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Hazarika, Abhijit

Publications and source records attributed to Hazarika, Abhijit.

13 records
All sourcesNASA NTRSDOE OSTI
DOE OSTI2023

Orientational Order in Spin-Cast Lead-Iodide Perovskite Nanocrystal Solids

Combined synthetic control over size and composition renders colloidal lead-halide perovskite nanocrystals a tunable platform for high-efficiency optoelectronic applications. However, the properties and operational stability of devices based on nanocrystal solids are often dictated by the method of the evaporation-induced assembly. Ubiquitous slow evaporation techniques can produce highly ordered nanocrystal domains but limit the prospects for scalable fabrication of continuous device layers, calling for investigation of approaches to more rapidly form ordered perovskite nanocrystal solids. Here, we study orientationally ordered lead-iodide perovskite nanocrystal solids prepared by conventional spin coating with molecular additives (excess ligand) to enhance ordering within the arrays. In situ X-ray scattering measurements reveal that orientational ordering occurs rapidly upon solvent removal during spin coating and can be further enhanced by manipulating the spin speed. We vary the additive ligand length and explore trade-offs between ordering and layered perovskite impurity formation. Arrays treated with the intermediate-length octylamine ligand exhibit increased in-plane electronic conductivity, suggesting orientational ordering and internanocrystal electronic coupling can be enhanced by the treatment. In conclusion, these results highlight the prospects of establishing long-range order in lead-halide perovskite nanocrystal solids by using simple and fast coating methods.

14 SOLAR ENERGY↗
DOE OSTI2023

Thermal tolerance of perovskite quantum dots dependent on A-site cation and surface ligand

A detailed picture of temperature dependent behavior of Cs x FA 1-x PbI 3 perovskite quantum dots across the composition range is constructed by performing in situ optical spectroscopic and structural measurements, supported by theoretical calculations that focus on the relation between A-site chemical composition and surface ligand binding. The thermal degradation mechanism depends not only on the exact chemical composition, but also on the ligand binding energy. The thermal degradation of Cs-rich perovskite quantum dots is induced by a phase transition from black γ-phase to yellow δ-phase, while FA-rich perovskite quantum dots with higher ligand binding energy directly decompose into PbI 2 . Quantum dot growth to form large bulk size grain is observed for all Cs x FA 1-x PbI 3 perovskite quantum dots at elevated temperatures. In addition, FA-rich quantum dots possess stronger electron-longitudinal optical phonon coupling, suggesting that photogenerated excitons in FA-rich quantum dots have higher probability to be dissociated by phonon scattering compared to Cs-rich quantum dots.

14 SOLAR ENERGY↗
DOE OSTI2022

Giant spin-selective bandgap renormalization in CsPbBr 3 colloidal nanocrystals

The spin-dependence of the strongly correlated phenomena and many-body interactions play an important role in quantum information science. In particular, it is quite interesting but unclear how the spin degree of freedom ramifies the bandgap renormalization, one of the fundamental many-body phenomena. We report the first room-temperature observation of giant spin-selective bandgap renormalization (SS-BGR) in CsPbBr 3 colloidal nanocrystals using time-resolved circularly polarized femtosecond pump-probe spectroscopy. The SS-BGR results from many-body interactions among carriers with the same spin that renormalize their joint density of states by 57+/- 1 meV, visualized here as photoinduced absorption (PIA) below band-edge transition energy when the pump and probe are co-polarized. The hallmark result of spectrally resolved SS-BGR is in stark contrast to the usually submerged signal in II-VI and III-V semiconductors and is 3 orders of magnitude larger than that observed in Ge/SiGe quantum wells, highlighting the unique and beneficial band structure of the metal-halide semiconductors. We propose that the PIA, due to SS-BGR, can be used to describe the spin-polarization and spin-relaxation dynamics. The experimental and theoretical findings open up new possibilities for optical manipulation of spin degrees of freedom and their many-body interactions in metal-halide perovskite nanocrystals for potential room-temperature applications.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2022

Colloidal Quantum Dot Solar Cells: Progressive Deposition Techniques and Future Prospects on Large-area Fabrication

Colloidally grown nanosized semiconductors yield extremely high-quality optoelectronic materials. Many examples have pointed to near perfect photoluminescence quantum yields, meaning non-radiative optical processes (losses) can be completely suppressed allowing for technology-leading materials as high purity color centers in display technology and emitters in LEDs. Furthermore, because of high chemical yield, and improved understanding of the surfaces, these materials, particularly colloidal quantum dots (QDs) can also be ideal candidates for other optoelectronic applications. Given the strong drive toward carbon neutrality and limiting climate change, electricity from solar photovoltaics will play a large role in the power generation sector. QDs have been developed and shown dramatic improvements over the past 15 years as photoactive materials in photovoltaics and have unique potential with various innovative deposition properties which could lead to exceptionally low-cost and high-performance devices. Once the key issues related to charge transport in optically thick arrays are addressed, QD-based photovoltaic technology could become a better candidate for practical application. In this article, w e show how the possibilities of different deposition techniques could bring QD-based solar cells to the industrial level and discuss the challenges for perovskite QD solar cells in particular, to achieve large-area fabrication for further advancing technology to solve pivotal energy and environmental issues.

14 SOLAR ENERGY↗
DOE OSTI2021

Perovskite nanocrystals and methods of making the same

The present disclosure relates to a perovskite that includes A1-xA′xBX3, where A is a first cation, A′ is a second cation, B is a third cation, X is a first anion, and 0<1−x≤1. In some embodiments of the present disclosure, the perovskite may further include a second anion (X′) such that the perovskite includes A1-xA′xB(X1-zX′z)3, where 0<z≤1. In some embodiments of the present disclosure, the perovskite may further include a fourth cation (A*) such that the perovskite includes A1-x-yA′xA*yB(X1-zX′z)3, where 0<y≤1. In some embodiments of the present disclosure, the perovskite may further include a fifth cation (B′) such that the perovskite includes A1-x-yA′xA*yB1-aB′a(X1-zX′z)3, where 0<a≤1.

Luther, Joseph Matthew↗
DOE OSTI2020

Polaron and Spin Dynamics in Organic–Inorganic Lead Halide Perovskite Nanocrystals

Long-lived carrier population and spin-based behavior in lead halide perovskite nanocrystals (NCs) are very interesting for implementing photovoltaic devices with efficiencies exceeding the Shockley–Queisser limit and quantum information processing, respectively. However, a comprehensive understanding of polaron-mediated charge carrier interactions and an accurate description of the spin-polarized states for spintronics are still lacking. Herein, the carrier and spin interactions are studied under controlled conditions in FAPbI 3 and Cs 0.01 FA 0.99 Pb(Br 0.11 I 0.89 ) 3 NCs through ultrafast transient absorption (TA) spectroscopy. At early timescales, TA spectrum shows an asymmetric derivative feature originating from the hot carrier-induced spectral redshift in FAPbI 3 NCs (55 ± 3 meV) and Cs0.01FA0.99Pb(Br0.11I0.89)3 NCs (54 ± 2 meV) at the bandedges that stabilizes to 9 ± 1 and 11 ± 2 meV, respectively, at 1 ps due to the polaron formation. The kinetic analysis indicates that the polaron populations in FAPbI3 and Cs 0.01 FA 0.99 Pb(Br 0.11 I 0.89 ) 3 NCs decay with an average lifetime of 657 ± 34 and 532 ± 28 ps, respectively. The circular polarization-resolved TA reveals that polaron formation can control spin relaxation in NCs, thus providing a powerful tool to explore the development of their prospective applications in spintronics.

36 MATERIALS SCIENCE↗
DOE OSTI2020

Beyond Strain: Controlling the Surface Chemistry of CsPbI 3 Nanocrystal Films for Improved Stability against Ambient Reactive Oxygen Species

Colloidal halide perovskite nanocrystals (NCs) have the possibility of easy scale-up due to their batch synthesis and have demonstrated excellent optoelectronic properties. In particular, perovskite NCs have remarkably high photoluminescence quantum yields in solution and as thin films and impressive open circuit voltages in photovoltaic devices. Despite these promising results, little work has been done to understand the stability of CsPbI 3 NCs for optoelectronic device applications. It has been previously shown that the ligands impart tensile surface strain, which stabilizes the black three-dimensional (3D) perovskite phase against phase degradation, making CsPbI 3 NCs some of the most structurally robust inorganic halide perovskites to date. However, understanding exactly how CsPbI 3 NCs degrade under ambient conditions is critical. Additionally, we demonstrate that the degradation mechanism of NCs is unique from, and 2 orders of magnitude slower than, their polycrystalline thin-film counterparts. Under specific conditions, CsPbI 3 NC films show a compositional instability instead of the phase instability seen in large grain CsPbI 3 . This is mediated through reactions with superoxide and other reactive oxygen species, which are initiated from surface defect states, O 2 and light. We then use this mechanistic insight to identify multiple strategies to prolong the lifetimes of CsPbI 3 NC films, by going beyond surface strain to mitigate key surface chemistries. We demonstrate that (1) minimizing the number of surface defects (2) using an alkylammonium bromide ligand surface treatment and (3) encapsulation with an oxygen scavenging layer all increase NC film lifetimes by inhibiting various steps in the photo-oxidation degradation reaction.

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗
DOE OSTI2020

FA x Cs 1–x PbI 3 Nanocrystals: Tuning Crystal Symmetry by A-Site Cation Composition

Nanoscale semiconductors show remarkably tunable properties. For metal halide perovskite (MHP) nanocrystals (NCs), surface energy and lattice strain stabilize desirable MHP compositions and crystallographic phases that are unstable in the bulk. We report an X-ray scattering study of the average room-temperature crystal structure of ~15 nm FA x Cs 1–x PbI 3 (FA = formamidinium) NCs. All compositions crystallize in the perovskite structure; however, the average structure lowers in symmetry from the a (cubic) to ß (tetragonal) to γ (orthorhombic) perovskite phases with decreasing x (Cs addition). The corresponding a- to ß- and ß- to γ-phase transitions occur between x = 0.75–0.5 and x = 0.25–0.1, respectively. Furthermore, structural refinements also indicate large octahedral tilt angles (10–30°) in the ß- and γ-phases and an increase in (pseudo)cubic unit cell volume upon FA addition. This work establishes the composition–structure relationship for FA x Cs 1–x PbI 3 NCs and demonstrates the ability to target average crystal symmetry with facile synthetic control.

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗
DOE OSTI2020

Metal Halide Perovskites in Quantum Dot Solar Cells: Progress and Prospects

“Go big or go home” could never be a truer statement, especially when it comes to energy. The world population is increasing, the energy use per person is growing more rapidly, and the total electricity use per person is growing even more quickly than that. To handle this demand, energy production must be increased, and it is vital for renewable sources to be used. Currently three quarters of a terawatt of power generated from photovoltaics (PVs) has been installed. At a somewhat optimistic average power-conversion efficiency of 18%, the distributed surface area of high purity optoelectronic grade semiconductor photovoltaic panels pointed at the sun is substantially larger than the area of the whole state of Rhode Island or nearly double the land mass of Luxembourg. While photovoltaic production continues to ramp to immense proportions, perovskite semiconductors are poised to greatly complement existing technology. On the other end of the size spectrum, the study of semiconductor nanocrystals or quantum dots (QDs) has led to advanced structures and deeper understanding within halide perovskite semiconductors. In this article, we show how the development of nanoscale metal halide perovskite semiconductors have gained prominence surpassing all other QD materials in terms of efficiency, and are becoming a platform for further improving technology to solve big energy challenges.

14 SOLAR ENERGY↗
DOE OSTI2020

Bright trion emission from semiconductor nanoplatelets

The trion, a quasiparticle comprising one exciton and an additional charge carrier, offers unique opportunities for generating spin-photon interfaces that can be used in developing quantum networks. Trions are also actively sought after for integrated optoelectronic devices including photovoltaics, photodetectors, and spintronics. However, formation of trions in strongly confined low-dimensional materials is often deemed detrimental. This is because trion emission in such materials is typically prohibited due to the predominant nonradiative Auger recombination processes. Semiconductor nanoplatelets with their strong confinement in the thickness direction and extended lateral geometries exhibit large exciton coherence sizes and reduced carrier-carrier interactions that may enable unprecedented trion properties. In this paper, we perform optical spectroscopic studies of individual CdSe nanoplatelets at cryogenic temperatures and observe bright trion emission with intensities comparable to that of neutral exciton emission. We perform carrier dynamics studies of the nanoplatelets and find that due to their extended lateral geometry, the fast radiative decay rate of the nanoplatelets at cryogenic temperatures is comparable to the inhibited Auger recombination rate, leading to the bright trion emission. Our tight-binding theory further reveals distinct size-tunable trion emission in the nanoplatelets that is advantageous for efficient trion emission. These properties make semiconductor nanoplatelets potential candidates as photon sources for optoelectronic and quantum logic devices.

36 MATERIALS SCIENCE↗
DOE OSTI2020

CsI-Antisolvent Adduct Formation in All-Inorganic Metal Halide Perovskites

The excellent optoelectronic properties shown by hybrid organic/inorganic metal halide perovskites are all predicated on precisely controlling the exact nucleation and crystallization dynamics that occur during film formation. In general, high-performance thin films are obtained by a method commonly called solvent engineering (or antisolvent quench) processing. The solvent engineering method removes excess solvent, but importantly leaves behind solvent that forms chemical adducts with the lead-halide precursor salts. These adduct-based precursor phases control nucleation and the growth of the polycrystalline domains. There has not yet been a comprehensive study comparing the various antisolvents used in different perovskite compositions containing cesium. In addition, there have been no reports of solvent engineering for high efficiency in all-inorganic perovskites such as CsPbI 3 . In this work, inorganic perovskite composition CsPbI 3 is specifically targeted and unique adducts formed between CsI and precursor solvents and antisolvents are found that have not been observed for other A-site cation salts. These CsI adducts control nucleation more so than the PbI 2 -dimethyl sulfoxide (DMSO) adduct and demonstrate how the A-site plays a significant role in crystallization. The use of methyl acetate (MeOAc) in this solvent engineering approach dictates crystallization through the formation of a CsI-MeOAc adduct and results in solar cells with a power conversion efficiency of 14.4%.

14 SOLAR ENERGY↗