| 1. |
- 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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| 2. |
- Campana, Pietro, et al.
(författare)
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Solar irradiance distribution under vertically mounted agrivoltaic systems – Model development, validation, and applications for microclimate assessment
- 2022
-
Tidskriftsartikel (övrigt vetenskapligt/konstnärligt)abstract
- ccurate assessment of the Photosynthetically Active Radiation (PAR) and Global Horizontal Irradiance (GHI) at the crop level is paramount for accurately assessing the energy balances and the crop yield under agrivoltaic systems. The shadings produced by the photovoltaic modules and structures of the agrivoltaic systems cause a non-homogeneous distribution of PAR and GHI at the crop level. It is thus essential to calculate their distribution at a high spatial resolution within the agrivoltaic field. Using field data and commercial software, we have validated a high spatial resolution model (i.e., 25 cm × 25 cm) for PAR and GHI distribution within a vertical agrivoltaic system. The results show good model accuracy with coefficients of determination higher than 0.98 when comparing GHI from the model developed in this study and commercial software based on ray tracing. Model applications are presented with consideration to the computation of surface temperature, evapotranspiration, crop yield, and soil moisture. The model developed in this study shows good agreement in terms of crop yield computations as compared to a previously published model for agrivoltaic systems simulations and optimization (Campana et al., 2021). The added value of the model presented in this study consists in performing high spatial resolution computations of microclimatic parameters and crop yield within the agrivoltaic field.
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| 3. |
- Comparotto, Corrado, et al.
(författare)
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Chalcogenide Perovskite BaZrS3 : Thin Film Growth by Sputtering and Rapid Thermal Processing
- 2020
-
Ingår i: ACS Applied Energy Materials. - : AMER CHEMICAL SOC. - 2574-0962. ; 3:3, s. 2762-2770
-
Tidskriftsartikel (refereegranskat)abstract
- Tandem solar cells based on hybrid organic-inorganic metal halide perovskites have reached efficiencies up to 28%, but major concerns for long-term stability and the presence of Pb have raised interest in searching for fully earth-abundant, intrinsic chemically stable, and nontoxic alternatives. With a direct band gap around 1.8 eV and stability in air up to at least 500 degrees C, BaZrS3 is a promising candidate. This work presents the first approach of synthesizing a thin film of such compound by sputtering at ambient temperature with a subsequent rapid thermal process. Despite the short fabrication time, the width of the XRD diffraction peaks and the energy and distribution of the photoluminescence response show comparable crystalline quality to that from bulk synthesis methods. Good crystallization required around 900 degrees C. Such a high temperature could be incompatible with fabrication of tandem solar cells.
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| 4. |
- Comparotto, Corrado, et al.
(författare)
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Synthesis of BaZrS3 Perovskite Thin Films at a Moderate Temperature on Conductive Substrates
- 2022
-
Ingår i: ACS Applied Energy Materials. - : American Chemical Society (ACS). - 2574-0962. ; 5:5, s. 6335-6343
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Tidskriftsartikel (refereegranskat)abstract
- Chalcogenide perovskites are being considered for various energy conversion applications, not least photovoltaics. BaZrS3 stands out for its highly stable, earth-abundant, and nontoxic nature. It exhibits a very strong light-matter interaction and an ideal band gap for a top subcell in a two-junction photovoltaic device. So far, thin-film synthesis-necessary for proper optoelectronic characterization as well as device integration-remains underdeveloped. Sputtering has been considered, among others, but the need for an annealing step of at least 900 degrees C has been a cause for concern: such a high temperature could lead to damaging the bottom layers of prospective tandem devices. Still, a solid-state fabrication route has already demonstrated that BaZrS3 can form at much lower temperatures if excess S is present. In this work, sputtered Ba-Zr precursors capped by SnS are sulfurized at under 600 degrees C for 20 min. Although some Sn is still present at the surface after sulfurization, the resulting crystalline quality is comparable to samples synthesized at much higher temperatures. The results are rationalized, and the effect of key process variables is examined. This study represents the first successful synthesis of BaZrS3 perovskite that is compatible with conductive substrates-an important step forward for device integration.
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| 5. |
- Davydova, Alexandra, et al.
(författare)
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The Single Phase Region in Cu2ZnSnS4 Thin Films from Theory and Combinatorial Experiments
- 2018
-
Ingår i: Chemistry of Materials. - : American Chemical Society (ACS). - 0897-4756 .- 1520-5002. ; 30:14, s. 4624-4638
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Tidskriftsartikel (refereegranskat)abstract
- Cu2ZnSnS4 (CZTS) is hoped to be a future, earth-abundant absorber material for thin film solar cells, but performance remains below the level needed for commercialization. In this work, the size of the single phase region of CZTS obtained from thin film synthesis methods is explored, to determine the scope available for defect engineering and thereby future improvements in solar cell performance. A chemical model for the single phase region is presented, based on equilibria between defect complexes in the CZTS phase and the other solid- and gas-phase components present during synthesis. The model predicts a variable single phase region size, depending on the partial pressures of SnS and S2. The model is verified by analysis of combinatorial thin-film CZTS samples prepared with different synthetic conditions and characterized by Raman and compositional mapping. We conclude that typical synthesis strategies for CZTS are not capable of accessing the full range of the CZTS single phase region since the required partial pressure of S2 is very large. The important implication is that our understanding of CZTS defect chemistry from experimental studies is incomplete and that scope exists for tuning the defect properties toward better solar cell performance.
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| 6. |
- Davydova, Alexandra, et al.
(författare)
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Thio-olivine Mn2SiS4 thin films by reactive magnetron sputtering : Structural and optical properties with insights from first principles calculations
- 2018
-
Ingår i: Materials & design. - : ELSEVIER SCI LTD. - 0264-1275 .- 1873-4197. ; 152, s. 110-118
-
Tidskriftsartikel (refereegranskat)abstract
- Thio-olivines such as (Fe,Mn)(2)(Si,Ge)S-4 have been proposed as candidate earth-abundant materials for single and multi-junction solar cells. In this work we present the first investigation of Mn2SiS4 thin films prepared by reactive magnetron sputtering deposition, using a composition grading approach. Precursor instability in ambient conditions is observed, revealing the oxidation/hydrolysis of Si-S bonds from the as-deposited film as a blocking mechanism for the ternary compound formation. Structural, morphological and optical properties of the annealed Mn2SiS4 films are reported for the first time. Resulting Mn2SiS4 films have orthorhombic Pnma structure and are polycrystalline. Raman active modes at 325 nm excitation are observed at 262, 320, 400 and 464 cm(-1). From room temperature photoluminescence at 532 nm excitation the band gap is estimated to be about 1.9 eV, but a high optical absorption coefficient of > 10(4) cm(-1) was only obtained at E > 2.8 eV.First principles calculations are used for better understanding of opto-electronic properties. From the calculations, Mn2SiS4 is suggested to have a band gap of about 1.73-1.86 eV depending on the magnetic configuration of Mn and slight indirect nature. The slow absorption onset is interpreted by strong anisotropy due to one of the components of the dielectric function.
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| 7. |
- Englund, Sven, 1987-
(författare)
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Alternative back contacts for CZTS thin film solar cells
- 2020
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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.
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| 8. |
- Englund, Sven, et al.
(författare)
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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.
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| 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.
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| 10. |
- Englund, Sven, et al.
(författare)
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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.
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| 11. |
- Ericson, Tove, 1983-, et al.
(författare)
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Annealing behavior of reactively sputtered precursor films for Cu2ZnSnS4 solar cells
- 2013
-
Ingår i: Thin Solid Films. - : Elsevier. - 0040-6090 .- 1879-2731. ; 535, s. 22-26
-
Tidskriftsartikel (refereegranskat)abstract
- Reactively sputtered Cu–Zn–Sn–S precursor films are prepared and recrystallized by rapid thermal processing to generate Cu2ZnSnS4 solar cell absorber layers. We study how the film properties are affected by substrate heating and composition. The stress, density and texture in the films were measured. Compressive stress was observed for the precursors but did not correlate to the deposition temperature, and had no influence on the properties of the annealed films or solar cells. However, the substrate temperature during precursor deposition had a large effect on the behavior during annealing and on the solar cell performance. The films deposited at room temperature had, after annealing, smaller grains and cracks, and gave shunted devices. Cracking is suggested to be due to a slightly higher sulfur content, lower density or to minor differences in material quality. The grain size in the annealed films seems to increase with higher copper content and higher precursor deposition temperature. The best device in the current series gave an efficiency of 4.5%.
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| 12. |
- Ericson, Tove, et al.
(författare)
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Reactive sputtering of precursors for Cu2ZnSnS4 thin film solar cells
- 2012
-
Ingår i: Thin Solid Films. - : Elsevier BV. - 0040-6090 .- 1879-2731. ; 520:24, s. 7093-7099
-
Tidskriftsartikel (refereegranskat)abstract
- The quaternary semiconductor Cu2ZnSnS4 (CZTS) is a possible In-free replacement for Cu(In,Ga)Se-2. Here we present reactive sputtering with the possibility to obtain homogeneous CZTS-precursors with tunable composition and a stoichiometric quantity of sulfur. The precursors can be rapidly annealed to create large grained films to be used in solar cells. The reactive sputtering process is flexible, and morphology, stress and metal and sulfur contents were varied by changing the H2S/Ar-flow ratio, pressure and substrate temperature. A process curve for the reactive sputtering from CuSn and Zn targets is presented. The Zn-target is shown to switch to compound mode earlier and faster compared to the CuSn-target. The precursors containing a stoichiometric amount of sulfur exhibit columnar grains, have a crystal structure best matching ZnS and give a broad peak, best matching CZTS, in Raman scattering. In comparing process gas flows it is shown that the sulfur content is strongly dependent on the H2S partial pressure but the total pressures compared in this study have little effect on the precursor properties. Increasing the substrate temperature changes the film composition due to the high vapor pressures of Zn, SnS and S. High substrate temperatures also give slightly denser and increasingly oriented films. The precursors are under compressive stress, which is reduced with higher deposition temperatures. (C) 2012 Elsevier B.V. All rights reserved.
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| 13. |
- Ericson, Tove, 1983-, et al.
(författare)
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Zn(O,S) Buffer Layers and Thickness Variations of CdS Buffer for Cu2ZnSnS4 Solar Cells
- 2014
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Ingår i: IEEE Journal of Photovoltaics. - 2156-3381. ; 4:1, s. 465-469
-
Tidskriftsartikel (refereegranskat)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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| 14. |
- Kosyak, Volodymyr, et al.
(författare)
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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
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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.
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| 15. |
- Kubart, Tomas, et al.
(författare)
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Reactive magnetron sputtering of precursors for CZTS solar cells
- 2012
-
Konferensbidrag (refereegranskat)abstract
- At the moment, CIGS (CuInGaSe2) solar cells show the highest efficiency among industrial scale produced thin film solar cells. Given the present strong increase in production, however, the availability and price of indium will become an issue because of its low abundance in Earth's crust. Therefore, there is strong interest in alternative indium free absorber materials. Kesterites CZTS (Cu2ZnSn(SxSe1-x)4) attracted most attention owing to the fact that relatively high efficiencies have already been demonstrated and also due to the similarity to CIGS. In this contribution we report on reactive sputtering for deposition of CZTS precursors. In order to avoid Sn loss at elevated temperatures a two stage process, synthesis of precursor films at relatively low substrate temperature followed by annealing, is used. Depositions are performed by pulsed DC magnetron sputtering from two targets, a CuSn alloy and Zn, in a mixture of Ar and H2S. The film structure is evaluated by X-ray diffraction and Raman spectroscopy while the composition is analysed by RBS, XRF, EDS and EPMA. Internal stress is measured by deflection of thin substrates. Characteristics of the deposition process are discussed with respect to the discharge power, total pressure, substrate temperature and H2S flow on the film structure and composition.Sulphur incorporation can be readily controlled by H2S flow with the structure changing from amorphous to columnar with increasing S content. The main issue encountered in the depositions is related to the film composition as the ratio between Cu and Sn does not correspond to the target composition. This effect is discussed in detail with respect to the sputtering and transport through the gas phase. Finally, material properties after annealing are briefly summarized.
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| 16. |
- Kubart, Tomas, et al.
(författare)
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Reactive sputtering of Cu2ZnSnS4 thin films - Target effects on the deposition process stability
- 2014
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Ingår i: Surface & Coatings Technology. - : Elsevier BV. - 0257-8972 .- 1879-3347. ; 240, s. 281-285
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Tidskriftsartikel (refereegranskat)abstract
- Cu2ZnSnS4 (TS) is a promising material for thin film solar cells which contains only abundant elements. This work focuses on the stability of elemental composition of films deposited by reactive sputtering of CuSn alloy targets in H2S. Long equilibration times of several hours were observed. The main reason is the formation of a thick Cu2S layer on the target surface. Especially in areas with low erosion rate, the Cu2S thickness reaches up to 700 pm and is accompanied by a preferential loss of Sn from the target. Based on the results, it is suggested that the formation of Cu2S may be limited either by more uniform erosion of the target surface or by reduction of the H2S partial pressure.
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| 17. |
- 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
-
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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| 18. |
- 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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| 19. |
- 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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| 20. |
- 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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| 21. |
- 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
-
Ingår i: Solar Energy Materials and Solar Cells. - : Elsevier BV. - 0927-0248 .- 1879-3398. ; 149, s. 170-178
-
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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| 22. |
- Mukherjee, Soham, et al.
(författare)
-
Interplay between Growth Mechanism, Materials Chemistry, and Band Gap Characteristics in Sputtered Thin Films of Chalcogenide Perovskite BaZrS3
- 2023
-
Ingår i: ACS Applied Energy Materials. - : American Chemical Society (ACS). - 2574-0962. ; 6:22, s. 11642-11653
-
Tidskriftsartikel (refereegranskat)abstract
- The prototypical chalcogenide perovskite BaZrS3, characterized by its direct band gap, exceptionally strong light-harvesting ability, and good carrier transport properties, provides fundamental prerequisites for a promising photovoltaic material. This inspired the synthesis of BaZrS3 in the form of thin films, using sputtering and rapid thermal processing, aimed at device fabrication for future optoelectronic applications. Using a combination of short- and long-range structural information from X-ray absorption spectroscopy (XAS) and X-ray diffraction (XRD), we have elucidated how, starting from a random network of Ba, Zr, and S atoms, thermal treatment induces crystallization and growth of BaZrS3 and explained its impact on the observed photoluminescence (PL) properties. We also provide a description of the electronic structure and substantiate the surface material chemistry using a combination of depth-dependent photoelectron spectroscopy (PES) using hard X-ray (HAXPES) and traditional Al K alpha radiation. From the knowledge of the optical band gap of BaZrS3 thin films, synthesized at an optimal temperature of 900 C-degrees, and our estimation of the valence band edge position with respect to the Fermi level, one may conclude that these semiconductor films are intrinsic in nature with a slight n-type character. A detailed understanding of the growth mechanism and electronic structure of BaZrS3 thin films helps pave the way toward their utilization in photovoltaic applications.
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| 23. |
- Paneta, Valentina, et al.
(författare)
-
Ion-beam based characterization of TiN back contact interlayers for CZTS(e), thin film solar cells
- 2019
-
Ingår i: Nuclear Instruments and Methods in Physics Research Section B. - : Elsevier BV. - 0168-583X .- 1872-9584. ; 450, s. 262-266
-
Tidskriftsartikel (refereegranskat)abstract
- Time-of-Flight Elastic Recoil Detection Analysis (ToF-ERDA) and Time-of-Flight Medium-Energy Ion Scattering (ToF-MEIS) have been employed to investigate the potential of TiN thin films as intermediate layers on Mo back contact in CZTS(e) solar cells. TiN films of various thicknesses (20, 50 and 200 nm) were prepared with reactive DC magnetron sputtering and atomic layer deposition on Mo/SLG (soda-lime glass) substrates and annealed ex situ in either S or Se atmosphere. In situ annealing of the samples to different temperatures was also performed in the MEIS setup together with subsequent ToF-MEIS and ERDA analysis. The results of the sample and interlayer composition profiles, layer quality and thickness distributions are discussed in context with complementary experimental findings partially obtained previously by X-ray Photoelectron Spectroscopy (XPS), X-ray Diffraction (XRD), Scanning Electron Microscopy and Scanning Transmission Electron Microscopy- Electron Energy Loss Spectroscopy (STEM - EELS).
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| 24. |
- Platzer-Bjorkman, Charlotte, et al.
(författare)
-
Diffusion of Fe and Na in co-evaporated Cu(In, Ga) Se-2 devices on steel substrates
- 2013
-
Ingår i: Thin Solid Films. - : Elsevier BV. - 0040-6090 .- 1879-2731. ; 535, s. 188-192
-
Tidskriftsartikel (refereegranskat)abstract
- In this work we study impurity diffusion into Cu(In, Ga) Se-2 from stainless steel substrates with and without Cr diffusion barriers using secondary ion mass spectrometry. For these substrate configurations we compare cases with and without adding NaF as a sodium precursor. A clear increase in impurity diffusion from the substrate is observed for samples with NaF. Devices made using our micro pilot line show the expected correlation between Fe content, Na content and efficiency, but the highest device efficiency obtained for steel substrates is still slightly below that of the glass substrate reference. We discuss reasons for the observed device performance.
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| 25. |
- Platzer Björkman, Charlotte, 1976-, et al.
(författare)
-
Back and front contacts in kesterite solar cells : state-of-the-art and open questions
- 2019
-
Ingår i: Journal of Physics. - : Institute of Physics Publishing (IOPP). - 2515-7655. ; 1:4
-
Tidskriftsartikel (refereegranskat)abstract
- We review the present state-of-the-art within back and front contacts in kesterite thin film solar cells, as well as the current challenges. At the back contact, molybdenum (Mo) is generally used, and thick Mo(S, Se)2 films of up to several hundred nanometers are seen in record devices, in particular for selenium-rich kesterite. The electrical properties of Mo(S, Se)2 can vary strongly depending on orientation and indiffusion of elements from the device stack, and there are indications that the back contact properties are less ideal in the sulfide as compared to the selenide case. However, the electronic interface structure of this contact is generally not well-studied and thus poorly understood, and more measurements are needed for a conclusive statement. Transparent back contacts is a relatively new topic attracting attention as crucial component in bifacial and multijunction solar cells. Front illuminated efficiencies of up to 6% have so far been achieved by adding interlayers that are not always fully transparent. For the front contact, a favorable energy level alignment at the kesterite/CdS interface can be confirmed for kesterite absorbers with an intermediate [S]/([S]+[Se]) composition. This agrees with the fact that kesterite absorbers of this composition reach highest efficiencies when CdS buffer layers are employed, while alternative buffer materials with larger band gap, such as Cd1−x Zn x S or Zn1−x Sn x O y , result in higher efficiencies than devices with CdS buffers when sulfur-rich kesterite absorbers are used. Etching of the kesterite absorber surface, and annealing in air or inert atmosphere before or after buffer layer deposition, has shown strong impact on device performance. Heterojunction annealing to promote interdiffusion was used for the highest performing sulfide kesterite device and air-annealing was reported important for selenium-rich record solar cells.
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| 26. |
- Platzer-Björkman, Charlotte, et al.
(författare)
-
Influence of precursor sulfur content on film formation and compositional changes in Cu2ZnSnS4 films and solar cells
- 2012
-
Ingår i: Solar Energy Materials and Solar Cells. - : Elsevier BV. - 0927-0248 .- 1879-3398. ; 98, s. 110-117
-
Tidskriftsartikel (refereegranskat)abstract
- Cu2ZnSnS4 (CZTS) thin films are made using sulfurisation of co-sputtered metallic and sulfur-containing precursor films. The CZTS grain size is larger for metallic precursors than for sulfur-containing precursors while more uniform films with fewer voids are obtained in the latter case. During sulfurisation of precursors with tin-excess in closed quartz ampoules, tin is lost from the films with greater losses from metallic precursors. We suggest that the reduced grain size and the reduced tin-loss for sulfur-containing precursors can be explained by a larger number of CZTS nuclei being formed early in the sulfurisation process. In sulfur containing precursors with large tin excess, SnS2 is observed together with CZTS, and a tin-rich bottom layer segregates. This indicates that tin-diffusion in CZTS is relatively slow. Solar cell devices made for a range of compositions at and around stoichiometric CZTS show highest efficiencies in two compositional groups; Zn-rich and Cu-poor/Sn-rich, while close to stoichiometric material gives poor devices. Devices including the tin-rich bottom layer show efficiencies of up to 3.2%. The role of secondary phases such as ZnS and SnS2 on device performance is discussed.
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| 27. |
- Platzer Björkman, Charlotte, 1976-, et al.
(författare)
-
Reactive sputtering of CZTS
- 2015
-
Ingår i: Copper zinc tin sulfide-based thin film solar cells. - : John Wiley & Sons. - 9781118437872 ; , s. 203-220
-
Bokkapitel (övrigt vetenskapligt/konstnärligt)
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| 28. |
- Platzer-Björkman, Charlotte, et al.
(författare)
-
Reduced interface recombination in Cu2ZnSnS4 solar cells with atomic layer deposition Zn1-xSnxO buffer layers
- 2015
-
Ingår i: Applied Physics Letters. - : AIP Publishing. - 0003-6951 .- 1077-3118. ; 107:24
-
Tidskriftsartikel (refereegranskat)abstract
- Cu2ZnSnS4 (CZTS) solar cells typically include a CdS buffer layer in between the CZTS and ZnO front contact. For sulfide CZTS, with a bandgap around 1.5 eV, the band alignment between CZTS and CdS is not ideal ("cliff-like"), which enhances interface recombination. In this work, we show how a Zn1-xSnxOy (ZTO) buffer layer can replace CdS, resulting in improved open circuit voltages (V-oc) for CZTS devices. The ZTO is deposited by atomic layer deposition (ALD), with a process previously developed for Cu(In,Ga)Se-2 solar cells. By varying the ALD process temperature, the position of the conduction band minimum of the ZTO is varied in relation to that of CZTS. A ZTO process at 95 degrees C is found to give higher Voc and efficiency as compared with the CdS reference devices. For a ZTO process at 120 degrees C, where the conduction band alignment is expected to be the same as for CdS, the Voc and efficiency is similar to the CdS reference. Further increase in conduction band minimum by lowering the deposition temperature to 80 degrees C shows blocking of forward current and reduced fill factor, consistent with barrier formation at the junction. Temperature-dependent current voltage analysis gives an activation energy for recombination of 1.36 eV for the best ZTO device compared with 0.98 eV for CdS. We argue that the Voc of the best ZTO devices is limited by bulk recombination, in agreement with a room temperature photoluminescence peak at around 1.3 eV for both devices, while the CdS device is limited by interface recombination.
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| 29. |
- Ren, Yi, et al.
(författare)
-
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
-
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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| 30. |
- Ren, Yi, et al.
(författare)
-
Evolution of Na-S(-O) compounds on Cu2ZnSnS4 absorber surface and its effect on CdS growth
- 2016
-
Ingår i: 2016 IEEE 43rd Photovoltaic Specialists Conference (PVSC). - New York : IEEE. - 9781509027248 ; , s. 2255-2257
-
Konferensbidrag (refereegranskat)abstract
- Na-containing surface compounds is likely to form during the formation of CZTS absorber. Still, the understanding of any potential influence on buffer layer growth and device performance is limited. In this work, we observed that NaxS compound can possibly form on the CZTS surface after annealing, and negatively affect the growth of the subsequent CdS buffer. The NaxS compound is oxidized to Na2SO4 via air exposing the annealed CZTS surface, which allows greatly improved quality of the CdS layer. This provides new insights for improving the CdS/CZTS interface during the fabrication of CZTS solar cells.
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| 31. |
- Ren, Yi, et al.
(författare)
-
Evolution of Na-S(-O) compounds on the Cu2ZnSnS4 absorber surface and their effects on CdS thin film growth
- 2016
-
Ingår i: ACS Applied Materials and Interfaces. - : American Chemical Society (ACS). - 1944-8244 .- 1944-8252. ; 8:28, s. 18600-18607
-
Tidskriftsartikel (refereegranskat)abstract
- Formation of Na-containing surface compounds is an important phenomenon in the Cu2ZnSnS4 (CZTS) quaternary material synthesis for solar cell applications. Still, identification of these compounds and the understanding of their potential influence on buffer layer growth and device performance are scarce. In this work, we discovered that the evolution of Na-S(-O) compounds on the CZTS surface substantially affect the solution/CZTS interface during the chemical bath deposition of CdS buffer film. We showed that Na2S negatively affects the growth of CdS, and that this compound is likely to form on the CZTS surface after annealing. It was also demonstrated that the Na2S compound can be oxidized to Na2SO4 by air exposure of the annealed CZTS surface or be removed using water dipping instead of the commonly used KCN etching process, resulting in significantly better quality of the CdS layer. Lastly, 6.5% CZTS solar cells were fabricated with air exposure treatment without incorporation of the KCN etching process. This work provides new insight into the growth of the CdS/CZTS interface for solar cell applications and opens new possibilities for improving likewise Cd-free buffer materials that are grown with a similar chemical bath deposition process.
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| 32. |
- Ren, Yi, et al.
(författare)
-
Influence of the Cu2ZnSnS4 absorberthickness on thin film solar cells
- 2015
-
Ingår i: Physica status solidi. A, Applied research. - : Wiley. - 0031-8965 .- 1521-396X. ; 212:12, s. 2889-2896
-
Tidskriftsartikel (refereegranskat)abstract
- In this study, we investigate the influence of absorber thickness on Cu2ZnSnS4 (CZTS) solar cells, ranging from 500 to 2000 nm, with nearly constant metallic composition. Despite the observed ZnS and SnS phases on the surface and backside of all absorber films, scanning electron microscopy, Raman scattering, and X-ray diffraction show no large variations in material quality for the different thicknesses. The open-circuit voltage (V-oc), short-circuit current and overall power conversion efficiency of the fabricated devices show an initial improvement as the absorber thickness increases but saturate when the thickness exceeds 750 nm. External quantum efficiency (EQE) measurements suggest that the current is mainly limited by collection losses. This can result from non-optimal bulk quality of the CZTS absorber (including the presence of secondary phases), which is apparently further reduced for the thinnest devices. The observed saturation of V-oc agrees with the expected influence from strong interface recombination. Finally, an effective collection depth of 750-1000 nm for the minority carriers generated in the absorber can be estimated from EQE, indicating that the proper absorber thickness for our device process is approximately 1000 nm. Performance could be improved for thicker films, if the collection depth can be increased.
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| 33. |
- Ren, Yi, et al.
(författare)
-
Investigation of the SnS/Cu2ZnSnS4 interfaces in Kesterite Thin-Film Solar Cells
- 2017
-
Ingår i: ACS Energy Letters. - : American Chemical Society (ACS). - 2380-8195. ; 2:5, s. 976-981
-
Tidskriftsartikel (refereegranskat)abstract
- Kesterite Cu2ZnSnS4 (CZTS), having only earth abundant elements, is a promising solar cell material. Nevertheless, the impact of the SnS secondary phase, which often forms alongside CZTS synthesis at high annealing temperature, on CZTS solar cells is poorly studied. We confirm, by means of X-ray diffraction, Raman scattering, and energy dispersive X-ray spectroscopy mapping, that this phase tends to segregate at both the surface and the back side of annealed CZTS films with Cu-poor and Zn-rich composition. Using electron beam-induced current measurements, it is further demonstrated that the formation of SnS on the CZTS surface is harmful for solar cells, whereas the SnS phase can be beneficial for solar cells when it segregates on the CZTS rear. This positive contribution of SnS could stem from a passivation effect at the CZTS/SnS rear interface. This work opens new possibilities for an alternative interface development for kesterite-based photovoltaic technology.
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| 34. |
- Ren, Yi, et al.
(författare)
-
Reactively sputtered films in the CuxS–ZnS–SnSy system : From metastability to equilibrium
- 2015
-
Ingår i: Thin Solid Films. - : Elsevier BV. - 0040-6090 .- 1879-2731. ; 582, s. 208-214
-
Tidskriftsartikel (refereegranskat)abstract
- Cu2ZnSnS4 is a promising photovoltaic absorber containing earth abundant elements. Using a two stage process, low temperature reactive co-sputtering followed by heat treatment, we have previously achieved a 7.9% efficient solar cell. Because the sputtered precursors contain non-equilibrium phases with unusual crystal structures, it is crucial to understand their nature and their conversion into Cu2ZnSnS4 (and secondary phases) during heat treatment. In this study, we report phase analysis of reactively sputtered binary and ternary sulfides in the CuxS–ZnS–SnSy system before and after annealing. In the as deposited films, Raman spectroscopy with 532 and 325 nm excitation wavelengths reveals expected phases for the binaries (CuS, ZnS and SnS2) and the ternary (Cu2SnS3), and unique metastable phases for the Cu–Zn–S and Zn–Sn–S precursors. Upon annealing, the non-equilibrium phases disappear, accompanied by additional chemical changes. Excess S content in the films is removed, and in the Sn–S and Zn–Sn–S films, further S loss from decomposition of SnSx (x > 1) and CuS respectively generates SnS and CuxS (x > 1). Due to the presence of SnS vapor, Cu2ZnSnS4 is generated from the Cu–Zn–S precursor. Additionally, the range of sulfur partial pressure in the annealing process is estimated according to the temperature–pressure phase diagram. This gives us useful insight allowing better control of annealing conditions.
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| 35. |
|
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| 36. |
- Rudisch, Katharina
(författare)
-
Defect Engineering in Kesterite Materials for Thin Film Solar Cells
- 2020
-
Doktorsavhandling (övrigt vetenskapligt/konstnärligt)abstract
- Cu2ZnSnS4 has great potential to be applied as an earth abundant and non-toxic absorber material in thin film solar cells, based on its suitable optical properties. However, several challenges have prevented the achievable efficiencies from exceeding 12.6 %, which is well below marketable efficiencies compared to competing solar cell technologies. One of the struggles in the development of Cu2ZnSnS4 solar cells is the high number of harmful defects leading to severe potential fluctuations. This thesis investigates different strategies of defect engineering in Cu2ZnSnS4, in particular to reduce Cu-Zn disorder.Cu2ZnSnS4 thin films are produced by a two-step process, where Cu-Zn-Sn-S precursors are deposited by co-sputtering and then annealed at high temperature to yield crystalline films. The material properties are investigated with Raman spectroscopy, photoluminescence and spectrophotometry.In the scope of this thesis, the following approaches to defect engineering are investigated: thermal treatments, varying partial pressures during the annealing step, and cation exchange to form the compound Cu2MnSnS4. Thermal treatments substantially enhance the degree of order in Cu2ZnSnS4. However, for the first time the severe limitations of such treatments are shown, indicating their insufficiency to reduce cation disorder to a level where potential fluctuations no longer affect Cu2ZnSnS4 solar cells. Furthermore, the stannite Cu2MnSnS4 suffers from cation disorder just like kesterite Cu2ZnSnS4 demonstrating that cation disorder is not restricted to the kesterite crystal structure and posing new challenges for finding new solar cell materials.On the other hand, the presented results demonstrate a strong composition dependence of the ordering kinetics. Compositions with high densities of vacancies or interstitials significantly enhance the ordering rate by reducing the activation energy while the critical temperature is constant for the investigated compositions. Furthermore, the important effect of S2 and SnS partial pressures during the annealing step of the fabrication is predicted from chemical models and experimentally verified by investigation of composition-spread Cu2ZnSnS4 thin films. Increasing both partial pressures leads to higher solubility of vacancies in Sn-rich Cu2ZnSnS4 further amplifying the positive effect of composition on the order-disorder transition. Investigation of composition-spread thin films further revealed the interplay between material properties and composition as well as secondary phases. In particular, the photoluminescence yield was drastically enhanced in the presence of SnSx secondary phases. This thesis discusses these results in the context of the current understanding of Cu2ZnSnS4.
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| 37. |
- Rudisch, Katharina, et al.
(författare)
-
Order-disorder transition in B-type Cu2ZnSnS4 and limitations of ordering through thermal treatments
- 2016
-
Ingår i: Applied Physics Letters. - : AIP Publishing. - 0003-6951 .- 1077-3118. ; 108:23
-
Tidskriftsartikel (refereegranskat)abstract
- B-type Cu2ZnSnS4 (CZTS) thin films with varying degrees of cation order were produced and examined with resonant Raman spectroscopy. Simulations based on Vineyard's theory of order allowed kinetic analysis of the final degree of order after the applied thermal treatments. Combining the results from the simulations and the resonant Raman spectra, the kinetic parameters within the Vineyard model for the order-disorder transition in B-type CZTS were determined, as well as a method which allows quantification of the degree of order based on resonant Raman spectra. The knowledge gained about the order-disorder transition in B-type CZTS allowed the prediction of a best practice thermal treatment for high ordering. This further leads to awareness about practical limits of thermal treatments regarding the cation ordering in B-type CZTS, and suggests that such treatments are not able to produce the high cation order necessary to sufficiently reduce detrimental potential fluctuations. Published by AIP Publishing.
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| 38. |
- Rudisch, Katharina, et al.
(författare)
-
Prospects for defect engineering in Cu2ZnSnS4 solar absorber films
- 2020
-
Ingår i: Journal of Materials Chemistry A. - : Royal Society of Chemistry (RSC). - 2050-7488 .- 2050-7496. ; 8:31, s. 15864-15874
-
Tidskriftsartikel (övrigt vetenskapligt/konstnärligt)abstract
- Complex compound semiconductors, such as the emerging solar cell material Cu2ZnSn(S,Se)4 (CZTS), present major challenges in terms of understanding and controlling growth processes and defect formation.This study aims to shed light upon the complicated interplay of synthesis conditions and the CZTS thin film properties. Composition-spread thin films are fabricated in different atmospheric conditionsduring the annealing step. The span of the single-phase region is identified by a phase analysiscombining XRD and Raman mapping. The phase characterization is strengthened by STEM-EDXanalysis. Our results show that the stability of the CZTS phase is strongly affected by the processconditions which is observed by a shift in the secondary phase boundaries and different levels of maximum cation ordering achieved in the different samples. With regard to the photoluminescence intensity, all investigated samples show the same trends: regions with Cu3SnS4 secondary phase show the lowest intensity, while the presence of SnSx secondary phases greatly enhances the photolumines-cence intensity. The single-phase region features an overall low photoluminescence intensity without a remarkable composition dependence and we propose the presence of deep defects in absence of secondary phases that limit the radiative recombination. We discuss implications for future efforts in defect engineering toward improving the efficiency of CZTS thin film devices.
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| 39. |
- Rudisch, Katharina, et al.
(författare)
-
Structural and Electronic Properties of Cu2MnSnS4 from Experiment and First-Principles Calculations
- 2019
-
Ingår i: Physica status solidi. B, Basic research. - : Wiley-VCH Verlagsgesellschaft. - 0370-1972 .- 1521-3951. ; 256:7
-
Tidskriftsartikel (refereegranskat)abstract
- Cu2MnSnS4 shares several promising properties with the widely investigated Cu2ZnSnS4 for photovoltaic applications such as containing only earth abundant and non-toxic elements, and suitable absorption characteristics for absorber materials. Thin film Cu2MnSnS4 samples with various cation compositions are co-sputtered reactively followed by a high temperature anneal. Formation of Cu2MnSnS4 and co-existence of several secondary phases is verified by XRD and Raman. Our investigation of the crystal structure based on first-principles DFT confirms that stannite crystal structure is preferred over kesterite, although, further verification considering cation disorder is needed. The direct band gap of Cu2MnSnS4 is calculated as 1.52 eV (1.62 eV) for stannite (kesterite), which coincides with the range of the measured band gaps from spectrophotometry of 1.42-1.59 eV. After further annealing treatments below 240 degrees C, the absorption shows reversible changes: the band gap blue-shifts and the Urbach tail energy is reduced. It is concluded that, just like Cu2ZnSnS4, disorder also occurs in Cu2MnSnS4. The implications of our findings are discussed and related to the current understanding of cation disorder in Cu2ZnSnS4 and related compounds. Furthermore, for the first time first-principles DFT investigations are presented for the thiospinel Cu2MnSn3S8 which is observed experimentally as a secondary phase in Sn-rich Cu2MnSnS4 thin films.
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| 40. |
- Rudisch, Katharina, et al.
(författare)
-
The effect of stoichiometry on Cu-Zn ordering kinetics in Cu2ZnSnS4 thin film
- 2018
-
Ingår i: Journal of Applied Physics. - : AIP Publishing. - 0021-8979 .- 1089-7550. ; 123:16
-
Tidskriftsartikel (refereegranskat)abstract
- Cu-Zn disorder in Cu2ZnSnS4 (CZTS) may be responsible for the large open circuit voltage deficit in CZTS based solar cells. In this study, it was investigated how composition-dependent defect complexes influence the order-disorder transition. A combinatorial CZTS thin film sample was produced with a cation composition gradient across the sample area. The graded sample was exposed to various temperature treatments and the degree of order was analyzed with resonant Raman spectroscopy for various compositions ranging from E- and A-type to B-, F-, and C-type CZTS. We observe that the composition has no influence on the critical temperature of the order-disorder transition, but strongly affects the activation energy. Reduced activation energy is achieved with compositions with Cu/Sn > 2 or Cu/Sn < 1.8 suggesting an acceleration of the cation ordering in the presence of vacancies or interstitials. This is rationalized with reference to the effect of point defects on exchange mechanisms. The implications for reducing disorder in CZTS thin films are discussed in light of the new findings.
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| 41. |
- Scragg, Jonathan, 1983-, et al.
(författare)
-
A Detrimental Reaction at the Molybdenum Back Contact in Cu2ZnSn(S,Se)4 Thin-Film Solar Cells
- 2012
-
Ingår i: Journal of the American Chemical Society. - : American Chemical Society (ACS). - 0002-7863 .- 1520-5126. ; 134:47, s. 19330-19333
-
Tidskriftsartikel (refereegranskat)abstract
- Experimental proof is presented for a hitherto undetected solid-state reaction between the solar cell material Cu2ZnSn(S,Se)4 (CZTS(e)) and the standard metallic back contact, molybdenum. Annealing experiments combined with Raman and transmission electron microscopy studies show that this aggressive reaction causes formation of MoS2 and secondary phases at the CZTS|Mo interface during thermal processing. A reaction scheme is presented and discussed in the context of current state-of-the-art synthesis methods for CZTS(e). It is concluded that alternative back contacts will be important for future improvements in CZTS(e) quality.
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| 42. |
- Scragg, Jonathan, 1983-, et al.
(författare)
-
A low-temperature order-disorder transition in Cu2ZnSnS4 thin films
- 2014
-
Ingår i: Applied Physics Letters. - : AIP Publishing. - 0003-6951 .- 1077-3118. ; 104:4, s. 041911-
-
Tidskriftsartikel (refereegranskat)abstract
- Cu2ZnSnS4(CZTS) is an interesting material for sustainable photovoltaics, but efficiencies are limitedby the low open-circuit voltage. A possible cause of this is disorder among the Cu and Zn cations, aphenomenon which is difficult to detect by standard techniques. We show that this issue can beovercome using near-resonant Raman scattering, which lets us estimate a critical temperature of 533±10 K for the transition between ordered and disordered CZTS. These findings have deepsignificance for the synthesis of high-quality material, and pave the way for quantitative investigationof the impact of disorder on the performance of CZTS-based solar cells.
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| 43. |
|
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| 44. |
- Scragg, Jonathan, 1983-, et al.
(författare)
-
Cu–Zn disorder and band gap fluctuations in Cu2ZnSn(S,Se)4 : Theoretical and experimental investigations
- 2016
-
Ingår i: Physica status solidi. B, Basic research. - : Wiley. - 0370-1972 .- 1521-3951. ; 253:2, s. 247-254
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Tidskriftsartikel (refereegranskat)abstract
- Cu2ZnSn(S,Se)4 (CZTS(e)) solar cells suffer from low-open-circuit voltages that have been blamed on the existence of band gap fluctuations, with different possible origins. In this paper, we show from both theoretical and experimental standpoints that disorder of Cu and Zn atoms is in all probability the primary cause of these fluctuations. First, quantification of Cu–Zn disorder in CZTS thin films is presented. The results indicate that disorder is prevalent in the majority of practical samples used for solar cells. Then, ab initio calculations for different arrangements and densities of disorder-induced [CuZn + ZnCu] defect pairs are presented and it is shown that spatial variations in band gap of the order of 200 meV can easily be caused by Cu–Zn disorder, which would cause large voltage losses in solar cells. Experiments using Raman spectroscopy and room temperature photoluminescence combined with in situ heat-treatments show that a shift in the energy of the dominant band-to-band recombination pathway correlates perfectly to the order-disorder transition, which clearly implicates Cu–Zn disorder as the cause of band gap fluctuations in CZTS. Our results suggest that elimination or passivation of Cu–Zn disorder could be very important for future improvements in the efficiency of CZTS(e)-based solar cells.
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| 45. |
- Scragg, Jonathan J., et al.
(författare)
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Chemical Insights into the Instability of Cu(2)ZnSnS(4) Films during Annealing
- 2011
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Ingår i: Chemistry of Materials. - : American Chemical Society (ACS). - 0897-4756 .- 1520-5002. ; 23:20, s. 4625-4633
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Tidskriftsartikel (refereegranskat)abstract
- Cu(2)ZnSnS(4) (CZTS) shows great potential for cheap, efficient photovoltaic devices. However, one problem during synthesis of CZTS films is the loss of Sn as a result of decomposition and evaporation of SnS. This paper uses kinetic models to show that the mechanism of the decomposition reaction probably occurs in at least two stages; first, a loss of sulfur which causes dissociation of the structure into binary sulfides, and only then the evaporation of SnS. Knowledge of the reaction mechanism helps to identify the driving force for decomposition as arising from the relative instability of Sn(IV) in CZTS against reduction; this theory is backed up by thermodynamic data. The volatility of SnS further exaggerates the decomposition by rendering it irreversible. This insight, alongside experimental data, allows prediction of the annealing conditions required to stabilize CZTS surfaces. A fundamental incompatibility of CZTS with high-temperature, vacuum-based processing is exposed, distinguishing it from related indium-containing compounds. This offers an explanation as to why the most efficient CZTS devices to-date all arise from "two-stage" fabrication processes involving low temperature deposition followed by annealing at high pressure, and provides key information for designing successful annealing strategies.
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| 46. |
- Scragg, Jonathan J., et al.
(författare)
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Effects of Back Contact Instability on Cu2ZnSnS4 Devices and Processes
- 2013
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Ingår i: Chemistry of Materials. - : American Chemical Society (ACS). - 0897-4756 .- 1520-5002. ; 25:15, s. 3162-3171
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Tidskriftsartikel (refereegranskat)abstract
- Cu2ZnSnS4 (CZTS) is a promising material for thin film solar cells based on sustainable resources. This paper explores some consequences of the chemical instability between CZTS and the standard Mo “back contact” layer used in the solar cell. Chemical passivation of the back contact interface using titanium nitride (TiN) diffusion barriers, combined with variations in the CZTS annealing process, enables us to isolate the effects of back contact chemistry on the electrical properties of the CZTS layer that result from the synthesis, as determined by measurements on completed solar cells. It is found that instability in the back contact is responsible for large current losses in the finished solar cell, which can be distinguished from other losses that arise from instabilities in the surface of the CZTS layer during annealing. The TiN-passivated back contact is an effective barrier to sulfur atoms and therefore prevents reactions between CZTS and Mo. However, it also results in a high series resistance and thus a reduced fill factor in the solar cell. The need for high chalcogen pressure during CZTS annealing can be linked to suppression of the back contact reactions and could potentially be avoided if better inert back contacts were to be developed.
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| 47. |
- Scragg, Jonathan, 1983-, et al.
(författare)
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Rapid annealing of reactively sputtered precursors for Cu2ZnSnS4 solar cells
- 2013
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Ingår i: Progress in Photovoltaics. - : Wiley. - 1062-7995 .- 1099-159X. ; 22:1, s. 10-17
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Tidskriftsartikel (refereegranskat)abstract
- Cu2ZnSnS4 (CZTS) is a promising thin-film absorber material that presents some interesting challenges in fabrication when compared with Cu(In,Ga)Se2. We introduce a two-step process for fabrication of CZTS films, involving reactive sputtering of a Cu-Zn-Sn-S precursor followed by rapid annealing. X-ray diffraction and Raman measurements of the sputtered precursor suggest that it is in a disordered, metastable CZTS phase, similar to the high-temperature cubic modification reported for CZTS. A few minutes of annealing at 550 °C are sufficient to produce crystalline CZTS films with grain sizes in the micrometer range. The first reported device using this approach has an AM1.5 efficiency of 4.6%, with Jsc and Voc both appearing to be limited by interface recombination.
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| 48. |
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| 49. |
- Scragg, Jonathan, 1983-, et al.
(författare)
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Thermodynamic Aspects of the Synthesis of Thin-Film Materials for Solar Cells
- 2012
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Ingår i: ChemPhysChem. - : Wiley. - 1439-4235 .- 1439-7641. ; 13:12, s. 3035-3046
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Tidskriftsartikel (refereegranskat)abstract
- A simple and useful thermodynamic approach to the prediction of reactions taking place during thermal treatment of layers of multinary semiconductor compounds on different substrates has been developed. The method, which uses the extensive information for the possible binary compounds to assess the stability of multinary phases, is illustrated with the examples of Cu(In,Ga)Se2 and Cu2ZnSnSe4 as well as other less-studied ternary and quaternary semiconductors that have the potential for use as absorbers in photovoltaic devices.
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| 50. |
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