Band Alignment of II-VI Semiconductors

The CdS/ZnO interface is of particular importance in Cu(In,Ga)Se2 thin film solar cells because it is used in the standard cell configuration (Fig. 4.2). A first experimental determination of the band alignment at the ZnO/CdS interface has been performed by Ruckh et al. [102]. The authors have used ex-situ sputter-deposited ZnO films as substrates. The interface formation was investigated by stepwise evaporation of the CdS compound from an effusion cell. Photoelectron spectroscopy revealed a valence band offset of A Vb = 1.2eV. An identical value of 1.18eV has been derived using first-principles calculations [103]. With the bulk band gaps of CdS and ZnO of 2.4 and 

respectively, this leads to a conduction band offset 0.3 eV, with the conduction band minimum of CdS being above the one of ZnO. This value is frequently used in the literature for modeling of the Cu(In,Ga)Se2 thin film solar cells [14,104,105]. 

The agreement between the calculations and the various experimental results is excellent. This indicates the wide applicability of the calculated band alignments. The general behavior also confirms the original value given 

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