Browsing by Author "Oikkonen, L. E."

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  • Effect of sodium incorporation into CuInSe2 from first principles
    (2013) Oikkonen, L. E.; Ganchenkova, M. G.; Seitsonen, A. P.; Nieminen, Risto M.
     School of Science | A1 Alkuperäisartikkeli tieteellisessä aikakauslehdessä
    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.
  • Hydrogen interaction with fullerenes: From C[sub 20] to graphene
    (2011) Vehviläinen, T. T.; Ganchenkova, M. G.; Oikkonen, L. E.; Nieminen, Risto M.
     School of Science | A1 Alkuperäisartikkeli tieteellisessä aikakauslehdessä
    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.
  • Mass transport in CuInSe2 from first principles
    (2013) Oikkonen, L. E.; Ganchenkova, M. G.; Seitsonen, A. P.; Nieminen, Risto M.
     School of Science | A1 Alkuperäisartikkeli tieteellisessä aikakauslehdessä
    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.
  • Redirecting focus in CuInSe2 research towards selenium-related defects
    (2012) Oikkonen, L. E.; Ganchenkova, M. G.; Seitsonen, A. P.; Nieminen, Risto M.
     School of Science | A1 Alkuperäisartikkeli tieteellisessä aikakauslehdessä
    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 VCu or InCu 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.