Reference : Influence of interface conditioning and dopants on Cd-free buffers for Cu(In,Ga)(S,Se...
Dissertations and theses : Doctoral thesis
Physical, chemical, mathematical & earth Sciences : Physics
Physics and Materials Science
Influence of interface conditioning and dopants on Cd-free buffers for Cu(In,Ga)(S,Se)2 solar cells
Hönes, Christian mailto [University of Luxembourg > Faculty of Science, Technology and Communication (FSTC) > Physics and Materials Science Research Unit]
University of Luxembourg, ​​Luxembourg
Docteur en Physique
Siebentritt, Susanne mailto
Wirtz, Ludger mailto
Dale, Phillip mailto
Schock, Hans-Werner mailto
Hackenberg, Jürgen mailto
[en] Cu(In,Ga)(S,Se)2 ; thin film solar cells ; Cd-free buffer layers ; zinc oxysulfide ; indium sulfide ; chemical bath deposition
[en] In the search for a non-toxic replacement of the commonly employed CdS buffer layer for Cu(In,Ga)(S,Se)2 (CIGSSe) based solar cells, indium sulfide thin films, deposited via thermal evaporation, and chemical bath deposited (CBD) Zn(O,S) thin films are promising materials. However, while both materials have already been successfully utilized in highly efficient cells, solar cells with both materials usually need an ill-defined post-treatment step in order to reach maximum efficiencies, putting them at a disadvantage for mass production.
In this thesis the influence of interface conditioning and dopants on the need for post-treatments is investigated for both materials, giving new insights into the underlying mechanisms and paving the way for solar cells with higher initial efficiencies.
First, CIGSSe solar cells with In2S3 thin film buffer layers, deposited by thermal evaporation, are presented in chapter 3. The distinctive improvement of these buffer layers upon annealing of the completed solar cell and the change of this annealing behavior when the CIGSSe surface is treated with wet-chemical means prior to buffer layer deposition is investigated. Additional model simulations lead to a two-part explanation for the observed effects, involving a reduction of interface recombination, and the removal of a highly p-doped CIGSSe surface layer.
chapter 4 introduces a novel, fast process for the deposition of Zn(O,S) buffer layers on submodule sized substrates. The resulting solar cell characteristics and the effects of annealing and prolonged illumination are discussed within the framework of theoretical considerations involving an electronic barrier for generated charge carriers. The most important influences on such an electronic barrier are investigated by model simulations and an experimental approach to each parameter. This leads to an improved window layer deposition process, absorber optimization, and intentional buffer layer doping, all reducing the electronic barrier and therefore the necessity for post-treatments to some extent.
The energetic barrier discussed above may be avoided altogether by effective interface engineering. Therefore, the controlled incorporation of indium as an additional cation into CBD-Zn(O,S) buffer layers by means of a newly developed alkaline chemical bath deposition process is presented in chapter 5. With increasing amount of incorporated indium, the energetic barrier in the conduction band can be reduced. This is quantitatively assessed by a combination of photoelectron spectroscopy measurements and the determination of the buffer layer's optical band gap. This barrier lowering leads to less distorted current--voltage characteristics and efficiencies above 14 %, comparable to CdS reference cells, without extensive light-soaking.
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