Molecular Photon Upconversion Solar Cells Using Multilayer Assemblies: Progress and Prospects.

Molecular photon upconversion via triplet-triplet annihilation (TTA-UC) is an intriguing strategy to increase solar cell efficiencies and surpass the Shockley-Quiesser (SQ) limit. In this Perspective, we recount our group's efforts to harness TTA-UC by directly incorporating metal ion linked multilayers of acceptor and sensitizer molecules into an organic-inorganic hybrid solar cell architecture. These self-assembled multilayers facilitate both upconverted emission and photocurrent generation from the upconverted state with a record contribution of 0.158 mA cm-2 under 1 sun solar flux. We recount the progression toward this record and the mechanistic insights learned along the way, summarize the rate- and efficiency-limiting events, and outline improvements that must be made to produce a viable TTA-UC solar cell that can surpass the SQ limit. We also discuss the potential impact that efficient TTA-UC and photocurrent generation could have on existing record solar cells.