On the Characterization and Manipulation of Interfaces in Organic and Hybrid Electronic Devices

Organic electronics comprises a field of study at the intersection of chemistry, physics, electrical engineering, and materials science focused on the development of electronic devices in which the active charge transporting materials are composed of organic conjugated molecules. This field has grown out of an interest in harnessing many attributes of organic materials not readily available to inorganic semiconductors, including: low synthesis temperatures for organic compounds; a nearly infinite combination of chemical moieties with similar conjugated character; and ease of fabricating thin films of organic compounds through both vacuum and solution processes. These properties make the fabrication of low-cost, highly-customizable electronics commercially viable, despite their inferior carrier transport to crystalline inorganic semiconductors. This key hurdle—understanding charge transport in organic molecules and thin films made from them—has become a primary research objective in the field. Understanding charge transport in organic electronic devices spans analysis across various size scales, each contributing to the observed behavior of an electronic device: * The chemical structure of the constituent conjugated molecules (Ås) * The arrangement of these molecules into ordered and disordered regions within a thin film (10s of Ås) * The configuration of the thin film within the working device (100s of Ås) At each of these scales, the concept of an interface acquires new meaning, scaling from van der Waals forces between molecules, to grain boundaries in polycrystalline materials, and incrementally to device-scale junctions between dissimilar materials. Because each of these interfaces can promote or inhibit carrier transport within an electronic device, a complete understanding of carrier transport in organic semiconductors (OSCs) demands comprehensive characterization of interfaces at each of these scales. The subject of this thesis is a critical examination of the insulator-OSC interface in the context of several electronic device architectures. The properties of this interface are of paramount importance in organic field-effect transistors (OFETs), where the low intrinsic carrier mobilities of OSCs renders them highly susceptible to even the most marginal deviations from an ideal interface. As a result, transistor switching characteristics quickly carry through to circuit-level reliability and power consumption. This dissertation aims to demonstrate the use of existing materials in new ways for exercising nanoscale control over this interface, with an eye towards understanding their individual and collective charge transport behavior. Chapter 1 reviews the state of the art in control over the threshold voltage of OFETs, of which two methods—dipolar self-assembled monolayers (SAMs) and electrostatic poling—are considered in the subsequent chapters. Chapter 2 details the use of SAMs of dipolar alkylsilanes as a surface treatment for tuning VT, reducing leakage currents, and improving switching efficiency. Increases in field-effect transconductance in SAM-treated OFETs are shown to be consistent with the presence of additional surface states. Chapter 3 details an approach to decouple the relative contributions of the insulator/SAM and SAM/OSC interfaces from the capacitive responses of the OFET multilayer, and is compared to recent theoretical predictions of increased energetic disorder in SAM-treated OSC layers. Increased mobility of equilibrium carriers as measured with charge extraction are compared to OFET measurements and are shown to further reinforce the notion that larger molecular dipoles contribute to enhanced carrier transport through changes in the energetic disorder at the insulator/OSC interface. In Chapter 4 electrostatic poling, or gate stressing, of lateral OFETs is explored. A Poisson’s equation model is applied to surface potential images of stressed lateral OFETs and shown to accurately predict the observed threshold voltage shift. Lastly, Chapter 5 presents future directions for the study of SAM-treated interfaces using charge extraction, with a focus on the use of SAMs as remedial layers for marginal quality OSCs. In addition, the potential of surface potential-derived charge densities for sensing applications is discussed

Charge-Carrier Dynamics in 2D Hybrid Metal–Halide Perovskites

Hybrid metal–halide perovskites are promising new materials for use in solar cells; however, their chemical stability in the presence of moisture remains a significant drawback. Quasi two-dimensional (2D) perovskites that incorporate hydrophobic organic interlayers offer improved resistance to degradation by moisture, currently still at the cost of overall cell efficiency. To elucidate the factors affecting the optoelectronic properties of these materials, we have investigated the charge transport properties and crystallographic orientation of mixed methylammonium (MA)–phenylethylammonium (PEA) lead iodide thin films as a function of the MA-to-PEA ratio and, thus, the thickness of the “encapsulated” MA lead–halide layers. We find that monomolecular charge-carrier recombination rates first decrease with increasing PEA fraction, most likely as a result of trap passivation, but then increase significantly as excitonic effects begin to dominate for thin confined layers. Bimolecular and Auger recombination rate constants are found to be sensitive to changes in electronic confinement, which alters the density of states for electronic transitions. We demonstrate that effective charge-carrier mobilities remain remarkably high (near 10 cm2V−1s−1) for intermediate PEA content and are enhanced for preferential orientation of the conducting lead iodide layers along the probing electric field. The trade-off between trap reduction, electronic confinement, and layer orientation leads to calculated charge-carrier diffusion lengths reaching a maximum of 2.5 μm for intermediate PEA content (50%)

Femtosecond Dynamics of Photoexcited C 60 Films

The well known organic semiconductor C60 is attracting renewed attention due to its centimeter-long electron diffusion length and high performance of solar cells containing 95% fullerene, yet its photophysical properties remain poorly understood. We elucidate the dynamics of Frenkel and intermolecular (inter-C60) charge-transfer (CT) excitons in neat and diluted C60 films from high-quality femtosecond transient absorption (TA) measurements performed at low fluences and free from oxygen or pump-induced photodimerization. We find from preferential excitation of either species that the CT excitons give rise to a strong electro-absorption (EA) signal but are extremely short-lived. The Frenkel exciton relaxation and triplet yield strongly depend on the C60 aggregation. Finally, TA measurements on full devices with applied electric field allow us to optically monitor the dissociation of CT excitons into free charges for the first time and to demonstrate the influence of cluster size on the spectral signature of the C60 anion

Ultrafast Charge Dynamics in Dilute-Donor versus Highly Intermixed TAPC:C60 Organic Solar Cell Blends

Elucidating the interplay between film morphology, photophysics, and device performance of bulk heterojunction (BHJ) organic photovoltaics remains challenging. Here, we use the well-defined morphology of vapor-deposited di-[4-(N,N-di-p-tolyl-amino)-phenyl]cyclohexane (TAPC):C60 blends to address charge generation and recombination by transient ultrafast spectroscopy. We gain relevant new insights to the functioning of dilute-donor (5% TAPC) fullerene-based BHJs compared to molecularly intermixed systems (50% TAPC). First, we show that intermolecular charge transfer (CT) excitons in the C60 clusters of dilute BHJs rapidly localize to Frenkel excitons prior to dissociating at the donor:acceptor interface. Thus, both Frenkel and CT excitons generate photocurrent over the entire fullerene absorption range. Second, we selectively monitor interfacial and bulk C60 clusters via their electro-absorption, demonstrating an energetic gradient that assists free charge generation. Third, we identify a fast (< 1 ns) recombination channel, whereby free electrons recombine with trapped holes on isolated TAPC molecules. This can harm the performance of dilute solar cells, unless the electrons are rapidly extracted in efficient devices

Femtosecond Dynamics of Photoexcited C₆₀ Films

The well known organic semiconductor C₆₀ is attracting renewed attention due to its centimeter-long electron diffusion length and high performance of solar cells containing 95% fullerene, yet its photophysical properties remain poorly understood. We elucidate the dynamics of Frenkel and intermolecular (inter-C₆₀) charge-transfer (CT) excitons in neat and diluted C₆₀ films from high-quality femtosecond transient absorption (TA) measurements performed at low fluences and free from oxygen or pump-induced photodimerization. We find from preferential excitation of either species that the CT excitons give rise to a strong electro-absorption (EA) signal but are extremely short-lived. The Frenkel exciton relaxation and triplet yield strongly depend on the C₆₀ aggregation. Finally, TA measurements on full devices with applied electric field allow us to optically monitor the dissociation of CT excitons into free charges for the first time and to demonstrate the influence of cluster size on the spectral signature of the C₆₀ anion

Charge-carrier dynamics in 2D hybrid metal-halide perovskites

Hybrid metal halide perovskites are promising new materials for use in solar cells, however, their chemical stability in the presence of moisture remains a significant drawback. Quasi two-dimensional perovskites that incorporate hydrophobic organic interlayers offer improved resistance to degradation by moisture, currently still at the cost of overall cell efficiency. To elucidate the factors affecting the optoelectronic properties of these materials, we have investigated the charge transport properties and crystallographic orientation of mixed methylammonium (MA)/phenylethylammonium (PEA) lead iodide thin films as a function of MA:PEA and thus the thickness of the 'encapsulated' MA lead halide layers. We find that monomolecular charge-carrier recombination rates first decrease with increasing PEA fraction, most likely as a result of trap passivation, but then increase significantly as excitonic effects begin to dominate for thin confined layers. Bimolecular and Auger recombination rate constants are found to be sensitive to changes in electronic confinement, which alters the density of states for electronic transitions. We demonstrate that effective charge-carrier mobilities remain remarkably high (near 10 cm2/Vs) for intermediate PEA content and are enhanced for preferential orientation of the conducting lead-iodide layers along the probing electric field. The tradeoff between trap reduction, electronic confinement and layer orientation leads to calculated charge-carrier diffusion lengths reaching a maximum of 2.5 µm for intermediate PEA content (50%)