The role of the CdS buffer layer in the Cu(In,Ga)Se2 thin film solar cell
Sammanfattning: 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.
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