Defect and Interface Engineering for High-Performance Organic and Perovskite Photovoltaics

Abstract

Organic-inorganic hybrid perovskites (OIHPs) and organic π-conjugated materials have drawn attraction as next-generation semiconductor materials due to their distinctive attributes beyond conventional inorganic semiconductor materials: low-cost mass production via solution-based printing process, portable electronics using lightweight characteristics, and flexible device applications from soft and nanoductile mechanical nature. However, to be addressed for commercialization, there are many obstacles such as high device performance, long-term stability, and reliable quality maintenance. In particular, high-quality semiconductor materials are a crucial factor in overcoming these limitations. Therefore, in this thesis, we will construct the material engineering of OIHPs and organic π-conjugated materials for high-performance optoelectronic device applications.

Chapter 1 introduces a general and fundamental overview of the field of organic solar cells (OSCs) and OIHP-based solar cells (PSCs). First, the chemical and electronic structure of organic π-conjugated semiconductor materials is discussed. Second, the unique optoelectronic properties of OIHPs are described. Lastly, the basic concepts of photovoltaics such as device architecture and operating mechanism, which include the generation, transportation, and extraction of photogenerated charge carriers, are explained.

Chapter 2 provides material engineering of OIHP and organic π-conjugated semiconductor polymers for optoelectronic device application. In chapter 2-1, we describe the defect engineering of OIHP materials by introducing a zwitterionic amino acid additive in OIHP precursor solutions. The amino acid has positive (NH3+) and negative (COO-) charges, which are induced by the specific pH range of the solvent. This additive effectively passivates both Lewis base (cation vacancy or anion interstitial) and Lewis acid (anion vacancy or cation interstitial) defects in the OIHP, enlarging grain sizes and lengthening the charge-carrier lifetime. The power conversion efficiency (PCE) of both small-area (4.64 mm2) devices and large-area (9.06 cm2) submodules fabricated with this additive process is improved by increasing an open-circuit voltage. In addition, the additive process significantly enhances the storage stability of PSCs in ambient conditions. Furthermore, in chapter 2-2, we fabricated photoelectrochemical (PEC) cells for hydrogen production with amino acid additive-treated OIHP materials and a eutectic gallium indium alloy (EGaIn) encapsulation method. The EGaIn not only completely prohibit the OIHP materials from the external environment but also effectively transports the photogenerated charges in OIHP materials. Therefore, these optimal OIHP-based PEC cells have high efficiency and high operating stability under continuous 1-sun illumination in a sulfuric acid solution. In chapter 2-3, we control the energy level and crystallinity of π-conjugated semiconductor polymers via a random copolymer system regulating the molar ratio of monomers for demonstrating high-performance OSCs. The best PCE performance-based random copolymer, in this study, has a well-matched energy level and well-mixed morphology with the Y6 non-fullerene acceptor, leading to efficient charge carrier dissociation and suppressed recombination loss in the photoactive layer. Therefore, our copolymer systems will guide further development of π-conjugated semiconductor polymer for highly efficient OSCs.

Chapter 3 describes versatile interface engineering strategies of PSCs to suppress their recombination loss and enhance the device's performance. In chapter 3-1, we focus on the bottom interface of OIHP. Because the modified bottom interface strategy can control the grain growth of OIHP materials and passivate defect sites on OIHP and charge transport layer (CTL) at once. Therefore, to apply as the bottom interface layer, we newly synthesized a non-conjugated polymer. Our new interface material effectively suppressed the non-radiative recombination, which occurs in both bulk OIHP materials and their bottom interface. Further, the bottom interface passivated PSCs exhibited a high PCE of 24.4% with a reduced non-radiative voltage loss. Furthermore, the non-encapsulated PSCs also exhibited better operating stability under continuous AM 1.5G 1-sun irradiation. In chapter 3-2, we introduce the top surface passivation of 2D OIHP-based PSCs. The OIHP top surface has numerous defect sites due to the volatile species (e.g. small organic cations or halide anions). These defect sites hinder interlayer charge transport between the OIHP materials and the CTL. Herein, to passivate these defect sites, we introduced a simple solution-processable multifunctional small molecule as a passivation material on the top surfaces of 2D OIHP. This innovative treatment strategy effectively passivated the defects, which exist on the top surface and the grain boundaries of the 2D OIHP materials, and simultaneously induced vertical orientations of the 2D OIHP crystals, leading to efficient charge transport with reduced recombination loss. In addition, the optimal 2D OIHP-based PSCs exhibited a high PCE of 20.05% and remarkable long-term operating stability.

Chapter 4 provides an overall summary of the work and the conclusions drawn in this thesis.