Synthesis of Cr2O3/TiO2 Nanocomposite and its Application as the Blocking Layer in Solar Cells

In this study, Cr2O3/TiO2 nanoparticles were synthesized using sol-gel method. TiO2 as one of the most important semiconductor materials with a variety of applications in many fields including photocatalysis and solar cells combined with Cr2O3 as a mineral material and one of the basic oxides used as pigments to improve properties such as mechanical strength, thermal stability form the Cr2O3/TiO2 nanocomposite showing attractive applications in photocatalysis and solar cells. To this end, its application in solar cells has been investigated to testify its performance. The results were promising in the case of solar cell. Cr2O3/TiO2 nanocomposite solution formed a compact layer with low defects and grain boundaries while it was sprayed as blocking layer (TiO2) in superstrate structure CZTS solar cells (Glass/FTO/TiO2/In2S3/CZTS/carbon). Compared to individual TiO2 blocking layer, the asdeposited layer showed better quality and performance. X-Ray was used to confirm synthesized nanoparticles and their morphology was investigated by Field-Emission Scanning Electron Microscopy (FE-SEM).


Introduction
Semiconducting materials are of high interest for their attractive applications in fields of photocatalysis and solar cells [1]. CZTS solar cells are promising alternatives to the conventional high cost silicon solar cells and their counterparts CIGS solar cells [2]. CZTS solar cells are made of Copper, Zinc, Tin and Sulfide/Selenide as their raw materials which are abundant elements in earth crust with lower cost compared to CIGS solar cell's raw materials of Indium and Gallium [3]. Thus, it is highly desirable to improve the efficiency of low cost CZTS solar cells. One way to improve the efficiency of such solar cells is modifying different layers including the blocking layer made of up TiO 2 which acts as an electron transporter and as a barrier to prevent the cell from getting short-circuited [4,5]. Having large grain boundaries and many defects which is the main causes of electron hole recombination, makes TiO 2 an undesired choice for having an efficient solar cell for the future market and opens the way for a quest to find new materials as the alternatives although TiO 2 is the most widely used material in solar cells including provskite and other thin film solar cells [6]. There has been many works done by the others in the literature which describes the best the effect of a high quality blocking layer on the performance of solar cells. Among previews works on modification of TiO 2 blocking layer, Alexander Agrios reported an improvement in charge transfer by synthesizing a nanocomposite of ZnO/TiO 2 used in dye sensitized solar cells which they showed a different application of such nanocomposites as the blocking layer [7]. Other works include synthesis of such nanocomposites by Keisuke Kawata using polymers like polyaniline as the additive material to TiO 2 to make it a better electron transporter in dye sensitized solar cells [8]. To best of our knowledge, there hasn't been any report in the literature about the application of such nanocomposites in inorganic solar cells of CZTS or CIGS. Thin film inorganic semiconductor of Cr 2 O 3 has a wide variety of features including: high thermal stability and mechanical strength with low friction coefficient [9]. Cr 2 O 3 nanoparticles could be prepared in different sizes using different techniques including: sol-gel method [10], gas condensation [11], microwave plasma [12] with a variety of morphologies like thin films [13], porous microspheres [14], nanowires [15], nanotubes [16] and so on. Among all of these synthesis processes and morphologies, only few researches have been doing on the synthesis of round shape Cr 2 O 3 nanoparticles ranging from 5 to 200 nm which has the potential to be used as spraying materials having significant features like high temperature resistance which is very important factor in solar cells as well as corrosion resistance, wear resistance features which are beneficial in other applications [17,18]. Semiconductor TiO 2 is a low cost, non-toxic material with a wide range of applications in solar cells [19,20], photocatalyst [21][22][23], sensors [24,25] etc. which is readily available. Having a band gap of 3.2 eV, TiO 2 could be largely used in solar cells and photocatalysts [26]. Among the photocatalyst materials, synthesis of these two nanoparticles as combined materials, a nanocomposite of TiO 2 /Cr 2 O 3 forms which has different attractive characteristics for solar cell and photocatalytic applications [27].
In this work, we synthesized nanocomposite powder of TiO 2 /Cr 2 O 3 using sol gel method. Following SEM images, the synthetized powder showed good homogeneity with nanoparticles having an average size of about 35 nm. The as-prepared nanocomposite was used to evaluate its performance in photocatalytic and solar cell applications. To testify its performance in solar cells, we sprayed 4 ml of the solution on Fluorine-doped Tin Oxide (FTO) with water as the solvent at 450°C. The as-deposited layer was shown to be a compact layer with lower defects compared to TiO 2 which is always regarded as a layer full of defects effecting charge transfer due to high rate of electron recombination in grain boundaries and defects. To the best of our knowledge, this synthesized nanocomposite shows to be a promising alternative to its counterpart semiconductors with the same

Synthesis method
According to calculations carried out beforehand, for the preparation of 5 g of the desired nanoparticles, 6.402 g of Cr(NO 3 ) 2 .6H 2 O was weighed, then 3 ml Ethanol was added to the container and placed in the ultrasonic water bath for 5 min, then, after adding 3 ml Acetyl acetone it was left in the ultrasonic bath for 10 min, in the third stage, 6.15cc EG was added and ultrasonicated until the complete dissolution of color of Cr(NO 3 ) 2 .6H 2 O in the solution. At the end of this phase, the black was changed into aura green. In the end, as the most important step, 8.19 ml TPOT was added to the container and left for 30 min in ultrasonic bath.
In the next step, container was left in the oven at 65°C until it was transformed from gel phase into solid phase. After leaving the oven, the final product was crushed in a mortar and put in a furnace following a temperature program. After leaving the furnace, the green color of final product showed to be the initial sign of Cr 2 O 3 /TiO 2 nanoparticles synthesized [28].

Characterization and property measurements
XRD patterns to analyze crystal structure of powders were recorded on X-Ray diffractometer (STOE-STADV) using Cu Kα radiation (λ=1.5408). Surface morphology and sizes of products were performed by (FE-SEM).

Fabrication of solar cells
To testify the application prepared nanocomposite, we examined the superstrate CZTS solar cell structure ( Figure 1) as an inorganic thin film solar cell, one of the candidates to replace high cost CIGS solar cells. The whole fabrication procedure was done following former works in the literature [29]. A solution of these nanocomposites was prepared using non-toxic solvent of water to assure the safety of spray pyrolysis process following previews works in the literature. The FTO substrate was annealed up to 450°C and the solution was sprayed on the substrate using 4 ml of solution [30]. This was repeated for three times and the quality of the layer was examined by FE-SEM.

Results and Discussion
All diffraction peaks of Figure 2

Solar cell application
The morphology of the resulting powder was examined by FE-SEM. The FE-SEM micrograph in Figure 4 shows the morphology of assynthesized Cr 2 O 3 /TiO 2 nanopowder, revealing uniform spherical shapes and very small particles of nanosize. The calculated average particle size of Cr 2 O 3 /TiO 2 , Cr 2 O 3 , TiO 2 was 35 nm with a standard deviation of ± 13 nm, and consistent with the aforementioned size calculated by Scherrer´s relation. Nanocomposite of TiO 2 /Cr 2 O 3 could be used as a precursor solution in water to be sprayed as the blocking layer in photovoltaic applications [31]. The role of blocking layer is very crucial in solar cells as it acts both as an electron transporter by allying its band gap with the top layers and as a barrier to prevent the solar cell from getting short circuited as the top layers could penetrate through the bottom layers during the deposition process [32]. Having an optimum band gap of 3.2 eV, it is highly desirable to use it as blocking layer. The final deposited layer showed very compact, low defect islands under FE-SEM with a thickness of 120 nm to be favorable for electron transfer. As shown in Figure 4, this compact layer seems promising to be an alternative to the TiO 2 blocking layer as it is obvious that TiO 2 /Cr 2 O 3 nanocomposite forms a dense blocking layer with lower defects and grain boundaries compared to individual TiO 2 deposited layer as shown in Figure 5. To evaluate its performance we made CZTS superstrate solar cells using this compact layer as the blocking layer. The results were promising with an improvement in the efficiencies followed by an enhancement in J sc and constant V oc . Fill factor decreases as a result of increasing current. The results are shown in Table 2. The J-V curve for the best cell is shown in Figure 6.

Conclusion
In this study Cr 2 O 3 /TiO 2 nanoparticles were investigated with a new technique from sol-gel method by using Cr(NO 3 ) 2 .6H 2 O and TPOT synthesized Cr 2 O 3 /TiO 2 nanopowder, revealed uniform spherical shapes and very small particles. Investigating application of this nanocomposite in solar cells, it reveals that it has a very high potential to be used in different types of solar cells and boost efficiencies to reach better records in this promising field of energy. Although the new blocking layer slightly Improved low efficient superstrate CZTS solar cells, noticing the fact that CZTS solar cells are half efficient compared to provskite solar cells it can be interpreted that using such low defect blocking layer in provskite solar cells with TiO 2 blocking layer could improve the efficiencies more than it was shown here.