| 1. |
- Edoff, Marika, et al.
(author)
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Gas flow sputtering of Cu(In,Ga)Se-2 for thin film solar cells
- 2015
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In: 2015 IEEE 42ND PHOTOVOLTAIC SPECIALIST CONFERENCE (PVSC). - 9781479979448
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Conference paper (peer-reviewed)abstract
- Gas flow sputtering of Cu(In,Ga)Se-2 (CIGS) from two opposing Cu(In,Ga)Se-2 targets with slightly Cu-poor stoichiometry was performed, using i) selenium only provided by the target and ii) using additional selenium from an elemental source inside the sputtering system. In both cases the composition of the sputtered CIGS film was similar to the target. A sputter process without additional selenium supply led to poor cell results at about 2 % efficiency. After introducing a posttreatment in selenium atmosphere immediately after the sputter deposition, the cell results were dramatically improved to 12 %. With selenium added during the sputtering process, 13.7 % conversion efficiency was obtained without any post treatment. Gas flow sputtering uses a high gas flow to transport the material from the plasma to the growing film, thereby the atoms will be thermalized, similarly to in an evaporation process. Reactant gases can be supplied close to the substrate, outside the plasma, thereby reducing the risk for sputter damage.
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| 2. |
- Edoff, Marika, et al.
(author)
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Sputtering of highly adhesive Mo back contact layers for Cu(In,Ga)Se2 solar cells
- 2009
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Conference paper (pop. science, debate, etc.)abstract
- In this work the sputter process for back contact Mo layers was adjusted to increase the adhesive strength of the Mo layers to the glass substrate, while keeping a high deposition rate and high conductivity. Mo layers were fabricated using DC magnetron sputtering in an in-line sputtering system. The adhesive strength was tested with ultrasonic agitation. The combination of good adhesion and high deposition rate was obtained by using a double layer, where the thickness of the first adhesion layer was varied between 25 and 100 nm and sputtered at 15 mTorr, whereas the second bulk layer was varied between 300 and 600 nm and sputtered at 6 mTorr. Solar cells were prepared for all different thicknesses of the adhesive layer and showed similar performance.
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| 3. |
- Ericson, Tove, 1983-, et al.
(author)
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Zn(O,S) Buffer Layers and Thickness Variations of CdS Buffer for Cu2ZnSnS4 Solar Cells
- 2014
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In: IEEE Journal of Photovoltaics. - 2156-3381. ; 4:1, s. 465-469
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Journal article (peer-reviewed)abstract
- To improve the conduction band alignment and explore the influence of the buffer-absorber interface, we here investigate an alternative buffer for Cu2ZnSnS4 (CZTS) solar cells. The Zn(O, S) system was chosen since the optimum conduction band alignment with CZTS is predicted to be achievable, by varying oxygen to sulfur ratio. Several sulfur to oxygen ratios were evaluated to find an appropriate conduction band offset. There is a clear trend in open-circuit voltage Voc, with the highest values for the most sulfur rich buffer, before going to the blocking ZnS, whereas the fill factor peaks at a lower S content. The best alternative buffer cell in this series had an efficiency of 4.6% and the best CdS reference gave 7.3%. Extrapolating Voc values to 0 K gave activation energies well below the expected bandgap of 1.5 eV for CZTS, which indicate that recombination at the interface is dominating. However, it is clear that the values are affected by the change of buffer composition and that increasing sulfur content of the Zn(O, S) increases the activation energy for recombination. A series with varying CdS buffer thickness showed the expected behavior for short wavelengths in quantum efficiency measurements but the final variation in efficiency was small.
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| 4. |
- Frisk, Christopher, 1985-, et al.
(author)
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Optimizing Ga-profiles for highly efficient Cu(In,Ga)Se2 thin film solar cells in simple and complex defect models
- 2014
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In: Journal of Physics D. - : IOP Publishing. - 0022-3727 .- 1361-6463. ; 47:48, s. 485104-
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Journal article (peer-reviewed)abstract
- Highly efficient Cu(In,Ga)(S,Se)2 photovoltaic thin film solar cells often have a compositional variation of Ga to In in the absorber layer, here described as a Ga-profile. In this work we have studied the role of Ga-profiles in four different models, based on input data from electrical and optical characterizations of an in-house state-of-the-art Cu(In,Ga)Se2 (CIGS) solar cell with power conversion efficiency above 19 %. A simple defect model with mid-gap defects in the absorber layer was compared with models with Ga-dependent defect concentrations and amphoteric defects. In these models optimized single-graded Ga-profiles have been compared with optimized double-graded Ga-profiles. It was found that the defect concentration for effective Shockley-Read-Hall recombination is low for high efficiency CIGS devices and that the doping concentration of the absorber layer, chosen according to the defect model, is paramount when optimizing Ga-profiles. For optimized single-graded Ga-profiles the simulated power conversion efficiency, depending on the model, is 20.5-20.8 %, and the equivalent double-graded Ga-profiles yield 20.6-21.4 %, indicating that the bandgap engineering of the CIGS device structure can lead to improvements in efficiency. Apart from the effects of increased doping in the complex defect models, the results are similar when comparing the complex defect models to the simple defect models.
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| 5. |
- Jõgi, Indrek, et al.
(author)
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On the Structural and Optical Properties of Ultrathin Iron Oxide
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Other publication (other academic/artistic)abstract
- Nanostructured iron oxides and especially hematite are interesting for a wide range of applications ranging from gas sensors to renewable solar hydrogen production. A promising method for deposition of low dimensional films is atomic layer deposition, ALD. Although a potent technique, ALD of ultrathin films is sensitive to the substrate and temperature conditions where initial formation of islands and crystallites influences the properties of the films. In this work the optical and structural properties of iron oxide films in the thickness range of 6 nm to 50 nm have been investigated. Below 10 nm nominal film thickness we find island formation and phase dependent particle crystallization that impose difficulties for ALD deposition of phase pure iron oxides on non-lattice matching substrates. For films between 10-20 nm, post-annealing steps were found to be needed to recrystallize iron polymorphs to hematite whereas for films thicker than 20 nm, phase pure hematite can be formed directly with ALD with very low influence of the substrate. Analysis of the indirect and direct band gaps of the thinnest films show a quantum confined blue shift of the absorption edge.
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| 6. |
- Lindahl, Johan, 1984-, et al.
(author)
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The effect of substrate temperature on atomic layer deposited zinc tin oxide
- 2015
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In: Thin Solid Films. - : Elsevier BV. - 0040-6090 .- 1879-2731. ; 586, s. 82-87
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Journal article (peer-reviewed)abstract
- Zinc tin oxide (ZTO) thin films were deposited on glass substrates by atomic layer deposition (ALD), and the film properties were investigated for varying deposition temperatures in the range of 90 to 180 degrees C. It was found that the [Sn]/([Sn] + [Zn]) composition is only slightly temperature dependent, while properties such as growth rate, film density, material structure and band gap are more strongly affected. The growth rate dependence on deposition temperature varies with the relative number of zinc or tin containing precursor pulses and it correlates with the growth rate behavior of pure ZnO and SnOx ALD. In contrast to the pure ZnO phase, the density of the mixed ZTO films is found to depend on the deposition temperature and it increases linearly with about 1 g/cm(3) in total over the investigated range. Characterization by transmission electron microscopy suggests that zinc rich ZTO films contain small (similar to 10 nm) ZnO or ZnO(Sn) crystallites embedded in an amorphous matrix, and that these crystallites increase in size with increasing zinc content and deposition temperature. These crystallites are small enough for quantum confinement effects to reduce the optical band gap of the ZTO films as they grow in size with increasing deposition temperature.
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| 7. |
- Lindahl, Johan, et al.
(author)
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The effect of Zn1−xSnxOy buffer layer thickness in 18.0% efficient Cd-free Cu(In,Ga)Se2 solar cells
- 2013
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In: Progress in Photovoltaics. - : Wiley. - 1062-7995 .- 1099-159X. ; 21:8, s. 1588-1597
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Journal article (peer-reviewed)abstract
- The influence of the thickness of atomic layer deposited Zn1−xSnxOy buffer layers and the presence of an intrinsic ZnO layer on the performance of Cu(In,Ga)Se2 solar cells are investigated. The amorphous Zn1−xSnxOy layer, with a [Sn]/([Sn] + [Zn]) composition of approximately 0.18, forms a conformal and in-depth uniform layer with an optical band gap of 3.3 eV. The short circuit current for cells with a Zn1−xSnxOy layer are found to be higher than the short circuit current for CdS buffer reference cells and thickness independent. On the contrary, both the open circuit voltage and the fill factor values obtained are lower than the references and are thickness dependent. A high conversion efficiency of 18.0%, which is comparable with CdS references, is attained for a cell with a Zn1−xSnxOy layer thickness of approximately 13 nm and with an i-ZnO layer.
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| 8. |
- Mardani, Shabnam, 1983-, et al.
(author)
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Morphological instability of Ag films caused by phase transition in the underlying Ta barrier layer
- 2014
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In: Applied Physics Letters. - : American Institute of Physics (AIP). - 0003-6951 .- 1077-3118. ; 105, s. 071604-
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Journal article (peer-reviewed)abstract
- Wide-bandgap (WBG) semiconductor technologies are maturing and may provide increased deviceperformance in many fields of applications, such as high-temperature electronics. However, thereare still issues regarding the stability and reliability of WBG devices. Of particular importance isthe high-temperature stability of interconnects for electronic systems based on WBG-semiconductors. For metallization without proper encapsulation, morphological degradation canoccur at elevated temperatures. Sandwiching Ag films between Ta and/or TaN layers in this studyis found to be electrically and morphologically stabilize the Ag metallization up to 800C, com-pared to 600C for uncapped films. However, the barrier layer plays a key role and TaN is found tobe superior to Ta, resulting in the best achieved stability, whereas the difference between Ta andTaN caps is negligible. The b-to-a phase transition in the underlying Ta barrier layer is identifiedas the major cause responsible for the morphological instability observed above 600C. It isshown that this phase transition can be avoided using a stacked Ta/TaN barrier.
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| 9. |
- Schleussner, Sebastian, 1979-, et al.
(author)
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Development of gallium gradients in three‐ stageCu(In,Ga)Se2 co‐evaporation processes
- 2012
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In: Progress in Photovoltaics. - : John Wiley & Sons. - 1062-7995 .- 1099-159X. ; 20:3, s. 284-293
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Journal article (peer-reviewed)abstract
- We use secondary-ion mass spectrometry, X-ray diffraction and scanning electron microscopy to investigate the development over time of compositional gradients in Cu(In,Ga)Se2 thin films grown in three-stage co-evaporation processes and suggest a comprehensive model for the formation of the well-known ‘notch’ structure. The model takes into account the need for compensating Cu diffusion by movement of group-III ions in order to remain on the quasi-binary tie line and indicates that the mobilities of In and Ga ions differ. Cu diffuses towards the back in the second stage and towards the front in the third, and this is the driving force for the movement of In and Ga. The [Ga]/[In + Ga] ratio then increases in the direction of the respective Cu movement because In has a higher mobility at process conditions than has Ga. Interdiffusion of In and Ga can be considerable in the (In,Ga)2Se3 film of the first stage, but seems largely to cease in Cu(In,Ga)Se2 and shows no signs of being boosted by the presence of a Cu2Se layer.
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| 10. |
- Schleussner, Sebastian, 1979-, et al.
(author)
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Effect of gallium grading in Cu(In,Ga)Se2 solar-cell absorbers produced by multi-stage coevaporation
- 2011
-
In: Solar Energy Materials and Solar Cells. - : Elsevier BV. - 0927-0248 .- 1879-3398. ; 95:2, s. 721-726
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Journal article (peer-reviewed)abstract
- We investigate Cu(In,Ga)Se2 thin films grown in multi-stage coevaporation processes and solar cells fabricated from such absorbers. Despite some interdiffusion during film growth, Ga/(Ga+In) gradients defined via evaporation-profile variations in the process are to a good part retained in the finished film. This indicates that the bandgap can be engineered in this type of process by varying the evaporation profiles, and consequently also that unintended profile variations should be noted and avoided. With front-side gradients the topmost part of many grains seems to be affected by a higher density of lattice defects due to the strong change of gallium content under copper-poor growth conditions. Electrically, both back-side gradients and moderate front-side gradients are shown to yield an improvement of device efficiency. If a front-side gradient is too wide, though, it causes strong voltage-dependent collection and the fill factor is severely reduced.
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