| 1. |
- Igalson, M., et al.
(författare)
-
Excitation spectra of defect levels derived from photoinduced current transient spectroscopy - a tool for studying deep levels in Cu(In,Ga)Se2 compounds
- 2017
-
Ingår i: Thin Solid Films. - : Elsevier BV. - 0040-6090 .- 1879-2731. ; 633:SI, s. 227-230
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Tidskriftsartikel (refereegranskat)abstract
- Energy required for the optical excitation of carriers onto defect levels is a parameter that compliments thermal activation energy and helps to understand the electronic properties of defects under study. Here a modification of the photoinduced current transient spectroscopy (PICTS) based on phase-sensitive detection is proposed which makes possible to measure the excitation spectra of defect levels. The representative results of the excitation spectra of the epitaxial CuGaSe2 and polycrystalline Cu(In,Ga)Se2 thin films are presented. They illustrate the usefulness of the method as a tool for studying defect properties by providing data that supplement information derived from standard PICTS spectroscopy.
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| 2. |
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| 3. |
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| 4. |
- Aboulfadl, Hisham, 1986, et al.
(författare)
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Alkali Dispersion in (Ag,Cu)(In,Ga)Se2 Thin Film Solar Cells - Insight from Theory and Experiment
- 2021
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Ingår i: ACS Applied Materials & Interfaces. - : American Chemical Society (ACS). - 1944-8252 .- 1944-8244. ; 13:6, s. 7188-7199
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Tidskriftsartikel (refereegranskat)abstract
- Silver alloying of Cu(In,Ga)Se2 absorbers for thin film photovoltaics offers improvements in open-circuit voltage, especially when combined with optimal alkali-treatments and certain Ga concentrations. The relationship between alkali distribution in the absorber and Ag alloying is investigated here, combining experimental and theoretical studies. Atom probe tomography analysis is implemented to quantify the local composition in grain interiors and at grain boundaries. The Na concentration in the bulk increases up to ∼60 ppm for [Ag]/([Ag] + [Cu]) = 0.2 compared to ∼20 ppm for films without Ag and up to ∼200 ppm for [Ag]/([Ag] + [Cu]) = 1.0. First-principles calculations were employed to evaluate the formation energies of alkali-on-group-I defects (where group-I refers to Ag and Cu) in (Ag,Cu)(In,Ga)Se2 as a function of the Ag and Ga contents. The computational results demonstrate strong agreement with the nanoscale analysis results, revealing a clear trend of increased alkali bulk solubility with the Ag concentration. The present study, therefore, provides a more nuanced understanding of the role of Ag in the enhanced performance of the respective photovoltaic devices.
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| 5. |
- Aboulfadl, Hisham, et al.
(författare)
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Microstructural Characterization of Sulfurization Effects in Cu(In,Ga)Se2 Thin Film Solar Cells
- 2019
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Ingår i: Microscopy and Microanalysis. - : CAMBRIDGE UNIV PRESS. - 1435-8115 .- 1431-9276. ; 25:2, s. 532-538
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Tidskriftsartikel (refereegranskat)abstract
- Surface sulfurization of Cu(In,Ga)Se 2 (CIGSe) absorbers is a commonly applied technique to improve the conversion efficiency of the corresponding solar cells, via increasing the bandgap towards the heterojunction. However, the resulting device performance is understood to be highly dependent on the thermodynamic stability of the chalcogenide structure at the upper region of the absorber. The present investigation provides a high-resolution chemical analysis, using energy dispersive X-ray spectrometry and laser-pulsed atom probe tomography, to determine the sulfur incorporation and chemical re-distribution in the absorber material. The post-sulfurization treatment was performed by exposing the CIGSe surface to elemental sulfur vapor for 20 min at 500°C. Two distinct sulfur-rich phases were found at the surface of the absorber exhibiting a layered structure showing In-rich and Ga-rich zones, respectively. Furthermore, sulfur atoms were found to segregate at the absorber grain boundaries showing concentrations up to ∼7 at% with traces of diffusion outwards into the grain interior.
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| 6. |
- Assar, Alireza, et al.
(författare)
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Gettering in PolySi/SiOx Passivating Contacts Enables Si-Based Tandem Solar Cells with High Thermal and Contamination Resilience
- 2022
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Ingår i: ACS Applied Materials and Interfaces. - : American Chemical Society (ACS). - 1944-8244 .- 1944-8252. ; 14:12, s. 14342-14358
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Tidskriftsartikel (refereegranskat)abstract
- Multijunction solar cells in a tandem configuration could further lower the costs of electricity if crystalline Si (c-Si) is used as the bottom cell. However, for direct monolithic integration on c-Si, only a restricted number of top and bottom cell architectures are compatible, due to either epitaxy or high-temperature constraints, where the interface between subcells is subject to a trade-off between transmittance, electrical interconnection, and bottom cell degradation. Using polySi/SiOx passivating contacts for Si, this degradation can be largely circumvented by tuning the polySi/SiOx stacks to promote gettering of contaminants admitted into the Si bottom cell during the top cell synthesis. Applying this concept to the low-cost top cell chalcogenides Cu2ZnSnS4 (CZTS), CuGaSe2 (CGSe), and AgInGaSe2 (AIGSe), fabricated under harsh S or Se atmospheres above 550 degrees C, we show that increasing the heavily doped polySi layer thickness from 40 to up to 400 nm prevents a reduction in Si carrier lifetime by 1 order of magnitude, with final lifetimes above 500 mu s uniformly across areas up to 20 cm(2). In all cases, the increased resilience was correlated with a 99.9% reduction in contaminant concentration in the c-Si bulk, provided by the thick polySi layer, which acts as a buried gettering layer in the tandem structure without compromising the Si passivation quality. The Si resilience decreased as AIGSe > CGSe > CZTS, in accordance with the measured Cu contamination profiles and higher annealing temperatures. An efficiency of up to 7% was achieved for a CZTS/Si tandem, where the Si bottom cell is no longer the limiting factor.
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| 7. |
- Englund, Sven, et al.
(författare)
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Characterization of TiN back contact interlayers with varied thickness for Cu2ZnSn(S,Se)4 thin film solar cells
- 2017
-
Ingår i: Thin Solid Films. - : Elsevier BV. - 0040-6090 .- 1879-2731. ; 639, s. 91-97
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Tidskriftsartikel (refereegranskat)abstract
- TiN thin films have previously been used as intermediate barrier layers on Mo back contacts in CZTS(e) solar cells to suppress excessive reaction of the Mo in the annealing step. In this work, TiN films with various thickness (20, 50 and 200 nm) were prepared with reactive DC magnetron sputtering on Mo/SLG substrates and annealed, without CZTS(e) layers, in either S or Se atmospheres. The as-deposited references and the annealed samples were characterized with X-ray Photoelectron Spectroscopy, X-ray Diffraction, Time-of-Flight-Elastic Recoil Detection Analysis, Time-of-Flight-Medium-Energy Ion Scattering, Scanning Electron Microscopy and Scanning Transmission Electron Microscopy – Electron Energy Loss Spectroscopy. It was found that the as-deposited TiN layers below 50 nm show discontinuities, which could be related to the surface roughness of the Mo. Upon annealing, TiN layers dramatically reduced the formation of MoS(e)2, but did not prevent the sulfurization or selenization of Mo. The MoS(e)2 had formed near the discontinuities, both below and above the TiN layers. Another unexpected finding was that the thicker TiN layer increased the amount of Na diffused to the surface after anneal, and we suggest that this effect is related to the Na affinity of the TiN layers and the MoS(e)2 thickness.
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| 8. |
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| 9. |
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| 10. |
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| 11. |
- Keller, Jan, et al.
(författare)
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On the Paramount Role of Absorber Stoichiometry in (Ag,Cu)(In,Ga)Se2 Wide‐Gap Solar Cells
- 2020
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Ingår i: Solar RRL. - : Wiley. - 2367-198X. ; 4:12
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Tidskriftsartikel (refereegranskat)abstract
- This contribution evaluates the effect of absorber off‐stoichiometry in wide‐gap (Ag,Cu)(In,Ga)Se2 (ACIGS) solar cells. It is found that ACIGS films show an increased tendency to form ordered vacancy compounds (OVCs) with increasing Ga and Ag contents. Very little tolerance to off‐stoichiometry is detected for absorber compositions giving the desired properties of 1) an optimum bandgap (EG) for a top cell in tandem devices (EG = 1.6–1.7 eV) and at the same time 2) a favorable band alignment with a CdS buffer layer. Herein, massive formation of either In‐ or Ga‐enriched OVC patches is found for group I‐poor ACIGS. As a consequence, carrier transport and charge collection are significantly impeded in corresponding solar cells. The transport barrier appears to be increasing with storage time, questioning the long‐term stability of wide‐gap ACIGS solar cells. Furthermore, the efficiency of samples with very high Ga and Ag contents depends on the voltage sweep direction. It is proposed that the hysteresis behavior is caused by a redistribution of mobile Na ions in the 1:1:2 absorber lattice upon voltage bias. Finally, a broader perspective on OVC formation in the ACIGS system is provided and fundamental limitations for wide‐gap ACIGS solar cells are discussed.
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| 12. |
- Khavari, Faraz, et al.
(författare)
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Comparison of Sulfur Incorporation into CuInSe(2)and CuGaSe(2)Thin-Film Solar Absorbers
- 2020
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Ingår i: Physica Status Solidi (a) applications and materials science. - : WILEY-V C H VERLAG GMBH. - 1862-6300 .- 1862-6319. ; 217:22
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Tidskriftsartikel (refereegranskat)abstract
- Herein, sulfurization of CuInSe(2)and CuGaSe2(CGSe) absorber layers is compared to improve the understanding of sulfur incorporation into Cu(In,Ga)Se(2)films by annealing in a sulfur atmosphere. It is found for Cu-poor CuInSe(2)that for an annealing temperature of 430 degrees C, sulfur is incorporated into the surface of the absorber and forms an inhomogeneous CuIn(S,Se)(2)layer. In addition, at 530 degrees C, a surface layer of CuInS(2)is formed. In contrast, for Cu-poor CuGaSe(2)samples, S can only be introduced at 530 degrees C, mainly forming an alloy of CuGa(S,Se)(2), where no closed CuGaS(2)layer is found. In Cu-rich CuGaSe(2)samples, however, selenium is substituted by S already at 330 degrees C, which can be explained by a rapid phase transformation of Cu2 - xSe into Cu2 - x(S,Se). This transformation facilitates S in-diffusion and catalyzes CuGa(S,Se)(2)formation, likewise that previously reported to occur in CuInSe2. Finally, the Cu-poor CuInSe(2)solar cell performance is improved by the sulfurization step at 430 degrees C, whereas for the 530 degrees C sample, a decreasing fill factor and short-circuit current density are observed, indicating lower diffusion length accompanied by possible formation of an electron transport barrier. In contrast, the electrical characteristics deteriorate for all sulfurized Cu-poor CuGaSe(2)cells.
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| 13. |
- Khavari, Faraz, et al.
(författare)
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Post‐deposition sulfurization of CuInSe2 solar absorbers by employing sacrificial CuInS2 precursor layers
- 2022
-
Ingår i: Physica Status Solidi (a) applications and materials science. - : John Wiley & Sons. - 1862-6300 .- 1862-6319. ; 219:5
-
Tidskriftsartikel (refereegranskat)abstract
- Herein, a new route of sulfur grading in CuInSe2 (CISe) thin-film solar absorbers by introducing an ultrathin (<50 nm) sacrificial sputtered CuInS2 (CIS) layer on top of the CISe. Different CIS top layer compositions (Cu-poor to Cu-rich) are analyzed, before and after a high-temperature treatment in selenium (Se)- or selenium+sulfur (SeS)-rich atmospheres. An [S]/([S] + [Se]) grading from the surface into the bulk of the Se- and SeS-treated samples is observed, and evidence of the formation of a mixed CuIn(S,Se)2 phase by Raman analysis and X-ray diffraction is provided. The optical bandgap from quantum efficiency measurements of solar cells is increased from 1.00 eV for the CISe reference to 1.14 and 1.30 eV for the Se- and SeS-treated bilayer samples, respectively. A ≈150 mV higher VOC is observed for the SeS-treated bilayer sample, but the cell exhibits blocking characteristics resulting in lower efficiency as compared with the CISe reference. This blocking is attributed to an internal electron barrier at the interface to the sulfur-rich surface layer. The difference in reaction routes and possible ways to improve the developed sulfurization process are discussed.
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| 14. |
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| 15. |
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| 16. |
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| 17. |
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| 18. |
- Larsen, Jes K, et al.
(författare)
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Band Tails and Cu-Zn Disorder in Cu2ZnSnS4 Solar Cells
- 2020
-
Ingår i: ACS Applied Energy Materials. - : AMER CHEMICAL SOC. - 2574-0962. ; 3:8, s. 7520-7526
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Tidskriftsartikel (refereegranskat)abstract
- Cu2ZnSnS4 (CZTS) has attracted interest for applications in thin-film solar cells. In this study, the annealing process for CZTS fabrication is systematically varied, resulting in a large variation of materials properties. These variations are connected to the sulfur partial pressure during the annealing. A well-known phenomenon in CZTS is the presence of a high density of Cu-Zn antisite defect pairs, also known as Cu-Zn disorder. Faster Cu-Zn ordering occurs in samples with a similar starting composition annealed in an atmosphere with a higher sulfur partial pressure. This is explained by a higher density of vacancies in these samples. The results indicate that reduction of the vacancy concentration in CZTS annealed in insufficient sulfur partial pressure reduces diffusion, which results in more defective material with a higher density of tail states and poorer device performance.
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| 19. |
- Larsen, Jes K, et al.
(författare)
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Cadmium Free Cu2ZnSnS4 Solar Cells with 9.7% Efficiency
- 2019
-
Ingår i: Advanced Energy Materials. - : Wiley. - 1614-6832 .- 1614-6840. ; 9:21
-
Tidskriftsartikel (refereegranskat)abstract
- Cu2ZnSnS4(CZTS) thin-film solar cell absorbers with different bandgaps can be produced by parameter variation during thermal treatments. Here, the effects of varied annealing time in a sulfur atmosphere and an ordering treatment of the absorber are compared. Chemical changes in the surface due to ordering are examined, and a downshift of the valence band edge is observed. With the goal to obtain different band alignments, these CZTS absorbers are combined with Zn1−xSnxOy (ZTO) or CdS buffer layers to produce complete devices. A high open circuit voltage of 809 mV is obtained for an ordered CZTS absorber with CdS buffer layer, while a 9.7% device is obtained utilizing a Cd free ZTO buffer layer. The best performing devices are produced with a very rapid 1 min sulfurization, resulting in very small grains.
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| 20. |
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| 21. |
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| 22. |
- 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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| 23. |
- Larsen, Jes K, et al.
(författare)
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Formation of Ga2O3 barrier layer in Cu (InGa) Se2 superstrate devices with ZnO buffer layer
- 2013
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Ingår i: MRS Proceedings. - : Springer Science and Business Media LLC. - 0272-9172 .- 1946-4274. ; 1538, s. 67-72
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Tidskriftsartikel (refereegranskat)abstract
- The junction formation when Cu(InGa)Se2 is deposited onto ZnO in a superstrate configuration (glass/window/buffer/Cu(InGa)Se2/contact) is investigated by x-ray photoelectron spectroscopy and analysis of device behavior. When Cu(InGa)Se2 is deposited on ZnO, a Ga2O3 layer is formed at the interface. Approaches to avoid the formation of this unfavorable interlayer are investigated. This includes modifications of the process to reduce the thermal load during deposition and improvement of the thermal stability of the ZnO buffer layer. It was demonstrated that both lowering of the substrate deposition temperature and deposition of the ZnO buffer layer at elevated temperature limits the Ga2O3 formation. The presence of Ga2O3 at the junction does affect the device behavior, resulting in a kink in JV curves measured under illumination. This behavior is absent in devices with limited Ga2O3 formation.
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| 24. |
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| 25. |
- Larsen, Jes K, et al.
(författare)
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Interference effects in photoluminescence spectra of Cu2ZnSnS4 and Cu(In,Ga)Se2 thin films
- 2015
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Ingår i: Journal of Applied Physics. - : AIP Publishing. - 0021-8979 .- 1089-7550. ; 118:3
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Tidskriftsartikel (refereegranskat)abstract
- Photoluminescence (PL) is commonly used for investigations of Cu2ZnSnS(e)4 [CZTS(e)] and Cu(In,Ga)Se2 (CIGS) thin film solar cells. The influence of interference effects on these measurements is, however, largely overlooked in the community. Here, it is demonstrated that PL spectra of typical CZTS absorbers on Mo/glass substrates can be heavily distorted by interference effects. One reason for the pronounced interference in CZTS is the low reabsorption of the PL emission that typically occurs below the band gap. A similar situation occurs in band gap graded CIGS where the PL emission originates predominantly from the band gap minimum located at the notch region. Based on an optical model for interference effects of PL emitted from a thin film, several approaches to reduce the fringing are identified and tested experimentally. These approaches include the use of measured reflectance data, a calculated interference function, use of high angles of incidence during PL measurements as well as the measurement of polarized light near the Brewster angle.
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| 26. |
- Larsen, Jes K, et al.
(författare)
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Potential of CuS cap to prevent decomposition of Cu2ZnSnS4 during annealing
- 2015
-
Ingår i: Physica Status Solidi (a) applications and materials science. - : Wiley. - 1862-6300 .- 1862-6319. ; 212:12, s. 2843-2849
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Tidskriftsartikel (refereegranskat)abstract
- One of the challenges associated with processing of Cu2ZnSnS4 (CZTS) is the thermal decomposition reaction that causes loss of S and SnS from the absorber surface. To reduce the decomposition a sufficiently high SnS and S partial pressure must be supplied during annealing. The absorber surface can alternatively be protected with a thin cap. Aiming to obtain a more flexible process, CZTS precursors were capped with a thin CuS layer before annealing. The cap was subsequently removed with a KCN etch before device finishing. It was found that the cap coverage decreased during annealing, exposing a part of the absorber surface. At the same time, the initially Cu poor absorber took up Cu from the cap, ending up with a stoichiometric Cu content. Devices made from capped precursors or precursors annealed without sulfur had poor device characteristics. An increased doping density of almost one order of magnitude could be the reason for the very poor performance. CuS is therefore not a suitable cap material for CZTS. Other cap materials could be investigated to protect the CZTS absorber surface during annealing.
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| 27. |
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| 28. |
- Larsen, Jes K, et al.
(författare)
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Sulfurization of Co-Evaporated Cu(In,Ga)Se-2 as a Postdeposition Treatment
- 2018
-
Ingår i: IEEE Journal of Photovoltaics. - : IEEE-INST ELECTRICAL ELECTRONICS ENGINEERS INC. - 2156-3381 .- 2156-3403. ; 8:2, s. 604-610
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Tidskriftsartikel (refereegranskat)abstract
- It is investigated if the performance of Cu(In,Ga)Se-2 (CIGSe) solar cells produced by co-evaporation can be improved by surface sulfurization in a postdeposition treatment. The expected benefit would be the formation of a sulfur/selenium gradient resulting in reduced interface recombination and increased open-circuit voltage. In the conditions used here it was, however, found that the reaction of the CIGSe layer in a sulfur environment results in the formation of a CuInS2 (CIS) surface phase containing no or very little selenium and gallium. At the same time, a significant pile up of gallium was observed at the CIGSe/CIS boundary. This surface structure was formed for a wide range of annealing conditions investigated in this paper. Increasing the temperature or extending the time of the dwell stage had a similar effect on the material. The gallium enrichment and CIS surface layer widens the surface bandgap and therefore increases the open-circuit voltage. At the same time, the fill factor is reduced, since the interface layer acts as an electron barrier. Due to the balance of these effects, the conversion efficiency could not be improved.
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| 29. |
- Larsen, Jes K, et al.
(författare)
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Surface modification through air annealing Cu2ZnSn(S,Se)4 absorbers
- 2017
-
Ingår i: Thin Solid Films. - : Elsevier BV. - 0040-6090 .- 1879-2731. ; 633, s. 118-121
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Tidskriftsartikel (refereegranskat)abstract
- Recent studies demonstrate that air annealing can have a positive effect on the device performance of Cu2ZnSn(SxSe1-x)(4)[CZTSSe] solar cells. In this work air annealing of the selenium containing CZTSSe is compared to the pure sulfide CZTS. It is discovered that the selenium containing absorbers benefit from air annealing at higher temperatures than selenium free absorbers. The highest efficiency obtained utilizing the air annealing treatment on selenium containing absorbers is 9.7%. We find that the band gap is narrowed when air annealing, which is partially explained by increased Cu-Zn disorder. Furthermore Zn enrichment of the surface after etching is identified as a possible cause of enhanced device performance. It is additionally observed that elemental selenium present on the CZTSSe surface is reduced in the air annealing treatment. Selenium removal is another possible explanation for the enhanced performance caused by the air annealing treatment.
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| 30. |
- Li, Shu-Yi, et al.
(författare)
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Optical properties of Cu2ZnSn(SxSe1-x)(4) solar absorbers : Spectroscopic ellipsometry and ab initio calculations
- 2017
-
Ingår i: Applied Physics Letters. - : AIP Publishing. - 0003-6951 .- 1077-3118. ; 110:2
-
Tidskriftsartikel (refereegranskat)abstract
- Dielectric functions of Cu2ZnSn(SxSe1-x)(4) thin film absorbers with varied x were determined by spectroscopic ellipsometry and ab initio calculations. From the combination of experimental and theoretical studies, the fundamental interband transition energy E-0 (similar to 1-1.5 eV) and the next following transition energy E-1 (similar to 2-3 eV) were identified and found to blue-shift with increasing sulfur anion content, while keeping the energy separation E-1 - E-0 almost constant, similar to 1.4 eV from experiments, and 1 eV from theory. In addition, the average dielectric responses were found to decrease with sulfur anion content from both theoretical and experimental results. The Tauc optical bandgap value E-g determined on samples prepared on Mo and soda lime glass substrate showed a positive linear relationship between x and bandgap E-g. The bandgap bowing factor determined from the theoretical data is 0.09 eV. (C) 2017 Author(s).
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| 31. |
- Li, Shu-Yi, et al.
(författare)
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Optical properties of reactively sputtered Cu2ZnSnS4 solar absorbers determined by spectroscopic ellipsometry and spectrophotometry
- 2016
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Ingår i: Solar Energy Materials and Solar Cells. - : Elsevier BV. - 0927-0248 .- 1879-3398. ; 149, s. 170-178
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Tidskriftsartikel (refereegranskat)abstract
- We have determined for the first time the device-relevant optical constants of 500 nm and 800 nm-thick Cu2ZnSnS4 absorbers, grown on bare and Mo-coated soda-lime glass (SLG), using spectroscopic ellipsometry (SE). The composition, structure, phase purity and morphology were characterized by X-ray fluorescence, X-ray photoelectron spectroscopy depth profiling, X-ray diffraction, Raman spectroscopy, scanning-electron microscopy and atomic force microscopy. For the SE analysis, carefully determined sample characteristics were utilized to build a multilayer stack optical model, in order to derive the dielectric functions and refractive indices. The SE-derived absorption coefficients from CZTS/SLG samples were compared with those derived from complementary spectrophotometry measurements and found to be in good agreement. The bandgap determined from Tauc plots was E-g=1.57 +/- 0.02 eV. The absorption coefficients just above the bandgap were found to be a few 10(4) cm(-1) and to exceed 10(5) cm(-1) at energies above similar to 2.5 eV, which is much higher than previously found. The sub-bandgap k-value was found to be k similar to 0.05 or less, suggesting that a moderate band tail is present. Separate device characterization performed on identical samples allowed us to assign device efficiencies of, respectively, 2.8% and 5.3% to the 500 nm and 800 nm-thick samples featured in this study.
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| 32. |
- Platzer Björkman, Charlotte, 1976-, et al.
(författare)
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Ultrathin wide band gap kesterites
- 2022
-
Ingår i: Faraday discussions. - : Royal Society of Chemistry. - 1359-6640 .- 1364-5498. ; 239, s. 38-50
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Tidskriftsartikel (refereegranskat)abstract
- Kesterite Cu2ZnSnS4 (CZTS), used for thin film solar cells, has a band gap energy around 1.5–1.6 eV with possibilities for further increase through alloying. In some applications for wide band gap solar cells, reduced absorber thickness can be beneficial, to allow partial light transmission. Reduced thickness can also be beneficial to reduce bulk recombination, and so called ultrathin solar cells (<700 nm thick) have been studied for several materials systems. Here, we report performance for CZTS devices down to 250 nm thickness and show that performance loss from thickness reduction is relatively small, partly due to short minority carrier diffusion length. Insertion of thin passivation layers (Al2O3, SiO2 or HfO2) at the Mo/CZTS interface gives improved performance of ultrathin devices, from 4.7% to 5.6% efficiency for best performing cells having 250 nm thick CZTS with Mo as compared to Mo/Al2O3 back contact. The approach of NaF post deposition for making isolating passivation layers conductive is tested for the first time for CZTS and is shown to work. For fabrication of CZTS devices on transparent ITO back contact, the insertion of passivation layers can reduce diffusion of indium into CZTS, but device performance is lower than on Mo back contacts.
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| 33. |
- Regesch, David, et al.
(författare)
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Degradation and passivation of CuInSe2
- 2012
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Ingår i: Applied Physics Letters. - : AIP Publishing. - 0003-6951 .- 1077-3118. ; 101:11, s. 112108-
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Tidskriftsartikel (refereegranskat)abstract
- The degradation of CuInSe2 absorbers in ambient air is observed by the decay of the quasi-Fermi level splitting under well defined illumination with time. The decay is faster and stronger in absorbers with [Cu]/[In]<1 than in ones with a higher ratio. It can be attributed to the oxidation of the sample. Epitaxial films containing no Na show very similar trends, indicating that decay and oxidation are independent of the Na content. A standard CdS layer commonly used as buffer in solar cells, terminates the decay even over many months. Aged absorbers can be completely restored by a KCNetch.
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| 34. |
- Ren, Yi, et al.
(författare)
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Evolution of Cu2ZnSnS4 during Non-Equilibrium Annealing with Quasi-in Situ Monitoring of Sulfur Partial Pressure
- 2017
-
Ingår i: Chemistry of Materials. - : American Chemical Society (ACS). - 0897-4756 .- 1520-5002. ; 29:8, s. 3713-3722
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Tidskriftsartikel (refereegranskat)abstract
- Chalcogen-based materials like Cu2ZnSnS4 (CZTS) have attracted extensive attention for applications such as photovoltaics and water splitting. However, an inability to monitor the sulfur partial pressure (PS2) during the non-equilibrium annealing process at high temperatures complicates the synthesis of CZTS with controlled optoelectronic properties. Here we demonstrate that PS2 can be monitored by investigating the Sn–S phase transformation. We showed that PS2 drops considerably over the annealing time, causing gradual alterations in CZTS: (i) a change in defect type and (ii) evolution of ZnS and SnxSy phases. With additional ordering treatment, we observed that the low room-temperature photoluminescence energy usually seen in CZTS can result from insufficient PS2 during annealing. It is proven that remarkable Voc beyond 700 mV for solar cells with nonoptimal CdS buffer can be repeatedly achieved when CZTS is prepared under a sufficiently high PS2. An ordering treatment before CdS deposition can further improve Voc to 783 mV.
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| 35. |
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| 36. |
- Ross, N., et al.
(författare)
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Cu2ZnSn(S,Se)4 Solar Cell Absorbers from Diffusion of Selenium into Annealed Cu2ZnSnS4 Absorbers
- 2016
-
Ingår i: 2016 IEEE 43rd Photovoltaic Specialists Conference (PVSC). - New York : IEEE. - 9781509027248 ; , s. 492-497
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Konferensbidrag (refereegranskat)abstract
- Small grain compound-sputtered Copper Zinc Tin Sulfide (CZTS) precursors and large grain recrystallized CZTS solar cell absorbers are annealed at sub-recrystallization temperatures in selenium atmosphere to promote selenium diffusion into the CZTS films. Grazing incidence x-ray diffraction and Raman spectroscopy show the presence of two distinct sulfide and mixed sulfide/selenide phases, with the selenide-like phase more prominent at the front surface of the absorber. Only a narrow range of sulfur-to-selenium ratios is observed in the sulfide/selenide phase. Secondary ion mass spectrometry profiles show a compositional sulfur-selenium gradient for samples annealed at 450 degrees C. Scanning electron microscopy with energy dispersive X-ray spectroscopy reveals that the compositional gradient is formed by a spatial distribution of sulfide/selenide grains, embedded in the remaining sulfide absorber. Sulfur-selenium gradients within single absorber grains are not observed, indicating that the mixed sulfide/selenide phase nucleates and grows in competition with the existing sulfide phase, rather than forming by replacement of anions within existing crystals. IV and EQE measurements of devices fabricated from the selenized absorbers support this conclusion.
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| 37. |
- Saini, Nishant, 1989-
(författare)
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Band gap engineering in Cu2ZnGexSn1-xS4 thin film solar cells
- 2021
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Doktorsavhandling (övrigt vetenskapligt/konstnärligt)abstract
- Photovoltaics based on kesterite Cu2ZnSnS4 (CZTS) has attracted interest as a sustainable alternative to other thin film technologies due to their tunable material properties and earth-abundance. However, the efficiency is limited to 12.6 % for selenium-containing CZTS, with a large voltage-deficit due to tail states, deep defects, secondary phases etc.In this thesis, Ge incorporation in kesterite solar cells was utilized to reduce Sn-related deep defects and tune the band gap. CZTS, Cu2ZnGeS4 (CZGS), and mixed Cu2ZnGexSn1-xS4 (CZGTS) films, and solar cells were investigated. CZGS showed wurtzite-like phases when sputter-deposited in a sulfur-rich atmosphere using GeS target. The mixture of phases could not recrystallize to kesterite phase after annealing, so, metallic Ge target was utilized for CZGTS absorber fabrication. From varying the Ge content in CZGTS, it could be concluded that CZTS grain growth increased at a low concentration of Ge.Sulfurization of CZTS layers deposited on top of CZGS was done to achieve band gap grading. Glow Discharge Optical Emission Spectroscopy showed smooth grading while Scanning Tunneling Microscopy/Energy Dispersive Spectroscopy showed a separation between larger Sn-rich grains at the front and smaller Ge-rich grains at the back. For longer annealing times, recrystallization of the complete film was seen together with a smeared-out grading.Germanium-rich absorbers often delaminated during the etching of the annealed samples. Adhesive TiN interlayer was used on the Mo-coated Soda-Lime Glass substrate to avoid delamination with partial success. Ge-containing samples showed oxygen-rich grain boundaries and voids. Oxide removal during etching has a possible connection to the issues with adhesion.Solar cell performance was not improved for graded absorbers compared to CZTS. An increasing cliff-like band alignment with the buffer layer could negate the benefit of a band gap gradient since Ge diffused to the front. CZGS solar cells with alternative buffer layer were fabricated to investigate front interface improvement. Zn1-xSnxOy (ZTO) buffer layers were deposited at various temperatures and thicknesses. The open-circuit voltage (Voc) increased to 1.1 V for CZGS/ZTO solar cells; however, Voc was relatively insensitive to ZTO band gap variations. The current was generally low but improved with KCN-etching of the CZGS absorber before deposition of the ZTO buffer layer. A possible explanation for the device behavior is the presence of an oxide interlayer for non-etched devices.The backside interface recombination can be reduced using band gap grading as well with passivation layers. Therefore, ultrathin CZTS with oxide passivation layers of AlxOy or SiOx on the back contact was investigated. The solar cell parameters improved with the addition of a thin oxide layer, but blocking behavior increased with passivation layer thickness.
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| 38. |
- Saini, Nishant, et al.
(författare)
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Bandgap engineered Cu2ZnGexSn1−xS4 solar cells using an adhesive TiN back contact layer
- 2021
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Ingår i: Journal of Alloys and Compounds. - : Elsevier. - 0925-8388 .- 1873-4669. ; 880
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Tidskriftsartikel (refereegranskat)abstract
- Kesterite-based solar cells are mainly restricted by their lower than expected open-circuit voltage (Voc) due to non-radiative recombination. Therefore, an approach to reduce bulk and interface recombination through band gap grading to induce a back surface field is attempted. This contribution presents the challenges in the formation of compositional grading of the wide bandgap material Cu2ZnGexSn1−xS4 (CZGTS) and successful fabrication of solar cells with an additional adhesive TiN interlayer. It is observed that the TiN interlayer improves adhesion between CZGTS and the back contact. The microstructure of the Cu2ZnSnS4 (CZTS) film is significantly affected by the concentration of Ge, and the existence of a Ge concentration gradient is strongly correlated to the formation of smaller Ge-rich and larger Sn-rich grains. The bandgap grading is exploited with a moderate Ge concentration of up to (Ge/(Ge+Sn) = 0.25) in CZTS. As the Ge profile stretched all the way to the front interface, the cliff-like band alignment at the front interface of the absorber could negate the beneficial effect of Ge inclusion in the bulk and back interface of the absorber. Ordering the absorber can introduce an additional downward shift in the valence band. In one of the samples, the increased ordering and high concentration of Ge in CZTS are suggested to enhance the hole barrier at the back interface. It is concluded that the effect of the bandgap grading with Ge can only be realized with optimization of interface band alignment and back contact formation.
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| 39. |
- 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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| 40. |
- Saini, Nishant, et al.
(författare)
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Record 1.1 V Open-Circuit Voltage for Cu2ZnGeS4-Based Thin-Film Solar Cells Using Atomic Layer Deposition Zn1-xSnxOy Buffer Layers
- 2022
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Ingår i: Solar RRL. - : John Wiley & Sons. - 2367-198X. ; 6:2
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Tidskriftsartikel (refereegranskat)abstract
- The Cu2ZnGe X Sn1-X S4 (CZGTS) thin-film solar cells have a limited open-circuit voltage (V OC) due to bulk and interface recombination. Since the standard CdS buffer layer gives a significant cliff-like conduction band offset to CZGTS, alternative buffer layers are needed to reduce the interface recombination. This work compares the performance of wide bandgap Cu2ZnGeS4 (CZGS) solar cells fabricated with nontoxic Zn x Sn1–x O y (ZTO) buffer layers grown by atomic layer deposition under different conditions. The V OC of the CZGS solar cell improved significantly to over 1 V by substituting CdS with ZTO. However, V OC is relatively insensitive to ZTO bandgap variations. The short-circuit current is generally low but is improved with KCN etching of the CZGS absorber before deposition of the ZTO buffer layer. A possible explanation for the device behavior is the presence of an oxide interlayer for nonetched devices.
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| 41. |
- Sopiha, Kostiantyn, et al.
(författare)
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Off-stoichiometry in I-III-VI2 chalcopyrite absorbers : a comparative analysis of structures and stabilities
- 2022
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Ingår i: Faraday discussions. - : Royal Society of Chemistry (RSC). - 1359-6640 .- 1364-5498. ; 239:0, s. 357-374
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Tidskriftsartikel (refereegranskat)abstract
- Chalcopyrite Cu(In,Ga)Se-2 (CIGSe) solar absorbers are renowned for delivering high solar power conversion efficiency despite containing high concentration of lattice defects amounting to copper deficiencies of several atomic percent. The unique ability to incorporate this deficiency without triggering decomposition (i.e. "tolerance to off-stoichiometry") is viewed by many as the key feature of CIGSe. In principle, this property could benefit any solar absorber, but remarkably little attention has been paid to it so far. In this study, we assess the tolerance to off-stoichiometry of thin-film photovoltaic materials by carrying out ab initio analysis of group-I-poor ordered defect compounds (ODCs) in the extended family of I-III-VI systems (where I = Cu, Ag, III = Al, Ga, In, and VI = S, Se, Te). We analyze convex hulls and structural evolution with respect to group-I content, link them with experimental phase diagrams, and determine two empirical principles for the future identification of solar energy materials with high tolerance to off-stoichiometry. Practical implications for the deposition of I-III-VI absorbers are also discussed in light of our computational results and recent experimental findings.
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| 42. |
- Sopiha, Kostiantyn, et al.
(författare)
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Thermodynamic stability, phase separation and Ag grading in (Ag,Cu)(In,Ga)Se-2 solar absorbers
- 2020
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Ingår i: Journal of Materials Chemistry A. - : Royal Society of Chemistry. - 2050-7488 .- 2050-7496. ; 8:17, s. 8740-8751
-
Tidskriftsartikel (refereegranskat)abstract
- Gallium alloying and grading in Cu(In,Ga)Se-2 (CIGS) are well-established strategies for improving performance of thin-film solar cells by tailoring band profiles within the absorber. Similarly, Ag incorporation is considered to be an effective complementary route towards further advancement of the field. Herein, we explore thermodynamics of the formation of (Ag,Cu)(In,Ga)Se-2 (ACIGS) alloy. Using first-principles methods, we reveal the existence of a miscibility gap in the Ga-rich alloys at temperatures close to those employed for the co-evaporation growth. We demonstrate that this property can result in phase separation and the formation of Ag gradients throughout the film thickness. We prove experimentally that the phase separation can indeed occur during low-temperature growth and/or post-deposition treatments. Furthermore, we uncover the anticorrelation between Ag and Ga contents, and demonstrate thermodynamically-driven formation of [Ag]/([Ag] + [Cu]) gradients in films with a steep [Ga]/([Ga] + [In]) profile. Finally, we discuss how these phenomena can influence solar cell devices. The presented results are expected to provide fundamental insight into the physics of growth and processing of ACIGS absorbers, which could be utilized to further boost the efficiency of thin-film solar cells.
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| 43. |
- Steichen, Marc, et al.
(författare)
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Preparation of CuGaSe 2 absorber layers for thin film solar cells by annealing of efficiently electrodeposited Cu–Ga precursor layers from ionic liquids
- 2011
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Ingår i: Thin Solid Films. - : Elsevier BV. - 0040-6090 .- 1879-2731. ; 519:21, s. 7254-7258
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Tidskriftsartikel (refereegranskat)abstract
- CuGaSe2 absorber layers were prepared on molybdenum substrates by electrochemical codeposition of copper and gallium and subsequential annealing in selenium vapour. The electrodeposition was made from a deep eutectic based ionic liquid consisting of choline chloride/urea (Reline) with a plating efficiency of over 85%. The precursor film composition is controlled by the ratio of the copper to gallium fluxes under hydrodynamic conditions and by the applied deposition potential. X-ray diffraction reveals CuGa2 alloying during the electrodeposition and CuGaSe2 formation after annealing. Photoluminescence (PL) and photocurrent spectroscopy revealed the good opto-electronic properties of the CuGaSe2 absorber films. The absorber layers have been converted to full devices with the best device achieving 4.0 % solar conversion efficiency.
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| 44. |
- Suvanam, Sethu Saveda, et al.
(författare)
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Extreme radiation hard thin film CZTSSe solar cell
- 2018
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Ingår i: Solar Energy Materials and Solar Cells. - : Elsevier. - 0927-0248 .- 1879-3398. ; 185, s. 16-20
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Tidskriftsartikel (refereegranskat)abstract
- In this work, we have demonstrated the extreme radiation hardness of thin film CZTSSe solar cells. Thin film solar cells with CZTSSe, CZTS and CIGS absorber layers were irradiated with 3 MeV protons. No degradation in device parameters was observed until a displacement damage dose of 2 × 1010 MeV/g for CZTS and CZTSSe. CIGS solar cells degraded by 13% at the same dose. For the highest proton dose both the CZTSSe and CZTS degraded by 16% while CIGS suffered from 34% degradation in efficiency. The degradation in efficiency maybe attributed to the reduction in the minority carrier lifetime due to radiation induced lattice defects. Comparisons with previously available literature show that our CZTS technology has superior radiation hardness by about two orders of magnitude compared to existing state of the art Si and GaAs technology.
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| 45. |
- Xin, Peipei, et al.
(författare)
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Development of Cu(In,Ga)Se-2 superstrate devices with alternative buffer layers
- 2016
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Ingår i: Solar Energy Materials and Solar Cells. - : Elsevier BV. - 0927-0248 .- 1879-3398. ; 157, s. 85-92
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Tidskriftsartikel (refereegranskat)abstract
- Superstrate Cu(ln,Ga)Se-2 (CIGS) solar cells are prepared in the structure SLG/TCO/buffer/CIGS/Au with CIGS deposited onto the buffer layer by a single stage co-evaporation process. Three buffer materials - CdS deposited by chemical surface deposition, ZnSe and ZnO by RF magnetron sputtering - are tested in the superstrate structure. The best cell achieves 8.6% efficiency with the ZnO buffer after light soaking and forward bias treatments. The efficiency of devices with CdS or ZnSe buffers are less than 4%. The junction formation between the absorber and buffer layers are investigated by x-ray photoelectron spectroscopy, scanning electron microscopy and transmission electron microscopy. For CdS/CIGS and ZnSe/CIGS superstrate devices extensive inter-diffusion between the absorber and buffer layer under CIGS growth condition is the critical problem. For ZnO/CIGS superstrate cells GaxOy formation at the junction interface and unfavorable conduction band alignment are the main factors that limit the device performance.
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