(Invited) Advance of Dye-Sensitized Solar Cells with Organic Dyes

Dye-sensitized solar cells (DSSCs) have the potential difference between the reduction potential of the dye cation and the redox potential of electrolyte. The cobalt complex redox (Co-redox) electrolyte with the deeper redox potential is one of the candidate to avoid the energy loss for dye regeneration. However, Co-redox electrolyte could induce the fast recombination reaction at the interface between the TiO₂ and the electrolyte. The recombination reaction is the reduction reaction of the redox couple with the injected electron in TiO₂ and will be the causes of decreasing the open circuit voltage (V_oc). To prevent the recombination reaction, the dyes may play important role as the recombination blocker on the TiO₂ surfaces. For the reason, we studied first the effects of steric hindrance of the donor and the π-linker groups in donor-π-linker-acceptors (D-π-As) structured organic dyes on the recombination reaction. The steric hindrance at the donor part in the dye structure reduced the recombination reaction and increased the V_oc of the DSSCs. Conversely, the small steric hindrance at the π-linker groups showed the less recombination reaction because of the higher dye coverage on the TiO₂ surface. Furthermore, we studied the effects of the type of donor structure in dye structure on the DSSC performance, and compared the dyes with carbazole (MK-1), coumarin (MK-31), and triphenylamine (MK-88) and N-phenyl-carbazole (MK-90) as the typical donor structure. It found that the triphenylamine can block the recombination reaction due to the steric hindrance of the rotatable phenyl groups.

We also studied the stability of the DSSCs with organic dyes. The binding strength between the dye and the TiO₂ surface may affect the heat stability of the DSSCs. Carboxylic acids have generally been used as the anchoring groups for the adsorption of dyes on TiO₂ surfaces. However, a phosphonic acid anchor is expected to make more stable bonding to the TiO₂ surface because of its tridentate binding capability. In this study, we developed a new carbazole dye that employed a phosphonic acid anchor (MK-94) and studied the effects of the dye anchoring group on the heat stability of DSSCs at 85 °C by comparing MK-94 and MK-2 (a carbazole dye with a carboxylic acid anchoring group). In the initial, the DSSC with MK-94 exhibited a lower short circuit current (J_sc) but a higher Voc than MK-2. After the heat stressing, both dyes showed the decreases in J_sc and V_oc but these were suppressed with MK-94. The drop of the power conversion efficiency (PCE) with MK-94 was kept within 6% after 504 h of heating. The phosphonic acid anchor was effective in extending the long-term heat stability of the DSSCs.