Development of organometal halide perovskite solar cells with enhanced hydrostability and photostability

Abstract

As the global warming becomes severe and the traditional fuels of coal and petroleum are running out, necessity of new and renewable energy is growing more and more. Among the various new and renewable energy sources, solar energy is the most promising energy source because of its tremendous energy density and pollutant-free property. As a result, lots of researchers have studied how to effectively utilize the infinite solar energy. Solar cell, the device directly converting energy from the sunlight into electricity, is known to be the most efficient device in generating energies from the sun. Until now, Si-based solar cells are recording the highest power conversion efficiency (PCE) among the practically used solar cells (except for the extreme cases like solar cells in the aerospace research field), therefore they are most commonly industrialized in real life. However, the high cost-to-energy generation ratio of Si solar cells makes the difficulty in the propagation of Si solar cells. To achieve the economic competitiveness, other kinds of solar cells have been variously studied like thin film solar cells, dye-sensitized solar cells, quantum dot-sensitized solar cells, and tandem solar cells. In spite of passionate efforts, the perfect alternative solar cell to Si solar cells has not been developed yet. However, from just 5 years ago, a new kind of solar cell has been exhibiting incredibly excellent output performances and rapid progresses. It is called as perovskite solar cell (PSC) because organometal halide perovskite is the light absorbing material in the devices. On the basis of various advantages of perovskite sensitizer (excellent charge carrier conductivities, high light absorption coefficient, strong self-assembling properties, ambipolar characteristic, nearly ideal direct band gap energy, and very low exciton binding energy), PCEs of PSCs has increased in the strikingly rapid pace, from 3.8% in 2009 to more than 20% in 2015. Nowadays the PCE of PSC is comparable to the conventional Si-based solar cells, while the fabrication cost of PSC is extremely lower than that of Si-based solar cells. Because of these lots of advantages, PSCs are considered to be the promising next generation solar cells. However, there are some problems required to be solved before the industrialization of PSCs: long-term stability and light-vulnerability of PSCs. Most of currently reported PSCs are suffering from the instability to both external factors: moisture and UV-light. In this thesis research, the instability issue of PSC will be intensively discussed and solutions for the problems will be suggested. In the first chapter, fundamental background knowledge for understanding PSCs will be discussed in various points of view. Basic properties, working mechanisms, characteristics, and advantages of organometal halide perovskite are covered. In addition, the history of research of PSC (from the first report of PSC in 2009 to the present various research trends on PSCs) is introduced in detail. Various research trends are categorized into the each part of PSC, including electron transporting layer, perovskite absorber, hole-transporting layer, cathode, and the overall device architectures. Motivation and objectives of this thesis research are once more clarified at the end of the first chapter. The first chapter will be helpful to understand the overall background of the research on PSC and the following research themes of this thesis research. In the second chapter, designing a hydrophobic layer onto the PSC to improve the moisture-resistivity of PSC is described. Because organometal halide perovskites are extremely soluble to polar solvents like water, efficient blocking of penetration of atmospheric moistures is essential for the long-term stability of PSCs. Teflon, which is a widely used industrialized hydrophobic polymer based on poly-tetrafluoroethylene, is used as the moisture-repelling hydrophobic layer in PSCs. With the facile spin-coating step, Teflon thin layer is deposited onto the complete PSC and demonstrates considerably increased stability of PSCs in the long-term storage. Teflon-treated PSC exhibits 95% of the initial PCE after 30 days of storage, while the reference PSC shows continuous PCE degradation from 11.3% to 6.3% during the same period of storage. In the third chapter, improving the light-vulnerability of PSCs by introducing TiO2/CdS core-shell structure is covered. PCE degradation of PSCs under the continuous light illumination is a well-known severe problem of PSCs. Although the reason has not been completely revealed yet, activation of the oxygen vacancies in TiO2 layer by UV-light is suspicious of causing the degradation. Therefore, CdS thin layer is suggested as the appropriate shell materials for TiO2 electrodes, to passivate the activation of oxygen vacancies and prevent the direct contact of TiO2 electrodes and perovskite absorbers. The choice of CdS is a comprehensive decision considering the required virtues for the shell: n-type semiconductor, conduction band energy level between TiO2 and perovskite, acceptable carrier conductivity, and synthesis possibility. With the solution-based deposition method named SILAR (successive ionic layer absorption reaction), thin CdS shell with the thickness of ~5 nm is deposited onto the mesoporous TiO2 and utilized as the electron transporter of PSCs. Resultant PSC with the TiO2/CdS electrode demonstrates enhanced light-stability showing 80% of the initial PCE after 12 hours of light illumination, while the reference PSC records only 40% of the initial PCE. In the fourth chapter, another kind of approach to overcome the light-instability is suggested. Because TiO2 layer is now suspected of causing the PCE degradation under the light, all of TiO2 layer (mesoporous TiO2 layer and the hole-blocking compact TiO2 layer) in the PSC are eliminated in the research. Instead of TiO2 compact layer, thermally evaporated CdS thin layer plays the role of hole-blocking layer for the PSCs. The selection of CdS is based on the same reason with the previous research. After the optimization experiments for the appropriate thickness of CdS hole-blocking layer, stability of CdS-applied PSC to the light illumination is measured. As expected, PSC with the CdS exhibits 90% PCE maintenance after 12 hours of light illumination, proving its considerably increased light stability. In the thesis research, as described above development of perovskite solar cells stable to humidity and light is intensively studied, and considerably improved stabilities to the both external risk elements are demonstrated.