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1.	Comparotto, Corrado, et al. (författare) 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.
2.	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.
3.	Aboulfadl, Hisham, 1986, et al. (författare) Alkali Dispersion in (Ag,Cu)(In,Ga)Se2 Thin Film Solar Cells - Insight from Theory and Experiment 2021 Ingår i: ACS Applied Materials & Interfaces. - : American Chemical Society (ACS). - 1944-8252 .- 1944-8244. ; 13:6, s. 7188-7199 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.
4.	Comparotto, Corrado, et al. (författare) 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 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.
5.	Davydova, Alexandra, et al. (författare) 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 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.
6.	Davydova, Alexandra, et al. (författare) 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.
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) 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.
11.	Ericson, Tove, 1983-, et al. (författare) 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%.
12.	Ericson, Tove, 1983-, et al. (författare) Zn(O,S) Buffer Layers and Thickness Variations of CdS Buffer for Cu2ZnSnS4 Solar Cells 2014 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.
13.	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.
14.	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.
15.	Larsen, Jes K, et al. (författare) 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 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.
16.	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).
17.	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.
18.	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.
19.	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.
20.	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.
21.	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.
22.	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.
23.	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.
24.	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.
25.	Scragg, Jonathan, 1983-, et al. (författare) Thermodynamic Aspects of the Synthesis of Thin-Film Materials for Solar Cells 2012 Ingår i: ChemPhysChem. - : Wiley. - 1439-4235 .- 1439-7641. ; 13:12, s. 3035-3046 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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