Electroabsorption Spectroscopy Measurements of the Exciton Binding Energy, Electron–Hole Reduced Effective Mass, and Band Gap in the Perovskite CH₃NH₃PbI₃

We use electroabsorption (EA) spectroscopy to measure the exciton binding energy (<i>E</i>_B), electron–hole reduced effective mass (μ), and one-electron band gap (<i>E</i>_g) at the fundamental absorption edge of the hybrid organic–inorganic perovskite CH₃NH₃PbI₃ in its tetragonal phase at 300 K. By studying the second-harmonic EA spectra at the fundamental absorption edge we establish that the room-temperature EA response in CH₃NH₃PbI₃ follows the low-field Franz–Keldysh–Aspnes (FKA) effect. Following FKA analysis we find that μ = 0.12 ± 0.03<i>m</i>₀, <i>E</i>_B = 7.4 meV, and <i>E</i>_g = 1.633 eV. Our results provide direct experimental evidence that at room temperature primary transitions occurring in CH₃NH₃PbI₃ can essentially be described in terms of free carrier generation

Exploring the Enigma of 4709 Ennomos

Large Trojan asteroids are characteristically dark, having albedos that are typically in the range 0.03 to 0.08 (Fernandez et al., 2003). One notable exception is 4709 Ennomos with an unusually high measured albedo of about 0.13 (Fernandez et al., 2003). This corresponds to an albedo of more than 10 standard deviations above the mean of the group of 32 large Trojans sampled by Fernandez et al (2003). There are two main explanations for the anomalous albedo: Ennomos\u27s surface composition may truly be different from similarly-sized Trojans and be richer in more highly-reflective species, or the assumptions that go into the modeling used to derive diameter and albedo are inapplicable to Ennomos because of unusual physical or thermal properties. For the first hypothesis, so far only upper limits to compositional signatures have been found (e.g. Yang and Jewitt 2007). In this work we address the second hypothesis. One plausible explanation is that Ennomos’ rotation period is sufficiently fast or its thermal inertia is sufficiently high so as to preclude the use of a zero-thermal memory thermal model (Lebofsky and Spencer 1989, Harris 1998) i.e. the model actually used to calculate its albedo. An alternative explanation is that shape or topographic anomalies conspired to reduce the thermal emission, causing the model - which assumes a spherical body - to underestimate the diameter. To address these issues, we obtained BVRI time-series CCD photometry of Ennomos with the University of Hawaii\u27s 88 inch telescope on February 8, 9, and 10, 2003. The goals were to determine Ennomos’ rotation period, basic shape, and visible colors, and we will present these results. We will also discuss what the results imply about the nature of Ennomos\u27s surface

A Lightcurve and Color Analysis of Asteroid 4709 Ennomos

We will present results from our study of the Jovian Trojan asteroid 4709 Ennomos, an asteroid with an unusually high estimated albedo. Large Trojan asteroids (radius \u3e 25 km) have a mean V-band geometric albedo of 0.041 with very little variation (standard deviation = 0.007 ; Fernandez et. al. 2003). Smaller Trojan asteroids, with radius \u3c 25 km, have both higher albedo (mean = 0.12) and wider variation (standard deviation = 0.065; Fernandez et. al. 2010). Asteroid 4709 Ennomos has a radius of about 38 km and a geometric albedo of about 0.15: several standard deviations above the mean albedo of other large Trojans, but very similar to the albedos of small Trojans. One plausible explanation of Ennomos’ apparently high albedo is that its rotation period may be sufficiently fast so as to invalidate the use of a low-thermal memory thermal model to calculate its size and albedo--the model used for Ennomos. To test this hypothesis, we obtained time series CCD photometry of Ennomos’ light curve using the University of Hawaii 88-inch telescope on UT February 8 through 10, 2003. Analysis of Ennomos’ light curve and rotation period will determine if an isothermal latitude model is more appropriate. Since asteroids of Ennomos’ size, both Trojans and Main-Belt, tend to be relatively slow rotators, a high rotation speed would be unusual. We therefore also consider some of the other hypotheses to explain Ennomos’ high albedo. For example, comparing Ennomos’ colors to those of other asteroid groups can give clues to the reason for an elevated albedo. To this end, we also obtained BVRI colors of Ennomos during our 2003 observing run. We will present a comparison between Ennomos’ colors, other published large Trojan and small Trojan colors (e.g. Jewitt & Luu 1990), and small asteroid colors (e.g. Karlsson et al. 2009)

Application of the AutoClass Artificial Intelligence Program to Asteroidal Data

As our digital databases grow, datasets become less tractable and investigating alternative analysis techniques such as artificial intelligence algorithms becomes more important. One such program, AutoClass, which was developed by NASA\u27s Artificial Intelligence Branch, uses Bayesian classification theory to automatically choose the most probable classification distribution to describe a dataset. To investigate its usefulness to the Planetary community, we tested its ability to reproduce the taxonomic classes as defined by Tholen and Barucci (1989). We started our evaluation by entering all Tholen identified C, S, or X type Eight Color Asteroid Survey asteroids with a color difference error of less than +/- 0.05 magnitudes. Of these 406 asteroids, AutoClass was able to firmly classify 346 (85%), identifying the remaining 60 asteroids as belonging to more than one class. Of the 346 asteroids that AutoClass classified, all but 3 (\u3c1%) were classified as they had been in the Tholen classification scheme. The three that were misclassified had color errors estimated to be greater than +/- 0.04 magnitudes (though several other asteroids with such errors were classified correctly). To further test AutoClass, we expanded our reach to include all taxonomic types in the ECAS data, and further to include the nine wavelengths used to create the Bus and Binzel taxonomic superclasses (2002), with similar results. The initial successes of AutoClass and its ability to scan large domains for natural classes, showcase its exciting potential as a new discovery tool for Planetary scientists

Probing Charge Transfer Character in Modern Donor/Acceptor Materials via Electroabsorption Spectroscopy

We use electroabsorption (EA) spectroscopy to probe the charge transfer (CT) character in neat films and blends of donors and acceptors of interest for organic electronic applications. In particular, we compare the CT character in two polymer donor and non-fullerene acceptor blends, including 3,9-bis(2-methylene-((3-(1,1-dicyanomethylene)-6,7-difluoro)-indanone))-5,5,11,11-tetrakis(4-hexylphenyl)-dithieno[2,3-d:2′,3′-d′]-s-indaceno[1,2-b:5,6-b′]dithiophene (IT-4F) and 2,2′-((2Z,2′Z)-((12,13-bis(2-ethylhexyl)-3,9-diundecyl-12,13-dihydro-[1,2,5]thiadiazolo[3,4-e]thieno[2″,3″:4′,5′]thieno[2′,3′:4,5]pyrrolo[3,2-g]thieno[2′,3′:4,5]thieno[3,2-b]indole-2,10-diyl)bis(methanylylidene))bis(5,6-difluoro-3-oxo-2,3-dihydro-1H-indene-2,1-diylidene))dimalononitrile (Y6). Like classic polymer/fullerene blends, the blend based on IT-4F exhibits primarily first derivative-like EA features, suggesting localized exciton formation upon photoexcitation. However, the Y6-based blend has an EA spectrum that is dominated by second derivative-like features, consistent with CT character. We show that this signal originates primarily from Y6. We find that Y6 exhibits the highest dipole moment change (7.5 ± 2.5 D) of the molecules that comprise this study, consistent with a high degree of the CT character, and a relatively large polarization volume of 361 ± 70 Å3, consistent with strong electron delocalization. These results point to the origins of exceptional performance of organic photovoltaics (OPVs) based on Y6

Probing Charge Transfer Character in Modern Donor/Acceptor Materials via Electroabsorption Spectroscopy

We use electroabsorption (EA) spectroscopy to probe the charge transfer (CT) character in neat films and blends of donors and acceptors of interest for organic electronic applications. In particular, we compare the CT character in two polymer donor and non-fullerene acceptor blends, including 3,9-bis(2-methylene-((3-(1,1-dicyanomethylene)-6,7-difluoro)-indanone))-5,5,11,11-tetrakis(4-hexylphenyl)-dithieno[2,3-d:2′,3′-d′]-s-indaceno[1,2-b:5,6-b′]dithiophene (IT-4F) and 2,2′-((2Z,2′Z)-((12,13-bis(2-ethylhexyl)-3,9-diundecyl-12,13-dihydro-[1,2,5]thiadiazolo[3,4-e]thieno[2″,3″:4′,5′]thieno[2′,3′:4,5]pyrrolo[3,2-g]thieno[2′,3′:4,5]thieno[3,2-b]indole-2,10-diyl)bis(methanylylidene))bis(5,6-difluoro-3-oxo-2,3-dihydro-1H-indene-2,1-diylidene))dimalononitrile (Y6). Like classic polymer/fullerene blends, the blend based on IT-4F exhibits primarily first derivative-like EA features, suggesting localized exciton formation upon photoexcitation. However, the Y6-based blend has an EA spectrum that is dominated by second derivative-like features, consistent with CT character. We show that this signal originates primarily from Y6. We find that Y6 exhibits the highest dipole moment change (7.5 ± 2.5 D) of the molecules that comprise this study, consistent with a high degree of the CT character, and a relatively large polarization volume of 361 ± 70 Å3, consistent with strong electron delocalization. These results point to the origins of exceptional performance of organic photovoltaics (OPVs) based on Y6

Photoluminescence Lifetimes Exceeding 8 μs and Quantum Yields Exceeding 30% in Hybrid Perovskite Thin Films by Ligand Passivation

We study the effects of a series of post-deposition ligand treatments on the photoluminescence (PL) of polycrystalline methylammonium lead triiodide perovskite thin films. We show that a variety of Lewis bases can improve the bulk PL quantum efficiency (PLQE) and extend the average PL lifetime, ⟨τ⟩, with large enhancements concentrated at grain boundaries. Notably, we demonstrate thin-film PLQE as high as 35 ± 1% and ⟨τ⟩ as long as 8.82 ± 0.03 μs at solar equivalent carrier densities using tri-n-octylphosphine oxide-treated films. Using glow discharge optical emission spectroscopy and nuclear magnetic resonance spectroscopy, we show that the ligands are incorporated primarily at the film surface and are acting as electron donors. These results indicate it is possible to obtain thin-film PL lifetime and PLQE values that are comparable to those from single crystals by control over surface chemistry.publishe

Charge carrier coupling to the soft phonon mode in a ferroelectric semiconductor

Many crystalline solids possess strongly anharmonic “soft” phonon modes characterized by diminishing frequency as temperature approaches a critical point associated with a symmetry breaking phase transition. While electron–soft phonon coupling can introduce unique scattering channels for charge carriers in ferroelectrics, recent studies on the nonferroelectric lead halide perovskites have also suggested the central role of anharmonic phonons bearing resemblance to soft modes in charge carrier screening. Here we apply coherent phonon spectroscopy to directly study electron coupling to the soft transverse optical phonon mode in a ferroelectric semiconductor SbSI. Photogenerated charge carriers in SbSI are found to be exceptionally long lived and are associated with a transient electro-optical effect that can be explained by interactions between charge carriers and thermally stimulated soft phonon excitations. These results provide strong evidence for the role of electron–soft phonon coupling in the efficient screening of charge carriers and in reducing charge recombination rates, both desirable properties for optoelectronics.ISSN:2475-995