Doctoral thesis (Dissertations and theses)
Optical Detection of Deep Defects in Cu(In,Ga)Se2
Spindler, Conrad
2018
 

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Keywords :
Photoluminescence; Defects; Chalcopyrites
Abstract :
[en] The aim of this thesis is to shed light on the deep defect structure in Cu(In,Ga)Se2 by photoluminescence measurements and to propose a possible conclusive defect model by attributing experimental findings to a literature review of defect calculations from first principles. Epitaxial films are grown on GaAs by metal organic vapor phase epitaxy and characterized by photoluminescence at room or low temperature. In CuGaSe2, deep defect bands at ca. 1.1 eV and 1.23 eV are resolved. A model for the power law behavior in excitation dependent measurements of the peak intensities is derived, which leads to the experimental finding of two deep donor-like defects as a result. In Cu(In,Ga)Se2, the deeper band at around 1.1 eV remains constant in energy when more and more gallium is replaced by indium in the solid solution. For decreasing Ga-contents, the band gap is mainly lowered by a decrease of the conduction band energy. From fitting models for electron-phonon coupling, the dominating deep donor-like defect is determined at 1.3 eV above the valence band maximum. This level is proposed to be crucial for high Ga-contents when it is deep inside the band gap and most likely acts as a recombination center. At low Ga-contents it is resonant with the conduction band. The larger open circuit voltage deficits for high Ga-contents are proposed to stem at least partly from this defect which is qualitatively supported by simulations. Additionally another defect band at around 0.7 eV is observed for high Ga-contents at low temperatures and at 0.8 eV for low Ga-contents. The intensity of the 0.8 eV band seems to disappear in a sample with Cu-deficiency. In general, deep luminescence is always observed with similar energies in all Cu-rich compositions, independent of the Ga-content. The deep defect involved could explain inferior efficiencies of Cu-rich devices which show increased non-radiative recombination in general. It is further discussed that the same deep defect could be the origin of a level at 0.8 eV which is observed in several photo-capacitance measurements in literature. Based on the literature review for intrinsic defect calculations by hybrid-functionals, a possible defect model for shallow and deep defects is derived with a focus on those results, where different authors using different methods agree. By comparing the experimental results in the scope of this thesis, the deep defect found at 1.3 eV above the valence band is attributed to the GaCu antisites. The single (0/-1) charge transition of CuIn and CuGa is proposed to be the main shallow acceptor in the near-band-edge luminescence of Cu-rich compositions at 60 - 100 meV, whereas the second (-1/-2) charge transition is attributed to the deep 0.8 eV defect band. The present findings could be useful for the improvement of Cu(In,Ga)Se2 solar cells with stochiometric absorber compositions (Cu-rich growth) or with high band gaps (high Ga-content). Furthermore, the results show a very good agreement of experiment and recent theoretical calculations of defects, which can be seen as a promising relation between photoluminescence spectroscopy and predictions from theory for other complex materials.
Disciplines :
Physics
Author, co-author :
Spindler, Conrad ;  University of Luxembourg > Faculty of Science, Technology and Communication (FSTC) > Physics and Materials Science Research Unit
Language :
English
Title :
Optical Detection of Deep Defects in Cu(In,Ga)Se2
Alternative titles :
[en] Optical Detection of Deep Defects in Cu(In,Ga)Se2
Defense date :
07 June 2018
Number of pages :
164
Institution :
Unilu - University of Luxembourg, Luxembourg, Luxembourg
Degree :
Docteur en Physique
Focus Area :
Physics and Materials Science
FnR Project :
FNR5857739 - Optical Detection Of Deep Defects In Chalcopyrite Semiconductors, 2013 (01/02/2014-31/01/2017) - Susanne Siebentritt
Name of the research project :
ODD - Optical detection of deep defects in chalcopyrite semiconductors
Funders :
FNR - Fonds National de la Recherche [LU]
Available on ORBilu :
since 25 October 2018

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