Highly Asymmetric Tribenzonaphtho‐Condensed Porphyrazinatozinc Complex: An Efficient Near‐Infrared Sensitizer for Dye‐Sensitized Solar Cells

Abstract

Novel, highly asymmetric zinc phthalocyanine (ZnPc) derivatives (Zn‐tri‐PcNc) containing tribenzonaphtho‐condensed porphyrazine with one carboxyl and three tert‐butyl (tBu) or n‐butoxy (nBuO) peripheral substituents were synthesized by using 6‐carboxymethyl‐2,3‐dicyanonaphthalene and 4‐tBu‐ or nBuO‐phthalonitrile as starting agents. The new, highly asymmetric ZnPc containing tBu groups (Zn‐tri‐PcNc‐2 b) yielded a 3.56 % conversion efficiency in sensitizing TiO2‐based dye‐sensitized solar cells (DSSCs) under simulated AM 1.5G solar irradiation (100 mW cm−2), which is higher than that of the nBuO‐containing analogue Zn‐tri‐PcNc‐3 b (2.20 %). Moreover, Zn‐tri‐PcNc‐2 b displays 60.7 % of the maximum incident monochromatic photon‐to‐current conversion efficiency (IPCE) in the red/near‐IR region, which is much higher than that of Zn‐tri‐PcNc‐3 b (22.9 %), thus highlighting the potential of Zn‐tri‐PcNc‐2 b as a DSSC component.

Asymmetric zinc phthalocyanines with large steric hindrance as efficient red/near-IR responsive sensitizer for dye-sensitized solar cells

“…As can be seen from Fig. 1, the Q-bands of the solid-state UVevis absorption spectra for both asymmetric ZnPc derivatives show slightly broadening, but it still is very similar to the absorption spectra of the corresponding dye solution with no new shoulder appeared in the Q-band attributable to the molecule aggregates [12,13]. It indicates that the introduction of bulky 2,6-diphenylphenol or 2,6-diphenylthiophenol groups can effectively retard the ZnPc molecule aggregates without chenodeoxycholic acid (CDCA), which is extensively used to prevent the dye aggregates during the sensitization process [13].…”

“…Zn-tri-PcNc-4 exhibits much higher IPCE values in the red/near-IR region (especially in 550e770 nm) than Zn-tri-PcNc-5, also implying Zn-tri-PcNc-4 would contribute higher photocurrent than Zn-tri-PcNc-5. Moreover, the maximum IPCE value of Zn-triPcNc-4/Zn-tri-PcNc-5 is redshifted by~11/62 nm compared to that (699 nm) of Zn-tri-PcNc-2 [12,13], suggesting a difference in the "pushepull" balance in the ZnPc ring due to the three tert-butyl groups in Zn-tri-PcNc-2 replaced by six bulky electron-donating groups with larger steric hindrance in Zn-tri-PcNc-4/Zn-triPcNc-5, which could further influence the electronic interaction between the Pc ring and the TiO 2 owing to the adsorption of the carboxylic onto TiO 2 surface, and then the electron transfer from ZnPc excited singlet state to TiO 2 CB [15,16].…”

“…It is known that Soret-band usually stems from the electronic transition from a low-energy-level orbital to the LUMO of ZnPcs, and its redshift on TiO 2 film can be ascribed to the partly unoccupied molecule orbitals of the adsorbed dye molecules coupling with TiO 2 , which leads to the expansion of the p-conjugated system, and then to the redshift of Soret-band absorption [20]. It has been reported that the solidstate UVevis absorption spectra of ZnPcs usually show a new strong shoulder peaks (for example, at~640 nm) in the Q-band region due to the molecule aggregates [12,13], and the peaks at 680 nm are monomer peak of ZnPc molecules [21e25]. As can be seen from Fig.…”

“…These processes are attributed to successive removal/addition of one electron from/to the Pc ring-based orbitals. Dye energy levels (for instance, LUMO corresponds to the excitedstate oxidation potential, while the HOMO to the first oxidation potential) are crucial to the dye's electron injection and regeneration [12,29]. According to Leeuw's experience formula [30], the calculated LUMO/HOMO are shown in Table 1, and corresponding LUMO/HOMO levels of Zn-tri-PcNc-4 and Zn-tri-PcNc-5, I 3 -/I À redox potential and TiO 2 CB are plotted in Fig.…”

“…Density functional theory (DFT) calculations were performed on the asymmetric ZnPc derivatives within the framework of the DFT method at the B3LYP/6-31G(d) level, which has been proved to be suitable for calculating the molecular orbitals (MOs) of ZnPc derivatives [12]. The primal input structures of Zn-tri-PcNc-4 and Zntri-PcNc-5 were obtained by adding a benzene ring onto one of benzene rings of phthalocyanine and then attaching six diphenylphenoxy or diphenylphenylthiolate groups and two carboxylic acid groups onto the peripheral positions.…”