Application of first principles in materials science research

(1) Electronic structure and carrier transport properties

The structural and electronic properties are investigated to investigate photoelectric and thermoelectric properties, using the plane-wave projector-augmented wave method as implemented in the Vienna ab initio simulation (VASP) package. In the research of excited states of photoelectric materials, our group ensure the accuracy and computational efficiency by choosing the property exchange-correlation potentials, such as MBJ+U, HSE and G₀W₀. In the research of electronic transport of thermoelectric materials, our group develop two packages, BoltzTrap_VASP and Transoptic, based on Boltzmann Transport Theory. The original package, Transoptic, effectively solve the issue of “band crossing” in electrical transport calculations based on transition matrix elements, which is the only one solution for the large-scale electrical transport calculations using VASP package.

(2) Lattce dynamics and thermal transport

Structure is relaxed using the VASP code, which is based on the DFT. Vibrational properties and polarization information are calculated using the supercell force constant method by PHONON or Phonopy software packages. Lattice thermal conductivity is estimated by the modified Debye-Callaway model. 

Design and optimization of new type thermoelectric materials

(1) Thermoelectric performance designing and optimization in Cu2SnX3(X=S, Se)compounds

The investigation on existence of the three-dimensional (3D) hole conductive network in ternary diamond-like Cu₂SnX₃ (X=Se, S) semiconductors, and identify the features of the electronic structure responsible for good TE performance. We also provide results as a function of doping level to find the regime where the highest performance will be realized and estimate the maximum figure of merit, ZT. The highest ZT reaches 1.14 at 850 K for the sample Cu₂Sn_0.90In_0.10Se₃. The high ZT value deserves further investigation on these new systems with “Cu-Se conductive network” concept.

 (2) Design of high-performance pseudocubic thermoelectric materials

A new strategy to search for and design high-performance non-cubic TE materials is developed through the utilization of a rational pseudocubic structure that supports cubic-like degenerate electronic bands, via the coexistence of long-range cubic framework with localized short-range non-cubic lattice distortions. We identify a simple yet powerful selection rule based on maintaining the distortion parameter η near unity that is shown to be equivalent to minimizing the energy-splitting parameter ΔCF . With the computedη(ΔCF) vs. lattice parameter maps, one can easily establish the correct molar ratio for two or more chalcopyrite compounds to form a solid solution with the desirable η ≈ 1 value that assures excellent TE performance. The pseudocubic approach augmented by the unity-η rule and the   η-a map, is a new paradigm that points a clear direction how to design high-performing tetragonal chalcopyrite-based TE materials. Using this approach, we predict a series of novel highly efficient chalcopyrite TE materials and, on a selected subset of them, we verify experimentally that they indeed possess significantly enhanced zT values. The approach can be extended to other tetragonal semiconductors as well as other non-cubic materials with the aim to realize cubic-like degenerate electronic bands that support high TE performance. This work thus addresses the interest of researchers to broaden the scope of prospective TE materials especially among noncubic semiconductors

(3) Thermoelectric properties in Rashba spin-split systems

The Rashba effect is interesting for thermoelectrics because of the spin-splitting band structure. The dimensionality in density of states is reduced due to the unique Fermi topology. By using bulk BiTeI as an example, we theoretically and experimentally demonstrate that the quantum wells and bulk materials with the Rashba effect have a one- and two-dimensional-like thermopower, respectively. The thermopower is much higher as compared with that in spin-degenerate systems, because of the lower Fermi level at given carrier concentrations. A simple but direct relationship between the thermopower and the Rashba parameter is established for quantum wells and bulk materials. Meanwhile, the internal electric field in the Rashba system may lead to the lattice anharmonicity and low lattice thermal conductivity. We suggest that quantum wells and bulk materials with large Rashba effect may become potential candidates for high-performance thermoelectric applications. 

Design and optimization of new type photoelectric materials

(1) Photoelectrical properties of semiconductors

CIGS and CZTS are two types of high-efficiency thin film materials for solar cells. The exploration and optimization on new efficiency, non-toxic and low-cost materials is still the significant task in fundamental research. To overcome the difficulties of theoretical calculations in narrow-bandgap Cu-based semiconductors, our group develop the mBJ+U method, which is applied to calculate complex photoelectric properties, such as bandgaps, optical absorption, band offset. Our study demonstrated that Cu₂SiSe₃ and Cu₂ZnSiTe₄ have great potential for applications.

 (2) Hybrid organic-inorganic perovskite CH₃NH₃PbI₃

Hybrid organic-inorganic perovskite CH₃NH₃PbI₃ improves the best record efficiency for dye-sensitized solar cell to 20.1%. Based on deformation potential theory, we calculated the electronic structure of cubic, tetragonal and orthorhombic phases to study the effects of bonding and anti-bonding states on carrier transport. We found that the anti-bonding states lead to small effective mass of carriers and bonding states can reduce the coupling strength of the carrier to acoustic phonon. 

Lithium battery

（1） The designing and computation of catalysts in the cathode for charging reaction in a lithium-air battery

The performance of lithium-air batteries is limited by such as high charging voltage, poor rate capacity in spite of the ultrahigh theoretical energy density. The discharging product is Li₂O₂ which is hard to decompose due to the poor electric conductivity. Meanwhile its side reactions with the materials such as carbon-based cathode and electrolyte. Apart from the electric conductivity and side reactions, the intrinsic decomposition of Li₂O₂ is a key factor that limit the rate of the reaction, too. The barrier rate determinate step of decomposition of Li₂O₂ can be decreased by 0.4 eV with B-doped grapheme as the catalyst in the cathode; Co-doping catalyst with B and P not only reduces the barrier of the rate determinate by 0.70 eV, but also decreases the equilibrium voltage by 0.13 eV. In the active doped grapheme systems, the adsorption of Li₂O₂ can be regulated by the electro-negativity and base.

Meanwhile, we also did the research on transition metal oxides such as Co₃O₄. By the calculation of reaction pathways, the (111) O-rich surface exhibits high reactivity, the charging voltage can be decreased by 0.54 V and O₂ desorption barrier can be decreased by 0.45 eV. The catalytic effect arises from the electron deficiency oxygen-rich layer that can adsorb electrons from Li₂O₂.

（2） Structural and Li⁺ Migration Properties of Li₁₀GeP₂S₁₂

Through first-principles calculation, the metastable structures of Li₁₀GeP₂S₁₂ was investigated with tetrahedral Li⁺ considered. The mixing of octahedral and tetrahedral Li⁺ is advantageous for the stablization of Li₁₀GeP₂S_12. The tetrahedral Li⁺ can take part in the Li⁺ migration which makes it isotropic with similar barrier (about 0.20~0.30 eV) for c and ab directions. However, the Li⁺ migration modes are different with synergetic and hopping modes for c and ab directions, respectively. Besides, thermodynamics calculation determined three voltage plateaus for x at 14, 10, and 6 during charge/discharge of Li_xGeP₂S₁₂.