Direct-indirect character of the bandgap in methylammonium lead iodide perovskite.

Metal halide perovskites such as methylammonium lead iodide (CH3NH3PbI3) are generating great excitement due to their outstanding optoelectronic properties, which lend them to application in high-efficiency solar cells and light-emission devices. However, there is currently debate over what drives the second-order electron-hole recombination in these materials. Here, we propose that the bandgap in CH3NH3PbI3 has a direct-indirect character. Time-resolved photo-conductance measurements show that generation of free mobile charges is maximized for excitation energies just above the indirect bandgap. Furthermore, we find that second-order electron-hole recombination of photo-excited charges is retarded at lower temperature. These observations are consistent with a slow phonon-assisted recombination pathway via the indirect bandgap. Interestingly, in the low-temperature orthorhombic phase, fast quenching of mobile charges occurs independent of the temperature and photon excitation energy. Our work provides a new framework to understand the optoelectronic properties of metal halide perovskites and analyse spectroscopic data

Long-range corrected DFT calculations of charge-transfer integrals in model metal-free phthalocyanine complexes

An assessment of several widely used exchange--correlation potentials in computing charge-transfer integrals is performed. In particular, we employ the recently proposed Coulomb-attenuated model which was proven by other authors to improve upon conventional functionals in the case of charge-transfer excitations. For further validation, two distinct approaches to compute the property in question are compared for a phthalocyanine dimer

G-quadruplex organic frameworks

Two-dimensional covalent organic frameworks often π stack into crystalline solids that allow precise spatial positioning of molecular building blocks. Inspired by the hydrogen-bonded G-quadruplexes found frequently in guanine-rich DNA, here we show that this structural motif can be exploited to guide the self-assembly of naphthalene diimide and perylene diimide electron acceptors end-capped with two guanine electron donors into crystalline G-quadruplex-based organic frameworks, wherein the electron donors and acceptors form ordered, segregated π-stacked arrays. Time-resolved optical and electron paramagnetic resonance spectroscopies show that photogenerated holes and electrons in the frameworks have long lifetimes and display recombination kinetics typical of dissociated charge carriers. Moreover, the reduced acceptors form polarons in which the electron is shared over several molecules. The G-quadruplex frameworks also demonstrate potential as cathode materials in Li-ion batteries because of the favourable electron- and Li-ion-transporting capacity provided by the ordered rylene diimide arrays and G-quadruplex structures, respectively

Electronic structure of thienylene vinylene oligomers:Singlet excited states, triplet excited states, cations, and dications

This paper describes a quantum chemical study of the electronic structure of thienylene vinylene oligomers ranging in size from two thienylene rings (2TV) to 12TV. The geometries of the TV oligomers in the ground state, the lowest triplet state, and the singly and doubly oxidized states were optimized using density functional theory calculations. The electronic absorption spectra were obtained from configuration interaction calculations with an INDO/s reference wave function. Comparison with experimental data shows that the agreement is satisfactory, except for the triplet-triplet absorption spectra. For closed shell systems (ground state and doubly occupied state), the spectra were also calculated by time dependent density functional theory (TDDFT). TDDFT considerably underestimates the neutral singlet-singlet excitation energies for longer chains. The nature of the excited states for the TV radical cations was found to be more similar to that of thiophenes than to that of phenylene vinylenes, indicating that the sulfur atom has a marked influence on the pi-electron system. For the (singlet) absorption spectra of doubly oxidized TVs, the results from TDDFT calculations are surprisingly Good; they are also good for long chains. TDDFT calculations for doubly charged TVs also confirm the existence of a second, weak absorption band as has been found experimentally

VIS/NIR absorption spectra of positively charged oligo(phenylenevinylene)s and comparison with time-dependent density functional theory calculations

A combined experimental and theoretical study of the optical properties of positively charged unsubstituted and dialkoxy-substituted phenylenevinylene (PV) oligomers in solution is presented. Cations of PV oligomers were produced by irradiation of a solution with high-energy electron pulses. The optical absorption spectra were measured using time-resolved visible/near-infrared (VIS/NIR) spectroscopy in the range of 500-2100 nm (0.6-2.5 eV). The optical absorption spectra of positively charged PVs are compared with results from time-dependent density functional theory (TDDFT) calculations and previous semiempirical calculations. The experimental spectra of cations of partially dialkoxy-substituted PVs indicate the presence of a transition with a maximum below 0.6 eV. According to earlier semiempirical calculations, the energy of this transition exhibits an oscillating behavior as a function of the length of the oligomer. This was not observed experimentally. However, the monotonic decrease of the low-energy absorption band, as obtained from TDDFT calculations, is in agreement with the experimental findings

Mechanism of charge transport along zinc porphyrin-based molecular wires.

In this study charge transport along zinc porphyrin-based molecular wires is simulated, considering both bandlike and hopping mechanisms. It is shown that bandlike transport simulations yield significantly overestimated hole mobility values. On the basis of kinetic and thermodynamic considerations, it is inferred that charge transport along zinc porphyrin-based molecular wires occurs by small polaron hopping. Hole mobility values on the order of 0.1 cm(2) V(-1) s(-1) are found from small polaron hopping simulations, which agree well with previously reported experimental results. It is suggested that the experimentally observed increase of the charge carrier mobility on formation of supramolecular ladderlike structures is determined by two factors. One of these is an increase of charge transfer integrals between monomer units due to molecular wire planarization. A more important factor is the reduction of the amount of energetic disorder along the molecular wire and in its environment. General guidelines for determining the mechanism of charge transport along molecular wires are discussed

QM/MM study of the role of the solvent in the formation of the charge separated excited state in 9,9'-bianthryl

In this paper the role of the solvent in the formation of the charge-separated excited state of 9,9'-bianthryl (BA) is examined by means of mixed molecular mechanical/quantum mechanical (QM/MM) calculations. It is shown that in weakly polar solvents a relaxed excited state is formed with an interunit angle that is significantly smaller than 90 degrees. This relaxed excited state has a considerable dipole moment even in weakly polar solvents; for benzene and dioxane dipole moments of ca. 6 D were calculated, which is close to experimental data. These dipoles are induced by the solvent in the highly polarizable relaxed excited state of BA, and the dipole relaxation time is governed by solvent reorganizations. In polar solvent the charge separation is driven to completion by the stronger dipoles in the solvent and a fully charged separated excited state is formed with an interunit angle of 90 degrees

Direct-indirect character of the bandgap in methylammonium lead iodide perovskite.

Metal halide perovskites such as methylammonium lead iodide (CH3NH3PbI3) are generating great excitement due to their outstanding optoelectronic properties, which lend them to application in high-efficiency solar cells and light-emission devices. However, there is currently debate over what drives the second-order electron-hole recombination in these materials. Here, we propose that the bandgap in CH3NH3PbI3 has a direct-indirect character. Time-resolved photo-conductance measurements show that generation of free mobile charges is maximized for excitation energies just above the indirect bandgap. Furthermore, we find that second-order electron-hole recombination of photo-excited charges is retarded at lower temperature. These observations are consistent with a slow phonon-assisted recombination pathway via the indirect bandgap. Interestingly, in the low-temperature orthorhombic phase, fast quenching of mobile charges occurs independent of the temperature and photon excitation energy. Our work provides a new framework to understand the optoelectronic properties of metal halide perovskites and analyse spectroscopic data

Effects of the Environment on Charge Transport in Molecular Wires

Supramolecular engineering offers opportunities for creating polymer-based materials with tailored conductive properties. However, this requires an understanding of intermolecular interaction effects on intramolecular charge transport. We present a study of hole transport along molecular wires consisting of fluorene-p-biphenyl or Zn-porphyrin monomer units, in dilute solutions. The intramolecular hole mobility was studied by pulse radiolysis-time-resolved microwave conductivity. Experiments were supplemented by charge transport simulations employing a quantum-mechanical description of the hole and a classical description of the polymer and solvent dynamics. The model was parametrized using ab initio and molecular dynamics calculations. It was found that the solvent-induced energy disorder along a polymer chain in common solvents (benzene, cyclohexane, acetonitrile, water) is 1 eV, significantly greater than the values of 0.05-0.2 eV commonly cited in the literature. Environment-induced disorder of this magnitude has profound consequences for intramolecular charge transport. The hole initial state upon injection onto a molecular wire also influences the mobility. Experiments and simulations demonstrate that supramolecular modification of polymers (coordination, rotaxination) can significantly enhance or suppress charge transport. Incorporating a molecular level description of the immediate supramolecular and solvent environment into charge transport models improves their predictive potential, providing a valuable tool for material design. © 2012 American Chemical Society

Near-threshold optical gain using colloidal HgTe quantum dots

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