The role of the CdS buffer layer in the Cu(In,Ga)Se2 thin film solar cell
1998 (English)Doctoral thesis, comprehensive summary (Other academic)
The Cu(In,Ga)Se2 thin film solar cell is one of the most promising candidates for low-costand high-efficiency solar cells. To date efficiencies approaching 18% have been achieved whenincluding a thin CdS layer prepared by chemical bath deposition (CBD) between theCu(In,Ga)Se2 and ZnO layers. The success of this buffer layer has previously been explained in terms of modified CdS/Cu(In,Ga)Se2 interface properties. In this thesis the approach toelucidate the role of the CBD-CdS layer has been to find out any unique bulk properties of thisthin layer and if they have an influence on the solar cell performance.
Compositional analysis was carried out using X-ray Photoelectron Spectroscopy, Fourier Transform Infrared Spectroscopy, Rutherford Backscattering Spectrometry, and Secondary Ion Mass Spectrometry. It was found that CBD-CdS films made for high-efficiency Cu(In,Ga)Se2 solar cells contain as much as 10-15% oxygen in the form of O-H bonds (mainly H2O) and C=O bonds (CdCO3). Additionally, ca. 5% nitrogen in the form of C≡N bonds (CdNCN or CdSCN) is identified.
The influence of these impurities on the CBD-CdS film properties was investigated by Optical Spectroscopy and X-ray Diffraction. Films with such a high amount of impurities are accompanied by an optical bandgap shift and a softening of the absorption edge. These two phenomena are explained by a decreased lattice parameter and an increased density of defects.
The importance of the bulk properties (or the Impurities) in the CBD-CdS film on the Cu(In,Ga)Se2 solar cell performance was then studied by standard and temperature dependentcurrent-voltage. measurements. A trade-off in the number of impurities for optimal performance is found. By increasing the impurity content in the CBD-CdS buffer layer the open-circuit voltage increases, but finally a cross-over between the light and dark I-V curves appears. This cross-over is explained. by an enhanced density of defects (or traps) in the impurity rich CBD-CdS films. These results indicate that the bulk properties of the CBD-CdS layer play a more crucial role in the Cu(In,Ga)Se2 solar cell than recently suggested.
Other topics in this thesis include the origin, distribution and chemical stability of these impurities as well as a reinvestigation of the CdS/Cu(In,Ga)Se2 interface properties.
Place, publisher, year, edition, pages
Uppsala: Acta Universitatis Upsaliensis , 1998. , , 66 p.
Comprehensive Summaries of Uppsala Dissertations from the Faculty of Science and Technology, ISSN 1104-232X ; 403
Research subject Materials Science
IdentifiersURN: urn:nbn:se:uu:diva-1273ISBN: 91-554-4319-2OAI: oai:DiVA.org:uu-1273DiVA: diva2:160836
1998-11-20, lecture hall 2005, Ångström laboratory, Uppsala University, Uppsala, 13:15