centre for Advanced Photovoltaics (ACAP). The views expressed herein are not necessarily the views of the Australian Government, and the Australian Government does not accept responsibility for any information or advice contained herein. S. D. S. acknowledges funding from the People Programme (Marie Curie Actions) of the European Union's Seventh Framework Programme (FP7/2007-2013) under REA grant agreement number PIOF-GA-2013-622630. This work was partially supported by ANR JCJC project milliPICS, the Région Midi-Pyrénées under contract MESR 13053031, BLAPHENE project under IDEX program Emergence and Programme des Investissements d'Avenir under the program ANR-11-IDEX-0002-02, reference ANR-10-LABX-0037-NEXT. Part of the work has been supported by TERASPEC grant within IDEX Emergence program of University of Toulouse. Zhuo Yang and Nan Zhang hold a fellowship from the Chinese Scholarship Council (CSC). This work was supported by EPSRC (UK) via its membership to the EMFL (grant no. EP/N01085X/1). M. A. J. gratefully acknowledges Cambridge Materials Limited for a PhD scholarship

Local Strain Heterogeneity Influences the Optoelectronic Properties of Halide Perovskites

Halide perovskites are promising semiconductors for optoelectronics, yet thin films show substantial microscale heterogeneity. Understanding the origins of these variations is essential for mitigating parasitic losses such as non-radiative decay. Here, we probe the structural and chemical origins of the heterogeneity by utilizing scanning X-ray diffraction beamlines at two different synchrotrons combined with high-resolution transmission electron microscopy to spatially characterize the crystallographic properties of individual micrometer-sized perovskite grains in high-quality films. We reveal new levels of heterogeneity on the ten-micrometer scale (super-grains) and even ten-nanometer scale (sub-grain domains). By directly correlating these properties with their corresponding local time-resolved photoluminescence properties, we find that regions showing the greatest luminescence losses correspond to strained regions, which arise from enhanced defect concentrations. Our work reveals remarkably complex heterogeneity across multiple length scales, shedding new light on the defect tolerance of perovskites

Strongly Enhanced Photovoltaic Performance and Defect Physics of Air-Stable Bismuth Oxyiodide (BiOI)

Bismuth-based compounds have recently gained increasing attention as potentially nontoxic and defect-tolerant solar absorbers. However, many of the new materials recently investigated show limited photovoltaic performance. Herein, one such compound is explored in detail through theory and experiment: bismuth oxyiodide (BiOI). BiOI thin films are grown by chemical vapor transport and found to maintain the same tetragonal phase in ambient air for at least 197 d. The computations suggest BiOI to be tolerant to antisite and vacancy defects. All-inorganic solar cells (ITO|NiO$_x$|BiOI|ZnO|Al) with negligible hysteresis and up to 80% external quantum efficiency under select monochromatic excitation are demonstrated. The short-circuit current densities and power conversion efficiencies under AM 1.5G illumination are nearly double those of previously reported BiOI solar cells, as well as other bismuth halide and chalcohalide photovoltaics recently explored by many groups. Through a detailed loss analysis using optical characterization, photoemission spectroscopy, and device modeling, direction for future improvements in efficiency is provided. This work demonstrates that BiOI, previously considered to be a poor photocatalyst, is promising for photovoltaics.R.L.Z.H. thanks Magdalene College, Cambridge. L.C.L. and J.L.M.-D. thank the EPRSC Centre for Doctoral Training: New and Sustainable Photovoltaics, and the Cambridge Winton Programme for the Physics of Sustainability for funding. T.N.H. thanks the Cambridge Graphene Centre, funded by the EPSRC. K.H.L.Z. was supported by the Herschel Smith fellowship. The U.S.-based theory and synthesis portions of this work were supported primarily as part of the Center for Next Generation Materials by Design (CNGMD), an Energy Frontier Research Center funded by the DOE Office of Science, Basic Energy Sciences under Contract No. DE-AC36-08GO28308. The MIT-based characterization portion of this work was supported primarily through a TOTAL SA research grant funded through MITei, as well as a SusChem grant funded by the National Science Foundation (No. CBET-1605495). The TCSPC work was supported as part of the Center for Excitonics, an Energy Frontier Research Center funded by the U.S. Department of Energy, Office of Science, Office of Basic Energy Sciences under Award No. DE-SC0001088 (MIT). The computations were performed using resources sponsored by the Department of Energy’s Office of Energy Efficiency and Renewable Energy and located at the NREL. The authors also acknowledge the MRSEC Shared Experimental Facilities at MIT, supported by the National Science Foundation (No. DMF-08019762)

Device Optimization of Tris-Aluminum (Alq3) Based Bilayer Organic Light Emitting Diode Structures

In this work we present detailed analysis of the emitted radiation spectrum from tris(8-hydroxyquinoline) aluminum (Alq3) based bilayer OLEDs as a function of: the choice of cathode, the thickness of organic layers, and the position of the hole transport layer/Alq3 interface. The calculations fully take into account dispersion in the glass substrate, the indium tin oxide anode, and in the organic layers, as well as the dispersion in the metal cathode. Influence of the incoherent transparent substrate (1 mm glass substrate) is also fully accounted for. Four cathode structures have been considered: Mg/Ag, Ca/Ag, LiF/Al, and Ag. For the hole transport layer, N,N'-diphenyl-N,N'-(3-methylphenyl)-1,1'-biphenyl-4,4'-diamine (TPD) and N,N'-di(naphthalene-1-yl)-N,N'-diphenylbenzidine (NPB) were considered. As expected, emitted radiation is strongly dependent on the position of the emissive layer inside the cavity and its distance from the metal cathode. Although our optical model for an OLED does not explicitly include exciton quenching in vicinity of the metal cathode, designs placing the emissive layer near the cathode are excluded to avoid unrealistic results. Guidelines for designing devices with optimum emission efficiency are presented. Finally, several different devices were fabricated and characterized and experimental and calculated emission spectra were compared