Dye-sensitized Solar Cells Based on Silicon Dioxide Nanoparticles Photochemically Synthesized: A Comparative Study in the Concentration of the Dye-sensitized

INTRODUCTION 
Nanomaterials are quickly spread across all essential science and technology sectors, including electronics, aerospace, defence, medicine, and dentistry [1-4]. It means the design, synthesis, characterization, and use of nanometer-scale materials and tools [4,5]. Physical, chemical, and biological properties in nanoscales differ from individual bulk atoms and molecules [6-8]. This allows creating new groups of advanced materials and compounds that fulfil high technology applications requirements [9-12]. Because of its broad applications in electronic equipment, insulators catalyzes or pharmaceuticals; The scientific community has given silica nanoparticles intense study [14]. Nanoparticles from SiO2 Amorphous are used to produce electronic substrates, film substrates, insulators for electrical purposes, insulators, and humidity sensors [15-17]. For each of these products, silica particles play a different function. Some products rely on their quality on silica particles’ amount and scale [18]. Small-scale silica particles with a high purity like high-tech are essential. Industries such as biotechnology and photonics are highly demanding of this sort of material. The optical properties of silica nanoparticles can be observed for surface defects consistent with large surface/volume ratios[19,20]. 
Different techniques such as processing microemulsion, chemical vapour deposition (CVD), hydrothermal techniques, combustion synthesis, plasma-synthesis, sol-gel techniques, etc., have been applied to the synthesis of SiO2 nanoparticles [21-24]. Regardless of the synthesis process, the main emphasis was on particle size, morphology regulation, and particle surface [25]. Our method (photolysis method) is considered new in the synthesis of SiO2 nanoparticles, whereby we can control the particle size without any aggregation [26,27]. In a solar cell application, Improved photon absorption and load carriers’ production are the essential requirements in the form of DSSC. Therefore, because of their fundamental properties that can improve solar cells’ converting power, Nanomaterials are used in photovoltaic ( PV ) technology. They are found promising for visible spectral area light harvesting because of the improved electron mobility resulting from the generation of fast charging carrier [28-30]. Due to their unique physical and chemical properties, SiO2 nanoparticles have been used in solar cell applications. This material also has excellent electrical and optical properties [31].
Consequently, sensors, piezoelectric devices, fuel cells, anti-reflection coating, catalysts were used [32-34]. A dye-sensitized solar cell (DSSC) is a part of the 3rd solar cell generation. DSSC does not require high pure content and relatively low manufacturing costs [35]. It involves four main components impacting cell activity: photoanode, a counter electrode, Dye-sensitized, and electrolytes [36]. In this paper, silicon dioxide nanoparticles were synthesized by a new method (photolysis method) and usage as a photo-anode to Fabrication Dye-sensitized solar cell (DSSC).

MATERIALS AND METHODS 
All materials were purchased and used as received from Sigma-Aldrich. Throughout the preparation and purification steps. Tetraethylorthosilicate (purity 98%), acetic acid (purity 99.8%), absolute ethanol (EtOH purity 99.9%), Rhodamine 6G dye, and urea (purity 99.9%) have been used in this work.

Synthesis of silicon dioxide (SiO2 ) Nanoparticles 
UV irradiation was used as a source to synthesis SiO2 NPs by mixing 20 ml of Tetraethylorthosilicate with 60 ml of acetic acid\ water (1:5). The mixture was stirred for 5 minutes; then, 20 ml, 0.2 M of urea was added slowly to the above solution. The UV source is a mercury lamp (λ = 365 nm) operating at 125 W. The irradiation lamp was immersed inside the chemical reaction, as shown in Fig. 1. An ice bath cooled the system to control the temperature. After 30 minutes, a gel of white colour was formed, the gel was separated and washed several times by absolute ethanol, then dried at 100 oC and calcinated in an oven at 600 oC for 3 hours.  A white powder of silicon dioxide nanoparticles was obtained.

Fabrication of silicon dioxide-based on dye-sensitized solar cell
SiO2 nanoparticles were coated onto the indium-doped tin oxide (ITO) glass, resistance 8 ohm, and transmission 83%. ITO glass (2 x 2 x 1 mm) was washed with ethanol and de-ionized water several times with an ultrasonic bath for impurity clearance and dry using an air blower. SiO2 nanoparticles were coating accordingly; a colloidal solution of SiO2 nanoparticles had prepared by mixing 500 mg of the nano-powder with 20 ml of ethanol. The photoanode was done utilizing a dripper to cover the ITO-glass’s conductive face with a colloidal solution, then annealed at 250°C for 60 minutes in the air. 
The annealed film had immersed overnight at room temperature in the different concentrations (5, 10, 15, 20 mM) of  Rhodamine 6G dye (C28H31ClN2O3) using de-ionized water as a solvent [37]. Graphene -silver nanocomposite was prepared by hummer’s modified method [38].  Then, coated on the conductive side of ITO glass by immersed it overnight in a colloidal solution of 200 mg graphene -silver nanocomposite with 20 ml of ethanol and used as a counter electrode.  The dye-absorbed SiO2 nanoparticles coated ITO glass was clipped with a Graphene -silver nanocomposite (G-Ag) coated ITO glass (counter electrode)  to make a sandwich-type DSSC design. Finally, the liquid electrolyte (I- /I-3) solution was immersed in the system through the electrode counter gap. The Fabrication of silicon dioxide-based on the dye-sensitized solar cell is shown in Fig. 2.

Characterization 
X-ray diffraction of SiO2 nanoparticles was examined using (XRD-6000) which was operated at 30 mA and 40 kV to generate radiation at a wavelength of 1.5406 Å. JEOL JEM-2100 TEM measurement was used to study nanoparticles’ size and morphology. A drop of suspended nanoparticles was placed on the carbon-coated TEM grid for analysis. Shimadzu UV-Vis 160 V spectrometer measured the absorbance of SiO2 nanoparticles. 

RESULT AND DISCUSSION
Structure of SiO2 nanoparticles
As a part of this investigation, the diffraction angle 2θ of XRD analysis spanning the 5−80 degree range were carried out to test the obtained SiO2 nanoparticles, as shown in Fig. 3. The powder diffraction pattern indicates a typical broad peak at 2θ = 22°, which reveals the amorphous existence of silica [39]. The XRD pattern also shows that no ordered crystalline structure is present.
The small size and incomplete internal structure of synthesized powders may be responsible for this high XRD reflecting point. There is no other high impurity reflecting silica nanoparticles’ pureness. The XRD results can be used to determine the crystal size of SiO2 nanoparticles. In this work, the average size (D) of SiO2 nanoparticles was calculated using the Debye-Scherrer equation [40-44]: 

D = Kλ/ βcos                                                              (1)

Where 𝑘 denotes Scherrer constant that equals 0. 9, λ is the wavelength of the Cu-Kα radiation, β corresponds to line broadening in radians (the full width at half maximum, FWHM) and θ is the Bragg angle derived from the 2θ value corresponding to the maximum peak-intensity in the XRD pattern. The SiO2 nanoparticles diameter obtained using Eq. (1) was 11.79 nm. Thus, our experiment’s UV source was proved to produce SiO2 nanoparticles.

Transmission electron microscopy (TEM)
In SiO2 nanoparticle characterization, TEM was chosen because it produces a higher resolution and greater precision in particle size in contrast to others, including electron microscopy scanning. Fig. 4. shows high-scale TEM images on two different scales (50 and 100 nm) of SiO2 nanoparticles. Subsequent TEM characterization studies have verified the actual scale, shape, and morphology of nanoparticles. Furthermore, the images show that the SiO2 nanoparticles are quasi-spherical without aggregation. Based on these experiments, the average size of the nanoparticle 20.7 nm was achieved after the average XRD measurement of nanoparticle size. That has been consistent.

Optical properties of SiO2 nanoparticles
The optical band gap of SiO2 nanoparticles was tested using UV-vis spectroscopy in the range of 200–800 nm. Dispersed into de-ionized water by sonication for 5 min, the synthesized SiO2 nanoparticles obtained a uniform solution. Fig. 5 (a) reveals a SiO2 nanoparticles UV-visible spectrum. The spectrum shows a high absorption peak at 317 nm due to SiO2 nanoparticles surface Plasmon absorption. The absorption edge of SiO2 nanoparticles was at 363 nm. 
The optical band gap of SiO2 nanoparticles was calculated by Tauc equation [45]:  

(αhν)² = A(hν-E_g )                                                        (2)

where Eg = energy of the optical bandgap, α = absorbance, h = planks constant, ʋ = frequency of incident radiation, A = constant called the band tailing parameter.
Plotting (αhv)2 versus Eg based on the spectral response gives the extrapolated intercept, which corresponds to the bandgap energy values, as shown in Fig. 5 (b). The optical band gap energy of the SiO2 nanoparticles is measured to be 3.61 eV.
photovoltaic properties of DSSC based on SiO2 nanoparticles
The photovoltaic parameters of the dye-sensitized solar cell (DSSC) with different dye concentrations made by SiO2 nanoparticles are shown in Fig. 6. The results of these performances are summarized in Table 1. A solar simulator includes the DSSC, illuminated by a 100 mW / cm2 halogen lamp. The power conversion efficiency of DSSC was calculated by [40,46,47]:     
                                  
η = Pmax / Pin = Voc.Jsc.FF / Pin  * 100 %                     (3)

where ,  and   Represent the value of open-circuit photovoltage, the value of photo-current of short-circuit density, and incident light power, respectively. The fill factor (FF) is defined by [40]:
 
FF= Vmax. Jmax / Voc. Jsc                                               (4)
  
where  and  Represent the voltage and the current density at the maximum output power.
The DSSC values are calculated in Table 1. It was critical for the SiO2-based DSSC parameters because of the concentration sensitizing dye and small particles of synthesized SiO2 nanoparticles. The cell power conversion efficiency was increased with increased dye concentrations. The increased absorption may also explain the dye molecules’ high efficiency on the SiO2 surface. Therefore, SiO2 nanoparticles are promising to be used in potential photovoltaics as the process is easy, and the materials can quickly be prepared. There was a relatively low current density rating. The photo-current is the most critical parameter for calculating the overall system efficiency limit. The parent materials act differently because of their large surface area and surface energy when their particle size approaches the nano level. The synthesized SiO2 nanoparticles have an average particle size of approximately 20.7 nm. We can, therefore, expect substantial phytochemicals. A relatively small photo-current may be powered by different factors, such as small roughness factor, ow injection efficiency, photoanode reflection or dispersion, and charging performance.
Consequently, additional electron densities at higher light intensity were transferred to SiO2. Table 1 shows that the values η and Jsc increase as the light density applied increases. The increase in control generation is due to the rise in light intensity. The highest short circuit current and high open-circuit voltage were shown on our DSSC, with a 20 mM photosensor concentration. Due to the SiO2 nanoparticle molecular structure (favorable with electron/hole pair separation). The DSSC mechanism can be discussed, To enter the excited state, light passes through a transparent electrode and is absorbed by Rhodamine 6G dye. The excited electrons would then be pumped into the semiconductor SiO2 Nanoparticles conduction band and transferred to an external circuit. To complete a loop, the oxidized dye would be reduced by a redox pair in the electrolyte, which a counter electrode would then reduce with external circuit electrons. In comparing the SiO2 nanoparticles-based DSSC with previously reported DSSC [48-52], the obtained DSSC in this study can be regarded as an active photoanode with a counter electrode to fabrication SiO2 nanoparticles-based DSSC Which gives high conversion efficiency as a result of the preference of silicon oxide in dye solar cells.

CONCLUSIONS
The dye-sensitized solar cell (DSSCs( based on SiO2 nanoparticles was provided in this report. In particular, the nano-size  SiO2 powders have been synthesized by the photolysis method; This method has the advantage of giving us a small size of particles without any aggregation. TEM, XRD, and UV-visible have characterized the Synthesized nano-powders. 20.7 nm is the size of the average particles we got from the TEM measurement. The energy band gap was 3.61 ev. The effects on the DSSC power conversion efficiency have also been studied in the concentration of Dye-sensitized Rhodamine 6G. Cell power conversion efficiency was mainly increased at an increased dye concentration. The maximum efficiency was 2.00% at a concentration of 20 mM Rhodamine 6G dye under an input light intensity of 100 mW\cm2.

CONFLICT OF INTEREST
The authors declare that there are no conflicts of interest regarding the publication of this manuscript.