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Modeling and electrical characterization of Cu(In,Ga)Se2 and Cu2ZnSnS4 solar cells
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
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: urn:nbn:se:uu:diva-320308ISBN: 978-91-554-9909-9 (print)OAI: oai:DiVA.org:uu-320308DiVA: diva2:1089208
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
List of papers
1. Optimizing Ga-profiles for highly efficient Cu(In,Ga)Se2 thin film solar cells in simple and complex defect models
Open this publication in new window or tab >>Optimizing Ga-profiles for highly efficient Cu(In,Ga)Se2 thin film solar cells in simple and complex defect models
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2014 (English)In: Journal of Physics D: Applied Physics, ISSN 0022-3727, E-ISSN 1361-6463, Vol. 47, no 48, 485104- p.Article in journal (Refereed) Published
Abstract [en]

Highly efficient Cu(In,Ga)(S,Se)2 photovoltaic thin film solar cells often have a compositional variation of Ga to In in the absorber layer, here described as a Ga-profile. In this work we have studied the role of Ga-profiles in four different models, based on input data from electrical and optical characterizations of an in-house state-of-the-art Cu(In,Ga)Se2 (CIGS) solar cell with power conversion efficiency above 19 %. A simple defect model with mid-gap defects in the absorber layer was compared with models with Ga-dependent defect concentrations and amphoteric defects. In these models optimized single-graded Ga-profiles have been compared with optimized double-graded Ga-profiles. It was found that the defect concentration for effective Shockley-Read-Hall recombination is low for high efficiency CIGS devices and that the doping concentration of the absorber layer, chosen according to the defect model, is paramount when optimizing Ga-profiles. For optimized single-graded Ga-profiles the simulated power conversion efficiency, depending on the model, is 20.5-20.8 %, and the equivalent double-graded Ga-profiles yield 20.6-21.4 %, indicating that the bandgap engineering of the CIGS device structure can lead to improvements in efficiency. Apart from the effects of increased doping in the complex defect models, the results are similar when comparing the complex defect models to the simple defect models. 

Keyword
Solar cell, CIGS, modelling, simulation, optimizing, Ga-profile
National Category
Other Electrical Engineering, Electronic Engineering, Information Engineering
Research subject
Engineering Science with specialization in Electronics
Identifiers
urn:nbn:se:uu:diva-235838 (URN)10.1088/0022-3727/47/48/485104 (DOI)000344941100009 ()
Funder
Swedish Research Council, B0393101Swedish Energy Agency
Available from: 2014-11-17 Created: 2014-11-11 Last updated: 2017-04-18Bibliographically approved
2. Combining strong interface recombination with bandgap narrowing and short diffusion length in Cu2ZnSnS4 device modeling
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, 364-370 p.Article 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.

Keyword
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: 2017-04-18Bibliographically approved
3. Reduced interface recombination in Cu2ZnSnS4 solar cells with atomic layer deposition Zn1-xSnxO buffer layers
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, 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: 2017-04-18Bibliographically approved
4. CZTS solar cell device simulation with varying absorber thickness
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
Keyword
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
5. 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: 2017-04-18
6. Potential of CuS cap to prevent decomposition of Cu2ZnSnS4 during annealing
Open this publication in new window or tab >>Potential of CuS cap to prevent decomposition of Cu2ZnSnS4 during annealing
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2015 (English)In: Physica Status Solidi (a) applications and materials science, ISSN 1862-6300, E-ISSN 1862-6319, Vol. 212, no 12, 2843-2849 p.Article in journal (Refereed) Published
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.

Keyword
cap layer;Cu2ZnSnS4;kesterite;solar cells;thin films
National Category
Condensed Matter Physics Engineering and Technology
Identifiers
urn:nbn:se:uu:diva-263983 (URN)10.1002/pssa.201532420 (DOI)000366589900028 ()
Funder
Swedish Research CouncilSwedish Foundation for Strategic Research EU, FP7, Seventh Framework Programme, 316488
Available from: 2015-10-05 Created: 2015-10-05 Last updated: 2017-04-18Bibliographically approved
7. On the extraction of doping concentration from capacitance-voltage: A Cu2ZnSnS4 and ZnS sandwich structure
Open this publication in new window or tab >>On the extraction of doping concentration from capacitance-voltage: A Cu2ZnSnS4 and ZnS sandwich structure
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2017 (English)In: IEEE Journal of Photovoltaics, ISSN 2156-3381, E-ISSN 2156-3403, Vol. 7, no 5, 1421-1425 p.Article in journal (Refereed) Published
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.

Keyword
Admittance measurement, capacitance-voltage characteristics, kesterite, modeling, semiconductor device doping
National Category
Engineering and Technology
Identifiers
urn:nbn:se:uu:diva-320307 (URN)10.1109/JPHOTOV.2017.2711427 (DOI)000408160700034 ()
Funder
Swedish Research CouncilSwedish Energy AgencyKnut and Alice Wallenberg Foundation
Available from: 2017-04-18 Created: 2017-04-18 Last updated: 2017-10-03

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