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  • 1.
    Bilousov, Oleksandr V.
    et al.
    Uppsala University, Disciplinary Domain of Science and Technology, Technology, Department of Engineering Sciences, Solid State Electronics.
    Ren, Yi
    Uppsala University, Disciplinary Domain of Science and Technology, Technology, Department of Engineering Sciences, Solid State Electronics.
    Törndahl, Tobias
    Uppsala University, Disciplinary Domain of Science and Technology, Technology, Department of Engineering Sciences, Solid State Electronics.
    Donzel-Gargand, Olivier
    Uppsala University, Disciplinary Domain of Science and Technology, Technology, Department of Engineering Sciences, Solid State Electronics.
    Ericson, Tove
    Uppsala University, Disciplinary Domain of Science and Technology, Technology, Department of Engineering Sciences, Solid State Electronics.
    Platzer Björkman, Charlotte
    Uppsala University, Disciplinary Domain of Science and Technology, Technology, Department of Engineering Sciences, Solid State Electronics.
    Edoff, Marika
    Uppsala University, Disciplinary Domain of Science and Technology, Technology, Department of Engineering Sciences, Solid State Electronics.
    Hägglund, Carl
    Uppsala University, Disciplinary Domain of Science and Technology, Technology, Department of Engineering Sciences, Solid State Electronics.
    ALD of phase controlled tin monosulfide thin films2017Conference paper (Refereed)
    Abstract [en]

    Tin monosulfide (SnS) is a promising semiconductor material for low-cost conversion of solar energy, playing the role of absorber layer in photovoltaic devices. SnS is, due to its high optical damping, also an excellent semiconductor candidate for the realization of ultrathin (nanoscale thickness) plasmonic solar cells [1].

    Here, we present an important step to further control and understand SnS film properties produced using low temperature ALD with Sn(acac)2 and H2S as precursors. We show that the SnS film properties vary over a rather wide range depending on substrate temperature and reaction conditions, and that this is connected to the growth of cubic (π-SnS) and orthorhombic SnS phases. The optical properties of the two polymorphs differ significantly, as demonstrated by spectroscopic ellipsometry [2].

    1. C. Hägglund, G. Zeltzer, R. Ruiz, A. Wangperawong, K. E. Roelofs, S. F. Bent, ACS Photonics 3 (3) (2016) 456–463.

    2. O. V. Bilousov, Y. Ren, T. Törndahl, O. Donzel-Gargand , T. Ericson, C. Platzer-Björkman, M. Edoff, and C. Hägglund, ACS Chemistry of Materials  29 (7) (2017) 2969–2978.

  • 2.
    Bilousov, Oleksandr V.
    et al.
    Uppsala University, Disciplinary Domain of Science and Technology, Technology, Department of Engineering Sciences, Solid State Electronics.
    Ren, Yi
    Uppsala University, Disciplinary Domain of Science and Technology, Technology, Department of Engineering Sciences, Solid State Electronics.
    Törndahl, Tobias
    Uppsala University, Disciplinary Domain of Science and Technology, Technology, Department of Engineering Sciences, Solid State Electronics.
    Donzel-Gargand, Olivier
    Uppsala University, Disciplinary Domain of Science and Technology, Technology, Department of Engineering Sciences, Solid State Electronics.
    Ericson, Tove
    Uppsala University, Disciplinary Domain of Science and Technology, Technology, Department of Engineering Sciences, Solid State Electronics.
    Platzer-Björkman, Charlotte
    Uppsala University, Disciplinary Domain of Science and Technology, Technology, Department of Engineering Sciences, Solid State Electronics.
    Edoff, Marika
    Uppsala University, Disciplinary Domain of Science and Technology, Technology, Department of Engineering Sciences, Solid State Electronics.
    Hägglund, Carl
    Uppsala University, Disciplinary Domain of Science and Technology, Technology, Department of Engineering Sciences, Solid State Electronics.
    Atomic Layer Deposition of Cubic and Orthorhombic Phase Tin Monosulfide2017In: Chemistry of Materials, ISSN 0897-4756, E-ISSN 1520-5002, Vol. 29, no 7, p. 2969-2978Article in journal (Refereed)
    Abstract [en]

    Tin monosulfide (SnS) is a promising light-absorbing material with weak environmental constraints for application in thin film solar cells. In this paper, we present low-temperature atomic layer deposition (ALD) of high-purity SnS of both cubic and orthorhombic phases. Using tin(II) 2,4-pentanedionate [Sn(acac)(2)] and hydrogen sulfide (H2S) as precursors, controlled growth of the two polymorphs is achieved. Quartz crystal microbalance measurements are used to establish saturated conditions and show that the SnS ALD is self-limiting over temperatures from at least 80 to 160 degrees C. In this temperature window, a stable mass gain of 19 ng cm(-2) cycle(-1) is observed. The SnS thin film crystal structure and morphology undergo significant changes depending on the conditions. High-resolution transmission electron microscopy and X-ray diffraction demonstrate that fully saturated growth requires a large H2S dose and results in the cubic phase. Smaller H2S doses and higher temperatures favor the orthorhombic phase. The optical properties of the two polymorphs differ significantly, as demonstrated by spectroscopic ellipsometry. The orthorhombic phase displays a wide (0.3-0.4 eV) Urbach tail in the near-infrared region, ascribed to its nanoscale structural disorder and/or to sulfur vacancy-induced gap states. In contrast, the cubic phase is smooth and void-free and shows a well-defined, direct forbidden-type bandgap of 1.64 eV.

  • 3.
    Englund, Sven
    et al.
    Uppsala University, Disciplinary Domain of Science and Technology, Technology, Department of Engineering Sciences, Solid State Electronics. Uppsala universitet.
    Paneta, Valentina
    Uppsala University, Disciplinary Domain of Science and Technology, Physics, Department of Physics and Astronomy, Applied Nuclear Physics.
    Primetzhofer, Daniel
    Uppsala University, Disciplinary Domain of Science and Technology, Physics, Department of Physics and Astronomy, Applied Nuclear Physics.
    Ren, Yi
    Uppsala University, Disciplinary Domain of Science and Technology, Technology, Department of Engineering Sciences, Solid State Electronics.
    Donzel-Gargand, Olivier
    Uppsala University, Disciplinary Domain of Science and Technology, Technology, Department of Engineering Sciences, Solid State Electronics.
    Larsen, Jes K
    Uppsala University, Disciplinary Domain of Science and Technology, Technology, Department of Engineering Sciences, Solid State Electronics.
    Scragg, Jonathan J.
    Uppsala University, Disciplinary Domain of Science and Technology, Technology, Department of Engineering Sciences, Solid State Electronics.
    Platzer Björkman, Charlotte
    Uppsala University, Disciplinary Domain of Science and Technology, Technology, Department of Engineering Sciences, Solid State Electronics.
    Characterization of TiN back contact interlayers with varied thickness for Cu2ZnSn(S,Se)4 thin film solar cells2017In: Thin Solid Films, ISSN 0040-6090, E-ISSN 1879-2731, Vol. 639, p. 91-97Article in journal (Refereed)
    Abstract [en]

    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.

  • 4.
    Frisk, Christopher
    et al.
    Uppsala University, Disciplinary Domain of Science and Technology, Technology, Department of Engineering Sciences, Solid State Electronics.
    Ren, Yi
    Uppsala University, Disciplinary Domain of Science and Technology, Technology, Department of Engineering Sciences, Solid State Electronics.
    Li, Shu-Yi
    Uppsala University, Disciplinary Domain of Science and Technology, Technology, Department of Engineering Sciences, Solid State Electronics.
    Platzer-Björkman, Charlotte
    Uppsala University, Disciplinary Domain of Science and Technology, Technology, Department of Engineering Sciences, Solid State Electronics.
    CZTS solar cell device simulation with varying absorber thickness2015In: 2015 IEEE 42nd Photovoltaic Specialist Conference (PVSC). Proceedings, IEEE conference proceedings, 2015Conference paper (Refereed)
    Abstract [en]

    In this study the influence of absorber layer thickness on the trends of the four current-voltage (J-V) parameters for our CZTS solar cells is studied with simulations and compared with empirical data. In the case of dominating interface recombination we find that open-circuit voltage and fill-factor are largely unaffected of thickness variations 0.5 – 2.0 μm, whereas short-circuit current, and thereby efficiency, saturates (98 % of max) at >1.1 μm absorber thickness, in agreement with measurements. In the case of suppressed interface recombination all four J-V parameters exhibit strong thickness dependence at <0.5 μm due to back contact recombination.

  • 5.
    Frisk, Christopher
    et al.
    Uppsala University, Disciplinary Domain of Science and Technology, Technology, Department of Engineering Sciences, Solid State Electronics.
    Ren, Yi
    Uppsala University, Disciplinary Domain of Science and Technology, Technology, Department of Engineering Sciences, Solid State Electronics.
    Olsson, Jörgen
    Uppsala University, Disciplinary Domain of Science and Technology, Technology, Department of Engineering Sciences, Solid State Electronics.
    Törndahl, Tobias
    Uppsala University, Disciplinary Domain of Science and Technology, Technology, Department of Engineering Sciences, Solid State Electronics.
    Annoni, Filippo
    CNR, IMEM..
    Platzer Björkman, Charlotte
    Uppsala University, Disciplinary Domain of Science and Technology, Technology, Department of Engineering Sciences, Solid State Electronics.
    On the extraction of doping concentration from capacitance-voltage: A Cu2ZnSnS4 and ZnS sandwich structure2017In: IEEE Journal of Photovoltaics, ISSN 2156-3381, E-ISSN 2156-3403, Vol. 7, no 5, p. 1421-1425Article in journal (Refereed)
    Abstract [en]

    The capacitance-voltage (C-V) method is frequently used to evaluate the net doping of thin-film solar cells, an important parameter for the function of solar cells. However, complex materials such as kesterites are challenging to characterize. To minimize ambiguity when determining the apparent doping concentration (N-A) of Cu2ZnSnS4 (CZTS), we fabricated and investigated different structures: CZTS/ZnS metal-insulator-semiconductor (MIS) device, stand-alone CZTS and ZnS metal-sandwich structures, and CZTS solar cells. Characterization was carried out by means of admittance spectroscopy (AS) and C-V measurements. ZnS exhibits excellent intrinsic properties, and with the high-quality MIS sample we managed to successfully isolate the capacitive response of the CZTS itself. N-A, as extracted from the MIS structure, is found to be more reliable and four times higher compared with the solar cell, impacting any estimated collection efficiency substantially. Data herein presented also show that CZTS has a substantial low-frequency dispersive capacitance and the extraction of N-A depends on the chosen measurement frequency, symptoms of presence of deep defects. Furthermore, the CZTS/ZnS MIS structure is strongly resilient to leakage currents at both forward and reverse voltage bias where contribution from deep defects is minimized and maximized, respectively.

  • 6.
    Larsen, Jes K
    et al.
    Uppsala University, Disciplinary Domain of Science and Technology, Technology, Department of Engineering Sciences, Solid State Electronics.
    Li, Shuyi
    Uppsala University, Disciplinary Domain of Science and Technology, Technology, Department of Engineering Sciences, Solid State Electronics.
    Scragg, Jonathan
    Uppsala University, Disciplinary Domain of Science and Technology, Technology, Department of Engineering Sciences, Solid State Electronics.
    Ren, Yi
    Uppsala University, Disciplinary Domain of Science and Technology, Technology, Department of Engineering Sciences, Solid State Electronics.
    Hägglund, Carl
    Uppsala University, Disciplinary Domain of Science and Technology, Technology, Department of Engineering Sciences, Solid State Electronics.
    Heinemann, Marc
    Kretzschmar, Steffen
    Unold, Thomas
    Platzer-Björkman, Charlotte
    Uppsala University, Disciplinary Domain of Science and Technology, Technology, Department of Engineering Sciences, Solid State Electronics.
    Interference effects in photoluminescence spectra of Cu2ZnSnS4 and Cu(In,Ga)Se2 thin films2015In: Journal of Applied Physics, ISSN 0021-8979, E-ISSN 1089-7550, Vol. 118, no 3, article id 035307Article in journal (Refereed)
    Abstract [en]

    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.

  • 7.
    Larsen, Jes K
    et al.
    Uppsala University, Disciplinary Domain of Science and Technology, Technology, Department of Engineering Sciences, Solid State Electronics.
    Ren, Yi
    Uppsala University, Disciplinary Domain of Science and Technology, Technology, Department of Engineering Sciences, Solid State Electronics.
    Ross, Nils
    Uppsala University, Disciplinary Domain of Science and Technology, Technology, Department of Engineering Sciences, Solid State Electronics. Univ Oslo, Ctr Mat Sci & Nanotechnol, Box 1126, N-0318 Oslo, Norway..
    Särhammar, Erik
    Uppsala University, Disciplinary Domain of Science and Technology, Technology, Department of Engineering Sciences, Solid State Electronics.
    Li, Shiyu
    Uppsala University, Disciplinary Domain of Science and Technology, Technology, Department of Engineering Sciences, Solid State Electronics.
    Platzer Björkman, Charlotte
    Uppsala University, Disciplinary Domain of Science and Technology, Technology, Department of Engineering Sciences, Solid State Electronics.
    Surface modification through air annealing Cu2ZnSn(S,Se)4 absorbers2017In: Thin Solid Films, ISSN 0040-6090, E-ISSN 1879-2731, Vol. 633, p. 118-121Article in journal (Refereed)
    Abstract [en]

    Recent studies demonstrate that air annealing can have a positive effect on the device performance of Cu2ZnSn(SxSe1-x)(4)[CZTSSe] solar cells. In this work air annealing of the selenium containing CZTSSe is compared to the pure sulfide CZTS. It is discovered that the selenium containing absorbers benefit from air annealing at higher temperatures than selenium free absorbers. The highest efficiency obtained utilizing the air annealing treatment on selenium containing absorbers is 9.7%. We find that the band gap is narrowed when air annealing, which is partially explained by increased Cu-Zn disorder. Furthermore Zn enrichment of the surface after etching is identified as a possible cause of enhanced device performance. It is additionally observed that elemental selenium present on the CZTSSe surface is reduced in the air annealing treatment. Selenium removal is another possible explanation for the enhanced performance caused by the air annealing treatment.

  • 8.
    Larsen, Jes K
    et al.
    Uppsala University, Disciplinary Domain of Science and Technology, Technology, Department of Engineering Sciences, Solid State Electronics.
    Scragg, J. J.
    Uppsala University, Disciplinary Domain of Science and Technology, Technology, Department of Engineering Sciences, Solid State Electronics.
    Frisk, C
    Uppsala University, Disciplinary Domain of Science and Technology, Technology, Department of Engineering Sciences, Solid State Electronics.
    Ren, Y.
    Uppsala University, Disciplinary Domain of Science and Technology, Technology, Department of Engineering Sciences, Solid State Electronics.
    Platzer-Björkman, C.
    Uppsala University, Disciplinary Domain of Science and Technology, Technology, Department of Engineering Sciences, Solid State Electronics.
    CuS Cap to Prevent Decomposition of Cu2ZnSnS4 Precursors during Annealing2015Conference paper (Other academic)
    Abstract [en]

    Chemical decomposition of the CZTS surface during annealing is detrimental to device performance. Aiming to obtain more flexibility in the annealing process the surface is protected by a thin CuS cap.

  • 9.
    Larsen, Jes K
    et al.
    Uppsala University, Disciplinary Domain of Science and Technology, Technology, Department of Engineering Sciences, Solid State Electronics.
    Scragg, Jonathan JS
    Uppsala University, Disciplinary Domain of Science and Technology, Technology, Department of Engineering Sciences, Solid State Electronics.
    Frisk, Christopher
    Uppsala University, Disciplinary Domain of Science and Technology, Technology, Department of Engineering Sciences, Solid State Electronics.
    Ren, Yi
    Uppsala University, Disciplinary Domain of Science and Technology, Technology, Department of Engineering Sciences, Solid State Electronics.
    Platzer-Björkman, Charlotte
    Uppsala University, Disciplinary Domain of Science and Technology, Technology, Department of Engineering Sciences, Solid State Electronics.
    Potential of CuS cap to prevent decomposition of Cu2ZnSnS4 during annealing2015In: Physica Status Solidi (a) applications and materials science, ISSN 1862-6300, E-ISSN 1862-6319, Vol. 212, no 12, p. 2843-2849Article in journal (Refereed)
    Abstract [en]

    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.

  • 10.
    Li, Shu-Yi
    et al.
    Uppsala University, Disciplinary Domain of Science and Technology, Technology, Department of Engineering Sciences, Solid State Electronics.
    Hagglund, Carl
    Uppsala University, Disciplinary Domain of Science and Technology, Technology, Department of Engineering Sciences, Solid State Electronics.
    Ren, Yi
    Uppsala University, Disciplinary Domain of Science and Technology, Technology, Department of Engineering Sciences, Solid State Electronics.
    Scragg, Jonathan J. S.
    Uppsala University, Disciplinary Domain of Science and Technology, Technology, Department of Engineering Sciences, Solid State Electronics.
    Larsen, Jes K.
    Uppsala University, Disciplinary Domain of Science and Technology, Technology, Department of Engineering Sciences, Solid State Electronics.
    Frisk, Christopher
    Uppsala University, Disciplinary Domain of Science and Technology, Technology, Department of Engineering Sciences, Solid State Electronics.
    Rudisch, Katharina
    Uppsala University, Disciplinary Domain of Science and Technology, Technology, Department of Engineering Sciences, Solid State Electronics.
    Englund, Sven
    Uppsala University, Disciplinary Domain of Science and Technology, Technology, Department of Engineering Sciences, Solid State Electronics.
    Platzer-Bjorkman, Charlotte
    Uppsala University, Disciplinary Domain of Science and Technology, Technology, Department of Engineering Sciences, Solid State Electronics.
    Optical properties of reactively sputtered Cu2ZnSnS4 solar absorbers determined by spectroscopic ellipsometry and spectrophotometry2016In: Solar Energy Materials and Solar Cells, ISSN 0927-0248, E-ISSN 1879-3398, Vol. 149, p. 170-178Article in journal (Refereed)
    Abstract [en]

    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.

  • 11. Márquez-Prieto, J.
    et al.
    Ren, Yi
    Uppsala University, Disciplinary Domain of Science and Technology, Technology, Department of Engineering Sciences, Solid State Electronics.
    Miles, R.W.
    Pearsall, N.
    Forbes, I.
    The influence of precursor Cu content and two-stage processing conditions on the microstructure of Cu2ZnSnSe42015In: Thin Solid Films, ISSN 0040-6090, E-ISSN 1879-2731, Vol. 582, p. 220-223Article in journal (Refereed)
    Abstract [en]

    This paper reports the influence of processing temperature on the microstructure of Cu2ZnSnSe4 (CZTSe) absorber layers for temperatures between 380 and 550 °C produced using a 2-stage process. X-ray diffraction analysis showed the formation of Cu2ZnSnSe4 over this temperatures range. The Williamson–Hall method was used for microstructural analysis of the CZTSe absorbers, and this showed a progressive decrease of the micro-strain of the CZTSe with increasing selenisation temperature. The influence of precursor Cu content on the microstructure of the CZTSe was also studied. An increase of Cu content in the precursor is correlated to an increase in grain size and a decrease in micro-strain. Raman measurements show an asymmetrical broadening towards lower energies of the main 197 cm− 1 mode for Cu-poor compositions. This study provides an insight into the dependency of the crystallinity of CZTSe on composition and synthesis temperature.

  • 12.
    Ren, Yi
    Uppsala University, Disciplinary Domain of Science and Technology, Technology, Department of Engineering Sciences, Solid State Electronics.
    Annealing of Cu2ZnSn(S,Se)4 Thin Films: A Study of Secondary Compounds and Their Effects on Solar Cells2017Doctoral thesis, comprehensive summary (Other academic)
    Abstract [en]

    Kesterite Cu2ZnSnS4 (CZTS) is interesting as a sustainable photovoltaic technology due to its earth-abundant elements and suitable semiconducting properties. To date, a record efficiency of 12.6% has been achieved but further improvements are required to reach high efficiency for industrial implementation. Among the limiting issues is the understanding of the annealing process, which is crucial in promoting high material quality. In particular, the knowledge of the effects of segregated secondary compounds on solar cell performance is lacking.

    In contrast to formation of ZnS particles throughout CZTS film, it is notable that SnS forms and usually segregates on the CZTS top and rear surfaces. The influence of SnS on CZTS solar cells was studied by electron beam induced current measurements. It is found that SnS presence on the CZTS surfacecan introduce “dead area”, whereas it seems beneficial for solar cell current when accumulates on the CZTS rear. For SnS passivation and from investigation of the passivation effect from an Al2O3 thin layer at the CZTS rear, improvement in overall device performance could not be demonstrated, due to either poor CZTS bulk or non-optimal device structure. The limitation in CZTS bulk quality was shown from a thickness study where carrier collection saturated already about 700-1000 nm CZTS thickness.

    Formation of SnS alongside CZTS implies the anneal is limited by a deficient sulfur partial pressure (PS2). By looking into Sn-S phase transformations in SnS2 films after annealing, we find that PS2 drops rapidly over the annealing time, which could be well-correlated to a series of changes in CZTS material quality including secondary phase formations and defect modifications. It is shown that annealing CZTS under sufficiently high PS2 is critical for CZTS solar cells with high open circuit voltage (upto 783mV was reached), possibly due to the defect modification.

    Besides SnS, it is observed that NaxS compounds are also readily formed on CZTS surfaces, due to Na diffusion from the glass substrate during annealing. NaxS negatively affects the formation of the CdS/CZTS interface during chemical bath deposition. It can be removed by an oxidation process or wet chemical etching.

    List of papers
    1. Reactively sputtered films in the CuxS–ZnS–SnSy system: From metastability to equilibrium
    Open this publication in new window or tab >>Reactively sputtered films in the CuxS–ZnS–SnSy system: From metastability to equilibrium
    Show others...
    2015 (English)In: Thin Solid Films, ISSN 0040-6090, E-ISSN 1879-2731, Vol. 582, p. 208-214Article in journal, Meeting abstract (Refereed) Published
    Abstract [en]

    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.

    Keywords
    Copper zinc tin sulfide; Thin film solar cells; Reactive sputtering; Secondary phases; Raman scattering; X-ray diffraction; Kesterite
    National Category
    Other Materials Engineering Other Electrical Engineering, Electronic Engineering, Information Engineering
    Research subject
    Engineering Science with specialization in Electronics
    Identifiers
    urn:nbn:se:uu:diva-236734 (URN)10.1016/j.tsf.2014.10.076 (DOI)000352225900045 ()
    Funder
    EU, FP7, Seventh Framework Programme, 316488 (KESTCELLS)
    Available from: 2014-11-21 Created: 2014-11-21 Last updated: 2017-12-05Bibliographically approved
    2. Influence of the Cu2ZnSnS4 absorber thickness on thin film solar cells
    Open this publication in new window or tab >>Influence of the Cu2ZnSnS4 absorber thickness on thin film solar cells
    2015 (English)In: Physica status solidi. A, Applied research, ISSN 0031-8965, E-ISSN 1521-396XArticle in journal (Refereed) Published
    National Category
    Engineering and Technology
    Identifiers
    urn:nbn:se:uu:diva-265420 (URN)DOI: 10.1002/pssa.201532311 (DOI)
    Available from: 2015-10-28 Created: 2015-10-28 Last updated: 2017-12-01
    3. Investigation of the SnS/Cu2ZnSnS4 interfaces in Kesterite Thin-Film Solar Cells
    Open this publication in new window or tab >>Investigation of the SnS/Cu2ZnSnS4 interfaces in Kesterite Thin-Film Solar Cells
    Show others...
    2017 (English)In: ACS Energy Letters, E-ISSN 2380-8195, Vol. 2, no 5, p. 976-981Article in journal (Refereed) Published
    Abstract [en]

    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.

    National Category
    Engineering and Technology
    Identifiers
    urn:nbn:se:uu:diva-314941 (URN)10.1021/acsenergylett.7b00151 (DOI)000401500200005 ()
    Funder
    EU, FP7, Seventh Framework Programme, 316488Knut and Alice Wallenberg FoundationSwedish Foundation for Strategic Research Swedish Research Council
    Available from: 2017-02-07 Created: 2017-02-07 Last updated: 2017-06-30Bibliographically approved
    4. Rear Surface Optimization of CZTS Solar Cells by Use of a Passivation Layer With Nanosized Point Openings
    Open this publication in new window or tab >>Rear Surface Optimization of CZTS Solar Cells by Use of a Passivation Layer With Nanosized Point Openings
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    2016 (English)In: IEEE Journal of Photovoltaics, ISSN 2156-3381, E-ISSN 2156-3403, Vol. 6, no 1, p. 332-336Article in journal (Refereed) Published
    Abstract [en]

    Previously, an innovative way to reduce rear interface recombination in Cu(In, Ga)(S, Se)(2) (CIGSSe) solar cells has been successfully developed. In this work, this concept is established in Cu-2(Zn, Sn)(S, Se)(4) (CZTSSe) cells to demonstrate its potential for other thin-film technologies. Therefore, ultrathin CZTS cells with an Al2O3 rear surface passivation layer having nanosized point openings are fabricated. The results indicate that introducing such a passivation layer can have a positive impact on open-circuit voltage (V-OC; +17% rel.), short-circuit current (J(SC); +5% rel.), and fill factor (FF; +9% rel.), compared with corresponding unpassivated cells. Hence, a promising efficiency improvement of 32% rel. is obtained for the rear passivated cells.

    Keywords
    Aluminum oxide, Cu(In, Ga)(S, Se)(2), Cu-2(Zn, Sn)(S, Se)(4), nanosized point contacts, solar cells, surface passivation layer, thin-film
    National Category
    Environmental Engineering
    Identifiers
    urn:nbn:se:uu:diva-274909 (URN)10.1109/JPHOTOV.2015.2496864 (DOI)000367251900048 ()
    Funder
    Swedish Research CouncilSwedish Energy AgencyEU, FP7, Seventh Framework Programme, 300998EU, FP7, Seventh Framework Programme, 327367
    Available from: 2016-01-27 Created: 2016-01-26 Last updated: 2018-08-12Bibliographically approved
    5. Evolution of Cu2ZnSnS4 in non-equilibrium thermal processing with Quasi-in-situ Monitoring of Sulfur Partial Pressure
    Open this publication in new window or tab >>Evolution of Cu2ZnSnS4 in non-equilibrium thermal processing with Quasi-in-situ Monitoring of Sulfur Partial Pressure
    Show others...
    2017 (English)In: Chemistry of Materials, ISSN 0897-4756, E-ISSN 1520-5002Article in journal (Refereed) Published
    National Category
    Engineering and Technology
    Identifiers
    urn:nbn:se:uu:diva-314940 (URN)
    Available from: 2017-02-07 Created: 2017-02-07 Last updated: 2017-05-09
    6. Evolution of Na-S(-O) compounds on the Cu2ZnSnS4 absorber surface and their effects on CdS thin film growth
    Open this publication in new window or tab >>Evolution of Na-S(-O) compounds on the Cu2ZnSnS4 absorber surface and their effects on CdS thin film growth
    Show others...
    2016 (English)In: ACS Applied Materials and Interfaces, ISSN 1944-8244, E-ISSN 1944-8252, Vol. 8, no 28, p. 18600-18607Article in journal (Refereed) Published
    Abstract [en]

    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.

    National Category
    Engineering and Technology
    Identifiers
    urn:nbn:se:uu:diva-298673 (URN)10.1021/acsami.6b04978 (DOI)000380298400096 ()27356214 (PubMedID)
    Funder
    EU, FP7, Seventh Framework Programme, 316488Knut and Alice Wallenberg FoundationSwedish Foundation for Strategic Research Swedish Research Council
    Available from: 2016-07-06 Created: 2016-07-06 Last updated: 2017-11-28Bibliographically approved
    7. Surface modification through air annealing Cu2ZnSn(S,Se)4 absorbers
    Open this publication in new window or tab >>Surface modification through air annealing Cu2ZnSn(S,Se)4 absorbers
    Show others...
    2017 (English)In: Thin Solid Films, ISSN 0040-6090, E-ISSN 1879-2731, Vol. 633, p. 118-121Article in journal (Refereed) Published
    Abstract [en]

    Recent studies demonstrate that air annealing can have a positive effect on the device performance of Cu2ZnSn(SxSe1-x)(4)[CZTSSe] solar cells. In this work air annealing of the selenium containing CZTSSe is compared to the pure sulfide CZTS. It is discovered that the selenium containing absorbers benefit from air annealing at higher temperatures than selenium free absorbers. The highest efficiency obtained utilizing the air annealing treatment on selenium containing absorbers is 9.7%. We find that the band gap is narrowed when air annealing, which is partially explained by increased Cu-Zn disorder. Furthermore Zn enrichment of the surface after etching is identified as a possible cause of enhanced device performance. It is additionally observed that elemental selenium present on the CZTSSe surface is reduced in the air annealing treatment. Selenium removal is another possible explanation for the enhanced performance caused by the air annealing treatment.

    Keywords
    CZTS, Kesterite, Thin film solar cells, Surface modification, Passivation
    National Category
    Materials Engineering Physical Sciences
    Identifiers
    urn:nbn:se:uu:diva-330018 (URN)10.1016/j.tsf.2016.08.030 (DOI)000404802300023 ()
    Conference
    Symposium V on Thin Film Chalcogenide Photovoltaic Materials held at the 13th E-MRS Spring Meeting, MAY 02-06, 2016, Lille, FRANCE
    Note

    Surface modification through air annealing Cu2ZnSn(S,Se)(4) absorbers

    Available from: 2017-10-11 Created: 2017-10-11 Last updated: 2018-09-14Bibliographically approved
  • 13. Ren, Yi
    Influence of the Cu2ZnSnS4 absorber thickness on thin film solar cells2015In: Physica status solidi. A, Applied research, ISSN 0031-8965, E-ISSN 1521-396XArticle in journal (Refereed)
  • 14.
    Ren, Yi
    et al.
    Uppsala University, Disciplinary Domain of Science and Technology, Technology, Department of Engineering Sciences, Solid State Electronics.
    Richter, Michael
    Carl von Ossietzky Univ Oldenburg, Energy & Semicond Res Lab, Lab Chalcogenide Photovolta, D-26111 Oldenburg, Germany.
    Keller, Jan
    Uppsala University, Disciplinary Domain of Science and Technology, Technology, Department of Engineering Sciences, Solid State Electronics.
    Redinger, Alex
    Helmholtz Zentrum Berlin Mat & Energie HZB, Hahn Meitner Pl 1, D-14109 Berlin, Germany.
    Unold, Thomas
    Helmholtz Zentrum Berlin Mat & Energie HZB, Hahn Meitner Pl 1, D-14109 Berlin, Germany.
    Donzel-Gargand, Olivier
    Uppsala University, Disciplinary Domain of Science and Technology, Technology, Department of Engineering Sciences, Solid State Electronics.
    Scragg, Jonathan J. S.
    Uppsala University, Disciplinary Domain of Science and Technology, Technology, Department of Engineering Sciences, Solid State Electronics.
    Platzer Björkman, Charlotte
    Uppsala University, Disciplinary Domain of Science and Technology, Technology, Department of Engineering Sciences, Solid State Electronics.
    Investigation of the SnS/Cu2ZnSnS4 interfaces in Kesterite Thin-Film Solar Cells2017In: ACS Energy Letters, E-ISSN 2380-8195, Vol. 2, no 5, p. 976-981Article in journal (Refereed)
    Abstract [en]

    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.

  • 15.
    Ren, Yi
    et al.
    Uppsala University, Disciplinary Domain of Science and Technology, Technology, Department of Engineering Sciences, Solid State Electronics.
    Ross, Nils
    Uppsala University, Disciplinary Domain of Science and Technology, Technology, Department of Engineering Sciences, Solid State Electronics.
    Larsen, Jes K.
    Uppsala University, Disciplinary Domain of Science and Technology, Technology, Department of Engineering Sciences, Solid State Electronics.
    Rudisch, Katharina
    Uppsala University, Disciplinary Domain of Science and Technology, Technology, Department of Engineering Sciences, Solid State Electronics.
    Scragg, Jonathan J.
    Uppsala University, Disciplinary Domain of Science and Technology, Technology, Department of Engineering Sciences, Solid State Electronics.
    Platzer-Björkman, Charlotte
    Uppsala University, Disciplinary Domain of Science and Technology, Technology, Department of Engineering Sciences, Solid State Electronics.
    Evolution of Cu2ZnSnS4 during Non-Equilibrium Annealing with Quasi-in Situ Monitoring of Sulfur Partial Pressure2017In: Chemistry of Materials, ISSN 0897-4756, E-ISSN 1520-5002, Vol. 29, no 8, p. 3713-3722Article in journal (Refereed)
    Abstract [en]

    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 (P-S2) during the non equilibrium annealing process at high temperatures complicates the synthesis of CZTS with controlled optoelectronic properties. Here we demonstrate that P-S2 can be monitored by investigating the Sn-S phase transformation. We showed that P-S2 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 P-S2 during annealing. It is proven that remarkable V-oc beyond 700 mV for solar cells with nonoptimal CdS buffer can be repeatedly achieved when CZTS is prepared under a sufficiently high P-S2. An ordering treatment before CdS deposition can further improve V-oc to 783 mV.

  • 16.
    Ren, Yi
    et al.
    Uppsala University, Disciplinary Domain of Science and Technology, Technology, Department of Engineering Sciences, Solid State Electronics.
    Scragg, Jonathan
    Uppsala University, Disciplinary Domain of Science and Technology, Technology, Department of Engineering Sciences, Solid State Electronics.
    Edoff, Marika
    Uppsala University, Disciplinary Domain of Science and Technology, Technology, Department of Engineering Sciences, Solid State Electronics.
    Larsen, Jes
    Uppsala University, Disciplinary Domain of Science and Technology, Technology, Department of Engineering Sciences, Solid State Electronics.
    Platzer-Björkman, Charlotte
    Uppsala University, Disciplinary Domain of Science and Technology, Technology, Department of Engineering Sciences, Solid State Electronics.
    Evolution of Na-S(-O) compounds on the Cu2ZnSnS4 absorber surface and their effects on CdS thin film growth2016In: ACS Applied Materials and Interfaces, ISSN 1944-8244, E-ISSN 1944-8252, Vol. 8, no 28, p. 18600-18607Article in journal (Refereed)
    Abstract [en]

    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.

  • 17.
    Ren, Yi
    et al.
    Uppsala University, Disciplinary Domain of Science and Technology, Technology, Department of Engineering Sciences, Solid State Electronics.
    Scragg, Jonathan
    Uppsala University, Disciplinary Domain of Science and Technology, Technology, Department of Engineering Sciences, Solid State Electronics.
    Frisk, Christopher
    Uppsala University, Disciplinary Domain of Science and Technology, Technology, Department of Engineering Sciences, Solid State Electronics.
    Larsen, Jes
    Uppsala University, Disciplinary Domain of Science and Technology, Technology, Department of Engineering Sciences, Solid State Electronics.
    Li, Shuyi
    Uppsala University, Disciplinary Domain of Science and Technology, Technology, Department of Engineering Sciences, Solid State Electronics.
    Platzer-Björkman, Charlotte
    Uppsala University, Disciplinary Domain of Science and Technology, Technology, Department of Engineering Sciences, Solid State Electronics.
    Influence of the Cu2ZnSnS4 absorberthickness on thin film solar cells2015In: Physica status solidi. A, Applied research, ISSN 0031-8965, E-ISSN 1521-396X, Vol. 212, no 12, p. 2889-2896Article in journal (Refereed)
    Abstract [en]

    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.

  • 18.
    Ren, Yi
    et al.
    Uppsala University, Disciplinary Domain of Science and Technology, Technology, Department of Engineering Sciences, Solid State Electronics.
    Scragg, Jonathan J.
    Uppsala University, Disciplinary Domain of Science and Technology, Technology, Department of Engineering Sciences, Solid State Electronics.
    Edoff, Marika
    Uppsala University, Disciplinary Domain of Science and Technology, Technology, Department of Engineering Sciences, Solid State Electronics.
    Platzer Björkman, Charlotte
    Uppsala University, Disciplinary Domain of Science and Technology, Technology, Department of Engineering Sciences, Solid State Electronics.
    Evolution of Na-S(-O) compounds on Cu2ZnSnS4 absorber surface and its effect on CdS growth2016In: 2016 IEEE 43rd Photovoltaic Specialists Conference (PVSC), New York: IEEE, 2016, p. 2255-2257Conference paper (Refereed)
    Abstract [en]

    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.

  • 19.
    Ren, Yi
    et al.
    Uppsala University, Disciplinary Domain of Science and Technology, Technology, Department of Engineering Sciences, Solid State Electronics.
    Scragg, Jonathan J.
    Uppsala University, Disciplinary Domain of Science and Technology, Technology, Department of Engineering Sciences, Solid State Electronics.
    Ericson, Tove
    Uppsala University, Disciplinary Domain of Science and Technology, Technology, Department of Engineering Sciences, Solid State Electronics.
    Kubart, Tomas
    Uppsala University, Disciplinary Domain of Science and Technology, Technology, Department of Engineering Sciences, Solid State Electronics.
    Platzer-Björkman, Charlotte
    Uppsala University, Disciplinary Domain of Science and Technology, Technology, Department of Engineering Sciences, Solid State Electronics.
    Reactively sputtered films in the CuxS–ZnS–SnSy system: From metastability to equilibrium2015In: Thin Solid Films, ISSN 0040-6090, E-ISSN 1879-2731, Vol. 582, p. 208-214Article in journal (Refereed)
    Abstract [en]

    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.

  • 20.
    Rudisch, Katharina
    et al.
    Uppsala University, Disciplinary Domain of Science and Technology, Technology, Department of Engineering Sciences, Solid State Electronics.
    Ren, Yi
    Uppsala University, Disciplinary Domain of Science and Technology, Technology, Department of Engineering Sciences, Solid State Electronics.
    Platzer-Björkman, Charlotte
    Scragg, Jonathan
    Uppsala University, Disciplinary Domain of Science and Technology, Technology, Department of Engineering Sciences, Solid State Electronics.
    Order-disorder transition in B-type Cu2ZnSnS4 and limitations of ordering through thermal treatments2016In: Applied Physics Letters, ISSN 0003-6951, E-ISSN 1077-3118, Vol. 108, no 23, article id 231902Article in journal (Refereed)
    Abstract [en]

    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.

  • 21.
    van Duren, Stephan
    et al.
    Helmholtz Zentrum Berlin, Dept Struct & Dynam Energy Mat, D-14109 Berlin, Germany..
    Ren, Yi
    Uppsala University, Disciplinary Domain of Science and Technology, Technology, Department of Engineering Sciences, Solid State Electronics.
    Scragg, Jonathan
    Uppsala University, Disciplinary Domain of Science and Technology, Technology, Department of Engineering Sciences, Solid State Electronics.
    Just, Justus
    Helmholtz Zentrum Berlin, Dept Struct & Dynam Energy Mat, D-14109 Berlin, Germany..
    Unold, Thomas
    Helmholtz Zentrum Berlin, Dept Struct & Dynam Energy Mat, D-14109 Berlin, Germany..
    In Situ Monitoring of Cu2ZnSnS4 Absorber Formation With Raman Spectroscopy During Mo/Cu2SnS3/ZnS Thin-Film Stack Annealing2017In: IEEE Journal of Photovoltaics, ISSN 2156-3381, E-ISSN 2156-3403, Vol. 7, no 3, p. 906-912Article in journal (Refereed)
    Abstract [en]

    In recent years, Cu-2 ZnSn(S,Se)(4) (kesterite) has become increasingly popular as a sustainable alternative absorber material. Many processes for kesterite synthesis involve a high temperature annealing step (>450 degrees C). This study investigates the possibility of Raman spectroscopy as an in situ monitoring technique during high temperature annealing up to 550 degrees C. Temperature-dependent behavior of Cu2SnS3 (CTS) and Cu2ZnSnS4 (CZTS) was studied for reference purposes. The synthesis of CZTS was performed by annealing a stacked Mo/CTS/ZnS precursor on a glass substrate. Annealing of the precursor stack resulted in formation of kesterite and could be monitored in situ by its main A-mode at 338 cm(-1). At higher temperatures, this mode shifts to lower wavenumbers, is broadened and reduced in intensity. This can be attributed to combined effects of thermal expansion and anharmonic phonon coupling. The shift of the peak position is linearly proportional to the temperature. Thus, given proper calibration, fitting the peak position of the 338 cm(-1) mode during the process yields the sample temperature. Implementation of in situ monitoring with Raman spectroscopy would be a step forward toward desired process control and monitoring during this crucial high temperature annealing step in kesterite synthesis.

  • 22.
    van Duren, Stephan
    et al.
    Helmholtz Zentrum Berlin, Hahn Meitner Pl 1, D-14109 Berlin, Germany..
    Ren, Yi
    Uppsala University, Disciplinary Domain of Science and Technology, Technology, Department of Engineering Sciences, Solid State Electronics.
    Scragg, Jonathan
    Uppsala University, Disciplinary Domain of Science and Technology, Technology, Department of Engineering Sciences, Solid State Electronics.
    Just, Justus
    Helmholtz Zentrum Berlin, Hahn Meitner Pl 1, D-14109 Berlin, Germany..
    Unold, Thomas
    Helmholtz Zentrum Berlin, Hahn Meitner Pl 1, D-14109 Berlin, Germany..
    Raman spectroscopy study on in-situ monitoring of Cu2ZnSnS4 synthesis2015In: 2015 IEEE 42ND PHOTOVOLTAIC SPECIALIST CONFERENCE (PVSC), IEEE conference proceedings, 2015Conference paper (Refereed)
    Abstract [en]

    This study investigates the possibility of Raman spectroscopy as an in-situ monitoring tool for the synthesis of the solar cell material Cu2ZnSnS4 (CZTS) by annealing a stacked precursor ZnS/Cu2SnS3 on a Mo-coated glass substrate. Temperature dependent behaviour of Raman scattering for ZnS and Cu2SnS3 is studied. Both phases can still be detected at respectively 450 degrees C and 550 degrees C. Annealing of Mo/CTS/ZnS precursor stacks resulted in in-situ observation of kesterite CZTS formation with Raman spectroscopy. This is a step towards in-situ optical process control desired in kesterite fabrication.

  • 23.
    Vermang, Bart
    et al.
    Uppsala University, Disciplinary Domain of Science and Technology, Technology, Department of Engineering Sciences, Solid State Electronics.
    Ren, Yi
    Uppsala University, Disciplinary Domain of Science and Technology, Technology, Department of Engineering Sciences, Solid State Electronics.
    Donzel-Gargand, Olivier
    Uppsala University, Disciplinary Domain of Science and Technology, Technology, Department of Engineering Sciences, Solid State Electronics.
    Frisk, Christopher
    Uppsala University, Disciplinary Domain of Science and Technology, Technology, Department of Engineering Sciences, Solid State Electronics.
    Joel, Jonathan
    Uppsala University, Disciplinary Domain of Science and Technology, Technology, Department of Engineering Sciences, Solid State Electronics.
    Salome, Pedro
    Borme, Jerome
    Sadeswasser, Sascha
    Platzer-Björkman, Charlotte
    Uppsala University, Disciplinary Domain of Science and Technology, Technology, Department of Engineering Sciences, Solid State Electronics.
    Edoff, Marika
    Uppsala University, Disciplinary Domain of Science and Technology, Technology, Department of Engineering Sciences, Solid State Electronics.
    Rear Surface Optimization of CZTS Solar Cellsby Use of a Passivation Layer WithNanosized Point Openings2015In: IEEE Journal of Photovoltaics, ISSN 2156-3381, E-ISSN 2156-3403Article in journal (Refereed)
  • 24.
    Vermang, Bart
    et al.
    Uppsala University, Disciplinary Domain of Science and Technology, Technology, Department of Engineering Sciences, Solid State Electronics. Uppsala Univ, Angstrom Solar Ctr, S-75121 Uppsala, Sweden.;Univ Leuven, Dept Elect Engn, B-3001 Leuven, Belgium.;IMEC, Thin Film Photovolta, B-3001 Leuven, Belgium..
    Ren, Yi
    Uppsala University, Disciplinary Domain of Science and Technology, Technology, Department of Engineering Sciences, Solid State Electronics.
    Donzel-Gargand, Olivier
    Uppsala University, Disciplinary Domain of Science and Technology, Technology, Department of Engineering Sciences, Solid State Electronics.
    Frisk, Christopher
    Uppsala University, Disciplinary Domain of Science and Technology, Technology, Department of Engineering Sciences, Solid State Electronics.
    Joel, Jonathan
    Uppsala University, Disciplinary Domain of Science and Technology, Technology, Department of Engineering Sciences, Solid State Electronics.
    Salome, Pedro
    Int Iberian Nanotechnol Lab, Lab Nanostruct Solar Cells, P-4715330 Braga, Portugal..
    Borme, Jerome
    Int Iberian Nanotechnol Lab, Lab Nanostruct Solar Cells, P-4715330 Braga, Portugal..
    Sadewasser, Sascha
    Int Iberian Nanotechnol Lab, Lab Nanostruct Solar Cells, P-4715330 Braga, Portugal..
    Platzer-Bjorkman, Charlotte
    Uppsala University, Disciplinary Domain of Science and Technology, Technology, Department of Engineering Sciences, Solid State Electronics.
    Edoff, Marika
    Uppsala University, Disciplinary Domain of Science and Technology, Technology, Department of Engineering Sciences, Solid State Electronics.
    Rear Surface Optimization of CZTS Solar Cells by Use of a Passivation Layer With Nanosized Point Openings2016In: IEEE Journal of Photovoltaics, ISSN 2156-3381, E-ISSN 2156-3403, Vol. 6, no 1, p. 332-336Article in journal (Refereed)
    Abstract [en]

    Previously, an innovative way to reduce rear interface recombination in Cu(In, Ga)(S, Se)(2) (CIGSSe) solar cells has been successfully developed. In this work, this concept is established in Cu-2(Zn, Sn)(S, Se)(4) (CZTSSe) cells to demonstrate its potential for other thin-film technologies. Therefore, ultrathin CZTS cells with an Al2O3 rear surface passivation layer having nanosized point openings are fabricated. The results indicate that introducing such a passivation layer can have a positive impact on open-circuit voltage (V-OC; +17% rel.), short-circuit current (J(SC); +5% rel.), and fill factor (FF; +9% rel.), compared with corresponding unpassivated cells. Hence, a promising efficiency improvement of 32% rel. is obtained for the rear passivated cells.

  • 25.
    Vermang, Bart
    et al.
    Uppsala University, Disciplinary Domain of Science and Technology, Technology, Department of Engineering Sciences, Solid State Electronics. Univ Leuven, Dept Elect Engn, B-3001 Leuven, Belgium..
    Ren, Yi
    Uppsala University, Disciplinary Domain of Science and Technology, Technology, Department of Engineering Sciences, Solid State Electronics.
    Joel, Jonathan
    Uppsala University, Disciplinary Domain of Science and Technology, Technology, Department of Engineering Sciences, Solid State Electronics.
    Frisk, Christopher
    Uppsala University, Disciplinary Domain of Science and Technology, Technology, Department of Engineering Sciences, Solid State Electronics.
    Donzel-Gargand, Olivier
    Uppsala University, Disciplinary Domain of Science and Technology, Technology, Department of Engineering Sciences, Solid State Electronics.
    Salome, Pedro
    Int Iberian Nanotechnol Lab, Nanostruct Solar Cells, P-4715330 Braga, Portugal..
    Borme, Jerome
    Int Iberian Nanotechnol Lab, Nanostruct Solar Cells, P-4715330 Braga, Portugal..
    Sadewasser, Sascha
    Int Iberian Nanotechnol Lab, Nanostruct Solar Cells, P-4715330 Braga, Portugal..
    Platzer-Björkman, Charlotte
    Uppsala University, Disciplinary Domain of Science and Technology, Technology, Department of Engineering Sciences, Solid State Electronics.
    Edoff, Marika
    Uppsala University, Disciplinary Domain of Science and Technology, Technology, Department of Engineering Sciences, Solid State Electronics.
    Rear surface optimization of CZTS solar cells by use of a passivation layer with nano-sized point openings2015In: 2015 IEEE 42ND PHOTOVOLTAIC SPECIALIST CONFERENCE (PVSC), 2015Conference paper (Refereed)
    Abstract [en]

    Previously, an innovative way to reduce rear interface recombination of Cu(In,Ga)(S,Se)(2) (CIGSSe) solar cells has been successfully developed. In this work, this concept is established in Cu-2(Zn,Sn)(S,Se)(4) (CZTSSe) cells, to demonstrate its potential for other thin-film technologies. Therefore, ultra-thin CZTS cells with an Al2O3 rear surface passivation layer having nano-sized point openings are fabricated. The results indicate that introducing such a passivation layer can have a positive impact on open circuit voltage (V-OC; +49%(rel.)) or short circuit current (J(SC); +17%(rel.)), compared to corresponding unpassivated cells. Hence, a promising efficiency improvement of 52%(rel.) is obtained for the rear passivated cells.

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