There Is a Future for N-Heterocyclic Carbene Iron(II) Dyes in Dye-Sensitized Solar Cells: Improving Performance through Changes in the Electrolyte

Abstract

By systematic tuning of the components of the electrolyte, the performances of dye-sensitized solar cells (DSCs) with an N-heterocyclic carbene iron(II) dye have been significantly improved. The beneficial effects of an increased Li+ ion concentration in the electrolyte lead to photoconversion efficiencies (PCEs) up to 0.66% for fully masked cells (representing 11.8% relative to 100% set for N719) and an external quantum efficiency maximum (EQEmax) up to approximately 25% due to an increased short-circuit current density (JSC). A study of the effects of varying the length of the alkyl chain in 1-alkyl-3-methylimidazolium iodide ionic liquids (ILs) shows that a longer chain results in an increase in JSC with an overall efficiency up to 0.61% (10.9% relative to N719 set at 100%) on going from n-methyl to n-butyl chain, although an n-hexyl chain leads to no further gain in PCE. The results of electrochemical impedance spectroscopy (EIS) support the trends in JSC and open-circuit voltage (VOC) parameters. A change in the counterion from I− to [BF4]− for 1-propyl-3-methylimidazolium iodide ionic liquid leads to DSCs with a remarkably high JSC value for an N-heterocyclic carbene iron(II) dye of 4.90 mA cm−2, but a low VOC of 244 mV. Our investigations have shown that an increased concentration of Li+ in combination with an optimized alkyl chain length in the 1-alkyl-3-methylimidazolium iodide IL in the electrolyte leads to iron(II)-sensitized DSC performances comparable with those of containing some copper(I)-based dyes.

Design and Synthesis of Photoactive Iron N-Heterocyclic Carbene Complexes

“…These properties make [Fe(phtmib) 2 ] + an interesting candidate to be employed in photoredox catalysis or artificial photosynthetic approaches based on abundant metal photosensitizers and catalysts. Generally, since the discovery of the beneficial photophysical properties of FeNHC complexes, a shift towards more application-oriented research can be seen without the focus on the fundamental research basis being lost [44]. Hence, a working photocatalytic system for the reduction of protons to hydrogen gas has been presented, albeit with very low turnover numbers [24].…”

“…It should be stated that DSSCs are a multicomponent system and variation of different constituents and the manufacturing process can have a significant influence on the performance of the assembled cell. In recent studies DSSCs with up to 0.57% and later 0.66% efficiency were realized using [Fe(cpbmi) 2 ] 2+ as PS and I 3 − /I − as redox mediator but employing a co-adsorbent and systematically varying the solvent, an ionic liquid additive and further additives [43,44]. Very recently, the efficiency of DSSCs employing [Fe(cpbmi) 2 ] 2+ as a dye was pushed even higher to just above 0.9%, using an I 3 − /I − electrolyte with guanidinium thiocyanate and other additives on carefully manufactured semiconductor surfaces [45].…”

Photophysics and Photochemistry of Iron Carbene Complexes for Solar Energy Conversion and Photocatalysis

“…The current density of DSSCs with complex 5 as the photosensitizer was improved significantly in 2018 and further in 2019, by altering both solvent and additives (changing acetonitrile to 3-methoxypropanenitrile and adding an ionic liquid together with more Li + ), but keeping the redox mediator (I 3 − /I − ). A new record conversion efficiency of 0.66% was obtained [90,91]. The current record conversion efficiency for FeNHC-based DSSCs, approaching 1%, was obtained yet more recently by Marchini et al A blocking underlayer in the TiO 2 structure together with additives guanidinium thiocyanate and MgI 2 helped to stop recombination and accelerate the regeneration of the dye (redox mediator still I 3 − /I − ) [92].…”

Electrolyte Tuning in Iron(II)-Based Dye-Sensitized Solar Cells: Different Ionic Liquids and I2 Concentrations

“…After a report by Wärnmark and coworkers that the iron(II) NHC complex 1 ( Scheme 1 ) exhibited a long 3 MLCT lifetime of 9 ps [ 22 ], the group of Gros demonstrated that 1 performed in n -type DSCs with η = 0.13% [ 23 ]. Until very recently, complex 1 continued to be one of the most promising iron(II)-based sensitizers [ 15 , 24 ], and tuning the composition of the electrolyte has been crucial in improving the performance of DSCs sensitized by 1 [ 25 , 26 ]. Gros reported in 2020 that the presence of both Mg 2+ ions and guanidium thiocyanate in the electrolyte with an I – /I 3 – redox shuttle resulted in a high J SC of 3.3 mA cm –2 and η of 1%.…”

“…At the same time, the concentration of I – ions in the electrolyte is typically much higher than that of I 2 [ 28 ]. We have previously shown that an electrolyte composed of LiI (0.18 M), I 2 (0.05 M), and the IL 1-propyl-2,3-dimethylimidazolium iodide (PDMII, Scheme 1 , 0.60 M) in methoxypropionitrile (MPN) led to a photoconversion efficiency of up to 0.66% for fully masked DSCs [ 25 ]. At the time of publication [ 25 ], this was the highest observed overall efficiency for an iron(II)-NHC sensitizer.…”