Effect of flexible, rigid planar and non-planar donors on the performance of dye-sensitized solar cells

A systematic design and synthesis of three push–pull metal-free organic dyes employing N-alkylated diphenylamine (D1), carbazole (D2), and phenothiazine (D3) as donor moieties are presented in this paper. While, vinylbenzene, methoxybenzene, and cyanoacrylic acid were used as π-linker, auxiliary donor, and acceptor moieties, respectively for all three dyes. The study focusses on identifying the best donor among the three in terms of simple molecular design. TiO2-based dye-sensitized solar cells (DSSC) were fabricated with these dyes. Further, dyes and their deprotonated forms were investigated using density functional theory (DFT) and time-dependent DFT analyses to characterize their electronic and optical properties. The variation in power conversion efficiency (PCE) was observed owing to change in coplanarity from freely rotating diphenylamine, fused planar carbazole, and nonplanar phenothiazine. Cyclic voltammetry was performed on D1, D2, and D3 indicated that the substitution at C3 position is effective in stabilizing the radical cation formed upon exposure to light, thereby preventing it from dimerization and degradation. Results illustrated that the dye-containing carbazole as donor (D2d) delivered the best PCE, viz., 3.7%. This was further enhanced to 4.2% on addition of chenodeoxycholic acid as a coadsorbent.