Steric Effect of N‑Substituted Triphenylamine on Double-Anchored Phenothiazine Dye-Sensitized Solar Cells

Bulky N-substituents of 4-(hexyloxy) phenyl-substituted and 4-(hexylthio)phenyl-substituted triphenylamine (TPA) were successfully introduced into double-anchored phenothiazine (PTZ) dyes (TY1–TY3) as capping moieties for metal-free organic sensitizers in dye-sensitized solar cells (DSSCs). Among TY dyes, the N-substituent of 4-(hexyloxy)phenyl-substituted TPA provided the best steric matching to its double-anchored PTZ dye having a small π-bridge of thiophene (TY1) and thereby facilitated the most compact dye packing as well as formed the most integrated shelter layer on the TiO₂ surface. Effective dark current suppression supported by TY1 resulted in a good DSSC performance of 10.47% at 1 sun and 21.2% at 1 klux (Philips T5 lamp) with the presence of chenodeoxycholic acid (CDCA) as the co-adsorbent; this cell efficacy was higher than that of the reference N719-based (9.10% at 1 sun) and HL5-based DSSCs (8.34% at 1 sun).