In the search for better thermoelectric materials, metal phosphides have not been considered to be viable candidates so far, due to their large lattice thermal conductivity. Here we study thermoelectric...

Here, we report a first-principles study of lattice vibrations and thermal transport in Cs2SnI6, the archetypal compound in the family of vacancy-ordered double perovskites. We show that twofold rattlers of...

Perovskite solar cells have continued to fascinate over the past decade due to fast increasing power conversion efficiency and very low fabrication cost but still suffered from poor stability. Interface engineering is evolved to be one of the most promising solutions to the instability problem. In this work, we perform a first-principles study on the MAPbI 3 /CsPbI 3 interface system, aiming at clarifying the underlying mechanism of interfacial enhancement of solar cell performance. We devise the atomistic modeling of superlattices as increasing the number of included unit cells and carry out structural optimizations, revealing that the binding strength between the perovskite layers becomes stronger while the band gap decreases as the supercell size increases. Using enough large supercells of the interface system, we further estimate the formation energies of the interfacial vacancy defects and activation barriers for vacancy-mediated I atom migrations. Our calculations show the shallow transition states for most of the defects and the higher activation barriers for I atom migrations across the interface, providing an evidence of performance enhancement by the interface formation. By giving an insightful understanding of the MAPbI 3 /CsPbI 3 heterojunction, this work definitely contributes to the design of interface systems for high-performance solar cells.

All-inorganic halide perovskites have attracted a great interest as a promising light harvester of perovskite solar cells due to their enhanced chemical stability. In this work we investigate the material properties of solid solutions CsPb(I 1−x Br x ) 3 in cubic phase by applying the virtual crystal approximation approach within a density functional theory framework. First we check the validity of constructed pseudopotentials of the virtual atoms (X = I 1−x Br x ) by verifying that the lattice constants follow the linear function of mixing ratio. We then suggest an idea of using the hybrid HSE functional with linear increasing value of exact exchange term as increasing the Br content x, which produces the band gaps of CsPbX 3 in good agreement with the available experimental data. The calculated light absorption coefficients and reflectivity show the systematic varying tendency to the Br content. We calculate the phonon dispersions of CsPbX 3 , CsX and PbX 2 as slightly changing their volumes, revealing the phase instability of CsPbX 3 and calculating the thermodynamic potential function differences. By projecting Gibbs free energy differences onto the plane of ∆G = 0, we determine the P − T diagram for CsPbX 3 to be stable against the chemical decomposition, highlighting that the area of being stable extends gradually as the Br content increases.

Searching thermoelectric materials with high performance and low cost is now receiving special attention and great challenges in the field of material design. In this work, we perform first-principles lattice dynamics combined with temperature-induced anharmonic phonon renormalization and connected to the Boltzmann transport equation to predict thermoelectric performance in the cubic inorganic iodide perovskites CsBI3 (B = Pb, Sn, and Ge) at a high temperature of 700 K. Under stabilization of the cubic phase that exhibits strong anharmonic phonon modes at 0 K, our calculations show that at T = 700 K, these perovskites have ultralow lattice thermal conductivities below 0.6 W m–1 K–1 and high thermopower factors over 1.5 mW m–1 K–2, being comparable or superior to those of GeTe. Moreover, we find that cubic CsGeI3 and CsSnI3 have higher thermoelectric figure of merit ZT over 0.95 upon n-type doping, being attributed to the strong lattice anharmonicity and flat-dispersive electronic bands with high degeneracy.

Molybdenum disulfide (MoS2) attracts attention as a highly efficient and low-cost photocatalyst for hydrogen production but suffers from low conductance and high recombination rate of photogenerated charge carriers. In this work, we investigate the MoS2 heterostructures with graphene variants (GVs), including graphene, graphene oxide, and their boron- and nitrogen-doped variants, by first-principles calculations. A systematic comparison between graphene and graphene oxide composites is performed, and the contrary effect of B and N doping on interface function and hydrogen evolution is clarified. We find that upon the formation of the interfaces, some amount of electronic charge transfers from the GV side to the MoS2 layer, inducing the creation of an interface dipole and the reduction of work function, which is more pronounced in the graphene oxide composites. Moreover, our results reveal that N doping enhances the interface functions by forming donor-type interface states, whereas B doping reduces those functions by forming acceptor-type interface states. However, the B-doped systems exhibit a lower Gibbs free energy difference for hydrogen adsorption on the GV side than the N-doped systems, which deserves much consideration in the design of new functional photocatalysts.

Developing highly efficient photocatalysts for the hydrogen evolution reaction (HER) by solar-driven water splitting is a great challenge. Here, we study the atomistic origin of interface properties and the HER performance of all-inorganic iodide perovskite β-CsPbI 3 / 2H-MoS 2 heterostructures with interfacial vacancy defects using first-principles calculations. Both CsI/MoS 2 and PbI 2 /MoS 2 heterostructures have strong binding and dipole moment, which are enhanced by interfacial iodine vacancies (V I ). Because of the nature of type II heterojunctions, photogenerated electrons on the CsPbI 3 side are promptly transferred to the MoS 2 side where HER occurs, and sulfur vacancies (V S ) spoil this process, acting as surface traps. The formation energies of various defects are calculated by applying atomistic thermodynamics, identifying the growth conditions for promoting V I and suppressing V S formation. The HER performance is enhanced by forming interfaces with lower ΔG H values for hydrogen adsorption on the MoS 2 side, suggesting PbI 2 /MoS 2 with V I to be the most promising photocatalyst.

We propose lead-free potassium iodide perovskite solid solutions KBI3 with B-site mixing between Ge/Sn and Mg as potential candidates for photocatalysts based on systematic first-principles calculations.