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An Investigation of Meta-Stability in (Ag,Cu)(In,Ga)Se2 Solar Cells
Uppsala University, Disciplinary Domain of Science and Technology, Technology, Department of Materials Science and Engineering, Solar Cell Technology. (Solcellsteknik)ORCID iD: 0000-0001-9972-4655
2025 (English)Doctoral thesis, comprehensive summary (Other academic)
Description
Abstract [en]

This thesis summarises a series of works investigating the effect of Ag alloying on the long-term stability and meta-stability of wide-gap (Ag,Cu)(In,Ga)Se2 (ACIGS) thin-film solar cells. External quantum efficiency, current-voltage, and capacitance-based measurements have been the main characterisation techniques used to investigate these behaviours. Although Ag alloying facilitates high efficiencies and open-circuit voltages (VOC) in the wide-gap devices (despite the high Ga contents), it is revealed that for a [Ag]/([Ag]+[Cu]) ratio (AAC) in excess of 0.5, the behaviour of ACIGS devices changes significantly. Above the threshold, a strong dependence of net doping concentration (Nnet) on group-I/group-III stoichiometry (I/III) is observed, which in turn dictates performance due to low diffusion lengths in high-Ga absorbers. Close-stoichiometric (I/III>0.92) absorbers are almost fully depleted (depletion widths up to 2μm), whilst off-stoichiometric (I/III<0.92) absorbers have high Nnet (1016-1017cm−3). Lightsoaking treatments induce significant reductions in Nnet (up to two orders of magnitude), whilst dark storage and annealing lead to increases. The corresponding changes in the depletion widths are large, leading to significant variations in device performance. Independent of Ag contents, high Ga contents lead to VOC losses after lightsoaking. These meta-stable effects lead to a poor long-term stability in the devices, with one month’s dark storage resulting in 1-2% efficiency losses (absolute) and the persistent VOC losses contributing a further 1-2% loss (absolute). Additionally, a combination of high Ga and high Ag contents is seen to cause large voltage hysteresis in the devices, which is a further concern for the long-term stability and reliability of modules. It is suggested that defects in ordered vacancy compounds, which segregate to the front surface of the high-Ag ACIGS absorber layers, may explain the dependence of Nnet on I/III, whilst the meta-stable variations in Nnet are attributed to a currently unknown bulk defect. VOC losses after lightsoaking are also attributed to a meta-stable defect, with one candidate being the GaI antisite. Considering the full range of absorber compositions evaluated, an AAC of 0.2 is seen to provide the best balance between performance and stability for our high-Ga devices.

Place, publisher, year, edition, pages
Uppsala: Acta Universitatis Upsaliensis, 2025. , p. 96
Series
Digital Comprehensive Summaries of Uppsala Dissertations from the Faculty of Science and Technology, ISSN 1651-6214 ; 2481
Keywords [en]
(Ag, Cu)(In, Ga)Se2, Cu(In, Ga)Se2, Stability, Wide-Gap Chalcopyrite, Metastability, Capacitance Measurements, Admittance Spectroscopy
National Category
Other Materials Engineering
Research subject
Engineering Science with specialization in Electronics
Identifiers
URN: urn:nbn:se:uu:diva-544453ISBN: 978-91-513-2329-9 (print)OAI: oai:DiVA.org:uu-544453DiVA, id: diva2:1918351
Public defence
2025-02-07, Polhemsalen, Ångströmlaboratoriet, Lägerhyddsvägen 1, 751 03, Uppsala, 09:15 (English)
Opponent
Supervisors
Available from: 2025-01-16 Created: 2024-12-04 Last updated: 2025-01-16
List of papers
1. The Effect of Absorber Stoichiometry on the Stability of Widegap (Ag,Cu)(In,Ga)Se2 Solar Cells
Open this publication in new window or tab >>The Effect of Absorber Stoichiometry on the Stability of Widegap (Ag,Cu)(In,Ga)Se2 Solar Cells
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2022 (English)In: Physica status solidi. B, Basic research, ISSN 0370-1972, E-ISSN 1521-3951, Vol. 259, no 11, article id 2200104Article in journal (Refereed) Published
Abstract [en]

(Ag,Cu)(In,Ga)Se2 solar cells with bandgaps of ≈1.45 eV with a large spread in absorber stoichiometry are characterized with the intention of assessing the effect of composition on the stability of the devices. This material is observed to have a poor diffusion length, leading to very strong dependence upon the depletion region width for charge carrier collection. The depletion width is observed to depend strongly upon the stoichiometry value and shrinks significantly after an initial period of dark storage. It is also seen that the depletion width can be varied strongly through light-soaking and dry-heat treatments, with prolonged annealing leading to detrimental contraction and light soaking leading to expansion which increases current collection. The extent of depletion width variation in response to the treatments is also clearly linked to absorber stoichiometry. Consequently, the device performance, particularly the current output, exhibits a stoichiometry dependence and is considerably affected after each round of treatment. Possible causes of this behavior are discussed.

Place, publisher, year, edition, pages
Wiley-VCH Verlagsgesellschaft, 2022
National Category
Other Electrical Engineering, Electronic Engineering, Information Engineering
Identifiers
urn:nbn:se:uu:diva-496436 (URN)10.1002/pssb.202200104 (DOI)000847103800001 ()
Funder
Swedish Energy Agency, 48479−1Swedish Research Council, 2019−04793
Available from: 2023-02-13 Created: 2023-02-13 Last updated: 2024-12-04Bibliographically approved
2. Investigating the Role of Ag and Ga Content in the Stability of Wide-Gap (Ag,Cu)(In,Ga)Se-2 Thin-Film Solar Cells
Open this publication in new window or tab >>Investigating the Role of Ag and Ga Content in the Stability of Wide-Gap (Ag,Cu)(In,Ga)Se-2 Thin-Film Solar Cells
2023 (English)In: Physica status solidi. B, Basic research, ISSN 0370-1972, E-ISSN 1521-3951, Vol. 260, no 7, article id 2300170Article in journal (Refereed) Published
Abstract [en]

The stability of thin-film solar cells spanning a wide range of compositions within the (Ag,Cu)(In,Ga)Se-2 material system is evaluated over time, after dry-heat annealing and after light soaking, and the role of Ag and Ga content is explored. Ag-free CuInSe2 is relatively stable to annealing and storage, while Cu(In,Ga)Se-2 suffers a degradation of fill factor and carrier collection. High-Ga (Ag,Cu)(In,Ga)Se-2 suffers degradation of carrier collection after prolonged annealing, reducing the short-circuit current by approximate to 12%. Ga-free (Ag,Cu)InSe2 loses up to a third of open-circuit voltage and a quarter of fill factor after all treatments are applied. All samples suffer voltage losses after light soaking, with the Ga-free devices losing up to 50 mV and those containing Ga losing up to 90 mV. Ag incorporation leads to a significant reduction in doping, and a significant increase in the response of doping to treatments, with the depletion width of (Ag,Cu)(In,Ga)Se-2 samples expanding from approximate to 0.1 mu m as-grown to beyond 1.0 mu m after all treatments, compared to the Cu(In,Ga)Se-2 sample variation of approximate to 0.1-0.3 mu m. Connections between Ag content, doping instability, and performance degradation are discussed.

Place, publisher, year, edition, pages
Wiley-VCH Verlagsgesellschaft, 2023
Keywords
(Ag, Cu)(In, Ga)Se-2, Cu(In, stability, stoichiometry, wide-gap chalcopyrites
National Category
Condensed Matter Physics Other Electrical Engineering, Electronic Engineering, Information Engineering
Identifiers
urn:nbn:se:uu:diva-511086 (URN)10.1002/pssb.202300170 (DOI)000987151600001 ()
Funder
Swedish Energy Agency, 48479-1Swedish Research Council, 201904793
Available from: 2023-09-07 Created: 2023-09-07 Last updated: 2024-12-04Bibliographically approved
3. Ag-Dependent Behavior Threshold and Metastability in Wide-Gap (Ag,Cu)(In,Ga)Se2 Solar Cells
Open this publication in new window or tab >>Ag-Dependent Behavior Threshold and Metastability in Wide-Gap (Ag,Cu)(In,Ga)Se2 Solar Cells
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2024 (English)In: Solar RRL, E-ISSN 2367-198X, Vol. 8, no 11, article id 2400220Article in journal (Refereed) Published
Abstract [en]

Wide-gap, high-Ga (Ag,Cu)(In,Ga)Se2 thin-film solar cells with a wide range of Ag contents are fabricated and characterized before and after dark storage, dark annealing at 85 degrees C, and light soaking. A 1:4 ratio of Ag to Cu enhances initial device performance significantly, with excess Ag enhancing carrier collection at the expense of open-circuit voltage and fill factor for close-stoichiometric devices. For off-stoichiometric devices, increased open-circuit voltages are offset by losses in carrier collection. Efficiency degradation after treatments is typically increased with additional Ag alloying. A second observation is a behavior threshold identified slightly below an Ag to Cu ratio of 1:1. For compositions below the threshold, the doping response to light soaking and dark annealing is similar to that exhibited by low-Ga Cu(In,Ga)Se2. Above the threshold, light soaking reduces net doping and that dark annealing can even increase net doping. Furthermore, devices above the threshold exhibit a far greater doping responsivity than those below and display a strong dependence of initial performance and stability on group-I/group-III stoichiometry. A third observation is that all devices lose approximate to 1-2% (absolute) in efficiency after a 3 h light soak, indicating that this loss originates from the high-Ga content (1:3 In:Ga), rather than the Ag alloying.

Place, publisher, year, edition, pages
Wiley-VCH Verlagsgesellschaft, 2024
Keywords
(Ag, Cu)(In, Ga)Se-2, Cu(In, Ga)Se-2, stabilities, wide-gap chalcopyrites, metastabilities
National Category
Condensed Matter Physics Materials Chemistry
Identifiers
urn:nbn:se:uu:diva-541378 (URN)10.1002/solr.202400220 (DOI)001206646300001 ()
Funder
Swedish Energy Agency, 48479-1Swedish Research Council, 2019-04793Swedish Research Council, 2019-00207
Available from: 2024-11-05 Created: 2024-11-05 Last updated: 2024-12-04Bibliographically approved
4. Extended Thermal Admittance Spectroscopy for the Investigation of Composition-Dependent Meta-Stability Behaviours in Wide-Gap(Ag,Cu)(In,Ga)Se2 Solar Cells
Open this publication in new window or tab >>Extended Thermal Admittance Spectroscopy for the Investigation of Composition-Dependent Meta-Stability Behaviours in Wide-Gap(Ag,Cu)(In,Ga)Se2 Solar Cells
(English)Manuscript (preprint) (Other academic)
Keywords
(Ag, Cu)(In, Ga)Se2, Cu(In, Ga)Se2, Stability, Wide-Gap Chalcopyrite, Metastability, Admittance Spectroscopy
National Category
Other Physics Topics Other Engineering and Technologies not elsewhere specified
Identifiers
urn:nbn:se:uu:diva-544452 (URN)
Available from: 2024-12-04 Created: 2024-12-04 Last updated: 2024-12-04

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