하이브리드 광전소자 계면의 광물리학적 연구

Abstract

The solar cells converse the incident sun light to electricity using photoelectrochemical process. Numerous materials in DSCs showed the important roles in core charge transfer phenomena such as light harvesting, electron injection, electron transport, and dye regeneration. For the flexible photovoltaic device as the final goal of DSCs, tremendous researches are concentrated on improvement in the properties of the sensitizer, metal oxide, and electrolytes, and still on going. Although the all materials with the pros and cons can affect on the photovoltaic performance using their intrinsic properties, at the same time, the intrinsic properties can change the interfacial properties of faced materials. However, there are still lacks in the studies on the interaction between materials on the basis of intrinsic properties, fundamental principles, charge transfer mechanism, and correlation at the interfaces (such as solubility, aggregation, adsorption, and recombination) Therefore, we should intensively study deep into the understanding and controlling the behaviors/properties of materials In the optoelectronic device, the all charge transfers are not the independent but the stepwise processes from exciton generation, electron injection, electron collection, to dye regeneration, at the same time with the competitive recombination reaction. Eventually, this charge transfer mechanisms are aimed at the maximizing the photovoltaic performance by smooth progress in photoelectrochemical cycles. The charge transfers between different materials are of importance because there are interfacial resistance occurred by different charge mobilities of materials, whereas the charge transporting through the inside of material is only governed by the intrinsic properties of own material. To conclude, the control over the intrinsic properties of materials is closely correlated with the charge transfer mechanism of solar cells. Therefore, the correlation should be studied on the basis of the factors affecting the material properties and charge transfer discussed in the previous chapter. Furthermore, the conventional methods for understanding the photovoltaic performance generally focused on the electrochemical and photoelectrical characterization such as UV-Vis absorption spectroscopy, electrochemical impedance spectroscopy, cyclic voltammetry, and photocurrent–voltage measurement. However, the solar cells are based on the photon to electron conversion, thereafter the charge transporting through the interfaces and/or inside the materials. Therefore, the charge transfer mechanism of photovoltaic device should be characterized by the photoelectrochemical analysis. For this purpose, the laser system based transient absorption spectroscopy is the proper method to directly investigate the internal charge transfer mechanism. In Chapter 1, the general points of solar cell will be presented first of all such as the origin of solar cells related to global warming. On the basis of the principles and operating mechanism of various types of solar cells, the key issues of solar cells are arranged in accordance to the limitation in state-of-art works. Thereafter, the motivations s and hypothesis on this research will be addressed. In Chapter 2, this chapter consists of the all processes from the material preparation to device characterization. In the previous section, the all materials used in this study are enumerated and noted with the full name, abbreviation, and manufacturer. In the next section, the sample preparation and device fabrication processes are expressed as a sequence of device type i.e. liquid, quasi-solid-state, and solid-state solar cells in detail. Lastly the all characterization methods are arranged as a two parts. One is the characterization methods of complete device, the other one is the general methods for various samples depending on the special experiments. Moreover this part doesn’t include the laser spectroscopic study which is handled in the chapter 5. The specific experimental details are organized in the ‘Methodology’ sections of each chapter. In Chapter 3, among the numerous researches on controlling the material properties at the interfaces, the environmental, post-process factors are intensely investigated as addressed above. The reason is the positive points of the both methods well known in material science. However, those methods have positive and negative points at the same time compared to the roles of coadsorbents. In this part, after brief introduction of the effects of environment and post-process factors, intensely the features of various coadsorbents will be handled how the coadsorbents overcome the limitations in previous methods and improve the interfacial properties, especially in the field of DSCs. Thus, the roles of coadsorbents on the interfacial properties as well as the control over the properties of coadsorbents will be intensively emphasized and investigated in liquid type DSCs. In Chapter 4, on the basis of the behavior and properties of materials especially coadsorbents, the interfacial charge transfer can be controlled using the introduction of functional materials. We posited that multiple, porous passivation layers overlaying dye-sensitized TiO2 nanocrystalline particles. We successfully encapsulated N719-sensitized nanocrystalline TiO2 particles using surface-induced polymerization and cross-linking of the reactive coadsorbents, which resulted in anisometric growth of the polymer layer over the surface of the TiO2 electrode. We revealed that novel porous network polymer layer as interfacial functional materials over the N719-sensitized TiO2 particles not only change the chemically robust hydrophobic layer, but also physically block the desorption of N719 dyes from the surfaces of TiO2 electrodes. Therefore controlling the functional materials can effect on the prevention of approaching the large oxidizing species to dye cation, thereby inhibiting the charge recombination reactions. Finally, we demonstrated that the resulting DSC devices satisfied three criteria: (1) reduced dye loading, (2) simultaneous improvements in JSC and VOC, and (3) long-term stability furthermore, prevent desorption of N719 molecules to afford stability to the devices.. In Chapter 5, the principles and measurement of TAS and TMS are introduced to characterize the interfacial charge transfer phenomena in solar cells. The establishment and setup processes are accurately arranged with photographic data and equations in the appendix. With this TAS and TMS systems, we can obtain the lifetime of dye cation, hole extraction, charge density, charge mobility and conductivity, and luminescence lifetime. On the basis of the interactions between the materials, we can calculate the recombination rate, regeneration rate, regeneration quantum yield, PL quenching yield, and charge density dependent conductivity or mobility of materials. In Chapter 6, the key point of this study is the correlation between Controls of Material Properties & Charge Transfer Phenomena. In other words, in this part on the basis of previous discussions, it will be discussed with various example results how the 3 factors as controlling the materials’ behaviors/properties influence on the changes in charge transfer at the interfaces of solar cells. Especially the core charge transfer phenomena will be discussed with introduction of coadsorbents at the interfaces; (1) the light harvesting and electron injection kinetics correlated with either the suppretion of aggregation or intrinsic property of sensitizer (part-2 & -4), (2) dye cation regeneration reaction as the rate-determining step controlled by the changing the interaction possibilities of dye cations with redox couples (using material properties in part-4, using the functional coadsorbents in part-6), (3) recombination reactions as the main loss in photocurrent suppressed by the coadsorbents-assisted 3D-polymer network membrane in part-3, and using the functional coadsorbents in part 6)