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Li, Shuyi
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Publications (10 of 42) Show all publications
Li, S.-Y., Zamulko, S., Persson, C., Ross, N., Larsen, J. K. & Platzer-Björkman, C. (2017). Optical properties of Cu2ZnSn(SxSe1-x)(4) solar absorbers: Spectroscopic ellipsometry and ab initio calculations. Applied Physics Letters, 110(2), Article ID 021905.
Open this publication in new window or tab >>Optical properties of Cu2ZnSn(SxSe1-x)(4) solar absorbers: Spectroscopic ellipsometry and ab initio calculations
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2017 (English)In: Applied Physics Letters, ISSN 0003-6951, E-ISSN 1077-3118, Vol. 110, no 2, article id 021905Article in journal (Refereed) Published
Abstract [en]

Dielectric functions of Cu2ZnSn(SxSe1-x)(4) thin film absorbers with varied x were determined by spectroscopic ellipsometry and ab initio calculations. From the combination of experimental and theoretical studies, the fundamental interband transition energy E-0 (similar to 1-1.5 eV) and the next following transition energy E-1 (similar to 2-3 eV) were identified and found to blue-shift with increasing sulfur anion content, while keeping the energy separation E-1 - E-0 almost constant, similar to 1.4 eV from experiments, and 1 eV from theory. In addition, the average dielectric responses were found to decrease with sulfur anion content from both theoretical and experimental results. The Tauc optical bandgap value E-g determined on samples prepared on Mo and soda lime glass substrate showed a positive linear relationship between x and bandgap E-g. The bandgap bowing factor determined from the theoretical data is 0.09 eV. (C) 2017 Author(s).

National Category
Physical Sciences Engineering and Technology
Identifiers
urn:nbn:se:uu:diva-316037 (URN)10.1063/1.4973353 (DOI)000392835300025 ()
Funder
Knut and Alice Wallenberg FoundationSwedish Research CouncilSwedish Foundation for Strategic Research
Available from: 2017-02-24 Created: 2017-02-24 Last updated: 2017-11-29Bibliographically approved
Ericson, T., Larsson, F., Törndahl, T., Frisk, C., Larsen, J., Kosyak, V., . . . Platzer Björkman, C. (2017). Zinc-Tin-Oxide Buffer Layer and Low Temperature Post Annealing Resulting in a 9.0% Efficient Cd-Free Cu2ZnSnS4 Solar Cell. Solar RRL, 1(5), Article ID 1700001.
Open this publication in new window or tab >>Zinc-Tin-Oxide Buffer Layer and Low Temperature Post Annealing Resulting in a 9.0% Efficient Cd-Free Cu2ZnSnS4 Solar Cell
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2017 (English)In: Solar RRL, ISSN 2367-198X, Vol. 1, no 5, article id 1700001Article in journal (Refereed) Published
Abstract [en]

Zn1−xSnxOy (ZTO) has yielded promising results as a buffer material for the full sulfur Cu2ZnSnS4 (CZTS), with efficiencies continuously surpassing its CdS-references. ZTO can be deposited by atomic layer deposition (ALD), enabling tuning of the conduction band position through the choice of metal ratio or deposition temperature. Thus, an optimization of the conduction band alignment between ZTO and CZTS can be achieved. The ZTO bandgap is generally larger than that of CdS and can therefore yield higher currents due to reduced losses in the short wavelength region. Another advantage is the possibility to omit the toxic Cd. In this study, the ALD process temperature was varied from 105 to 165 °C. Current-blocked devices were obtained at 105 °C, while the highest open-circuit voltage and device efficiency was achieved for 145 °C. The highest fill factor was seen at 165 °C. The best efficiency reached in this study was 9.0%, which, to our knowledge, is the highest efficiency reported for Cd-free full-sulfur CZTS. We also show that the effect of heat needs to be taken into account. The results indicate that part of the device improvement comes from heating the absorber, but that the benefit of using a ZTO-buffer is clear.

National Category
Electrical Engineering, Electronic Engineering, Information Engineering
Identifiers
urn:nbn:se:uu:diva-321006 (URN)10.1002/solr.201700001 (DOI)
Funder
Swedish Energy Agency, 32787Swedish Foundation for Strategic Research , FFL12-0178Swedish Research Council, 2015-04558Knut and Alice Wallenberg Foundation, KAW 2012.0144
Available from: 2017-04-27 Created: 2017-04-27 Last updated: 2018-08-17Bibliographically approved
Frisk, C., Ericson, T., Li, S.-Y., Szaniawski, P., Olsson, J. & Platzer-Björkman, C. (2016). Combining strong interface recombination with bandgap narrowing and short diffusion length in Cu2ZnSnS4 device modeling. Solar Energy Materials and Solar Cells, 144, 364-370
Open this publication in new window or tab >>Combining strong interface recombination with bandgap narrowing and short diffusion length in Cu2ZnSnS4 device modeling
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2016 (English)In: Solar Energy Materials and Solar Cells, ISSN 0927-0248, E-ISSN 1879-3398, Vol. 144, p. 364-370Article in journal (Refereed) Published
Abstract [en]

In this work we establish a device model in SCAPS, incorporating bandgap narrowing, short minority carrier diffusion length and interface recombination. The model is based on a reference device with standard structure; sputtered Mo on soda lime glass, a reactively sputtered and annealed Cu2ZnSnS4 (CZTS) absorber layer, chemical bath deposited CdS and sputtered i-ZnO buffer layers, and front contact formed with sputtered ZnO:Al and an evaporated Ni/Al/Ni grid. The efficiency of the reference device is 6.7%. Model parameter values of the absorber layer are based on the analysis of temperature dependent current–voltage (JVT) measurements, capacitance–voltage (CV) and drive-level capacitance profiling (DLCP) measurements, performed on the reference device, and on the comparison of simulated and measured quantum efficiency (QE) and current–voltage (JV) performance. Additional parameters are taken from literature. The key elements, electron–hole pair generation and recombination in the absorber layer, are the main focus in this study. Reported values of the absorption coefficient of CZTS vary around one order of magnitude when comparing data from reflectance–transmission (RT) measurements with ellipsometry measurements, and calculations. Therefore, a modified semi-empirical absorption coefficient, extracted from RT and QE measurements, with the depletion width from CV and DLCP, is presented and used in this study. The dominating recombination path is evaluated with JVT   analysis and the zero Kelvin activation energy (EA,0) is extracted from both temperature dependent open circuit voltage (VOC) and from modified Arrhenius plots. In each case,is found to be substantially smaller than the bandgap energy, even when considering bandgap narrowing due to disorder, which is an indication that the deficit observed in our CZTS device dominated by interface recombination. Finally, a complete device model is established, with JV   and QE simulations in good agreement with corresponding measurements, where the interface has the biggest impact on the Voc deficit, but with clear contribution from bulk recombination, with minority carrier diffusion length 250 nm, and from bandgap narrowing, giving a lower than nominal bandgap energy of 1.35 eV.

Keywords
absorption coefficient, CZTS, interface recombination, kesterite, modeling, simulation.
National Category
Electrical Engineering, Electronic Engineering, Information Engineering
Identifiers
urn:nbn:se:uu:diva-268929 (URN)10.1016/j.solmat.2015.09.019 (DOI)000366223900047 ()
Funder
Swedish Energy Agency, 32787-3Swedish Research Council, B0393101
Available from: 2015-12-11 Created: 2015-12-11 Last updated: 2017-12-01Bibliographically approved
Li, S.-Y., Hagglund, C., Ren, Y., Scragg, J. J. S., Larsen, J. K., Frisk, C., . . . Platzer-Bjorkman, C. (2016). Optical properties of reactively sputtered Cu2ZnSnS4 solar absorbers determined by spectroscopic ellipsometry and spectrophotometry. Solar Energy Materials and Solar Cells, 149, 170-178
Open this publication in new window or tab >>Optical properties of reactively sputtered Cu2ZnSnS4 solar absorbers determined by spectroscopic ellipsometry and spectrophotometry
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2016 (English)In: Solar Energy Materials and Solar Cells, ISSN 0927-0248, E-ISSN 1879-3398, Vol. 149, p. 170-178Article in journal (Refereed) Published
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.

Keywords
Ellipsometry, Cu2ZnSnS4, Optical properties, Bandgap, Absorption coefficient, Thin film solar cell
National Category
Engineering and Technology
Identifiers
urn:nbn:se:uu:diva-294657 (URN)10.1016/j.solmat.2016.01.014 (DOI)000373539900023 ()
Funder
Knut and Alice Wallenberg FoundationSwedish Research CouncilSwedish Foundation for Strategic Research
Available from: 2016-06-02 Created: 2016-05-26 Last updated: 2017-11-30Bibliographically approved
Frisk, C., Ren, Y., Li, S.-Y. & Platzer-Björkman, C. (2015). CZTS solar cell device simulation with varying absorber thickness. In: 2015 IEEE 42nd Photovoltaic Specialist Conference (PVSC). Proceedings: . Paper presented at IEEE 42nd Photovoltaic Specialists Conference, 14-19 June 2015, New Orleans, LA, USA. IEEE conference proceedings
Open this publication in new window or tab >>CZTS solar cell device simulation with varying absorber thickness
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
Keywords
absorber layer, CZTS, device model, photovoltaic cells, simulations
National Category
Electrical Engineering, Electronic Engineering, Information Engineering
Identifiers
urn:nbn:se:uu:diva-268931 (URN)10.1109/PVSC.2015.7355794 (DOI)000369992900204 ()978-1-4799-7944-8 (ISBN)
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
Ren, Y., Scragg, J., Frisk, C., Larsen, J., Li, S. & Platzer-Björkman, C. (2015). Influence of the Cu2ZnSnS4 absorberthickness on thin film solar cells. Physica status solidi. A, Applied research, 212(12), 2889-2896
Open this publication in new window or tab >>Influence of the Cu2ZnSnS4 absorberthickness on thin film solar cells
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2015 (English)In: Physica status solidi. A, Applied research, ISSN 0031-8965, E-ISSN 1521-396X, Vol. 212, no 12, p. 2889-2896Article in journal (Refereed) Published
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.

National Category
Engineering and Technology
Identifiers
urn:nbn:se:uu:diva-268543 (URN)10.1002/pssa.201532311 (DOI)000366589900034 ()
Funder
EU, FP7, Seventh Framework Programme, 316488Knut and Alice Wallenberg FoundationSwedish Foundation for Strategic Research
Available from: 2015-12-07 Created: 2015-12-07 Last updated: 2017-12-01
Larsen, J. K., Li, S., Scragg, J., Ren, Y., Hägglund, C., Heinemann, M., . . . Platzer-Björkman, C. (2015). Interference effects in photoluminescence spectra of Cu2ZnSnS4 and Cu(In,Ga)Se2 thin films. Journal of Applied Physics, 118(3), Article ID 035307.
Open this publication in new window or tab >>Interference effects in photoluminescence spectra of Cu2ZnSnS4 and Cu(In,Ga)Se2 thin films
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2015 (English)In: Journal of Applied Physics, ISSN 0021-8979, E-ISSN 1089-7550, Vol. 118, no 3, article id 035307Article in journal (Refereed) Published
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.

National Category
Condensed Matter Physics Engineering and Technology
Identifiers
urn:nbn:se:uu:diva-258976 (URN)10.1063/1.4926857 (DOI)000358429200053 ()
Funder
Swedish Research CouncilEU, FP7, Seventh Framework Programme
Available from: 2015-07-23 Created: 2015-07-23 Last updated: 2017-12-04Bibliographically approved
Topalian, Z., Li, S.-Y., Niklasson, G. A., Granqvist, C. & Kish, L. B. (2015). Percolation noise at the metal-insulator transition of nanostructured VO2 films. In: Extended Abstracts: . Paper presented at Conference on Unsolved Problems of Noise (UPoN), Barcelona, Spanien, Juli 13-17.
Open this publication in new window or tab >>Percolation noise at the metal-insulator transition of nanostructured VO2 films
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2015 (English)In: Extended Abstracts, 2015Conference paper, Published paper (Refereed)
National Category
Engineering and Technology
Identifiers
urn:nbn:se:uu:diva-268305 (URN)
Conference
Conference on Unsolved Problems of Noise (UPoN), Barcelona, Spanien, Juli 13-17
Available from: 2015-12-03 Created: 2015-12-03 Last updated: 2018-08-30
Niklasson, G. A., Lansåker, P. C., Li, S.-Y. & Granqvist, C. G. (2015). Plasmonic thin films for application to inproved chromogenic windows. In: Nesheva, D; Chamati, H; Genova, J; Gesheva, K; Ivanova, T; Szekeres, A (Ed.), INERA Conference: Book of Abstracts. Paper presented at Light in nanoscience and nanotechnology, LNN 2015, October 20-22, 2015, Hissar, Bulgaria. , Article ID 012003.
Open this publication in new window or tab >>Plasmonic thin films for application to inproved chromogenic windows
2015 (English)In: INERA Conference: Book of Abstracts / [ed] Nesheva, D; Chamati, H; Genova, J; Gesheva, K; Ivanova, T; Szekeres, A, 2015, article id 012003Conference paper, Published paper (Refereed)
Abstract [en]

Nanocomposites consisting of noble metal nanoparticles in a transparent matrix exhibit plasmonic absorption in the visible wavelength range. On the other hand conducting oxide nanoparticles display a localized plasma absorption in the near infrared. These effects can be exploited in the design of energy-efficient windows in order to obtain improved performance. Electrochromic coatings that switch in the near infrared make use of the modulation of the plasma absorption of oxide nanoparticles due to charge insertion/extraction induced by an external voltage. Thermochromic nanocomposites are predicted to exhibit a much improved energy efficiency, as compared to thermochromics thin films. Plasmonic thermochromics switching in the near infrared has the potential to be significantly larger than in the case of a thin film. Very thin noble metal films are an interesting alternative to conducting oxides as transparent contacts to electrochromic devices. However, in this latter case plasmonic effects are to be avoided rather than exploited.

 

In order to model and optimize chromogenic devices, a good theoretical understanding of plasmonic effects is necessary. The optical properties of nanocomposites are commonly described by effective medium theories. They describe the effective dielectric function of the composite using as input the dielectric functions of the constituents and their respective volume fractions. However, theoretical modelling by effective medium theories is not straightforward since the effective dielectric function is also sensitively dependent on the actual microgeometry of the composite. These structural effects can be described by the so-called spectral density function. In this paper we describe recent work on three topics related to energy efficient chromogenic window coatings: (1) The performance limits of plasmonic electrochromic coatings, (2) the performance of thermochromics nanocomposites and (3) a study of the optical properties of ultrathin silver films by the spectral density formalism.

Series
Journal of Physics: Conference Series, ISSN 1742-6588 ; 682
National Category
Condensed Matter Physics Engineering and Technology
Identifiers
urn:nbn:se:uu:diva-266861 (URN)10.1088/1742-6596/682/1/012003 (DOI)000372173400003 ()
Conference
Light in nanoscience and nanotechnology, LNN 2015, October 20-22, 2015, Hissar, Bulgaria
Funder
Swedish Research Council
Available from: 2015-11-12 Created: 2015-11-12 Last updated: 2018-04-04Bibliographically approved
Platzer-Björkman, C., Frisk, C., Larsen, J., Ericson, T., Li, S., Scragg, J., . . . Törndahl, T. (2015). Reduced interface recombination in Cu2ZnSnS4 solar cells with atomic layer deposition Zn1-xSnxO buffer layers. Applied Physics Letters, 107(24), Article ID 243904.
Open this publication in new window or tab >>Reduced interface recombination in Cu2ZnSnS4 solar cells with atomic layer deposition Zn1-xSnxO buffer layers
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2015 (English)In: Applied Physics Letters, ISSN 0003-6951, E-ISSN 1077-3118, Vol. 107, no 24, article id 243904Article in journal (Refereed) Published
Abstract [en]

Cu2ZnSnS4 (CZTS) solar cells typically include a CdS buffer layer in between the CZTS and ZnO front contact. For sulfide CZTS, with a bandgap around 1.5 eV, the band alignment between CZTS and CdS is not ideal ("cliff-like"), which enhances interface recombination. In this work, we show how a Zn1-xSnxOy (ZTO) buffer layer can replace CdS, resulting in improved open circuit voltages (V-oc) for CZTS devices. The ZTO is deposited by atomic layer deposition (ALD), with a process previously developed for Cu(In,Ga)Se-2 solar cells. By varying the ALD process temperature, the position of the conduction band minimum of the ZTO is varied in relation to that of CZTS. A ZTO process at 95 degrees C is found to give higher Voc and efficiency as compared with the CdS reference devices. For a ZTO process at 120 degrees C, where the conduction band alignment is expected to be the same as for CdS, the Voc and efficiency is similar to the CdS reference. Further increase in conduction band minimum by lowering the deposition temperature to 80 degrees C shows blocking of forward current and reduced fill factor, consistent with barrier formation at the junction. Temperature-dependent current voltage analysis gives an activation energy for recombination of 1.36 eV for the best ZTO device compared with 0.98 eV for CdS. We argue that the Voc of the best ZTO devices is limited by bulk recombination, in agreement with a room temperature photoluminescence peak at around 1.3 eV for both devices, while the CdS device is limited by interface recombination.

National Category
Electrical Engineering, Electronic Engineering, Information Engineering
Identifiers
urn:nbn:se:uu:diva-268548 (URN)10.1063/1.4937998 (DOI)000367318600062 ()
Funder
Swedish Energy AgencySwedish Research CouncilSwedish Foundation for Strategic Research
Available from: 2015-12-07 Created: 2015-12-07 Last updated: 2018-08-17Bibliographically approved
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