Rights: © 2013 American Institute of Physics. This is the accepted version of the following article: Oikkonen, L. E. & Ganchenkova, M. G. & Seitsonen, A. P. & Nieminen, Risto M. 2013. Mass transport in CuInSe2 from first principles. Journal of Applied Physics. Volume 113, Issue 13. 133510/1-5. ISSN 0021-8979 (printed). DOI: 10.1063/1.4799064, which has been published in final form at http://scitation.aip.org/content/aip/journal/jap/113/13/10.1063/1.4799064

Rights: © 2011 American Physical Society (APS). This is the accepted version of the following article: Vehviläinen, T. T. & Ganchenkova, M. G. & Oikkonen, L. E. & Nieminen, Risto M. 2011. Hydrogen interaction with fullerenes: From C[sub 20] to graphene. Physical Review B. Volume 84, Issue 8. 085447/1-7. ISSN 1550-235X (electronic). DOI: 10.1103/physrevb.84.085447, which has been published in final form at http://journals.aps.org/prb/abstract/10.1103/PhysRevB.84.085447.

The electronic properties of high-efficiency CuInSe2 (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 degrees C (V-Cu-related complexes) or 400 degrees 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.

The wide scatter in experimental results has not allowed drawing solid conclusions on self-diffusion in the chalcopyrite CuInSe2 (CIS). In this work, the defect-assisted mass transport mechanisms operating in CIS are clarified using first-principles calculations. We present how the stoichiometry of the material and temperature affect the dominant diffusion mechanisms. The most mobile species in CIS is shown to be copper, whose migration proceeds either via copper vacancies or interstitials. Both of these mass-mediating agents exist in the material abundantly and face rather low migration barriers (1.09 and 0.20 eV, respectively). Depending on chemical conditions, selenium mass transport relies either solely on selenium dumbbells, which diffuse with a barrier of 0.24 eV, or also on selenium vacancies whose diffusion is hindered by a migration barrier of 2.19 eV. Surprisingly, indium plays no role in long-range mass transport in CIS; instead, indium vacancies and interstitials participate in mechanisms that promote the formation of antisites on the cation sublattice. Our results help to understand how compositional inhomogeneities arise in CIS.

The presence of small amounts of sodium has been shown to improve the electronic performance of Cu(In,Ga)Se2 (CIGS) solar cells, but the origins of this effect have not yet been fully resolved. In this work, we have addressed the questions involving the role of sodium in CuInSe2 (CIS) using density-functional-theory-based calculations. We find no direct way how the creation of Na-related point defects in bulk CIS would enhance p-type conductivity. Instead, we demonstrate that Na reduces copper mass transport due to the capture of copper vacancies by NaCu defects. This finding provides an explanation for experimental measurements where the presence of Na has been observed to decrease copper diffusion. The suggested mechanism can also impede VCu-related cluster formation and lead to measurable effects on defect distribution within the material.

Density-functional-theory calculations have often been used to interpret experimental observations of defects in CuInSe2 (CIS). In this work, we bring back under scrutiny conclusions drawn from earlier calculations employing the (semi) local-density approximation. We present hybrid-functional results showing that copper- or indium-related defects such as V-Cu or In-Cu do not create charge transition levels within the band gap in CIS. Instead, deep levels in CIS can only arise from selenium-related defects, which act as recombination centers in this material.

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.

The paper presents a systematic study of the trends in the interaction of hydrogen with carbon fullerenes versus their curvature, where graphene is taken as the limit of zero curvature. The efficiency of hydrogen incapsulation in fullerenes, penetration into them, and adsorption on their surface are analyzed and discussed. The effects on magnetism are also considered; in particular, it is shown that hydrogen adsorption to some fullerenes induces magnetism to initially nonmagnetic systems. In addition, highly hydrogen-saturated fullerenes are examined and the suitability of fullerenes for hydrogen storage is discussed.

In this paper, using first-principles calculations, we demonstrate that vacancies and E-centers (AsV, SbV) in silicon can co-exist in several metastable configurations with notably different relaxation patterns, which have very similar formation energies. Thus these vacancy-type defects can be considered as multi-symmetry defects in the sense that, at elevated temperatures, the probabilities to find vacancies in different stable configurations are comparable. From an experimental point of view, the co-existence of various symmetries can complicate the identification of the defect.