Metal halide perovskites have emerged as photoactive materials in solution-processed devices thanks to their unique properties such as high absorption coefficient, sharp absorption edge, long carrier diffusion lengths, and tunable bandgap, together with ease of fabrication. The single-junction perovskite solar cells have reached power conversion efficiencies of more than 25%. Although the efficiency of perovskite devices has increased tremendously in a very short time, the efficiency is still limited by carrier recombination at defects and interfaces. Thus, understanding these losses and how to reduce them is the way forward towards the Shockley-Queisser limit. This thesis aims to apply ultrafast optical spectroscopy techniques to investigate the recombination pathways in halide perovskites, and understand the charge extraction from perovskite to transport layers and nonradiative losses at the interface. The first part focuses on perovskite solar cells with planar n–i–p device architecture which offers significant advantages in terms of large scale processing, the potential use of flexible substrates, and applicability to tandems. In addition to the optimization of MAPbI3 solar cell fabrication using a modified sequential interdiffusion protocol, the photophysics of perovskites exposed to humid air and illumination are discussed. The MAPbI3 film processed with the addition of glycol ethers to the methylammonium iodide solution results in the control of PbI2 to perovskite conversion dynamics, thus enhanced morphology and crystallinity. For samples exposed to humid air and illumination, the formation of sub-bandgap states and increased trap-assisted recombination are observed, using highly-sensitive absorption and time-resolved photoluminescence measurements, respectively. It appears that such exposure primarily affects the perovskite surface. The second part discusses the hole extraction from Cs0.07Rb0.03FA0.765MA0.135PbI2.55Br0.45 to the polymeric hole transport layer and interfacial recombination using ultrafast transient absorption spectroscopy technique. To illustrate this, PDPP-3T was used as HTL, since its ground state absorption is red-shifted compared to the perovskite’s photobleach, thereby allowing direct probing of the interfacial hole extraction and recombination. Moreover, carrier diffusion is investigated by varying the perovskite film thickness, and carrier mobility is found to be 39 cm2V-1s-1. Finally, hole extraction is found to be one order of magnitude faster than the recombination at the interface.

京都大学 / 新制・課程博士 / 博士(理学) / 甲第22990号 / 理博第4667号 / 新制||理||1670(附属図書館) / 京都大学大学院理学研究科物理学・宇宙物理学専攻 / (主査)教授 山本 潤, 教授 石田 憲二, 教授 田中 耕一郎 / 学位規則第4条第1項該当 / Doctor of Science / Kyoto University / DFAM

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Attosecond pulses have been developed as a means for investigating phenomena that proceed on the order of the atomic unit of time (24 as). Unfortunately, these extreme ultraviolet (XUV) pulses by themselves contain too few photons to initiate nonlinear dynamics or dress states in an attosecond pump--attosecond probe scheme. As a result, most attosecond experiments thus far have featured complementary near infrared (NIR) femtosecond lasers for instigating electron dynamics. In order to access the benefits of all-attosecond measurements and open attosecond physics to new fields of exploration, the photon flux of these pulses must be increased. One way to boost the attosecond pulse energy is to scale up the energy of the NIR pulse responsible for driving high-harmonic generation (HHG). With generalized double optical gating (GDOG), isolated attosecond pulses can be generated with multi-cycle laser systems, wherein the pulse energy can be boosted more easily than in the few-cycle laser systems required by other gating methods. At the Institute for the Frontier of Attosecond Science and Technology (IFAST), this scalability was demonstrated using a 350 mJ, 15 fs (10 TW) Ti:sapphire laser, which was used to generate a 100 nJ XUV continuum. This represented an order-of-magnitude improvement over typical attosecond pulse energies achievable by millijoule-level few-cycle lasers. To obtain the microjoule-level attosecond pulse energy required for performing all-attosecond experiments, the attosecond flux generated by the IFAST 10 TW system was still deficient by an order of magnitude. To this end, the laser system was upgraded to provide joule-level output energies while maintaining pulse compression to 15 fs, with a targeted peak power of 200 TW. This was accomplished by adding an additional Ti:sapphire amplifier to the existing 10 TW system and implementing a new pulse compression system to accommodate the higher pulse energy. Because this system operated at a 10 Hz repetition rate, stabilization of the carrier-envelope phase (CEP) -- important for controlling attosecond pulse production -- could not be achieved using traditional methods. Therefore, a new scheme was developed, demonstrating the first-ever control of CEP in a chirped-pulse amplifier (CPA) at low repetition rates. Finally, a new variation of optical gating was proposed as a way to improve the efficiency of the attosecond pulse generation process. This method was also predicted to allow for the generation of isolated attosecond pulses with longer driving laser pulses, as well as the extension of the high-energy photon cut-off of the XUV continuum.

This dissertation is focused on ferromagnetic, multiferroics, and two-dimensional (2D) perovskites, exploring different unique collective magnetic and ferroic characters: (1) ferromagnetic thin film Co/Pd multilayers, (2) BaTiO3-BiFeO3 (BTO-BFO) a magneto-electric materials system, and (3) CuCl4 halide organic-inorganic perovskites. Low-power all-optical memory offers a unique opportunity to achieve ultra-fast magnetic switching in which the switching dynamics are not thermally mediated and occur on the order of the laser pulse. However, it is challenging to achieve a low-power optically excited magnetization precession angle above 90 degrees, which is required for magnetic switching. Co/Pd thin film multilayers were investigated for their potentially large perpendicular magnetic anisotropy (PMA) with three differing regimes of magnetic anisotropy: in-plane, weakly out-of-plane, and out-of-plane. Utilizing the time-resolved magneto-optical Kerr effect (TR-MOKE), we observed clear magnetic precession (on the order of a few GHz) with magnetic precession angle increasing (up to 4.5 degrees) for thinner Co samples which demonstrated stronger PMA. We observed a clear connection between PMA strength and precession amplitude as well as a large efficiency of energy transfer between spin and orbital subsystems for our strongest PMA sample. BTO-BFO is a strong room-temperature multiferroic with enhanced magneto-electric properties compared to BFO. We utilized time-resolved differential reflectivity (TR-DR) and TR-MOKE to observe strong coherent acoustic phonons in thin films as well as nanorods. Our nanorods showed additional modes (a new 20 GHz and 6 GHz mode) not observed in thin films including the fast 33 GHz mode which showed some weak tunability with high magnetic fields (up to 10 T). The observed tunability of these modes in an external magnetic field shows interesting coupling between magnetic moment and phononic modes which may be caused by the breaking of the spin-cycloid at the interface of the nanorods and the surface of the nanorods. We also observed second harmonic generation (SHG) emission which demonstrated a large enhancement in our nanorod structures with further observation of wavelength dependence. Finally, ferromagnetic resonance on our nanorod and thin film BTO-BFO structures indicated very weak Gilbert damping (on the order of 10−3), demonstrating the practicality of our structure for low-spin loss applications. Lastly, this dissertation focuses on a project around CuCl4 and CuCl2Br2 perovskites in which we observed time-dependent SHG. An increase in SHG as a function of infrared laser exposure is shown to coincide with changes in the crystal structure of the Cu perovskite materials. This increase in SHG was shown to last for a few days after hours of laser exposure indicating a slow hysteretic change to the crystal structure of the perovskites. / Doctor of Philosophy / Multifunctionality in materials is important for various applications including future mem- ory devices where ferromagnetism (collective magnetic order), ferroelectricity (collective electric polarization order), and piezoelectricity (collective strain order) can be implemented in a given device. This dissertation centers on three material systems for exploring ferroic orders: Co/Pd thin multilayers, BaTiO3-BiFeO3 (BTO-BFO) films and nano-rod arrays, and Cu halide organic-inorganic perovskite thin films and 2D structures. Co/Pd thin films demonstrate interesting ferromagnetic order with magnetic anisotropy in which the magnetization of the thin film has a preferred direction based on the thickness of the thin film. BTO-BFO demonstrates coupling between ferroelectric and antiferromagnetic order. The magnetic information may be controlled by applying electric fields or strain and Cu halide perovskites demonstrate potentially created ferroelectric order under long-term laser expo- sure with high ferroelectric switching speeds. Dynamics and nonlinear optical responses in these materials systems were explored with Ti:Sapphire pulsed lasers (∼ 100 fs). Our techniques allowed us for a better understanding of fast carrier and spin dynamics after optical excitation. Furthermore, nonlinear optics, in which two or more photons can be used to emit higher energy photons, were employed to explore the ferroelectric properties within these material systems. The results presented in this dissertation provided information on collective orders and fundamental interactions in several less-explored material systems.

<p> The interaction of light with semiconductors provides essential insight into their electronic and photonic properties. Excitons, excited electron-hole pairs, determine the optical response of nanomaterials and act as nanoscale energy carriers, making excitonic materials excellent candidates for optoelectronic, photovoltaic, and quantum devices. Unique phenomena can be brought about by using excitonic materials as building blocks in designing new systems and taking advantage of excitons’ dimensionality. For example, growing quantum dots into highly ordered arrays enhances exciton transport due to the strong dipolar coupling between excitons. Alternatively, forming vertical heterostructures between monolayer transition metal dichalcogenides introduces moiré superlattices, which localize the excitons introducing nonlinear interactions that be exploited for quantum information processing. Understanding these complex excitonic systems requires experimental tools capable of high spatial and temporal resolutions.</p> <p><br></p> <p>This thesis aims to contribute to understanding the complex excitons and charges formed at nanoscale interfaces with ultrafast techniques. In the discussed work, we take advantage of the 100s of fs time resolution and 10s of nm spatial precision to visualize exciton migration and dynamics associated with complex excitonic systems. First, we introduce the optical techniques needed to help us understand the fundamental photophysics of the studied systems (Chapter 2). Next, we provide an example of how we can use these methods to understand exciton coherence in perovskite quantum dot solids exhibiting superradiance (Chapter 3) and enhanced exciton transport (Chapter 4) due to low disorder and strong dipolar coupling. We also characterize and explore the behavior of highly excited excitons, Rydberg states, in transition metal dichalcogenides (Chapter 5). Then, we examine the properties of heterostructures formed between two monolayers of transition metal dichalcogenides exhibiting moiré superlattices and investigate the nonlinear exciton-exciton interactions modulated by the moiré potentials (Chapter 6). We also explore charge carrier behavior at interfaces of two different excitonic materials in molybdenum disulfide-single-wall carbon nanotube heterojunctions containing one- and two-dimensional excitons (Chapter 7). Finally, we visualize and quantify charge carrier migration across an alloyed cadmium sulfide and cadmium selenide lateral heterojunction (Chapter 8). We hope to give the reader a better understanding of these complex systems and open up new possibilities for their efficient use through the results presented in this thesis. </p>