The Formation of Ti-H Species at Interface Is Lethal to the Efficiency of TiO₂-Based Dye-Sensitized Devices

TiO₂-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 TiO₂-mediated dye-sensitized devices has not yet been determined. Here, by using isotopically labeled MAS-¹H NMR, ATR-FTIR spectroscopy (separate H/D and ⁴⁸Ti/⁴⁹Ti experiments), and ESR, we revealed that the accumulative formation of Ti-H species on the TiO₂ surface is the intrinsic cause of the PCE failure of TiO₂-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 TiO₂, which deteriorates the PCE mainly by reducing the electrical conductivity of the TiO₂ (by a maximum of ∼80%) and the hydrophilic nature of the TiO₂ surface (contact angle increased).