Increased Charge Transfer of Poly (ethylene Oxide) Based Electrolyte by Addition of Small Molecule and Its Application in Dye-sensitized Solar Cells

A Poly (ethylene oxide) based polymer electrolyte impregnated with 2-Mercapto benzimidazole was comprehensively characterized by XRD, UV–visible spectroscopy, FTIR as well as electrochemical impedance spectroscopy. It was found that the crystallization of PEO was dramatically reduced and the ionic conductivity of the electrolyte was increased 4.5 fold by addition of 2-Mercapto benzimidazole. UV–visible and FTIR spectroscopes indicated the formation of charge transfer complex between 2-Mercapto benzimidazole and iodine of the electrolyte. Dye-sensitized solar cells with the polymer electrolytes were assembled. It was found that both the photocurrent density and photovoltage were enhanced with respect to the DSC without 2-Mercapto benzimidazole, leading to a 60% increase of the performance of the cell. Introduction Dye-sensitized solar cells (DSC) have attracted great interest from both academic and industrial community since over 7% energy conversion efficiency was reported in 1991. Due to their merits of cost effectiveness, ease of preparation and relatively high efficiency, DSC is considered to be one of the most promising candidates for the next generation of cost-effective renewable energy resources [1,2]. Generally speaking, a typical DSC consists of dye coated TiO2 film, electrolyte containing I−/I3 − redox couple and platinum counter electrode. Under illumination, an electron generated by the dye is transferred to TiO2 thanks to the energy band matching between the dye excited state and the TiO2 conduction band. The electron is transferred to the current collector electrode (FTO based conductive glass). Simultaneously, the dye is regenerated by I of the electrolyte and I3 − in the electrolyte is reduced by the electron which reaches the counter electrode. Though over 11% conversion efficiency has been achieved with DSC with liquid electrolyte [3], the issues of leakage and evaporation resulting from the liquid electrolyte under long term continuous illumination has raised serious concerns of the stability of the device [4,5]. Such problems are expected to be overcome by employing ionic liquids or quasi-solid/solid medium [2–18]. The development of a gel polymer electrolyte for high performance and high stability DSC is of critical importance and high priority in terms of accelerating the commercialization process of DSC. It is well known that the performance of a DSC is strongly dependent on the transport property of the electrolyte. However, the change in the electrolytic properties has been attributed to the change in the mobility of the ions due to the change in the crystallinity of the polymer electrolyte [19,20]. The crystallization of the polymer can be reduced by addition of plasticizers and inert ceramic fillers, thus enhancing the ionic conductivity [21,22]. On the other hand, in order to ensure efficient dye regeneration and low recombination, the electrolyte should possess both high ionic conductivity and good wet-ability with the dye coated TiO2 film. Poly (ethylene oxide) (PEO) has been widely investigated for application in energy storage device because of its excellent stability [10,11]. However, this material is semicrystalline at room temperature and has conductivity of about 10−6 S/cm, far lower than the conductivity requirement for electrolyte used in DSC. In this work, we employed a small molecule 2-Mercapto benzimidazole (MB) to improve the ion transport property of PEO. It was found that the conductivity of polymer electrolyte was increased by nearly five-fold due to a significant reduction of the crystallization of the PEO and the formation of charge transfer associations between PEO–MB and I2. As a result, the performance and stability of the corresponding DSC with the polymer electrolyte modified with MB were significantly improved. 2. Experimental procedure 2.1. Preparation of polymer electrolyte for the DSC The polymer electrolyte was prepared by the following procedure. All chemicals were provided by Sigma–Aldrich unless otherwise stated. 0.3 g of PEO (Mw = 5,000,000), 0.06 g of KI, 0.0254 g of I2 and 0.014 g of 2-Mercapto benzimidazole (MB) (Merck) were dissolved in 25 ml dimethyl formamide (DMF) and were well mixed by rigorous magnetic stirring at 353 K for 3 h to form a viscous gel state electrolyte. Thin polymer electrolyte films were made by solution casting technique and were used for characterization studies. 2.2. Assemble of dye-sensitized solar cells The nanocrystalline opaque TiO2 film deposited on fluorine doped tin oxide (FTO, TEC15) and platinum coated FTO counter electrodes were provided by Dyesol Ltd. (Australia). The TiO2 film was sintered at 450 ◦C for 40 min and then cool down to 100 ◦C before being immersed into the dye bath with concentration of 0.3 mM N719 in ethanol overnight. The prepared polymer electrolyte of minimum quantity (≤400 _l) was cast on the anode (active area 0.88 cm2) and evaporated to dryness on a hot plate at 150 ◦C. DSC was assembled by sealing the electrolyte covered photoanode and Pt counter electrode with thermal plastics (Surlyn, 30 μm). A small fraction of polymer electrolyte was introduced into the cell in order to wet the photoanode so as to increase the segmental mobility [14] and also to prevent the formation of cavities on the photoanode during solvent evaporation through a predrilled hole in the cathode under vacuum condition [23]. The hole was then sealed with Bynel. 2.3. Characterization techniques The morphology of the electrolyte was monitored with a scanning electron microscope (SEM) by JEOL JSM 6362 model. The FT-IR spectra of the electrolyte were recorded with a Perkin-Elmer 1000 spectrometer by coating the electrolyte on KBr disc. The absorption characteristics of the electrolyte which was diluted in DMF were traced by a Varian UV–visible spectrometer (Cary 100). X-ray diffraction pattern measurements were done by Siefert Model SF60 XRD system with CuK_1 radiation. The electrochemical impedance spectroscopy (EIS) of the electrolyte was measured by a HP Precision LCR meter (4284A) over the frequency range of 20 Hz–1 MHz within the temperature range of 25–50 ◦C through assembling a symmetrical cell Ag disc/electrolyte film/Ag disc. The EIS data were analysed by Boukamp equivalent circuit program software [24,25]. 2.4. I–V performance The I–V performance of the DSC was evaluated with a solar simulator system equipped with a high pressure sodium vapour lamp (1000 W, Dyesol Pty Ltd. set-up model: a metal chamber with the light source and a cooling system to maintain the temperature. This was custom built as per Dyesol Ltd. specifications) and Keithley potentiostat source meter (Model 2400) using UPTS (Universal Photovoltaic Test System) Dyesol Ltd. I–V characterization software. The illumination intensity of the solar simulator was equivalent to 0.71 Sun which was calibrated with a silicon reference photodiode and corrected for any spectral mismatch with outdoor illumination [26]. The active area of the DSC was 0.88 cm2. 3. Results and discussion 3.1. Scanning electron microscope studies The morphology of the polymer electrolyte modified with MB by SEM is shown in Fig. 1. As can be seen, the electrolyte spreads uniformly on the surface of TiO2 film. The large circle patterns with diameter around 100 _m may be assigned to the large PEO molecule. The film shows large regions of inhomogeneity and may highlight a potential difficulty in penetrating into the nanopores of TiO2 film. 3.2. FT IR results The FT-IR spectrum recorded for the polymer electrolyte (PEO/I2/KI/MB) and pure PEO are shown in Fig. 2(a) and (b). The characteristics peaks of PEO at 2860 cm−1 and 1094 cm−1 for CH2 linkage and ether linkage groups can be seen, respectively, the characteristic peaks of MB are also observed in the spectrum. The peak at 3541 cm belongs to the anti-symmetric stretching of NH group and the peak at 2929 cm−1 represents the SH group of MB. The peaks at 1658 cm−1 and 865 cm−1 can be attributed to aromatic and di-substituted benzene. The infrared spectrum also indicates the coordination between PEO, MB and I2 via the thione sulphur atom. The thione tautomer (Scheme 1(a)) predominates in both the solid and solution states of MB due to the stronger electron attraction of sulphur than nitrogen [27]. When the carbon atom is directly linked to nitrogen, _(C S) is strongly coupled and as a result there are a number of different bands in the 1570–650 cm−1 region, each of which contains some contribution from the C S stretching mode as shown in Fig. 2 [28]. The multiple bands in the region 1150–1000 cm−1 may be due to the C S content [29]. The thiocarbonyl (C S) group is less polar than the carbonyl group and is a considerably weaker bond. But, as the carbon atom is coupled to a nitrogen atom, it becomes more polar and can donate electron to atom with strong electron affinity such as iodine. In the electrolyte studied here, the electron is donated from the sulphur into the LUMO of iodine [30]. 3.3. UV–visible spectroscopic studies The association of PEO with MB and I2 is further confirmed by the investigation of the UV– visible absorption of the electrolyte. Fig. 3 shows the UV–visible spectra of PEO/MB (a) and PEO/KI/I2/MB (b) as well as PEO/KI/I2 (c). The spectrum pattern of (a) at around 312 nm confirms the presence of a substituted aromatic ring structure, which may be due to the presence of MB. The peaks at 295 nm and 368 nm in (c) belong to the absorption of I3 [31,32]. However, after addition MB into PEO/I2/KI, both the peaks at 295 nm and 368 nm disappeared and a new peak at 286 nm with a shoulder at 291 nm appears (b) (Fig. 3 inset). This indicates the association between MB and iodine [27,29]. The ‘electron rich’ MB (Lewis-base donor) transfers electronic charge of S atom to ‘electron deficient’ iodine atom (Lewis-acid acceptor) forming charge transfer complex. This phenomenon has also been reported in pyrimidine derivatives based electrolyte systems [33,34]. Moreover, MB exists in solution as thiol-thione equilibrium as shown in Scheme 1. The presence of thione tautomeric is in favour of the formation of the charge transfer complex through reaction proposed in Scheme 1(b) and (c). Clearly, both association routes result in the decrease of the concentration of I3− and the increase of the concentration of I−. The observation in the UV– visible spectrum is consistent with the FTIR results discussed above. 3.4. X-ray diffraction studies The X-ray diffraction pattern of the electrolyte with and without MB is shown in Fig. 4(a) and (b). The sharp and intense XRD peaks observed at 2 theta= 20◦, 24◦ and 42◦, respectively, in Fig. 4(a) belong to PEO, indicating its crystalline nature. However, for the electrolyte containing MB, the intensity of the XRD pattern at 20◦, 24◦ and 42◦ dramatically decreases and the peaks become broader as illustrated in Fig. 4(b). This suggests the crystallization of the PEO is reduced with MB. It is well known that amorphous state of polymer is in favour of the carrier transport and thus the conductivity. In order clarify this; the ion conductivity of the electrolyte was investigated. 3.5. Conductivity measurements A symmetrical cell with the structure of Ag disc/electrolyte film/Ag disc was built for the evaluation of the conductivity of the electrolyte. The Nyquist plot of the electrochemical impedance spectrum of the polymer electrolyte with MB at 298 ◦C is illustrated in Fig. 5. The ionic conductivity was calculated according to Eq. (1):