Formation, migration, and clustering of point defects in CuInSe2 from first principles.

The electronic properties of high-efficiency CuInSe(2) (CIS)-based solar cells are affected by the microstructural features of the absorber layer, such as point defect types and their distribution. Recently, there has been controversy over whether some of the typical point defects in CIS--V(Cu), V(Se), In(Cu), Cu(In)--can form stable complexes in the material. In this work, we demonstrate that the presence of defect complexes during device operational time can be justified by taking into account the thermodynamic and kinetic driving forces acting behind defect microstructure formation. Our conclusions are backed up by thorough state-of-the-art calculations of defect interaction potentials as well as the activation barriers surrounding the complexes. Defect complexes such as In(Cu)-2V(Cu), In(Cu)-Cu(In), and V(Se)-V(Cu) are shown to be stable against thermal dissociation at device operating temperatures, but can anneal out within tens of minutes at temperatures higher than 150-200 °C (V(Cu)-related complexes) or 400 °C (antisite pair). Our results suggest that the presence of these complexes can be controlled via growth temperatures, which provides a mechanism for tuning the electronic activity of defects and the device altogether.

Vacancies in CuInSe2: new insights from hybrid-functional calculations.

We calculate the energetics of vacancies in CuInSe(2) using a hybrid functional (HSE06, HSE standing for Heyd, Scuseria and Ernzerhof), which gives a better description of the band gap compared to (semi)local exchange-correlation functionals. We show that, contrary to present beliefs, copper and indium vacancies induce no defect levels within the band gap and therefore cannot account for any experimentally observed levels. The selenium vacancy is responsible for only one level, namely, a deep acceptor level ε(0/2-). We find strong preference for V(Cu) and V(Se) over V(In) under practically all chemical conditions.

Multiscale study of hydrogen diffusion and clustering on carbon nanotube.

In this paper we report the results of a multiscale study of hydrogen clusterization at the surface of (10,0) carbon nanotube. For this purpose, a systematic study of the binding energies and migration barriers of hydrogen adatom and various close adatom pairs of has been undertaken using density-functional theory approach. The interaction between hydrogen atoms on the surface of nanotube is shown to be long ranged and anisotropic. On applying the obtained potential energy surfaces for lattice kinetic Monte Carlo simulations of chemisorbed hydrogen annealihg, a noticeable influence of the annealing conditions on cluster sizes, shapes and relative populations has bean revealed, which opens a possibility for the control of hydrogen clusterization kinetics. The effect on carbon nanotube electronic structure from hydrogen dimers and trimers most frequently met in lattice kinetic Monte Carlo simulations is discussed.

C20 based polymers: electronic and elastic properties and stability studies.

In this paper we propose several new C20 polymer structures, whose properties were studied using density-functional theory based calculations. New structures predicted in this work are shown to be energetically more favourable and stable than previously found structures in computational studies. The new carbon structure which we have named as a quasi-graphite phase with fcc type structures was found to be the most energetically favourable polymer structures. Stability of the polymers was studied using constant temperature and constant pressure techniques. All the predicted structures demonstrate high stability with respect to high temperatures and external loads. The elastic and electronic properties of the proposed structures are discussed.

Vacancies in wurtzite GaN and AlN.

Vacancies in wurtzite GaN and AlN are studied using a computational method which is based on the density functional theory (DFT) and takes into account the errors arising from use of finite-sized supercells and the DFT band gap underestimation. Negatively charged N vacancies in GaN and AlN are found to be stable, with formation energies similar to and higher than those of Ga and Al vacancies in n-type material under Ga- and Al-rich growth conditions, respectively. The localization and energies of the defect levels close to the computational conduction band edge are considered in detail.

Nitrogen vacancies as major point defects in gallium nitride.

We present results of ab initio calculations for vacancies and divacancies in GaN. Particular attention is paid to nitrogen vacancies and mixed Ga-N divacancies in negatively charged states, which in n-type GaN are found to be energetically comparable with gallium vacancies. We also demonstrate that the activation energy for self-diffusion over the nitrogen sublattice is lower than over the gallium one for all Fermi-level positions, which implies the nitrogen vacancies are major defects in samples annealed at high temperatures. Possibilities for direct observations of nitrogen vacancies through positron annihilation experiments are discussed.