Semiconductor CdO as a Blocking Layer Material on DSSC Electrode: Mechanism and Application

We fabricated dye-sensitized solar cells (DSSCs) with SnO2 nanoparticles coated with thin layers of CdO on the surfaces into a core-shell-type structure with various Cd/Sn ratios, and studied their effects on the DSSC performance as a function of the CdO shell thickness. CdO is an n-type semiconductor with a conduction band edge lower than that of SnO2 and, thus, has not been considered as a blocking layer material in the previous studies. The overall conversion efficiency of the DSSC was increased until the Cd/Sn ratio was 0.10, but was dramatically decreased when Cd/Sn was 0.20, compared with that of pure SnO2. The DSSC with Cd/Sn = 0.05 showed an efficiency of 3.25%, which is improved by 59% from that of pure SnO2. The influences of the CdO shells were studied by X-ray diffraction, nitrogen adsorption, dye adsorption, scanning and transmission electron microscopy, UV-vis diffuse reflectance spectroscopy, and electrochemical measurements. When the Cd-content is low, the flat band potential of the electrode, determined by the Mott-Schottky analyses, was negatively shifted, and the electron density accumulated into the electrode, determined by analyzing the chronoamperometry data, was decreased from those of pure SnO2. In addition, the amount of adsorbed dye molecules oil the SnO2 surface was increased. Since the CdO shell thickness is very small, the semiconductor properties of CdO do not appear in these samples. These changes of the physicochemical properties of SnO2 by the thin CdO shells improved the conversion efficiency and the effects of the CdO shells could be explained with the high basicity of CdO. However, when the Cd/Sn ratio was 0.20, the CdO shell became thick enough to show a narrower band gap and a lower conduction band edge than those SnO2, whose negative effects were more than offset those from the basicity of the CdO shell, and the conversion efficiency was greatly suppressed.