Effects of a dianion compound as a surface modifier on the back reaction of photogenerated electrons in TiO₂-based solar cells

The TiO₂ films were modified with a dianion compound, 1,2-ethanedisulfonic acid disodium salt (ESD), to give a negative charge (ethane sulfonate anion) on the TiO₂ surface, i.e., TiO₂-O-SO₂-CH₂-CH₂-SO₃ ⁻), and effects of repulsion between the negative charge and ions (I₃ ⁻) of the electrolyte on the performance of dye-sensitized solar cells (DSSCs) were investigated. The reference device without any modification showed a power conversion efficiency (PCE) of 9.89%, whereas for the device with ESD(20)-TiO₂/FTO, which was prepared by soaking bare TiO₂/FTO in an ESD solution for 20 min, the PCE was increased to 10.97%, due to an increase in both short-circuit current (J_sc) and open-circuit voltage(V_oc). It was verified from the measurements of electrochemical impedance, open-circuit voltage decay and dark current that the enhancement in the J_sc and V_oc values was attributed to the reduced back reaction between photoinjected electrons and I₃ ⁻ ions, resulting from the presence of the ethane sulfonate anions on the TiO₂ surface.