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  • 1.
    Chen, Rongzhen
    KTH, School of Industrial Engineering and Management (ITM), Materials Science and Engineering.
    Exploring the Electronic and Optical Properties of Cu(In,Ga) Se22015Licentiate thesis, comprehensive summary (Other academic)
  • 2.
    Chen, Rongzhen
    KTH, School of Industrial Engineering and Management (ITM), Materials Science and Engineering.
    First-Principles Study on Electronic and Optical Properties of Copper-Based Chalcogenide Photovoltaic Materials2017Doctoral thesis, comprehensive summary (Other academic)
    Abstract [en]

    To accelerate environmentally friendly thin film photovoltaic (PV) technologies, copper-based chalcogenides are attractive as absorber materials. Chalcopyrite copper indium gallium selenide (CIGS ≡ CuIn1–xGaxSe2) is today a commercially important PV material, and it is also in many aspects a very interesting material from a scientific point of view. Copper zinc tin sulfide selenide (CZTSSe ≡ Cu2ZnSn(S1–xSex)4) is considered as an emerging alternative thin film absorber material. Ternary Cu2SnS3 (CTS) is a potential absorber material, thus its related alloys Cu2Sn1–xGexS3 (CTGS) and Cu2Sn1–xSixS3 (CTSS) are attractive due to the tunable band gap energies. CuSb(Se1–xTex)2 and CuBi(S1–xSex)2 can be potential as ultra-thin (≤ 100 nm) film absorber materials in the future. In the thesis, analyses of these Cu-based chalcogenides are based on first-principles calculations performed by means of the projector augmented wave method and the full-potential linearized augmented plane wave formalisms within the density functional theory as implemented in the VASP and WIEN2k program packages, respectively.

    The electronic and optical properties of CIGS (x = 0, 0.5, and 1) are studied, where the lowest conduction band (CB) and the three uppermost valence bands (VBs) are parameterized and analyzed in detail. The parameterization demonstrates that the corresponding energy dispersions of the topmost VBs are strongly anisotropic and non-parabolic even very close to the Γ-point. Moreover, the density-of-states and constant energy surfaces are calculated utilizing the parameterization, and the Fermi energy level and the carrier concentration are modeled for p-type CIGS. We conclude that the parameterization is more accurate than the commonly used parabolic approximation. The calculated dielectric function of CuIn0.5Ga0.5Se2 is also compared with measured dielectric function of CuIn0.7Ga0.3Se2 collaborating with experimentalists. We found that the overall shapes of the calculated and measured dielectric function spectra are in good agreement. The transitions in the Brillouin zone edge from the topmost and the second topmost VBs to the lowest CB are responsible for the main absorption peaks. However, also the energetically lower VBs contribute significantly to the high absorption coefficient.

    CTS and its related alloys are explored and investigated. For a perfectly crystalline CTS, reported experimental double absorption onset in dielectric function is for the first time confirmed by our calculations. We also found that the band gap energies of CTGS and CTSS vary almost linearly with composition over the entire range of x. Moreover, those alloys have comparable absorption coefficients with CZTSSe. Cu2XSnS4 (X = Be, Mg, Ca, Mn, Fe, Ni, and Zn) are also studied, revealing rather similar crystalline, electronic, and optical properties. Despite difficulties to avoid high concentration of anti-site pairs disordering in all compounds, the concentration is reduced in Cu2BeSnS4 partly due to larger relaxation effects. CuSb(Se1–xTex)2 and CuBi(S1–xSex)2 are suggested as alternative ultra-thin film absorber materials. Their maximum efficiencies considering the Auger effect are ~25% even when the thicknesses of the materials are between 50 and 300 nm.

  • 3.
    Chen, Rongzhen
    et al.
    KTH, School of Industrial Engineering and Management (ITM), Materials Science and Engineering, Multiscale Materials Modelling.
    Persson, Clas
    KTH, School of Industrial Engineering and Management (ITM), Materials Science and Engineering, Multiscale Materials Modelling.
    Band structure and optical properties of CuInSe22014Conference paper (Refereed)
    Abstract [en]

    In this work, the electronic structure and dielectric function of chalcopyrite CuInSe2 are presented. The results are based on the full-potential linearized augmented plane wave (FPLAPW) method using the generalized gradient approximation (GGA) plus an onsite Coulomb interaction U of the Cu d states. The dielectric constant, absorption coefficient and refractive index are explored by means of optical response. The spin-orbit coupling effect is considered for the calculations of electronic structure and optical properties. We find that the results based on our calculation method have good agreement compared with experimental and other earlier simulations results.

  • 4.
    Chen, Rongzhen
    et al.
    KTH, School of Industrial Engineering and Management (ITM), Materials Science and Engineering.
    Persson, Clas
    KTH, School of Industrial Engineering and Management (ITM), Materials Science and Engineering, Applied Material Physics.
    Band-edge density-of-states and carrier concentrations in intrinsic and p-type CuIn1-xGaxSe22012In: Journal of Applied Physics, ISSN 0021-8979, E-ISSN 1089-7550, Vol. 112, no 10, p. 103708-Article in journal (Refereed)
    Abstract [en]

    The electronic structures of chalcopyrite CuIn1-xGaxSe2 have recently been reported to have strongly anisotropic and non-parabolic valence bands (VBs) even close to the Gamma-point VB maximum. Also, the lowest conduction band (CB) is non-parabolic for energies 50-100 meV above the CB minimum. The details in the band-edge dispersion govern the material's electrical properties. In this study, we, therefore, analyze the electronic structure of the three uppermost VBs and the lowest CB in CuIn1-xGaxSe2 (x = 0, 0.5, and 1). The parameterized band dispersions are explored, and the density-of-states (DOS) as well as the constant energy surfaces are calculated and analyzed. The carrier concentration and the Fermi energy E-F in the intrinsic alloys as functions of the temperature is determined from the DOS. The carrier concentration in p-type materials is modeled by assuming the presence of Cu vacancies as the acceptor type defect. We demonstrate that the non-parabolicity of the energy bands strongly affects the total DOS. Therefore, it is important to take into account full band dispersion of the VBs and CB when analyzing the free carrier concentration, like for instance, in studies of electronic transport and/or measurements that involve strong excitation conditions.

  • 5.
    Chen, Rongzhen
    et al.
    KTH, School of Industrial Engineering and Management (ITM), Materials Science and Engineering. University of Oslo, Norge.
    Persson, Clas
    KTH, School of Industrial Engineering and Management (ITM), Materials Science and Engineering. University of Oslo, Norge.
    Electronic and optical properties of Cu2 X SnS4 (X = Be, Mg, Ca, Mn, Fe, and Ni) and the impact of native defect pairs2017In: Journal of Applied Physics, ISSN 0021-8979, E-ISSN 1089-7550, Vol. 121, no 20, article id 203104Article in journal (Refereed)
    Abstract [en]

    Reducing or controlling cation disorder in Cu2ZnSnS4 is a major challenge, mainly due to low formation energies of the anti-site pair (Cu Zn - + Zn Cu +) and the compensated Cu vacancy (V Cu - + Zn Cu +). We study the electronic and optical properties of Cu2XSnS4 (CXTS, with X = Be, Mg, Ca, Mn, Fe, and Ni) and the impact of defect pairs, by employing the first-principles method within the density functional theory. The calculations indicate that these compounds can be grown in either the kesterite or stannite tetragonal phase, except Cu2CaSnS4 which seems to be unstable also in its trigonal phase. In the tetragonal phase, all six compounds have rather similar electronic band structures, suitable band-gap energies Eg for photovoltaic applications, as well as good absorption coefficients α(ω). However, the formation of the defect pairs (C u X + X Cu) and (V Cu + X Cu) is an issue for these compounds, especially considering the anti-site pair which has formation energy in the order of ∼0.3 eV. The (C u X + X Cu) pair narrows the energy gap by typically ΔEg ≈ 0.1-0.3 eV, but for Cu2NiSnS4, the complex yields localized in-gap states. Due to the low formation energy of (C u X + X Cu), we conclude that it is difficult to avoid disordering from the high concentration of anti-site pairs. The defect concentration in Cu2BeSnS4 is however expected to be significantly lower (as much as ∼104 times at typical device operating temperature) compared to the other compounds, which is partly explained by larger relaxation effects in Cu2BeSnS4 as the two anti-site atoms have different sizes. The disadvantage is that the stronger relaxation has a stronger impact on the band-gap narrowing. Therefore, instead of trying to reduce the anti-site pairs, we suggest that one shall try to compensate (C u X + X Cu) with (V Cu + X Cu) or other defects in order to stabilize the gap energy.

  • 6.
    Chen, Rongzhen
    et al.
    KTH, School of Industrial Engineering and Management (ITM), Materials Science and Engineering.
    Persson, Clas
    KTH, School of Industrial Engineering and Management (ITM), Materials Science and Engineering.
    Exploring the electronic and optical properties of Cu2Sn1-xGexS3 and Cu2Sn1-xSixS3 (x = 0, 0.5, and 1)2017In: Phys. Status Solidi BArticle in journal (Refereed)
    Abstract [en]

    To accelerate environmental friendly thin-film photovoltaic technologies, earth-abundant, non-toxic, and low-cost materials are demanded. We study the compounds of Cu2Sn1−xGexS3 and Cu2Sn1−xSixS3 (x = 0, 0.5, and 1) employing first-principles method within the density functional theory. The compounds have comparable band dispersions. The band-gap energies Eg can be tailored by cation alloying the Sn atoms with Ge or Si. The gap energies of Cu2Sn1−xGexS3 and Cu2Sn1−xSixS3, with x = 0, 0.5, and 1, vary almost linearly from 0.83 to 1.43 eV and 2.60 eV, respectively. However, the gap energy of Cu2SiS3 does not follow the linear relation for x > 0.8. The effective electron masses at the Γ-point of the lowest conduction band are almost isotropic for all materials, which are between 0.15m0 and 0.25m0. On the other hand, the effective hole masses of the topmost valence band show very strong anisotropy for all compounds. In the (010) direction, the hole masses are estimated to be between 1.01m0 and 1.85m0, while between 0.11m0 and 0.41m0 in the (001) direction. Calculations reveal that all compounds have high absorption coefficients that are comparable with that of Cu2ZnSnS4. The absorptions in the energy region from Eg + 0.5 eV to Eg + 1.0 eV are even higher for Ge- and Si-alloying of Cu2SnS3, compared with Cu2ZnSnS4. The high-frequency dielectric constants of the compounds are between 6.8 and 8.9. Cu2Sn1−xGexS3 and Cu2Sn1−xSixS3 can be considered as potential candidates for absorber materials in thin-film solar cells.

  • 7.
    Chen, Rongzhen
    et al.
    KTH, School of Industrial Engineering and Management (ITM), Materials Science and Engineering.
    Persson, Clas
    KTH, School of Industrial Engineering and Management (ITM), Materials Science and Engineering.
    High absorption coefficients of the CuSb(Se,Te)2 and CuBi(S,Se)2 alloys enable high efficient 100 nm thin-film photovoltaics2017In: EPJ Photovoltaics, ISSN 2105-0716Article in journal (Refereed)
    Abstract [en]

    We demonstrate that the band-gap energies Eg of CuSb(Se,Te)2 and CuBi(S,Se)2 can be optimized for high energy conversion in very thin photovoltaic devices, and that the alloys then exhibit excellent optical properties, especially for tellurium rich CuSb(Se1−xTex)2. This is explained by multi-valley band structure with flat energy dispersions, mainly due to the localized character of the Sb/Bi p-like conduction band states. Still the effective electron mass is reasonable small: mc ≈ 0.25m0 for CuSbTe2. The absorption coefficient α(ω) for CuSb(Se1−xTex)2 is at ħω = Eg + 1 eV as much as 5–7 times larger than α(ω) for traditional thin-film absorber materials. Auger recombination does limit the efficiency if the carrier concentration becomes too high, and this effect needs to be suppressed. However with high absorptivity, the alloys can be utilized for extremely thin inorganic solar cells with the maximum efficiency ηmax ≈ 25% even for film thicknesses d ≈ 50–150 nm, and the efficiency increases to ~30%if the Auger effect is diminished.

  • 8.
    Chen, Rongzhen
    et al.
    KTH, School of Industrial Engineering and Management (ITM), Materials Science and Engineering.
    Persson, Clas
    KTH, School of Industrial Engineering and Management (ITM), Materials Science and Engineering, Applied Material Physics.
    Parameterization of CuIn1-xGaxSe2 (x=0, 0.5, and 1) energy bands2011In: Thin Solid Films, ISSN 0040-6090, E-ISSN 1879-2731, Vol. 519, no 21, p. 7503-7507Article in journal (Refereed)
    Abstract [en]

    Parameterization of the electronic band structure of CuIn(1-x)Ga(x)Se(2) (x=0, 0.5, and 1) demonstrates that the energy dispersions of the three uppermost valence bands [E(j)(k); j=v1, v2, and v3] are strongly anisotropic and non-parabolic even very close to the Gamma-point valence-band maximum E,(0). Also the lowest conduction band E(c1) (k) is anisotropic and non-parabolic for energies similar to 0.05 eV above the band-gap energy. Since the electrical conductivity depends directly on the energy dispersion, future electron and hole transport simulations of CuIn(1-x)Ga(x)Se(2) need to go beyond the parabolic approximation of the bands. We therefore present a parameterization of the energy bands, the k-dependency of the effective electron and hole masses m(f)(k), and also an average energy-dependent approximation of the masses m(j)(E).

  • 9. Choi, S. G.
    et al.
    Chen, Rongzhen
    KTH, School of Industrial Engineering and Management (ITM), Materials Science and Engineering.
    Persson, Clas
    KTH, School of Industrial Engineering and Management (ITM), Materials Science and Engineering, Applied Material Physics.
    Kim, T. J.
    Hwang, S. Y.
    Kim, Y. D.
    Mansfield, L. M.
    Dielectric function spectra at 40 K and critical-point energies for CuIn0.7Ga0.3Se22012In: Applied Physics Letters, ISSN 0003-6951, E-ISSN 1077-3118, Vol. 101, no 26, p. 261903-Article in journal (Refereed)
    Abstract [en]

    We report ellipsometrically determined dielectric function ε spectra for CuIn0.7Ga0.3Se2 thin film at 40 and 300 K. The data exhibit numerous spectral features associated with interband critical points (CPs) in the spectral range from 0.74 to 6.43 eV. The second-energy-derivatives of ε further reveal a total of twelve above-bandgap CP features, whose energies are obtained accurately by a standard lineshape analysis. The ε spectra determined by ellipsometry show a good agreement with the results of full-potential linearized augmented plane wave calculations. Probable electronic origins of the CP features observed are discussed.

  • 10. Crovetto, Andrea
    et al.
    Chen, Rongzhen
    KTH, School of Industrial Engineering and Management (ITM), Materials Science and Engineering.
    Ettlinger, Rebecca Bolt
    Cazzaniga, Andrea Carlo
    Schou, Jorgen
    Persson, Clas
    KTH, School of Industrial Engineering and Management (ITM), Materials Science and Engineering.
    Hansen, Ole
    Dielectric function and double absorption onset of monoclinic Cu2SnS3: origin of experimental features explained by first-principles calculations2016In: Solar Energy Materials and Solar Cells, ISSN 0927-0248, E-ISSN 1879-3398, Vol. 154, p. 121-129Article in journal (Refereed)
    Abstract [en]

    In this work, we determine experimentally the dielectric function of monoclinic Cu2SnS3 (CTS) by spectroscopic ellipsometry from 0.7 to 5.9 eV. An experimental approach is proposed to overcome the challenges of extracting the dielectric function of Cu2SnS3 when grown on a glass/Mo substrate, as relevant for photovoltaic applications. The ellipsometry measurement reveals a double absorption onset at 0.91 eV and 0.99 eV. Importantly, we demonstrate that calculation within the density functional theory (DFT) confirms this double onset only when a very dense k-mesh is used to reveal fine details in the electronic structure, and this can explain why it has not been reported in earlier calculated spectra. We can now show that the double onset originates from optical transitions at the Gamma-point from three energetically close-lying valence bands to a single conduction band. Thus, structural imperfection, like secondary phases, is not needed to explain such an absorption spectrum. Finally, we show that the absorption coefficient of CTS is particularly large in the near-band gap spectral region when compared to similar photovoltaic materials. (C) 2016 Elsevier B.V. All rights reserved.

  • 11.
    Huang, Dan
    et al.
    Guangxi Univ, Guangxi Coll & Univ Key Lab Novel Energy Mat & Re, Guangxi Novel Battery Mat Res Ctr Engn Technol,Sc, Guangxi Key Lab Proc Non Ferrous Metall & Feature, Nanning 530004, Peoples R China.;Guilin Univ Elect Technol, Sch Mat Sci & Engn, Guangxi Collaborat Innovat Ctr Struct & Property, Guilin, Peoples R China..
    Jiang, Jing-Wen
    Guangxi Univ, Guangxi Coll & Univ Key Lab Novel Energy Mat & Re, Guangxi Novel Battery Mat Res Ctr Engn Technol,Sc, Guangxi Key Lab Proc Non Ferrous Metall & Feature, Nanning 530004, Peoples R China..
    Guo, Jin
    Guangxi Univ, Guangxi Coll & Univ Key Lab Novel Energy Mat & Re, Guangxi Novel Battery Mat Res Ctr Engn Technol,Sc, Guangxi Key Lab Proc Non Ferrous Metall & Feature, Nanning 530004, Peoples R China.;Guilin Univ Elect Technol, Sch Mat Sci & Engn, Guangxi Collaborat Innovat Ctr Struct & Property, Guilin, Peoples R China..
    Zhao, Yu-Jun
    South China Univ Technol, Dept Phys, Guangzhou 510640, Guangdong, Peoples R China.;South China Univ Technol, Key Lab Adv Energy Storage Mat Guangdong Prov, Guangzhou 510640, Guangdong, Peoples R China..
    Chen, Rongzhen
    KTH, School of Industrial Engineering and Management (ITM), Materials Science and Engineering.
    Persson, Clas
    KTH, School of Industrial Engineering and Management (ITM), Materials Science and Engineering.
    General rules of the sub-band gaps in group-IV (Si, Ge, and Sn)-doped I-III-VI2-type chalcopyrite compounds for intermediate band solar cell: A first-principles study2018In: Materials Science & Engineering: B. Solid-state Materials for Advanced Technology, ISSN 0921-5107, E-ISSN 1873-4944, Vol. 236, p. 147-152Article in journal (Refereed)
    Abstract [en]

    In this work, we have investigated Si, Ge and Sn doped at III-site(Ga or Al) in CuGaSe2, CuAlSe2, AgGaSe2, and AgAlSe2 as the candidates for intermediate band solar cell (IBSC), and demonstrated that the absolute energy levels of the intermediate band from a given group IV dopant in various Se-based chalcopyrite hosts do not show remarkable changes. This is resulted from the fact that the intermediate band originates from the same anti-bonding state of IV-s and Se-p states. The intermediate bands sequence of Ge* < Sn* < Si* from the different dopants in the same chalcopyrite host is explained by a simple model based on the atomic orbital energy and bond interaction. Furthermore, Sn-doped CuAlSe2 with the suitable main-gap and sub-gaps has been selected out as a potential candidate for IBSC, and alloying with isovalent cations to adjust to proper sub-band gaps has been demonstrated in Ge-doped (Ag,Cu)AlSe2 and Ag(Ga,Al)Se-2.( )

  • 12. Huang, Dan
    et al.
    Jiang, Jing-Wen
    Guo, Jin
    Zhao, Yu-Jun
    Chen, Rongzhen
    KTH, School of Industrial Engineering and Management (ITM), Materials Science and Engineering.
    Persson, Clas
    KTH, School of Industrial Engineering and Management (ITM), Materials Science and Engineering.
    Group-IV (Si, Ge, and Sn)-doped AgAlTe2 for intermediate band solar cell from first-principles study2017In: Semiconductor Science and Technology, ISSN 0268-1242, E-ISSN 1361-6641, Vol. 32, no 6, p. 065007-065014, article id 065007Article in journal (Refereed)
    Abstract [en]

    Earlier studies of chalcopyrites as the absorber for intermediate band solar cells (IBSCs) mainly focused on Cu-based compounds, whose intermediate band is usually empty due to its intrinsic p-type conductivity. This is not beneficial to the two sub-bandgap absorptions. In this paper, we demonstrate that the intermediate bands in group IV (Si, Ge, and Sn) doped AgAlTe2 are delocalized and mainly contributed by the anti-bonding state of group-IV elements s state and Te-p state. Overall, we suggest that Sn-doped AgAlTe2 should be a promising absorber candidate for IBSCs based on the theoretical efficiency and defect stability.

  • 13.
    Persson, Clas
    et al.
    KTH, School of Industrial Engineering and Management (ITM), Materials Science and Engineering.
    Chen, Rongzhen
    KTH, School of Industrial Engineering and Management (ITM), Materials Science and Engineering.
    Zhao, Hanyue
    KTH, School of Industrial Engineering and Management (ITM), Materials Science and Engineering.
    Kumar, Mukesh
    KTH, School of Industrial Engineering and Management (ITM), Materials Science and Engineering.
    Huang, Dan
    KTH, School of Industrial Engineering and Management (ITM), Materials Science and Engineering.
    Electronic Structure and Optical Properties from First-Principles Modeling2015In: Copper Zinc Tin Sulfide-Based Thin-Film Solar Cells, John Wiley & Sons, 2015, p. 75-105Chapter in book (Other academic)
    Abstract [en]

    This chapter explores the electronic and optical properties of the kesterite and stannite phases of Cu2ZnSnS4 (CZTS) and Cu2ZnSnSe4 (CZTSe) by means of first-principles modeling. It analyzes the optical properties of CZT(S,Se), in terms of the dielectric function and the optical absorption coefficient. The chapter demonstrates the application of density functional theory (DFT) in conjunction with the Kohn-Sham (KS) equation, utilizing the generalized gradient approximation (GGA), the screened hybrid functional, and the single-electron excitation GW approach, as implemented in the Wien2k and VASP software packages. In the theoretical research on CZT(S,Se) several methods and implementations have been utilized, and it is not surprising to find somewhat deviating results for calculations using similar computational approaches. The chapter briefly discusses these methods. It also describes the crystal and electronic structures of kesterite and stannite phases of CZTS and CZTSe.

  • 14. Zamulko, S.
    et al.
    Chen, Rongzhen
    KTH, School of Industrial Engineering and Management (ITM), Materials Science and Engineering. University of Oslo, Norway.
    Persson, Clas
    KTH, School of Industrial Engineering and Management (ITM), Materials Science and Engineering. University of Oslo, Norway.
    Investigation of the structural, optical and electronic properties of Cu2Zn(Sn,Si/Ge)(S/Se)4 alloys for solar cell applications2017In: Physica status solidi. B, Basic research, ISSN 0370-1972, E-ISSN 1521-3951, Vol. 254, no 6, article id e201700084Article in journal (Refereed)
    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 &gt; 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.

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