Despite prolonged scientific efforts to unravel the hydration structures of ions in water, many open questions remain, in particular concerning the existences and structures of ion clusters in 1∶1 strong electrolyte aqueous solutions. A combined ultrafast 2D IR and pump/probe study through vibrational energy transfers directly observes ion clustering in aqueous solutions of LiSCN, NaSCN, KSCN and CsSCN. In a near saturated KSCN aqueous solution (water/KSCN molar ratio ¼ 2.4∕1), 95% of the anions form ion clusters. Diluting the solution results in fewer, smaller, and tighter clusters. Cations have significant effects on cluster formation. A small cation results in smaller and fewer clusters. The vibrational energy transfer method holds promise for studying a wide variety of other fast short-range molecular interactions.T he solution properties of ions in water are relevant to a wide range of systems, including electrochemistry, biological environments, and atmospheric aerosols (1, 2). For more than 100 yr, tremendous scientific efforts have been devoted to unravel the hydration structures of ions in water (1-11). However, many fundamental questions remain open, in particular concerning the existence, concentration, and structure of ion clusters in 1∶1 strong electrolyte aqueous solutions. Whether strong 1∶1 electrolytes (especially salts of Na þ and K þ ) form ion pairs or clusters in water has been considered a key issue for understanding many important problems, e.g., the excess ionic activity in 1∶1 electrolytes (12), ion dependent conformational and binding equilibria of nucleic acids (13), the concentration difference between Na þ and K þ in living cells, protein denaturation by salts (14, 15), and ion concentration dependent properties of ion channels (16).The properties of aqueous solutions of 1∶1 strong electrolytes deviate from the ideal dilute solution at extremely low concentrations (<10 −5 M). The deviations were generally believed to be caused by the attraction between ions of opposite charge and the repulsion of ions of the same charge, leading to the development of the Debye-Hückel theory (17, 18). However, this theory begins to fail at a very low concentration (∼10 −3 M), as the assumptions upon which the theory was based become invalid. The formation of ion pairs containing two ions of opposite charge has been proposed to be primarily responsible for this failure (1, 2). Recently, calculations from molecular dynamics (MD) simulations, suggested that, clusters with more than one ion of the same charge which are traditionally viewed as unlikely, could be a major factor contributing to the nonideality of solutions at medium or high concentrations (12,19). However, these predicted ion clusters cannot be investigated by the usual tools for probing molecular structures and particle sizes in liquids, e.g., X-ray or neutron diffraction (20), or the dynamic light scattering (19,21), because the contribution of ion-ion correlations to the total scattering pattern is too small compared to the contributions ...

With a certified efficiency as high as 25.2%, perovskite has taken the crown as the highest efficiency thin film solar cell material. Unfortunately, serious instability issues must be resolved before perovskite solar cells (PSCs) are commercialized. Aided by theoretical calculation, an appropriate multifunctional molecule, 2,2‐difluoropropanediamide (DFPDA), is selected to ameliorate all the instability issues. Specifically, the carbonyl groups in DFPDA form chemical bonds with Pb2+ and passivate under‐coordinated Pb2+ defects. Consequently, the perovskite crystallization rate is reduced and high‐quality films are produced with fewer defects. The amino groups not only bind with iodide to suppress ion migration but also increase the electron density on the carbonyl groups to further enhance their passivation effect. Furthermore, the fluorine groups in DFPDA form both an effective barrier on the perovskite to improve its moisture stability and a bridge between the perovskite and HTL for effective charge transport. In addition, they show an effective doping effect in the HTL to improve its carrier mobility. With the help of the combined effects of these groups in DFPDA, the PSCs with DFPDA additive achieve a champion efficiency of 22.21% and a substantially improved stability against moisture, heat, and light.

Waiting time dependent rotational anisotropies of SCN − anions and water molecules in alkali thiocyanate (XSCN, X = Li, Na, K, Cs) aqueous solutions at various concentrations were measured with ultrafast infrared spectroscopy. It was found that cations can significantly affect the reorientational motions of both water molecules and SCN − anions. The dynamics are slower in a solution with a smaller cation. The reorientational time constants follow the order of Li + > Na + > K + ≃ Cs + . The changes of rotational time constants of SCN − at various concentrations scale almost linearly with the changes of solution viscosity, but those of water molecules do not. In addition, the concentration-dependent amplitudes of dynamical changes are much more significant in the Li + and Na + solutions than those in the K + and Cs + solutions. Further investigations on the systems with the ultrafast vibrational energy exchange method and molecular dynamics simulations provide an explanation for the observations: the observed rotational dynamics are the balanced results of ion clustering and cation/anion/water direct interactions. In all the solutions at high concentrations (>5 M), substantial amounts of ions form clusters. The structural inhomogeneity in the solutions leads to distinct rotational dynamics of water and anions. The strong interactions of Li + and Na + because of their relatively large charge densities with water molecules and SCN − anions, in addition to the likely geometric confinements because of ion clustering, substantially slow down the rotations of SCN − anions and water molecules inside the ion clusters. The interactions of K + and Cs + with water or SCN − are much weaker. The rotations of water molecules inside ion clusters of K + and Cs + solutions are not significantly different from those of other water species so that the experimentally observed rotational relaxation dynamics are only slightly affected by the ion concentrations.

Vibrational energy transfer from the first excited state (2635 cm(-1)) of the O-D stretch of deuterated water (D(2)O) to the 0-1 transition (2075 cm(-1)) of the CN stretch of potassium selenocyanate (KSeCN) in their 2.5:1 liquid mixture was observed with a multiple-mode two dimensional infrared spectroscopic technique. Despite the big energy mismatch (560 cm(-1)) between the two modes, the transfer is still very efficient with a time constant of 20 ps. The efficient energy transfer is probably because of the large excitation coupling between the two modes. The coupling is experimentally determined to be 176 cm(-1). An approximate analytical equation derived from the Landau-Teller formula is applied to calculate the energy transfer rate with all parameters experimentally determined. The calculation results are qualitatively consistent with the experimental data.

Vibrational energy transfer from the first excited state 2252 cm−1 of the C–D stretch of deuterated chloroform DCCl3 to the 0-1 transition 2155 cm−1 of the CN stretch of phenyl selenocyanate C6H5SeCN in their 1:1 liquid mixture was observed with a pump/probe two-color two dimensional infrared spectroscopic technique. The mode-specific energy transfer can occur mainly because of the long vibrational lifetime of the CN stretch first excited state 300 ps and the relatively strong hydrogen-bond between the C–D and CN calculated H-bond formation energy in gas phase −5.4 kcal /mol. The mode-specific energy transfer is relatively low efficient only 2%, which is mainly because of the relatively short vibrational lifetime 9 ps of the C–D stretch first excited state and the big donor/acceptor energy mismatch 97 cm−1 and the slow transfer kinetics 1 /kCD→CN=330 ps. © 2010 American Institute of Physics. doi:10.1063/1.342917

The donor/acceptor energy mismatch and vibrational coupling strength dependences of interionic vibrational energy transfer kinetics in electrolyte aqueous solutions were investigated with ultrafast multiple-dimensional vibrational spectroscopy. An analytical equation derived from the Fermi's Golden rule that correlates molecular structural parameters and vibrational energy transfer kinetics was found to be able to describe the intermolecular mode specific vibrational energy transfer. Under the assumption of the dipole-dipole approximation, the distance between anions in the aqueous solutions was obtained from the vibrational energy transfer measurements, confirmed with measurements on the corresponding crystalline samples. The result demonstrates that the mode-specific vibrational energy transfer method holds promise as an angstrom molecular ruler.

devices due to their outstanding semiconductor properties. [1,2] In spite of their high efficiencies, however, the volatile organic components in the hybrid perovskite, such as methylammonium (MA) and formamidinium (FA), suffer from poor thermal and chemical stability issues, and they can be easily released under high temperature and/or react with oxygen and water to form superoxides and hydrates, thus eventually breaking the perovskite structure into MAI/FAI and PbI 2 . [3,4] Due to this instability issue, it is very challenging to prepare state-of-the-art hybrid perovskite solar cells (PSCs) in the ambient environment. Accordingly, the instability issues of hybrid perovskites hinder their commercial application.The replacement of organic cations with inorganic cations, such as Cs, could overcome the instability issue of the organicinorganic hybrid perovskite to endow the perovskite with better compositional stability. [5,6] Therefore, inorganic PSCs based on CsPbX 3 (X: I, Br, or mixed halides) have been developed rapidly in the past few years. However, the preparation of highefficiency inorganic PSCs in ambient air is still intractable, especially under high humidity conditions, since the crystallization, High temperature stable inorganic CsPbX 3 (X: I, Br, or mixed halides) perovskites with their bandgap tailored by tuning the halide composition offer promising opportunities in the design of ideal top cells for high-efficiency tandem solar cells. Unfortunately, the current high-efficiency CsPbX 3 perovskite solar cells (PSCs) are prepared in vacuum, a moisture-free glovebox or other low-humidity conditions due to their poor moisture stability. Herein, a new precursor system (HCOOCs, HPbI 3 , and HPbBr 3 ) is developed to replace the traditional precursors (CsI, PbI 2 , and PbBr 2 ) commonly used for solar cells of this type. Both the experiments and calculations reveal that a new complex (HCOOH•Cs + ) is generated in this precursor system. The new complex is not only stable against aging in humid air ambient at 91% relative humidity, but also effectively slows the perovskite crystallization, making it possible to eliminate the popular antisolvent used in the perovskite CsPbI 2 Br film deposition. The CsPbI 2 Br PSCs based on the new precursor system achieve a champion efficiency of 16.14%, the highest for inorganic PSCs prepared in ambient air conditions. Meanwhile, high air stability is demonstrated for an unencapsulated CsPbI 2 Br PSC with 92% of the original efficiency remaining after more than 800 h aging in ambient air.

Specific ion effects on the nonlinear optical response from the water molecules at the air/sodium halide solution interfaces are measured using non-resonant surface second harmonic generation (SHG). Procedures have been developed to monitor and remove the impurities in the salt solution samples to ensure measurement of small changes in the SHG signal. Quantitative polarization analysis of the measured SHG data indicated that the average orientation of the interfacial water molecules changed only slightly around 40 degrees with the increase of the bulk concentration of the three sodium halides, namely NaF, NaCl and NaBr, from that of the neat air/water interface. The observed significant SHG signal increase with the bulk salt concentration is attributed to the overall increase of the thickness of the interfacial water molecular layer, following the order of NaBr > NaCl approximately NaF. The absence of the electric-field-induced SHG (EFISHG) effect indicated that the electric double layer at the salt aqueous solution interface is much weaker than that predicted from the molecular dynamics (MD) simulations. These results provided quantitative data to the specific anion effects on the interfacial water molecules of the electrolyte aqueous solution, not only for the larger and more polarizable Br(-) anion, but also for the smaller and less polarizable F(-) and Cl(-) anions.