Robust High-performance Dye-sensitized Solar Cells Based on Ionic Liquid-sulfolane Composite Electrolytes

Novel ionic liquid-sulfolane composite electrolytes based on the 1,2,3-triazolium family of ionic liquids were developed for dye-sensitized solar cells. The best performing device exhibited a short-circuit current density of 13.4 mA cm−2, an open-circuit voltage of 713 mV and a fill factor of 0.65, corresponding to an overall power conversion efficiency (PCE) of 6.3%. In addition, these devices are highly stable, retaining more than 95% of the initial device PCE after 1000 hours of light- and heat-stress. These composite electrolytes show great promise for industrial application as they allow for a 14.5% improvement in PCE, compared to the solvent-free eutectic ionic liquid electrolyte system, without compromising device stability.

correlation exists between the viscosity of the electrolyte and the volume percentage of sulfolane in the electrolyte 12 . Therefore, to achieve the best photovoltaic performance, 50 vol% of sulfolane was added to electrolytes E1 to E4, to obtain a second set of electrolytes (coded ES1 to ES4). The triiodide diffusion coefficients in each electrolyte were determined via cyclic voltammetry and by applying equation (1), see Table 1. While there does not appear to be a clear trend between the triiodide diffusion coefficients and the length of the alkyl chain on the ionic liquid cation, the results show that addition of 50 vol % sulfolane to the electrolytes results in a considerable increase in the triiodide diffusion coefficient, in some cases by up to 6 times (ES1). A higher triiodide diffusion coefficient is typically indicative of better overall device performance due to faster transport and dye regeneration kinetics 13 .    -D). This difference may be ascribed to an increase in the redox energy of the electrolyte and a negative shift of the conduction band due to the use of sulfolane in the electrolyte, as described previously 12 .   The best performing electrolytes, i.e. ES2, ES3 and ES4, were then used to fabricate DSCs with a double layer TiO 2 film (8 + 5 μ m), affording devices I, J and K. The application of double layer TiO 2 films as photoanodes, in conjunction with these lower viscosity electrolytes, led to enhanced PCEs in most cases, with the best performing device delivering 6.3% and 8.4% power conversion efficiencies at 100 mW cm −2 and 10 mW cm −2 , respectively (device I).
In order to investigate the long-term stability of these sulfolane-based DSCs, devices I, J and K were subjected to an accelerated aging test at 60 °C under constant full sun illumination (100 mW cm −2 ) for 1000 hours. Figure 2 shows the I-V performance of devices I, J and K before and after light-soaking, measured at 50 mW cm −2 illumination. The variations in the photovoltaic parameters, measured at 50 mW cm −2 , are presented in Table 3.
Although a 30 mV drop in the V OC was observed in devices I and J, this was partially compensated for by a slight increase in the J SC values, thus allowing for the retention of more than 95% of the initial device PCE after the 1000 hours accelerated aging test. The initial J SC values for devices I, J and K were in the range of 7-8 mA cm −2 , and the V OC values were in the range of 660-680 mV, when measured under standard AM 1.5G illuminations at 50 mW cm −2 . After aging the devices under full sunlight intensity at 60 °C, the changes in the J SC values observed were minor compared to the decrease in the V OC values. After light-soaking treatment, a decrease in V OC of 36, 24, and 22 mV was observed for devices I, J, and K, respectively (Fig. 2). The drop in V OC is in agreement with the changes observed in the dark current of the devices during the light-soaking tests (Fig. 3). In general, the aged devices show a higher dark current compared to the freshly prepared devices, presumably originating from a lower recombination resistance or from a downward shift in the TiO 2 conduction band 14 .
Electrochemical impedance spectroscopy (EIS) studies were undertaken to help understand the changes in the photovoltaic parameters of devices I, J and K during the extended light-soaking treatment. EIS measurements give direct access to information regarding the charge transfer resistance of the TiO 2 /dye/electrolyte interface, the transport resistance for electrons in the mesoporous TiO 2 , and the chemical capacitance from the filling of trap states in the mesoporous metal oxide. EIS investigations were performed in the dark on devices I-K. The EI spectra were fitted according to the transmission line model [15][16][17] . The apparent electron lifetime (τ n ) and transport time (τ trans ) were estimated using the transport and recombination resistance, R trans and R ct respectively, in conjunction with the chemical capacitance (C chem ) of the TiO 2 (τ n = R ct × C chem and τ trans = R trans × C chem ) [15][16][17] .
The main parameters (charge transfer resistance, charge transport resistance and chemical capacitance of the mesoporous TiO 2 film) extracted from the Nyquist plots by the transmission line model for devices I, J and K are presented in Fig. 4. After aging, a shift in the chemical capacitance of approximately 36, 35, and 37 mV is observed in devices I, J, and K, respectively (Fig. 4). These values indicate a shift in the conduction band edge of the TiO 2 and are close to the observed differences in the V OC before and after aging.
Plotting the electron lifetime τ n against the chemical capacitance (to rule out the shifts in the conduction band of different devices and compare the recombination properties at a similar charge density) the main feature that changes the V OC is the shift of the TiO 2 conduction band (Fig. 5). For device I there is not much difference in the electron lifetime during the aging process. The electron lifetime increases slightly in devices J and K when aged, further reducing the loss of voltage due to a downward shift of the conduction band. Thus, the main change in V OC originates from the conduction band shift, which is partly compensated for in devices J and K by the increase in electron lifetime. While the length of the alkyl chain on the triazolium cation does not appear to significantly influence the position of the conduction band, a longer chain length does seem to favor longer electron lifetimes.

Discussion
The triiodide diffusion coefficients of each electrolyte, as determined by cyclic voltammetry, confirm our hypothesis that addition of 50 vol % sulfolane to solvent-free eutectic ionic liquid electrolytes results in the lowering of their overall viscosity, in some cases by up to 6 times. This is desirable for efficient electron transfer kinetics within the The recombination resistance (R ct ) clearly mirrors the observed tendencies of the dark current. Also, the change in the chemical capacitance can be observed. device, and contributes to higher device PCEs, which were also observed experimentally. Device B, which contained electrolyte E2, gave a power conversion efficiency of 5.5% at full sun, while device F, which contained electrolyte ES2, gave a power conversion efficiency of 6.3% at full sun. This represents a 14.5% improvement when going from the solvent-free eutectic ionic liquid electrolyte system to the ionic liquid-sulfolane composite electrolyte system. Dilution of the ionic liquid-based electrolytes with sulfolane is therefore a simple and straightforward method to reduce overall device costs while improving device performance.
Long-term stability tests performed on DSCs containing the novel triazolium ionic liquid-sulfolane composite electrolytes showed that the sulfolane-based systems are in fact very robust, retaining more than 95% of their initial PCE after 1000 hours of continuous light-and heat-stress (devices I and J).
In conclusion, we have developed novel triazolium ionic liquid-sulfolane composite electrolytes for application in dye-sensitized solar cells. Devices employing these new electrolytes exhibit very good power conversion efficiencies and device stability, comparable to existing benchmarks.
Photovoltaic measurements and accelerated aging tests. An AM 1.5 solar simulator equipped with a 450 W Xenon lamp (Oriel, USA) was used for all photovoltaic measurements. To obtain the I-V curves, an external bias was applied to the cell and the generated photocurrent was measured with a Keithley model 2400 digital source meter. All devices were masked to attain an illuminated active area of 0.159 cm 2 . For the accelerated aging tests, the devices were placed under continuous full sun illumination (100 mW cm −2 ) at 60 °C, in the presence of a UV cut-off filter. The devices were kept under open circuit conditions throughout the experiment, and were periodically removed for measurements.
Electrochemical impedance spectroscopy (EIS) measurements. EIS measurements were performed with a BioLogic SP300 potentiostat providing a voltage modulation of 15 mV in the desired frequency range (1 MHz to 0.1 Hz). The EIS measurements of the devices made for the determination of the diffusion coefficient were performed at 0 V. Z-view software (v2.8b, Scribner Associates Inc.) was used to analyze the impedance data on the basis of Randles circuit. The impedance spectra of the measured devices were recorded at bias potentials varying from 0 to 750 mV in 50 mV steps. The impedance spectra of the DSCs were analyzed on the basis of the transmission line model [15][16][17] .