The efficiency of photoconversion systems, such as organic photovoltaic (OPV) cells, is largely controlled by a series of fundamental photophysical processes occurring at the interface before carrier collection. A profound understanding of ultrafast interfacial charge transfer (CT), charge separation (CS), and charge recombination (CR) is the key determinant to improving the overall performances of photovoltaic devices. The discussion in this dissertation primarily focuses on the relevant parameters that are involved in photon absorption, exciton separation, carrier transport, carrier recombination and carrier collection in organic photovoltaic devices. A combination of steady-state and femtosecond broadband transient spectroscopies was used to investigate the photoinduced charge carrier dynamics in various donor-acceptor systems. Furthermore, this study was extended to investigate some important factors that influence charge transfer in donor-acceptor systems, such as the morphology, energy band alignment, electronic properties and chemical structure. Interestingly, clear correlations among the steady-state measurements, time-resolved spectroscopy results, grain alignment of the electron transporting layer (ETL), carrier mobility, and device performance are found. In this thesis, we explored the significant impacts of ultrafast charge separation and charge recombination at donor/acceptor (D/A) interfaces on the performance of a conjugated polymer PTB7-Th device with three fullerene acceptors: PC71BM, PC61BM and IC60BA. Time-resolved laser spectroscopy and high-resolution electron microscopy can illustrate the basis for fabricating solar cell devices with improved performances. In addition, we studied the effects of the incorporation of heavy metals into π-conjugated chromophores on electron transfer by monitoring the triplet state lifetime of the oligomer using transient absorption spectroscopy, as understanding the mechanisms controlling intersystem crossing and photoinduced electron transfer dynamics is required to improve the device performance of solar cells. Here, we evaluated the effects of incorporating Pt(II) on intersystem crossing and photoinduced electron transfer by comparing and analyzing the photoexcited dynamics of DPP-Pt(II)(acac) and metal-free DPP with different acceptors such as TCNE, TMPyP, and TPyP.

This thesis studies properties of hydrogel, which arises on the basis of electrostatic and hydrophobic interactions between hyaluronan chain and micelles of cationic surfactant. A native sodium hyaluronan at molecular weight 750–1 000 kDa and a cationic surfactant CTAB (cetyltrimethylammonium bromide) were used. This hydrogel was assessed as a material for drug delivery systems. The hydrogels were made by mixing 200mM CTAB with 0.5% hyaluronan, both dissolved in 0.15M aqueous solution of NaCl simulating physiological solution. Methods used in this study were steady-state and time-resolved fluorescence spectroscopy, more accurately time-resolved emission spectra (TRES) and deconvolution of steady-state emission spectra of a whole sample by means of parameters gained from fluorescence intensity decays at a set of wavelenghts. Selected systems were investigated by three fluorescent probes, prodan, laurdan and rhodamine 6G. The first two mentioned probes were in hydrogel localized only within micelles in three different microenvironments. Rhodamine 6G pointed out that in hydrogel the aqueous environment is significantly restricted in comparison to purely micellar solution. In addition, rhodamine informed about less available micelle surfaces, caused by hyaluronan chains occupation. There were no interactions between the probes and hyaluronan chains. Freshly made hydrogels showed almost the same results as after a week of maturation under its supernatant.

This diploma thesis deals with the possibilities of multiparametric fluorescence spectroscopy, since the main objective of this experiment was to evaluate the possibilities of multiparametric measurements in the fluorescence spectroscopy laboratory. A suitable fluorescence probe was proposed for this type of experiment that shows high sensitivity for pH changes in the environment, SNARF-4F AM, based on a literature research. The fluorophore was dissolved in solutions of different pH and this system was examined using a time-resolved spectrofluorimeter. The method named TRES (time-resolved emission spectra) was used to obtain the emission spectra of the probe and to find the emission maximum. Fluorescence intensity decay measurements as a function of wavelengths allowed to create deconvolution of the emission spectra, which provided information about the fluorescent lifetime and the relative representation of the states of probes in the solution. Later, the probe was dissolved in solutions of different density and pH - this system served for anisotropic measurements, during which the individual correlation-rotational times of the fluorophore were obtained. The obtained results were then used as the basis for multiparametric analysis, which was performed by using a fluorescence correlation microscope and a spectrograph. This combination allows to measure the necessary fluorescence parameters in one step. A standard operating procedure was created for the spectrograph’s control. On the basis of the obtained information the suitability, accuracy and sensitivity of the multiparametric analysis were qualified.

Nous avons travaillé sur le développement d’un instrument de mesure de la luminescence induite par un faisceau d’ions lourds (nucléons  12) et d’énergie de l’ordre du MeV/nucléons. Basé sur une méthode de comptage de photons uniques obtenus par coïncidences, le dispositif permet d’obtenir sur 16 voies à la fois un spectre en énergie dans le domaine proche UV-visible-proche IR (185-920 nm) et la réponse temporelle sur la gamme ns-µs, avec un échantillonnage de 100 ps. Des mesures en température peuvent être réalisées depuis la température ambiante jusqu’à 30K.Ce travail met particulièrement l’accent sur les méthodes d’extraction des données : Une fois montrée la nécessité de déconvoluer les signaux, on s’intéresse dans un premier temps à évaluer différents profils instrumentaux modélisés et reconstruit à partir de mesures. A cet effet, un travail de caractérisation temporelle de chaque constituant du dispositif est mené. Puis ces profils instrumentaux sont utilisés dans deux méthodes de déconvolution par moindres carrés d’abord puis par maximum d’entropie ensuite.Deux matériaux types sont testés : Le Titanate de Strontium pour l’étude de la dynamique de l’excitation électronique, et un scintillateur plastique commercial, le BC400, pour l’étude du vieillissement et de la baisse des performances en fonction de la fluence. Dans les deux cas on a pu mettre en évidence la présence d’une composante ultra rapide de constante de temps subnanoseconde. / We developed an instrument for measuring the luminescence induced by a heavy ion beam (nucleons  12) and energy in the range of MeV / nucleon. Based on a single photon counting method obtained by coincidences, the device can provide in the same run a 16-channel energy spectrum in the UV-visible- IR region (185-920 nm) and a time-resolved response in the range of ns up to µs for each channel. Temperature measurements can be performed from room temperature down to 30K.This work places particular emphasis on data extraction methods: Once the need to deconvolve the signals demonstrated the evaluation of different instrument profiles (simulated and reconstructed from measurements) leads to a systematic temporal characterization of each component of the device. Then, these instrumental profiles are used in two deconvolution methods: least squares first followed by maximum entropy method.Two typical materials are tested: the Strontium Titanate for the study of the dynamics of the electronic excitation, and a commercial scintillator, the BC400, for the study of the aging and the decrease of performances with fluence. In both cases, we have been able to highlight the presence of an ultrafast component of subnanosecond time constant.

Since its inception a quarter of a century ago, Kelvin probe force microscopy (KPFM) has enabled studying contact potential differences (CPDs) on the nanometre scale. However, current KPFM investigations are limited by the bandwidth of its constituent electronic loops to the millisecond regime. To overcome this limitation, pump-probe-driven Kelvin probe force microscopy (pp-KPFM) is introduced that exploits the non-linear electric interaction between tip and sample. The time resolution surpasses the electronic bandwidth and is limited by the length of the probe pulse. In this work, probe pulse lengths as small as 4.5 ns have been realized. These probe pulses can be synchronized to any kind of pump pulses. The first system investigated with pp-KPFM is an electrically-driven organic field-effect transistor (OFET). Here, charge carrier propagation in the OFET channel upon switching the drain-source voltage is directly observed and compared to simulations based on a transmission line model. Varying the charge carrier density reveals the impeding influence of Schottky barriers on the maximum switching frequency. The second system is an optically-modulated silicon homojunction. Here, the speed of surface photovoltage (SPV) build-up is accessed and compared to timeaveraged results. Due to slow trap states, the time-averaged method is found to lack comprehensiveness. In contrast, pp-KPFM exposes two intensity-dependent recombination times on the same timescale — high-level Shockley-Read-Hall recombination in the bulk and heat-dominated recombination in the surface layer — and a delay of the SPV decay with rising frequency, which is attributed to charge carrier retention at nanocrystals. The third system is a DCV5T-Me:C60 bulk heterojunction. The SPV dynamics is probed and compared to measurements via open-circuit corrected transient charge carrier extraction by linearly increasing voltage. Both methods reveal an exponential onset of the band bending reduction that is attributed to the charge carrier diffusion time in DCV5T-Me, and a double exponential decay, hinting at different recombination paths in the studied organic solar cell. The above-mentioned experiments demonstrate that pp-KPFM surpasses conventional KPFM when it comes to extracting dynamic device parameters such as charge carrier retention and recombination times, and prove that pp-KPFM is a versatile and reliable tool for studying electrodynamics on nanosurfaces.

京都大学 / 0048 / 新制・課程博士 / 博士(理学) / 甲第18494号 / 理博第4009号 / 新制||理||1578(附属図書館) / 31380 / 京都大学大学院理学研究科物理学・宇宙物理学専攻 / (主査)准教授 中 暢子, 教授 田中 耕一郎, 教授 金光 義彦 / 学位規則第4条第1項該当 / Doctor of Science / Kyoto University / DGAM

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In this work, we elucidated recombination kinetics in organic and hybrid semiconductors by steady-state and time-resolved PL spectroscopy. Using these simple and very flexible experimental techniques, we probed the infrared emission from recombining free charge carriers in metal–halide perovskites, as well as the deep blue luminescence from intramolecular charge-transfer states in novel OLED emitters. We showed that similar state diagrams and kinetic models accurately describe the dynamics of excited species in these very different material systems. In Chapters 4 and 5, we focused on lead iodide perovskites (MAPI and FAPI), whose comparatively developed deposition techniques suited the systematic material research. In MAPI, we harnessed the anomalous dependence of transient PL on the laser repetition rate in order to investigate the role of interfaces with the commonly used charge-selective layers: PC60BM, spiro-MeOTAD, and P3HT. The film was deposited on a large precut substrate and separated into several parts, which were then covered with the charge-selective layers. Thereby, the same bulk perovskite structure was maintained for all samples. Consequently, we were able to isolate interface-affected and bulk carrier recombination. The first one dominated the fast component of PL decay up to 300 ns, whereas the last was assigned to the remaining slow component. The laser repetition rate significantly prolonged PL decay in MAPI with additional interfaces while shortening the charge carrier lifetime in the pristine film. We qualitatively explained this effect by a kinetic model that included radiative electron–hole recombination and nonradiative trap-assisted recombination. All in all, we showed that the apparent PL lifetime in MAPI is to large extend defined by the laser repetition rate and by the adjacent interfaces. Further, we studied photon recycling in MAPI and FAPI. We monitored how the microscopic PL transforms while propagating through the thin perovskite film. The emission was recorded within 5orders of magnitude in intensity up to 70μm away from the excitation spot. The Beer–Lambert law previously failed to describe the complex interplay of the intrinsic PL spectrum and the additional red-shifted peak. Therefore, we developed a general numerical model that accounts for self-absorption and diffusion of the secondary charge carriers. A simulation based on this model showed excellent agreement with the experimental spatially resolved PL maps. The proposed model can be applied to any perovskite film, because it uses easily measurable intrinsic PL spectrum and macroscopic absorption coefficient as seeding parameters. In Chapter 6, we conducted an extensive photophysical study of a novel compact deep blue OLED emitter, SBABz4, containing spiro-biacridine and benzonitrile units. We also considered its single-donor monomer counterpart, DMABz4, in order to highlight the structure–property relationships. Both compounds exhibited thermally activated delayed fluorescence (TADF), which was independently proven by oxygen quenching and temperature-dependent transient PL measurements. The spiro-linkage in the double-donor core of SBABz4 rendered its luminescence pure blue compared to the blue-green emission from the single-donor DMABz4. Thus, the core-donor provided desirable color tuning in the deep blue region, as opposed to the common TADF molecular design with core-acceptor. Using PL lifetimes and efficiencies, we predicted EQEmax = 7.1% for SBABz4-based OLED, whereas a real test device showed EQEmax = 6.8%. Transient PL was recorded from the solutions and solid films in the unprecedentedly broad dynamic range covering up to 6orders of magnitude in time and 8orders of magnitude in intensity. The stretched exponent was shown to fit the transient PL in the films very well, whereas PL decay in dilute solution was found purely exponential. When the emitter was embedded in the host matrix that prevented aggregation, its TADF properties were superior in comparison with the pure SBABz4 film. Finally, using temperature-dependent transient PL data, we calculated the TADF activation energy of 70 meV. To sum up, this Thesis contributes to the two fascinating topics of the last decade’s material research: perovskite absorbers for photovoltaics and TADF emitters for OLEDs. We were lucky to work with the emerging systems and tailor for them new models out of the well-known physical concepts. This was both exciting and challenging. In the end, science of novel materials is always a mess. We hope that we brought there a bit of clarity and light. / Im Rahmen dieser Arbeit wurden Rekombinationsmechanismen in organischen und hybriden Halbleitern mittels statischer und zeitaufgelöster Photolumineszenz-Spektroskopie untersucht. Diese einfachen und flexiblen experimentellen Methoden erlaubten es, sowohl die infrarote Emission rekombinierender freier Ladungsträger in Perowskiten als auch die blaue Lumineszenz intramolekularer Ladungstransferzustände in neuartigen OLED-Emittern zu erforschen. Es wurde gezeigt, dass das Verhalten angeregter Ladungsträger in sehr unterschiedlichen Materialsystemen durch vergleichbare Zustandsdiagramme und kinetische Modelle beschrieben werden kann. Kapitel 4 und 5 legen den Fokus auf Bleiiodid-Perowskite (MAPI und FAPI), deren vergleichsweise etablierte Herstellungsmethode systematische Untersuchungen erlaubt. In MAPI wurde die anomale Abhängigkeit transienter PL von der Repetitionsrate des Lasers verwendet, um die Bedeutung der Grenzflächen zwischen Perowskitschicht und den gängigsten ladungsselektiven Schichten PC60BM, spiro-MeOTAD und P3HT zu untersuchen. Dafür wurde die Perowskitschicht auf ein Substrat aufgebracht, dieses in mehrere gleiche Stücke geteilt und anschließend mit einer jeweils unterschiedlichen ladungsselektiven Schicht bedeckt. Dies sicherte die Vergleichbarkeit der aktiven Schicht der verschiedenen Proben. Durch diesen Ansatz konnten der Einfluss des aktiven Materials als auch der seiner Grenzflächen auf die Ladungsträgerrekombination getrennt beobachtet werden. Ersterer dominierte den schnellen Anteil des PL-Abfalls, letzterer den langsamen Anteil. Die Repetitionsrate des Lasers verlangsamte den PL-Abfall in MAPI-Filmen mit zusätzlichen Grenzflächen signifikant, während sie die Lebensdauer der Ladungsträger in reinen MAPI- Filmen verkürzte. Dieser Effekt konnte durch ein qualitatives Modell erklärt werden, welches strahlende Elektron–Loch-Rekombination sowie nichtstrahlende Rekombination über Ladungsträgerfallen miteinbezieht. Insgesamt konnte gezeigt werden, dass die PL- Lebensdauer in MAPI stark von der Laserrepetitionsrate sowie von Grenzflächeneffekten abhängig ist. Des Weiteren wurde der Photon-Recycling-Effekt in MAPI und FAPI untersucht. Dafür wurde verfolgt, wie sich die lokale PL mit ihrer Ausbreitung durch den dünnen Perowskitfilm verändert. Die Emission konnte bis zu 70 μm entfernt von der Anregung gemessen werden, bei einer Abnahme der Intensität um fünf Größenordnungen. Mit reiner Anwendung des Lambert–Beer’sches Gesetzes konnte das auftretende komplexe Zusammenspiel des ursprünglichen Spektrums mit einer zusätzlichen rotverschobenen Emission nicht erklärt werden. Deshalb wurde ein allgemeines numerisches Modell entwickelt, das sowohl Selbstabsorption als auch die Diffusion sekundärer Ladungsträger berücksichtigt. Entsprechende Simulationen zeigten hervorragende Übereinstimmung mit räumlich aufgelösten experimentellen PL-Messungen. Das Modell kann auf jeden Perwoskitfilm angewendet werden, da die nötigen Parameter auf dem einfach messbaren intrinsischen PL- Spektrum und dem makroskopischen Absorptionskoeffizienten des jeweiligen Films beruhen. In Kapitel6 wird die umfangreiche photophysikalische Untersuchung eines neuartigen kompakten blauen OLED-Emitters, SBABz4, welcher Spiro-Biacridine und Benzonitril-Einheiten enthält, beschrieben. Auch sein Gegenstück DMABz4, als einfacher Donator, wurde betrachtet, um Zusammenhänge zwischen Struktur und Materialeigenschaften hervorzuheben. Beide Verbindungen zeigten thermisch-aktivierte verzögerte Fluoreszenz (TADF), welche unabhängig voneinander sowohl durch Sauerstoff- Fluoreszenzlöschung als auch durch temperaturabhängige transiente PL-Messungen nachgewiesen wurde. Die Spiro-Bindung im Inneren des zweifachen Donators SBABz4 führten zu einer, im Vergleich zur blaugrünen Emission des einfachen Donators DMABz4, reinen blauen Lumineszenz. Im Gegensatz zum Aufbau üblicher TADF-Molekülen mit zentralem Akzeptor, erlaubt in diesem Fall der zentrale Donator also die gewünschte Farbeinstellung im tiefblauen Bereich. Mit Hilfe von PL-Lebensdauern und -Effizienzen wurde eine EQEmax von 7.1% für SBABz4-basierte OLEDs abgeschätzt, während ein reales Testexemplar eine EQEmax von 6.8% aufzeigte. Transiente PL wurde für Lösungen sowie für feste Filme in einem beispiellos großen, dynamischen Bereich von sechs Größenordnungen in Zeit und acht Größenordnungen in Intensität aufgenommen. Die transiente PL der Filme lässt sich gut durch eine gestreckte Exponentialfunktion anpassen, während der PL-Abfall der Lösung rein exponentiell verläuft. Die Einbettung des Emitters in der Gast-Matrix, die Aggregieren verhinderte, führten zu gegenüber dem reinen SBABz4-Film überlegenen TADF- Eigenschaften. Zuletzt wurde die TADF Aktivierungsenergie von 70 meV unter alleiniger Verwendung der temperaturabhängigen transienten PL berechnet. Zusammengefasst steuert diese Doktorarbeit einen Beitrag zu zwei der faszinierendsten Themen der Materialforschung des letzten Jahrzehnts bei: Perowskitabsorbern für die Photovoltaik und TADF-Emittern für OLEDs. Diese Arbeit erlaubte es mit aufkommenden Systemen zu arbeiten und neue Modelle aus bekannten physikalischen Konzepten für sie zu entwickeln. Dies war sowohl spannend als auch anspruchsvoll. Letztlich ist Forschung an neuartigen Materialien immer ein großes Durcheinander. Hoffentlich konnte durch diese Arbeit jedoch ein wenig mehr Klarheit geschaffen werden.

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