| Sumario: | TiO-based dye-sensitization cycle is one of the basic strategies for the development of solar energy applications. Although the power conversion efficiency (PCE) of dye-sensitized devices has been improved through constant attempts, the intrinsically fatal factor that leads to the complete failure of the PCE of TiO-mediated dye-sensitized devices has not yet been determined. Here, by using isotopically labeled MAS-H NMR, ATR-FTIR spectroscopy (separate H/D and Ti/Ti experiments), and ESR, we revealed that the accumulative formation of Ti-H species on the TiO surface is the intrinsic cause of the PCE failure of TiO-based dye-sensitization devices. Such a Ti-H species is generated from the reduction of hydrogen ions (mostly released from dye carboxyl groups or organic electrolyte) accompanied by electron injection on the surface of TiO, which deteriorates the PCE mainly by reducing the electrical conductivity of the TiO (by a maximum of ∼80%) and the hydrophilic nature of the TiO surface (contact angle increased).
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