Efficient, low-cost, and robust alternatives to traditional wafer-based inorganic solar cells will allow for large-scale implementation of solar-energy conversion technologies. Dye-sensitized solar cells (DSSCs) are one option, but their energy-conversion efficiencies are less than half of those reported for state-of-the-art solar cells. In my presentation I will report on my research group’s recent success in demonstrating a new design and operational mechanism for DSSCs, hereafter referred to as a dye and catalyst co-functionalized solar cell (DCFSC). The difference between DSSCs and DCFSCs is that in DCFSCs an electrocatalyst is also present at the mesoporous metal-oxide surface to drive the multiple-electron-transfer chemistry of state-of-the-art redox shuttles, such as I–/I3–, at low overpotential. This allows use of dyes with higher-energy ground states therefore extending their absorption properties into the near-infrared spectral region and projected efficiencies to beyond 20%. Specifically, we utilized weakly oxidizing Os–based coordination compounds, whose Os(III/II) redox potential cannot thermodynamically drive iodide oxidation via a single-electron-transfer mechanism. Instead, after excited-state injection from Os* and prior to recombination with electrons in TiO2, the DCFSC relies on (i) rapid self-exchange electron-transfer reactions between Os(III) and Os(II) species across the metal-oxide surface to ultimately deliver two redox equivalents to surface-anchored catalysts and (ii) rapid multiple-electron-transfer iodide-oxidation electrocatalysis. Using nanosecond transient absorption spectroscopy we recently demonstrated criterion (i) using mesoporous TiO2 thin films co-anchored with metal–polypyridyl coordination compound dyes and bis(triarylamine) model (catalyst) compound acceptors. This was the first report that unequivocally showed such behavior under conditions of low-fluence pulsed-laser excitation. Moreover, we satisfied both criteria (i) and (ii) by demonstrating photovoltaic action from a DCFSC containing a discontinuous film of photoelectrochemically deposited poly(3,4-ethylenedioxythiophene) (PEDOT) hole-conducting polymer, whereas a DSSC (without the PEDOT) did not exhibit a photo-effect. These two proof-of-concept demonstrations validate the new proposed architecture.

New mechanisms in dye-sensitized solar cells: Catalyzing two-electron-transfer halide redox chemistry at sensitized TiO2

251st National Meeting (2016)

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