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The impressive photovoltaic performance of hybrid iodide CH3NH3PbI3 perovskite relies, among other factors, on the optimal alignment of the electronic energy levels of the semiconductor with respect to conventional hole transporting (HTM) and electron transporting (ETM) materials. Here, we first report on density functional theory electronic structure calculations of slab models of the (001) surface aiming to assess how the perovskite valence and conduction band edge (VBE and CBE) energies depend on the nature of the surface exposed to vacuum. We find that the surface termination plays a critical role in determining the energies of the frontier crystal orbitals, with PbI-terminated surface showing VBE and CBE energy ∼1 eV below the corresponding levels in the methylammonium-terminated surfaces. We next build perovskite/C60 interfaces based on two such surfaces and discuss the associated electronic structure in light of recent experimental data. The two interfaces are rather inert showing limited band bending/shifts with respect to the isolated components, in line with photoelectron spectroscopy data. They, however, yield very different electron extraction energies, possibly explaining the different behaviors reported in the literature. [ABSTRACT FROM AUTHOR]

We report a combined experimental and theoretical study on cationic Ir(III) complexes for OLED applications and describe a strategy to tune the phosphorescence wavelength and to enhance the emission quantum yields for this class of compounds. This is achieved by modulating the electronic structure and the excited states of the complexes by selective ligand functionalization. In particular, we report the synthesis, electrochemical characterization, and photophysical properties of a new cationic Ir(III) complex, [Ir(2,4-difluorophenylpyridine)2(4,4′-dimethylamino-2,2′- bipyridine)](PF6) (N969), and compare the results with those reported for the analogous [Ir(2-phenylpyridine)2(4,4′-dimethylamino-2,2′-bipyridine)](PF6) (N926) and for the prototype [Ir(2-phenylpyridine)2(4,4′-tert-butyl-2,2′-bipyridine)]- (PF6) complex, hereafter labeled N925. The three complexes allow us to explore the (CAN) and (NAN) ligand functionalization: considering N925 as a reference, we investigate in N926 the effect of electron-releasing substituents on the bipyridine ligand, while in N969, we investigate the combined effect of electron-releasing substituents on the bipyridine ligand and the effect of electron-withdrawing substituents on the phenylpyridine ligands. For N969 we obtain blue-green emission at 463 nm with unprecedented high quantum yield of 85% in acetonitrile solution at room temperature. To gain insight into the factors responsible for the emission color change and the different quantum yields, we perform DFT and TDDFT calculations on the ground and excited states of the three complexes, characterizing the excited-state geometries and including solvation effects on the calculation of the excited states. This computational procedure allows us to provide a detailed assignment of the excited states involved in the absorption and emission processes and to rationalize the factors determining the efficiency of radiative and nonradiative deactivation pathways in the investigated complexes. This work represents an example of electronic structure-driven tuning of the excited-state properties, thus opening the way to a combined theoretical and experimental strategy for the design of new iridium(III) phosphors with specific target characteristics. [ABSTRACT FROM AUTHOR]

The reactivity of the isomeric oxo-Mn(V)-2-tetra-N-methylpyridyl (2-TMPyP) and oxo-Mn(V)-4-tetra-N-methylpyridyl (4-TMPyP) porphyrins has been investigated by a combined experimental and theoretical approach based on density functional theory. The unusual higher reactivity of the more electron-rich 4-TMPyP species appears to be related to both the higher basicity of its oxo ligand, compared to that of the 2-TMPyP isomer, and the smaller low-spin-high-spin promotion energy of 4-TMPyP, compared to that of 2-TMPyP, because of the stabilization of the A2u orbital in the latter isomer. Therefore, in a two-state energy profile involving crossing of the initial singlet and final quintet potential energy surfaces, the 4-TMPyP isomer should be kinetically favored. The calculated differences in the singlet-quintet gaps for the 2-TMPyP and 4-TMPyP systems compare well with the measured differences in the activation energies for two isomeric porphyrins. Both effects, proton affinity and electron-promotion energy, contribute to reduce the reactivity of the more electrophilic oxidant when electron-withdrawing groups are closer to the active site, contrary to the usual expectations based on simple chemical reactivity correlations. These theoretical results are in accord with new experimental data showing OMn(V)OH pKa5 of 7.5 and 8.6 for the isomeric 2-TMPyP and 4-TMPyP systems, respectively. [ABSTRACT FROM AUTHOR]

We present a combined density functional/time-dependent density functional study of the electronic structure and optical absorption spectrum of the charge-transfer sensitizer cis-[Ru(4,4′-COOH-2,2′-bpy)2(NCS)2] in water solution. To simulate the effect of different pH values, calculations have been performed for both the title complex and the corresponding tetra-deprotonated species, cis-[Ru(4,4′-COO-2,2′-bpy)2(NCS)2]4−. The experimentally observed blue-shift of the spectrum at high pH is well reproduced by our theoretical model and appears to be related to destabilization of the bipyridines π* LUMOs as a result of the increased electron density on the deprotonated carboxylic groups. [Copyright &y& Elsevier]

Since their first appearance, organic-inorganic perovskite absorbers have been capturing the attention of the scientific community. While high efficiency devices highlight the importance of band level alignment, very little is known on the origin of the strong n-doping character observed in the perovskite. Here, by means of a highly accurate photoemission study, we shed light on the energy alignment in perovskite-based devices. Our results suggest that the interaction with the substrate may be the driver for the observed doping in the perovskite samples. [ABSTRACT FROM AUTHOR]

• Different Hamiltonian, basis sets and DFT functionals were compared. • Inclusion of the spin orbit coupling and Hartree Fock exchange is essential. • The Pb I bond has a partial covalent character. • PbI 2 and MAPbI 3 solid state materials can be treated with the same theoretical approach. A detailed knowledge of the basic electronic interactions in lead halide perovskites components (PbI 2 and methylammonium iodide) can possibly drive enhanced solar cell efficiency. We report an extensive investigation on the electronic structure and nature of the chemical bond in the PbI 2 perovskite precursor, both in gas and solid state, together with a comparison with available experimental data, which allows to effectively calibrate the computational framework, along with gaining basic understanding on the nature of the Pb I chemical bond. Inclusion of spin orbit coupling and calibrated HF exchange contribution to the DFT hybrid functional are proved essential for an accurate description of the electronic structure of both molecular and solid state PbI 2. Such computational framework, calibrated on the model PbI 2 system, can be directly translated to the accurate description of the electronic band structure of the prototypical methylammonium lead-iodide perovskite, setting the basis for the trustful modelling of different lead-halide perovskites. [ABSTRACT FROM AUTHOR]

We report on ab initio electronicstructure and Car–Parrinellomolecular dynamics simulations for several structural models of theprototype MAPbI3perovskite for solar cells applications.We considered both configurations having a preferred orientation ofthe MA cations, giving rise to a net dipole alignment, and configurationswith an isotropic distribution of the MA cations, respectively representativeof polar (ferroelectric) and apolar (antiferroelectric) structures.Our calculations demonstrate the preferred stability of a set of polarstructures over apolar ones, with an energy difference within 0.1eV and a conversion barrier within 0.2 eV per unit cell (four MAPbI3), thus possibly accessible at room temperature. Ferroelectric-likeorientations lead to a quasi I4cmstructure for the inorganic component, characterized by lack ofinversion symmetry, while the antiferroelectric-like orientationsare associated to a quasi I4/mcmstructure. Ab initio molecular dynamics simulations on the polarstructures show no molecular rotations in the investigated time-scale,while several MA rotations are observed in the same time scale forthe considered apolar structure, which is thus characterized by ahigher disorder. The I4cmand I4/mcmtypes of structure have markedlydifferent band structures, despite showing a relatively small bandgap variation. Simulations carried out on finite surface slabs demonstratethat a net orientation of the MA cations gives rise to a strong bendingin the valence and conduction bands, which could definitely assistcharge separation and reduce carrier recombination, provided one isable to effectively stabilize polar compared to apolar structures.We believe our results could contribute an important step toward anin-depth comprehension of the basic properties of organohalide perovskites,assisting a further optimization of their photovoltaic response. [ABSTRACT FROM AUTHOR]

The photophysical and electrochemical properties of the novel complexes [Ir(ppy)2(5-X-1 ,10-phen)][PF6] (ppy = 2-phenylpyridine, phen = phenanthroline, X = NMe2, NO2), [lr(pq)2(5-X-1,10-phen)][PF6) (pq = 2-phenyiquinoline, X = H, Me, NMe2, NO2), [lr(ppy)2(4-Me,7-Me-1,10-phen)][PF6], [Ir(ppy)2(5-Me,6-Me-1,10-phen)][PF6], [lr(ppy)2(2-Me,9-Me-1,10-phen)][PF6], and [lr(pq)2(4-Ph,7-Ph-1,10-phen)][PF6] have been investigated and compared with those of the known reference complexes [lr(ppy)2(4-Me or 5-H or 5-Me-1,10-phen)][PF6] and [Ir(ppy)2(4-Ph,7-Ph-1,10-phen)][PF6], showing how the nature and number of the phenanthroline substituents tune the color of the emission, its quantum yield, and the emission lifetime, It turns out that the quantum yield is strongly dependent on the nonradiative decay. The geometry, ground state, electronic structure, and excited electronic states of the investigated complexes have been calculated on the basis of density functional theory (DFT) and time-dependent DFT approaches, thus substantiating the electrochemical measurements and providing insight into the electronic origin of the absorption spectra and of the lowest excited states involved in the light emission process. These results provide useful guidelines for further tailoring of the photophysical properties of ionic lr(III) complexes. [ABSTRACT FROM AUTHOR]