Application of rice husk as a carbon source for substitution of sensitizer and counter electrode material in dye-sensitized solar cells

The third generation of solar cells is dye-sensitized solar cells (DSSC). DSSC can convert solar energy into electrical energy. The main components of DSSC include working electrode, counter electrode, sensitizer, and electrolyte. Substitution of commercial chemicals for the sensitizer (N719) and the counter electrode (platinum) of DSSC will lower the cost of DSSC fabrication. This study focuses on Nitrogen-Doped Carbon Quantum Dots (N-CQDs) synthesized from rice husks as a sensitizer. Then, DSSC with sensitizer of N719 (Pt-1) and N-CQDs (Pt-2) were compared. Furthermore, the N-CQDs residue in solid carbon was used as the counter electrode material using different sensitizers N719 (C-1) and N-CQDs (C-2). Based on the results, the FTIR characterization of N-CQDs showed the functional groups N-H, O-H, C=O, and CN. The UV-VIS characterization of N-CQDs had absorbance in the wavelength range of 200-400 nm. Pt-2 has an efficiency value of 0.258%, Jsc of 2.188 mA/cm2 Voc of 0.6 volts, and FF of 0.20. C-2 has an efficiency of 0.0010%, Jsc of 0.042 mA/cm2 Voc of 0.131 volts, and FF of 0.17. This confirms that N-CQDs can be used as a sensitizer and counter electrode material, although the results are not as good as commercial chemicals.


Introduction
Energy is a necessity for human survival, such as fossil energy.Unfortunately, fossil energy does not last long.According to the Indonesian Ministry of Energy and Mineral Resources, Oil reserves from 8.21 billion barrels in 2008 fell to around 3.8 billion barrels in 2019 and natural gas reserves in 2008 amounted to 170 Triliun Standard Cubic Feet (TSCF) and continued to fall to around 77.29 TSCF in 2019 [1].Dependence on fossil energy causes this energy source to become depleted.
Solar energy is a high-potential solution to overcome the energy crisis.It can be converted into electrical energy through solar cells [2].Solar cells are an alternative solution to energy and environmental problems because they are environmentally friendly, clean, renewable, and have high efficiency in terms of performance.As technology develops, the third generation of solar cells is dyesensitized solar cells (DSSC).DSSC is a solar cell consisting of working electrode, dye, electrolyte, and counter electrode [3].The substrate of the working electrode and counter electrode is FTO (fluorinatedtin oxide) glass.The widely used materials are N719 as a photosensitizer and iodide-based as an electrolyte.Additionally, platinum (Pt) is used as the counter electrode material and titanium dioxide (TiO2) is used as the working electrode material.
Ruthenium dye (N719) is the most widely used because it has a good absorbance value for absorbing visible light [4].However, ruthenium dye is considered quite expensive.Substitution with other materials that are abundantly available can lower dye costs, such as carbon nanoparticles in nitrogen-doped carbon quantum dots (N-CQDs).N-CQDs are carbon nanoparticles with a doped of nitrogen metal.This material is a fluorescence nanoparticle based on a carbon core with high stability.The use of N-CQDs as a co-sensitizer resulted in a DSSC performance of 3.92% efficiency with a carbon source from citric acid and nitrogen source from urea using microwave-assisted synthesis [5].Zhao et al successfully converted strawberry powders as carbon sources added NH3.H2O as nitrogen sources to become N-CQDS through hydrothermal synthesis.DSSC using N719 achieves conversion efficiency of 8.09% and it increase to 9.29% with co-sensitized N-CQDs [6].
Carbon quantum dots (CQDs) can be synthesized from various carbon sources such as sugar, glucose, cellulose, starch, organic molecules, and biomass waste containing functional groups such as carboxyl and hydroxyl.Meanwhile, Indonesia has abundant forest and agricultural product resources.One of the most abundant agricultural products is rice.However, the large number of rice agricultural products produces a lot of waste.In addition, the utilization of rice agricultural waste has not been maximized due to technical and economic factors.One of the rice agricultural wastes is rice husk which comes from the husk of the rice seed.Rice husk has a rich content of carbon elements so that it has the potential to make CQDs.
Therefore, in this research, N-CQDs were synthesized from rice husks with the addition of ethylenediamine (nitrogen source) which is processed through hydrothermal synthesis.Furthermore, the residues of N-CQDs in the form of solid carbon were used as counter electrode material.

Synthesis N-CQDs from Rice Husk
Rice husks were washed using distilled water and then dried in an oven at 60°C.The dried rice husks were grounded in a blender to a powder.The powder samples were cleaned of inorganic impurities using 0.1 M HCl before centrifuging at 4000 rpm and dried in an oven at 60°C.After that, 2 grams of rice husk powder was mixed with 1 ml of ethylenediamine (EDA) and 30 ml of distilled water, then transferred into a Teflon-lined autoclave to undergo a hydrothermal process for 12 hours at 200°C.EDA is used as a functionalizing agent for the amino groups.After heating, the mixture was cooled at room temperature for 24 hours before filtering through a vacuum filter.Furthermore, residue-free N-CQDs were obtained.At the same time, the residue of N-CQDs in the form of solid carbon can be used as a counter electrode material.

Fabrication of DSSC
FTO glass is used as an electrode conductive glass substrate.Previously, the FTO glass was sonicated in alcohol using an ultrasonic cleaner for 10 minutes, then immersed in a 40mm TiCl4 solution and heated on a hotplate at 70°C for 30 minutes.Afterward, the FTO glass was rinsed with ethanol and heated again on the hotplate at 450°C for 30 minutes.
The working electrode was prepared by depositing TiO2 T/SP paste onto the FTO glass with active layer (1 cm x 1 cm) using the doctor blade method.The FTO glass that had been coated with TiO2 T/SP paste was heated on a hot plate at 150°C for 30 minutes.Furthermore, the glass was coated again with TiO2 R/SP paste on top of the TiO2 T/SP layer and heated on a hotplate at 450ᵒC for 45 minutes.Posttreatment was carried out by immersing the electrode in 40mm TiCl4 solution and heating it on a hotplate with a temperature of 70°C for 30 minutes, then rinsing it with ethanol and heating it again on a hotplate with a temperature of 450°C for 30 minutes.The working electrode was immersed in two variations, (1) 0.5 mM N719 and (2) N-CQDs solution for 24 hours in a closed container.
Counter electrodes were made in two variations by depositing (1) platinum ( 2) solid carbon on the FTO glass with an active layer of 1 cm x 1 cm.Solid carbon paste was made by mixing solid carbon with ethanol, acetic acid, and triton-x.Platinum paste and solid carbon paste was deposited on the FTO glass using the doctor blade method.Then the working and counter electrodes are heated in a furnace at 450°C for 45 minutes.Electrolyte was prepared by mixing 0.5 grams of urea and 5 mL of iodolyte.
DSSC was assembled by sequentially arranging working electrode and counter electrode.Then, an electrolyte was injected into DSSC.There are four variations in this study which are shown in detail in Table 1

FTIR Characterization
Based on the Fourier transform infrared test results of N719 ethanol solution and N-CQDs at wavenumbers between 4000-500 cm -1 .The transmittance peaks were obtained in the variant wavelengths indicating various functional bonds, which can be seen in the Figure 1.In FTIR testing of N-CQDs, the transmittance peaks are obtained at wavelengths 3347 cm -1 represented N-H and O-H from amine and hydroxyl group stretching vibration.Meanwhile, the peaks at 1636 cm -1 assign to C=O stretching vibration in carboxylate and aromatic ring skeleton vibration [7].The low absorption peak at 1200-1600 cm -1 indicates the aromatic CN heterocycles [8].The N-H functional group is absorbed as a secondary amino group from ethylenediamine compounds and C=O functional group is an alkene group from carbon in rice husk.FTIR results confirmed a content of nitrogenated surface groups in the N-CQDs [9].
Meanwhile, in testing the N719 dye, the transmittance peaks are at the wavelength 3399 cm -1 indicates N-H functional group.The presence of the band at approximately 2971 cm −1 is due to the C-H bond stretching.COO− symmetric stretching is found at wavelength 1365 cm −1 [10].The emergence of a band at around 1072 cm -1 ascribs to C-O group vibrations from dye molecules.The components at 1630 cm −1 indicate the carbon double bonds (C=C) linked the aromatic carbon bond stretching (C=C) in the molecules of the N719 dye [11].This compound also has more varied bond peaks because it has a more complex content than N-CQDs.

UV-Visible Characterization
UV-Vis spectrophotometer was carried out to review the absorbance of the analyzed compound.The UV-Vis spectra of N719 ethanol solution and N-CQDs is shown in Figure 2.

Figure 2. UV-Vis Test Results for N719 and N-CQDs
The results of the characterization of the absorbance spectrum of the N719 showed that the light absorption was at a wavelength of 248 nm, 309 nm, 375 nm, and 507 nm.Similar absorption peaks were obtained from the experimental results of Portillo-Cortez et al. and Wen et al.According to reports, the bands at 309 nm (UV region) corresponds to intraligand (π−π*) transitions [12] and the two peaks at 507 and 375 nm are assigned to metal-to-ligand charge-transfer (MLCT) origin [13].
The N-CQDs compound has an absorbance peak at a wavelength of 209.These data prove that N-CQDs have absorbance in the 200-400 nm wavelength range.These results are almost the same as the research conducted by Shejale et al. with the absorbance values of N-CQDs in the wavelength range of 200-450 nm [5].

SEM Characterization
Scanning Electron Microscope (SEM) characterization was used to analyze the morphological structure of N-CQDs residue in the form of carbon.This solid carbon will be used as a counter electrode material.SEM can provide an overview of surface morphology, which is specific to carbon particles and pores on the surface of solid carbon.The morphology of solid carbon can be seen from the SEM test results with magnifications of 5,000x and 10,000x shown in the following Figure 3. From the figure, the morphology of solid carbon is irregular in shape and micron in size.The chemical element of carbon solids was obtained through EDX characterization.From the Table 2, it can be seen that the element carbon has a very dominant percentage.Carbon solids contain the element carbon of 70.84%.There is other elements are 27.4% oxygen and 1.76% silicon confirming that SiO2 is the major mineral substance in solid carbon [14].

I-V Meter Characterization
DSSC performance with variation of sensitizer of N719 (Pt-1) and N-CQDs (Pt-2) using platinum (Pt) counter electrode was shown in Table 3 and Figure 4.
Table 3 The use of N719 is well known as a sensitizer in DSSC.DSSC with N719 as a sensitizer have an efficiency of 2.23% (Pt-1).While the use of N-CQDs as a sensitizer is indicated by Pt-2 with an efficiency of 0.258%.This proves that N-CQDs from rice husks can be used as sensitizer for DSSC.However, the efficiency of Pt-2 is lower than that of Pt-1 (around 11.5% compared to Pt-1).The lower performance of N-CQDs than N719 is consistent with the UV-Vis characterization results.N719 have a broaden absorbance range compared to N-CQDs resulting the short current density (Jsc) of Pt-1 higher than that of Pt-2.The Jsc value is also influenced by the amount of sensitizer loading and electron injection rate at the sensitizer/TiO2 interface [15].Furthermore, decreasing Rs (series resistance) and increasing Rsh (shunt resistance) will improve overall DSSC performance.The fill factor (FF) value will be proportional to Rsh, and vice versa to Rs. Rsh of Pt-2 is smaller than Rsh of Pt-1, so the FF value of Pt-2 is smaller than FF of Pt-1.Rs is related to the interfacial contact resistance and the sheet resistance of the counter electrode.In addition, the charge recombination loss inside the DSSC is represented by Rsh [16].DSSC performance with variation of sensitizer N719 (C-1) and NCQDs (C-2) using solid carbon as counter electrode material can be observed in Table 4 and Figure 5. Platinum as the counter electrode material used in DSSC has the best performance.The widespread use of platinum in counter electrode is hampered by large-scale applications because it is expensive [17].Therefore, it is necessary to develop materials used for platinum substitution.One way is to use solid carbon as counter electrodes.The efficiency results for C-1 and C-2 were 0.0038% and 0.0010%, respectively.This confirms that solid carbon can be used as a counter electrode material for DSSC, although its performance is lower than that of platinum.The low performance of DSSC with counter electrodes from carbon is due to the micron particle size, so the surface area and active sites are small [18].

Conclusion
In this research, Pt-2 can produce electric current and voltage on the DSSC, which confirms that N-CQDs can be used as a substitute for the N719 sensitizer.However, the highest performance was obtained on the Pt-1 with the Pt/Iodolyte/N719/TiO2 arrangement with an efficiency value of 2.23% with a Jsc of 5.27 mA/cm 2 and a Voc of 0.706 volts.The UV-Vis characterization results are consistent with N-CQDs less effective performance than N719.Lower energy is produced by N-CQDs compared

Table 1 .
. Component of samples

Table 2 .
EDX for Solid Carbon

Table 4 .
I-V Meter Results for DSSC with Carbon as Counter Electrodes to N719 because they have a narrower absorbance range.Furthermore, solid carbon can be used as a substitute for counter electrode materials, although the resulting DSSC performance is low. 7