Photon upconversion via triplet–triplet annihilation (TTA-UC) is a process that converts two lower-energy photons to a higher-energy photon, which is expected to increase the maximum solar cell efficiency beyond the Shockley–Queisser limit. To incorporate TTA-UC into a dye-sensitized solar cell (DSSC), we used a co-adsorption approach, in which both a TTA-UC donor with an alkyl carboxylic acid chain and a TTA-UC acceptor dye were adsorbed onto mesoporous TiO 2 . Incident photon-to-current conversion efficiency spectra and excitation intensity dependence indicated that a photocurrent was generated under irradiation at wavelengths above 490 nm by the TTA-UC mechanism. The power-conversion efficiency of the DSSC was increased to 0.72%, and the photocurrent contributed by TTA-UC was 0.036 mA cm –2 under 1 sun irradiation.