| 1. |
- Aboulfadl, Hisham, et al.
(författare)
-
Microstructural Characterization of Sulfurization Effects in Cu(In,Ga)Se2 Thin Film Solar Cells
- 2019
-
Ingår i: Microscopy and Microanalysis. - : CAMBRIDGE UNIV PRESS. - 1435-8115 .- 1431-9276. ; 25:2, s. 532-538
-
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.
|
|
| 2. |
- Assar, Alireza, et al.
(författare)
-
Gettering in PolySi/SiOx Passivating Contacts Enables Si-Based Tandem Solar Cells with High Thermal and Contamination Resilience
- 2022
-
Ingår i: ACS Applied Materials and Interfaces. - : American Chemical Society (ACS). - 1944-8244 .- 1944-8252. ; 14:12, s. 14342-14358
-
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.
|
|
| 3. |
- Babucci, Melike, et al.
(författare)
-
Depth-Dependent Atomic-Scale Structural Changes in (Ag,Cu)(In,Ga)Se2 Absorbers Relevant for Thin-Film Solar Cells
- 2023
-
Ingår i: ACS Applied Energy Materials. - : American Chemical Society (ACS). - 2574-0962. ; 6:18, s. 9264-9275
-
Tidskriftsartikel (refereegranskat)abstract
- Alloying a Cu(In,Ga)Se-2 (CIGS) solar cell absorber with silver to form (Ag,Cu)(In,Ga)Se-2 (ACIGS) is an effective route for improving the performance of CIGS-based thin-film solar cells by increasing the optical band gap and open-circuit voltage. While the role of Ag on the solar cell's performance and crystal structure has been analyzed, important gaps in our understanding remain, especially regarding the atomistic (short-range) structure. Previous X-ray absorption spectroscopy (XAS) results have shown that local atomic arrangements in Ag-free CIGS deviate from the long-range crystallographic structure deduced from X-ray diffraction (XRD). However, it is unclear how these structural deviations evolve with Ag alloying, particularly in the presence of Ga depth gradient. In this work, we employ angle-resolved XAS to probe the local environment of Se atoms within different depths of ACIGS absorbers with varying Ag content and Ga depth gradient. By complementing XAS results with X-ray diffraction measurements for long-range structures, glow discharge optical emission spectroscopy for elemental profiles, and scanning transmission electron microscopy for morphologies, changes in element-specific bond lengths, cell parameters, and anion displacement depending on compositions of Group [I] (Cu, Ag) and Group [III] (In, Ga) elements were mapped. The results suggest that the local atomic arrangement of the investigated ACIGS thin-film solar cell samples is depth-dependent and deviates from the long-range crystallographic structure. Possible reasons include tetragonal distortion or the presence of other phases or off-stoichiometry compounds. For the sample with the highest Ag content, increased bond lengths of Se-Group [I] atoms and Se-Ga are observed from the absorber bulk toward the near-absorber/buffer interface, whereas, in Ag-free CIGS, no significant changes are found. Results further indicate nonlinear anion displacement with Ag addition in the absorber bulk or with depth composition variation, which is likely to affect the electronic properties of solar cells. These findings offer a better understanding of the atomic-scale properties of ACIGS absorbers in actual thin-film solar cells containing in-depth composition variations.
|
|
| 4. |
- Bilousov, Oleksandr V., et al.
(författare)
-
ALD of phase controlled tin monosulfide thin films
- 2017
-
Konferensbidrag (refereegranskat)abstract
- Tin monosulfide (SnS) is a promising semiconductor material for low-cost conversion of solar energy, playing the role of absorber layer in photovoltaic devices. SnS is, due to its high optical damping, also an excellent semiconductor candidate for the realization of ultrathin (nanoscale thickness) plasmonic solar cells [1].Here, we present an important step to further control and understand SnS film properties produced using low temperature ALD with Sn(acac)2 and H2S as precursors. We show that the SnS film properties vary over a rather wide range depending on substrate temperature and reaction conditions, and that this is connected to the growth of cubic (π-SnS) and orthorhombic SnS phases. The optical properties of the two polymorphs differ significantly, as demonstrated by spectroscopic ellipsometry [2].1. C. Hägglund, G. Zeltzer, R. Ruiz, A. Wangperawong, K. E. Roelofs, S. F. Bent, ACS Photonics 3 (3) (2016) 456–463.2. O. V. Bilousov, Y. Ren, T. Törndahl, O. Donzel-Gargand , T. Ericson, C. Platzer-Björkman, M. Edoff, and C. Hägglund, ACS Chemistry of Materials 29 (7) (2017) 2969–2978.
|
|
| 5. |
- Bilousov, Oleksandr V., et al.
(författare)
-
Atomic Layer Deposition of Cubic and Orthorhombic Phase Tin Monosulfide
- 2017
-
Ingår i: Chemistry of Materials. - : AMER CHEMICAL SOC. - 0897-4756 .- 1520-5002. ; 29:7, s. 2969-2978
-
Tidskriftsartikel (refereegranskat)abstract
- Tin monosulfide (SnS) is a promising light-absorbing material with weak environmental constraints for application in thin film solar cells. In this paper, we present low-temperature atomic layer deposition (ALD) of high-purity SnS of both cubic and orthorhombic phases. Using tin(II) 2,4-pentanedionate [Sn(acac)(2)] and hydrogen sulfide (H2S) as precursors, controlled growth of the two polymorphs is achieved. Quartz crystal microbalance measurements are used to establish saturated conditions and show that the SnS ALD is self-limiting over temperatures from at least 80 to 160 degrees C. In this temperature window, a stable mass gain of 19 ng cm(-2) cycle(-1) is observed. The SnS thin film crystal structure and morphology undergo significant changes depending on the conditions. High-resolution transmission electron microscopy and X-ray diffraction demonstrate that fully saturated growth requires a large H2S dose and results in the cubic phase. Smaller H2S doses and higher temperatures favor the orthorhombic phase. The optical properties of the two polymorphs differ significantly, as demonstrated by spectroscopic ellipsometry. The orthorhombic phase displays a wide (0.3-0.4 eV) Urbach tail in the near-infrared region, ascribed to its nanoscale structural disorder and/or to sulfur vacancy-induced gap states. In contrast, the cubic phase is smooth and void-free and shows a well-defined, direct forbidden-type bandgap of 1.64 eV.
|
|
| 6. |
- Bras, Patrice, et al.
(författare)
-
Ga-grading and Solar Cell Capacitance Simulation of an industrial Cu(In,Ga)Se2 solar cell produced by an in-line vacuum, all-sputtering process
- 2017
-
Ingår i: Thin Solid Films. - : Elsevier BV. - 0040-6090 .- 1879-2731. ; 636, s. 367-374
-
Tidskriftsartikel (refereegranskat)abstract
- Cadmium-free Cu(In,Ga)Se-2 (CIGS) solar cells are fabricated on stainless steel substrate using an industrial, inline vacuum, all sputtering process. The absorber layer is deposited from compound CIGS targets and crystallized simultaneously by high temperature processing. In-depth compositional and structural characterization of the chalcopyrite material is conducted and a Solar Cell Capacitance Simulator (SCAPS) model for the complete device is set-up. Ga-grading of the absorber through the successive use of different CIGS target compositions and resulting in solar cell performance enhancement is shown. At the research and development scale, efficiency values of 15.1% and 13.2% are reported for 1 cm(2) and 225 cm(2) total area solar cells, respectively. Successful transfer to production is also demonstrated. A series of a hundred 225 cm(2) solar cells produced following an optimized process including the Ga grading studied in the present contribution average at 14.8% total area efficiency.
|
|
| 7. |
- Englund, Sven, 1987-
(författare)
-
Alternative back contacts for CZTS thin film solar cells
- 2020
-
Doktorsavhandling (övrigt vetenskapligt/konstnärligt)abstract
- In this thesis, alternative back contacts for Cu2ZnSnS4 (CZTS) thin film solar cells were investigated. Back contacts for two different configurations were studied, namely traditional single-junction cells with opaque back contacts and transparent back contacts for possible use in either tandem or bifacial solar cell configuration.CZTS is processed under chemically challenging conditions, such as high temperature and high chalcogen partial pressure. This places great demands on the back contact. Mo is the standard choice as back contact, but reacts with chalcogens to form MoS(e)2 while the CZTS decomposes, mainly into detrimental secondary phases. Thin MoS(e)2 is assumed to be beneficial for the electrical contact, but excessive thickness is detrimental to solar cell performance. The back contact acts as diffusion medium for Na during annealing when soda-lime glass is used as substrate. Na influences both defect passivation and doping in CZTS and increases the efficiency of the solar cells. The ability of the back contact to facilitate Na diffusion is an important property that must be monitored.Titanium nitride (TiN) as an interlayer between the opaque molybdenum (Mo) and CZTS as well as complete replacement of Mo with TiN back contacts were investigated. TiN was found to be chemically stable in typical anneal conditions. Formation of MoS(e)2 was observed only in areas where the TiN interlayers did not fully cover the Mo, following from the surface roughness of Mo and insufficient step-coverage of the sputter-deposition of TiN. Thick TiN interlayers (200 nm) were found to increase the diffusion of Na to the absorber layer from the glass substrate. For precursors annealed in sulfur atmosphere, improved device efficiency was observed for increased TiN thickness.Transparent back contacts can be used in either tandem configurations where two or more absorber materials are used to more efficiently use different parts of the solar spectra, or in bifacial solar cells to allow light to reach the absorber layer from two sides and thus increase the photocurrent. Thus far only a few studies have investigated transparent back contact materials in CZTS solar cell devices. Antimony-doped tin oxide (ATO) was studied as a transparent back contact for CZTS. Annealing of bare ATO resulted in complete reaction with S to form Sn–S compounds. When annealed below the CZTS, ATO was found to be stable at low temperature (<550 °C), and in some aspects even improved its properties. ATO back contacts resulted in significantly increased formation of Sn–S secondary phases on the CZTS absorber surface compared to the Mo reference. Sn–S secondary compounds on the absorber surface made it challenging to obtain good device performance. Adhesion and device behavior could be improved by pre-addition of NaF on the precursor prior to annealing.
|
|
| 8. |
- Englund, Sven, et al.
(författare)
-
Antimony-Doped Tin Oxide as Transparent Back Contact in Cu2ZnSnS4 Thin-Film Solar Cells
- 2019
-
Ingår i: Physica Status Solidi (a) applications and materials science. - : Wiley. - 1862-6300 .- 1862-6319. ; 216:22
-
Tidskriftsartikel (refereegranskat)abstract
- Antimony-doped tin oxide (Sn2O3:Sb, ATO) is investigated as a transparent back contact for Cu2ZnSnS4 (CZTS) thin-film solar cells. The stability of the ATO under different anneal conditions and the effect from ATO on CZTS absorber growth are studied. It is found that ATO directly exposed to sulfurizing anneal atmosphere reacts with S, but when covered by CZTS, it does not deteriorate when annealed at T < 550 degrees C. The electrical properties of ATO are even found to improve when CZTS is annealed at T = 534 degrees C. At T = 580 degrees C, it is found that ATO reacts with S and degrades. Analysis shows repeatedly that ATO affects the absorber growth as large amounts of Sn-S secondary compounds are found on the absorber surfaces. Time-resolved anneal series show that these compounds form early during anneal and evaporate with time to leave pinholes behind. Device performance can be improved by addition of Na prior to annealing. The best CZTS device on ATO back contact herein has an efficiency of 2.6%. As compared with a reference on a Mo back contact, a similar open-circuit voltage and short-circuit current density are achieved, but a lower fill factor is measured.
|
|
| 9. |
- Englund, Sven, et al.
(författare)
-
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
-
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.
|
|
| 10. |
- Englund, Sven, et al.
(författare)
-
Cu2ZnSn(S,Se)4 from annealing of compound co-sputtered precursors : Recent results and open questions
- 2018
-
Ingår i: Solar Energy. - : Elsevier BV. - 0038-092X .- 1471-1257. ; 175, s. 84-93
-
Tidskriftsartikel (refereegranskat)abstract
- Cu2ZnSn(S,Se)(4) (CZTS) thin film solar cells have reached efficiencies of up to 12.6% and current research is focused on understanding reasons for device limitations. At Uppsala University, a CZTS synthesis route based on compound sputtering and annealing in elemental vapors is used. Variation of chemical composition and annealing conditions is used as a tool to try to understand defect-related material- and device properties. Front and back contacts are also studied with focus on energy band matching at the hetero-interface using atomic layer deposition buffer layers and chemical stability of the back contact. In this review, we discuss recent work from our group, with reference to related work in the literature and with regards to areas for future work.
|
|
| 11. |
- Englund, Sven, et al.
(författare)
-
TiN Interlayers with Varied Thickness in Cu2ZnSnS(e)(4) Thin Film Solar Cells : Effect on Na Diffusion, Back Contact Stability, and Performance
- 2018
-
Ingår i: Physica Status Solidi (a) applications and materials science. - : Wiley. - 1862-6300 .- 1862-6319. ; 215:23
-
Tidskriftsartikel (refereegranskat)abstract
- In this study, interlayers with varied thickness of TiN between Cu2ZnSnS(e)(4) (CZTS(e)) absorbers and Mo on soda-lime glass substrates are investigated for CZTS(e) thin film solar cells. Na diffusion is analyzed using Secondary Ion Mass Spectrometry and it is found that the use of thick TiN interlayers facilitates Na diffusion into the absorbers. The CZTS(e)/TiN/Mo interfaces are scrutinized using Transmission Electron Microscopy (TEM) Electron Energy Loss Spectroscopy (EELS). It is found that diffusion of chalcogens present in the precursor occurs through openings, resulting from surface roughness in the Mo, in the otherwise chemically stable TiN interlayers, forming point contacts of MoS(e)(2). It is further established that both chalcogens and Mo diffuse along the TiN interlayer grain boundaries. Solar cell performance for sulfur-annealed samples improved with increased thickness of TiN, and with a 200 nm TiN interlayer, the solar cell performance is comparable to a typical Mo reference. Pure TiN bulk contacts are investigated and shown to work, but the performance is still inferior to the TiN interlayer back contacts. The use of thick TiN interlayers offers a pathway to achieve high efficiency CZTS(e) solar cells on highly inert back contacts.
|
|
| 12. |
- Ericson, Tove, 1983-, et al.
(författare)
-
Zinc-Tin-Oxide Buffer Layer and Low Temperature Post Annealing Resulting in a 9.0% Efficient Cd-Free Cu2ZnSnS4 Solar Cell
- 2017
-
Ingår i: Solar RRL. - : Wiley. - 2367-198X. ; 1:5
-
Tidskriftsartikel (refereegranskat)abstract
- Zn1−xSnxOy (ZTO) has yielded promising results as a buffer material for the full sulfur Cu2ZnSnS4 (CZTS), with efficiencies continuously surpassing its CdS-references. ZTO can be deposited by atomic layer deposition (ALD), enabling tuning of the conduction band position through the choice of metal ratio or deposition temperature. Thus, an optimization of the conduction band alignment between ZTO and CZTS can be achieved. The ZTO bandgap is generally larger than that of CdS and can therefore yield higher currents due to reduced losses in the short wavelength region. Another advantage is the possibility to omit the toxic Cd. In this study, the ALD process temperature was varied from 105 to 165 °C. Current-blocked devices were obtained at 105 °C, while the highest open-circuit voltage and device efficiency was achieved for 145 °C. The highest fill factor was seen at 165 °C. The best efficiency reached in this study was 9.0%, which, to our knowledge, is the highest efficiency reported for Cd-free full-sulfur CZTS. We also show that the effect of heat needs to be taken into account. The results indicate that part of the device improvement comes from heating the absorber, but that the benefit of using a ZTO-buffer is clear.
|
|
| 13. |
- Frisk, Christopher, 1985-, et al.
(författare)
-
On the extraction of doping concentration from capacitance-voltage : A Cu2ZnSnS4 and ZnS sandwich structure
- 2017
-
Ingår i: IEEE Journal of Photovoltaics. - 2156-3381 .- 2156-3403. ; 7:5, s. 1421-1425
-
Tidskriftsartikel (refereegranskat)abstract
- The capacitance-voltage (C-V) method is frequently used to evaluate the net doping of thin-film solar cells, an important parameter for the function of solar cells. However, complex materials such as kesterites are challenging to characterize. To minimize ambiguity when determining the apparent doping concentration (N-A) of Cu2ZnSnS4 (CZTS), we fabricated and investigated different structures: CZTS/ZnS metal-insulator-semiconductor (MIS) device, stand-alone CZTS and ZnS metal-sandwich structures, and CZTS solar cells. Characterization was carried out by means of admittance spectroscopy (AS) and C-V measurements. ZnS exhibits excellent intrinsic properties, and with the high-quality MIS sample we managed to successfully isolate the capacitive response of the CZTS itself. N-A, as extracted from the MIS structure, is found to be more reliable and four times higher compared with the solar cell, impacting any estimated collection efficiency substantially. Data herein presented also show that CZTS has a substantial low-frequency dispersive capacitance and the extraction of N-A depends on the chosen measurement frequency, symptoms of presence of deep defects. Furthermore, the CZTS/ZnS MIS structure is strongly resilient to leakage currents at both forward and reverse voltage bias where contribution from deep defects is minimized and maximized, respectively.
|
|
| 14. |
- Grini, S., et al.
(författare)
-
Dynamic Impurity Redistributions in Kesterite Absorbers
- 2020
-
Ingår i: Physica status solidi. B, Basic research. - : Wiley. - 0370-1972 .- 1521-3951.
-
Tidskriftsartikel (refereegranskat)abstract
- Cu2ZnSn(S,Se)4 is a promising nontoxic earth-abundant solar cell absorber. To optimize the thin films for solar cell device performance, postdeposition treatments at temperatures below the crystallization temperature are normally performed, which alter the surface and bulk properties. The polycrystalline thin films contain relatively high concentrations of impurities, such as sodium, oxygen and hydrogen. During the treatments, these impurities migrate and likely agglomerate at lattice defects or interfaces. Herein, impurity redistribution after air annealing for temperatures up to 200 °C and short heavy water treatments are studied. In addition, nonuniformities of the sodium distribution on a nanometer and micrometer scale are characterized by atom probe tomography and secondary ion mass spectrometry, respectively. Sodium and oxygen correlate to a greater extent after heat treatments, supporting strong binding between the two impurities. Redistributions of these impurities occur even at room temperature over longer time periods. Heavy water treatments confirm out-diffusion of sodium with more incorporation of oxygen and hydrogen. It is observed that the increased hydrogen content does not originate from the heavy water. The existence of an “ice-like” layer on top of the Cu2ZnSnS4 layer is proposed.
|
|
| 15. |
- Grini, Sigbjörn, et al.
(författare)
-
Low temperature incorporation of selenium in Cu2ZnSnS4 : Diffusion and nucleation
- 2018
-
Ingår i: Thin Solid Films. - : ELSEVIER SCIENCE SA. - 0040-6090 .- 1879-2731. ; 665, s. 159-163
-
Tidskriftsartikel (refereegranskat)abstract
- Band gap grading of Cu2ZnSn(S, Se)(4) (CZTSSe) solar cells can be achieved by varying the S-r = [S]/([S]+[Se]) ratio in the absorber layer with depth. One approach is a two-step annealing process where the absorber is first sulfurized to Cu2ZnSnS4 (CZTS) followed by selenization to form CZTSSe. However, once nucleation of CZTSSe initiates, the rapid interchange of S and Se limits the control over the Sr ratio with depth. Here, we have studied incorporation of Se into CZTS and observed the behavior of Se below and up to the nucleation temperature of CZTSSe. Se diffusion at 337 and 360 degrees C is dominated by grain boundary diffusion while some increase of Se is also seen in the region from 100 to 800 nm from the surface. After selenization at 409 degrees C, recrystallization is observed and CZTSSe grains are formed. The recrystallization is more rapid for a smaller average grain size and is facilitated by diffusion of Na from the back contact. The grain boundary diffusion is identified with secondary ion mass spectrometry measurements by measuring the accumulation in the CZTS/Mo interface for three samples with different average grain size.
|
|
| 16. |
- Grini, Sigbjörn, et al.
(författare)
-
Secondary ion mass spectrometry as a tool to study selenium gradient in Cu2ZnSn(S,Se)4
- 2017
-
Ingår i: Physica Status Solidi. C, Current topics in solid state physics. - : Wiley. - 1610-1634 .- 1610-1642. ; :6
-
Tidskriftsartikel (refereegranskat)abstract
- Secondary ion mass spectrometry (SIMS) has been utilized to study compositional gradients in compound-sputtered and annealed Cu2ZnSn(S,Se)(4) (CZTSSe). SIMS image depth profiling shows a non-uniform spatial distribution of selenium and supports a mechanism where selenization is accompanied by grain growth rather than substitution of selenium for sulfur. Furthermore, SIMS depth profiles of S and Se using O-2(+) primary ions and detecting molecular ions of the MCs+ type using Cs+ primary ions have been compared, where a linear relationship between the sulfur and selenium concentration suitable for compositional analysis is observed for concentrations with an Se/(S+Se) ratio in the range from 0.25 to 0.65. 3D image of the spatial Se distribution in a 20 x 20 mu m(2) grid measured with SIMS image depth profiling.
|
|
| 17. |
- Grini, Sigbjörn, et al.
(författare)
-
Strong Interplay between Sodium and Oxygen in Kesterite Absorbers : Complex Formation, Incorporation, and Tailoring Depth Distributions
- 2019
-
Ingår i: Advanced Energy Materials. - : WILEY-V C H VERLAG GMBH. - 1614-6832 .- 1614-6840. ; 9:27
-
Tidskriftsartikel (refereegranskat)abstract
- Sodium and oxygen are prevalent impurities in kesterite solar cells. Both elements are known to strongly impact performance of the kesterite devices and can be connected to efficiency improvements seen after heat treatments. The sodium distribution in the kesterite absorber is commonly reported, whereas the oxygen distribution has received less attention. Here, a direct relationship between sodium and oxygen in kesterite absorbers is established using secondary ion mass spectrometry and explained by defect analyses within the density functional theory. The calculations reveal a binding energy of 0.76 eV between the substitutional defects Na-Cu and O-S in the nearest neighbor configuration, indicating an abundance of Na Symbol of the Klingon Empire O complexes in kesterite absorbers at relevant temperatures. Oxygen incorporation is studied by introducing isotopic O-18 at different stages of the Cu2ZnSnS4/Mo/soda-lime glass baseline processing. It is observed that oxygen from the Mo back contact and contaminations during the sulfurization are primary contributors to the oxygen distribution. Indeed, unintentional oxygen incorporation leads to immobilization of sodium. This results in a strong correlation between sodium and oxygen, in excellent agreement with the theoretical calculations. Consequently, oxygen availability should be monitored to optimize postdeposition heat treatments to control impurities in kesterite absorbers and ultimately, the solar cell efficiency.
|
|
| 18. |
|
|
| 19. |
- Keller, Jan, et al.
(författare)
-
Effect of Cu content on post‐sulfurization of Cu(In,Ga)Se2 films and corresponding solar cell performance
- 2019
-
Ingår i: Physica Status Solidi (a) applications and materials science. - : Wiley. - 1862-6300 .- 1862-6319. ; 216:20
-
Tidskriftsartikel (refereegranskat)abstract
- Herein, the effect of the initial copper content of co‐evaporated Cu(In1−x,Gax)Se2 (CIGS) absorber films on the impact of a post‐annealing step in elemental sulfur atmosphere is studied. The Cu concentration is varied over a wide range ([Cu]/[III] = CGI = 0.57–1.23), allowing to identify composition‐dependent trends in phase formation, chemical rearrangements, and solar cell performance after sulfurization. For all samples, a ternary CuInS2 layer forms at the surface. In addition, sulfur 1) is incorporated in randomly distributed CuIn(S,Se)2 mixed crystals underneath CuInS2; 2) diffuses into multidimensional defects (e.g., dislocations and grain boundaries); and 3) is bound in Na–In–S surface plates. It is found that Cu‐poor absorber composition (CGI ≤ 0.82) favors CuInS2 growth as compared with close‐stoichiometric CIGS films, driven by a faster diffusion of Cu toward the surface. For Cu‐rich absorbers (CGI > 1), Se—S exchange is significantly accelerated, presumably by the presence of Cu2−xSe phases reacting to Cu2−xS and eventually catalyzing CuInS2 formation. Finally, open‐circuit voltage (VOC), fill factor (FF), and efficiency (η) of corresponding solar cells increase after sulfurization with increasing CGI until stoichiometry is reached. The result is explained by a mitigated Cu depletion of the absorber bulk after sulfurization for close‐stoichiometric CIGS.
|
|
| 20. |
- Keller, Jan, et al.
(författare)
-
Heavy Alkali Treatment of Post‐Sulfurized Cu(In,Ga)Se2 Layers : Effect on Absorber Properties and Solar Cell Performance
- 2020
-
Ingår i: Solar RRL. - : Wiley. - 2367-198X. ; 4:9
-
Tidskriftsartikel (refereegranskat)abstract
- This contribution evaluates a sequential post‐deposition treatment of Cu(In,Ga)Se2 (CIGS) films, consisting of 1) a post‐sulfurization in elemental S‐atmosphere and 2) a subsequent treatment by heavy alkali fluorides (Alk‐PDT). First, the effect of the sulfurization step on the corresponding solar cell performance is investigated and optimum process parameters, leading to an efficiency improvement, are identified. Losses in carrier collection observed after S‐incorporation are attributed to an increased grain boundary (GB) recombination. It is found that the corresponding reduction in short‐circuit current density can be mitigated by a RbF‐ or KF‐PDT, supposedly by depleting GBs in Cu. However, in strong contrast to non‐sulfurized CIGS, the Alk‐PDT results in a lower open‐circuit voltage and distortions in the current–voltage (I –V ) characteristics for sulfurized absorbers. Possible explanations are the absence of a wide‐gap surface phase and/or air exposure between the post‐treatment steps. It is further proposed that a back contact barrier may be responsible for the distortions in I–V.
|
|
| 21. |
- Khavari, Faraz, et al.
(författare)
-
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.
|
|
| 22. |
- Kosyak, Volodymyr, et al.
(författare)
-
Calculation of point defect concentration in Cu2ZnSnS4 : Insights into the high-temperature equilibrium and quenching
- 2017
-
Ingår i: Journal of Applied Physics. - : AMER INST PHYSICS. - 0021-8979 .- 1089-7550. ; 122:3
-
Tidskriftsartikel (refereegranskat)abstract
- Herein, we study the native point defect equilibrium in Cu2ZnSnS4 (CZTS) by applying a statistical thermodynamic model. The stable chemical- potential space (SCPS) of CZTS at an elevated temperature was estimated directly, on the basis of deviations from stoichiometry calculated for the different combinations of chemical potential of the components. We show that the SCPS is narrow due to high concentration of (V-Cu(-) Zn-Cu(+)) complex which is dominant over other complexes and isolated defects. The CZTS was found to have p-type conductivity for both stoichiometric and Cu-poor/Zn-rich composition. It is established that the reason for this is that the majority of donor-like Zn-Cu(+) antisites are involved in the formation of (V-Cu(-) Zn-Cu(+)) complex making Cu-Zn dominant and providing p- type conductivity even for Cu-poor/Zn-rich composition. However, our calculation reveals that the hole concentration is almost insensitive to the variation of the chemical composition within the composition region of the single-phase CZTS due to nearly constant concentration of dominant charged defects. The calculations for the full equilibrium and quenching indicate that hole concentration is strongly dependent on the annealing temperature and decreases substantially after the drastic cooling. This means that the precise control of annealing temperature and post-annealing cooling rate are critical for tuning the electrical properties of CZTS.
|
|
| 23. |
- Kosyak, Volodymyr, et al.
(författare)
-
Current-voltage and capacitance study of light-induced metastabilities in CuZnSnSSe thin film solar cells
- 2020
-
Ingår i: Journal of Physics D. - : IOP Publishing. - 0022-3727 .- 1361-6463. ; 53:18
-
Tidskriftsartikel (refereegranskat)abstract
- White, red and blue light-induced metastabilities in Cu2ZnSnS(4)(Se-4) solar cells were investigated by temperature dependent current-voltage measurements, drive level capacitance profiling, impedance and thermal admittance spectroscopy. A set of devices were studied where white and blue light soaking at room temperature led to degradation of the device performance, while after red light soaking the solar cell efficiency did not change. We observed a significant effect of light soaking on capacitance data measured in both low and high-temperature ranges for these devices. In particular, the net doping concentration extracted from drive-level capacitance profiling substantially increased after light soaking treatments. Low and high-temperature capacitance steps observed in the reference capacitance-frequency spectra were assigned to Fermi level pinning and bulk defects, correspondingly. Light soaking with different-wavelength light led to a shift of both steps toward the high-frequency range, and hence a decrease in the thermal admittance activation energies. A low-frequency 'inductive' loop was detected in the impedance spectra after light soaking, regardless of wavelength. It was proposed that the appearance of the 'inductive' loop is due to the formation of a negative electric field at the highly defected CdS/Cu2ZnSnS(4)(Se-4) hetero-interface. This result also leads us to conclude that such electric field is responsible for the metastable behaviour of these devices at room temperature, while the low temperature metastable changes might have a different origin. We also discuss the methodology for electrical characterization of the metastable solar cells in detail.
|
|
| 24. |
- Larsen, Jes K, et al.
(författare)
-
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
-
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.
|
|
| 25. |
- Larsen, Jes K, et al.
(författare)
-
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.
|
|
