Doctoral thesis (Dissertations and theses)
Characterization of the surface properties of polycrystalline Cu(In,Ga)Se2 using a combination of scanning probe microscopy and X-ray photoelectron spectroscopy
Kameni Boumenou, Christian
2022
 

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Keywords :
Cu(In,Ga)Se2, front and rear surface properties, grain boundaries, solar cells, chemical and post deposition treatments, scanning probe microscopy, X-ray photoelectron spectroscopy
Abstract :
[en] Polycrystalline Cu(In,Ga)Se2 (CIGSe) exhibit excellent properties for high power conversion efficiency (PCE) thin film solar cells. In recent years, photovoltaic cells made from CIGSe reached a PCE of 23.4\%, surpassing that of multicrystalline silicon photovoltaic cells. Nevertheless, the changes in surface composition and electronic properties of the absorbers after various solution-based surface treatments are still under intensive investigation and are widely discussed in the literature. In this thesis, the front, the rear surface properties as well as the impact of post-deposition treatments (PDT) on CIGSe absorbers with different elemental compositions were analyzed by scanning tunneling microscopy and spectroscopy, Kelvin probe force microscopy, and X-ray photoelectron spectroscopy. I show that potassium cyanide (KCN) etching reduces the Cu content at the surface of Cu-rich absorbers substantially. The reduction of the Cu-content is accompanied with the formation of a large number of defects at the surface. Scanning tunneling spectroscopy measurements showed that most of these defects could be passivated with Cd ions. A semiconducting surface and no changes in the density of states were measured across the grain boundaries. In addition to the defect passivation an increase in surface band bending due to the substitution of Cu vacancies by Cd ions, which act as shallow donor defects was observed. As in the case of the front surface, the analyses carried out on the back surface of Cu-rich absorbers showed that a detrimental CuxSe secondary phase was also formed at the interface between the MoSe2 layer and CISe absorber after growth. This CuxSe secondary phase at the back contact was not present in Cu-poor absorbers. Regarding the alkali metal post-treated absorbers, I show that the occurrence of an enlarged surface bandgap, often reported on CIGSe absorbers after PDT treatment is only present after H2O rinsing. After ammonia (NH4OH) washing, which is always applied before buffer layer deposition, all the high bandgap precipitates disappeared and an increased amount of an ordered vacancy compound was observed. The thesis thereby gives a comprehensive overview of the CIGSe surfaces after various chemical and post deposition treatments.
Research center :
Physics and Materials Research Unit
Disciplines :
Physics
Author, co-author :
Kameni Boumenou, Christian ;  University of Luxembourg > Faculty of Science, Technology and Medecine (FSTM)
Language :
English
Title :
Characterization of the surface properties of polycrystalline Cu(In,Ga)Se2 using a combination of scanning probe microscopy and X-ray photoelectron spectroscopy
Defense date :
11 May 2022
Number of pages :
213
Institution :
Unilu - University of Luxembourg, Luxembourg, Luxembourg
Degree :
Docteur en Physique
Promotor :
President :
Jury member :
Sadewasser, Sascha
Carron, Romain
Schmidt, Thomas 
Focus Area :
Physics and Materials Science
FnR Project :
FNR11244141 - Surface And Interface Science On Photovoltaic Materials, 2016 (15/03/2017-14/09/2022) - Alex Redinger
Funders :
FNR - Fonds National de la Recherche [LU]
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since 20 June 2022

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