Harnessing molecular photon upconversion at subsolar irradiance using dual sensitized self-assembled trilayers

Photon upconversion via triplet-triplet annihilation (TTA-UC) is an intriguing strategy to potentially increase maximum solar cell efficiencies from 33% to greater than 43%. TTA-UC has been incorporated into a dye-sensitized solar cell (DSSC) but the low solar energy conversion efficiency can be attributed, at least in part, to the relatively narrow absorption features of the sensitizer molecule. Here we incorporate multiple sensitizers into a TTA-UC DSSC using bilayer and trilayer self-assembly via metal ion linkages. The sensitizers' complementary absorption features increase broad-band light absorption and work cooperatively to achieve peak TTA-UC efficiency at sub-solar irradiance (<1 sun or <100 mW cm(-2)). The trilayer DSSC, with high sensitizer density and directional energy transfer cascade towards the charge seperation interface, exhibited the highest efficiency yet reported for harvesting low energy light in an integrated TTA-UC solar cell (1.2 x 10(-3)%).