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

We fabricated dye-sensitized solar cells (DSSCs) with SnO 2 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 SnO 2 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 SnO 2 . The DSSC with Cd/Sn = 0.05 showed an efficiency of 3.25%, which is improved by 59% from that of pure SnO 2 . 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 SnO 2 . In addition, the amount of adsorbed dye molecules on the SnO 2 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 SnO 2 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 SnO 2, whose negative effects were more than offset those from the basicity of the CdO shell, and the conversion efficiency was greatly suppressed.