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Search indexed NASA NTRS and DOE OSTI research on propulsion, heat transfer, battery materials and energy systems. Follow report and document links to the original sources.

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At least 865 records · Page 48
All sourcesNASA NTRSDOE OSTI
DOE OSTI2011

In situ measurement of power conversion efficiency and molecular ordering during thermal annealing in P3HT:PCBM bulk heterojunction solar cells

Bulk heterojunction organic solar cells hold much promise as commercially viable sources of renewable energy due to their relatively inexpensive fabrication. Developing a fundamental knowledge of how processing conditions influence solar power conversion efficiency will enable rational and efficient design, optimization, and control of new organic solar cell materials. In this report, we use a combination of in situ current–voltage measurements and grazing-incidence wide-angle X-ray scattering experiments at elevated temperature to correlate the changes in photoconversion efficiency to the changes in the molecular ordering of a poly(3-hexylthiophene):[6,6]-phenyl-C₆₁-butyric acid methyl ester (P3HT:PCBM) bulk heterojunction active layer. In situ measurements of current–voltage characteristics were used to optimize the power conversion efficiency and the resulting thermal processing was in agreement with studies from repeated heating and cooling cycles. The improvements in short circuit current with thermal annealing were correlated to an increase in the population of face-on oriented crystallites of P3HT rather than improvements in molecular ordering of PCBM.

14 SOLAR ENERGY↗
DOE OSTI2014

Singlet fission efficiency in tetracene-based organic solar cells

Singlet exciton fission splits one singlet exciton into two triplet excitons. Using a joint analysis of photocurrent and fluorescence modulation under a magnetic field, we determine that the triplet yield within optimized tetracene organic photovoltaic devices is 153% ± 5% for a tetracene film thickness of 20 nm. The corresponding internal quantum efficiency is 127% ± 18%. These results are used to prove the effectiveness of a simplified triplet yield measurement that relies only on the magnetic field modulation of fluorescence. Despite its relatively slow rate of singlet fission, the measured triplet yields confirm that tetracene is presently the best candidate for use with silicon solar cells.

36 MATERIALS SCIENCE↗
DOE OSTI2014

First-principles study of vacancy-assisted impurity diffusion in ZnO

Group-III elements act as donors in ZnO when incorporated on the Zn site. Their incorporation and behavior upon annealing is governed by diffusion, which proceeds mainly through a vacancy-assisted process. We report first-principles calculations for the migration of Al, Ga, and In donors in ZnO, based on density functional theory using a hybrid functional. From the calculated migration barriers and formation energies, we determine diffusion activation energies and estimate annealing temperatures. Impurity-vacancy binding energies and migration barriers decrease from Al to In. Activation energies for vacancy-assisted diffusion are lowest for In and highest for Al.

36 MATERIALS SCIENCE↗
DOE OSTI2012

Indium and impurity incorporation in InGaN films on polar, nonpolar, and semipolar GaN orientations grown by ammonia molecular beam epitaxy

The effects of NH{sub 3} flow, group III flux, and substrate growth temperature on indium incorporation and surface morphology have been investigated for bulk InGaN films grown by ammonia molecular beam epitaxy. The incorporation of unintentional impurity elements (H, C, O) in InGaN films was studied as a function of growth temperature for growth on polar (0001) GaN on sapphire templates, nonpolar (1010) bulk GaN, and semipolar (1122), (2021) bulk GaN substrates. Enhanced indium incorporation was observed on both (1010) and (2021) surfaces relative to c-plane, while reduced indium incorporation was observed on (1122) for co-loaded conditions. Indium incorporation was observed to increase with decreasing growth temperature for all planes, while being relatively unaffected by the group III flux rates for a 1:1 Ga:In ratio. Indium incorporation was found to increase at the expense of a decreased growth rate for higher ammonia flows; however, smooth surface morphology was consistently observed for growth on semipolar orientations. Increased concentrations of oxygen and hydrogen were observed on semipolar and nonpolar orientations with a clear trend of increased hydrogen incorporation with indium content.

36 MATERIALS SCIENCE↗
DOE OSTI2014

Origin of electrons emitted into vacuum from InGaN light emitting diodes

The mechanism responsible for efficiency droop in InGaN light-emitting diodes (LEDs) has long been elusive due to indirect measurement techniques used for its identification. Auger recombination is unique among proposed efficiency droop mechanisms, in that it is the only mechanism capable of generating hot carriers. In a previous study [J. Iveland et al., Phys. Rev. Lett. 110, 177406 (2013)], we performed electron energy analysis of electrons emitted into vacuum from a forward biased InGaN LED that had been brought into negative electron affinity by cesiation. Three peaks were observed in the energy spectrum of vacuum emitted electrons. In this Letter, we unambiguously identify the origin of the peaks. The two higher energy peaks correspond to accumulation of electrons transported to the surface in the bulk Γ and side L conduction band valleys. The L-valley peak is a direct signature of a hot Auger electron population. The lower energy peak results from surface photoemission induced by the internal LED light emitted from the InGaN quantum wells. Two control experiments were performed. In the first, a simple GaN pn junction generated only a single Γ peak in electroemission. In the second, selective detection of the photoemission from an LED under modulated light excitation and DC electrical injection confirms that only the low energy peak is photogenerated and that LED light is incapable of generating Γ or L-valley peaks, the latter only occurring due to the Auger effect in the LED active region.

42 ENGINEERING↗
DOE OSTI2012

Three-dimensional photonic crystals by large-area membrane stacking

We designed and analyzed a “mesh-stack” three-dimensional photonic crystal of a 12.4% bandgap with a dielectric constant ratio of 12:1. The mesh-stack consists of four offset identical square-lattice air-hole patterned membranes in each vertical period that is equal to the in-plane period of the square lattice. This design is fully compatible with the membrane-stacking fabrication method, which is based on alignment and stacking of large-area single-crystal membranes containing engineered defects. A bandgap greater than 10% is preserved as long as the membranes are subjected to in-plane misalignment less than 3% of the square period. By introducing a linear defect with a nonsymmorphic symmetry into the mesh-stack, we achieved a single-mode waveguide over a wide bandwidth.

36 MATERIALS SCIENCE↗