Direct Observation of Dynamic Symmetry Breaking above Room Temperature in Methylammonium Lead Iodide Perovskite

Lead halide perovskites such as methylammonium lead triiodide (MAPI) have outstanding optical and electronic properties for photovoltaic applications, yet a full understanding of how this solution processable material works so well is currently missing. Previous research has revealed that MAPI possesses multiple forms of static disorder regardless of preparation method, which is surprising in light of its excellent performance. Using high energy resolution inelastic X-ray (HERIX) scattering, we measure phonon dispersions in MAPI and find direct evidence for another form of disorder in single crystals: large amplitude anharmonic zone-edge rotational instabilities of the PbI_6 octahedra that persist to room temperature and above, left over from structural phase transitions that take place tens to hundreds of degrees below. Phonon calculations show that the orientations of the methylammonium couple strongly and cooperatively to these modes. The result is a non-centrosymmetric, instantaneous local structure, which we observe in atomic pair distribution function (PDF) measurements. This local symmetry breaking is unobservable by Bragg diffraction, but can explain key material properties such as the structural phase sequence, ultra low thermal transport, and large minority charge carrier lifetimes despite moderate carrier mobility.Comment: 30 pages, 11 figure

Study of Rainout of Radioactivity in Illinois: Tenth Progress Report

published or submitted for publicationis peer reviewedOpe

In situ interface engineering for probing the limit of quantum dot photovoltaic devices.

Quantum dot (QD) photovoltaic devices are attractive for their low-cost synthesis, tunable band gap and potentially high power conversion efficiency (PCE). However, the experimentally achieved efficiency to date remains far from ideal. Here, we report an in-situ fabrication and investigation of single TiO2-nanowire/CdSe-QD heterojunction solar cell (QDHSC) using a custom-designed photoelectric transmission electron microscope (TEM) holder. A mobile counter electrode is used to precisely tune the interface area for in situ photoelectrical measurements, which reveals a strong interface area dependent PCE. Theoretical simulations show that the simplified single nanowire solar cell structure can minimize the interface area and associated charge scattering to enable an efficient charge collection. Additionally, the optical antenna effect of nanowire-based QDHSCs can further enhance the absorption and boost the PCE. This study establishes a robust 'nanolab' platform in a TEM for in situ photoelectrical studies and provides valuable insight into the interfacial effects in nanoscale solar cells

Acid precipitation: origins and distribution in the United States

Ope

Multiple Exciton Generation in Quantum Dot Solar Cells

Photovoltaics are limited in their power conversion efficiency (PCE) by very rapid relaxation of energetic carriers to the band edge. Therefore, photons from the visible and ultraviolet parts of the spectrum typically are not efficiently converted into electrical energy. One approach that can address this is multiple exciton generation (MEG), where a single photon of sufficient energy can generate multiple excited electron-hole pairs. This process has been shown to be more efficient in quantum dots than bulk semiconductors, but it has never been demonstrated in the photocurrent of a solar cell. In order to demonstrate that multiple exciton generation can address fundamental limits for conventional photovoltaics, I have developed prototype devices from colloidal PbS and PbSe quantum dot inks. I have characterized both the colloidal suspensions and films of quantum dots with the goal of understanding what properties determine the efficiency of the solar cell and of the MEG process. I have found surface chemistry effects on solar cells, photoluminescence, and MEG, and I have found some chemical treatments that lead to solar cells showing MEG. These devices show external quantum efficiency (EQE) greater than 100% for certain parts of the solar spectrum, and I extract internal quantum efficiency (IQE) consistent with previous measurements of colloidal suspensions of quantum dots. These findings are a small first step toward breaking the single junction Shockley-Queisser limit of present-day first and second generation solar cells, thus moving photovoltaic cells toward a new regime of efficiency

Effect of Contrail Cirrus on Surface Weather Conditions in the Midwest - Phase I

published or submitted for publicationis peer reviewedOpe

Study of Atmospheric Pollution Scavenging: Sixteenth Progress Report

published or submitted for publicationis peer reviewedOpe

The chemistry of precipitation for the island of Hawaii during the HAMEC Project.

The 1995 ISWS Publications Catalog entry for ISWS CR 361 lists this work. The CR series statement has been added to this record on the basis of this information even though there is no reference to the ISWS series in the work itself.During the Hawaii Mesoscale Energy and Climate Project (HAMEC), a program of precipitation chemistry measurements was conducted using daily and sequential collection techniques. Determination of pH and conductivity were made on site for the daily samples, and all samples were shipped to the Illinois State Water Survey for analysis of major ions. During the period of study,June 11-24, 1980, over 300 sequential rain samples were collected at the Hilo site. The acidity of these samples was very similar to the 5-year acidity distribution of Hilo with a median pH of 4.7. Enrichment calculations showed that the chloride and magnesium levels were consistent with seawater ratios; potassium and calcium levels were 20%-40% greater and sulfate was more than 100% in excess of levels expected from seawater ratios. The daily samples collected at nine sites from Hilo (60 m) to Mauna Loa Observatory (3400 m) showed a steady decrease in seasalt constituents with elevation. The acidity also decreased with elevation, a condition that did not follow the increase with elevation measured over the previous 5-year sampling program. It was also found that nitrate decreased less rapidly with elevation than did excess sulfate. Implications of these HAMEC results are interpreted in terms of long-range transport. Differences in field and laboratory pH measurements are explained as an indication of unmeasured inorganic acids. At the time of the field pH measurements (within 24 hours of the rainfall) the organic acids were calculated to contribute hydrogen ion concentrations that were from 35% to 282% of the levels from strong mineral acids.NOAAOpe

Illinois Precipitation Enhancement Program, Phase 1: Interim Report for 1 September 1971 - 30 June 1972

published or submitted for publicationis peer reviewedOpe