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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 379 records · Page 21
All sourcesNASA NTRSDOE OSTI
DOE OSTI2019

GeI 2 Additive for High Optoelectronic Quality CsPbI 3 Quantum Dots and Their Application in Photovoltaic Devices

Trioctylphosphine (TOP)-based syntheses of CsPbI 3 perovskite quantum dots (QDs) yield unprecedented high photoluminescence quantum yield (PL QY), lower Stokes shifts, and longer carrier lifetimes due to their enhanced crystallinity. This synthetic route relies on a heavily Pb-rich condition or a large Pb:Cs molar ratio in precursor solution to produce QDs with appropriate stoichiometry as well as to guarantee a good colloidal stability. The high Pb condition is achieved by a high concentration of PbI 2 prepared in TOP. Here we find such Pb-rich strategies can be avoided by providing additional iodine ions using other metal halide salts. In particular GeI 2 , which contrary to PbI 2 , readily dissolves in TOP. CsPbI 3 QDs prepared using PbI 2 /GeI 2 combination show near-unity PL QY and improved chemical stability compared to the previous synthetic route. Furthermore, we find no sign of Ge incorporation in the QDs (compositionally or energetically). The ensuing QD solar cells deliver power conversion efficiency of 12.15% and retain 85% of its peak performance after storage over 90 days. The PbI 2 /GeI 2 dual-source iodine synthetic approach presented here represents a more rational and robust route to high-quality CsPbI 3 QDs. to guarantee a good colloidal stability. The high Pb condition is achieved by a high concentration of PbI2 prepared in TOP. Here we find such Pb-rich strategies can be avoided by providing additional iodine ions using other metal halide salts. In particular GeI 2 , which contrary to PbI 2 , readily dissolves in TOP. CsPbI 3 QDs prepared using PbI 2 /GeI 2 combination show near-unity PL QY and improved chemical stability compared to the previous synthetic route. Furthermore, we find no sign of Ge incorporation in the QDs (compositionally or energetically). The ensuing QD solar cells deliver power conversion efficiency of 12.15% and retain 85% of its peak performance after storage over 90 days. The PbI 2 /GeI 2 dual-source iodine synthetic approach presented here represents a more rational and robust route to high-quality CsPbI 3 QDs.

14 SOLAR ENERGY↗
DOE OSTI2017

Role of Lead Vacancies for Optoelectronic Properties of Lead-Halide Perovskites

Methylammonium lead iodide perovskite materials have been shown to be efficient in photovoltaic devices. The current fabrication process has not been perfected, leaving defects such as site vacancies, which can trap charge and have a detrimental effect on photogenerated charge carriers. Here focus is placed on the effect a Pb site vacancy has on the charge-carrier dynamics following photoexcitation. The electronic structure of materials with vacancies is often found in open-shell configurations with unpaired electrons in the conduction/valence bands To accurately describe unpaired electrons, spin-polarized and noncollinear spin calculations are performed on both neutral and charged vacancy systems. This work presents spin-polarized and noncollinear spin ground-state electronic structures and nonradiative rates of charge-carrier relaxation and introduces an extension to a novel procedure to compute photoluminescence spectra for open-shell models. This study describes nonadiabatic dynamics of MAPbI 3 models within a Redfield formalism focusing on the role of a Pb vacancy defect on electronic relaxation processes. Results show the vacancy of the Pb ion introducing a new energy state within the unblemished material band gap region. Finally, this additional unoccupied state is expected to increase the nonradiative relaxation lifetime of the excited electron, allowing for a longer lifetime of the charge carrier and increased opportunity for secondary relaxation mechanisms or collection to take place.

14 SOLAR ENERGY↗
DOE OSTI2017

Role of Water in the Reversible Optoelectronic Degradation in Hybrid Perovskites at Low Pressure

There is no doubt about the potential offered by the low-cost fabrication and high efficiency of hybrid organic–inorganic perovskite solar cells. However, the service lifetimes of these devices must be increased from months to years to capitalize on their potential. The archetypal hybrid perovskite for solar cells, methylammonium lead iodide (CH 3 NH 3 PbI 3 , abbreviated MAPI), readily degrades in ambient atmosphere under standard operating conditions. Understanding the origin and effects of this degradation can pave the way to better engineer photovoltaic devices and the perovskite material itself. Herein we present the effects of varying pressure on the electrical performance of MAPI solar cells. Solar cell parameters, especially open circuit voltage, are significantly affected by the total ambient pressure and present an unexpected reversible behavior upon pressure cycling. We complement photoluminescence studies as a function of ambient atmosphere and temperature with first-principles density functional theory (DFT) calculations. The results suggest that the reversible intercalation of water in MAPI is a necessary component underlying this behavior.

14 SOLAR ENERGY↗
DOE OSTI2022

Surface and Optoelectronic Properties of Ultrathin Trigonal Selenium: A Density Functional Theory Study with van der Waals Correction

Selenium is a crucial earth-abundant and non-toxic semiconductor with a wide range of applications across the semiconductor industries. Selenium has drawn attention from scientific communities for its wide range of applicability: from photovoltaics to imaging devices. Its usage as a photosensitive material largely involves the synthesis of the amorphous phase (a-Se) via various experimental techniques. However, the ground state crystalline phase of this material, known as the trigonal selenium (\textit{t}-Se), is not extensively studied for its optimum electronic and optical properties. In this work, we present density functional theory (DFT) based systematic studies on the ultra-thin $(10\overline{1}0)$ surface slabs of \textit{t}-Se. We report the surface energies, work function, electronic and optical properties as a function of number of layers for $(10\overline{1}0)$ surface slabs to access its suitability for applications as a photosensitive material.

46 INSTRUMENTATION RELATED TO NUCLEAR SCIENCE AND ↗
DOE OSTI2025

Deposition-Dependent Coverage and Performance of Phosphonic Acid Interface Modifiers in Halide Perovskite Optoelectronics

In this work, we study the effect of various deposition methods for phosphonic acid interface modifiers commonly pursued as self-assembled monolayers in high-performance metal halide perovskite photovoltaics and light-emitting diodes. We compare the deposition of (2-(3,6-diiodo-9H-carbazol-9-yl)ethyl)phosphonic acid onto indium tin oxide (ITO) bottom contacts by varying three parameters: the method of deposition, specifically spin coating or prolonged dip coating; ITO surface treatment via HCl/FeCl3 etching; and use in combination with a second modifier, 1,6-hexylenediphosphonic acid. We demonstrate that varying these modification protocols can impact time-resolved photoluminescence carrier lifetimes and quasi-Fermi level splitting of perovskite films deposited onto the phosphonic acid-modified ITO. Ultraviolet photoelectron spectroscopy shows an increase in the effective work function after phosphonic acid modification and clear evidence for photoemission from carbazole functional groups at the ITO surface. We used X-ray photoelectron spectroscopy to probe differences in phosphonic acid coverage on the metal oxide contact and show that perovskite samples grown on ITO with the highest phosphonic acid coverage exhibit the longest carrier lifetimes. Finally, we establish that device performance follows these same trends. These results indicate that the reactivity, heterogeneity, and composition of the bottom contact help to control recombination rates and therefore power conversion efficiencies. ITO etching, prolonged deposition times for phosphonic acids via dip coating, and the use of a secondary, more hydrophilic bisphosphonic acid all contribute to improvements in surface coverage, carrier lifetime, and device efficiency. Furthermore, these improvements each have a positive impact, and we achieve the best results when all three strategies are implemented.

contact engineering↗
DOE OSTI2019

Electrocatalytic and Optoelectronic Characteristics of the Two-Dimensional Titanium Nitride Ti 4 N 3 T x MXene

A relatively new class of two-dimensional (2D) materials called MXenes have garnered tremendous interest in the field of energy storage and conversion. Thus far nearly all MXenes reported experimentally have been described as metals, with a lone report of a mixed-metal carbide phase exhibiting semiconducting character. Here, we report the optical, electrocatalytic, and electrical properties of the 2D Ti 4 N 3 T x MXene (Tx = basal plane surface terminating groups) and show that this material exhbits both metallic and semiconducting behaviors. We provide complete structural characterization of exfoliated Ti 4 N 3 T x MXene and assign T x = O and/or OH and find that this material is susceptible to surface oxidation. Optical experiments indicate that the exfoliated Ti 4 N 3 T x MXene forms a hybrid with a thin surface oxide layer resulting in visible light absorbtion at energies greater than ~2.0 eV and an excitation wavelength-dependent defect-state emission over a broad range centered at ~2.9 eV. As an electrocatalyst for the hydrogen evolution reaction, the exfoliated Ti 4 N 3 T x shows an overpotential of ~300 mV at -10 mA cm -2 and a Tafel slope of ~190 mV dec -1 . Finally, we observe semiconducting behavior at temperatures below ~90 K from temperature-dependent transport measurements under 5 T magnetic field likely resulting from the thin oxide layer. These results unveil the intriguing optical, electrocatalytic, and electrical properties of this 2D Ti 4 N 3 T x MXene that expands the potential of these new 2D materials into electrocatalysis and (opto)electronic applications.

2D materials↗