Abstract
This study examined the synthesis of the n-type nanostructured titanium dioxide semiconductor using a combined sol-gel/solvothermal method at 200°C, varying the concentrations of H2O and HCl used as a catalyst for the hydrolysis of the titanium isopropoxide precursor. A white powder of TiO2 nanoparticles was obtained via the solvothermal process. Scanning electron microscopy revealed a spherical morphology of the TiO2 nanoparticles, with their diameter ranging from 2 to 7 microns as the HCl concentration increases. High-resolution electron microscopy and X-ray diffraction showed that the spheres are mesoporous titanium oxide (TiO2m) composed of nanocrystals with an anatase crystalline phase whose crystallite diameter grows from 8 to 13 nm as the HCl concentration increases. On the contrary, optimizing the H2O concentration enabled a decrease in the crystallite size of TiO2m and increases in the surface area and the energy band gap of TiO2m. The enlarged surface area enabled an increase in the number of contact points between TiO2m and the dye of dye-sensitized solar cells (DSSCs), resulting in a better solar cell performance. The white powder was used to prepare a TiO2m film via the screen-printing technique, which was used in the development of DSSC. The performance parameters of the DSSC (ISC, VOC, FF, and η%) were correlated with the synthesis parameters of TiO2m. This correlation showed that H2O and HCl greatly influence the semiconductor properties of TiO2m, along with the short-circuit current JSC and the conversion efficiency η% of the DSSC.
Highlights
Dye-sensitized solar cells (DSSCs) were developed at the beginning of the 90s [1], and today, they represent a mature technology with high marketing potential due to their acceptable stability, high performance under different lighting conditions, relatively low cost of production, and low toxicity [2,3,4]. e principle of operation of these cells is based on the separation of electrical charge by the junction between semiconductor materials with different electrical conductivities [5]. e active electrode of a dye-sensitized solar cells (DSSCs) uses an n-type mesoporous oxide semiconductor with a large surface area as its main component—in most cases, titanium oxide (TiO2)— whose crystalline anatase phase is most often used [6,7,8,9,10,11]
A DSSC has a structure of the type SnO2 : F/TiO2c/TiO2m/N719/I−/I3−/Pt/SnO2 : F, where SnO2 : F represents a transparent conductor of high electrical conductivity and optical transmittance, TiO2c is a thin layer of TiO2 called the compact layer, TiO2m is a mesoporous n-type semiconductor layer, N719 is a ruthenium dye, I−/I3− is the redox pair, and Pt/SnO2 : F represents a platinum counter electrode deposited on the transparent conductor SnO2 : F
During the synthesis of TiO2m nanoparticles via the solvothermal method, the volume of hydrochloric acid (HCl) as a catalyst for hydrolysis was varied in relation to the total solution volume (% (v/v)) according to the following values: 0.5, 1.0, 1.5, 2.0, 2.5, and 3.0% (v/v). e H2O concentration was fixed during this process at 1.9% (v/v). e TiO2m nanoparticles prepared using this method were characterized by X-ray diffraction and FE-SEM. is experiment showed that HCl, as a catalyst for the hydrolysis of titanium isopropoxide, plays a fundamental role in the properties of TiO2m nanoparticles synthesized during the sol-gel/solvothermal process as described below
Summary
Dye-sensitized solar cells (DSSCs) were developed at the beginning of the 90s [1], and today, they represent a mature technology with high marketing potential due to their acceptable stability, high performance under different lighting conditions, relatively low cost of production, and low toxicity [2,3,4]. e principle of operation of these cells is based on the separation of electrical charge by the junction between semiconductor materials with different electrical conductivities [5]. e active electrode of a DSSC uses an n-type mesoporous oxide semiconductor with a large surface area as its main component—in most cases, titanium oxide (TiO2)— whose crystalline anatase phase is most often used [6,7,8,9,10,11]. Some authors synthesized TiO2m precursor spheres from a sol-gel solution in the presence of hexadecylamine, dried them in the air, and subjected them to solvothermal treatment in a mixture of water and ethanol [31, 32] Another method used to form titania (anatase-rutile) spheres is to add titanium tetraisopropoxide to ethylene glycol to form a white suspension, drying it at 80°C for 10 h. The synthesis of TiO2m spheres is performed via a sol-gel/solvothermal process, in which the structural, morphological, optical, and electrical properties of the spheres are essentially studied according to the concentration of hydrochloric acid (as a catalyst for the hydrolysis of the titanium isopropoxide precursor), the H2O content, and the temperature during the synthesis. The surface area of TiO2m, pore size distribution, crystallite size, and forbidden energy band gap Eg are studied. is study examines the application of TiO2m in the development of DSSC and the performance parameters (short-circuit current ISC, open-circuit voltage VOC, fill factor FF, and conversion efficiency η%) as a function of the synthesis parameters of the n-type TiO2m semiconductor
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