Earth-abundant Metal Complexes for Solar Cells - From Molecular Design to Working DSSC

International Conference on Hybrid and Organic Photovoltaics
Proceedings of International Conference on Hybrid and Organic Photovoltaics 2015 (HOPV15)

Poster, Pavel Chábera, 131
Publication date: 5th February 2015

Metal-to-ligand charge transfer (MLCT) excited states play a key role in the photophysics and photochemistry of transition metal complexes. Although endowed with intense MLCT absorption, abundant and environmentally benign Fe^II polyimine ([FeN₆]²⁺) materials have unfortunately long been regarded as unexploitable in energy conversion applications, due to the low-lying metal-centered (MC) ⁵T₂ high-spin state that deactivates ^1,3MLCT manifolds in unity yield within the 100 fs regime. Following our initial success to destabilize the ⁵T₂ state and extend the ³MLCT lifetime of FeNHC complexes [1], we report the efficient synthesis of a heteroleptic Fe^II complex (1) based on sequentially furnishing the Fe^II center with the benchmark 2,2’-bipyridine (bpy) and the more strongly s-donating mesoionic ligand, 4,4’-bis(1,2,3-triazol-5-ylidene) (btz). Complex 1, the first hexa-coordinate mononuclear octahedral transition metal complex of btz, was comprehensively characterized and compared to [Fe(bpy)₃]²⁺ and [Fe(bpy)(CN)₄]²⁻ (CN=cyanide, complex 2), by electrochemistry, static and ultrafast spectroscopies and quantum chemical (QC) calculations. The combination of the mesoionic nature of btz and the heteroleptic structure was demonstrated to effectively destabilize the metal-centered (MC) states relative to the triplet metal-to-ligand charge transfer (³MLCT) state in 1, rendering it a lifetime of 13 ps, the longest to date reported of a photochemically stable Fe^II complex. Based on DFT and TD-DFT calculations the deactivation of the ³MLCT state is proposed to proceed via the ³MC state that strongly couples with the singlet ground state. The calculations show that the exceptionally long excited-state lifetime is achieved for this Fe complex through a significant destabilization of both triplet and quintet metal-centered scavenger states compared to other Fe^II complexes. In addition, a shallow ³MLCT potential energy surface with a low-energy transition path from the ³MLCT to ³MC and facile crossing from the ³MC state to the ground state are identified as key features for the excited-state deactivation (see figure for more details). The first generation of our FeNHC complexes [1] was functionalized with COOH groups for attachment to a nanostructured TiO₂ electrode and electron injection and recombination was characterized using ultrafast optical and THz spectroscopy, as well as EPR. A solar cell was built and I-V characteristics measured. Together, all these measurements show that a functioning solar cell can be built based on a FeNHC complex sensitizer.
Heteroleptic FeII complex (1), Reference compound (2) and Energy landscape schematically illustrating deactivation process (right panel)
[1] Y. Liu, T. Harlang, S. E. Canton, P. Chábera, K. Suárez-Alcántara, A. Fleckhaus, D. A. Vithanage, E. Göransson, A. Corani, R. Lomoth, V. Sundström and K. Wärnmark, Chem. Commun. 2013, 49, 6412-6414. [2] L. A. Fredin, M. Pápai, E. Rozsályi, G. Vankó, K. Wärnmark, V. Sundström and P. Persson, J. Phys. Chem. Lett. 2014, 5, 2066-2071. [3] Y. Liu et al., Chem. Eur. J. 2014, 20, 1 – 13

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