The influence of alkyl chains on the performance of DSCs employing iron(ii) N-heterocyclic carbene sensitizers

The photovoltaic performances of DSCs employing two new iron(ii) N-heterocyclic carbene (NHC) sensitizers are presented. The presence of n-butyl side chains had a significant impact on DSC performace. The improvement in DSC performance up to 0.93–0.95% was observed for a new heteroleptic sensitizer bearing one carboxylic acid anchoring group. The photovoltaic performance was remarkably affected by sensitization time and by a presence/absence of coadsorbent on the semiconductor surface. The highest photoconversion efficiencies (PCE) were achieved for DSCs sensitized over 17.5 hours without addition of coadsorbents. However, for a shorter dipping time of 4 hours, the presence of chenodeoxycholic acid improved the PCE from 0.46% (no coadsorbents) to 0.74%, respectively. The performance of DSCs based on a new homoleptic complex bearing two n-butyl side chains and a carboxylic acid anchor on each NHC-ligand was improved from 0.05 to 0.29% via changes in dye-bath concentration and sensitization time. The changes in the dye load on the semiconductor surface depending on the sensitization conditions were confirmed using solid-state UV-Vis spectroscopy and thermogravimetric analysis. Electrochemical impedance spectroscopy was used to gain information about the processes occurring at the different interfaces in the DSCs. The impedance response was strongly affected by the immersion time of the photoanodes in the dye-bath solutions. In the case of the homoleptic iron(ii) complex, a Gerischer impedance was observed after 17.5 hours immersion. Shorter dipping times resulted in a decrease in the resistance in the system. For the heteroleptic complex, values of the chemical capacitance and electron lifetime were affected by the immersion time. However, the diffusion length was independent of sensitization conditions.


General considerations for measurements
Current density-voltage (J-V) measurements were made by irradiating from the photoanode side 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.
UV-Vis spectra were recorded on Shimadzu UV-2600 spectrophotometer. Solid-state UV-Vis spectra were measured on a VARIAN Cary-5000 spectrophotometer. Transparent TiO 2 electrodes were used as a reference for the solid-state absorption spectroscopic 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 and fitted using ZView® sofware from Scribner Associates Inc.
N719 dye was purchased from Solaronix. Commercial working electrodes (opaque), platinum counter electrodes (Test Cell Platinum Electrodes Drilled) hot-melt sealing foil (Test Cell Gaskets, made from Meltonix 1170-60 sealing film, 60 microns thick) were obtained from Solaronix as well as. The conducting silver paint (colloidal suspension, 0.5 troy oz.) was obtained from SPI. HPLC grade solvents were used for solar cell fabrication, and were purchased from HPLC VWR and J.T. Baker.
Thermogravimetric analysis (TGA) was performed on a TGA5500 instrument (TA Instruments) coupled to a Discovery II MS, Cirrus 3, Mass Spectrometer, DMS. The analysis was carried under nitrogen, using a Barchart scanning method in the mass range 10-125. In all the experiments, the temperature of the TGA instrument was initially stabilized at 30°C for 10 min followed by heating at a rate of 10°C/min to 120°C. This temperature was maintained for 30 min. Afterwards each sample was heated at a rate of 10°C/min to 900°C. After 30 min at 900°C a sample was cooled down to ambient temperature.

Synthesis and characterization of complexes 2 and 3
Starting materials for synthesis were obtained in reagent grade from Avocado Research Chemicals Ltd, Sigma-Aldrich, Fluorochem, Alfa-Aesar, TCI, Carl Roth and Acros Organics. Dry solvents (crown cap or AcroSeal®) were purchased from Acros Organics. NMR solvents were obtained from Cambridge Isotope Laboratories Inc. and Apollo. Fluka silica gel 60 was used for flash chromatography. Thin layer chromatography (TLC) was performed with aluminium sheets covered with silica gel 60 (Merck). 1 H, 13 C{ 1 H} and 19 F{ 1 H} NMR spectra were recorded on Bruker Avance III-500 NMR spectrometers; spectra were recorded at 295 K. 1 H and 13 C spectra were referenced with respect to δ(TMS) = 0 ppm. High resolution mass spectrometry (HRMS) was performed on Bruker maXis 4G instrument. FTIR spectra were recorded on a Perkin Elmer UATR Two spectrophotometer. A flame-dried flask was charged with 2,6-dibromopyridine (1.0 equiv, 5.00 mmol, 1.18 g) under nitrogen. The flask was evaporated and refilled with nitrogen. Anhydrous THF (50 mL) was added and the solution was cooled to -30°C. TMPMgCl·LiCl (1.2 equiv, 6.00 mmol, 6.0 mL) was added dropwise for 15 min and the reaction mixture was stirred at the same temperature for 3 h. Afterwards the reaction was quenched with dry ice until gas evaluation stopped and the mixture was allowed to warm to room temperature overnight. The pH was adjusted (pH≈12) with 1M NaOH solution and the mixture was extracted with AcOEt (3 x 15 mL). The aq. layer was acidified with 1M H 2 SO 4 to pH = 2 and extracted with AcOEt (3 x 15 mL). The combined organic fractions were dried over MgSO 4 and the solvent was removed under reduced pressure to obtain 2,6-dibromoisonicotinic acid as a white solid (1.65 mmol, 462 mg, 33%).

Electric circuit model used for fitting EIS experimental data
Fig. S 6. The circuit model consists of five elements and includes a series resistance (Rs), a resistance (R Pt ) and a constant phase element (CPE Pt ) to model a counter electrode, an extended distributed element (DX1) to represent the mesoporous TiO 2 /electrolyte interface as a transmission line model, and a Warburg element (Ws) to represent the diffusion of the electrolyte.