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Alternative Back Contacts for CZTS Thin Film Solar Cells

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Alternative Back Contacts for CZTS Thin Film Solar Cells Digital Comprehensive Summaries of Uppsala Dissertations from the Faculty of Science and Technology 1900 Alternative back contacts for CZTS thin film solar cells SVEN ENGLUND ACTA UNIVERSITATIS UPSALIENSIS ISSN 1651-6214 ISBN 978-91-513-0866-1 UPPSALA urn:nbn:se:uu:diva-403583 2020 Dissertation presented at Uppsala University to be publicly examined in Häggsalen, Ångströmlaboratoriet, Regementsvägen 1, Uppsala, Friday, 20 March 2020 at 10:15 for the degree of Doctor of Philosophy. The examination will be conducted in English. Faculty examiner: Dr. Levent Gütay (University of Oldenburg). Abstract Englund, S. 2020. Alternative back contacts for CZTS thin film solar cells. (Alternativa bakkontakter för CZTS tunnfilmssolceller). Digital Comprehensive Summaries of Uppsala Dissertations from the Faculty of Science and Technology 1900. 106 pp. Uppsala: Acta Universitatis Upsaliensis. ISBN 978-91-513-0866-1. 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. Keywords: CZTS, thin film solar cells, back contacts, passivation, interface, titanium nitride, ATO, antimony-doped tin oxide, transparent back contact Sven Englund, Department of Materials Science and Engineering, Solar Cell Technology, Box 534, Uppsala University, SE-751 21 Uppsala, Sweden. © Sven Englund 2020 ISSN 1651-6214 ISBN 978-91-513-0866-1 urn:nbn:se:uu:diva-403583 (http://urn.kb.se/resolve?urn=urn:nbn:se:uu:diva-403583) Till mitt älskade Sverige To my beloved Sweden 致瑞典，我的挚爱 List of Papers This thesis is based on the following papers, which are referred to in the text by their Roman numerals. I. Englund, S., Saini, N. Platzer-Björkman C. (2018) Cu2ZnSn(S,Se)4 from annealing of compound co-sputtered pre- cursors – Recent results and open questions. Solar Energy, 175:84–93 II. Englund, S., Paneta, V., Primetzhofer, D., Ren, Y., Donzel-Gar- gand, O., Larsen, J.K., Scragg, J., Platzer-Björkman C. (2017) Characterization of TiN back contact interlayers with varied thickness for Cu2ZnSn(S,Se)4 thin film solar cells, Thin Solid Films, 639:91-97. III. Paneta. V., Englund, S., Suvanam, S., Scragg, J., Platzer-Björk- man. C., Primetzhofer, D., (2019) Ion-beam based characteriza- tion of TiN back contact interlayers for CZTS(e) thin film solar cells, Nuclear Instruments and Methods in Physics Research Sec- tion B: Beam Interactions with Materials and Atoms, 450:262- 266 IV. Englund, S., Grini, S., Donzel-Gargand, O., Paneta, V., Kosyak, V., Primetzhofer, D., Scragg, J. J. S., Platzer-Björkman, C. (2018) TiN Interlayers with Varied Thickness in Cu2ZnSnS(e)4 Thin Film Solar Cells: Effect on Na Diffusion, Back Contact Stability, and Performance, Physica Status Solidi a, 215(23):1800491 V. Englund, S., Kubart, T., Keller, J., Moro M. V., Primetzhofer, D., Suvanam, S. S., Scragg, J. J. S., Platzer-Björkman, C. (2019) An- timony-doped Tin Oxide as Transparent Back Contacts in CZTS Thin Film Solar Cells, Physica Status Solidi a, 2019, 1900542 Reprints were made with permission from the respective publishers. Personal contributions to the papers I. Contributions mainly related to the back contact part; plan- ning, literature review, writing, figures. II. Definition of the research project, literature review, planning, sample preparation (deposition, annealing, STEM lamellae), characterization and analysis (SEM, XPS, XRD), discussion and writing with input from co-authors. III. Literature review, part of planning, sample preparation (sput- ter deposition, annealing), some characterization (XPS), dis- cussion and involved in writing process. IV. Definition of the research project, literature review, planning, sample preparation (deposition of back contacts, annealing, STEM lamellae of interlayer sample), analysis (SEM, XPS, IV, QE, stress), discussion and writing with input from co- authors. V. Part in definition of the research project, literature review, planning, sample preparation (deposition of back contacts, CZTS and NaF, annealing,), analysis (resistivity, XRD, GDOES, IV, QE), discussion and writing with input from co- authors. Contents 1. Introduction ......................................................................................... 13 1.1 Motivation and aim of the thesis ........................................................ 16 2. Background – solar energy and solar cells .......................................... 18 2.1 Solar energy ........................................................................................ 18 2.2 Semiconductors .................................................................................. 20 2.3 Band gap of solar cell materials ......................................................... 22 2.4 Solar cell devices ................................................................................ 26 2.4.2 Doping and formation of pn-junctions ........................................ 26 2.4.4 Comparing properties of solar cell devices ................................. 27 3. CZTS thin film solar cells – fabrication and components ......................... 32 3.2 Deposition techniques and processing ............................................... 33 3.2.1 Sputtering .................................................................................... 33 3.2.2 Other deposition techniques ....................................................... 35 3.2.3 Annealing .................................................................................... 36 3.3 The CZTS solar cell stack .................................................................. 37 3.3.1 Front metal contact grid .............................................................. 37 3.3.2 Window layer.............................................................................. 37 3.3.3 Absorber layer ............................................................................ 38 3.3.4 Back contact................................................................................ 43 3.3.5 Substrate ..................................................................................... 47 4. Characterization methods .......................................................................... 48 4.1 Ion Beam Analysis ............................................................................. 48 4.1.1 Rutherford backscattering spectrometry ..................................... 49 4.1.2 Time-of-Flight Medium-Energy Ion Scattering ......................... 51 4.1.3 Time-of-Flight Elastic Recoil Detection Analysis ...................... 51 4.1.4 Particle Induced X-ray Emission ................................................ 53 4.2 Electron microscopy and X-ray based analysis .................................. 54 4.2.1 Scanning electron microscopy .................................................... 54 4.2.2 Scanning transmission electron
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