Effect of thickness on structural, electrical, and electrochemical properties of platinum/titanium bilayer counterelectrode

Abstract

To strengthen the adhesion of platinum layer to counterelectrodes of dye sensitized solar cells, titanium thin film has been utilized as adhesion layer between platinum and substrate [Wei et al., Appl. Phys. Lett. 90, 153122 (2007)]. In our study, platinum/titanium bilayer counterelectrodes were fabricated by magnetron sputtering. The structural, electrical, and electrochemical properties of the bilayer counterelectrode were studied by varying the thickness of both platinum and titanium layers. The influence of both electrochemical active surface and crystallite size on charge transfer resistance of the bilayer counterelectrode was explored. From these evaluations, it was found that the as-prepared titanium layer was almost amorphous, and intermetallic phase was formed between platinum and titanium layers. As expected, sheet resistance of the electrode decreased with the increase in deposition time of both platinum and titanium layers. Metal titanium was found to be inert in the catalyzing reduction in tri-iodide. With the increment of deposition time of platinum layers, charge transfer resistance of the electrode decreased quickly first, and then reached a plateau after which it changed slightly. This outcome differs from that reported before. Careful examination showed that such a behavior was related closely to the electrochemical active surface area of electrode and platinum crystallite size. For a given electrolyte, charge transfer resistance decreased with the increase in electrochemical active surface and increased with the increment of the latter. Thus, a competition was observed between the two factors, which supports the suggestion of former researchers.