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

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.


Fabrication of DSCs
Each working commercial TiO2 electrode (opaque, Solaronix) was rinsed with EtOH and dried on a heating plate at 450 °C for 30 min. The electrodes were cooled to 60 °C and immersed in an iron(II) dye 2 (0.5 mM) acetonitrile solution containing chenodeoxycholic acid (0.1 mM) overnight. The reference working electrode was made by dipping a commercial electrode in an 0.3 mM EtOH solution of N719 (Solaronix) overnight. After soaking in the dye-baths, the electrodes were washed with the same solvent as used in the dye-bath (in case of MeCN the electrodes were washed second time with acetone) and dried with a heat gun.
Commercial counter electrodes from Solaronix (Test Cell Platinum Electrodes Drilled) were rinsed with EtOH and dried on a heating plate at 450 °C for 30 min. The TiO2 electrodes and Pt counter-electrodes were assembled using thermoplast hot-melt sealing foil (Solaronix, Test Cell Gaskets, made from Meltonix 1170-60 sealing film, 60 microns thick) by heating while pressing them together. The electrolytes were introduced into DSCs by vacuum backfilling through a hole drilled in the counter electrode and this was then sealed using hotmelt sealing foil and a cover glass.
The solar cell measurements used fully masked cells using black coloured copper sheet with a single aperture placed over the screen printed dye-sensitized TiO2 square. The area of the aperture in the mask was smaller than the active area of the dye-sensitized TiO2 (0.36 cm 2 ). For complete masking, black tape was also applied over the edges and rear of the cell. Current density-voltage (J-V) measurements were made by irradiating from behind with a LOT Quantum Design LS0811 instrument (100 mW cm -2 = 1 sun at AM 1.5) and the simulated light power was calibrated with a silicon reference cell.
The external quantum efficiency (EQE) measurements were performed on a Spe-Quest quantum efficiency setup from Rera Systems (Netherlands) equipped with a 100W halogen lamp (QTH) and a lambda 300 grating monochromator from Lot Oriel. The monochromatic light was modulated to 1 Hz using a chopper wheel from ThorLabs. The cell response was amplified with a large dynamic range IV converter from CVI Melles Griot and then measured with a SR830 DSP Lock-In amplifier from Stanford Research.

EIS measurements
For the EIS measurements a ModuLab® XM PhotoEchem photoelectrochemical measurement system from Solartron Analytical was used. The impedance was measured at the open-circuit potential of the cell at a light intensity of 22 mW cm -2 (590 nm) in the frequency range 0.05 Hz to 100 kHz using an amplitude of 10 mV. The impedance data were analysed using ZView® sofware from Scribner Associates Inc. Table S1. Parameters for multiple DSCs using electrolytes with different concentration of lithium salts and ILs.