An Iron‐Based Photosensitizer with Extended Excited‐State Lifetime: Photophysical and Photovoltaic Properties

Abstract

Herein, we report a homoleptic iron complex bearing tridentate bis‐carbene (CNC) ligands designed for sensitization of TiO2 photoanodes. Its excited state has been characterized by ultra‐fast transient spectroscopy and time‐dependent density functional theory (TD‐DFT) computations, which reveal a record triplet metal‐to‐ligand charge‐transfer (3MLCT) excited‐state lifetime (16 ps). The new dye was efficiently chemisorbed on TiO2 and promoted electron injection and photocurrent generation in a dye‐sensitized solar cell upon solar irradiation.

NHC-Based Iron Sensitizers for DSSCs

“…As a result, the 3 MLCT state has a lower energy in C2 as compared to C1, leading to quasi-degenerated 3 MLCT and 3 MC equilibrium geometries in the former complex, and hence to the important increase of excited-state lifetime. The photophysical properties of C1 and C2 were also theoretically investigated using density functional theory (DFT) and the time-dependent (TD)-DFT techniques, employing the Hamprecht-Cohen-Tozer-Handy (HCTH) exchange and correlation functional [30]. The simulated absorption spectra are in good agreement with the experimental recordings.…”

“…(a) (b) The photophysical properties of C1 and C2 were also theoretically investigated using density functional theory (DFT) and the time-dependent (TD)-DFT techniques, employing the HamprechtCohen-Tozer-Handy (HCTH) exchange and correlation functional [30]. The simulated absorption spectra are in good agreement with the experimental recordings.…”

“…Nevertheless, an exceptional increase of the excited-state lifetime of C1 was achieved. In fact, complex C1 attained a 9-ps MLCT lifetime, while that of [Fe(tpy) 2 ] 2+ was 145 fs only [30]. Despite the lower covering of the 500-800 nm window, which is where the solar spectral irradiance is maximum, this result opened up the door for this class of complexes to be employed in PV applications.…”

“…These three ESA bands are characteristic of the 3 MLCT state in Fe(II) complexes [24,31]. A detailed analysis of the spectral dynamics allows us to establish the following photoinduced reaction scenario [30]:…”

“…We first decided to tune the MLCT state levels by introducing electron-withdrawing groups onto the ligands as in C2 (Scheme 1) [30]. The interest of investigating the effect of carboxylic groups was two-fold: (i) the electron-withdrawing effect was expected to stabilize the MLCT states and thus decrease the MLCT transition gap and red-shift the absorption band; (ii) carboxylic groups also allow further grafting of the sensitizer onto the semiconductor surface.…”