Effect of Sodium Hydroxide Pretreatment of NiO_x Cathodes on the Performance of Squaraine‐Sensitized p‐Type Dye‐Sensitized Solar Cells

Abstract

Squaraines are full‐organic dyes employed as sensitizers in p‐type dye‐sensitized solar cells (p‐DSSC). Their absorption spectrum shows a wide tunability that ranges from visible to NIR. Sensitization in the NIR region is crucial for exploiting a particularly intense portion of the solar spectrum. In this work three squaraines will be presented and tested as sensitizers in NiO‐based p‐type DSSC O4_C2, O4_C4 and O4_C12). The structures of the dyes differ for the length of the alkyl side chain (C2, C4 and C12). Alkyl side chains improve the solubility of the dye, influence the extent of dye loading on the electrode and affect the overall efficiency of devices. The generally low stability of squaraines represents a critical issue in view of their employment as sensitizers of p‐DSSC. Such a problem becomes even more evident when this class of molecules is bound onto an acidic surface like the one of the photocathode here employed: non‐stoichiometric nickel oxide (NiOx). NiOx possesses a quite acidic character because of the high surface concentration of Ni(III) sites. To buffer the surface acidity of NiOx due to the presence of high‐valence states of nickel, we considered the electrode pretreatment with sodium hydroxide (NaOH) prior to sensitization. This assures a major stability of the solar cell. At the same time the chemisorbed hydroxyl moieties act as passivating agents of the Ni(III) sites thus diminishing the surface concentration of sites for dye anchoring.

Effect of Sensitization on the Electrochemical Properties of Nanostructured NiO

“…In all the experiments, the electrolyte solution was LiClO4 0.2 M in ACN (Acetonitrile). All figures have been plotted with the application Kaleidagraph 3.6, and their photoelectrochemical characterization has been already described in previous works [36,85,86]. …”

“…The structures of the nine dyes here employed as sensitizers of NiO are reported in the Supplementary Materials ( Figures S1-S3). The colorants here considered are P1 [75][76][77][78], Fast Green (FG) [79,80], erythrosine B (ERY) [3,[81][82][83], the series of the differently substituted squaraines DS_35, DS_44, and DS_46 [84,85], and the series of squaraines VG_1, VG_10, and VG_11, which differ to the extent of electronic conjugation [86].…”

Nanocomposites of Nickel Oxide and Zirconia for the Preparation of Photocathodes with Improved Performance in p-Type Dye-Sensitized Solar Cells

“…An alternative route is the direct modification of the photocathode. In a previous paper we showed that the treatment of NiO surface with soda has a twofold effect: it reduces the surface concentration of superficial Ni 3+ sites and passivates the NiO surface prior sensitization 22 . The success of this method has been confirmed by the achievement of a less dark film.…”

Sodium Hydroxide Pretreatment as an Effective Approach to Reduce the Dye/Holes Recombination Reaction in P-Type DSCs

“…Some of us have previously reported the effect of a soda pretreatment on a NiO electrode surface on the photoelectrochemical properties of squaraine-based p-type DSC (Bonomo et al, 2018b). The chemisorption of the hydroxyl moieties on the NiO surface has been shown to lower the dye-loading leading to a lower photocurrent density.…”

“…Here we introduce the synthesis and the applications of a new squaraine dye (VG21-C12) as a sensitizer for p-DSC in comparison to the derivative O4-C12 as a reference (Figure 1) (Bonomo et al, 2018b).…”