>Magister Scientiae - MSc / Tin dioxide (SnO2) thin films are a worthy candidate for an electron transport layer (ETL) in perovskite solar cells, due to its suitable energy level, high electron mobility of 240 cm2 v-1 s- 1, desirable band gap of 3.6 - 4.0 eV, and ultimately proves to be suited for a low temperature thermal oxidation technique for ETL production. A variety of methods are available to prepare SnO2 thin films such as spin and dip coating and chemical bath deposition. However, the customary solid-state method, which incorporates thermal decomposition and oxidation of a metallic Sn precursor compound in an oxygen abundant atmosphere prevails to be low in cost, is repeatable and allows for large-scale processing.

Halide perovskites have recently emerged as the state-of-the-art semiconductors with the unique combination of outstanding optoelectronic properties and facile solution synthesis. Within only a decade of research, they have witnessed a remarkable success in photovoltaics and shown great potential for applications in light-emitting devices, photodetectors, and high-energy sensors. Yet, the majority of current perovskite-based devices still rely on polycrystalline thin films which, as will be discussed in Chapter 2, exhibit inferior charge transport characteristics and increased tendency to chemical degradation compared to their single-crystalline analogues. In this regard, unburdened from the effects of grain boundaries, single crystals demonstrate the upper limits of semiconductor performance. Their study is, thus, important from both fundamental and practical aspects, which present the major objectives of this dissertation. In Chapter 3, we study the intrinsic charge transport and recombination characteristics of single crystals of formamidinium lead halide perovskites. While, in Chapter 4, we investigate the mechanistic origins of rapid synthesis of halide perovskite single crystals by inverse temperature crystallization. Understanding the nucleation and growth mechanisms of halide perovskites enables us to design strategies toward integrating their single crystals into device applications. Namely, in Chapters 5 and 6, we demonstrate crystal engineering approaches for tailoring the thicknesses and facets of halide perovskite single crystals to make them suitable for, respectively, vertical and planar architecture optoelectronic devices. The findings of this dissertation are expected to benefit future studies on fundamental characterization of halide perovskites, as well as motivate researchers to develop perovskite-based optoelectronic devices with better crystallinity, performance and stability.

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>Magister Scientiae - MSc / Perovskite solar cells have attracted a tremendous amount of research interest in the scientific community recently, owing to their remarkable performance reaching up to 22% power conversion efficiency (PCE) in merely 6 to 7 years of development. Numerous advantages such as reduced price of raw materials, ease of fabrication and so on, have contributed to their increased popularity.

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.

Depuis 5 ans, les pérovskites hybrides organiques-inorganiques sont apparues comme une nouvelle classe de semiconducteurs possédant des propriétés optoélectroniques très intéressantes pour les dispositifs photovoltaïques et émetteurs de lumière. Cette thèse porte sur une étude expérimentale de spectroscopie optique, qui s’inscrit dans le champ d’exploration des propriétés optiques et des effets excitoniques des pérovskites hybrides CH3NH3PbX3 avec X = I ou Br. Nous avons étudié les propriétés optiques de couches minces déposées par spin-coating et de monocristaux élaborés en solution. Les couches minces présentent une structure granulaire et une densité élevée de défauts qui induisent une grande variabilité des propriétés optiques. L’étude des monocristaux nous a permis de mettre en évidence les propriétés intrinsèques du matériau : émission d’excitons libres, couplage électron-phonon, dynamique de recombinaison des porteurs de charge. De plus, nous avons exploré l’impact de la transition de phase orthorhombique-tétragonale sur les propriétés optiques de CH3NH3PbI3. Enfin, nous avons quantifié l’effet de la réabsorption sur les propriétés d’émission des pérovskites hybrides. L’estimation précise de cet effet est particulièrement importante pour l’interprétation des propriétés optiques des pérovskites hybrides et explique la grande hétérogénéité des résultats dans la littérature. / In the last five years, hybrid organic-inorganic perovskites have emerged as a novel class of semiconductors owing to their interesting electronic and optical properties for photovoltaic and light-emitting devices. This thesis reports an experimental study using optical spectroscopy to explore the optical properties and excitonic effects of hybrid perovskites such as CH3NH3PbX3 with X = I or Br.We studied the optical properties of spin-coated thin films and solution processed single crystals. Thin films present a granular structure and a high density of defects which induce a great variability of the optical properties. The study of single crystals allows us to highlight the intrinsic properties of material: free exciton emission, electron-phonon coupling and charge carriers recombination dynamics. Besides, we have investigated the impact of the orthorhombic-tetragonal phase transition on the optical properties of CH3NH3PbI3. Finally, we have quantified the effect of reabsorption on the emission properties of hybrid perovskites. The accurate estimate of this effect is particularly important for the interpretation of the optical properties of hybrid perovskites and explains the great heterogeneity of the results in the literature.

京都大学 / 0048 / 新制・課程博士 / 博士(工学) / 甲第18996号 / 工博第4038号 / 新制||工||1622(附属図書館) / 31947 / 京都大学大学院工学研究科材料化学専攻 / (主査)教授 田中 勝久, 教授 平尾 一之, 教授 三浦 清貴 / 学位規則第4条第1項該当 / Doctor of Philosophy (Engineering) / Kyoto University / DGAM

京都大学 / 0048 / 新制・課程博士 / 博士(工学) / 甲第19684号 / 工博第4139号 / 新制||工||1639(附属図書館) / 32720 / 京都大学大学院工学研究科機械理工学専攻 / (主査)教授 北村 隆行, 教授 立花 明知, 教授 鈴木 基史 / 学位規則第4条第1項該当 / Doctor of Philosophy (Engineering) / Kyoto University / DFAM

京都大学 / 0048 / 新制・課程博士 / 博士(工学) / 甲第19735号 / 工博第4190号 / 新制||工||1646(附属図書館) / 32771 / 京都大学大学院工学研究科物質エネルギー化学専攻 / (主査)教授 陰山 洋, 教授 安部 武志, 教授 阿部 竜 / 学位規則第4条第1項該当 / Doctor of Philosophy (Engineering) / Kyoto University / DGAM

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