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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 721 records · Page 40
All sourcesNASA NTRSDOE OSTI
DOE OSTI2016

Direct Observation of a Charge-Transfer State Preceding High-Yield Singlet Fission in Terrylenediimide Thin Films

Singlet exciton fission (SF) in organic chromophore assemblies results in the conversion of one singlet exciton (S 1 ) into two triplet excitons (T 1 ), provided that the overall process is exoergic, i.e., E (S 1 ) > 2 E (T 1 ). In this paper, we report on SF in thin polycrystalline films of two terrylene-3,4:11,12-bis(dicarboximide) (TDI) derivatives 1 and 2 , which crystallize into two distinct π-stacked structures. Femtosecond transient absorption spectroscopy (fsTA) reveals a charge-transfer state preceding a 190% T 1 yield in films of 1 , where the π-stacked TDI molecules are rotated by 23° along an axis perpendicular to their π systems. In contrast, when the TDI molecules are slip-stacked along their N–N axes in films of 2 , fsTA shows excimer formation, followed by a 50% T 1 yield.

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗
DOE OSTI2016

Influence of Anion Delocalization on Electron Transfer in a Covalent Porphyrin Donor–Perylenediimide Dimer Acceptor System

Photodriven electron transfer from a donor excited state to an assembly of electronically coupled acceptors has been proposed to enhance charge transfer efficiency in functional organic electronic materials. However, the circumstances under which this may occur are difficult to investigate in a controlled manner in disordered donor–acceptor materials. Here we investigate the effects of anion delocalization on electron transfer using zinc meso-tetraphenylporphyrin (ZnTPP) as a donor and a perylene-3,4:9,10-bis(dicarboximide) dimer as the acceptor (PDI 2 ). The PDI units of the dimer are positioned in a cofacial orientation relative to one another by attachment of the imide group of each PDI to the 4- and 5-positions of a xanthene spacer. Furthermore, the distal imide group of one PDI is linked to the para-position of one ZnTPP phenyl group to yield ZnTPP-PDI 2 . The data for the dimer are compared to two different ZnTPP-PDI monomer reference systems designed to probe electron transfer to each of the individual PDI molecules comprising PDI 2 . The electron transfer rate from the ZnTPP lowest excited singlet state to PDI 2 is increased by 50% relative to that in ZnTPP-PDI, when the data are corrected for the statistics of having two electron acceptors. Femtosecond transient IR absorption spectroscopy provides evidence that the observed enhancement in charge separation results from electron transfer producing a delocalized PDI 2 anion.

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗
DOE OSTI2016

Importance of Reducing Vapor Atmosphere in the Fabrication of Tin-Based Perovskite Solar Cells

Tin-based halide perovskite materials have been successfully employed in lead-free perovskite solar cells, but the tendency of these materials to form leakage pathways from p-type defect states, mainly Sn 4+ and Sn vacancies, causes poor device reproducibility and limits the overall power conversion efficiencies. Here, we present an effective process that involves a reducing vapor atmosphere during the preparation of tin-based halide perovskite solar cells to solve this problem, using MASnI 3 , CsSnI 3 and CsSnBr 3 as the representative absorbers. This process enables the fabrication of remarkably improved solar cells with power conversion efficiencies of 3.89%, 1.83% and 3.04% for MASnI 3 , CsSnI 3 and CsSnBr 3 respectively. The reducing vapor atmosphere process results in more than 20% reduction of Sn 4+ /Sn 2+ ratios, which leads to greatly suppressed carrier recombination, to a level comparable to their lead-based counterparts. These results mark an important step towards a deeper understanding of the intrinsic tin-based halide perovskite materials, paving the way to the realization of low-cost and lead-free Sn-based halide perovskite solar cells.

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗
DOE OSTI2017

Slow Equilibration between Spectroscopically Distinct Trap States in Reduced TiO 2 Nanoparticles

Understanding the nature of charge carriers in nanoscale titanium dioxide is important for its use in solar energy conversion, photocatalysis, and other applications. UV-irradiation of aqueous, colloidal TiO 2 nanoparticles in the presence of methanol gives highly reduced suspensions. Two distinct types of electron traps were observed and characterized by EPR and optical spectroscopies. The relative populations of the states depend on temperature, indicating a small energy difference, $ΔH$° = 3.0 ± 0.6 kcal/mol (130 ± 30 meV). Interconversion between the electron traps occurs slowly over the course of minutes to hours within the temperature range studied here, 0-50 °C. The slow time scale implies that interconversion involves changes in structure or stoichiometry, not just the movement of electrons. This occurrence of slow structural modification with changes in trap state occupancy is likely a general feature of reduced TiO 2 systems at thermodynamic equilibria or photostationary states and should be considered in the design of TiO 2 -containing devices.

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗
DOE OSTI2017

Exciton Absorption Spectra by Linear Response Methods: Application to Conjugated Polymers

The theoretical description of the time-evolution of excitons requires, as an initial step, the calculation of their spectra, which has been inaccessible to most users due to the high computational scaling of conventional algorithms and accuracy issues caused by common density functionals. Previously (J. Chem. Phys.2016, 144, 204105), we developed a simple method that resolves these issues. Our scheme is based on a two-step calculation in which a linear-response TDDFT calculation is used to generate orbitals perturbed by the excitonic state, and then a second linear-response TDDFT calculation is used to determine the spectrum of excitations relative to the excitonic state. Herein, we apply this theory to study near-infrared absorption spectra of excitons in oligomers of the ubiquitous conjugated polymers poly(3-hexylthiophene) (P3HT), poly(2-methoxy-5-(2-ethylhexyloxy)-1,4-phenylenevinylene) (MEH-PPV), and poly(benzodithiophene-thieno[3,4-b]thiophene) (PTB7). For P3HT and MEH-PPV oligomers, the calculated intense absorption bands converge at the longest wavelengths for 10 monomer units, and show strong consistency with experimental measurements. The calculations confirm that the exciton spectral features in MEH-PPV overlap with those of the bipolaron formation. In addition, our calculations identify the exciton absorption bands in transient absorption spectra measured by our group for oligomers (1, 2, and 3 units) of PTB7. For all of the cases studied, we report the dominant orbital excitations contributing to the optically active excited state–excited state transitions, and suggest a simple rule to identify absorption peaks at the longest wavelengths. We suggest our methodology could be considered for further developments in theoretical transient spectroscopy to include nonadiabatic effects, coherences, and to describe the formation of species such as charge-transfer states and polaron pairs.

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗
DOE OSTI2017

In Situ Synthesis of Highly Dispersed and Ultrafine Metal Nanoparticles from Chalcogels

We report a unique reaction type for facile synthesis of ultrafine and well-dispersed Pt nanoparticles supported on chalcogel surfaces. The nanoparticles are obtained by in-situ Pt 2+ reduction of a chalcogel network formed by the metathesis reaction between K 2 PtCl 4 and Na 4 SnS 4 . The rapid catalytic ability of the chalcogelsupported Pt nanoparticles is demonstrated in a recyclable manner by using 4-nitrophenol reduction as a probe reaction.

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗
DOE OSTI2017

Dopant-Free Tetrakis-Triphenylamine Hole Transporting Material for Efficient Tin-Based Perovskite Solar Cells

Developing dopant-free hole transporting layers (HTLs) is critical in achieving high-performance and robust state-of-the-art perovskite photovoltaics, especially for the air-sensitive tin-based perovskite systems. The commonly used HTLs require hygroscopic dopants and additives for optimal performance, which adds extra cost to manufacturing and limits long-term device stability. Here we demonstrate the use of a novel tetrakis-triphenylamine (TPE) small molecule prepared by a facile synthetic route as a superior dopant-free HTL for lead-free tin-based perovskite solar cells. The best-performing tin iodide perovskite cells employing the novel mixed-cation ethylenediammonium/formamidinium with the dopant-free TPE HTL achieve a power conversion efficiency as high as 7.23%, ascribed to the HTL’s suitable band alignment and excellent hole extraction/collection properties. This efficiency is one of the highest reported so far for tin halide perovskite systems, highlighting potential application of TPE HTL material in low-cost high-performance tin-based perovskite solar cells.

14 SOLAR ENERGY↗
DOE OSTI2018

Hybrid Dion–Jacobson 2D Lead Iodide Perovskites

The three-dimensional hybrid organic–inorganic perovskites have shown huge potential for use in solar cells and other optoelectronic devices. Although these materials are under intense investigation, derivative materials with lower dimensionality are emerging, offering higher tunability of physical properties and new capabilities. Here, we present two new series of hybrid two-dimensional (2D) perovskites that adopt the Dion–Jacobson (DJ) structure type, which are the first complete homologous series reported in halide perovskite chemistry. Lead iodide DJ perovskites adopt a general formula A'A n–1 PbnI 3 n +1 (A' = 3-(aminomethyl)piperidinium (3AMP) or 4-(aminomethyl)piperidinium (4AMP), A = methylammonium (MA)). These materials have layered structures where the stacking of inorganic layers is unique as they lay exactly on top of another. With a slightly different position of the functional group in the templating cation 3AMP and 4AMP, the as-formed DJ perovskites show different optical properties, with the 3AMP series having smaller band gaps than the 4AMP series. Analysis on the crystal structures and density functional theory (DFT) calculations suggest that the origin of the systematic band gap shift is the strong but indirect influence of the organic cation on the inorganic framework. Fabrication of photovoltaic devices utilizing these materials as light absorbers reveals that (3AMP)(MA) 3 Pb 4 I 13 has the best power conversion efficiency (PCE) of 7.32%, which is much higher than that of the corresponding (4AMP)(MA) 3 Pb 4 I 13 .

14 SOLAR ENERGY↗
DOE OSTI2018

Charge Carrier Activity on Single-Particle Photo(electro)catalysts: Toward Function in Solar Energy Conversion

Understanding the fundamental properties of charge carriers on the surface of semiconductor photo(electro)catalysts is key to the rational design of efficient photo(electro)catalytic devices for sunlight-driven energy conversion. Here high spatial resolution information is always desirable because of the ubiquitous heterogeneity in semiconductor particles. In this Perspective, we review the latest advances in nanoscale imaging and quantitative analysis of charge carrier activities on individual semiconductor particles down to subparticle resolution, covering the approaches of single-molecule super-resolution fluorescence imaging, scanning electron microscopy, and photoluminescence microscopy. We further highlight direct, operando functional assessments of their performances toward the targeted photo(electro)-catalytic processes through single- and subparticle photocurrent measurements. Here, we also discuss the significance of establishing quantitative relations between the desired functions of photo(electro)catalysts and their surface charge carrier activities. These fundamental relations can provide guiding principles for rationally engineering photo(electro)catalytic systems, for example with cocatalysts, for a broad range of applications.

14 SOLAR ENERGY↗