Dye-Sensitized Nanocrystalline TiO<INF>2</INF> Solar Cells Based on Ruthenium(II) Phenanthroline Complex Photosensitizers

We have synthesized four carboxylated Ru(II) phenanthroline complexes with different numbers of carboxyl groups, cis-bis(4,7-dicarboxy-1,10-phenanthroline)dithiocyanato ruthenium(II) (Ru(dcphen)<INF>2</INF>(NCS)<INF>2</INF>), cis-bis(4-monocarboxy-1,10-phenanthroline)dithiocyanato ruthenium(II) (Ru(mcphen)<INF>2</INF>(NCS)<INF>2</INF>), cis-(4,7-dicarboxy-1,10-phenanthroline)(1,10-phenanthroline)dithiocyanato ruthenium(II) (Ru(dcphen)(phen)(NCS)<INF>2</INF>), and cis-(4-monocarboxy-1,10-phenanthroline)(1,10-phenanthroline)dithiocyanato ruthenium(II) (Ru(mcphen)(phen)(NCS)<INF>2</INF>), as photosensitizers for oxide semiconductor solar cells. We have studied photovoltaic properties of dye-sensitized nanocrystalline semiconductor solar cells based on Ru phenanthroline complexes and an iodine redox electrolyte. The photovoltaic performance of the TiO<INF>2</INF> solar cell sensitized by Ru(dcphen)<INF>2</INF>(NCS)<INF>2</INF>(TBA)<INF>2</INF> exceeded that of ZnO, SnO<INF>2</INF>, and In<INF>2</INF>O<INF>3</INF> solar cells. A solar energy to electricity conversion efficiency (η) of 6.6% was obtained under the standard AM 1.5 irradiation (100 mW cm^-2, JIS A class) with a short-circuit photocurrent density (J<INF>sc</INF>) of 12.5 mA cm^-2, an open-circuit photovoltage (V<INF>oc</INF>) of 0.74 V, and a fill factor (ff) of 0.71. Monochromatic incident photon to current conversion efficiency was 78% at 526 nm. Deoxycholic acid as a coadsorbate and decreasing film thickness improved V<INF>oc</INF> especially due to suppression of the dark current reaction corresponding to the reduction of triiodide ions with injected electrons. The improved photovoltaic property due to the added coadsorbate suggests that some aggregates of the Ru complex suppress efficient electron injection to the semiconductor. The position and number of carboxyl groups attached to the phenanthroline ligand as an anchor affect photosensitizer performance significantly, suggesting that the anchoring configuration of Ru phenanthroline complexes on the semiconductor surface is important to efficient photovoltaic cell performance. Two carboxyl groups attached to phenanthroline ligands are necessary for effective electron injection.