A Study on Charge Collecting Property of Semiconducting Oxide Materials in Photoelectric Energy Conversion System

Abstract

Nanostructured transparent n-type and p-type semiconducting oxides were synthesized and designed for enhancing device performance in photoelectric energy conversion system. The synthesized nanostructures were used as photoelectrodes and applied to photovoltaic devices, particularly in emerging photovoltaic (PV) system. The correlation between structural/compositional design of the photoelectrodes and their charge collecting property was explored. Concretely, inorganic semiconductor sensitized solar cells (ISSCs) and organic-inorganic hybrid perovskite solar cells (PSCs), which are acceptable as typical emerging PVs in economic terms for environment conservation and energy generation, were studied. These ISSCs and PSCs currently face significant challenges in improving energy conversion efficiency and environmental stability. Since n-type and p-type semiconducting oxide based photoelectrodes are essential in charge collecting ability of photovoltaic devices, control and design of fundamental properties of semiconducting oxides are needed to overcome the above challenges. From this point of view, this thesis proposes two strategies for efficient charge collecting property in ISSCs and PSCs. First is structural design of semiconducting oxides. Second is compositional design of semiconducting oxides for effective charge injection from light absorbers to electron acceptors. First, n-type titanium dioxide (TiO2) and p-type nickel oxide (NiO) semiconducting oxides were explored to investigate the structural effect on electron-hole recombination and overall charge collecting property in emerging PV devices. For TiO2, hierarchically organized nano-architectures containing both large surface area and open channels of pores were synthesized on glass substrates using pulsed laser deposition (PLD) system. Growth mechanism of the architectures was studied and their charge collecting property in ISSCs was investigated. It was found that the morphology of photoelectrode strongly influenced the device performance. The open channels of pores between the TiO2 domains which generated by the unique form of the architecture induced the infiltration of inorganic semiconductors (IS) and liquid polysulfide electrolytes more rapidly, resulting in higher light harvesting (efficient absorption of IS) and longer electron lifetime compared to the conventional TiO2 nanoparticle film. The enhanced light harvesting and charge collecting property led to the enhancement of overall energy conversion efficiency of the devices. Additionally, the larger surface area of nano-architecture further enhanced the light harvesting efficiency. For NiO, highly transparent, (111)-preferred oriented, columnshaped nanostructured films were grown on glass substrates and the optimum microstructure of NiO as hole transporting layer (HTL) for PSCs was investigated using PLD system. The device performance was explored using a p-i-n type PSCs, which is based on NiO/MAPbI3/PCBM configuration. Compared to that of densely-packed NiO thin film or porous film, the overall efficiency was significantly enhanced with the nanostructured NiO film. The enhanced performance was attributed to the suppressed recombination rate during the extraction and transportation of the dissociated carriers. Second, compositional design of TiO2 nanostructured films was explored to enhance the charge injection property between light absorbing materials and electron accepting semiconducting oxides in ISSCs. From the basis that the energy difference between the conduction band (CB) of IS and TiO2 serves as the driving force for electron injection and that the doping metal cation into TiO2 lattice alters the CB position of TiO2, the enhanced charge injection ability of TiO2 was expected by Niobium (Nb) doping. The relationship between the Nb doping and their effects on the final photovoltaic device performance were investigated. As a consequence, the generated photocurrents were greatly improved by Nb doping compared to the undoped ISSC. The enhanced photocurrents and overall energy conversion efficiency were attributed to the efficient electron injection between IS and Nb doped TiO2, resulted from the enlarged energy difference of the CB edge between IS and TiO2 by introduction of Nb dopants. This thesis focused on understanding of the relation between the structural/compositional design of semiconducting oxides and their effect on charge collecting property in photoelectric energy conversion system. Through the study, this thesis proposes a guideline of designing the semiconducting oxide based photoelectrode and a possibility to solve the faced challenges in the fields of emerging PVs.