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First-Principles Study on Electronic and Optical Properties of Copper-Based Chalcogenide Photovoltaic Materials
KTH, School of Industrial Engineering and Management (ITM), Materials Science and Engineering.ORCID iD: 0000-0002-7297-7262
2017 (English)Doctoral 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.

Place, publisher, year, edition, pages
Stockholm: KTH Royal Institute of Technology, 2017. , 97 p.
Keyword [en]
density functional theory, electronic structure, dielectric function, absorption coefficient, copper-based chalcogenides, ultra-thin film
National Category
Materials Engineering
Research subject
Materials Science and Engineering
Identifiers
URN: urn:nbn:se:kth:diva-207626ISBN: 978-91-7729-396-5 (print)OAI: oai:DiVA.org:kth-207626DiVA: diva2:1097373
Public defence
2017-06-12, Sal D3, Lindstedtsvägen 5, Stockholm, 13:15 (English)
Opponent
Supervisors
Note

QC 20170523

Available from: 2017-05-23 Created: 2017-05-22 Last updated: 2017-05-24Bibliographically approved
List of papers
1. Parameterization of CuIn1-xGaxSe2 (x=0, 0.5, and 1) energy bands
Open this publication in new window or tab >>Parameterization of CuIn1-xGaxSe2 (x=0, 0.5, and 1) energy bands
2011 (English)In: Thin Solid Films, ISSN 0040-6090, E-ISSN 1879-2731, Vol. 519, no 21, 7503-7507 p.Article in journal (Refereed) Published
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).

Keyword
CuInSe(2), CuGaSe(2), Chalcopyrite, Solar cells, Band structure, Electronic structure, Effective mass
National Category
Physical Sciences
Identifiers
urn:nbn:se:kth:diva-45289 (URN)10.1016/j.tsf.2010.12.216 (DOI)000295347700088 ()2-s2.0-80052151715 (Scopus ID)
Funder
Swedish Research Council
Note

QC 20111028

Available from: 2011-10-28 Created: 2011-10-28 Last updated: 2017-05-23Bibliographically approved
2. Band-edge density-of-states and carrier concentrations in intrinsic and p-type CuIn1-xGaxSe2
Open this publication in new window or tab >>Band-edge density-of-states and carrier concentrations in intrinsic and p-type CuIn1-xGaxSe2
2012 (English)In: Journal of Applied Physics, ISSN 0021-8979, E-ISSN 1089-7550, Vol. 112, no 10, 103708- p.Article in journal (Refereed) Published
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.

Keyword
N-Type, Electronic-Properties, Solar-Cells, Thin-Films, Cuinse2, Cugase2, Transport, Germanium, Crystals, Silicon
National Category
Other Physics Topics
Identifiers
urn:nbn:se:kth:diva-109640 (URN)10.1063/1.4767120 (DOI)000311969800064 ()2-s2.0-84870697429 (Scopus ID)
Funder
Swedish Research Council
Note

QC 20130109

Available from: 2013-01-09 Created: 2013-01-08 Last updated: 2017-05-23Bibliographically approved
3. Dielectric function spectra at 40 K and critical-point energies for CuIn0.7Ga0.3Se2
Open this publication in new window or tab >>Dielectric function spectra at 40 K and critical-point energies for CuIn0.7Ga0.3Se2
Show others...
2012 (English)In: Applied Physics Letters, ISSN 0003-6951, E-ISSN 1077-3118, Vol. 101, no 26, 261903- p.Article in journal (Refereed) Published
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.

Keyword
Interband Critical-Points, Temperature-Dependence, Spectroscopic Ellipsometry, Thin-Films, Electronic-Structure, Cuin1-Xgaxse2, Cuinse2
National Category
Physical Sciences
Identifiers
urn:nbn:se:kth:diva-116662 (URN)10.1063/1.4773362 (DOI)000312830700025 ()2-s2.0-84871730503 (Scopus ID)
Note

QC 20130123

Available from: 2013-01-23 Created: 2013-01-22 Last updated: 2017-05-23Bibliographically approved
4. Dielectric function and double absorption onset of monoclinic Cu2SnS3: origin of experimental features explained by first-principles calculations
Open this publication in new window or tab >>Dielectric function and double absorption onset of monoclinic Cu2SnS3: origin of experimental features explained by first-principles calculations
Show others...
2016 (English)In: Solar Energy Materials and Solar Cells, ISSN 0927-0248, E-ISSN 1879-3398, Vol. 154, 121-129 p.Article in journal (Refereed) Published
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.

Place, publisher, year, edition, pages
Elsevier, 2016
Keyword
CTS, Cu2SnS3, Optical properties, Band gap, Ellipsometry
National Category
Materials Engineering
Identifiers
urn:nbn:se:kth:diva-189906 (URN)10.1016/j.solmat.2016.04.028 (DOI)000378569600016 ()2-s2.0-84969542894 (Scopus ID)
Note

QC 20160808

Available from: 2016-08-08 Created: 2016-07-25 Last updated: 2017-05-23Bibliographically approved
5. Exploring the electronic and optical properties of Cu2Sn1-xGexS3 and Cu2Sn1-xSixS3 (x = 0, 0.5, and 1)
Open this publication in new window or tab >>Exploring the electronic and optical properties of Cu2Sn1-xGexS3 and Cu2Sn1-xSixS3 (x = 0, 0.5, and 1)
2017 (English)In: Phys. Status Solidi BArticle in journal (Refereed) Published
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.

Keyword
absorption coefficient, band-gap energy, Cu2GeS3, Cu2SiS3, Cu2SnS3
National Category
Materials Engineering
Identifiers
urn:nbn:se:kth:diva-207608 (URN)10.1002/pssb.201700111 (DOI)000403292300009 ()2-s2.0-85017473217 (Scopus ID)
Note

QC 20170523

Available from: 2017-05-22 Created: 2017-05-22 Last updated: 2017-06-30Bibliographically approved
6. Electronic and optical properties of Cu2 X SnS4 (X = Be, Mg, Ca, Mn, Fe, and Ni) and the impact of native defect pairs
Open this publication in new window or tab >>Electronic and optical properties of Cu2 X SnS4 (X = Be, Mg, Ca, Mn, Fe, and Ni) and the impact of native defect pairs
2017 (English)In: Journal of Applied Physics, ISSN 0021-8979, E-ISSN 1089-7550, Vol. 121, no 20, 203104Article in journal (Refereed) Published
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.

Place, publisher, year, edition, pages
American Institute of Physics (AIP), 2017
Keyword
kesterite, stannite, electronic structure, optical properties, native defects
National Category
Materials Engineering
Identifiers
urn:nbn:se:kth:diva-207610 (URN)10.1063/1.4984115 (DOI)2-s2.0-85019981692 (Scopus ID)
Note

QC 20170523

Available from: 2017-05-22 Created: 2017-05-22 Last updated: 2017-06-21Bibliographically approved
7. High absorption coefficients of the CuSb(Se,Te)2 and CuBi(S,Se)2 alloys enable high efficient 100 nm thin-film photovoltaics
Open this publication in new window or tab >>High absorption coefficients of the CuSb(Se,Te)2 and CuBi(S,Se)2 alloys enable high efficient 100 nm thin-film photovoltaics
2017 (English)In: EPJ Photovoltaics, ISSN 2105-0716Article in journal (Refereed) Accepted
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.

National Category
Materials Engineering
Identifiers
urn:nbn:se:kth:diva-207612 (URN)
Note

QC 20170523

Available from: 2017-05-22 Created: 2017-05-22 Last updated: 2017-05-23Bibliographically approved

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