Dissertations and theses : Doctoral thesis
Physical, chemical, mathematical & earth Sciences : Chemistry
Physical, chemical, mathematical & earth Sciences : Physics
Crossay, Alexandre [University of Luxembourg > Faculty of Science, Technology and Communication (FSTC) > Physics and Materials Science Research Unit]
University of Luxembourg, ​​Luxembourg
Docteur en Chimie
Dale, Phillip mailto
Lagerwall, Jan mailto
Perez Rodriguez, Alejandro
Colombara, Diego mailto
Grand, Pierre-Philippe
[en] solar cell ; thin film ; kesterite ; electrodeposition ; zinc selenide
[en] Thin films of Kesterite Cu2ZnSnSe4 (CZTSe) are prepared via a low energy cost and high material efficiency process, to be potentially used as light absorbers in solar cell devices. The fabrication process involves two main steps: (i) formation of a metallic stack of Cu/Sn/Zn by sequential electrodeposition of Cu, Sn and Zn onto glass/Mo substrates; (ii) reactive annealing at 550°C in presence of Se and SnSe powders to form Kesterite. This thesis mainly aims at understanding the mechanisms of metal alloying and selenization occurring during step (ii), and their effects on the microstructure of the final film, the presence of secondary phases and their distribution in the thin films synthesized. The second objective is to understand their effects on the solar cells parameters.
The stoichiometry of the precursor layers Cu/Sn/Zn is deliberately chosen to be Cu-poor and Zn-rich (Cu/(Zn+Sn)<1 and Zn/Sn>1), as it allows to reach the best power conversion efficiencies. Under these conditions, Kesterite, SnSe2 and ZnSe are expected. However, a study of different compositions shows that the predominant phases present are only Kesterite and ZnSe. SnSe2 is not present because this phase is unstable under the conditions of selenization, which leads to a self-regulation of tin content via gas phase exchange of SnSe during the selenization.
Analyses of the selenization of Cu/Sn/Zn layers at short times and lower temperatures allow to deconstruct the mechanism of Kesterite formation into sequential steps. Because of the diffusion of metals and the formation of alloys, a reorganization of metals is observed in the thin films. The layers are then composed of Sn, Cu-Sn and Cu-Zn phases mainly, which are found to be segregating at large scales of tens of micrometers. During selenium incorporation, a tin self-regulation process is established, in which tin is depleted during the first stages of selenization, and then tin is replenished. ZnSe segregates at the surface of the absorber layer as large islands of 10-20 micrometers. By analyzing a specific position of a sample after the different process steps, it is shown that the segregation of ZnSe at this large scale is originating in the segregation of metals during alloying.
Because of the presence of ZnSe on the surface of the films, part of the photocurrent generated in the absorber layer is not collected, which decrases the short circuit current of the devices. In this sense, a linear decrease of short circuit current is observed when the ZnSe molar ratio is increasing, and confirmed by external quantum efficiency (EQE) measurements showing a decrease of current collected through the whole range of photon energies. An optimal molar ratio of ZnSe/(CZTSe + ZnSe)=0.2 is found. Below this value, the short circuit current decreases, probably due to the formation of other types of harmful secondary phases such as Cu2SnSe3 or Cu2Se.
A strong decrease of open circuit voltage and fill factor of the solar cells is proved to be related to the formation of blisters in the thin films, which result in the creation of pinholes due to their fragility. Formation of these blisters is supposed to originate from hydrogen evolution under the Cu layer during the electrodeposition process.
Finally, a study of an additional process of prealloying between the steps of electrodeposition and selenization is presented, which demonstrates the possibility to increase the open circuit voltage of the solar cells by varying the time of this alloying step. A best power conversion efficiency of 7.2% is achieved via this method, which is close to the highest value of 9.1% reported for an electrodeposition-based process of Kesterite synthesis.
FP7 ; 284486 - SCALENANO - Development and scale-up of nanostructured based materials and processes for low cost high efficiency chalcogenide based photovoltaics

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