Photophysical Study on Charge Transfer Kinetics in Hybrid Solar Cells

Abstract

p-n junction and the p-i-n solar cells were two main models for understanding of operating mechanism of solar cells until the 1990s. The development of nanostructured solar cells such as dye-sensitized solar cells (DSSCs), bulk heterojunction organic photovoltaics (OPVs) or organic-inorganic hybrid perovskite solar cells (PSCs) introduced a series of new device structures. Research into PSCs has made tremendous progress over the last decade, leading to devices that achieve certified power conversion efficiencies exceeding 20%. Long charge diffusion length in micrometre range is the key factor that lead the great achievement for PSCs and has enabled thin-film photovoltaic device architectures. In Chapter 1, the general introduction to charge extraction kinetics in hybrid solar cell was presented. Based on the principles and operating mechanism of charge extraction process, importance of charge extraction study was described General information and introduction about time-resolved photoluminescence (TRPL) measurement that is most distinct experiment for investigating charge extraction kinetics were presented. Thereafter the research motivations on this research will be addressed. In Chapter 2, I demonstrated the parameters that can influence the electron extraction in planar perovskite solar cells (PSCs). I could demonstrate the dominant factor for efficient electron extraction in PSCs through symmetrical design of experiment condition, which control the character for electron extraction including free energy difference (∆G) values between the ETL and perovskites, electron mobility (μe) of the ETL, and quality of physical contact between the ETL and fluorine-doped tin oxide (FTO). Spin-coated SnO2 and TiO2, anodized-TiO2, and bilayered electron transport layers (ETL) composed of SnO2 and TiO2 or SnO2 on a-TiO2 (SnO2@a-TiO2) were employed to control variable. These varied ∆G values between the ETL and perovskites, μe of the ETL, and quality of physical contact between the ETL and fluorine-doped tin oxide (FTO). Among the various ETLs, the bilayered ETL (SnO2@a-TiO2) gives a large ∆G as well as defect-free physical contact. This study emphasizes that a large ∆G value plays an important role in electron extraction. More importantly, the defect-free physical contact is also crucial for achieving improved electron extraction. In Chapter 3, I studied complex PL decay behavior obtained from TRPL measurement. I reported a dynamic trapping model describing the trapping process of a free electron in association with diffusion. The origin of the stretched exponential decay behavior of the trapping process, which appears in the fast component of TRPL, is described phenomenologically and further clarified through a simulation study. I also presented a simulation study analyzing the electron extraction process. I investigated the influence of the electron injection coefficient and electron mobility on the electron extraction process through simulation study. I found that the electron injection coefficient at the interface between perovskite and the electron transport layer is more important determinant than the electron mobility of the perovskite in electron extraction process. In Chapter 4, I could separate the hole extraction process into the phase regarding the hole injection process and the phase regarding the hole diffusion process, and calculate more reasonable hole injection coefficient (kI) and hole diffusion coefficient (Dh) from the systematic experiment design producing difference in the initial hole density distribution. The phase about diffusion process also satisfy the boundary conditions that is the requirement for solving the one-dimensional diffusion limited quenching model equation, displaying the distinct mono-exponential PL decay curve. In this chapter, I revealed how the kI , Dh, and the initial hole density distribution affect the hole extraction kinetics in perovskite. The Dh value which calculated by our phenomenological analysis shows 4 or 5 times difference when the effective lifetime or average lifetime of fast component were employed as the time constant of hole extraction kinetics. In Chapter 5, I summarized my research and proposed a direction of research according to my academic results.