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Reduced graphene oxide on the performance of solid-state dye-sensitized solar cell

Dye-sensitized solar cell (DSSC) is a third generation solar cell which provides a good research platform for future photovoltaic technologies. DSSC can be categorized into two types based on the nature of its electrolyte, namely liquid and solid-state. In general, DSSC with liquid-based electrolyte has risk of electrolyte leakage. Therefore, the utilization of solid-state hole transport materials instead of liquid electrolytes can overcome the leakage issue of liquid-based DSSC. Despite of this advantage, the performance of solid-state dye-sensitized solar cell (SS-DSSC) is low compared to the cell that uses liquid electrolyte. To remedy this, doping with foreign materials is used to enhance the performance of the solar cell. Hence, this work focuses on the effect of reduced graphene oxide (RGO) on the performance of SS-DSSC utilizing TiO2 nanoparticles. The electrode samples were characterized with scanning electrode microscopy, energy dispersive spectroscopy, UV-Visible spectroscopy, IV test, and electrochemical impedance spectroscopy (EIS). By introducing RGO into SS-DSSC, the efficiency and fill factor was improved by at least 15%. Moreover, the presence of RGO increased the electron lifetime with higher recombination resistance as observed from EIS result.


Introduction
With the increasing number of world population, the demand for energy will also increase in tandem. The usage of fossil fuels has resulted in fast climate change due to global warming. Therefore, accessing to environmental friendly energy resources has become crucial to cater the increase demand of clean energy. Solar energy is an effective alternative and clean energy source for sustainable development.
Dye-sensitized solar cell (DSSC) is a third generation solar cell where its working principle depends on the photo-excited electron movement from the dye molecules to the external circuit and looping back to the dye molecules through the electrolyte or hole transport layer (HTM) [1]. The entire process occurs under light exposure.
In general, DSSC can be categorized into two types, which are liquid and solid-state (SS) types [2]. Liquid-based DSSC is known to have problems of possible leakage. Therefore, the utilization of solidstate HTM instead of liquid electrolytes can overcome this issue. However, the current performance of solid-state DSSC is low compared to the cell that uses liquid electrolyte. The performance of solidstate cell could be further improved with the incorporation of graphene-based material within the active layer [3].
Graphene based materials that have good electrical, optical, mechanical properties have been used in DSSC as transparent electrode instead of the usual fluorine doped tin oxide as well as used as photo-anode. Graphene is largely applied in DSSC due to its transparency and stability to visible light [4]. Previous study has shown that graphene could lower the band gap of TiO2 into visible range of wavelength, thus increasing the performance of DSSC [5]. When graphene is added into electrolyte, the performance of DSSC improved due to reduced potential loss (as a result of incompatibility of HOMO level of dye molecule and redox potential of electrolyte) and fast charge transport [3].
On the other hand, reduced graphene oxide (RGO), which is a graphene-like sheets of graphene oxide where its oxygen-containing groups have been discarded, has shown to improve the performance of DSSC when used as counter electrode [6]. In this work, the effect of RGO to the performance of DSSC was investigated. RGO was added into TiO2 paste to form a composite blend for anode electrode application. The properties of the samples were characterized and their performance was assessed. This study is hoped to provide some insight on the effect of RGO to the performance of SS-DSSC since it is not frequently explored.

Methods
Anode electrode was fabricated by firstly spin coating a titania blocking layer at 3000 rpm for 10 s. This was followed by sintering at 450°C for half an hour. After that, a mixture of RGO-TiO2 paste was deposited on top of the sintered layer using doctor blade method. Different RGO-TiO2 compositions were studied: pure TiO2, 0.1 wt% RGO-TiO2, 0.2 wt% RGO-TiO2 and 0.3 wt% RGO-TiO2. The deposited layer was then sintered at 450°C for 30 min. The as prepared anode electrode was soaked overnight in ruthenium dye ethanolic solution for dye attachment.
After dye soaking, the electrode was cleaned. Then 22L of electrolyte was deposited on top of the dyed RGO-TiO2 layer before spin coating at 2000 rpm for 20 s and left to solidify. The electrolyte consists of 80 mg of spiro-OMeTAD dissolved in 0.5 ml chlorobenzene with 4-tert-butylpyridine and LiTFSI in acetonitrile. The coated anode electrode was then assembled with counter electrode which was coated with thin layer of platinum precursor. The counter electrode was heated at 450°C prior usage.

Characterization
Anode electrode samples without dye attachment were characterized with Hitachi S-4800 scanning electron microscope coupled with energy dispersive spectroscopy, Cary 100 Conc UV-Vis spectrophotometer and Shimadzu XRD 6000 diffractometer. The performances of the assembled cells were measured with Keithley electrometer under 1 sun illumination. Their impedance responses were recorded with Zive SP1 electrochemical workstation under dark condition bias at open-circuit voltage.

Results and discussion
The anode electrodes were first characterized with SEM and EDX in order to confirm the nanostructure size of the active layer. Figure 1 shows the surface morphology of the anode samples under 40,000x magnification. All the images show distinct crystalline structure of TiO2. With addition of RGO, the porosity of the particles TiO2 is observed to have slightly reduced.  The presence of RGO can be confirmed by the increase of carbon element in EDS results. Figure 2 shows the elemental composition of TiO2 sample and 0.3 wt% RGO-TiO2 sample. The higher carbon content in RGO-TiO2 sample indicates that rich carbon substance has been successfully added into the sample; in this case it's RGO. The carbon atomic content increased from 3.53% for undoped TiO2 sample to 19.72% for 0.3 wt % RGO-TiO2. The crystallinity of the samples was further confirmed with XRD measurement (result not shown here). There's no appreciable difference in terms of peak intensity and broadening. Other study also confirmed this observation [7]. Thus, the inclusion of up to 0.3 wt% RGO in the TiO2 layer does not change the crystallinity of the TiO2 layer, which is important for the dye anchoring. The effect of RGO to the light absorbing ability was analyzed with UV-Vis spectrophotometer. Figure 3 shows the visible light absorbance of RGO-TiO2 and undoped TiO2 electrode samples, with and without dye attachment. Only one set of RGO-TiO2 sample is shown (i.e. 0.3 wt% RGO-TiO2) to avoid cluttering of the spectra. It can be observed that the absorption rate is improved with the presence of RGO in dye-sensitized TiO2 sample compared with that without RGO. The effect is prominent in the region of 400-500 nm wavelength. The increased of light absorbance suggests a decreased in band gap. With the inclusion of RGO, it is thought to have improved the diffuse absorption and reflectance properties of TiO2 under visible light.
The effect of doping RGO with TiO2 on the performance of SS-DSSC was further analyzed by characterizing its current density against voltage (J-V) responses. Table 1 shows the J-V results while its corresponding data is shown in Figure 4 (left). Only 0.3 wt% RGO-TiO2 sample is shown as its performance is the highest among the RGO doped samples. On the other hand, the charge transport  Table 1 as well.   The fill factor (FF) of the cells are within the range of 0.3 to 0.4 range which suggests that addition of RGO has negligible effect on it. FF is usually responsive to the series resistance in the solar cell that is generally derived from the resistance of the transparent conducting oxide, electrolyte as well as counter electrode. From Table 1, cell with RGO content has improved short-circuit current density (JSC) at the value of 5.72 mA/cm 2 compared to normal cell of 5.40 mA/cm 2 . Consequently, the efficiency of SS-DSSC with RGO is higher than the normal SS-DSSC. The overall performance is low To understand the performance of the devices, the charge transport and recombination properties are analyzed. Such information can be acquired from Nyquist plot where the internal resistance and impedance can be derived from the equivalent circuit corresponding to the device [8]. The equivalent circuit for this SS-DSSC consists of a resistor coupled with two pairs of parallel resistor and constant phase element. From the Nyquist plot, it is apparent that SS-DSSC with 0.3 wt% RGO, the central arc is wider with respect to undoped TiO2 sample. Therefore, with the inclusion of RGO, the electron lifetime and recombination resistance of SS-DSSC are improved, where the product of resistance and true capacitance is higher. This observation is beneficial in designing better and efficient photoanode structure where the addition of RGO has a role as performance enhancer.

Conclusion
RGO doped SS-DSSC has been successfully fabricated and investigated. The efficiency of the RGO doped SS-DSSC is higher when compared to undoped device where more than 15% improvement in efficiency was observed. The improved performance is supported by better light absorbance as well as longer electron lifetime of the device. Nevertheless, further study is required to ascertain the limit of the doping amount of RGO in SS-DSSC.