Publicação

Deposition and characterization of CdS and ZnO:Al thin films for Cu(In,Ga)Se solar cells

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Resumo:	The purpose of this thesis is to establish a baseline methodology for the optimized deposition of CdS thin films by chemical bath deposition (CBD) and ZnO:Al thin films by atomic layer deposition (ALD), for the development of Cu(In,Ga)Se2 (CIGS) solar cells at INL facilities. A background research on several optimized parameters is presented and taken in consideration for both layers’ growth methodologies. A detailed study on how the chemical bath deposition parameters affect the properties of the resulting CdS thin film is made. The reproducibility of CdS layers with thickness 50-70 nm is successfully shown. The studied parameters that were found to heavily affect the deposition were: i) the bath temperature; ii) the pH (indirectly, through the amount of ammonia); iii) the deposition time. A colour-thickness pattern established a naked-eye projection of the grown sample’s thickness, which allows for a quick and quantitative evaluation of the deposited films. Further XRD measurements are presented, as well as a statistical particle analysis with the software ImageJ applied to optical microscopy images of the samples. Based on these analysis, a modelling for the transition between ion-by-ion deposition and cluster-by-cluster deposition was presented. Further works compass the development of a working sample holder and the production of a detailed experimental protocol. Concerning the ZnO:Al depositions, ALD, which is a potential industrial friendly technique, was used. The study focused on evaluating the growth rates of the system and on understanding the thickness effects on the transmittance and resistance of the resultant films. Two aluminium doping quantities were also studied and an increase of the bandgap energy is confirmed with increasing aluminium doping. The sample with the lowest sheet resistance of 80 Ω.□-1 of this work had a thickness of 735 nm, while presenting transmittance values averaging above 90 % for wavelengths values between 400-1200 nm. As part of understanding the thickness effects, scanning electron microscopy, atomic force microscopy and X-ray diffraction analysis were performed, revealing a correlation between the (110) planar orientation and the resistivity values. Both CdS and ZnO:Al layers are then individually incorporated in (externally manufactured) CIGS solar cells and are characterized. The solar cell results showed that the CIGS layer does not survive the harsh growth conditions of the ALD processing, which demonstrates that more improvements in this technique are needed. Two solar cell devices with 60 nm and a 210 nm CdS layers present fill factors of 0.60 and 0.50 respectively, and efficiency values of 11.6 % and 9.8 %. These results demonstrate better electrical performances for the solar cell with the optimized 60 nm CdS layer, presenting an increase of 1.8 % efficiency with a 150 nm decrease in thickness, highlighting the importance of controlling the CdS deposition procedure.
Autores principais:	Salvador, Paulo Miguel Babo Cunha
Assunto:	Semiconductors Thin film solar cells CdS buffer ZnO:Al TCO CIGS Semicondutores Células solares de filmes finos ZnO:Al óxido condutor e transparente
Ano:	2017
País:	Portugal
Tipo de documento:	dissertação de mestrado
Tipo de acesso:	acesso aberto
Instituição associada:	Universidade do Minho
Idioma:	inglês
Origem:	RepositóriUM - Universidade do Minho