Electrolyte-Dependent Photovoltaic Responses in Dye-Sensitized Solar Cells Based on an Osmium(II) Dye of Mixed Denticity
By Onicha, Anthony C. & Castellano, Felix N.
Published in The Journal of Physical Chemistry C
2010
Abstract
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(tBu3tpy)(dcbpyH2)(NCS)]PF6 (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 (JSC), open-circuit voltage (VOC), 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 JSC in the Os(II)-based devices accompanied by corresponding decreases in VOC. 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 JSC without concomitantly lowering the VOC. These observations are discussed and directly compared in parallel to devices based on the benchmark sensitizer N3, cis-Ru(dcbpyH2)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%.