Cyclometalated Fe(II) Complexes as Sensitizers in Dye-Sensitized Solar Cells

Abstract

Dye-sensitized solar cells (DSSCs) often utilize transition metal-based chromophores for light absorption and semiconductor sensitization. Ru(II)-based dyes are among the most commonly used sensitizers in DSSCs. As ruthenium is both expensive and rare, complexes based on cheaper and more abundant iron could serve as a good alternative. In this study, we investigate Fe(II)-bis(terpyridine) and its cyclometalated analogues, in which pyridine ligands are systematically replaced by aryl groups, as potential photosensitizers in DSSCs. We employ density functional theory at the B3LYP/6-31G*,SDD level to obtain the ground state electronic structure of these complexes. Quantum dynamics simulations are utilized to study interfacial electron transfer between the Fe(II) photosensitizers and a titanium dioxide semiconductor. We find that cyclometalation stabilizes the singlet ground state of these complexes by 8-19 kcal/mol but reduces the electron density on the carboxylic acid attached to the aryl ring. The results suggest that cyclometalation provides a feasible route to increasing the efficiency of Fe(II) photosensitizers but that care should be taken in choosing the substitution position for the semiconductor anchoring group.

Excited‐State Kinetics of an Air‐Stable Cyclometalated Iron(II) Complex

“…Still, our time‐resolved measurements confirm the theoretical calculations of Dixon et al . and Jakubikova et al . predicting a 3 MC< 3 MLCT< 5 MC ordering of states in 2 , which is our third conclusion.…”

“…Consequently, the Kohn–Sham molecular orbital diagrams (Figure ) obtained by DFT calculations with MeCN as the solvent using the SMD solvation model show a significant destabilization of the HOMO‐t 2g levels of reduced degeneracy, in agreement with the strong π‐donor properties of the cyclometalating ligand. The LUMO is located at the tpy ligand in 2 , whereas the pbpy − ligand only contributes to molecular orbitals of higher energy, similar to the Ru II congener .…”

“…In contrast, quantum chemical predictions provided by Jakubikova et al. and Dixon et al . for the effects of exchanging an N‐donor by a cyclometalating ligand could not be confirmed experimentally so far.…”

“…In the electronic absorption spectra, 2 shows a strong panchromatic absorption up into the NIR region (Figure ) with broadened peaks due to the lower symmetry and concomitant larger t 2g splitting in comparison to 3 . The MLCT maximum of 2 at λ max =618 nm ( ϵ max =0.92 10 4 m −1 cm −1 ) is redshifted compared to that of 3 ( λ max =552 nm, ϵ max =1.12 10 4 m −1 cm −1 ) due to the destabilized t 2g orbitals (Figure ).…”

Are we on a path to solar cells that utilize iron?

“…While challenging, there is good precedence for optimization of dye energetics through synthetic design and the probability for success is quite high. In this regard, it is interesting to note that independent calculations by Mukherjee et al 4 predicted a priori that iron carbene compounds would indeed be useful for solar energy conversion. With synthesis guided by theory, the possibility for efficient DSSCs based on iron seems highly likely.…”