Cyanoacetic acid tethered thiophene for well-matched LUMO level in Ru(II)-terpyridine dye sensitized solar cells

• We have synthesized two new Ru–tpy sensitizers, SY-01 and SY-02, based on the molecular design strategy of tethering a strong ED π-spacer thiophene between the strong EA cyanoacetic acid and pyridine of the terpyridyl ligand.
• These dyes have similar LUMO level to N749. Both dyes exhibited more intense absorption and showed better molar extinction coefficients compared with that of N749 owing to the extended π-conjugation and strong auxochrome resulting from the thiophene moiety.
• Density functional theory calculations reveal that the LUMO is distributed over the terpyridine and thienyl moieties, which enhances the light-harvesting capability and is appropriate for smooth electron injection from the dye to the TiO2 conduction band.
• The excited-state oxidation potentials are sufficiently more negative than the conduction-band edge of the TiO2 electrode, which facilitates the effective electron injection into TiO2 from the excited state of the dyes. Both computational and electrochemical studies reveal that the LUMO energy levels and band gaps of both dyes are similar to those of N749, which is due to the diminishing of the electron density provided by the thiophene π-spacer by the strong electron accepting cyanoacetic acid anchoring group.
• DSC based on SY-02 increases both JSC and VOC with η of 7.5%, which is 26% higher than that (5.84%) of N719-based DSC, and much higher than that of N749 under the same cell fabrication conditions. The increased η might be attributed to the increase in both IPCE and electron life time (5.38 ms) measured by electrochemical impedance for SY-02 compared with that (4.61 ms) of N719.

We synthesized two new terpyridine based Ru sensitizers (SY-01 and SY-02) with the lowest unoccupied molecular orbital (LUMO) levels of these two dyes are well-matched with that of the black dye (N749). Both sensitizers capable of exhibiting metal-to-ligand charge transfer band show a higher molar extinction coefficients (ε ∼ 5.0–6.8 × 103 M−1 cm−1) than that (4.2 × 103 M−1 cm−1) of N749 because of the enhanced absorption arising from thiophene moiety. Density functional theory calculations reveal that LUMOs of SY-01 and SY-02 are distributed over the terpyridine and thienyl moieties. The excited-state oxidation potentials, E°ox*, obtained by the electrochemical studies, range from −0.98 to −0.96 V, are sufficiently more negative than the conduction band edge of the TiO2 electrode (ca.−0.5 V). The increased conjugation conferred to SY-01 and SY-02 dyes through π-expansion by thiophene moiety increases light-harvesting and energy conversion efficiency in comparison with N749.

We have designed and successfully synthesized strong electron donating thiophene attached between central pyridine ring and cyanoacetic acid of the terpyridine ligand based dyes (SY-01 and SY-02) for well-matched LUMO level in Ru(II) terpyridine dye sensitized solar cells. These dyes show higher molar extinction coefficients and better photon to electricity conversion efficiencies compared to that of the black dye.Figure optionsDownload as PowerPoint slide