Efficiency Enhancement of Sensitized Solar Cells through the Size Control of Mesoporous Spherical TiO2

Abstract

Harvesting energy directly from sunlight using photovoltaic system for future energy production has been the concern of all photovoltaic researchers. In the search for new photovoltaic technology, thin film solar cells appear to have significant potential as a low cost alternative to conventional solid-state junction solar cells. Among them, dye-sensitized solar cells (DSSCs) are the most advanced and they are currently on the verge of commercialization. Typically, the DSSC consists of a mesoporous oxide film, dye molecules, a redox couple and a Pt coated counter electrode. Instead of using a dye, sensitization of a mesoporous oxide electrode can be achieved by using a narrow band-gap semiconductor quantum dot (QD). Quantum dot-sensitized solar cells (QDSSCs) have also attracted great attention from better light absorption than its counterpart DSSCs, the notable advantage of QDSSCs has multiple exciton generation (MEG) through the impact ionization effect. However, further improving the conversion efficiency of these solar cells remains a challenge. Until recently, numerous researches have been trying to improve their efficiency in the field of DSSCs or QDSSCs. One of the notable approach for effective performance of DSSCs

is the tailoring of oxide morphology and photo electrode structure. Recently, considerable improvements of conversion efficiency were achieved by using mesoporous spherical oxide particles that include ZnO aggregates, nano-embossed hollow spherical TiO2 and nanoporous TiO2 bead. These porous spherical structures simultaneously promote light scattering and the dye uptake of the electrode, thereby increasing photo conversion efficiency.In this dissertation, we proposed a new synthetic route for mesoporous spherical TiO2 (MS TiO2) having tunable secondary particle diameters together with pore diameters. The synthesized MS TiO2 was successfully used for photovoltaic performance.First, Size-tunable MS TiO2 with high surface area have been prepared through combination of ？dilute mixing？-driven hydrolysis of titanium (iv) tetraethoxide and solvo-thermal treatment. The hierarchically-structured MS TiO2 are characterized by scanning electron microscopy (SEM), transmission electron microscopy (TEM), X-ray diffraction (XRD), and nitrogen sorption analysis. Using three different MS TiO2 (587, 757, and 1554 nm in diameter) as a scattering overlayer on a transparent nanocrystalline TiO2 film, bi-layered dye-sensitized solar cells (DSSCs) have been fabricated. Since the MS TiO2 particles are comprised of ~10 nm-nanocrystallites that cluster together to form large secondary spheres and they can also function as light scatterers without sacrificing the surface area for dye-uptake. As a result, the present MS TiO2？based cells perfom a noticeable improvement in the overall efficiency: maximum 9.37 % versus 6.80 % for the reference cell made of a TiO2 nanocrystalline film.Secondly, mesoporous TiO2 (MS TiO2) with big pore and primary particle diameter (with a diameter of 1190 ？ 60 nm) was synthesized by the similar method proposed above with the small addictive change in solvothermal treatment

and applied to CdS quantum-dot-sensitized solar cells (QDSSCs). The synthesized Mesoporous spherical TiO2 offers high internal reflectance in the visible region and efficient pore accessibility. A conversion efficiency of 1.9 % was achieved by CdS QDSSCs composed of the MS TiO2 photoanode, which corresponds to ~58 % improvement as compared with the values obtained from the conventional devices made with 20-nm-sized nanocrystalline TiO2 under AM 1.5 illumination of 100 mW cm-2.