Photon upconversion via triplet–triplet annihilation (TTA-UC) is a promising strategy for increasing maximum theoretical solar cell efficiencies by a factor of 1.3. However, one factor limiting integrated TTA-UC solar cell performance is the transmission window between sensitizer and acceptor molecule absorption. Here we demonstrate that the incorporation of a singlet sensitizer (SS) into a self-assembled trilayer is an effective means of harnessing that previously transmitted light. A record TTA-UC photocurrent density of 0.315 mA cm–2 under 1 sun irradiation was achieved and is attributed to directional SS-to-sensitizer singlet energy transfer and sensitizer-to-acceptor triplet energy transfer, followed by TTA, excited-state electron transfer into TiO2, and regeneration by a redox mediator in solution. These results demonstrate that singlet sensitization-enhanced self-assembled trilayers are a promising strategy for enhancing broad-band light absorption and improving the performance of TTA-UC solar cells.

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Self-assembly of sensitizer and acceptor molecules has recently emerged as a promising strategy to facilitate and harness photon upconversion via triplet-triplet annihilation (TTA-UC). In addition to the energetic requirements, the structure and relative orientation of these molecules can have a strong influence on TTA-UC rates and efficiency. Here we report the synthesis of five different acceptor molecules composed of an anthracene core functionalized with 9,10- or 2,6-phenyl, methyl, or directly bound phosphonic acid groups and their incorporation into self-assembled bilayers on a ZrO2 surface. All five films facilitate green-to-blue photon upconversion with Φuc as high as 0.0023. The efficiency of TTA, and not triplet energy transfer, fluorescence, or losses via FRET, was primarily responsible for dictating the Φuc emission. Even for molecules having similar photophysical properties, variation in the position of the phosphonic acid resulted in dramatically different ΦTTA, Ith values, γTTA, and D. Interestingly, we observed a strong linear correlation between ΦTTA and the Ith value but the cause of this relationship, if any, is unclear.