Electrolyte-Dependent Photovoltaic Responses in Dye-Sensitized Solar Cells Based on an Osmium(II) Dye of Mixed Denticity

This work focuses upon the effects of electrolyte composition, specifically the role of Li + and I − ions, on the resultant photovoltaic performance of dye-sensitized solar cells (DSCs) based on a new Os(II) polypyridine complex of mixed denticity, [Os( t Bu 3 tpy)(dcbpyH 2 )(NCS)]PF 6 ( 1 ). Photophysical and electrochemical characterization of 1 confirmed the suitability of this dye to serve as a sensitizer for regenerative DSCs on mesoscopic titania films. Device photovoltaic performances were assessed by measuring external quantum efficiencies as a function of wavelength, and current−voltage curves, the latter under simulated AM1.5G one-sun illumination. Varying the concentration of LiI in the redox electrolyte affects the short-circuit photocurrent ( J SC ), open-circuit voltage ( V OC ), fill factor (ff), power conversion efficiency (η), and external quantum efficiency (EQE) of the individual devices. Increasing the concentration of LiI results in enormous increases in J SC in the Os(II)-based devices accompanied by corresponding decreases in V OC . Independently increasing the concentration of I − by using either tetrabutylammonium iodide (TBAI) or the ionic liquid 1- n -propyl-3-methylimidazolium iodide (PMII) was found to increase J SC without concomitantly lowering the V OC . These observations are discussed and directly compared in parallel to devices based on the benchmark sensitizer N3, cis- Ru(dcbpyH 2 ) 2 (NCS) 2 . The combined results suggest that the photovoltaic performance of Os(II)-based DSCs can indeed be optimized by simply modifying the composition of redox electrolytes used in the operational sandwich cells. An abundance of I − appears to be crucial for the effective regeneration of oxidized surface-bound osmium sensitizers and therefore for the production of higher photocurrents in these devices. We note that select devices based on 1 represent the record power conversion efficiency for an Os(II)-based DSC measured under simulated AM1.5G one-sun illumination, η = 4.7%.