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

A detailed picture of temperature dependent behavior of Cs_xFA_1-xPbI₃ 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 PbI₂. Quantum dot growth to form large bulk size grain is observed for all Cs_xFA_1-xPbI₃ 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.

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, allowing for technology-leading materials such as high purity color centers in display technology. 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 urgent necessity toward carbon neutrality, electricity from solar photovoltaics will play a large role in the power generation sector. QDs are developed and shown dramatic improvements over the past 15 years as photoactive materials in photovoltaics with various innovative deposition properties which can 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 can become a better candidate for practical application. In this article, the authors show how the possibilities of different deposition techniques can 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.

Ionic Liquids Modulating CsPbI3 Colloidal Quantum Dots Enable Improved Mobility for High-Performance Solar Cells

Colloidal all-inorganic CsPbI₃ perovskite quantum dots (PQDs) have shown tremendous potential in photovoltaic applications in recent years due to their outstanding optoelectronic properties that general metal halide perovskites offer, along with the added advantages that originates from size reduction and the quantum confinement effect. However, the issue of low carrier mobility in PQD films caused by insulating organic ligands capped on the PQD surface still remains to be addressed while aiming for high-efficiency PQD solar cells. Herein, we propose a novel strategy that takes benefits of ionic liquids, which can offer the high polarity and the electron donating ability to boost the mobility of PQD films in photovoltaic devices. Specifically, 1-propyl-3-methylimidazolium iodide to modulate the colloidal CsPbI₃ PQD surface and couple QDs is demonstrated for the first time. The lone pair electrons on the nitrogen of the imidazole ring within the ionic liquid binds to the empty nonbonding surface orbitals of CsPbI₃ PQDs while the long-chain insulating ligands are replaced, which enables not only efficient charge transport but also reduced defect density in the assembled PQD solid films. The resulting CsPbI₃ PQD solar cell shows a significant increase in efficiency with suppressed hysteresis, indicating the impressive potential of this strategy for developing highly efficient PQD solar cells.

Strategies to Achieve High Circularly Polarized Luminescence from Colloidal Organic-Inorganic Hybrid Perovskite Nanocrystals

Colloidal metal halide perovskite nanocrystals (NCs) with chiral ligands are outstanding candidates as a circularly polarized luminescence (CPL) light source due to many advantages such as high photoluminescence quantum efficiency, large spin-orbit coupling, and extensive tunability via composition and choice of organic ligands. However, achieving pronounced and controllable polarized light emission remains challenging. Here, we develop strategies to achieve high CPL responses from colloidal formamidinium lead bromide (FAPbBr3) NCs at room temperature using chiral surface ligands. First, we show that replacing a portion of typical ligands (oleylamine) with short chiral ligands ((R)-2-octylamine) during FAPbBr3 NC synthesis results in small and monodisperse NCs that yield high CPL with average luminescence dissymmetry g-factor, glum = 6.8 × 10-2. To the best of our knowledge, this is the highest among reported perovskite materials at room temperature to date and represents around 10-fold improvement over the previously reported colloidal CsPbClxBryI3-x-y NCs. In order to incorporate NCs into any optoelectronic or spintronic application, the NCs necessitate purification, which removes a substantial amount of the chiral ligands and extinguishes the CPL signals. To circumvent this issue, we also developed a postsynthetic ligand treatment using a different chiral ligand, (R-/S-)methylbenzylammonium bromide, which also induces a CPL with an average glum = ±1.18 × 10-2. This postsynthetic method is also amenable for long-range charge transport since methylbenzylammonium is quite compact in relation to other surface ligands. Our demonstrations of high CPL and glum from both as-synthesized and purified perovskite NCs at room temperature suggest a route to demonstrate colloidal NC-based spintronics.

Conductivity Tuning via Doping with Electron Donating and Withdrawing Molecules in Perovskite CsPbI3 Nanocrystal Films

Doping of semiconductors enables fine control over the excess charge carriers, and thus the overall electronic properties, crucial to many technologies. Controlled doping in lead-halide perovskite semiconductors has thus far proven to be difficult. However, lower dimensional perovskites such as nanocrystals, with their high surface-area-to-volume ratio, are particularly well-suited for doping via ground-state molecular charge transfer. Here, the tunability of the electronic properties of perovskite nanocrystal arrays is detailed using physically adsorbed molecular dopants. Incorporation of the dopant molecules into electronically coupled CsPbI₃ nanocrystal arrays is confirmed via infrared and photoelectron spectroscopies. Untreated CsPbI₃ nanocrystal films are found to be slightly p-type with increasing conductivity achieved by incorporating the electron-accepting dopant 2,3,5,6-tetrafluoro-7,7,8,8-tetracyanoquinodimethane (F₄ TCNQ) and decreasing conductivity for the electron-donating dopant benzyl viologen. Time-resolved spectroscopic measurements reveal the time scales of Auger-mediated recombination in the presence of excess electrons or holes. Microwave conductance and field-effect transistor measurements demonstrate that both the local and long-range hole mobility are improved by F₄ TCNQ doping of the nanocrystal arrays. The improved hole mobility in photoexcited p-type arrays leads to a pronounced enhancement in phototransistors.

High efficiency perovskite quantum dot solar cells with charge separating heterostructure

Metal halide perovskite semiconductors possess outstanding characteristics for optoelectronic applications including but not limited to photovoltaics. Low-dimensional and nanostructured motifs impart added functionality which can be exploited further. Moreover, wider cation composition tunability and tunable surface ligand properties of colloidal quantum dot (QD) perovskites now enable unprecedented device architectures which differ from thin-film perovskites fabricated from solvated molecular precursors. Here, using layer-by-layer deposition of perovskite QDs, we demonstrate solar cells with abrupt compositional changes throughout the perovskite film. We utilize this ability to abruptly control composition to create an internal heterojunction that facilitates charge separation at the internal interface leading to improved photocarrier harvesting. We show how the photovoltaic performance depends upon the heterojunction position, as well as the composition of each component, and we describe an architecture that greatly improves the performance of perovskite QD photovoltaics.

Metal halide perovskites have wide tunability in both material and device structure. Here Zhao et al. fabricate heterojunctions of colloidal perovskite quantum dots with different composition using layer-by-layer deposition and demonstrate improved photovoltaic performance with enhanced photocarrier harvesting.

Perovskite Quantum Dot Photovoltaic Materials beyond the Reach of Thin Films: Full-Range Tuning of A-Site Cation Composition

We present a cation-exchange approach for tunable A-site alloys of cesium (Cs⁺) and formamidinium (FA⁺) lead triiodide perovskite nanocrystals that enables the formation of compositions spanning the complete range of Cs_{1- x}FA _xPbI₃, unlike thin-film alloys or the direct synthesis of alloyed perovskite nanocrystals. These materials show bright and finely tunable emission in the red and near-infrared range between 650 and 800 nm. The activation energy for the miscibility between Cs⁺ and FA⁺ is measured (∼0.65 eV) and is shown to be higher than reported for X-site exchange in lead halide perovskites. We use these alloyed colloidal perovskite quantum dots to fabricate photovoltaic devices. In addition to the expanded compositional range for Cs_{1- x}FA _xPbI₃ materials, the quantum dot solar cells exhibit high open-circuit voltage ( V_OC) with a lower loss than the thin-film perovskite devices of similar compositions.

Rainbow Emission from an Atomic Transition in Doped Quantum Dots

Although semiconductor quantum dots are promising materials for displays and lighting due to their tunable emissions, these materials also suffer from the serious disadvantage of self-absorption of emitted light. The reabsorption of emitted light is a serious loss mechanism in practical situations because most phosphors exhibit subunity quantum yields. Manganese-based phosphors that also exhibit high stability and quantum efficiency do not suffer from this problem but in turn lack emission tunability, seriously affecting their practical utility. Here, we present a class of manganese-doped quantum dot materials, where strain is used to tune the wavelength of the dopant emission, extending the otherwise limited emission tunability over the yellow-orange range for manganese ions to almost the entire visible spectrum covering all colors from blue to red. These new materials thus combine the advantages of both quantum dots and conventional doped phosphors, thereby opening new possibilities for a wide range of applications in the future.

Ultranarrow and widely tunable Mn2+-Induced photoluminescence from single Mn-doped nanocrystals of ZnS-CdS alloys

Extensively studied Mn-doped semiconductor nanocrystals have invariably exhibited photoluminescence over a narrow energy window of width ≤150  meV in the orange-red region and a surprisingly large spectral width (≥180  meV), contrary to its presumed atomic-like origin. Carrying out emission measurements on individual single nanocrystals and supported by ab initio calculations, we show that Mn PL emission, in fact, can (i) vary over a much wider range (∼370  meV) covering the deep green--deep red region and (ii) exhibit widths substantially lower (∼60-75  meV) than reported so far, opening newer application possibilities and requiring a fundamental shift in our perception of the emission from Mn-doped semiconductor nanocrystals.

Near-room-temperature colossal magnetodielectricity and multiglass properties in partially disordered La2NiMnO6

We report magnetic, dielectric, and magnetodielectric responses of the pure monoclinic bulk phase of partially disordered La2NiMnO6, exhibiting a spectrum of unusual properties and establish that this compound is an intrinsically multiglass system with a large magnetodielectric coupling (8%-20%) over a wide range of temperatures (150-300 K). Specifically, our results establish a unique way to obtain colossal magnetodielectricity, independent of any striction effects, by engineering the asymmetric hopping contribution to the dielectric constant via the tuning of the relative-spin orientations between neighboring magnetic ions in a transition-metal oxide system. We discuss the role of antisite (Ni-Mn) disorder in emergence of these unusual properties.

Subacute toxicity of anilofos, a new organophosphorus herbicide, in male rats: effect on some physical attributes and acetylcholinesterase activity

In acute toxicity study, rats showed dose-dependent signs of cholinergic hyperactivity and behavioural alterations. Maximum intensity of symptoms was not associated with mortality. Oral LD50 was 1681 mg/kg. In subacute toxicity study, rats were orally administered 50, 100 or 200 mg/kg of anilofos once daily for 28 days. Signs and symptoms were observed mainly with 200mg/kg. At this dose, anilofos induced hypothermia and progressive weight loss. None of the anilofos-treated rats died. Weight of brain, lung, testis was not altered, while of liver, heart, spleen and kidney increased. Anilofos inhibited cholinesterase (ChE) activities of erythrocyte (41-67%), plasma (36%), blood (37-64%), brain (63-73%) and liver (28-48%). Total protein was decreased in plasma and liver. Results indicate moderate toxic potential of anilofos in mammals, substantial contribution of CNS-mediated effects in causing anilofos toxicity and no direct relationship between hypothermia and level of ChE inhibition.

Subacute toxicity of anilofos, a new organophosphorus herbicide in male rats: effect on lipid peroxidation and ATPase activity

Effects of anilofos on lipid peroxidation--an index of oxidative stress, ATPase activity--an integral part of active transport mechanisms for cations, GSH level and GST activity were evaluated in blood (erythrocyte/plasma), brain and liver of male rats after daily oral exposure to 50, 100 or 200 mg/kg for 28 days. None of the doses increased lipid peroxidation. The lowest dose, rather, produced marginally significant decrease in peroxidation in liver. Different doses of anilofos decreased GSH content and activities of GST and ATPases. Inhibition of total ATPase (34-44%) and Na+-K+-ATPase (45-52%) activities was maximum in liver, while that of Mg2+-ATPase (46-56%) was more in erythrocyte. Results indicate that anilofos may not cause oxidative damage to cell membrane in repeatedly exposed animals and may cause neuronal/cellular dysfunction by affecting ionic transport across cell membrane.

Effect of isoproturon pretreatment on the biochemical toxicodynamics of anilofos in male rats

Anilofos and isoproturon are important herbicides of organophosphorus and substituted phenylurea groups, respectively. Isoproturon is an inducer of hepatic drug-metabolizing enzymes. Animals and humans have the potential to be exposed to the mixture of these intentionally introduced environmental xenobiotics, but toxicological interactions between these herbicides are not known. Effects of isoproturon pretreatment (675 mg/kg/day for 3 consecutive days) on the toxic actions of anilofos administered orally as a single dose (850 mg/kg) were evaluated by determining some biochemical attributes in blood (erythrocyte/plasma), brain and liver of rats. Anilofos or isoproturon alone or in combination failed to produce any noticeable signs of cholinergic hyperactivity and behavioural alterations. Isoproturon did not potentiate the anticholinesterase action of anilofos in blood and liver. Inhibition of brain acetylcholinesterase was significantly protected. No significant alteration in anilofos-mediated production of lipid peroxidation was observed in erythrocyte and brain of isoproturon-pretreated rats, but it was significantly increased in liver. Anilofos did not affect GSH and GST. The isoproturon-mediated increase in GSH levels of brain (threefold) and liver (3.6-fold) was also not affected following combined administration. GST activity was increased in liver of rats given isoproturon alone (fourfold) or in combination with anilofos (2.8-fold). Activities of total ATPase, Mg2+-ATPase and Na+-K+-ATPase were not affected in rats given either anilofos alone or herbicides in sequence. With these treatments, there were no alterations in the protein content of plasma, brain and liver. Overall findings of the study indicate that isoproturon pretreatment does not alter the toxicity of anilofos, the GSH-GST metabolic pathway may not have a significant implication in the detoxification of anilofos and the production of a reactive oxygen species may be a factor in mediating anilofos toxicity.

HIV infection among the high risk groups of Upper Assam

Serum samples of 9350 individuals belonging to different high risk groups were tested for HIV Infection by ELISA and western blot technique. 9 samples were found to be positive. Two of them belonged to indigenous people of Assam and the infection was transfusion/transplant associated and acquired outside the state during the course of medical treatment. Two were IDUs from Nagaland requiring treatment in a local hospital at Dibrugarh, Assam. Five were from floating population temporarily residing in Assam with history of heterosexual promiscuity. Overall seropositivity rate was 0.97/1000. It is felt that HIV infection in Upper Assam has not penetrated deeply and is at a manageable level and the spread of infection can be prevented through IEC programmes.