What Limits Photon Upconversion on Mesoporous Thin Films Sensitized by Solution-Phase Absorbers?

Photon upconversion by sensitized triplet–triplet annihilation (UC-STTA) is a promising strategy for breaking the Shockley–Queisser limit for efficiency of single-threshold solar cells, and in particular dye-sensitized solar cells (DSSCs). Here, we report on a heterogeneous UC system, where the annihilating dyes (“emitters”) are bound to a ZrO 2 nanostructured film and the light absorbing dyes (“sensitizers”) are free in solution. A comparative study of four different emitter dyes was conducted, all of them derivatives of the well-known UC-STTA emitter dye 9,10-diphenylanthracene (DPA), and in every case, the sensitizer dye was platinum(II) octaethylporphyrin (PtOEP). The physical separation of emitter and sensitizer molecules in two different phases makes homogeneous triplet–triplet annihilation among sensitizers in solution a significant loss channel at high excitation intensity and low emitter surface coverage. For the studied emitter dyes, the number and type of anchor groups, and the solubility of the emitter dye in the employed solvents, are the determining factors of the UC output. The signal evolves in time and with light exposure due to emitter desorption and light-induced endoperoxide formation. These results can guide the way toward a better understanding of UC-STTA on nanocrystalline metal oxides and its development for solar energy applications.