For more than 20 years, scientists have studied solar cells made from organic semiconductors. Throughout this time, device structures have evolved from bilayer devices to bulk heterojunction (BHJ) devices and even though efficiencies are approaching 10%, scientists still know relatively little about the transport of charge carriers and recombination mechanisms in these materials. Novel structures, based on lateral BHJ solar cells, have proven to be versatile tools to study transport and recombination mechanisms. In addition, these structures can easily be employed by researchers and solar cell manufacturers to determine the quality and measure the improvement of their materials. For these studies, poly(3-hexylthiophene) (P3HT):[6,6]-phenyl C61-butyric acid methyl ester (PCBM) has been employed due to its wide use among researchers as well as potential for commercialization. DC photocurrent measurements as a function of device length have yielded the mobility-lifetime product and the generation rate of free carriers within these BHJ devices. In addition to these parameters, the recombination rate as a function of light intensity provides information about the mechanisms of recombination. For example, by measuring the recombination rate as a function of applied electric field and light intensity we have found that recombination is unimolecular in nature and shifts to bimolecular at increased electric field strengths. Additionally, the mobility-lifetime product, generation rate, and recombination mechanism have been studied as a function of applied electric field, illumination spectrum, illumination intensity, etc. This information has provided much insight on physics of the P3HT:PCBM material system which did not exist before these studies. / text

Organische Halbleiter besitzen neue, bemerkenswerte Materialeigenschaften, die sie für die grundlegende Forschung wie auch aktuelle technologische Entwicklung (bsw. org. Leuchtdioden, org. Solarzellen) interessant werden lassen. Aufgrund der starken konformative Freiheit der konjugierten Polymerketten führt die Vielzahl der möglichen Anordnungen und die schwache intermolekulare Wechselwirkung für gewöhnlich zu geringer struktureller Ordnung im Festkörper. Die Morphologie hat gleichzeitig direkten Einfluss auf die elektronische Struktur der organischen Halbleiter, welches sich meistens in einer deutlichen Reduktion der Ladungsträgerbeweglichkeit gegenüber den anorganischen Verwandten zeigt. So stellt die Beweglichkeit der Ladungen im Halbleiter einen der limitierenden Faktoren für die Leistungsfähigkeit bzw. den Wirkungsgrad von funktionellen organischen Bauteilen dar. Im Jahr 2009 wurde ein neues auf Naphthalindiimid und Bithiophen basierendes Dornor/Akzeptor Copolymer vorgestellt [P(NDI2OD‑T2)], welches sich durch seine außergewöhnlich hohe Ladungsträgermobilität auszeichnet. In dieser Arbeit wird die Ladungsträgermobilität in P(NDI2OD‑T2) bestimmt, und der Transport durch eine geringe energetischer Unordnung charakterisiert. Obwohl dieses Material zunächst als amorph beschrieben wurde zeigt eine detaillierte Analyse der optischen Eigenschaften von P(NDI2OD‑T2), dass bereits in Lösung geordnete Vorstufen supramolekularer Strukturen (Aggregate) existieren. Quantenchemische Berechnungen belegen die beobachteten spektralen Änderungen. Mithilfe der NMR-Spektroskopie kann die Bildung der Aggregate unabhängig von optischer Spektroskopie bestätigt werden. Die Analytische Ultrazentrifugation an P(NDI2OD‑T2) Lösungen legt nahe, dass sich die Aggregation innerhalb der einzelnen Ketten unter Reduktion des hydrodynamischen Radius vollzieht. Die Ausbildung supramolekularen Strukturen nimmt auch eine signifikante Rolle bei der Filmbildung ein und verhindert gleichzeitig die Herstellung amorpher P(NDI2OD‑T2) Filme. Durch chemische Modifikation der P(NDI2OD‑T2)-Kette und verschiedener Prozessierungs-Methoden wurde eine Änderung des Kristallinitätsgrades und gleichzeitig der Orientierung der kristallinen Domänen erreicht und mittels Röntgenbeugung quantifiziert. In hochauflösenden Elektronenmikroskopie-Messungen werden die Netzebenen und deren Einbettung in die semikristallinen Strukturen direkt abgebildet. Aus der Kombination der verschiedenen Methoden erschließt sich ein Gesamtbild der Nah- und Fernordnung in P(NDI2OD‑T2). Über die Messung der Elektronenmobilität dieser Schichten wird die Anisotropie des Ladungstransports in den kristallographischen Raumrichtungen von P(NDI2OD‑T2) charakterisiert und die Bedeutung der intramolekularen Wechselwirkung für effizienten Ladungstransport herausgearbeitet. Gleichzeitig wird deutlich, wie die Verwendung von größeren und planaren funktionellen Gruppen zu höheren Ladungsträgermobilitäten führt, welche im Vergleich zu klassischen semikristallinen Polymeren weniger sensitiv auf die strukturelle Unordnung im Film sind. / Organic semiconductors are in the focus of recent research and technological development (eg. for organic light-emitting diodes and solar cells) due to their specific and outstanding material properties. The strong conformational freedom of conjugated polymer chains usually leads to a large number of possible geometric arrangements while weak intermolecular interactions additionally lead to poor structural order in the solid state. At the same time the morphology of those systems has direct influence on the electronic structure of the organic semiconductor which is accompanied by a significant reduction of the charge carrier mobility in contrast to their inorganic counterparts. In that way the transport of charges within the semiconductor represents one of the main limiting factors regarding the performance and efficiency of functional organic devices. In 2009 Facchetti and coworkers presented a novel conjugated donor/acceptor copolymer based on naphthalene diimide and bithiophene [P(NDI2OD‑T2)] which was characterized by an outstanding charge carrier mobility. In this work the mobility of electrons and holes in the bulk of P(NDI2OD‑T2) is determined by single carrier devices and the time-of-flight technique. The results imply a low energetic disorder in these polymer layers. While the material was initially expected to be mainly amorphous, a detailed study of the photophysical properties of P(NDI2OD‑T2) shows that precursors of supramolecular assemblies (aggregates) are already formed in polymer solution. Quantum-chemical calculations support the occurring optical changes. NMR spectroscopy was applied to independently prove the formation of chain aggregates in commonly used organic solvents. The investigation of P(NDI2OD‑T2) solutions by analytical ultracentrifugation implies that aggregation mainly proceeds within single polymer chains by reduction of the hydrodynamic radius. To understand the influence of the chemical structure, pre-aggregation and crystal packing of conventional regioregular P(NDI2OD-T2) on the charge transport, the corresponding regioirregular polymer RI-P(NDI2OD-T2) was synthesized. By combining optical, X-ray, and transmission electron microscopy data, a quantitatively characterization of the aggregation, crystallization, and backbone orientation of all of the polymer films was possible, which was then correlated to the electron mobilities in electron-only diodes. The anisotropy of the charge transport along the different crystallographic directions is demonstrated and how the mobility depends on π-stacking but is insensitive to the degree or coherence of lamellar stacking. The comparison between the regioregular and regioirregular polymers also shows how the use of large planar functional groups leads to improved charge transport, with mobilities that are less affected by chemical and structural disorder with respect to classic semicrystalline polymers such as poly(3-hexylthiophene).

Melanins are a class of bio-macromolecules that are found throughout the biosphere. They fulfill various functions in human beings, which makes them a long studied substance in medicine and biology. Furthermore they possess a set of rare and special physico-chemical properties which include featureless broad band absorption in the UV-Vis spectrum and condensed phase electrical conduction. Many scientists have interpreted their findings in terms of an amorphous semiconductor model, but this was done under the a priori assumption that charge transport in melanin is electronic. However, a very strong dependence of melanin’s electrical properties on its level of hydration has recently led to speculations that the dominant charge carrier for high hydration is of protonic rather than electronic nature. This thesis will present direct evidence for electronic charge transport, found by investigating the influence of different environmental parameters on the conductivity of melanin. It will furthermore be shown that the hydration dependent conductivity of melanin can be understood in terms of a dielectric response model for an amorphous semiconductor. This establishment of the major charge carrier is an important step in the on-going effort to fully map the structure-property relationship of melanin and will help to understand its function in vivo. With the ultimate goal to make use of melanin’s fascinating properties, thin films, a new class of device has been characterized and investigated. These thin films, while exhibiting melanin’s characteristics, show improved mechanical stability, a very uniform morphology and a much faster response than standard pellet samples. They are furthermore applicable to standard polymer processing techniques which brings technological applications within reach.

abstract: Studying charge transport through single molecules tethered between two metal electrodes is of fundamental importance in molecular electronics. Over the years, a variety of methods have been developed in attempts of performing such measurements. However, the limitation of these techniques is still one of the factors that prohibit one from gaining a thorough understanding of single molecule junctions. Firstly, the time resolution of experiments is typically limited to milli to microseconds, while molecular dynamics simulations are carried out on the time scale of pico to nanoseconds. A huge gap therefore persists between the theory and the experiments. This thesis demonstrates a nanosecond scale measurement of the gold atomic contact breakdown process. A combined setup of DC and AC circuits is employed, where the AC circuit reveals interesting observations in nanosecond scale not previously seen using conventional DC circuits. The breakdown time of gold atomic contacts is determined to be faster than 0.1 ns and subtle atomic events are observed within nanoseconds. Furthermore, a new method based on the scanning tunneling microscope break junction (STM-BJ) technique is developed to rapidly record thousands of I-V curves from repeatedly formed single molecule junctions. 2-dimensional I-V and conductance-voltage (G-V) histograms constructed using the acquired data allow for more meaningful statistical analysis to single molecule I-V characteristics. The bias voltage adds an additional dimension to the conventional single molecule conductance measurement. This method also allows one to perform transition voltage spectra (TVS) for individual junctions and to study the correlation between the conductance and the tunneling barrier height. The variation of measured conductance values is found to be primarily determined by the poorly defined contact geometry between the molecule and metal electrodes, rather than the tunnel barrier height. In addition, the rapid I-V technique is also found useful in studying thermoelectric effect in single molecule junctions. When applying a temperature gradient between the STM tip and substrate in air, the offset current at zero bias in the I-V characteristics is a measure of thermoelectric current. The rapid I-V technique allows for statistical analysis of such offset current at different temperature gradients and thus the Seebeck coefficient of single molecule junctions is measured. Combining with single molecule TVS, the Seebeck coefficient is also found to be a measure of tunnel barrier height. / Dissertation/Thesis / Ph.D. Electrical Engineering 2012

abstract: Charge transport in molecular systems, including DNA (Deoxyribonucleic acid), is involved in many basic chemical and biological processes. Studying their charge transport properties can help developing DNA based electronic devices with many tunable functionalities. This thesis investigates the electric properties of double-stranded DNA, DNA G-quadruplex and dsDNA with modified base. First, double-stranded DNA with alternating GC sequence and stacked GC sequence were measured with respect to length. The resistance of DNA sequences increases linearly with length, indicating a hopping transport mechanism. However, for DNA sequences with stacked GC, a periodic oscillation is superimposed on the linear length dependence, indicating a partial coherent transport. The result is supported by the finding of delocalization of the highest occupied molecular orbitals of Guanines from theoretical simulation and by fitting based on the Büttiker’s theory. Then, a DNA G4-duplex structures with a G-quadruplex as the core and DNA duplexes as the arms were studied. Similar conductance values were observed by varying the linker positions, thus a charge splitter is developed. The conductance of the DNA G-tetrads structures was found to be sensitive to the π-stacking at the interface between the G-quadruplex and DNA duplexes by observing a higher conductance value when one duplex was removed and a polyethylene glycol (PEG) linker was added into the interface. This was further supported by molecular dynamic simulations. Finally, a double-stranded DNA with one of the bases replaced by an anthraquinone group was studied via electrochemical STM break junction technique. Anthraquinone can be reversibly switched into the oxidized state or reduced state, to give a low conductance or high conductance respectively. Furthermore, the thermodynamics and kinetics properties of the switching were systematically studied. Theoretical simulation shows that the difference between the two states is due to a difference in the energy alignment with neighboring Guanine bases. / Dissertation/Thesis / Doctoral Dissertation Chemistry 2016

São apresentadas as soluções aproximadas de alguns problemas de transporte de carga em dielétricos, inexpugnáveis ainda a um tratamento rigoroso, usando-se o Método de Galerkin. Com ele reduz-se o sistema de equações a derivadas parciais, que descrevem o transporte na presença de armadilhas, em um sistema de equações diferenciais ordinárias que são, então, integradas numericamente. Sempre que possível, a solução aproximada é comparada com alguma exata ou quase-exata, como a que se obtém da integração numérica direta do sistema de equações a derivadas parciais com o Método das Diferenças Finitas. Três diferentes condições de contorno são empregadas aqui: circuito aberto, curto circuito e circuito fechado com uma voltagem aplicada entre os eletrodos; em alguns casos considera-se temperatura variável. Este método requer que seja escollhida a priori, a forma da distribuição de carga livre; verifica-se que a corrente é mais sensível a esta distribuição do que o potencial de superfície, que sempre resulta muito próximo do exato, mesmo quando a aproximação parece grosseira. / Approximate solutions for some problems of charge transport in dielectrics, unsolved yet by exact methods, are presented using Galerkin\'s Method. This allows to transforming the system of partial differential equations, describing transport with trapping, into a system of ordinary differential equations which are, then, integrated numerically. Whenever possible, a comparison is made between this approximate solution with some exact or quasi-exact solution as, for example, that obtained from the direct numerical integrated of the system of partial differential equations using the Finite Difference Method. Three different boundary conditions are considered here: open circuit, short circuit and closed circuit with a voltage applied between the electrodes; in some cases the temperature was allowed to vary. Use of Galerkin\'s Method requires a priori choice of the free charge distribution; there results that the current is more sensitive to this distribution than the surface potential which leads to good results even when the approximation seems crude.

L'isolement du graphène a suscité un grand intérêt vers la recherche d’applications potentielles de ce matériau unique et d'autres matériaux bidimensionnels (2D) pour l'électronique, l'optoélectronique, la spintronique et de nombreux autres domaines. Par rapport au graphène, les dichalcogenides de métaux de transition (TMD) 2D offrent l'avantage d'être des semi-conducteurs, ce qui permettrait de les utiliser pour des circuits logiques. Au cours des dix dernières années, de nombreux développements ont déjà été réalisés dans ce domaine où les opportunités et les défis coexistent. Cette thèse présente les résultats de simulations de transport quantique d’une nouvelle structure de dispositif logique à très faible consommation à base de matériaux bidimensionnels : le transistor à effet tunnel à base d’hétérostructures verticales de TMDs 2D. A cause de leur petite taille, ces dispositifs sont intrinsèquement dominés par des effets quantiques. Par conséquent, l’adoption d’une théorie générale du transport s’impose. Le choix se porte ici sur la méthode des fonctions de Green hors équilibre (NEGF), une approche largement utilisée pour la simulation du transport électronique dans les nanostructures. Dans la première partie de cette thèse, les matériaux 2D, leur synthèse et leurs applications sont brièvement introduits. Ensuite, le formalisme NEGF est illustré. Cette méthode est ensuite utilisée pour la simulation de deux structures de transistor à effet tunnel vertical basées sur l’hétérojonction van der Waals de Mos2 et WTe2. La description du système se base sur un modèle de masse effective calibré avec des résultats ab-initio (afin de reproduire la structure de bandes dans l’intervalle d’énergie intéressé par les simulations de transport) et aux mesures expérimentales de mobilité (pour le couplage électron-phonon). Les résultats non seulement démontrent la possibilité d’obtenir une forte pente sous seuil avec ce type de transistors, mais présentent une étude de la physique qui en détermine les performances en fonction de leur géométrie et de l’interaction entre électrons et phonons. Dans la dernière partie, les effets du malignement rotationnel entre les deux couches 2D sont investigués. Expérimentalement, ce type de désordre est difficile à éviter et peut considérablement affecter les performances du transistor. Par le moyen de simulations quantiques précises et d’analyses physiques, cette thèse montre les défis à relever dans la conception des transistors à effet tunnel à base de matériaux 2D performants. / The successful isolation of graphene in 2004 has attracted great interest to search for potential applications of this unique material and other newborn members of the two-dimensional (2D) family in electronics, optoelectronics, spintronics and other fields. Compared to graphene, the 2D transition metal dichalcogenides (TMDs) have the advantage of being semiconductors, which would allow their use for logic devices. In the past ten years, significant developments have been made in this area, where opportunities and challenges co-exist.This thesis presents the results of quantum transport simulations of novel 2D-material-based tunnel field-effect transistors for ultra-low-power digital applications. Due to their size, such devices are intrinsically dominated by quantum effects. This requires the adoption of a fairly general theory of transport, such as the nonequilibrium Green's functions (NEGF) formalism, which is a method extensively used for the simulation of electron transport in nanostructures.In the first part of this thesis, a brief introduction about the 2D materials, their synthesis and applications is presented. Then, the NEGF formalism is concisely reviewed. This approach is applied to the simulation of two different models of vertical tunnel field-effect transistors based on 2D-TMD van der Waal heterojunctions (MoS2 and WTe2). To properly describe the system, a coupled effective mass Hamiltonian has been implemented and carefully calibrated to experimental measurements and density functional theory to reproduce the band structure in the energy range of interest for the simulations.This thesis not only demonstrates the ultra-steep subthreshold slope potentially expected for these devices, but also provides a physical insight into the impact of the transistor geometry on its performances. In the last and more exploratory part of the manuscript, the effect of rotational misalignment within the two layers of the heterostructure is investigated. Experimentally, such a disorder is difficult to avoid, and it can substantially affect the device performances.Through accurate quantum simulations and deep physical analysis, this study sheds light on the design challenges to be addressed for the development of efficient tunnel field-effect transistors based on 2D materials.

The goal of this thesis is to generate fundamental understandings of charge transport behaviors of composites consisting of garnet structured Al substituted Li7La3Zr2O12 (LLZO) electrolyte and LiCoO2 electrode. In order to take full advantage of all-solid-state batteries, bulk type composite electrodes should be introduced to increase energy and power density. However, the charge utilization of bulk type composite electrodes is quite low. Understanding ionic conduction behavior is, therefore, important for improving the performance of all-solid-state batteries, because ion conduction within solids depends on effective pathways. Electronic conductivity can be easily compensated by adding carbon black, but ionic conductivity can only depend on composites electrode itself. Here, we show that electronic and ionic conductivities of composites consisting of LiCoO2 and Al doped LLZO can be achieved separately. 3D reconstructed image obtained from focused ion beam-scanning electron microscope (FIB-SEM) demonstrates that porosity, percolation, and grain boundaries often play antagonistic roles in controlling the charge transport behaviors in the composite electrodes, resulting in an overall conductivity dominated by electrons. This work suggests an approach to optimize electronic and ionic conductivities for bulk type composite electrodes, which may eventually be utilized in all-solid-state batteries.

The Thesis reports charge transport studies on conducting polymers, polymer carbon nanotube composites and organic semiconductor devices. Conducting and semiconducting polymers consisting of π-conjugated chains have attracted considerable attention as they combine the optoelectronic properties of semiconductors with mechanical properties and processing advantages of plastics. The chemical/electrochemical/photodoping of these semiconducting polymers can tune the Fermi levels and conductivity in a controlled way, and hence the properties of devices can be easily tailored to suit in several applications. Carbon nanotube (CNT) is another another novel promising material for electronic/optoelectronic applications. Lately there has been a great interest in developing composites of polymer and CNTs to utilize the advantages of both CNTs and polymers. The inclusion of CNTs in polymers improves the mechanical, electrical and thermal properties since the aspect ratio (ratio of length to diameter) is very large, as well its density is rather low. The Thesis consists of 6 chapters. First chapter is a brief introduction of general and transport properties of conducting polymers and polymer-carbon nanotube composites. In Chapter 2, the sample preparation and experimental techniques used in this work are discussed. The charge transport in poly(3,4-ethylenedioxythiophene)-poly(styrenesulfonate) (PEDOT-PSS) is presented in Chapter 3. Chapter 4 focuses on the transport measurements in the polymer-CNT composite samples. Chapter 5 elaborates the ac and dc characterization of organic field-effect transistors (OFETs). And chapter 6 presents the conclusion and future directions of the work that has been presented in the Thesis. Chapter 1: In the scientific and technological revolution of the last few years, the study of high performance materials has been steadily increasing including the study of carbon-based materials. Conducting polymers have special properties that are interesting for this new technology. The charge transport in conjugated polymers is important to optimize the performance of devices. The discovery of CNTs with exceptional thermal, mechanical, optical, electrical and structural properties has facilitated the synthesis of new type of nanocomposites with very interesting properties. Nanocomposites represent a guest-host matrix consisting of easily processible functionalized conjugated polymer as host, incorporating CNTs as fillers with versatile electronic and magnetic properties, which provide a wide range of technological applications. To optimize their electrical properties it is essential to understand the charge transport mechanism in detail. Chapter 2: The multi-wall carbon nanotubes (MWNTs) grown by thermal chemical vapor deposition (CVD) are mixed with a 1:1 mixture of 98% H2SO4 and 70% HNO3 to produce sulfonic acid functionalized multi-wall carbon nanotubes (s-MWNTs). The s-MWNTs are dispersed in a solution of Nafion by ultrasonication and then cast on a glass substrate and slowly dried by moderate heating to obtain the composite films. Polyaniline (PANI)-MWNT composites were obtained by carrying out the chemical synthesis of nanofibrilar PANI in the presence of CNTs. This water dispersible PANIMWNT composite contains well segregated MWNTs partially coated by nanofibrilar PANI. The ac and dc charge transport measurements suggest hopping transport in these materials. OFETs are fabricated with pentacene, poly(2,5-bis(3-tetradecylthiophen-2-yl)thieno[3,2-b]thiophene)(PBTTT) and poly(3-hexylthiophene) (P3HT) as active materials. A novel technique is used to characterize the acphotoresponse of these OFETs. Chapter 3: Charge transport studies on PEDOT-PSS have been carried out and found that it correlates with the morphology. The dc conductivity of PEDOT–PSS shows enhanced delocalization of the carriers upon the addition of dimethyl sulfoxide (DMSO) and this is attributed to the extended chain conformation. PEDOT-PSS is known to form a phase-segregated material comprising highly conducting PEDOT grains that are surrounded by a sea of weakly ionic-conducting PSS and a wide variation in the charge transport properties of PEDOT-PSS films is attributed to the degree of phasesegregation of the excess insulating polyanion. The magnetotransport and temperature dependent ac transport parameters across different conducting grades of PEDOT-PSS processed with DMSO were compared. Depending on the subtle alterations in morphology, the transport at low temperatures is shown to vary from the hopping regime (Baytron P) to critical regime of the metal-insulator transition (Baytron PH510) There is a significant positive magnetoresistance (MR) for P–films, but this is considerably less in case of PH510-film. From the low temperature ac conductance it is found that the onset frequency for PH510 is nearly temperature independent, whereas in P type it is strongly temperature dependent, again showing the superior transport in PH510. The presence of ‘shorter network connections’ together with a very weak temperature dependence down to ~ 5 K, suggest that the limitation on transport in PH510 arises from the connectivity within the PEDOT-rich grain rather than transport via the PSS barriers. Chapter 4: DC and AC charge transport properties of Nafion s-MWNT and PANI-MWNT composites are studied. Such a detailed investigation is required to optimize the correlation among morphology and transport properties in these composites towards applications in field-effect transistors, antistatic coating, electromagnetic shielding, etc. The conductivity in Nafion s-MWNT shows a percolative transport with percolation threshold pc = 0.42 whereas such a sharp percolation is absent in PANI-MWNT composite since the conduction via PANI matrix smears out the onset of rapid increase in conductivity. Three-dimensional variable range hopping (VRH) transport is observed in Nafion s-MWNT composites. The positive and negative MR data on 10 wt. % sample are analyzed by taking into account forward interference mechanism (negative MR) and wave-function shrinkage (positive MR), and the carrier scattering is observed to be in the weak limit. The electric-field dependence, measured to high fields, follows the predictions of hopping transport in high electric-field regime. The ac conductivity in 1 wt. % sample follows a power law: ( )  A s , and s decreases with increasing temperature as expected in the correlated barrier hopping (CBH) model. In general, Mott’s VRH transport is observed in PANI-MWNT samples. It is found that the MWNTs are sparingly adhered with PANI coatings, and this facilitates inter-tube hopping at low temperatures. The negative MR of MWNT-PANI composites suggest that the electronic transport at low temperatures is dominated by MWNT network. AC impedance measurements at low temperatures with different MWNT loading show that ac conductivity become temperature independent as the MWNT content increases. The onset frequency for the increase in conductivity is observed to be strongly dependent on the MWNT weight percentage, and the ac conductivity can be scaled onto a master curve given by  ( )  0[1 k( 0 )s ]. Chapter 5: Organic field-effect transistors (OFETs) based on small molecules and polymers have attracted considerable attention due to their unique advantages, such as low cost of fabrication, ease of processing and mechanical flexibility. Impedance characterization of these devices can identify the circuit elements present in addition to the source-drain (SD) channel, and the bottlenecks in charge transport can be identified. The charge carrier trapping at various interfaces and in the semiconductor can be estimated from the dc and ac impedance measurements under illumination. The equivalent circuit parameters for a pentacene OFET are determined from low frequency impedance measurements in the dark as well as under light illumination. The charge accumulation at organic semiconductor–metal interface and dielectric semiconductor interface is monitored from the response to light as an additional parameter to find out the contributions arising from photovoltaic and photoconductive effects. The shift in threshold voltage is due to the accumulation of photogenerated carriers under SD electrodes and at dielectric–semiconductor interface, and also this dominates the carrier transport. Similar charge trapping is observed in an OFET with PBTTT as the active material. This novel method can be used to differentiate the photophysical phenomena occurring in the bulk from that at the metal-semiconductor interface for the polymer. Chapter 6: The conclusions from the various works presented in the thesis are coherently summarized in this chapter. Thoughts for future directions are also summed up.

Insight into the nanoscale carrier transport in the rapidly developing class of solutionprocessed semiconductors known as metal halide perovskites is the focal point for these studies. Further advancement in fundamentally understanding photophysical processes associated with charge carrier transport is needed to realize the true potential of perovskites for photovoltaic applications. In this work, we study photogenerated carrier transport to understand the underlying transport behavior of the material on the 10s to 100s nanometer lengthscales. To study these processes, we employ a temporally-resolved and spatially-resolved technique, known as transient absorption microscopy, to elucidate the charge carrier dynamics and propagation associated with metal halide perovskites. This technique provides a simultaneous high temporal resolution (200 fs) and spatial resolution (50 nm) to allow for direct visualization of charge carrier migration on the nanometer length scale. There are many obstacles these carriers encounter between photogeneration and charge collection such as morphological effects (grain boundaries) and carrier interactions (scattering processes). We investigate carrier transport on the nanoscale to understand how morphological effects influence the materials transport behavior. Morphological defects such as voids and grain boundaries are inherently small and traditionally difficult to study directly. Further, because carrier cooling takes place on an ultrafast time scale (fs to ps), the combined spatial and temporal resolution is necessary for direct probing of hot (non-equilibrium) carrier transport. Here we investigate a variety of ways to enhance carrier transport lengthscales by studying how non-equilibrium carriers propagate throughout the material, as well as, carrier cooling mechanisms to extend the non-equilibrium regime. For optoelectronic devices based on polycrystalline semiconducting thin films, grain boundaries are important to consider since solution-based processing results in the formation of well-defined grains. In Chapter 3, we investigate equilibrium carrier transport in metal halide perovskite thin films that are created via the highly desired solution processing method. Carrier transport across grain boundaries is an important process in defining efficiency due to the literary discrepancies on whether the grains limit carrier transport or not. In this work, we employ transient absorption microscopy to directly measure carrier transport within and across the boundaries. By selectively imaging sub-bandgap states, our results show that lateral carrier transport is slowed down by these states at the grain boundaries. However, the long carrier lifetimes allow for efficient transport across the grain boundaries. The carrier diffusion constant is reduced by about a factor of 2 for micron-sized grain samples by the grain boundaries. For grain sizes on the order of ∼200 nm, carrier transport over multiple grains has been observed within a time window of 5 ns. These observations explain both the shortened photoluminescence lifetimes at the boundaries as well as the seemingly benign nature of the grain boundaries in carrier generation. The results of this work provide insight into why this defect tolerant material performs so well. Photovoltaic performance (power conversion efficiency) is governed by the ShockleyQueisser limit which can be overcame if hot carriers can be harvested before they thermalize. To convert sunlight to usable electricity, the photogenerated charge carriers need to migrate long distances and or live long enough to be collected. It is unclear whether these hot carriers can migrate a long enough distance for efficient collection. In Chapter 4, we report direct visualization of hot-carrier migration in methylammonium lead iodide (CH3NH3PbI3) thin films by ultrafast transient absorption microscopy. This work demonstrates three distinct transport regimes. (i) Quasiballistic transport, (ii) nonequilibrium transport, and (iii) diffusive transport. Quasiballistic transport was observed to correlate with excess kinetic energy, resulting in up to 230 nanometers of transport distance that could overcome grain boundaries. The nonequilibrium transport persisted over tens of picoseconds and ~600 nanometers before reaching the diffusive transport limit. These results suggest potential applications of hot-carrier devices based on hybrid perovskites to ultimately overcome the Shockley-Queisser limit. In the next work, we investigated a way to extend non-equilibrium carrier lifetime, which ultimately corresponds to an accelerated carrier transport. From the knowledge of the hot carrier transport work, we showed a proof of concept that the excess kinetic energy corresponds to long range carrier transport. To further develop the idea of harvesting hot carriers, one must investigate a way to make the carriers stay hot for a longer period (i.e. cool down slower). In Chapter 5, we slow down the cooling of hot carriers via a phonon bottleneck, which points toward the potential to overcome the Shockley-Queisser limit. Open questions remain on whether the high optical phonon density from the bottleneck impedes the transport of these hot carriers. We show a direct visualization of hot carrier transport in the phonon bottleneck regime in both single crystalline and polycrystalline lead halide perovskites, more specifically, a relatively new class of alkali metal doped perovskites (RbCsMAFA), which has one of the highest power conversion efficiencies. Remarkably, hot carrier diffusion is enhanced by the presence of a phonon bottleneck, the exact opposite from what is observed in conventional semiconductors such as GaAs. These results showcase the unique aspects of hot carrier transport in hybrid perovskites and suggest even larger potential for hot carrier devices than previously envisioned by the initial results presented in Chapter 4. The final chapter will be divided into two sections, as we summarize and highlight our collaborative efforts towards homogenization of carrier dynamics via doping perovskites with alkali metals and our work on two-dimensional hybrid quantum well perovskites. Further studies on the champion solar cell (RbCsMAFA) were performed to elucidate the role inorganic cations play in this material. By employing transient absorption microscopy, we show that alkali metals Rb+ and Cs+ are responsible for inducing a more homogenous halide (Iand Br- ) distribution, despite the partial incorporation into the perovskite lattice. This translates into improved electronic dynamics, including fluorescence lifetimes above 3 µs and homogenous carrier dynamics, which was visualized by ultrafast microscopy. Additionally, there is an improvement in photovoltaic device performance. We find that while Cs cations tend to distribute homogenously across the perovskite grain, Rb and K cations tend to phase segregate at precursor concentrations as low as 1%. These precipitates have a counter-productive effect on the solar cell, acting as recombination centers in the device, as argued from electron beam-induced current measurements. Remarkably, the high concentration of Rb and Cs agglomerations do not affect the open-circuit voltage, average lifetimes, and photoluminescence distribution, further indicating the perovskite’s notorious defect tolerance. A new class of high-quality two dimensional organic-inorganic hybrid perovskite quantum wells with tunable structures and band alignments was studied. By tuning the functionality of the material, the strong self-aggregation of the conjugated organic molecules can be suppressed, and 2D organic-halide perovskite superlattice crystals and thin films can be easily obtained via onestep solution-processing. We observe energy transfer and charge transfer between adjacent organic and inorganic layers, which is extremely fast and efficient (as revealed by ultrafast spectroscopy characterizations). Remarkably, these 2D hybrid perovskite superlattices are stable, due to the protection of the bulky hydrophobic organic groups. This is a huge step towards the practicality of using perovskites for optoelectronics, since stability is always a huge concern with water-sensitive materials. The molecularly engineered 2D semiconductors are on par with III-V quantum wells and are promising for next-generation electronics, optoelectronics, and photonics.