Joongoo Kang

Halide perovskite materials for solar cells: a theoretical review

Halide perovskites have recently emerged as promising materials for low-cost, high-efficiency solar cells. The efficiency of perovskite-based solar cells has increased rapidly, from 3.8% in 2009 to 19.3% in 2014, by using the all-solid-state thin-film architecture and engineering cell structures with mixed-halide perovskites. The emergence of perovskite solar cells revolutionized the field not only because of their rapidly increased efficiency, but also flexibility in material growth and architecture. The superior performance of the perovskite solar cells suggested that perovskite materials possess intrinsically unique properties. In this review, we summarize recent theoretical investigations into the structural, electrical, and optical properties of halide perovskite materials in relation to their applications in solar cells. We also discuss some current challenges of using perovskites in solar cells, along with possible theoretical solutions.

H-related defect complexes in HfO2: A model for positive fixed charge defects

Based on first-principles theoretical calculations, we investigate the hydrogenation effect on the defect properties of oxygen vacancies (VO) in HfO2. A defect complex of VO and H behaves as a shallow donor for a wide range of Fermi levels, with a positive charge state, and this complex is energetically stable against its dissociation into VO and H. We suggest that the VO–H complex is responsible for the formation of positive fixed charges, which neutralize negative fixed charges during the postannealing process of SiOx/HfO2 stack.

Interlayer coupling and optoelectronic properties of ultrathin two-dimensional heterostructures based on graphene, MoS2 and WS2

Unique optoelectronic properties and interlayer coupling are observed in the artificial two-dimensional (2D) heterostructures based on graphene, MoS2 and WS2 monolayers. In the graphene–WS2 heterostructures, substantial photoluminescence (PL) quenching and significant stiffening phonon modes emerge due to strong interlayer coupling. Such hybrid systems also exhibit gate-tunable current rectification behavior with a maximum rectification ratio of 103. In addition, the ambipolar properties originating from their constituents and enhanced photo-switching properties with a maximum on/off ratio of 103 were also observed. The MoS2–WS2 heterostructures exhibit light emission quenching of WS2 while unchanged emission of MoS2. Such a phenomenon is due to the weak interlayer coupling and inefficient charge transfer process. The enhanced optoelectronic performances suggest that the ultrathin 2D heterostructures have great potential in the future architectural design of novel optoelectronic devices.

Microstructure, optical property, and electronic band structure of cuprous oxide thin films

Cuprous oxide (Cu2O) thin films were grown via radio frequency sputtering deposition at various temperatures. The dielectric functions and luminescence properties of the Cu2O thin films were measured using spectroscopic ellipsometry and photoluminescence, respectively. High-energy peaks were observed in the photoluminescence spectra. Several critical points (CPs) were found using second derivative spectra of the dielectric functions and the standard critical point model. The electronic band structure and the dielectric functions were calculated using density functional theory, and the CP energies were estimated to compare with the experimental data. We identified the high-energy photoluminescence peaks to quasi-direct transitions which arose from the granular structures of the Cu2O thin films.

Origin of the diverse melting behaviors of intermediate-size nanoclusters: theoretical study of AlN (N = 51-58, 64).

Microscopic understanding of thermal behaviors of metal nanoparticles is important for nanoscale catalysis and thermal energy storage applications. However, it is a challenge to obtain a structural interpretation at the atomic level from measured thermodynamic quantities such as heat capacity. Using first-principles molecular dynamics simulations, we reproduce the size-sensitive heat capacities of Al(N) clusters with N around 55, which exhibit several distinctive shapes associated with diverse melting behaviors of the clusters. We reveal a clear correlation of the diverse melting behaviors with cluster core symmetries. For the Al(N) clusters with N = 51-58 and 64, we identify several competing structures with widely different degree of symmetry. The conceptual link between the degree of symmetry (e.g., T(d), D(2d), and C(s)) and solidity of atomic clusters is quantitatively demonstrated through the analysis of the configuration entropy. The size-dependent, diverse melting behaviors of Al clusters originate from the reduced symmetry (T(d) → D(2d) → C(s)) with increasing the cluster size. In particular, the sudden drop of the melting temperature and appearance of the dip at N = 56 are due to the T(d)-to-D(2d) symmetry change, triggered by the surface saturation of the tetrahedral Al(55) with the T(d) symmetry.

Shape Control of Al Nanoclusters by Ligand Size

It is a challenge to synthesize clusters having a certain shape associated with a desirable property. In this study, we perform density functional calculations on ligand-protected Al(7) and Al(77) clusters. It is found that small ligands such as NH(2) still prefer the compact structure of bare Al clusters. However, large ligands such as N(SiMe(3))(2) stabilize the experimentally observed shell-like structures due to the steric effect. This is different from the Ga(84) cluster case where small ligands can stabilize the experimental shell-like Ga(84) cluster. Our study suggests that the shape, and thus the properties, of clusters (for instance, C(3v) Al(7) cluster has a finite dipole moment in contrast to the centrosymmetric D(3d) cluster) can be controlled by using ligands with different sizes.

A Unified Understanding of the Thickness-Dependent Bandgap Transition in Hexagonal Two-Dimensional Semiconductors

Many important layered semiconductors, such as hexagonal boron nitride (hBN) and transition-metal dichalcogenides (TMDs), are derived from a hexagonal lattice. A single layer of such hexagonal semiconductors generally has a direct bandgap at the high-symmetry point K, whereas it becomes an indirect, optically inactive semiconductor as the number of layers increases to two or more. Here, taking hBN and MoS2 as examples, we reveal the microscopic origin of the direct-to-indirect bandgap transition of hexagonal layered materials. Our symmetry analysis and first-principles calculations show that the bandgap transition arises from the lack of the interlayer orbital couplings for the band-edge states at K, which are inherently weak because of the crystal symmetries of hexagonal layered materials. Therefore, it is necessary to judiciously break the underlying crystal symmetries to design more optically active, multilayered semiconductors from hBN or TMDs.

Enhanced dihydrogen adsorption in symmetry-lowered metal–porphyrin-containing frameworks

Porphyrin is a very important component of natural and artificial catalysis and oxygen delivery in blood. Here, we report that, based on first-principles density-functional calculations, a hydrogen molecule can be adsorbed non-dissociatively onto Ti-, V-, and Fe-porphyrins, similar to oxygen adsorption in heme-containing proteins, with a significant energy gain, greater than 0.3 eV per H(2). The dihydrogen-heme complex will be non-magnetic, as is oxyhemoglobin. In contrast to the backward electron donation of Fe(III)-O(2)(-) in oxyhemoglobin, the dihydrogen binding originates from electron donation from H(2) to the Fe(II). We have identified that the local symmetry of the transition metal center of porphyrins uniquely determines the binding strength, and, thus, one can even manipulate the strength by intentionally and systematically breaking symmetry.

P-type doping of lithium peroxide with carbon sheets

The interaction of lithium peroxide (Li2O2) with carbon electrodes in Li-air batteries is studied with model systems of graphene-intercalated Li2O2, using density functional theory (DFT) methods. Although both the Li2O2 bulk and its stoichiometric surface structures (without single O atoms) are insulating, the incorporation of graphene sheets into the Li2O2 introduces hole states in the oxygen orbitals due to the electron transfer from the anti-bonding O2 orbitals to the graphene sheets. This indicates that carbon sheets not only provide conducting channels by themselves, but they also open new channels in Li2O2.

Microscopic Theory of Hysteretic Hydrogen Adsorption in Nanoporous Materials

Understanding gas adsorption confined in nanoscale pores is a fundamental issue with broad applications in catalysis and gas storage. Recently, hysteretic H(2) adsorption was observed in several nanoporous metal-organic frameworks (MOFs). Here, using first-principles calculations and simulated adsorption/desorption isotherms, we present a microscopic theory of the enhanced adsorption hysteresis of H(2) molecules using the MOF Co(1,4-benzenedipyrazolate) [Co(BDP)] as a model system. Using activated H(2) diffusion along the small-pore channels as a dominant equilibration process, we demonstrate that the system shows hysteretic H(2) adsorption under changes of external pressure. For a small increase of temperature, the pressure width of the hysteresis, as well as the adsorption/desorption pressure, dramatically increases. The sensitivity of gas adsorption to temperature changes is explained by the simple thermodynamics of the gas reservoir. Detailed analysis of transient adsorption dynamics reveals that the hysteretic H(2) adsorption is an intrinsic adsorption characteristic in the diffusion-controlled small-pore systems.