NHC-Based Iron Sensitizers for DSSCs

Abstract

Nanostructured dye-sensitized solar cells (DSSCs) are promising photovoltaic devices because of their low cost and transparency. Ruthenium polypyridine complexes have long been considered as lead sensitizers for DSSCs, allowing them to reach up to 11% conversion efficiency. However, ruthenium suffers from serious drawbacks potentially limiting its widespread applicability, mainly related to its potential toxicity and scarcity. This has motivated continuous research efforts to develop valuable alternatives from cheap earth-abundant metals, and among them, iron is particularly attractive. Making iron complexes applicable in DSSCs is highly challenging due to an ultrafast deactivation of the metal-ligand charge-transfer (MLCT) states into metal-centered (MC) states, leading to inefficient injection into TiO 2 . In this review, we present our latest developments in the field using Fe(II)-based photosensitizers bearing N-heterocyclic carbene (NHC) ligands, and their use in DSSCs. Special attention is paid to synthesis, photophysical, electrochemical, and computational characterization.

Impact of the fac/mer Isomerism on the Excited-State Dynamics of Pyridyl-carbene Fe(II) Complexes

“…We can unambiguously rule out that the metal-centered quintet 5 T 2 participates in the relaxation scheme since the latter is characterised by a sharp (10-15 nm broad) excited state absorption in the 340-360 nm range for tridentate ligands. 10,12 No sign of such an ESA is indicated by the DA spectra ( Fig. 5).…”

Is Iron the New Ruthenium?

“…This is the key concept behind the use of NHC ligands for Fe II complexes (Figure 2d), as first investigated in the context of obtaining long-lived 3 MLCT excited states by Wärnmarka nd co-workers. [14] This very promising area has been reviewed, [23] ands everald ifferent research groups have made important recent contributions.…”

Excited-State Relaxation in Luminescent Molybdenum(0) Complexes with Isocyanide Chelate Ligands

“…The molecular and the electronic structures of these compounds are reminiscent of Fe II and Ru II polypyridine complexes, which have been investigated extensively in the past. Until now, no convincing case of steady-state MLCT luminescence from a Fe II complex has been reported despite significant advances in extending their 3 MLCT lifetimes in recent years [18][19][20][21][22][23]; hence, our Cr 0 complex currently seems to be the only example of a first-row d 6 -metal complex showing MLCT luminescence in solution at room temperature under steady-state photo-irradiation [24]. The Mo 0 diisocyanide complexes are not only emissive, but they can furthermore be employed in photoredox catalysis of thermodynamically challenging reductions, which cannot be performed with more widely known photoreductants such as fac-[Ir(ppy)3] (ppy = 2-phenylpyridine) [25].…”