| 1. |
- Keller, Jan, et al.
(författare)
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Wide-gap (Ag,Cu)(In,Ga)Se2 solar cells with different buffer materials—A path to a better heterojunction
- 2020
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Ingår i: Progress in Photovoltaics. - : Wiley. - 1062-7995 .- 1099-159X. ; 28:4, s. 237-250
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Tidskriftsartikel (refereegranskat)abstract
- This contribution concerns the effect of the Ag content in wide-gap AgwCu1-wIn1-xGaxSe2 (ACIGS) absorber films and its impact on solar cell performance. First-principles calculations are conducted, predicting trends in absorber band gap energy (Eg) and band structure across the entire compositional range (w and x). It is revealed that a detrimental negative conduction band offset (CBO) with a CdS buffer can be avoided for all possible absorber band gap values (Eg = 1.0–1.8 eV) by adjusting the Ag alloying level. This opens a new path to reduce interface recombination in wide-gap chalcopyrite solar cells. Indeed, corresponding samples show a clear increase in open-circuit voltage (VOC) if a positive CBO is created by sufficient Ag addition. A further extension of the beneficial compositional range (positive CBO at buffer/ACIGS interface) is possible when exchanging CdS with Zn1-ySnyOz, because of its lower electron affinity (χ). Nevertheless, the experimental results strongly suggest that at present, residual interface recombination still limits the performance of solar cells with optimized CBO, which show an efficiency of up to 15.1% for an absorber band gap of Eg = 1.45 eV.
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| 2. |
- Larsen, Jes K., et al.
(författare)
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Experimental and Theoretical Study of Stable and Metastable Phases in Sputtered CuInS2
- 2022
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Ingår i: Advanced Science. - : Wiley. - 2198-3844. ; 9:23
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Tidskriftsartikel (refereegranskat)abstract
- The chalcopyrite Cu(In,Ga)S2 has gained renewed interest in recent years due to the potential application in tandem solar cells. In this contribution, a combined theoretical and experimental approach is applied to investigate stable and metastable phases forming in CuInS2 (CIS) thin films. Ab initio calculations are performed to obtain formation energies, X-ray diffraction (XRD) patterns, and Raman spectra of CIS polytypes and related compounds. Multiple CIS structures with zinc-blende and wurtzite-derived lattices are identified and their XRD/Raman patterns are shown to contain overlapping features, which could lead to misidentification. Thin films with compositions from Cu-rich to Cu-poor are synthesized via a two-step approach based on sputtering from binary targets followed by high-temperature sulfurization. It is discovered that several CIS polymorphs are formed when growing the material with this approach. In the Cu-poor material, wurtzite CIS is observed for the first time in sputtered thin films along with chalcopyrite CIS and CuAu-ordered CIS. Once the wurtzite CIS phase has formed, it is difficult to convert into the stable chalcopyrite polymorph. CuIn5S8 and NaInS2 accommodating In-excess are found alongside the CIS polymorphs. It is argued that the metastable polymorphs are stabilized by off-stoichiometry of the precursors, hence tight composition control is required.
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| 3. |
- Larsen, Jes Kipkoech, et al.
(författare)
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Investigation of AgGaSe2 as a Wide Gap Solar Cell Absorber
- 2021
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Ingår i: ACS Applied Energy Materials. - : American Chemical Society (ACS). - 2574-0962. ; 4:2, s. 1805-1814
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Tidskriftsartikel (refereegranskat)abstract
- The compound AgGaSe2 has received limited attention as a potential wide gap solar cell material for tandem applications, despite its suitable band gap. This study aims to investigate the potential of this material by deposition of thin films by co-evaporation and production of solar cell devices. Since AgGaSe2 has a very low tolerance to off-stoichiometry, reference materials of possible secondary phases in the Ag2Se-Ga2Se3 system were also produced. Based on these samples, it was concluded that X-ray diffraction is suited to distinguish the phases in this material system. An attempt to use Raman spectroscopy to identify secondary phases was less successful. Devices were produced using absorbers containing the secondary phases likely formed during co-evaporation. When grown under slightly Ag-rich conditions, the Ag9GaSe6 secondary phase was present along with AgGaSe2, which resulted in devices being shunted under illumination. When absorbers were grown under Ag-deficient conditions, the AgGa5Se8 secondary phase was observed, making the device behavior dependent on the processing route. Deposition with a three-stage evaporation (Ag-poor, Ag-rich, and Ag-poor) resulted in AgGa5Se8 layers at both front and back surfaces, leading to charge carrier blocking in devices. Deposition of the absorber with a one-stage process, on the other hand, caused the formation of AgGa5Se8 locally extended through the entire film, but no continuous layer was found. As a consequence, these devices were not blocking and achieved an efficiency of up to 5.8%, which is the highest reported to date for AgGaSe2 solar cells.
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| 4. |
- Saini, Nishant, et al.
(författare)
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Germanium Incorporation in Cu2ZnSnS4 and Formation of a Sn–Ge Gradient
- 2019
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Ingår i: Physica Status Solidi (a) applications and materials science. - : Wiley. - 1862-6300 .- 1862-6319. ; 216:22
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Tidskriftsartikel (refereegranskat)abstract
- Alloying of Cu2ZnSnS4 (CZTS) with Ge can potentially promote grain growth and suppress the formation of Sn‐related defects. Herein, a two‐step fabrication route based on compound co‐sputtering and sulfurization at a high temperature is used to prepare Ge‐incorporated CZTS (Cu2ZnGexSn1 − xS4 [CZGTS]). For Cu2ZnGeS4 (CZGS), films deposited using elemental Ge and binary GeS targets are compared. The recrystallization is shown to be promoted for the absorbers deposited using Ge target, possibly due to lower sulfur content in the precursor suppressing the formation of wurtzite‐like phases during sputtering. The grain growth and crystallinity in CZGTS are slightly improved for x = 0.2 but not for higher concentration of the incorporated Ge. Owing to the composition‐dependent electronic properties, compositionally graded CZGTS films may be beneficial for reducing recombination towards the back contact. Hence, herein, the successful formation of a steep concentration gradient with Ge and Sn is demonstrated by the deposition of a CZGS/CZTS precursor stack followed by sulfurization with varying time periods.
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| 5. |
- Sopiha, Kostiantyn V, et al.
(författare)
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First-Principles Mapping of the Electronic Properties of Two-Dimensional Materials for Strain-Tunable Nanoelectronics
- 2019
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Ingår i: ACS APPLIED NANO MATERIALS. - : American Chemical Society (ACS). - 2574-0970. ; 2:9, s. 5614-5624
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Tidskriftsartikel (refereegranskat)abstract
- Herein, we demonstrate that first-principles calculations can be used for mapping of the electronic properties of two-dimensional (2d) materials with respect to nonuniform strain. By investigating four representative single-layer 2d compounds with different symmetries and bonding characters, namely, 2d-MoS2, phosphorene, alpha-Te, and beta-Te, we reveal that such a mapping can be an effective guidance for advanced strain engineering and the development of strain-tunable nano electronics devices, including transistors, sensors, and photodetectors. Thus, we show that alpha-Te and beta-Te are considerably more elastic compared to the 2d compounds with strong chemical bonding. In the case of beta-Te, mapping uncovers the existence of curious regimes where nonuniform deformations allow one to achieve unique localization of band edges in momentum space that cannot be realized under either uniform or uniaxial deformations. For all other systems, the strain mapping is shown to provide deeper insight into the known trends of band-gap modulation and direct-indirect transitions under strain. Hence, we prove that the standard way of analyzing selected strain directions is insufficient for some 2d systems, and a more general mapping strategy should be employed instead.
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