Thesis (Ph. D.)--University of Chicago, Dept. of Chemistry, March 1999. / Includes bibliographical references. Also available on the Internet.

<div>Recently, atomically thin two-dimensional (2D) layered materials such as graphene and transition metal dichalcogenides (TMDCs) have emerged as a new class of materials due to their unique electronic structures and optical properties at the nanoscale limit. 2D materials also hold great promises as building blocks for creating new heterostructures for optoelectronic applications such as atomically thin photovoltaics, light emitting diodes, and photodetectors. Understanding the fundamental photo-physics process in 2D semiconductors and heterostructures is critical for above-mentioned applications. </div><div>In Chapter 1, we briefly describe photo-generated charge carriers in two-dimensional (2D) transition metal dichalcogenides (TMDCs) semiconductors and heterostructures. Due to the reduced dielectric screening in the single-layer or few-layer of TMDCs semiconductors, Columbo interaction between electron and hole in the exciton is greatly enhanced that leads to extraordinary large exciton binding energy compared with bulk semiconductors. The environmental robust 2D excitons provide an ideal platform to study exciton properties in TMDCs semiconductors. Since layers in 2D materials are holding by weak van de Waals interaction, different 2D layers could be assembled together to make 2D heterostructures. The successful preparation of 2D heterostructures paves a new path to explore intriguing optoelectronic properties.</div><div>In Chapter 2, we introduce various optical microscopy techniques used in our work for the optical characterization of 2D semiconductors and heterostructures. These optical imaging tools with high spatial and temporal resolution allow us to directly track charge and energy flow at 2D interfaces.</div><div>Exciton recombination is a critical factor in determining the efficiency for optoelectronic applications such as semiconductor lasers and light-emitting diodes. Although exciton dynamics have been investigated in different 2D semiconductor, large variations in sample qualities due to different preparation methods have prevented obtaining intrinsic exciton lifetimes from being conclusively established. In Chapter 3, we study exciton dynamics in 2D TMDCs semiconductors using ultrafast PL and transient absorption microscopy. Here we employ 2D WS2 semiconductor as a model system to study exciton dynamics due to the low defect density and high quantum yield of WS2. We mainly focus on how the exciton population affects exciton dynamics. At low exciton density regime, we demonstrate how the interlayer between the bright and dark exciton populations influence exciton recombination. At high exciton density regime, we exhibit significant exciton-exciton annihilation in monolayer WS2. When comparing with the bilayer and trilayer WS2, the exciton-exciton annihilation rate in monolayer WS2 increases by two orders of magnitude due to enhanced many-body interactions at single layer limit. </div><div>Long-range transport of 2D excitons is desirable for optoelectronic applications based on TMDCs semiconductors. However, there still lacks a comprehensive understanding of the intrinsic limit for exciton transport in the TMDCs materials currently. In Chapter 4, we employ ultrafast transient absorption microscopy that is capable of imaging excitons transport with ~ 200 fs temporal resolution and ~ 50 nm spatial precision to track exciton motion in 2D WS2 with different thickness. Our results demonstrate that exciton mobility in single layer WS2 is largely limited by extrinsic factors such as charge impurities and surface phonons of the substrate. The intrinsic phonon-limited exciton transport is achieved in WS2 layers with a thickness greater than 20 layers.</div><div>Efficient photocarrier generation and separation at 2D interfaces remain a central challenge for many optoelectronic applications based on 2D heterostructures. The structural tunability of 2D nanostructures along with atomically thin and sharp 2D interfaces provides new opportunities for controlling charge transfer (CT) interactions at 2D interfaces. A largely unexplored question is how interlayer CT interactions contribute to interfacial photo-carrier generation and separation in 2D heterostructures. In Chapter 5, we present a joint experimental and theoretical study to address carrier generation from interlayer CT transitions in WS2-graphene heterostructures. We use spatially resolved ultrafast transient absorption microscopy to elucidate the role of interlayer coupling on charge transfer and photo-carrier generation in WS2-graphene heterostructures. These results demonstrate efficient broadband photo-carrier generation in WS2-graphene heterostructures which is highly desirable for atomically thin photovoltaic and photodetector applications based on graphene and 2D semiconductors.</div><div>CT exciton transport at heterointerfaces plays a critical role in light to electricity conversion using 2D heterostructures. One of the challenges is that direct measurements of CT exciton transport require quantitative information in both spatial and temporal domains. In order to address this challenge, we employ transient absorption microscopy (TAM) with high temporal and spatial resolution to image both bright and dark CT excitons in WS2-tetrance and CVD WS2-WSe2 heterostructure. In Chapter 6, we study the formation and transport of interlayer CT excitons in 2D WS2-Tetracene vdW heterostructures. TAM measurements of CT exciton transport at these 2D interfaces reveal coexistence of delocalized and localized CT excitons. The highly mobile delocalized CT excitons could be the key factor to overcome large CT exciton binding energy in achieving efficient charge separation. In Chapter 7, we study stacking orientational dependent interlayer exciton recombination and transport in CVD WS2-WSe2 heterostructures. Temperature-dependent interlayer exciton dynamics measurements suggest the existence of moiré potential that localizes interlayer excitons. TAM measurements of interlayer excitons transport reveal that CT excitons at WS2-WSe2 heterointerface are much more mobile than intralayer excitons of WS2. We attributed this to the dipole-dipole repulsion from bipolar interlayer excitons that efficiently screen the moiré potential fluctuations and facilitate interlayer exciton transport. Our results provide fundamental insights in understanding the influence of moiré potential on interlayer exciton dynamics and transport in CVD WS2-WSe2 heterostructures which has important implications in optoelectronic applications such as atomically thin photovoltaics and light harvesting devices. </div><div><br></div>

Key issues in improving small molecule organic solar cells (SMOSC) are the need for new absorber materials and optimized active layer morphology. This thesis deals with the improvement of SMOSC on the donor material side. Promising donor materials (D) are provided by dicyanovinyl endcapped oligothiophenes DCV2-nT (n = 3, . . . , 6) synthesized in the group of Prof. Bäuerle at the University of Ulm. Here, DCV2-nT (n = 3, 5) with different alkyl side chains are characterized. Side chain variations mainly influence the aggregation of molecules in pristine films as well as in blend films with the commonly used acceptor (A) fullerene C60. With changes in the layer morphology, important physical properties in thin film like absorption spectra, energy levels, as well as excited state properties are changed. The focus of this work are excited state properties accessed by photoinduced absorption spectroscopy (PIA). PIA probes the long living excited states in pristine and blend films, i. e. triplet excitons, anions, and cations. For a series of four dicyanovinyl-terthiophenes DCV2-3T (without side chains, with two methyl, two butyl, and four butyl side chains) a systematic study of the effect of alkyl side chains on the aggregation in neat and blend film is discussed. In consequence the efficiency of the energy transfer mechanism between DCV2-3T and C60 is affected. It turns out that in solution spectra and cyclic voltammetry (CV) measurements, the variation of alkyl side chains has almost no influence. However, in thin film there is strong impact on the molecular arrangement confirmed by strongly varying absorption spectra, ionization potentials, and surface roughnesses. Furthermore, PIA measurements reveal that the energy transfer efficiency between D and A in general decreases with increasing side chain length, but is most efficient for a compound with methyl side chains. For blends of dicyanovinyl-quinquethiophenes (DCV2-5T) with C60, the layer morphology is influenced by two different methods. On one hand substrate heating is applied while deposition of the active layer, on the other hand DCV2-5Ts with different alkyl side chains (four methyl and four butyl side chains) are used. Deposition on a heated substrate (80°C) results in an improved solar cell performance, assigned to the formation of a sufficient phase separation of D and A phase in the active layer. This leads to reduced recombination losses and closed percolation paths. The morphological change can be correlated to an increased lifetime of cations. In blends deposited on a heated substrate, the donor cation lifetime increases by almost one order of magnitude from around 10 μs to ≈ 80 μs. This increase of carrier lifetime is both detected optically by PIA as well as electrically by impedance spectroscopy. The increase in lifetime is consequently assigned to a better spatial separation of positive and negative charges induced by the phase separation. Comparing DCV2-5T with methyl and butyl side chains results in a similar effect: The dicyanovinyl-quinquethiophene with methyl side chains leads to an improved solar cell device performance compared to devices comprising the compound with butyl side chains as donor. The improved device performance is again accompanied by an increase in cation lifetime detected by PIA.:Contents Publications 1. Introduction 2. Organic semiconductors 2.1. Introduction 2.2. Optical excitations in organic semiconductors 2.2.1. Energy levels: single molecules to molecular solids 2.2.2. Absorption and emission spectra 2.3. Transport in organic semiconductors 2.3.1. Exciton motion 2.3.2. Charge transport 2.3.3. Amorphous organic semiconductors 3. Organic photovoltaics 3.1. Introduction 3.2. Solarenergyconversion 3.2.1. Quasi Fermi levels 3.2.2. p-n junction 3.3. Organic solar cells 3.3.1. Charge generation mechanisms 4. Experimental methods 4.1. Sample preparation 4.2. Photoinduced absorption spectroscopy 4.2.1. PIA setup 4.2.2. Recombination dynamics 4.3. Solar cell characterization 4.3.1. External quantum efficiency 4.3.2. J-V characteristics 4.4. Absorption and emission spectroscopy 4.5. Determination of energy levels 4.5.1. Ultraviolet photo electron emission spectroscopy 4.5.2. Cyclic voltammetry 4.6. Atomic force microscopy 4.7. Density functional theory calculations 4.8. Impedance spectroscopy 5. Dicyanovinyl-oligothiophenes 5.1. Introduction 5.2. The DCV2-nT:C60 interface 5.3. Processability 6. Side chain variations on DCV2-3T 6.1. Introduction 6.2. Density functional theory calculations 6.2.1. Excited state transitions 6.3. Absorption and Emission in solution and thin film 6.3.1. Blend layer absorption spectra 6.3.2. Photoluminescence spectra of neat and blend films 6.4. Energy levels of the DCV2-3T series 6.5. Atomic force microscopy 6.6. Photoinduced absorption spectroscopy 6.6.1. PIA signatures of charged states 6.6.2. Recombination dynamics 6.6.3. Efficiency of the ping pong effect 6.7. Conclusion 7. Influencing the morphology of DCV2-5T:C60 blend layers 7.1. Introduction 7.2. Properties of the DCV2-5T:C60 interface 7.2.1. Analysis of the DCV2-5T triplet transition 7.2.2. Analysis of the DCV2-5T cation transitions 7.2.3. Suggested energy level scheme for neat and blend layer 7.3. Temperature evolution of excited state properties 7.4. Effect of substrate heating on excited state lifetime and generation rate 7.4.1. Solar cell devices 7.4.2. Photoinduced absorption 7.4.3. Impedance spectroscopy 7.5. Conclusion 8. Side chain variations on DCV2-5T 8.1. Introduction 8.2. Atomic force microscopy 8.3. Energy levels 8.4. Mip solar cells 8.4.1. Flat heterojunctions 8.4.2. Bulk heterojunctions 8.4.3. Discussion of Voc 8.5. Photoinduced absorption 8.5.1. Comparison at room temperature 8.6. Conclusion 9. Conclusion and Outlook 9.1. Conclusion 9.2. Outlook A. Appendix Bibliography / Die Entwicklung neuer Absorber-Materialien sowie die Morphologie der photo- aktiven Schicht sind zentrale Themen hinsichtlich der Optimierung organischer Solarzellen aus kleinen Molekülen. In der vorliegenden Arbeit werden diese beiden Aspekte von Seiten des Donor-Materials (D) her behandelt. Die Material- klasse der Dicyanovinyl-Oligothiophene DCV2-nT(n=3,...,6) (synthetisiert in der Arbeitsgruppe von Prof. Bäuerle an der Universität Ulm) dient dabei als Ausgangspunkt. Insbesondere werden DCV2-nT-Moleküle (n = 3, 5) mit verschiedenen Alkyl-Seitenketten charakterisiert. Die Variation der Seitenketten beeinflusst in erster Linie die Anordnung der Moleküle in Einzel- sowie in Mischschichten mit dem typischerweise verwendeten Akzeptor-Material Fulleren C60 (A). Als Folge der Schichtmorphologie ändern sich physikalische Eigenschaften wie u. a. Absorptions- spektren, Energieniveaus sowie die Eigenschaften angeregter Zustände. Angeregte Zustände, wie Triplett-Exzitonen, Anionen und Kationen werden in dieser Arbeit mittels photoinduzierter Absorptionsspektroskopie (PIA) charakterisiert. Anhand einer Serie von vier Dicyanovinyl-Tertiophenen DCV2-3T (ohne Seiten- ketten, mit zwei Methyl-, zwei Butyl-, und vier Butyl-Seitenketten) werden systematisch Einflüsse der Seitenketten auf die Aggregation der Moleküle in Einzel- und Mischschichten untersucht. Besonderes Augenmerk liegt dabei auf dem Effekt der Seitenketten auf den Energie-Transfer-Mechanismus zwischen D und A. In Lösungsmittelspektren und Cyclovoltammetrie-Messungen ist fast keine Änderung durch die Seitenketten erkennbar. Im Dünnfilm hingegen besteht ein starker Einfluss auf die molekulare Anordnung, erkennbar in einer starken Variation der Absorptionsspektren, Ionisationspotentiale und Oberflächen-Topographie. PIA- Messungen zeigen weiterhin, dass im Allgemeinen die Effizienz des Energie-Transfer- Mechanismus mit zunehmender Länge der Alkyl-Ketten abnimmt. Der effizienteste Transfer besteht jedoch für die Verbindung mit Methyl-Seitenketten. In Mischschichten aus Dicyanovinyl-Quinquethiophenen (DCV2-5T) und C60 werden hier zwei Methoden zur Beeinflussung der Schichtmorphologie verfolgt. Zum einen wird die aktive Schicht auf einem geheizten Substrat abgeschieden, zum anderen werden DCV2-5T-Moleküle mit Methyl- und Butyl-Seitenketten als Donor verwendet. Das Abscheiden der aktiven Schicht auf einem geheizten Substrat (80 °C) führt zu einer verbesserten Solarzellenleistung, was auf die Bildung einer hin- reichenden Phasenseparation von D- und A-Phasen in der aktiven Schicht zurückzuführen ist. Die Phasenseparation bewirkt eine Reduktion von Rekombinationsverlusten und die Bildung geschlossener Perkolationspfade. Die morphologische Änderung korreliert mit einem Anstieg der Ladungsträger-Lebensdauer um fast eine Größenordnung von etwa 10 μs auf ≈ 80 μs. Der Anstieg kann sowohl optisch durch PIA, als auch elektrisch mittels Impedanz-Spektroskopie detektiert werden. Eine höhere Lebensdauer der Ladungsträger kann letztlich auf eine größere räumlichen Separation der positiven und negativen Ladungsträger zurückgeführt werden, induziert durch die Phasenseparation. Ein Vergleich von DCV2-5T-Molekülen mit Methyl- und Butyl-Seitenketten führt zu ähnlichen Resultaten: Solarzellen mit DCV2-5T substituiert mit Methyl- Seitenketten sind effizienter als die der butyl-substituierten Moleküle. Dies korreliert wiederum mit einer signifikant erhöhten Lebensdauer der Ladungsträger in Mischschichten der methyl-substituierten Verbindung.:Contents Publications 1. Introduction 2. Organic semiconductors 2.1. Introduction 2.2. Optical excitations in organic semiconductors 2.2.1. Energy levels: single molecules to molecular solids 2.2.2. Absorption and emission spectra 2.3. Transport in organic semiconductors 2.3.1. Exciton motion 2.3.2. Charge transport 2.3.3. Amorphous organic semiconductors 3. Organic photovoltaics 3.1. Introduction 3.2. Solarenergyconversion 3.2.1. Quasi Fermi levels 3.2.2. p-n junction 3.3. Organic solar cells 3.3.1. Charge generation mechanisms 4. Experimental methods 4.1. Sample preparation 4.2. Photoinduced absorption spectroscopy 4.2.1. PIA setup 4.2.2. Recombination dynamics 4.3. Solar cell characterization 4.3.1. External quantum efficiency 4.3.2. J-V characteristics 4.4. Absorption and emission spectroscopy 4.5. Determination of energy levels 4.5.1. Ultraviolet photo electron emission spectroscopy 4.5.2. Cyclic voltammetry 4.6. Atomic force microscopy 4.7. Density functional theory calculations 4.8. Impedance spectroscopy 5. Dicyanovinyl-oligothiophenes 5.1. Introduction 5.2. The DCV2-nT:C60 interface 5.3. Processability 6. Side chain variations on DCV2-3T 6.1. Introduction 6.2. Density functional theory calculations 6.2.1. Excited state transitions 6.3. Absorption and Emission in solution and thin film 6.3.1. Blend layer absorption spectra 6.3.2. Photoluminescence spectra of neat and blend films 6.4. Energy levels of the DCV2-3T series 6.5. Atomic force microscopy 6.6. Photoinduced absorption spectroscopy 6.6.1. PIA signatures of charged states 6.6.2. Recombination dynamics 6.6.3. Efficiency of the ping pong effect 6.7. Conclusion 7. Influencing the morphology of DCV2-5T:C60 blend layers 7.1. Introduction 7.2. Properties of the DCV2-5T:C60 interface 7.2.1. Analysis of the DCV2-5T triplet transition 7.2.2. Analysis of the DCV2-5T cation transitions 7.2.3. Suggested energy level scheme for neat and blend layer 7.3. Temperature evolution of excited state properties 7.4. Effect of substrate heating on excited state lifetime and generation rate 7.4.1. Solar cell devices 7.4.2. Photoinduced absorption 7.4.3. Impedance spectroscopy 7.5. Conclusion 8. Side chain variations on DCV2-5T 8.1. Introduction 8.2. Atomic force microscopy 8.3. Energy levels 8.4. Mip solar cells 8.4.1. Flat heterojunctions 8.4.2. Bulk heterojunctions 8.4.3. Discussion of Voc 8.5. Photoinduced absorption 8.5.1. Comparison at room temperature 8.6. Conclusion 9. Conclusion and Outlook 9.1. Conclusion 9.2. Outlook A. Appendix Bibliography