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First-principles study of dislocations in Cu(In,Ga)Se2 solar cell absorbers

Barragan-Yani, Daniel Antonio :
First-principles study of dislocations in Cu(In,Ga)Se2 solar cell absorbers.
[Online-Edition: https://tuprints.ulb.tu-darmstadt.de/8195]
Technische Universität , Darmstadt
[Ph.D. Thesis], (2018)

Official URL: https://tuprints.ulb.tu-darmstadt.de/8195

Abstract

Among the thin-film solar cells, the maximum efficiencies are achieved by devices that use Cu(In,Ga)Se2 as absorber. However, this fact should not mask that there is room for improvement, if we could mitigate the main sources of efficiency loss in this solar cell type, which are induced by lattice defects. Therefore, a more complete picture of the nature of defects in Cu(In,Ga)Se2-based solar cells would help to improve the growth process in such way that detrimental defects are avoided and the efficiency increased. In order to achieve this goal, first-principles calculations provide valuable insights that complement experimental studies and can also be used as predictive tools. These calculations have been and continue to be successfully used for the case of point and planar defects in Cu(In,Ga)Se2-based solar cells.

However, a defect type that has been studied to a lesser extent are lattice dislocations. The aim of this thesis is to carry out a complete study of the structural and electronic properties of Frank partials and perfect dislocations in CuInSe2 and CuGaSe2 . Results from this study allow us to solve, at least partially, the puzzle of Cu(In,Ga)Se2-based solar cells which exhibit decent efficiencies and at the same time have a very high dislocation density. Specifically, in the case of Frank partials our results suggest that these cores prefer to be non-stoichiometric and, as a consequence, are expected to be highly detrimental. Therefore, this defect type should not be present in a fully grown and highly efficient device. Furthermore, we relate the beneficial effect of the Cu-rich stage of the three-stage co-evaporation process used to deposit the absorber in high-efficiency devices with the disappearance of these loops. In the case of stoichiometric perfect dislocations, our results show that their electrical activity is related to the presence of cation-cation or anion-anion "wrong" bonds in the cores. Moreover, we found that cation-rich α-cores are active in the Cu(In,Ga)Se2 semiconductor alloy, whereas the anion-rich β-cores are not. These results, along with the study of sodium segregation tendency into the electrically active cores, are put in perspective with respect to the experimental findings and structural models available in literature.

Item Type: Ph.D. Thesis
Erschienen: 2018
Creators: Barragan-Yani, Daniel Antonio
Title: First-principles study of dislocations in Cu(In,Ga)Se2 solar cell absorbers
Language: English
Abstract:

Among the thin-film solar cells, the maximum efficiencies are achieved by devices that use Cu(In,Ga)Se2 as absorber. However, this fact should not mask that there is room for improvement, if we could mitigate the main sources of efficiency loss in this solar cell type, which are induced by lattice defects. Therefore, a more complete picture of the nature of defects in Cu(In,Ga)Se2-based solar cells would help to improve the growth process in such way that detrimental defects are avoided and the efficiency increased. In order to achieve this goal, first-principles calculations provide valuable insights that complement experimental studies and can also be used as predictive tools. These calculations have been and continue to be successfully used for the case of point and planar defects in Cu(In,Ga)Se2-based solar cells.

However, a defect type that has been studied to a lesser extent are lattice dislocations. The aim of this thesis is to carry out a complete study of the structural and electronic properties of Frank partials and perfect dislocations in CuInSe2 and CuGaSe2 . Results from this study allow us to solve, at least partially, the puzzle of Cu(In,Ga)Se2-based solar cells which exhibit decent efficiencies and at the same time have a very high dislocation density. Specifically, in the case of Frank partials our results suggest that these cores prefer to be non-stoichiometric and, as a consequence, are expected to be highly detrimental. Therefore, this defect type should not be present in a fully grown and highly efficient device. Furthermore, we relate the beneficial effect of the Cu-rich stage of the three-stage co-evaporation process used to deposit the absorber in high-efficiency devices with the disappearance of these loops. In the case of stoichiometric perfect dislocations, our results show that their electrical activity is related to the presence of cation-cation or anion-anion "wrong" bonds in the cores. Moreover, we found that cation-rich α-cores are active in the Cu(In,Ga)Se2 semiconductor alloy, whereas the anion-rich β-cores are not. These results, along with the study of sodium segregation tendency into the electrically active cores, are put in perspective with respect to the experimental findings and structural models available in literature.

Place of Publication: Darmstadt
Divisions: 11 Department of Materials and Earth Sciences
11 Department of Materials and Earth Sciences > Material Science
11 Department of Materials and Earth Sciences > Material Science > Materials Modelling
Date Deposited: 25 Nov 2018 20:55
Official URL: https://tuprints.ulb.tu-darmstadt.de/8195
URN: urn:nbn:de:tuda-tuprints-81954
Referees: Albe, Prof. Dr. Karsten and Klein, Prof. Dr. Andreas and Scheer, Prof. Dr. Roland and Krupke, Prof. Dr. Ralph
Refereed / Verteidigung / mdl. Prüfung: 3 September 2018
Alternative Abstract:
Alternative abstract Language
Bei Dünnschichtsolarzellen werden die maximalen Effizienzwerte von Ger\"aten erreicht, die Cu(In,Ga)Se2 als Absorber verwenden. Diese Tatsache sollte jedoch nicht darüber hinwegtäuschen, dass es hier immer noch Verbesserungspotential gibt. Die Hauptquellen für Effizienzverluste in diesem Solarzellentyp werden durch Gitterdefekte erzeugt. Daher würde ein vollständigeres Verständnis dieser Defektart in Cu(In,Ga)Se2-basierten Solarzellen helfen, den Wachstumsprozess so zu verbessern, dass schädliche Defekte vermieden und die Effizienz gesteigert wird. Um dieses Ziel zu erreichen, liefern First-Principles-Berechnungen wertvolle Erkenntnisse, die experimentelle Studien ergänzen und auch als prädiktive Werkzeuge genutzt werden können. Diese Berechnungen wurden und werden erfolgreich für den Fall von Punkt- und Planardefekten in Cu(In,Ga)Se2-basierten Solarzellen durchgeführt. Ein Defekttyp, der bis jetzt in geringerem Maße untersucht wurde, sind jedoch Gitterversetzungen. Das Ziel dieser Arbeit ist es, eine vollständige Studie über die strukturellen und elektronischen Eigenschaften von Frank Partialversetzungen und perfekten Versetzungen in CuInSe2 und CuGaSe2 durchzuführen. Die Ergebnisse dieser Studie erlauben es uns, zumindest teilweise, das Rätsel zu lösen, warum Cu(In,Ga)Se2-basierten Solarzellen mit einer sehr hohen Versetungsdichte trotzdem einen guten Wirkungsgrad aufweisen können. Insbesondere im Falle von Frank Partialversetzungen deuten unsere Ergebnisse darauf hin, dass diese Kerne eine nicht-stöchiometrische Konfiguration bevorzugen und daher hochgradig schädlich sind. Deshalb sollte dieser Fehlertyp am Ende des Herstellungsprozesses nicht vorhanden sein um ein hocheffizienten Solarzelle zu erzeugen. Wir konnten den positiven Effekt der Cu-reichen Stufe des dreistufigen Co-Verdampfungsprozesses, mit dem der Absorber in hocheffizienten Geräten abgeschieden wird, mit dem Verschwinden dieser Partialversetzungen verknüpfen. Im Falle von stöchiometrischen perfekten Versetzungen zeigen unsere Resultate, dass deren elektrische Aktivität mit dem Vorhandensein von Kationen-Kationen oder Anionen-Anionen "falschen" Bindungen in den Kernen zusammenhängt. Außerdem haben wir herausgefunden, dass kationenreiche α-Kerne in Cu(In,Ga)Se2 Halbleitern elektrisch aktiv sind, während die anionreichen β-Kerne es nicht sind. Diese Ergebnisse, zusammen mit unserer Untersuchung der Tendenz zur Natriumanreicherung in den elektrisch aktiven Kernen, werden in Bezug zu experimentellen Befunden und Strukturmodelle in der Literatur gesetzt.German
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