Iron sensitizer converts light to electrons with 92% yield

Abstract

Solar energy conversion in photovoltaics or photocatalysis involves light harvesting, or sensitization, of a semiconductor or catalyst as a first step. Rare elements are frequently used for this purpose, but they are obviously not ideal for large-scale implementation. Great efforts have been made to replace the widely used ruthenium with more abundant analogues like iron, but without much success due to the very short-lived excited states of the resulting iron complexes. Here, we describe the development of an iron-nitrogen-heterocyclic-carbene sensitizer with an excited-state lifetime that is nearly a thousand-fold longer than that of traditional iron polypyridyl complexes. By the use of electron paramagnetic resonance, transient absorption spectroscopy, transient terahertz spectroscopy and quantum chemical calculations, we show that the iron complex generates photoelectrons in the conduction band of titanium dioxide with a quantum yield of 92% from the (3)MLCT (metal-to-ligand charge transfer) state. These results open up possibilities to develop solar energy-converting materials based on abundant elements.

NHC-Based Iron Sensitizers for DSSCs

“…Despite their good optical and photophysical properties, both the poor performance of the C4 complex and the extremely low photocurrents measured for C2 point to a nonoptimal interfacial charge separation process, possibly associated with a fast recombination mechanism occurring in the pico-and nanosecond timescale. These suggestions suppose that the relatively high injection yield reported by Wärnmark [26] for C2 grafted on TiO2 is valid in our systems. Indeed, and as compared to the typical 50 ps time window required for complete charge injection of the Ru N719 dye [39], the excited-state lifetimes of C2 and C4 in acetonitrile (16 and 26 ps, respectively), should afford a PCE (Photon Conversion Efficiency) > 1%.…”

“…Recently, as shown by Wärnmark and coworkers, N-heterocyclic carbenes (NHC) have been proven to promote notable improvements of 3 MLCT lifetimes, thanks to strong σ-donating effects inducing the destabilization of MC states [25][26][27][28]. While no DSSCs have been built from the newly designed complexes, this work has fostered the revival of research towards the development of iron-based sensitizers for DSSCs, which has been almost asleep since the pioneering works by Ferrere using the Fe(dcbpy) 2 (CN) 2 dye [14].…”

Synthesis and complexes of imidazolinylidene-based CCC pincer ligands

“…5 ,6 Increasingly NHC-based pincer complexes are also becoming recognised for their useful photophysical properties: ruthenium-based C, for example, is notable for microsecond 3 MLCT excited-state lifetimes, three orders of magnitude higher than [Ru(terpyridine) 2 ] 2+ . 7,8 Chart 1: Selected examples of NHC-based pincer complexes…”

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

“…The time constant for excited state electron injection into the mesoporous TiO 2 thin films commonly used in DSSCs was found to be about 3 ps, which is consistent with 490% reported yield. 2 Indeed, a variety of spectroscopic measurements provided strong evidence of nearly quantitative excited state interfacial electron transfer to TiO 2 . It is abundantly clear that an iron compound has finally been discovered that efficiently transfers electrons when illuminated with visible light!…”

“…In the final analysis, iron consistently continues to disappoint and one can safely conclude that it is best to keep iron out of solar cells. Until now Harlang et al 2 found that the iron compound shown in Figure 1 harvests sunlight through most of the visible region with subsequent excited state electron transfer to a TiO 2 semiconductor with efficiencies 490%. Such a breakthrough in efficiency is remarkable, particularly when one considers the decades of prior research that failed to accomplish anything even close.…”