D−π–A-Structured Porphyrins with Extended Auxiliary π‑Spacers for Highly Efficient Dye-Sensitized Solar Cells

Zn­(II)-porphyrin dyes (SGT-030 and SGT-031) with extended auxiliary π-spacers in the donor (D) part have been prepared and applied to dye-sensitized solar cells (DSSCs). The porphyrin dyes contained the same D–ethynyl–zinc porphyrinyl (ZnP)–ethynyl–benzothiadiazole-acceptor platform, but their donor groups varied from phenylene (Ph) in SGT-053 as a reference dye to the thieno­[3,2-b]­benzothiophene (TBT) and 4-hexyl-4H-thieno­[3,2-b]­indole (TI) moieties in SGT-030 and SGT-031, respectively. The effects of the extended auxiliary π-spacer in the D−π–A-structured porphyrin sensitizers on the molecular and photovoltaic properties were investigated via photophysical and electrochemical experiments as well as theoretical calculations. With the trend in conjugation length (Ph TBT ≈ TI) and the donating ability of the π-spacer (Ph TBT TI), the absorption maxima and molecular absorptivity increased in the order SGT-053 (Ph) (TBT) (TI). The incorporation of TBT and TI promoted significant enhancements in the light-harvesting properties by reducing the energy gap and efficiently improving electronic communication. The DSSCs based on SGT-030 (10.80%) and SGT-031 (10.89%) with coadsorption of 4-(3,6-bis­(4-((2-ethylhexyl)­oxy)­phenyl)-9H-carbazol-9-yl)­benzoic acid in conjunction with the [Co­(bpy)3]2+/3+-based electrolyte showed better power conversion efficiency than that of SGT-053 (9.10%). Electrochemical impedance spectroscopy analysis unveiled that the difference in Jsc and Voc originates mainly from the twisted orientation between D and ZnP by the introduction of TBT and TI. This result indicated that the introduction of an extended auxiliary π-spacer in the donor part is a rational molecular design approach to improve photovoltaic performance by enhancing the light-harvesting ability and hindering charge recombination on the TiO2 photoanode

Influence of the π‑Bridge-Fused Ring and Acceptor Unit Extension in D−π–A-Structured Organic Dyes for Highly Efficient Dye-Sensitized Solar Cells

Three new D−π–A-structured organic dyes, coded as SGT-138, SGT-150, and SGT-151, with the expansion of π-conjugation in the π-bridge and acceptor parts have been developed to adjust HOMO/LUMO levels and to expand the light absorption range of organic dyes. Referring to the SGT-137 dye, the π-bridge group was extended from the 4-hexyl-4H-thieno[3,2-b]indole (TI) to the 9-hexyl-9H-thieno[2′,3′:4,5]thieno[3,2-b]indole (TII), and the acceptor group was extended from (E)-3-(4-(benzo[c][1,2,5]thiadiazol-4-yl)phenyl)-2-cyanoacrylic acid (BTCA) to (E)-3-(4-(benzo[c][1,2,5]thiadiazol-4-ylethynyl)phenyl)-2-cyanoacrylic acid (BTECA), where TII was introduced as a π-bridging unit for the first time. It was determined that both extensions are promising strategies to enhance the light-harvesting ability. They present several features, such as (i) efficiently intensifying the extinction coefficient and expanding the absorption bands; (ii) exhibiting enhanced intramolecular charge transfer in comparison with the SGT-137; and (iii) being favorable to photoelectric current generation of dye-sensitized solar cells (DSSCs) with cobalt electrolytes. In particular, the π-spacer extension from TI to TII was useful for modulating the HOMO energy levels, while the acceptor extension from BTCA to BTECA was useful for modulating the LUMO energy levels. These phenomena could be explained with the aid of density functional theory calculations. Finally, the DSSCs based on new SGT-dyes with an HC-A1 co-adsorbent presented good power conversion efficiencies as high as 11.23, 11.30, 11.05, and 10.80% for SGT-137, SGT-138, SGT-150, and SGT-151, respectively. Furthermore, it was determined that the use of the bulky co-adsorbent, HC-A1, can effectively suppress the structural relaxation of dyes in the excited state, thereby enhancing the charge injection rate of SGT-dyes. The observations in time-resolved photoluminescence were indeed consistent with the variation in the PCE, quantitatively

Highly Robust Hybrid Photocatalyst for Carbon Dioxide Reduction: Tuning and Optimization of Catalytic Activities of Dye/TiO₂/Re(I) Organic–Inorganic Ternary Systems

Herein we report a detailed investigation of a highly robust hybrid system (sensitizer/TiO₂/catalyst) for the visible-light reduction of CO₂ to CO; the system comprises 5′-(4-[bis­(4-methoxy­methyl­phenyl)­amino]­phenyl-2,2′-dithiophen-5-yl)­cyano­acrylic acid as the sensitizer and (4,4′-bis­(methyl­phosphonic acid)-2,2′-bipyridine)­Re^I(CO)₃Cl as the catalyst, both of which have been anchored on three different types of TiO₂ particles (s-TiO₂, h-TiO₂, d-TiO₂). It was found that remarkable enhancements in the CO₂ conversion activity of the hybrid photocatalytic system can be achieved by addition of water or such other additives as Li⁺, Na⁺, and TEOA. The photocatalytic CO₂ reduction efficiency was enhanced by approximately 300% upon addition of 3% (v/v) H₂O, giving a turnover number of ≥570 for 30 h. A series of Mott–Schottky (MS) analyses on nanoparticle TiO₂ films demonstrated that the flat-band potential (<i>V</i>_fb) of TiO₂ in dry DMF is substantially negative but positively shifts to considerable degrees in the presence of water or Li⁺, indicating that the enhancement effects of the additives on the catalytic activity should mainly arise from optimal alignment of the TiO₂ <i>V</i>_fb with respect to the excited-state oxidation potential of the sensitizer and the reduction potential of the catalyst in our ternary system. The present results confirm that the TiO₂ semiconductor in our heterogeneous hybrid system is an essential component that can effectively work as an electron reservoir and as an electron transporting mediator to play essential roles in the persistent photocatalysis activity of the hybrid system in the selective reduction of CO₂ to CO

D‑π‑A Structured Porphyrin and Organic Dyes with Easily Synthesizable Donor Units for Low-Cost and Efficient Dye-Sensitized Solar Cells

This study aimed to develop low-cost D-π-A structured porphyrin and organic dyes with easily synthesizable donor units instead of the conventional complex multistep synthetic donor unit of Hexyloxy-BPFA [bis(7-(2,4-bis(hexyloxy)phenyl)-9,9-dimethyl-9H-fluoren-2-yl)amine] used in SGT-021 and SGT-149 as well-known record cosensitizers with an extremely high power conversion efficiency (PCE). The design strategy concerned the easier synthesis of low-cost donor units with inversion structures in donor groups via donor structural engineering, particularly by changing the position of the fluorene and phenylene units in the donor moiety while keeping the π-bridge and acceptor unit unchanged, leading to the synthesis of two D-π-A structured porphyrins [SGT-021(D0) and SGT-021(D)] and one D-π-A structured organic sensitizer [SGT-149(D)] for dye-sensitized solar cells (DSSCs). Specifically, porphyrin SGT-021(D0) incorporated two hexyl chains into the 9-position of each fluorene, while SGT-021(D) and SGT-149(D) substituted two hexyloxy chain units to the terminal position of each fluorene in the donor groups of porphyrin dyes. The effect of the position of the fluorene and phenylene units in the donor moiety on the photochemical and electrochemical properties, as well as the photovoltaic performance, was compared with the reference dyes of SGT-021 and SGT-149, previously reported by the research group. After optimizing the DSSC devices, SGT-021(D) and SGT-021(D0) achieved a high PCE of 11.6 and 10.5%, respectively, while SGT-149(D) exhibited a little lower PCE of 10.3% under the standard AM 1.5G light intensity. The cell performance of DSSC devices based on SGT-021(D) and SGT-149(D) was inferior to the corresponding reference dyes of SGT-021 and SGT-149 due to their lower donating ability of Hexyloxy-BPFA than Hexyloxy-BFPA