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  • 151. Tomas, S. A.
    et al.
    Stolik, S.
    Palomino, R.
    Lozada, R.
    Persson, Clas
    KTH, Skolan för industriell teknik och management (ITM), Materialvetenskap, Tillämpad materialfysik.
    Ahuja, R.
    Pepe, I.
    da Siva, A. F.
    Optical properties of rhodamine 6G-doped TiO2 sol-gel films2005Ingår i: Journal de Physique IV: Colloque, ISSN 1155-4339, E-ISSN 1764-7177, Vol. 125, s. 415-417Artikel i tidskrift (Refereegranskat)
    Abstract [en]

    The optical properties of titania (TiO2) thin films prepared by the sol-gel process and doped with rhodamine 6G were studied by Photoacoustic Spectroscopy. Rhodamine 6G-doping was achieved by adding 0.01%, 0.02%, 0.05% y 0.1% mol rhodamine to a solution that contained titanium isopropoxide as precursor. Two absorption regions were distinguished in the absorption spectrum of a typical rhodamine 6G-doped TiO2 film. A shift of these bands occured as a function of rhodamine 6G-doping concentration. In addition, the optical absorption and band gap energy for rutile-phase TiO2 films were calculated employing the full-potential linearized augmented plane wave method. A comparison of these calculations with experimental data of TiO2 films prepared by sol-gel at room temperature was performed.

  • 152. Tomas, S. A.
    et al.
    Stolik, S.
    Palomino, R.
    Lozada, R.
    Persson, Clas
    KTH, Skolan för industriell teknik och management (ITM), Materialvetenskap, Tillämpad materialfysik.
    Pepe, I.
    da Silva, A. F.
    Influence of rhodamine 6G doping on the optical properties of TiO2 sol-gel films2005Ingår i: Journal of Applied Physics, ISSN 0021-8979, E-ISSN 1089-7550, Vol. 98, nr 7Artikel i tidskrift (Refereegranskat)
    Abstract [en]

    Amorphous titanium dioxide (TiO2) thin films doped with rhodamine 6G (R6G) were deposited on glass substrates by the sol-gel process. The optical properties of the films were characterized by photoacoustic, excitation, and fluorescence spectroscopies. The absorption spectra of the R6G-doped TiO2 films exhibited two well-defined absorption regions: an absorption band over 2.0 eV attributed to rhodamine 6G and a band above 3.0 eV corresponding to TiO2 absorption. While the onset of the R6G absorption band was shifted by 0.06 eV towards lower energies as the R6G doping concentration increased within the interval of 0.01-0.10 mol %, the onset to high absorption (TiO2 band) for the doped films decreased only by 0.01 eV within the same interval. In addition, the optical absorption of undoped rutile-phase bulk TiO2 was calculated and compared to the experimental results. The estimated theoretical value of rutile TiO2 sample was 3.0 eV. This theoretical result shows good agreement when compared with the experimental data of undoped TiO2 sol-gel films, as well as the undoped TiO2 film prepared by sputtering.

  • 153. van Teeffelen, S.
    et al.
    Persson, Clas
    KTH, Tidigare Institutioner                               , Materialvetenskap.
    Eriksson, O.
    Johansson, Börje
    KTH, Tidigare Institutioner                               , Materialvetenskap.
    Doping-induced bandgap narrowing in Si rich n- and p-type Si1-xGex2003Ingår i: Journal of Physics: Condensed Matter, ISSN 0953-8984, E-ISSN 1361-648X, Vol. 15, nr 3, s. 489-502Artikel i tidskrift (Refereegranskat)
    Abstract [en]

    The shifts of the fundamental and optical bandgap energies as functions of dopant concentration in heavily n-type and p-type doped Si1-xGex (x less than or equal to 0.3) have been investigated theoretically. The band structure of the intrinsic crystal was described by the k . p-perturbation method, where the Kohn-Luttinger parameters were determined from a first-principles and full-potential band-structure calculation. The doping-induced effects on the bandgap were thereafter calculated using a zero-temperature Green function formalism within the random phase approximation and with a local field correction of Hubbard. We found only small effects on the bandgap energies due to variation of composition x. The calculated bandgap narrowing of Si and of Si0.82Ge0.18 were found to be in good agreement with photoluminescence measurements.

  • 154. Zamulko, S.
    et al.
    Chen, Rongzhen
    KTH, Skolan för industriell teknik och management (ITM), Materialvetenskap. University of Oslo, Norway.
    Persson, Clas
    KTH, Skolan för industriell teknik och management (ITM), Materialvetenskap. University of Oslo, Norway.
    Investigation of the structural, optical and electronic properties of Cu2Zn(Sn,Si/Ge)(S/Se)4 alloys for solar cell applications2017Ingår i: Physica status solidi. B, Basic research, ISSN 0370-1972, E-ISSN 1521-3951, Vol. 254, nr 6, artikel-id e201700084Artikel i tidskrift (Refereegranskat)
    Abstract [en]

    The crystalline structural, electronic and optical properties of the alloys Cu2ZnSn1−xGexS4, Cu2ZnSn1−xSixS4, Cu2ZnSn1−xGexSe4 and Cu2ZnSn1−xSixSe4 are calculated by first-principles using both the generalized gradient approximation and a hybrid functional approach. We find that the electronic band structures are qualitatively very similar for these alloys. The band-gap energy Eg(x) (for x = 0, 0.125, 0.25, 0.5, 0.75, 0.875 and 1) increases almost linearly with Ge and Si substitution. However, for very Si rich Cu2ZnSn1−xSixS4 alloys (but not for Cu2ZnSn1−xSixSe4) there is an abrupt increase of Eg(x) for x > 0.96. We therefore analyse this effect by calculating the electronic structures for x = 0.93, 0.96 and 1. We find that the Sn-like states form localised density-of-states below the conduction band edge in Cu2ZnSn1−xSixS4, while corresponding states resonate more with the conduction bands in Cu2ZnSn1−xSixSe4. The effect in S-based alloys is a direct consequence of the energetically high conduction band edge for Cu2ZnSiS4 in combination with energetically low Sn-like states. Furthermore, the calculated dielectric constants are relatively similar for all alloy configurations. Overall however, our results suggest that it is possible to use Si and Ge as alloying element in quaternary Cu2ZnSnS4 to improve the photovoltaic properties.

  • 155.
    Zamulko, Sergiy
    et al.
    Univ Oslo, Ctr Mat Sci & Nanotechnol, POB 1048 Blindern, NO-0316 Oslo, Norway..
    Berland, Kristian
    Univ Oslo, Ctr Mat Sci & Nanotechnol, POB 1048 Blindern, NO-0316 Oslo, Norway..
    Persson, Clas
    KTH, Skolan för industriell teknik och management (ITM), Materialvetenskap. University of Oslo, P. O. Box 1048 Blindern, Oslo, NO-0316, Norway.
    Optical Properties of Cu2ZnSn(SxSe1-x)(4) by First-Principles Calculations2018Ingår i: Physica Status Solidi (a) applications and materials science, ISSN 1862-6300, E-ISSN 1862-6319, Vol. 215, nr 17, artikel-id 1700945Artikel i tidskrift (Refereegranskat)
    Abstract [en]

    Structural, electronic, and optical properties of Cu2ZnSn(SxSe1-x)(4) semiconductors are studied theoretically for different concentration of S and Se anions. The optical properties are calculated at three levels of theory, in the generalized gradient approximation (GGA), meta-GGA, and with a hybrid functional. The GGA and meta-GGA calculations are corrected with an on-site Coulomb U-d term. Lattice constants, dielectric constants, and band-gaps are found to vary almost linearly with the concentration of S. The authors also show that a dense sampling of the Brillouin zone is required to accurately account for the shape of the dielectric function, which is hard to attain with hybrid functionals. This issue is resolved with a recently developed kp based interpolation scheme, which allows us to compare results of the hybrid functional calculations on an equal footing with the GGA and meta-GGA results. We find that the hybrid functionals provide the overall best agreement with the experimental dielectric function.

  • 156.
    Zhao, Hanyue
    et al.
    KTH, Skolan för industriell teknik och management (ITM), Materialvetenskap.
    Kumar, Mukesh
    KTH, Skolan för industriell teknik och management (ITM), Materialvetenskap.
    Persson, Clas
    KTH, Skolan för industriell teknik och management (ITM), Materialvetenskap, Tillämpad materialfysik.
    Density functional theory study of ordered defect Cu-(In,Ga)-Se compounds2012Ingår i: Physica Status Solidi. C, Current topics in solid state physics, ISSN 1610-1634, E-ISSN 1610-1642, Vol. 9, nr 7, s. 1600-1603Artikel i tidskrift (Refereegranskat)
    Abstract [en]

    CuIn1-xGaxSe2 is used as absorber material in thin-film photovoltaics. Presence of defect arrays of 2V(Cu)(-) + X-Cu(2+) complexes (X = In or Ga) in the material results in the formation of ordered defect phases, like for instance Cu(In1-xGax)(3)Se-5 and Cu(In1-xGax)(5)Se-8. An understanding of these defects phases is of considerably importance since they can strongly affect the performance of the solar cell. Density functional theory calculation with a hybrid functional is carried out to study the electronic structures and optical properties of these ordered defect phases. Fundamental properties, such as the band bowing, the band alignment and the absorption coefficients, are presented, and their dependence on material composition is discussed.

  • 157.
    Zhao, Hanyue
    et al.
    KTH, Skolan för industriell teknik och management (ITM), Materialvetenskap.
    Persson, Clas
    KTH, Skolan för industriell teknik och management (ITM), Materialvetenskap, Tillämpad materialfysik.
    Optical properties of Cu(In,Ga)Se(2) and Cu(2)ZnSn(S,Se)(4)2011Ingår i: Thin Solid Films, ISSN 0040-6090, E-ISSN 1879-2731, Vol. 519, nr 21, s. 7508-7512Artikel i tidskrift (Refereegranskat)
    Abstract [en]

    The optical properties of CuInSe(2), CuGaSe(2), Cu(2)ZnSnS(4), and Cu(2)ZnSnSe(4) are investigated using three different first-principles methods, namely the generalized gradient approximation by Perdew. Burke, and Ernzerhof (PBE), the hybrid Hartree-Fock-like functional by Heyd. Scuseria, and Emzerhof (HSE), and a Green's function approach (GW). The density-of-states, the complex dielectric function epsilon(omega) = epsilon(1)(omega) + i epsilon(2)(omega), and the optical absorption coefficient alpha(omega) are determined, providing fundamental understanding of these materials. We find that even though the PBE method generates fairly accurate effective crystal potentials, the HSE and GW methods improve considerably the band-gap energies E(g) and also the localization of the semicore states, thereby describing the optical properties much better. Furthermore, we also present optimized convergence parameters for the self-consistent HSE calculation in order to reduce the computational time of this orbital-dependent method.

  • 158. Zhao, Y. J.
    et al.
    Persson, Clas
    Lany, S.
    Zunger, A.
    Why can CuInSe2 be readily equilibrium-doped n-type but the wider-gap CuGaSe2 cannot?2004Ingår i: Applied Physics Letters, ISSN 0003-6951, E-ISSN 1077-3118, Vol. 85, nr 24, s. 5860-5862Artikel i tidskrift (Refereegranskat)
    Abstract [en]

    The wider-gap members of a semiconductor series such as diamond-->Si-->Ge or AlN-->GaN-->InN often cannot be doped n-type at equilibrium. We study theoretically if this is the case in the chalcopyrite family CuGaSe2-->CuInSe2, finding that: (i) Bulk CuInSe2 (CIS, E-g=1.04 eV) can be doped at equilibrium n-type either by Cd or Cl, but bulk CuGaSe2 (CGS, E-g=1.68 eV) cannot; (ii) result (i) is primarily because the Cu-vacancy pins the Fermi level in CGS farther below the conduction band minimum than it does in CIS, as explained by the doping limit rule; (iii) Cd doping is better than Cl doping, in that Cd-Cu yields in CIS a higher net donor concentration than Cl-Se; and (iv) in general, the system shows massive compensation of acceptors (Cd-III,V-Cu) and donors (Cl-Se,Cd-Cu,In-Cu).

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