S YNTHESIS OF T IN O XIDE N ANOCRYSTALLITES WITH V ARIOUS C ALCINATION T EMPERATURES U SING C O -P RECIPITATION M ETHOD WITH L OCAL T IN C HLORIDE P RECURSOR

Indonesia is one of the largest tin metal producers in the world


INTRODUCTION
Richard Feynman's 1959 scientific speech entitled "There is Plenty Room at the Bottom" paved the way for the rapid development of nanotechnology in today's modern world, from the processes of synthesis and characterization to its applications [1]. An example of its development is the application of metal oxide in gas sensors [2], lithium-ion batteries [3], and solar cells [4]. One of the metal oxides with excellent potential for the development of nanotechnology in Indonesia is SnO2 (tin oxide). Indonesia has scored in tin production through PT. Timah Tbk, with an increase of 128.7% in 2019 and a total production of 73,390 tons from the previous 33,444 tons in 2018 [5].
The use of nanomaterials, especially in the field of renewable energy, has continued to grow until now. It is such a significant opportunity to utilize solar energy sources in Indonesia. With an average daily intensity of solar radiation of 4.8 kWh/m2, Indonesia has the potential to generate more than 207 gigawatts of electrical energy from solar energy sources, beating out hydro, hydrothermal, wind, and ocean waves [6]. The advancement of solar cells, particularly PSC (perovskite solar cells), is needed as an alternative to renewable energy in Indonesia. Concerning this strategic point, SnO2 could be utilized as an ETL (electron transport layer) material in PSCs.
SnO2 was chosen as an alternative ETL material due to its excellent electron mobility properties (240 cm 2 /(V.s)) [7]. In addition, SnO2 can also be synthesized at low temperatures and produce high efficiency and good PSC stability [8]. Among the various chemical methods for metal oxide nanoparticle preparation, the coprecipitation method has shown good advantages, such as not requiring high temperatures and pressures. Also, filtration and washing can eliminate any impurities left over from the reaction process [9].
In this paper, SnO2 nanocrystallites were synthesized using local SnCl4 (tin chloride) from PT. Timah Industri by co-precipitation method and calcination at 300, 400, 500, and 600 °C. According to the literature review, no similar research has been conducted on synthesizing SnO2 nanocrystallites for PSC applications from indigenous Indonesian materials. The characterization of the synthesized samples was carried out using XRD (x-ray diffraction), SEM (scanning electron microscope), and UV-Vis spectrophotometer. The PSC device was fabricated and consisted of SnO2 nanocrystallites as the ETL, with a planar heterojunction architecture of ITO/SnO2/CH3NH3PbI3/Spiro-OMeTAD/Au used for the performance test.

Synthesis and Characterization of SnO2
Nanocrystallites In water solvent, 0.2 M SnCl4 solution and 0.8 M NH4OH solution were mixed and stirred until they formed a pH 9 homogenous solution. After that, the precipitate process was carried out for approximately 3 days until a white precipitate was formed, which was then filtered using Whatman filter paper No. 41 and washed with 100 ml of distilled water, and then the precipitate was separated again by the centrifugation method. The white precipitate was washed three times until the pH was close to 7 (neutral). The sample was then dried in an oven at 110 °C for 24 hours to obtain clear crystals of SnO2, which were then mashed using a mortar to form a powder. Furthermore, the SnO2 powder was calcined in a muffle furnace for 3 hours at 300, 400, 500, and 600 °C to be further analyzed using XRD (x-ray diffraction), SEM (scanning electron microscope), and UV-Vis spectrophotometer.
The estimation of the average crystallite size of SnO2 nanocrystallites was calculated according to the following Scherrer's equation [10]: Where D is the average crystallite size (nm), k is the Scherrer constant (0.9), λ is the wavelength of X-rays, CuKα (1.5406 Å), β is the full width at half maximum/FWHM (radian), and θ is the diffraction angles (degrees).
The optical band gap energy (Eg) of synthesized materials was calculated from absorbance data by using Tauc's equation [11]: (2) Where α is the absorbance coefficient, hν is the incident photon's energy, A is the proportional constant, and Eg is the optical band gap energy. The exponent value of n indicates the nature of the electron transition in the material, which can be ½ for the direct transition and 2 for the indirect transitions [12].

Fabrication of Perovskite Solar Cell
Preparation of the ITO glass substrate begins the first stage of PSC fabrication. The 1.5 × 2 cm ITO glass was covered with duct tape, and the rest section was etched with HCl for 15 minutes. Next, the duct tape was removed, and the ITO glass substrate was washed using an ultrasonic Synthesis of Tin Oxide Nanocrystallites.../ Norbert Egan Christo Panthoko | 11 cleaner for 20 minutes with distilled water, acetone, and ethanol, respectively [13]. The second stage is the deposition of the SnO2 layer onto the ITO glass, prepared in the first stage. SnO2 slurry was made by mixing the SnO2 powder with distilled water, acetone, ethanol, and PEG in a ratio of 3:2:4:2:2. This SnO2 slurry solution was pulverized with a mortar so that it was evenly distributed. The slurry was then deposited onto the ITO glass by the doctor blade method and annealed in the open air at 110 °C for 1 hour [13]. The next stage is the coating of perovskite material (CH3NH3PbI3 0.8 M), obtained by mixing 276 mg PbI2 with 470 mg CH3NH3PbI3. The mixed powder was dissolved in 5 mL of DMF and heated at 80 °C for 2 hours. A 100 µL perovskite solution was then dripped onto SnO2coated ITO glass and allowed to stand for one minute before spin coating at 3000 rpm for 30 seconds. The SnO2@perovskite-coated ITO glass was heated at 150 °C for 1 hour in a muffle furnace [13].
Next is the HTL (hole transport layer) coating, which was obtained by dissolving 70 mg of Spiro-OMeTAD in 1 mL of chlorobenzene. After that, the solution was dripped onto SnO2@perovskite-coated ITO glass and spincoated for 30 seconds at 3000 rpm. Finally, a thin gold (Au) film was applied to the PSC (perovskite solar cells) configuration using the sputtering method at a pressure of 10-1 mbar and a current of 10 mA for 120 seconds [13]. The structure of the fabricated PSC layer is shown in Fig. 1. PSC performance testing was conducted to obtain the I-V curve using a semiconductor parameter analyzer (HP Agilent 4145B) with a standard illumination intensity of 100 mW/cm 2 . The power conversion efficiency (PCE, η) was then calculated using the equation [14]: Where η is the PCE, is the fill factor, is the short-circuit photocurrent, is the open circuit voltage, and is the illumination intensity used during the test (100 mW/cm 2 ). While the value of FF was obtained by using the equation [14]: (4) Where and are the current and voltage obtained from the maximum power on the I-V curve, which was obtained from the test results.

Crystal Structure Analysis
The formation of SnO2 nanocrystallites begins with the formation of a white precipitate, Sn(OH4), in the gel phase, which results from the reaction of SnCl4 with NH4OH (Eq. (5)). When the white gel is dried, the water vapor is released, leaving behind white SnO2 crystals (Eq. (6)) [15].  Figure 2 shows the x-ray diffraction pattern of SnO2 nanocrystallite samples synthesized by the co-precipitation method with no calcination and with calcination at temperatures ranging from 300 to 600 °C. The diffraction peaks on the graph correspond to the crystal planes (110), (101), and (211) in the ICDD database No. 01-077-0447. This indicates that the synthesized samples had a rutile phase with a tetragonal structure. The graph depicts the sample's crystallinity increases with increasing calcination temperature. This shows that the calcination temperature substantially affect the formation of purified SnO2 crystals.
According to the XRD graph, SnO2 nanocrystallite samples that were calcined at temperatures higher than 300 °C have more excellent crystallinity than samples that were calcined at 300 °C or not calcined. When the calcination temperature is increased, the intensity of the diffraction peak becomes higher and sharper, and the width becomes narrower. This indicates that the precursor obtains sufficient energy to form SnO2 crystals [15]. The results of calculating the average crystallite size of SnO2 nanocrystallites with the Scherrer equation (Eq. (1)) are presented in Table 1. The average crystallite size of samples that were not calcined and were calcined at temperatures ranging from 300 to 600 °C has increased from 1.19 to 13.75 nm.  Figure 3(a) depicts the optical absorbance spectrum of SnO2 nanocrystallites samples. The curve shows a high absorbance in the ultraviolet region and a low absorbance in the visible light region. The absorbance peak of the sample shifted to the visible light region, or redshifted, as the calcination temperature increased from 300 to 500 °C. This phenomenon affects the optical band gap energy (Eg) of the SnO2 nanocrystallites samples.

Optical Properties Analysis
According to the calculation results of the Tauc method (Eq. (2)) in Table 2, the optical band gap energy tends to decrease with increasing calcination temperature, from 3.57 eV (uncalcined) to 2.81 eV (calcined at 500 °C). However, the optical band gap energy was discovered to rise 3.10 eV in the 600 C calcined sample. This conforms to the absorbance spectrum curve of the sample, which also underwent a blueshift. effects, the band gap energy of the nanoparticle will decrease with increasing crystallite size and vice versa [16]. In general, the decrease in band gap energy in the SnO2 samples was consistent with the XRD analysis results, which demonstrated that the average crystallite size grows as the calcination temperature rises.
In the case of the 600 C calcined sample, the band gap energy has increased again. Meanwhile, the crystallite size has increased due to the received heat energy driving force. There is a possibility that other phases may emerge in minor quantities, which are undetectable by XRD testing but perform an important role in influencing the mechanism of electron excitation from the valence band to the conduction band of SnO2 nanocrystallites in this study. These nanocrystallites were discovered by UV-Vis spectroscopy at the absorption edge and represented by the optical band gap energy measurement results. However, additional characterization with more advanced tools, such as XANES and XPS, is required to confirm this. Another explanation that can be related is the influence of the calcination temperature, which has a part in controlling the intrinsic defect in the SnO2 crystal [17]. The higher the calcination temperature, the more significant the defect reduction, particularly oxygen vacancy, which increases the optical band gap energy [18]. This also happened in a study by Kamble et al., [19], which confirmed an increase in optical band gap energy due to defect reduction of SnO2 nanocrystallites after the calcination process at 800 °C for 2 hours.
As ETL (electron transport layer) materials, SnO2 nanocrystallite samples must have high transmittance. So that photons can easily pass through the ETL to the perovskite layer, where electron charge excitation occurs. The overall performance of the PSC (perovskite solar cells) improves with increasing transmittance of the ETL material [20]. For further analysis, the absorbance spectrum of the SnO2 nanocrystallite samples was processed to obtain the percentage of transmittance using the Lambert-Beer equation as follows [21].
Where T is the percentage of transmittance (%) and A is the absorbance.
As shown in Figure 3(b), the above calculation results were then plotted as a curve between the transmittance percentage and wavelength. According to the transmittance curve, it can be seen that the percentage of transmittance of all SnO 2 nanocrystallites samples increased in the visible light area. Higher transmittance intensities were discovered in SnO 2 nanocrystallite samples that were calcined at 300 and 400 °C. However, the curve for the 300 °C calcined sample has a greater range of light transmittance areas than the curve for the 400 °C calcined sample. This indicates that more photons of light are transmitted to the ETL layer and excite electrons in the perovskite layer in that sample. Therefore, the ETL transmittance performance of the 300 °C calcined sample is greater than that of other samples.

Surface Morphology Analysis
The surface morphology of the synthesized samples is observable in Fig. 4. According to the SEM observations, the samples tend to aggregate. due to the use of mortar in refining clear SnO2 crystals into fine SnO2 powder, which has a very high surface energy that leads to the formation of agglomerates. This result can also be correlated to forming strong hydrogen bonds in the precipitate during the synthesis process [22].

Perovskite Solar Cell Performance
The PSC configuration in this study consisted of ITO/SnO2/CH3NH3PbI3/Spiro-OMeTAD/Au. From the results of processing the I-V curve (Fig.  5), it was determined that only the SnO2 nanocrystallites sample calcined at 300 °C could serve as an ETL layer and produced the highest PCE (power conversion efficiency), which was 0.0024% (Table 3). In this study, uncalcined and calcined SnO2 nanocrystallite samples at temperatures other than 300 °C produced no difference in the I-V curve and thus did not affect the PCE.  [7] used a commercial tin precursor (Alfa Aesar) to produce SnO2 nanoparticles as ETL through a lowtemperature solution process. They achieved a high PCE of almost 20%. The work by Liu et al., [23] uses the same tin precursor and achieves high PCE of 19.38%.
The PSC device fabrication process using SnO2 nanocrystallite samples, apart from the 300 °C calcined sample, had problems with uneven depositions of the SnO2 layer onto the ITO glass using the doctor blade method. In addition, the perovskite layer fabrication process carried out in the open air and not in an inert environment also contributed to the low PCE obtained. It has been reported in other studies that most high-efficiency PSC devices are obtained by manufacturing a perovskite layer under inert conditions using a glove box [24].
It is recommended to preserve the substrate at an increased temperature for future fabrication of perovskite solar cell layers using doctor blade deposition in order to obtain a pinhole-free, uniform, and smooth coating. This will increase the solvent evaporation rate, promoting nucleation, and crystal growth [25].

CONCLUSION
SnO2 nanocrystallites have been successfully synthesized using a local tin chloride precursor through the co-precipitation method and calcination treatment at various temperatures of 300, 400, 500, and 600 °C. The crystal structure characterization revealed that the higher the calcination temperature, the higher the crystallinity and the larger the average size of the formed SnO2 nanocrystallites. The increasing size of crystallites also causes a reduction in band gap energy due to quantum effects. Thus, the calcination treatment in the co-precipitation method affects the characteristics of the resulting SnO2 nanocrystallites. The SnO2 nanocrystallites have been applied as an ETL material in PSC with a planar n-i-p heterojunction configuration composed of ITO/SnO2/CH3NH3PbI3/Spiro-OMeTAD/Au. The highest PSC efficiency was achieved by PSC composed of SnO2 nanocrystalline with a calcination temperature of 300 °C, which was 0.0024%. This result is following the transmittance level of the sample, which has a wider range of transmittance areas compared to other samples so that more photon energy can pass through the ETL and ultimately improve the overall PSC performance. In this study, the PSC efficiency is still very low. This is because of limitations during making the device, such as the thickness of the layers of the different structures that make up the solar cells and the non-inert condition in which the device is made.