Controlling Light Harvesting and Charge Collection Properties of Titanium Oxide Semiconductor for Improving Photovoltaic Device Performance

Abstract

The global primary energy demand is increased rapidly, then the development of low cost and sustainable energy sources is desirable. Solar cells are the prominent method to convert solar energy into electricity, directly. Among the various type of solar cells, organic-inorganic hybrid solar cells is considered as the 4th generation solar cells due to the low cost, high efficient and the excellent stability that come from the merits of organic and inorganic materials. In hybrid-solar cells system, titanium oxide semiconductor, i.e. anatase TiO2, is responsible for loading of light absorber materials, in this study N719 dye, electron collecting from the conduction band/LUMO of light absorber materials and electron transport to TCO electrode. In terms of the research of metal oxide, their surface area, surface defect, and electrochemical properties, like electronic band structure and conductivity, are the most important properties on the study of metal oxide development for high efficient solar cells. Therefore, it is highly desirable to design of the appropriate titanium oxide semiconductor for each light absorber materials or cell architectures. From this point of view, this thesis proposed three strategies for improving the light harvesting in Dye-sensitized solar cells (DSCs), enhancing the electron injection in planar heterojunction flexible perovskite solar cells (PSCs), and improving the electron transport in mesoscopic heterojunction PSCs, which strategies were designed from the view point of light absorber and cell structure. First, the effects of surface area increment in scattering layer were investigated. Generally applicable anatase TiO2 decoration method for various anatase TiO2 host was suggested by the TSSG process, and its feasibility as a surface area improved scattering layer in photoelectrode of a DSC was demonstrated. The enhanced photovoltaic properties were discussed on the view point of increased electron generation and preserving of electron transport properties at the thick TiO2 scattering Second, the surface-defect free low-temperature processed TiOx charge collecting layer for high efficient flexible perovskite solar cells were investigated. For the purpose, the low-temperature processed TiOx thin film was deposited on PET/ITO flexible substrate using ALD, and its charge transfer properties at the interface of TiOx/perovskite in solar cell devices were investigated using time-resolved photoluminescence and electrical impedance spectroscopy. In addition, the mechanical behavior of this flexible device were examined under various bending radius, systemically. Finally, the effects of niobium (Nb) doping into titanium oxide on electronic band structure and on photovoltaic properties of mesoscopic perovskite solar cell systems were explored. Light Nb doping (0.5 at% and 1.0 at%) increased optical band gap very slightly, but heavy doping (5.0 at%) distinctively decreased it. Relative Fermi level position from conduction band minimum of the lightly Nb doped TiO2 (NTO) is similar with undoped TiO2, while that of the heavily doped NTO decreased as much as ~ 0.3 eV. The lightly doped NTO-based PSCs exhibit 10% higher power conversion efficiency than undoped-TiO2-based PSCs, and 52% than the heavily doped NTO-based PSCs (from 8.8% to 13.4%). The excellent performance of the lightly doped NTO-based cells was attributed to fast electron injection/transport and preserved electron lifetime, verified by transient photocurrent decay and impedance studies. This thesis focused on the understanding the relation between the extrinsic/intrinsic properties, i.e. enlarging surface area, control of electronic band structure, and surface defect less TiOx formation, of titanium oxide and the photovoltaic properties through the demonstrating in solar cells, such as DSCs or PSCs. As a result, each extrinsic/intrinsic properties of titanium oxide are led to improve the light harvesting, electron transporting, and electron injection, respectively. The results and interpretations given in this thesis are expected to offer the guideline of designing desirable photoelectrode for high efficiency solar cells.