Electrochemical impedance and X-ray photoelectron spectroscopic analysis of dye-sensitized liquid electrolyte based SnO 2/ZnO solar cell

Abstract

A dye-sensitized solar cell based on interconnected SnO 2 nanoparticle matrix covered with a thin outer shell of ZnO, N719 dye, I −/I 3 − in acetonitrile liquid electrolyte system and lightly platinized FTO counter electrode shows significantly enhanced performance when compared to similar cells made with either pristine SnO 2 or pristine ZnO interconnected nanoparticles. Attempts have been made to investigate the reasons for such an improvement using the information obtained from X-ray photoelectron spectroscopy (XPS) and the electrochemical impedance spectroscopy (EIS). The XPS results reveal that the interconnected nanoparticluar SnO 2 matrix surfaces are fully covered by a ∼1 nm thick outer shell of a ZnO layer. EIS results disfavour the idea of direct injection of electrons from the excited dye molecules across the thin outer shell of ZnO into the conduction band of SnO 2 but supports the fact that electrons are first injected to the CB of ZnO and subsequently to the CB of SnO 2 particles both involving trapping and detrapping at each stage. The electron transport along the interconnected SnO 2 nanoparticles also involves anomalous diffusion characterized by a straight line of inclination greater than 45° in the complex impedance plot. This anomalous diffusion is attributed to the trap mediated electron transport.