Studies on the Light-Induced Phase Transition of CsPbBr3 Metal Halide Perovskite Materials.

We investigate the internal mechanism of the light-induced phase transition of CsPbBr3 perovskite materials via density functional theory simulations. Although CsPbBr3 tends to appear in the orthorhombic structure, it can be changed easily by external stimulus. We find that the transition of photogenerated carriers plays the decisive role in this process. When the photogenerated carriers transit from the valence band maximum to conduction band minimum in the reciprocal space, they actually transit from Br ions to Pb ions in the real space, which are taken away by the Br atoms with higher electronegativity from Pb atoms during the initial formation of the CsPbBr3 lattice. The reverse transition of valence electrons leads to the weakening of bond strength, which is proved by our calculated Bader charge, electron localization function, and integral value of COHP results. This charge transition releases the distortion of the Pb-Br octahedral framework and expands the CsPbBr3 lattice, providing possibilities to the phase transition from the orthorhombic structure to tetragonal structure. This phase transition is a self-accelerating positive feedback process, increasing the light absorption efficiency of the CsPbBr3 material, which is of great significance for the widespread promotion and application of the photostriction effect. Our results are helpful to understand the performance of CsPbBr3 perovskite under a light irradiation environment.

Studies on the photoelectronic properties of a manganese (Mn)-doped lead-free double perovskite.

Taking Cs2NaBiCl6, Cs2AgInCl6 and Cs2AgBiCl6 as examples of lead-free double perovskites (DPs), we study the photoluminescence (PL) properties of Mn-doped DPs. The electron localization function (ELF) reveals the more ionic nature of the Na-Cl bond in Cs2NaBiCl6 than that of the Ag-Cl bond in Cs2AgBiCl6. Bader charge calculations confirm the nominal +2 valence state of Mn ions in both DPs. Mn2+ ions introduce two defect levels in the band gap of the Cs2NaBiCl6 host, accounting for the d-d transition (4T1-6A1 transition) of Mn2+ and thus the subsequent orange PL. The changes of the crystal field and their influences on the emission energy of Mn2+ ions in different DPs are evaluated by calculating the Racah parameters (B and C) and the crystal field strength (Dq) obtained from energies of the terms of d5 in the Cs2NaBiCl6:Mn2+ and Cs2AgInCl6:Mn2+ systems. The results show that Dq in Cs2AgInCl6:Mn2+ is stronger than that in Cs2NaBiCl6:Mn2+. The analyses on bonding interactions of the Mn-Cl bond via ELF and the integrated projected pCOHP also confirm the stronger ionic bonding interactions and thus the boost of the crystal field strength in the Cs2AgInCl6:Mn2+ system, which results in the blue-shift of the Mn2+ introduced PL peak from Cs2AgInCl6 to Cs2NaBiCl6. Our results provide a new strategy to modulate the emission wavelengths, i.e., tuning the crystal field.

Investigations of modulation effect of co-metal ions on the optical properties of the hybrid double perovskites (MA)2AgBi1-xSbxBr6.

Composition engineering plays an important role in generating novel properties and decreasing the lead (Pb) toxicity for halide perovskite materials. To find out the modulation effect introduced by the composition engineering, namely,B'-site co-metal ions, in (MA)2AgBi1-xSbxBr6systems with various Bi/Sb ratios ofx= 0, 0.25, 0.75, 1.00, series of theoretical simulations and analyses are carried out. For the (MA)2AgBi1-xSbxBr6systems, the Goldschmidt tolerance factortand the octahedral factorµindicate that all samples are in a standard double perovskite structure with alternating AgBr6and Bi/SbBr6octahedra. The calculated electronic structures show that the band gap of (MA)2AgBi1-xSbxBr6decreases with the increase of Sb content, but the indirect band gaps are maintained for all samples. By analyses of the imaginary partɛ2(ω) of dielectric function and the absorption spectra, we find that all (MA)2AgBi1-xSbxBr6systems show absorption in the visible-light region. All these results indicate that the composition engineering adopted in this paper is an effective strategy to modulate the optical properties of (MA)2AgBi1-xSbxBr6systems and may open a new way to put it into applications in the fields of solar cells and other optoelectronic devices.

Modulation Effect Generated by A Cations in Hybrid A2BB'X6 Double Halogen Perovskite Materials.

Double perovskite A2BB'X6, including all-inorganic and hybrid organic-inorganic composition, show great potential applications. The role of A cations (organic molecules or inorganic ions) in the double perovskite is distinct from that in the standard perovskite. Therefore, we carried out systematic analyses of the geometric and electronic structures of Cs2AgBiBr6 and (MA)2AgBiBr6 (MA = CH3NH3) double perovskites. Cs2AgBiBr6 maintains the standard cubic double perovskite lattice. While MA molecules prefer to align in the [110] direction in (MA)2AgBiBr6 and give rise to obvious lattice distortion. The band gap of (MA)2AgBiBr6 is slightly less than that of Cs2AgBiBr6. Because of the spherical or quasi-spherical wave functions of the s/d orbitals, the lattice distortion and the transverse shift between Ag/Bi and Br induced by MA molecules do not change the composition of the band edges. But the complex bonding interactions between MA and the inorganic frameworks make the Ag-Br or Bi-Br bond lengths no longer identical values, so the bond strength and the energy level of each bonding state are dispersed and the band is expanded, which reduces the band gap of the hybrid organic-inorganic double perovskite (MA)2AgBiBr6. Making the role of A cations in the A2BB'X6 double perovskite clear, we could find an excellent double perovskite to put forward their applications.

Investigation of the Mn dopant-enhanced photoluminescence performance of lead-free Cs2AgInCl6 double perovskite crystals.

The lead-free double perovskite Cs2AgInCl6 is a potential candidate for LEDs, the photoluminescence performance of which is reinforced greatly by Mn doping. Here, we analyzed the geometric, electronic and photoluminescence properties of Mn-doped Cs2AgInCl6 by means of first-principle calculations. We found that in the interior of Cs2AgInCl6, the Mn dopant formed defect complexes by substituting an Ag atom and generating an Ag vacancy (MnAgVAg) owing to the charge balance and the weak distortion of the metal octahedra. The MnAgVAg defect introduced two defect bands in the forbidden gap, which was contributed predominantly by the 3d orbitals of the Mn2+ ions. The electron transition of the Mn2+ ions from the first excited state to the ground state, i.e., from 4T1 to 6A1 states, gives rise to the PL spectrum that is lower than the bandgap. Therefore, we show that the Mn dopant indeed reinforces the PL performance of Cs2AgInCl6 greatly and is beneficial for its use as an LED material.

DFT Simulations of the Vibrational Spectrum and Hydrogen Bonds of Ice XIV.

It is always a difficult task to assign the peaks recorded from a vibrational spectrum. Herein, we explored a new pathway of density functional theory (DFT) simulation to present three kinds of spectra of ice XIV that can be referenced as inelastic neutron scattering (INS), infrared (IR), and Raman experimental spectrum. The INS spectrum is proportional to the phonon density of states (PDOS) while the photon scattering signals reflect the normal vibration frequencies near the Brillouin zone (BZ) center. Based on good agreements with the experimental data, we identified the relative frequency and made scientific assignments through normal vibration modes analysis. The two hydrogen bond (H-bond) peaks among the ice phases from INS were discussed and the dynamic process of the H-bond vibrations was found to be classified into two basic modes. We deduced that two H-bond modes are a general rule among the ice family and more studies are ongoing to investigate this subject.

Optimization of exciton currents in photosynthetic systems.

In an approach analogous to that used to treat electronic currents in semiconductor quantum dots, we investigate the exciton current in a pigment network that is sandwiched between two exciton reservoirs, also known as the emitter and the acceptor. Employing the master equation for the reduced density matrix, the exciton current is obtained analytically for a two-site model, and numerically for an eight-site Fenna-Matthews-Olson (FMO) subunit model. It is found that, to maximize the exciton current with a specific network configuration, there exist optimal emitter temperatures and exciton transfer rates between the network and the reservoirs. The steady state current in the FMO model is consistent with the trapping time calculated by network optimization in the one-exciton picture. The current optimization with respect to various control parameters is discussed for the FMO model. At and below the biologically relevant transfer rate 1 ps(-1), the FMO network is more efficient for excitation energy transfer than the two-site model. Beyond this scale, the FMO network shows robustness with respect to the interplay with the reservoirs.

Validity of time-dependent trial states for the Holstein polaron.

Validity of three time-dependent trial states of the Holstein Hamiltonian, namely the D(2), Merrifield and D Ansätze has been examined in detail with regards to their deviations from the exact solution to the time-dependent Schrödinger equation. Linear absorption spectra are also calculated as an additional indicator of the Ansatz validity. It is found that the D Ansatz is the most accurate trial state of the three, and all Ansätze fail to provide an adequate description of system dynamics in the weak-coupling regime.