Carbazole dye with phosphonic acid anchoring groups for long-term heat stability of dye-sensitized solar cells

In dye-sensitized solar cells (DSSCs), the binding strength between the dye and 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 effect more stable bonding to a 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. We studied the effect of the dye anchoring group on the heat stability of DSSCs by comparing MK-94, MK-2 (a carbazole dye with a carboxylic acid anchoring group), and Z-907 (a well-known, robust Ru complex dye with a carboxylic acid anchoring group). In the initial performance assessment, the DSSC with MK-94 exhibited a lower short circuit current (j_sc) but a higher open circuit voltage (ΔV_oc) than MK-2. With respect to heat stability, decreases in j_sc and ΔV_oc were suppressed with MK-94, and the drop of the power conversion efficiency (PCE) was kept within 6% after 504 h of heating. In addition, MK-94 demonstrated the longest electron lifetime and largest charge transfer resistance than the other dyes during the stability test. In contrast, the PCE decreased 27% from the initial value in the DSSC with MK-2. Thus, the phosphonic acid anchor was effective in extending the long-term heat stability of the DSSCs