Negative Capacitance and Inverted Hysteresis: Matching Features in Perovskite Solar Cells.

Negative capacitance in the low-frequency domain and inverted hysteresis are familiar features in perovskite solar cells, which origin is still under discussion. Here we use impedance spectroscopy to analyze these responses in methylammonium lead bromide cells treated with lithium cations at the electron-selective layer/perovskite interface and in iodide devices exposed to different relative humidity conditions. Employing the surface polarization model, we obtain a time constant associated with the kinetics of the interaction of ions/vacancies with the surface, τkin, in the range of 100-102 s for all the cases exhibiting both features. These interactions lead to a decrease in the overall recombination resistance, modifying the low-frequency perovskite response and yielding a flattening of the cyclic voltammetry. As a consequence of these results we find that negative capacitance and inverted hysteresis lead to a decrease in the fill factor and photovoltage values.

Computation provides chemical insight into the diverse hydride NMR chemical shifts of [Ru(NHC)4(L)H]0/+ species (NHC = N-heterocyclic carbene; L = vacant, H2, N2, CO, MeCN, O2, P4, SO2, H-, F- and Cl-) and their [Ru(R2PCH2CH2PR2)2(L)H]+ congeners.

Relativistic density functional theory calculations, both with and without the effects of spin-orbit coupling, have been employed to model hydride NMR chemical shifts for a series of [Ru(NHC)4(L)H]0/+ species (NHC = N-heterocyclic carbene; L = vacant, H2, N2, CO, MeCN, O2, P4, SO2, H-, F- and Cl-), as well as selected phosphine analogues [Ru(R2PCH2CH2PR2)2(L)H]+ (R = iPr, Cy; L = vacant, O2). Inclusion of spin-orbit coupling provides good agreement with the experimental data. For the NHC systems large variations in hydride chemical shift are shown to arise from the paramagnetic term, with high net shielding (L = vacant, Cl-, F-) being reinforced by the contribution from spin-orbit coupling. Natural chemical shift analysis highlights the major orbital contributions to the paramagnetic term and rationalizes trends via changes in the energies of the occupied Ru dπ orbitals and the unoccupied σ*Ru-H orbital. In [Ru(NHC)4(η2-O2)H]+ a δ-interaction with the O2 ligand results in a low-lying LUMO of dπ character. As a result this orbital can no longer contribute to the paramagnetic shielding, but instead provides additional deshielding via overlap with the remaining (occupied) dπ orbital under the Lz angular momentum operator. These two effects account for the unusual hydride chemical shift of +4.8 ppm observed experimentally for this species. Calculations reproduce hydride chemical shift data observed for [Ru(iPr2PCH2CH2PiPr2)2(η2-O2)H]+ (δ = -6.2 ppm) and [Ru(R2PCH2CH2PR2)2H]+ (ca. -32 ppm, R = iPr, Cy). For the latter, the presence of a weak agostic interaction trans to the hydride ligand is significant, as in its absence (R = Me) calculations predict a chemical shift of -41 ppm, similar to the [Ru(NHC)4H]+ analogues. Depending on the strength of the agostic interaction a variation of up to 18 ppm in hydride chemical shift is possible and this factor (that is not necessarily readily detected experimentally) can aid in the interpretation of hydride chemical shift data for nominally unsaturated hydride-containing species. The synthesis and crystallographic characterization of the BArF4- salts of [Ru(IMe4)4(L)H]+ (IMe4 = 1,3,4,5-tetramethylimidazol-2-ylidene; L = P4, SO2; ArF = 3,5-(CF3)2C6H3) and [Ru(IMe4)4(Cl)H] are also reported.

Polarization Switching and Light-Enhanced Piezoelectricity in Lead Halide Perovskites.

We investigate the ferroelectric properties of photovoltaic methylammonium lead halide CH3NH3PbI3 perovskite using piezoelectric force microscopy (PFM) and macroscopic polarization methods. The electric polarization is clearly observed by amplitude and phase hysteresis loops. However, the polarization loop decreases as the frequency is lowered, persisting for a short time only, in the one second regime, indicating that CH3NH3PbI3 does not exhibit permanent polarization at room temperature. This result is confirmed by macroscopic polarization measurement based on a standard capacitive method. We have observed a strong increase of piezoelectric response under illumination, consistent with the previously reported giant photoinduced dielectric constant at low frequencies. We speculate that an intrinsic charge transfer photoinduced dipole in the perovskite cage may lie at the origin of this effect.

Capacitive Dark Currents, Hysteresis, and Electrode Polarization in Lead Halide Perovskite Solar Cells.

Despite spectacular advances in conversion efficiency of perovskite solar cell many aspects of its operating modes are still poorly understood. Capacitance constitutes a key parameter to explore which mechanisms control particular functioning and undesired effects as current hysteresis. Analyzing capacitive responses allows addressing not only the nature of charge distribution in the device but also the kinetics of the charging processes and how they alter the solar cell current. Two main polarization processes are identified. Dielectric properties of the microscopic dipolar units through the orthorhombic-to-tetragonal phase transition account for the measured intermediate frequency capacitance. Electrode polarization caused by interfacial effects, presumably related to kinetically slow ions piled up in the vicinity of the outer interfaces, consistently explain the reported excess capacitance values at low frequencies. In addition, current-voltage curves and capacitive responses of perovskite-based solar cells are connected. The observed hysteretic effect in the dark current originates from the slow capacitive mechanisms.

A C(3v)-symmetrical tribenzotriquinacene-based threefold N-heterocyclic carbene. Coordination to rhodium(I) and stereoelectronic properties.

A novel tribenzotriquinacene-based tris-NHC has been obtained and coordinated to rhodium. The new ligand displays a unique rigid C3v symmetry. The electrochemical analysis of the tri-rhodium complex reveals that the three metals are essentially disconnected.

Substitution reactions in dinuclear Ru-Hbpp complexes: an evaluation of through-space interactions.

The synthesis of new dinuclear complexes of the general formula in,in-{[Ru(II)(trpy)(L)](µ-bpp)[Ru(II)(trpy)(L')]}(3+) [bpp(-) is the bis(2-pyridyl)-3,5-pyrazolate anionic ligand; trpy is the 2,2':6',2″-terpyridine neutral meridional ligand, and L and L' are monodentate ligands; L = L' = MeCN, 3a(3+); L = L' = 3,5-lutidine (Me(2)-py), 3c(3+); L = MeCN, L' = pyridine (py), 4(3+)], have been prepared and thoroughly characterized. Further, the preparation and isolation of dinuclear complexes containing dinitrile bridging ligands of the general formula in,in-{[Ru(II)(trpy)](2)(µ-bpp)(µ-L-L)}(3+) [µ-L-L = 1,4-dicyanobutane (adiponitrile, adip), 6a(3+); 1,3-dicyanopropane (glutaronitrile, glut), 6b(3+); 1,2-dicyanoethane (succinonitrile; succ), 6c(3+)] have also been carried out. In addition, a number of homologous dinuclear complexes previously described, containing the anionic bis(pyridyl)indazolate (bid(-)) tridentate meridional ligand in lieu of trpy, have also been prepared for comparative purposes. In the solid state, six complexes have been characterized by X-ray crystallography, and in solution, all of them have been spectroscopically characterized by NMR and UV-vis spectroscopy. In addition, their redox properties have also been investigated by means of cyclic voltammetry and differential pulse voltammetry and show the existence of two one-electron waves assigned to the formation of the II,III and III,III species. Dinitrile complexes 6a(3+), 6b(3+), and 6c(3+) display a dynamic behavior involving their enantiomeric interconversion. The energy barrier for this interconversion can be controlled by the number of methylenic units between the dinitrile ligand. On the other hand, pyridyl complexes in,in-{[Ru(II)(T)(py)](2)(µ-bpp)}(n+) (T = trpy, n = 3, 3b(3+); T = bid(-), n = 1, 3b'(+)) and 3c(3+) undergo two consecutive substitution reactions of their monodentate ligands by MeCN.The substitution kinetics have been monitored by (1)H NMR and UV-vis spectroscopy and follow first-order behavior with regard to the initial ruthenium complex. For the case of 3b(3+), the first-order rate constant k(1) = (2.9 ± 0.3) × 10(-5) s(-1), whereas for the second substitution, the k obtained is k(2) = (1.7 ± 0.7) × 10(-6) s(-1), both measured at 313 K. Their energies of activation at 298 K are 114.7 and 144.3 kJ mol(-1), respectively. Density functional theory (DFT) calculations have been performed for two consecutive substitution reactions, giving insight into the nature of the intermediates. Furthermore, the energetics obtained by DFT calculations of the two consecutive substitution reactions agree with the experimental values obtained. The kinetic properties of the two consecutive substitution reactions are rationalized in terms of steric crowding and also in terms of through-space interactions.

Intramolecular oxidation of the alcohol functionalities in hydroxyalkyl-N-heterocyclic carbene complexes of iridium and rhodium.

A series of hydroxyalkyl-functionalized imidazolium salts have been coordinated to Rh and Ir to afford the corresponding MCp*-(NHC) (Cp*=pentamethylcyclopentadienyl) complexes. The reactivity of the new complexes has been studied with special attention to the transformations that deal with the alcohol functionality. The metal-mediated intramolecular transformations allowed the formation of several products that resulted from the oxidation of the alcohols to aldehydes and esters. All the new complexes have been fully characterized, and the crystal structures of the most representative complexes have been resolved.

The influence of N-heterocyclic carbenes (NHC) on the reactivity of [Ru(NHC)(4)H](+) with H(2), N(2), CO and O(2).

The five-coordinate ruthenium N-heterocyclic carbene (NHC) hydrido complexes [Ru(IiPr(2)Me(2))(4)H][BAr(F) (4)] (1; IiPr(2)Me(2)=1,3-diisopropyl-4,5-dimethylimidazol-2-ylidene; Ar(F)=3,5-(CF(3))(2)C(6)H(3)), [Ru(IEt(2)Me(2))(4)H][BAr(F) (4)] (2; IEt(2)Me(2)=1,3-diethyl-4,5-dimethylimidazol-2-ylidene) and [Ru(IMe(4))(4)H][BAr(F) (4)] (3; IMe(4)=1,3,4,5-tetramethylimidazol-2-ylidene) have been synthesised following reaction of [Ru(PPh(3))(3)HCl] with 4-8 equivalents of the free carbenes at ambient temperature. Complexes 1-3 have been structurally characterised and show square pyramidal geometries with apical hydride ligands. In both dichloromethane or pyridine solution, 1 and 2 display very low frequency hydride signals at about delta -41. The tetramethyl carbene complex 3 exhibits a similar chemical shift in toluene, but shows a higher frequency signal in acetonitrile arising from the solvent adduct [Ru(IMe(4))(4)(MeCN)H][BAr(F) (4)], 4. The reactivity of 1-3 towards H(2) and N(2) depends on the size of the N-substituent of the NHC ligand. Thus, 1 is unreactive towards both gases, 2 reacts with both H(2) and N(2) only at low temperature and incompletely, while 3 affords [Ru(IMe(4))(4)(eta(2)-H(2))H][BAr(F) (4)] (7) and [Ru(IMe(4))(4)(N(2))H][BAr(F) (4)] (8) in quantitative yield at room temperature. CO shows no selectivity, reacting with 1-3 to give [Ru(NHC)(4)(CO)H][BAr(F) (4)] (9-11). Addition of O(2) to solutions of 2 and 3 leads to rapid oxidation, from which the Ru(III) species [Ru(NHC)(4)(OH)(2)][BAr(F) (4)] and the Ru(IV) oxo chlorido complex [Ru(IEt(2)Me(2))(4)(O)Cl][BAr(F) (4)] were isolated. DFT calculations reproduce the greater ability of 3 to bind small molecules and show relative binding strengths that follow the trend CO >> O(2) > N(2) > H(2).

Formation of [Ru(NHC)4(eta(2)-O2)H]+: an unusual, high frequency hydride chemical shift and facile, reversible coordination of O2.

Reaction of the purple tetrakiscarbene ruthenium cation [Ru(I(i)Pr(2)Me(2))(4)H](+) (1, I(i)Pr(2)Me(2) = 1,3-diisopropyl-4,5-dimethylimidazol-2-ylidene) with oxygen affords the pink eta(2)-O(2) hydride species [Ru(I(i)Pr(2)Me(2))(4)(eta(2)-O(2))H](+) (2). 2 displays (i) a very facile, reversible O(2) coordination and (ii) an unexpectedly positive hydride chemical shift, and both these features can be predicted and explained by density functional theory (DFT) calculations.

Ru complexes that can catalytically oxidize water to molecular dioxygen.

The main objective of this review is to give a general overview of the structure, electrochemistry (when available), and catalytic performance of the Ru complexes, which are capable of oxidizing water to molecular dioxygen, and to highlight their more relevant features. The description of the Ru catalysts is mainly divided into complexes that contain a Ru-O-Ru bridging group and those that do not. Finally a few conclusions are drawn from the global description of all of the catalysts presented here, and some guidelines for future catalyst design are given.

Palladium-NHC complexes do catalyse the diboration of alkenes: mechanistic insights.

Novel catalytic activation of the B-B bond by palladium(II)-NHC complexes in presence of a mild base (NaOAc) and an excess of diboron reagent enables chemoselective 1,2-diboration of alkenes, suggesting the heterolytic cleavage of diboron rather than oxidative addition of a B-B bond to the metal.

Coordination versatility of pyridine-functionalized N-heterocyclic carbenes: a detailed study of the different activation procedures. Characterization of new Rh and Ir compounds and study of their catalytic activity.

Three different reaction procedures for the coordination of N-n-butyl-N'-(2-pyridylmethyl)imidazolium salt have produced new N-heterocyclic complexes of Rh and Ir. The direct reaction of the imidazolium salt with [IrCl(cod)](2) provides a NHC-Ir(III)-H complex, while transmetalation from a silver-NHC complex and deprotonation with NEt(3) give new NHC complexes of M(I) and M(III) when reacting with [MCl(cod)](2) or [MCl(coe)(2)](2) (M = Rh, Ir). The crystal structures of the biscarbene Rh(III) and Ir(III) complexes are described. The catalytic properties of the compounds obtained have been tested in the hydrosilylation of acetylenes, the cyclization of acetylenic carboxylic acids, and hydrogen transfer to ketones.

Synthesis and reactivity of new chelate-N-heterocyclic biscarbene complexes of ruthenium.

The carbene-ligand precursors methylenebis(N-alkylimidazolium) iodide (alkyl = methyl, neo-pentyl) and ethylenebis(N-methylimidazolium) chloride have been used in the preparation of several new Ru(II)-p-cymene complexes where the ligand behaves as mono- and bidentate. The molecular structures of the two biscarbene-complexes are reported. From the data reported, we can conclude that steric reasons (mainly the bisimidazolium linkers, methylene/ethylene) are the main factors determining both reactivity and synthetic difficulties of the products reported.

Carbene complexes of rhodium and iridium from tripodal N-heterocyclic carbene ligands: synthesis and catalytic properties.

Two tripodal trisimidazolium ligand precursors have been tested in the synthesis of new N-heterocyclic carbene rhodium and iridium complexes. [Tris(3-methylbenzimidazolium-1-yl)]methane sulfate gave products with coordination of the decomposed precursor. [1,1,1-Tris(3-butylimidazolium-1-yl)methyl]ethane trichloride (TIMEH(3)(Bu)) coordinated to the metal in a chelate and bridged-chelate form, depending on the reaction conditions. The crystal structures of two of the products are described. The compounds resulting from the coordination with TIME(Bu) were tested in the catalytic hydrosilylation of terminal alkynes.