Superior photovoltaics/optoelectronics of two-dimensional halide perovskites

Taking advantage of the excellent stability and photoelectric properties,two-dimensional (2D) organic-inorganic halide perovskites have been widely researched a...     

The PV-Researcher's Siren: Hybrid metal halide perovskites

Metal-halide perovskite semiconductors are certainly one of the hottest topic in solar energy conversion. Optimization of both the absorber material and device architecture has led to an astoundingly rapid increase in the reported device efficiencies. Initially developed in the context of dye-sensitized solar cell research, metal-halide perovskite devices now reach efficiency values and hence need to be compared to more conventional photovoltaic technologies such as silicon, copper indium gallium diselenide and cadmium telluride. Strong direct band gap absorption, long charge carrier diffusion length, ease and flexibility in processing at low temperatures and facile tunability makes these materials ideal for solar energy conversion applications. This review will both reflect on favorable properties of these hybrid and ionic semiconductors as well as reflecting on lead toxicity, material and device stability as the most critical issues that need to be solved in order for these materials to become technologically viable.     

Using steric hindrance to design new inhibitors of class C beta-lactamases

beta-lactamases confer resistance to beta-lactam antibiotics such as penicillins and cephalosporins. However, beta-lactams that form an acyl-intermediate with the enzyme but subsequently are hindered from forming a catalytically competent conformation seem to be inhibitors of beta-lactamases. This inhibition may be imparted by specific groups on the ubiquitous R(1) side chain of beta-lactams, such as the 2-amino-4-thiazolyl methoxyimino (ATMO) group common among third-generation cephalosporins. Using steric hindrance of deacylation as a design guide, penicillin and carbacephem substrates were converted into effective beta-lactamase inhibitors and antiresistance antibiotics. To investigate the structural bases of inhibition, the crystal structures of the acyl-adducts of the penicillin substrate amoxicillin and the new analogous inhibitor ATMO-penicillin were determined. ATMO-penicillin binds in a catalytically incompetent conformation resembling that adopted by third-generation cephalosporins, demonstrating the transferability of such sterically hindered groups in inhibitor design.     

Recent strategies to improve moisture stability in metal halide perovskites materials and devices

At present,the stability of the new generation of solar cells based on hybrid perovskites is the bottleneck for their practical applications.Photochemical effects,high temperature,ultraviolet light,humidity and other known or still unknown factors might cause reduction of effectiveness or even irreversible loss of materials properties due to decomposition of functional layers within perovskite solar cells(PSCs).These factors alone have a serious impact on each component of the device,while their combinations lead to much more complicated effects and consequences.This review focuses on the stability of PSCs and the degradation of the device in a humid environment.We assess the instability factors and deep-seated principles of evolution of the device structure in a humidity environment with the emphasis on the influence on their interrelations.The related solutions are reviewed from the perspective of the encapsulation,perovskite active layer,carrier transport layer and electrodes.Combined with the latest research,we believe that the waterproof strategy of PSCs requires either tight encapsulation or thorough modifications in the device itself.Therefore,it is important to develop feasible strategies to improve the overall device stability over humid according to the target characteristics of various devices.     

Quantum mechanical size and steric hindrance

A quantum mechanical definition of molecular size and shape is formulated from the electronic second moment of the Hartree-Fock wave function. The shape tensor is defined to be invariant with respect to the origin. The geometric average of the eigenvalues of the tensor correlates very well with van der Waals and Bragg-Slater radii. There is also a close linear relationship between this definition of molecular size and molecular volumes determined computationally using isodensity contours. Furthermore, this definition of molecular size is effective in predicting the steric effects of substituents, as predicted by existing methods, such as the modified Taft Ee(s), P-values, and n-values.     

High quality large single crystals of metal halide perovskites for optoelectronic applications

正As the technological development of large single-crystalline wafers have revolutionized many industries including electronics and photovoltaics,one can predict that the availability of large single-crystalline perovskite crystals and wafers can revolutionize its broad applications in photodetectors,solar cells,LEDs,lasers,etc.In 2015,Liu et al.[1]at Shaanxi Normal University developed a reactive inverse-temperature crystallization(RITC)method and harvested high quality MAPbI_3 single     

Recent insights for achieving mixed halide perovskites without halide segregation

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Employ ionic liquid to stabilize black-phase formamidinium perovskites

正The certified power conversion efficiency (PCE) of perovskite solar cells which have emerged as the most promising candidates for next-generation thin-film photovoltaics has exceeded 25%.Among all available compositions,formamidinium lead iodide perovskite(FAPbI_3) has a theoretically higher efficiency than the other perovskite analogues due to its narrower bandgap [1,2] and hence has attracted widespread attention.However,due to the large size of the formamidinium cation,the black-phase of FAPbI_3 is metastable at room temperature and tends to transform to non-photoactiveδ-phase [3],which greatly limits its applications.Therefore,stabilizing the black-phase of FAPbI_3 at room temperature has become a great challenge in the community,which urgently needs a remedy.     

Hyperpolarizability of donor-acceptor azines subject to push-pull character and steric hindrance

The push-pull character of two series of donor-acceptor azines has been quantified by ¹³C, ¹⁵N chemical shift differences of the partial C(1)N(1) and N(2)C(2) double bonds in the central linking C(1)N(1)-N(2)C(2) unit and by the quotient of the occupations of the bonding π and anti-bonding π^∗ orbitals of these bonds. Excellent correlation of the latter push-pull parameter with the corresponding bond lengths d_CN strongly recommend both the occupation quotients π^∗/π and the corresponding bond lengths as reasonable sensors for quantifying the push, pull character along the CN-NC linking unit, for the donor-acceptor quality of the two series of azines and for the molecular hyperpolarizability ß₀ of these compounds. Within this context, reasonable conclusions concerning the interplay of steric hindrance in the chromophore, push-pull character and hyperpolarizability of the azines and their application as NLO materials will be drawn.     

Conjugation, hybridization, and steric hindrance in relation to bond lengths

Three theories which have been proposed to explain the observed shortening of a single bond when it is adjacent to a double bond are discussed. Possible predictions from these theories are examined, especially for comparison with various quantities measurable by microwave spectroscopy. It is concluded that the steric theory is probably untenable, that some conjugation appears necessary to explain observed barries to internal rotation, and that it is difficult to find testable predictions from the hybridization theory.     

Radical-carbanion cyclo-coupling in armed aromatics: overriding steric hindrance to ring closure

Omega-(2-Halophenyl)alkyl-2-oxazolines were prepared and reacted via base promoted intramolecular coupling of radical with carbanionic centres to yield 1-phenyl-1-oxazolino-indan and -tetralin derivatives containing quaternary C-atoms.     

Anomeric effects versus steric hindrance to ionic solvation in protonated glucosylanilines and cyclohexylanilines

The so-called reverse anomeric effect is the preference of cationic substituents for the equatorial position on a pyranose ring, but it is not consistent with current theories of molecular structure or with previous studies designed to test it. To probe this further, the N-protonation-induced shifts of the anomeric equilibrium in a series of N-(tetra-O-methylglucopyranosyl)anilines have been measured with high precision through an NMR titration method that compares basicities of alpha and beta anomers in a mixture of the two. For comparison, the N-protonation-induced shifts of the cis/trans equilibrium in N-(4-tert-butylcyclohexyl)anilines have also been measured by this same method. In both series, there is a shift of the equilibrium toward equatorial upon N-protonation, consistent with steric hindrance to ionic solvation. This shift is smaller for the glucosylanilines than for the cyclohexylanilines, consistent with an enhancement of the normal anomeric effect that counters the steric hindrance and reduces the shift toward the equatorial beta anomer. Moreover, the shift toward equatorial increases slightly but detectably with electron-withdrawing substituents on the cyclohexylaniline, which fine-tune the steric hindrance to ionic solvation. In contrast, the shift decreases for the glucosylanilines. This is consistent with an enhancement of the normal anomeric effect due to a more localized positive charge, rather than with a reverse anomeric effect. These results thus define the substituent dependence of the anomeric effect.     

Molecular engineering of inorganic halide perovskites and HTMs for photovoltaic applications

A comprehensive study was performed for the design of ABX₃ perovskites, (A = Li, K, Na, B = Ge, Sn, Pb, X = F, Cl, Br, I) and organic hole transfer materials, HTMs (Fu-2a, Fu-2b, Fu-2c, and Dm-Q) for efficient perovskite solar cells (PSCs) through quantum chemistry calculations. Photovoltaic characteristics of the investigated perovskites are strongly affected by the halide anions. The results reveal that reducing the exciton binding energy of perovskites enhances the rate of the formation/dissociation of holes and electrons so F-based perovskites are superior from this viewpoint. Additionally, the electron and hole injection processes are more favorable in the case of the F-based perovskites in comparison with other studied perovskites. Moreover, spectroscopic properties of the perovskites demonstrate that KSnCl₃, NaSnCl₃, and F-based perovskites exhibit a greater ability of the light-harvesting and incident photon to current conversion efficiency. Ultimately, based on diverse analyses, F-based perovskites, KSnCl₃ and NaSnCl₃ are the preferred candidates to be applied in the PSCs due to an excellent incident photon to current conversion efficiency, light-harvesting efficiency, short circuit current, and solar cell final efficiency.     

Organic-inorganic perovskites containing trivalent metal halide layers: the templating influence of the organic cation layer

Thin sheetlike crystals of the metal-deficient perovskites (H2AEQT)M2/3I4 [M = Bi or Sb; AEQT = 5,5"'-bis-(aminoethyl)-2,2':5',2':5',2'-quaterthiophene] were formed from slowly cooled ethylene glycol/2-butanol solutions containing the bismuth(III) or antimony(III) iodide and AEQT.2HI salts. Each structure was refined in a monoclinic (C2/m) subcell, with the lattice parameters a = 39.712(13) A, b = 5.976(2) A, c = 6.043(2) A, beta = 92.238(5) degrees, and Z = 2 for M = Bi and a = 39.439(7) A, b = 5.952(1) A, c = 6.031(1) A, beta = 92.245(3) degrees, and Z = 2 for M = Sb. The trivalent metal cations locally adopt a distorted octahedral coordination, with M-I bond lengths ranging from 3.046(1) to 3.218(3) A (3.114 A average) for M = Bi and 3.012(1) to 3.153(2) A (3.073 A average) for M = Sb. The new organic-inorganic hybrids are the first members of a metal-deficient perovskite family consisting of (Mn+)2/nV(n-2)/nX4(2-) sheets, where V represents a vacancy (generally left out of the formula) and the metal cation valence, n, is greater than 2. The organic layers in the AEQT-based organic-inorganic hybrids feature edge-to-face aromatic interactions among the rigid, rodlike quaterthiophene moieties, which may help to stabilize the unusual metal-deficient layered structures.     

A kinetic model for steric hindrance effects on quaternization of poly(vinylpyridines)

Quaternization reactions of poly(vinylpyridines) with alkyl halides show retardation in excess of that predicted by the classical second-order kinetics. Based on the classical collision and transition state theories, a kinetic model has been developed to quantify such retardation, in which the overall reaction rate is characterized by a rate constant k₀ of the intrinsic reactivity between a pyridyl group and an alkyl halide group, and by a steric hindrance effect parameter α. The latter accounts for the degree to which the rate of collisions of reactants is reduced, or to which the freedom of movement of the reactants in the transition state is restricted as the reaction proceeds. The resulting kinetic expression has been validated using experimental results reported in the literature and those of our own. The functional dependence of k₀ and α values on the nature of poly(vinylpyridines) and that of alkyl halides is explained. Other factors affecting k₀ and α, including changes in macromolecular dimensions and/or in the distribution of residue environment, quality of solvent, and reaction temperature, are discussed. © 1993 John Wiley & Sons, Inc.     

Colloidal nano-MOFs nucleate and stabilize ultra-small quantum dots of lead bromide perovskites

The development of synthetic routes to access stable, ultra-small (i.e. <5 nm) lead halide perovskite (LHP) quantum dots (QDs) is of fundamental and technological interest. The considerable challenges include the high solubility of the ionic LHPs in polar solvents and aggregation to form larger particles. Here, we demonstrate a simple and effective host–guest strategy for preparing ultra-small lead bromide perovskite QDs through the use of nano-sized MOFs that function as nucleating and host sites. Cr₃O(OH)(H₂O)₂(terephthalate)₃ (Cr-MIL-101), made of large mesopore-sized pseudo-spherical cages, allows fast and efficient diffusion of perovskite precursors within its pores, and promotes the formation of stable, ∼3 nm-wide lead bromide perovskite QDs. CsPbBr₃, MAPbBr₃ (MA⁺ = methylammonium), and (FA)PbBr₃ (FA⁺ = formamidinium) QDs exhibit significantly blue-shifted emission maxima at 440 nm, 446 nm, and 450 nm, respectively, as expected for strongly confined perovskite QDs. Optical characterization and composite modelling confirm that the APbBr₃ (A = Cs, MA, FA) QDs owe their stability within the MIL-101 nanocrystals to both short- and long-range interfacial interactions with the MOF pore walls.

We demonstrate a simple and effective host–guest strategy for preparing ultra-small lead bromide perovskite QDs through the use of nano-sized MOFs that function as nucleating and host sites.     

Synthesis, characterization, and steric hindrance comparisons of selected transition and main group metal β-ketoiminato complexes

The coordination preference of the ketoiminato ligand, RN(H)(C(Me))₂C(Me)O, (R = 2,6-diisopropylphenyl, (Dipp)), L¹, and RN(H)C(Me)CHC(Me)O, R = C₂H₄NEt₂, L², have been investigated with a range of d and p block metal halides, (or alkyls), to compare and contrast products obtained from the bulky ketoiminato ligand, L¹, versus the less bulky, but multidentate ligand, L². The products have been characterized by X-ray crystallography along with other spectroscopic techniques and show how the preferred metal geometry remains constant for products with either ligand, but the steric protection offered by the individual ligands governs the nuclearity of the products, affording monomers, dimers and tetramers.     

Doped all-inorganic cesium zirconium halide perovskites with high-efficiency and tunable emission

Doping enables manipulation of both the electrical and optical properties of halide perovskites.Herein,we incorporated Te⁴⁺ into Cs₂ZrCl₆ single crystal,simultaneously preserving the vacancy-ordered structure,to obtain an efficient yellow-emitting perovskite with a near-unity photoluminescence quantum yield(PLQY≈97.6%).Te⁴⁺ doping modifies the hue and emission color of pristine Cs₂ZrCl₆,generates new absorption channels,and successfully extends the excitation energy from<280 nm to 360-450 nm range.Detailed spectral characterizations,including ultrafast femtosecond transient absorption measurements,reveal that the bright yellow light is derived from triplet self-trapped excitons.Moreover,further tuning doping concentration enables Te-doped Cs₂ZrCl₆ single crystals to exhibit efficient warm white light emission.This work provides a new perspective for the development and design of stable lead-free perovskites with highly efficient luminescence.     

Novel push-pull π-conjugated compounds suffering steric hindrance between donor and acceptor subunits

A novel push-pull compound with a 1,3-bis(dicyanomethylene)indan-2-ylidene moiety as an acceptor subunit has been prepared along with its two derivatives. These molecules are severely distorted from its planar structure and in solution undergo dynamic interconversion between bent structures. The structural features and UV-vis spectroscopic results indicate that the zwitter-ionic nature of the molecule is enhanced in the ground state, which is also supported theoretically by density functional calculations.