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CZTS solar cell device simulation with varying absorber thickness
Uppsala University, Disciplinary Domain of Science and Technology, Technology, Department of Engineering Sciences, Solid State Electronics. (Ångström Solar Cell Group)ORCID iD: 0000-0002-4125-4002
Uppsala University, Disciplinary Domain of Science and Technology, Technology, Department of Engineering Sciences, Solid State Electronics.
Uppsala University, Disciplinary Domain of Science and Technology, Technology, Department of Engineering Sciences, Solid State Electronics.
Uppsala University, Disciplinary Domain of Science and Technology, Technology, Department of Engineering Sciences, Solid State Electronics.
2015 (English)In: 2015 IEEE 42nd Photovoltaic Specialist Conference (PVSC). Proceedings, IEEE conference proceedings, 2015Conference paper, Published 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.

Place, publisher, year, edition, pages
IEEE conference proceedings, 2015.
Series
IEEE Photovoltaic Specialists Conference, ISSN 0160-8371
Keyword [en]
absorber layer, CZTS, device model, photovoltaic cells, simulations
National Category
Electrical Engineering, Electronic Engineering, Information Engineering
Identifiers
URN: urn:nbn:se:uu:diva-268931DOI: 10.1109/PVSC.2015.7355794ISI: 000369992900204ISBN: 978-1-4799-7944-8 (print)OAI: oai:DiVA.org:uu-268931DiVA: diva2:881710
Conference
IEEE 42nd Photovoltaic Specialists Conference, 14-19 June 2015, New Orleans, LA, USA
Available from: 2015-12-11 Created: 2015-12-11 Last updated: 2017-04-18Bibliographically approved
In thesis
1. Modeling and electrical characterization of Cu(In,Ga)Se2 and Cu2ZnSnS4 solar cells
Open this publication in new window or tab >>Modeling and electrical characterization of Cu(In,Ga)Se2 and Cu2ZnSnS4 solar cells
2017 (English)Doctoral thesis, comprehensive summary (Other academic)
Abstract [en]

In this thesis, modeling and electrical characterization have been performed on Cu(In,Ga)Se2 (CIGS) and Cu2ZnSnS4 (CZTS) thin film solar cells, with the aim to investigate potential improvements to power conversion efficiency for respective technology. The modeling was primarily done in SCAPS, and current-voltage (J-V), quantum efficiency (QE) and capacitance-voltage (C-V) were the primary characterization methods. In CIGS, models of a 19.2 % efficient reference device were created by fitting simulations of J-V and QE to corresponding experimental data. Within the models, single and double GGI = Ga/(Ga+In) gradients through the absorber layer were optimized yielding up to 2 % absolute increase in efficiency, compared to the reference models. For CIGS solar cells of this performance level, electron diffusion length (Ln) is comparable to absorber thickness. Thus, increasing GGI towards the back contact acts as passivation and constitutes largest part of the efficiency increase. For further efficiency increase, majority bottlenecks to improve are optical losses and electron lifetime in the CIGS. In a CZTS model of a 6.7 % reference device, bandgap (Eg) fluctuations and interface recombination were shown to be the majority limit to open circuit voltage (Voc), and Shockley-Read-Hall (SRH) recombination limiting Ln and thus being the majority limit to short-circuit current and fill-factor. Combined, Eg fluctuations and interface recombination cause about 10 % absolute loss in efficiency, and SRH recombination about 9 % loss, compared to an ideal system. Part of the Voc-deficit originates from a cliff-type conduction band offset (CBO) between CZTS and the standard CdS buffer layer, and the energy of the dominant recombination path (EA) is around 1 eV, well below Eg for CZTS. However, it was shown that the CBO could be adjusted and improved with Zn1-xSn­xOy buffer layers. Best results gave EA = 1.36 eV, close to Eg = 1.3-1.35 eV for CZTS as given by photoluminescence, and the Voc-deficit decreased almost 100 mV. Experimentally by varying the absorber layer thickness in CZTS devices, the efficiency saturated at <1 μm, due to short Ln, expected to be 250-500 nm, and narrow depletion width, commonly of the order 100 nm in in-house CZTS. Doping concentration (NA) determines depletion width, but is critical to device performance in general. To better estimate NA with C-V, ZnS and CZTS sandwich structures were created, and in conjunction with simulations it was seen that the capacitance extracted from CZTS is heavily frequency dependent. Moreover, it was shown that C-V characterization of full solar cells may underestimate NA greatly, meaning that the simple sandwich structure might be preferable in this type of analysis. Finally, a model of the Cu2ZnSn(S,Se)4 was created to study the effect of S/(S+Se) gradients, in a similar manner to the GGI gradients in CIGS. With lower Eg and higher mobility for pure selenides, compared to pure sulfides, it was seen that increasing S/(S+Se) towards the back contact improves efficiency with about 1 % absolute, compared to the best ungraded model where S/(S+Se) = 0.25. Minimizing Eg fluctuation in CZTS in conjunction with suitable buffer layers, and improving Ln in all sulfo-selenides, are needed to bring these technologies into the commercial realm.

Place, publisher, year, edition, pages
Uppsala: Acta Universitatis Upsaliensis, 2017. 86 p.
Series
Digital Comprehensive Summaries of Uppsala Dissertations from the Faculty of Science and Technology, ISSN 1651-6214 ; 1514
National Category
Engineering and Technology
Identifiers
urn:nbn:se:uu:diva-320308 (URN)978-91-554-9909-9 (ISBN)
Public defence
2017-06-08, Polhemsalen, Ångströmlaboratoriet, Läderhyddsvägen 1, Uppsala, 09:15 (English)
Opponent
Supervisors
Funder
Knut and Alice Wallenberg FoundationSwedish Energy AgencySwedish Research Council
Available from: 2017-05-18 Created: 2017-04-18 Last updated: 2017-06-07

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