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Slow hot carrier cooling in halide perovskites holds the key to the development of hot carrier (HC) perovskite solar cells. For accurate modeling and pragmatic design of HC materials and devices, it is essential that HC temperatures are reliably determined. A common approach involves fitting the high-energy tail of the main photobleaching peak in a transient absorption spectrum with a Maxwell-Boltzmann distribution. However, this approach is problematic because of complications from the overlap of several photophysical phenomena and a lack of consensus in the community on the fitting procedures. Herein, we propose a simple approach that circumvents these challenges. Through tracking the broadband spectral evolution and accounting for bandgap renormalization and spectral line width broadening effects, our method extracts not only accurate and consistent carrier temperatures but also other important parameters such as the quasi-Fermi levels, bandgap renormalization constant, etc. Establishing a reliable method for the carrier temperature determination is a step forward in the study of HCs for next-generation perovskite optoelectronics. NRF (Natl Research Foundation, S’pore) MOE (Min. of Education, S’pore) Accepted version

Bismuth double perovskite Cs2AgBiBr6 strong electron–acoustic phonon interactions derail its photovoltaic aspirations., Bismuth-based double perovskite Cs2AgBiBr6 is regarded as a potential candidate for low-toxicity, high-stability perovskite solar cells. However, its performance is far from satisfactory. Albeit being an indirect bandgap semiconductor, we observe bright emission with large bimolecular recombination coefficient (reaching 4.5 ± 0.1 × 10−11 cm3 s−1) and low charge carrier mobility (around 0.05 cm2 s−1 V−1). Besides intermediate Fröhlich couplings present in both Pb-based perovskites and Cs2AgBiBr6, we uncover evidence of strong deformation potential by acoustic phonons in the latter through transient reflection, time-resolved terahertz measurements, and density functional theory calculations. The Fröhlich and deformation potentials synergistically lead to ultrafast self-trapping of free carriers forming polarons highly localized on a few units of the lattice within a few picoseconds, which also breaks down the electronic band picture, leading to efficient radiative recombination. The strong self-trapping in Cs2AgBiBr6 could impose intrinsic limitations for its application in photovoltaics.

In this work, the solvation of a nitrosyl ruthenium complex, [(CH3)4N][RuCl3(qn)(NO)] (with qn = deprotonated 8-hydroxyquinoline), which is a potential NO-releasing molecule in the bio-environment,...

Vibrational energy transfer in transition metal complexes with flexible structures in condensed phases is of central importance to catalytical chemistry processes. In this work, two molecules with different metal atoms, M(CO)5Br (where M = Mn, Re), were used as model systems, and their axial and radial carbonyl stretching modes as infrared probes. The central-metal effect on intramolecular vibrational energy redistribution (IVR) in M(CO)5Br was investigated in polar and nonpolar solvents. The linear infrared (IR) peak splitting between carbonyl vibrations increases as the metal atom changes from Mn to Re. The waiting-time dependent two-dimensional infrared diagonal- and off-diagonal peak amplitudes reveal a faster IVR process in Re(CO)5Br than in Mn(CO)5Br. With the aid of density functional theory (DFT) calculations, the central-metal effect on IVR time linearly correlates with the vibrational coupling strength between the two involved modes. In addition, the polar solvent is found to accelerate the IVR process by affecting the anharmonic vibrational potentials of a solute vibration mode.

Microemulsions are transparent, thermodynamically stable liquid mixtures of oil, water and surfactant. They typically exist in the form of micellar and bicontinuous structures, which appear to be equilibrium systems but are actually complex in structure and are difficult to characterize at the molecular level. In this work, potassium ferrocyanide [K4Fe(CN)6] and tungsten hexacarbonyl [W(CO)6] were chosen as two probe molecules for water and organic phases respectively to simultaneously explore the structures and dynamics of commonly prepared reverse micellar and bicontinuous microemulsion structures using aerosol OT (AOT) as the surfactant and isooctane as the oil phase. Even though these two structures have quite different solvent environments due to the varying geometry and boundary conditions, the steady-state infrared spectra of the C[triple bond, length as m-dash]N- stretching mode in the water phase are quite similar, and so are those of the C[triple bond, length as m-dash]O stretching mode in the oil phase. However, vibrational and anisotropic relaxation dynamics obtained from infrared pump-probe spectroscopy and spectral diffusion dynamics extracted from two-dimensional infrared spectroscopy of the two infrared chromophores in the two probe molecules are found to be quite sensitive to whether the probe molecules are in pure solvent or in a restricted microemulsion environment. The results in the water phase and oil phase are discussed separately. Our work demonstrated that ultrafast nonlinear infrared spectroscopy can be used to differentiate the structural details at the molecular level for different microemulsion systems.