Introduction: Ventricular ectopic beats and corrected QT interval (QTc) prolongation are both relatively common entities that are typically benign. It is difficult to predict subsequent dysrhythmias from either electrocardiogram (ECG) feature. The combination of both features may better predict the risk of torsades de pointes. We highlight a case of torsades preceded by a bizarre bigeminal rhythm with QTc prolongation likely caused by memantine use and hypokalemia. 

Case Report: An 84-year-old female presented to the emergency department with a fall. A syncope workup revealed an ECG demonstrating bigeminy with a prolonged QTc interval. Several minutes after obtaining the ECG, the patient went into torsades. She had multiple subsequent cardiac arrests during the rest of her hospital stay. This case report details the importance of recognizing ventricular bigeminy in the context of QTc prolongation as a harbinger of torsades. 

Conclusion: While premature ventricular contractions including bigeminy may be a benign finding, when accompanied by prolonged QTc intervals, they warrant immediate investigation and treatment of potential underlying pathology to prevent torsades and subsequent cardiac arrest. 

Since the early 1900's, synthetic insulating polymers (plastics) have slowly taken over the role that traditional materials like wood or metal have had as basic components for construction, manufactured goods, and parts. Plastics allow for high throughput, low temperature processing, and control of bulk properties through molecular modifications. In the same way, pi-conjugated organic molecules are emerging as a possible substitute for inorganic materials due to their electronic properties. The semiconductive nature of pi-conjugated materials make them an attractive candidate to replace inorganic materials, primarily due to their promise for low cost and large-scale production of basic semiconducting devices such as light-emitting diodes, solar cells, and field-effect transistors.

Before organic semiconductors can be realized as a commercial product, several hurdles must be cleared. The purpose of this dissertation is to address three distinct properties that dominate the functionality of devices harnessing these materials: (1) optical properties, (2) charge injection, and (3) charge transport. First, it is shown that the electron injection barrier in the emissive layer of polymer light-emitting diodes can be significantly reduced by processing of novel conjugated oligoelectrolytes or deoxyribonucleic acid atop the emissive layer. Next, the charge transport properties of several polymers could be modified by processing them from solvents containing small amounts of additives or by using regioregular and enantiopure chemical structures.

It is then demonstrated that the optical and electronic properties of Lewis basic polymer structures can be readily modified by interactions with strongly electron-withdrawing Lewis acids. Through red-shifted absorption, photoluminescence, and electroluminescence, a single pi-conjugated backbone can be polychromatic. In addition, interaction with Lewis acids can remarkably p-dope the hole transport of the parent polymer, leading to a two-orders of magnitude increase in the hole mobility. Finally, the hole, electron, and double carrier transport in solar cell devices are studied in a bid to examine the correlations between bulk morphologies and free carrier recombination.

The sum of these works help to create new pathways for the synthesis and design of new pi-conjugated materials and device architectures. All of this is in hopes of achieving higher performance and more stable devices to rival inorganic systems.

The motto of University of California is “let there be light.” But how do we know whether there is light, especially the light that we cannot see with our eyes? The answer is photodetectors. Photodetection has long play an indispensable role in modern technologies. The research endeavors on organic photodetectors (OPDs) have been continuously increasing in the recent years since they promise great potential as a platform for photosensing, imaging, and optical communication. Compared to the established crystalline inorganic counterparts, OPDs show their unique attractiveness: tunable spectral activity, low-temperature and low-cost fabrication, light weight and mechanical flexibility, large-area solution/printing processibility, and versatile integration into complicated optoelectronics. In this thesis, three studies are carried out to enhance the sensing performance, as well as to better understand the device physics of bulk heterojunction (BHJ) OPDs. Notwithstanding the exciting progress made thus far for OPDs, efficient broadband detection, especially in the near infrared (NIR) region, still needs to be improved due to the challenges in simultaneously securing high photoresponse and low dark/noise current for narrow bandgap systems. For the first part, the focus is placed on achieving high NIR sensing performance for BHJ organic photodiodes. A novel ultranarrow bandgap (~1.2 eV) small molecule is used, which present the highest responsivity of 0.45-0.52 A/W in the spectral region of 920 – 940 nm. With well suppressed charge injection under reverse bias using thick junction approach, high NIR specific detectivity of ~1012 Jones can be obtained even at a large bias of 2 V. Subsequently, we touch an instability problem found for inverted structure OPDs, where a large inconsistency of dark current can be universally found between before- and after-illumination conditions, negatively impacting the crucial figures-of-merit such as specific detectivity. Systematic control tests reveal that the widely used zinc oxide electron transport layer has caused the issue, which is triggered by exposure to high energy photons. We identify an alternative electron transport layer, optimized as “multilayer” tin oxide film, which is not only immune to the illumination-induced dark current inconsistency, but also simultaneously retain well suppressed low dark/noise current, uncompromised photoresponse, and rapid response speed. With the success of regular photodiode based OPDs studied in the first two parts, the focus is then shifted to introducing photomultiplication (PM) gain to BHJ OPDs, aiming at external quantum efficiency over 100%. This goal is accompanied by the targets of simultaneously achieving broad spectral response, low operating voltage, and applicable photoresponse speed, the aspects where the current approaches of making PM OPDs show shortcomings. Introduction of electron trapping agent (tetracyanoquinodimethane, TCNQ) into the BHJ blend has resulted in PM OPDs that can deliver not only large EQE over 104 % at low operation voltages, but also a wide span of spectral response from ultraviolet to NIR. The origin of the observed large photocurrent gain is revealed to originate from the interfacial metal-organic interaction and the enhanced electron trapping in the bulk of the BHJ layer, both induced by the intentionally added TCNQ. These effects essentially convert the device behavior from that of a regular photodiode to a photoconductor with excessive trapping sites of electrons.

Organic photovoltaic (OPV) devices have the potential to be a cost-efficient, clean, and renewable energy source. A fundamental process in OPV devices which directly impacts the device performance is the diffusion length of Coulombically bound electron-hole pairs or excitons. It is therefore important to investigate how chemical structure and processing conditions impact the exciton diffusion length. This study is difficult to perform with the current body of literature since compounds in separate works differ by a number of chemical modifications. Comparisons between works are further complicated by the use of different techniques to measure the exciton diffusion length. To resolve these issues, the first aim of this dissertation is to compare the fabrication, measurement, and analysis procedure for six different techniques to measure exciton diffusion length. We find that bulk quenching techniques are preferred over surface quenching techniques which require elaborate fabrication procedures, multiple measurements, and a number of assumptions in the analysis process. The second aim of this dissertation is to investigate how chemical structure and processing conditions impact the exciton diffusion length. We find that decreasing the conjugation length in a small molecule leads to an enhancement in the exciton diffusion length while replacing the linear alkyl chains with bulky ethyl hexyl groups has no significant effect. Lastly, we show that processing films with a high boiling point solvent leads to an enhancement in the exciton trap density which directly reduces the exciton diffusion length.

Ionic-organic ratchets have the ability to rectify alternating electric fields into DC current to sustain low power electronics, such as RFID devices. Until now however, organic electronic ratchets have only been realized using p-type organic semiconductors. The development and performance of organic n-type devices have historically lagged far behind their p-type equivalents, largely due to the high susceptibility of n-type organic semiconductors to electron trapping. A previously developed charge pump model predicts an output current for ionic-organic ratchets that is linearly dependent on frequency, and agrees well with data from p-type devices. In this model, the capacitance of the device is assumed to be constant over the measured frequency range. N-type ratchets based on PCBM and N2200 exhibit output currents that deviate from a linear relationship with frequency. Impedance spectroscopy measurements suggest that this deviation is caused by the presence of ions and traps which modify the frequency dependence of the device capacitance. This work demonstrates the first n-type organic electronic ratchet, capable of producing an output power comparable to its p-type counterparts.

Organic semiconductors have been widely applied to electronic devices including organic solar cells (OSCs), and organic photodetectors (OPDs) in the last decades for their lightweight, flexibility, and great industry potential for high throughput solution processing and roll-to-roll printing technologies. However, traditional solvents used for these organic photovoltaics processing conditions are commonly highly toxic halogenated organic solvents like chlorobenzene (CB), o-dichlorobenzene (o-DCB), and chloroform (CF) which are not only harmful to human health upon exposure but can also cause long-term effects on the ecosystem. Consequently, research on halogen-free/ “green solvents” (such as xylene, 2-methyltetrahydrofuran (2-MeTHF), methanol, water, etc.) processable organic semiconductors are essential to moving organic photovoltaics from bench chemistry towards real-life applications. In this thesis, three studies are carried out to enhance the sensing performance, as well as to better understand the device physics of bulk heterojunction (BHJ) OPVs and OPDs. The design, fabrication, optimization, as well as characterization of green-solvent processing organic semiconductors, have been discussed in detail including OPV and OPD systems. The characterization of the solid-state properties of the optimized active layers in these organic electronic devices is further included. The bulk-heterojunction (BHJ) thin films processed from green solvents were characterized with different techniques at different length scales, including atomic force microscopy (AFM), transmission electron microscopy (TEM), grazing-incidence wide-angle X-ray scattering (GIWAXS) and solid-state nuclear magnetic resonance (ssNMR) spectroscopy. The first study gains insights into the structure–processing–property correlation of a PPDT2FBT: PC61BM bulk-heterojunction (BHJ) system processed from a green solvent, ortho-xylene (o-XY). This system is investigated in comparison with the same blend processed from a traditional halogenated solvent, chlorobenzene (CB). The optimized PPDT2FBT:PC61BM devices processed from o-XY can achieve a noteworthy higher power conversion efficiency (PCE) owing to a higher short-circuit current density and fill factor. The second study develop a series of 2-MeTHF processing OPV systems based on newly designed bulk heterojunction (BHJ) consisting of PM7-D3/D5 donors and Y-series non-fullerene acceptors with systematically changing in their side chain length and positions. The optimized 2-MeTHF casting device based on PM7-D3:PTI04 exhibits the best PCE of approaching 15%, which is comparable with the well-known PM6:Y6 system processing from traditional halogenated solvents. The third study reports a designed bulk heterojunction (BHJ) consisting of PM7-D5 donor and Y12 non-fullerene acceptor processed from 2-methyltetrahydrofuran (2-MeTHF) sets a record in light detectivity, which is also comparable with commercially available silicon-based photodiodes. Newly designed PM7-D5:Y12 OPD can be employed in wearable self-powered devices to monitor heart rate and blood oxygen saturation. This development of a framework for a detailed understanding of the structure-processing-property relationship provides insight into the mechanisms that lead to the improved performance of green-solvent-processed organic electronics.

Organic photovoltaic devices (OPVs) have been progressively developed as a promising green energy source. The solar-to-electrical power conversion efficiency (PCE) of the OPVs has been steadily enhanced in the last few decades by advanced chemical designs and device structures/architectures. A state-of-the-art class of photoactive materials recently contributed to a drastic increase in PCEs, reaching almost 20%. Compared to the other PVs such as inorganic-based and perovskite PVs, the OPVs have unique merits: thinness, lightweight, flexibility, and ease to tune color and transparency. These characteristics have attracted the commercialization of the OPVs in various fields (e.g. incorporating with clothing and aesthetic windows). However, in contrast to the remarkable PCE improvements, the OPVs are still suffering from short lifetime under environmental operating conditions (e.g. intense sunlight, high temperature, etc.), which is a hurdle to encourage wide-ranging commercial products of the OPVs. The lack of understanding of fundamental degradation processes and mechanisms within an entire OPV has delayed the development of long-term stable OPVs. In the aspect of chemical synthesis, many efforts have introduced durable photoactive materials (thermally stable and/or photostable chemicals), but synthetic steps often require complicated processes and high cost, and provide low yield and toxicity which is another potential environmental issue. This dissertation demonstrates a comprehensive understanding of possible degradation mechanisms within an OPV device, and suggests a facile method to improve the lifetime of the OPVs based on existing materials. In the first study, we discussed both the physical and chemical degradation processes of layers, especially, at interfaces between adjacent layers within an OPV. Molecular-level insights are provided into the impact of different metal top electrodes (the most commonly used aluminum (Al) and silver (Ag)) on the interfacial morphology and stability of photoactive layers in PM6:Y6 bulk-heterojunction (BHJ) OPVs. OPVs with an Al top electrode exhibit inferior stability compared to Ag electrode devices upon thermal aging, whereby thermal stress induces the diffusion of both Al and Ag atoms to the PM6:Y6 BHJ layer. Multiscale characterizations (X-ray photoelectron, solid-state nuclear magnetic resonance, and electron paramagnetic resonance spectroscopy) suggest the different local chemical environments of PM6 and Y6 moieties in PM6:Y6/Al-contact. By comparison, the Ag atoms do not adversely affect PM6:Y6 BHJ morphology and the associated device physics. Next, a common cross-linking method is applied for the organic BHJ photoactive to address a fundamental morphological stability issue with high performing small molecule non-fullerene acceptors (NFAs). The molecular diffusion and/or aggregation of the NFAs due to thermal energy alter optimum BHJ morphology which is required to yield high PCE, leading to undesirable morphology and hence reducing PCE. Herein, a photochemically activated multi-bridged azide cross-linker (6Bx) restricts molecular aggregation and crystallization of the Y6, a representative high performing NFA, in solid state. Solid-state magnetic resonance and infrared spectroscopy analyses demonstrate that the 6Bx molecules closely interact with Y6 moieties within the PM6:Y6 blend, facilitating molecularly cross-linked Y6 regions. Consequently, 0.05 wt% 6Bx cross-linked PM6:Y6 BHJ OPVs retain 93.4% of initial power conversion efficiency upon thermal aging at 85 °C for 1680 h. This dissertation comprehensively examines physical and chemical reactions/interactions within an entire OPV device by multiscale characterizations and device physics, therefore will provide guidance to develop stable and efficient organic electronics.

Even though significant breakthroughs with unprecedented power conversion efficiencies (PCEs) approaching 18% have been achieved for polymer:non-fullerene acceptor (NFA) organic solar cells (OSCs) recently, not many studies in the literature have focused on acquiring a comprehensive understanding of the underlying mechanisms governing these novel systems. The common knowledge in the OSC community is that to achieve high performances, there needs to be a minimization of the voltage losses due to charge generation, recombination, and energetic disorder, as well as an optimal control of the bulk heterojunction morphology for beneficial charge transport and extraction. In practice, it is extremely challenging to obtain such parameters simultaneously and even more complex to delineate device photophysics in polymer:NFA blends comprehensively. Furthermore, tracing origins of the differences in device photophysics to the subtle differences in energetics and morphology can be particularly complicated. This dissertation encompasses four studies that unify approaches to understand the complicated device photophysics of organic semiconductor devices. Firstly, a systematic study of a series of polymer:NFA blends is conducted to unify and correlate the cumulative effects of i) voltage losses ii) charge generation efficiencies, iii) non-geminate recombination and extraction dynamics, and iv) nuanced morphological differences with device performances. Most importantly, a deconvolution of the major loss processes in polymer:NFA blends and their connections to the complex BHJ morphology and energetics are established. Secondly, the device photophysics in the high performing PM6:Y6 blend system is investigated. This blend system is found to exhibit low voltage losses coupled with moderate non-geminate recombination and exceptional extraction that can explain its high efficiencies of over 15%. Thirdly, a study is conducted to understand the role of morphology in the key operating processes of high performing polymer:NFA organic solar cells. Varying polymer molecular weight fractions is used as a tool to exert fine control over the interfacial and bulk morphology. This study provides an avenue to understand two fundamental and complex questions that are relevant to the OSC community: i) the role of the nature of the D:A BHJ interface on charge generation and recombination processes, and ii) the factors affecting charge extraction and transport in OSCs. The results from this work provide recommendations on the significant bulk and interfacial morphological features that are critical in optimizing charge generation, recombination, and extraction processes to give high performances, expediting the pathway to the commercialization of OSCs in the near future. Lastly, the DOS distribution widths of two structurally unique organic semiconducting polymers are characterized by using a combination of techniques that have not be explored together. These include, temperature-dependent current density-voltage (J-V) measurements, Kelvin probe measurement (KP) of band bending, and energy-resolved electrochemical impedance spectroscopy (ER-EIS). A quantitative correlation between energetic disorder from band‐bending measurements and charge transport is established, providing direct experimental evidence that charge‐carrier mobility in disordered materials is compromised due to the relaxation of carriers into the tail states of the DOS. Distinction and quantification of locally ordered and disordered regions of thin films at an atomic level was achieved using solid‐state NMR spectroscopy.

Organic Photovoltaics (OPVs) harvest energy directly from sunlight. They comprise a molecular or polymeric carbon-based donor:acceptor blend (bulk heterojunction or BHJ) in between two electrodes. OPVs underwent tremendous progress in the past few years, now reaching Power Conversion Efficiencies (PCEs) of over 19%. In contrast to their well-established inorganic counterparts, OPVs are lightweight, thin, and flexible, and they can be tuned on the molecular level, allowing the modification of color and transparency. These unique properties make OPVs ideal candidates for integrated energy harvesting solutions as their optical tunability and solution-processability are favorable for integration into buildings. However, to date, OPVs do not meet the requirements for widespread commercialization. I) High-performing systems suffer from reproducibility challenges due to batch-to-batch variations and II) a systematic in-depth understanding of the structure-property relationships is absent. Lastly, III) Semitransparent OPVs (ST-OPVs) still suffer from poor performances even though theoretical calculations predict high PCEs even for systems with 100% average visible transmittance (AVT). This work focuses on the understanding of these limitations through a multidimensional approach, including both experimental and simulation-based methods. After an introduction to OPVs in Chapter 1, various techniques used in this work are discussed in Chapter 2. Chapter 3 investigates residual catalysts traces as a possible cause for batch-to-batch variations, addressing I) and II). The systematic addition of Pd(PPh3)4 to PTB7-Th:IOTIC-4F devices and its effect on the morphology and the optoelectronic processes is studied. A drop in performance is observed that is due to altered material properties and different loss mechanisms, but the system showed robustness to 0.75% Pd(PPh3)4, an amount typically not exceeded after purification. Next, Chapter 4 presents a new approach to unravel the optoelectronic processes under short-circuit conditions, facilitating the study of II). We propose a new method to obtain the geminate prefactor Pg, the mobility-lifetime product µτ, and the extraction efficiency η, using only standard measurements and simulations. Our simple method also predicts the optimal device configuration and active layer thickness with greater accuracy than optical transfer matrix simulations, providing a fast and cost-effective alternative to the experimental device optimization. The following chapters focus on narrow-band gap ST-OPVs for integrated energy harvesting solutions, addressing III). Chapter 5 identifies changes in ST-OPVs that are concomitant with increased transparency. Reduced generation rates and altered generation rate profiles lead to a reduced open-circuit voltage (Voc) and changes in the recombination dynamics. We show that high-purity and low-trap-density active layers are crucial. Furthermore, transparent devices are sensitive to shunt-leakage, highlighting the need for high-quality active layers even more. The impact of surface recombination decreases with increased AVT and limitations due to high series resistance decrease, suggesting considering a wider range of transparent electrode materials. Chapter 6 focuses on the interfacial recombination in the narrow-band gap system PCE10:COTIC-4F. Our findings indicate that ZnO is the most suitable front electrode due to low interfacial recombination, efficient charge extraction, and favorable energy level alignment. Other electrodes are studied, including ZnO functionalized with PFN-Br, PEDOT:PSS and CPE-K. The present results show that interfacial recombination plays a significant role in narrow band gap OPV systems and future research in this direction will be necessary to overcome the PCE bottleneck arising from surface traps. Lastly, Chapter 7 introduces an experimental approach to obtain OPVs with highly transparent active layers, addressing II) and III). Devices with donor concentrations of 40% to 20% show high average visible transmittance (AVT) values of 64% to 77%. The AVT increases with lower donor concentration due to reduced visible range donor absorption and increased near-IR acceptor absorption. We investigate morphology, charge generation, charge recombination, and charge extraction, and propose further optimization of the interfaces and active layer morphology.

The Field Effect Transistor (FET) is inarguably the most important circuit element in modern electronics. Metal-Oxide FETs (MOSFETs), the most common type of FET, are integrated in microprocessors in almost all electronic devices: mobile phones, computers, digital cameras, and digital printers, to name a few. MOSFETs are normally fabricated on top of rigid single crystalline silicon, which requires high temperature processing (~1000 oC). Recently, a need has emerged for flexible electronics in a variety of applications. Examples include form-fitting healthcare-monitoring devices, flexible displays, and flexible radio frequency identification tags (RFID). Organic FETs (OFETs) are viable candidates for flexible electronics because they are based on semiconducting π-conjugated materials, including small molecules and polymers, which can be solution-processed at low temperature on flexible substrates. Solution-processing may enable the use of high throughput methods such as roll-to-roll coating and inkjet printing for low-cost manufacturing. In the molecular perspective, the limitless ability to tune the properties of these materials just by a small modification of the conjugated backbone or sidechains makes them attractive to both academic research and industrial manufacturing. Between the two materials, semiconducting polymers offer better potential for the formation and mechanical properties of thin films, compared to their small molecule counterparts.

For OFETs to be industrially viable, however, they must first have high charge carrier mobility. Recent advances in molecular designs and device engineering have seen significant increases in the mobility of OFETs fabricated with conjugated polymer (or PFETs). In this dissertation, it is shown that obtaining polymer-chain alignment is critical to improve the mobility of PFETs. In addition, the charge transport mechanism is investigated to explain the high mobility in PFETs with aligned polymers. Most importantly, the mechanism of electrical instability and non-ideality (i.e. the double-slope) of high mobility PFETs with a certain degree of ambipolarity is unraveled.

Firstly, the alignment of polymer chains inside polymer fiber bundles is revealed by high-resolution atomic force microscopy (AFM). This alignment is enabled by nano-grooves of ca. 50 nm wide and 2-5 nm deep on SiO2 substrates used for fabricating the PFETs. Mobility of charge transport along the direction of the polymer fiber is an order of magnitude higher than that of charge transport perpendicular to the fiber direction. It indicates that aligned polymer chains facilitate fast intrachain charge transport.

Secondly, the charge transport mechanism is determined to be the thermally-activated hopping of charge carriers. This is an important finding because it has been speculated that band transport is possible in OFETs fabricated from well-aligned polymer fibers. With a normal range of molecular weight (30 kDa to 100 kDa), which is feasible for industrial scale-up, the stretched length of the corresponding polymer chain, ranging from 50 – 150 nm, is not enough to cross the full channel length of the OFETs. In addition, polymer chains in a solution-processed thin film are likely to have kinks and twists that disrupt the perfect electronic coupling necessary for band transport. It implies that polymers should be designed to facilitate not only chain alignment but also the strong electronic coupling in the π-π stacking direction between chains for efficient hoppings between chains with low energetic barriers.

Finally, the electrical instability and its associated non-ideal device characteristics is thoroughly investigated. Electrical stability is as important as high charge carrier mobility for OFETs to be commercialized in large-scale production. The use of low bandgap donor-acceptor (D-A) polymers for obtaining high mobility introduces undesirable electron current in p-type PFETs. The effect of electron transport and trapping on hole conduction in p-type OFETs has not been addressed. In this work, p-type PFETs fabricated with SiO2 dielectrics and with a certain degree of ambipolarity exhibit electrical instability (a change in current upon bias stressing) and non-ideality (double-slope in transfer curves) of hole current. It is determined that electron trapping and the subsequent formation of -SiO- charges on SiO2/polymer interface are the principle origins of the instability and the double-slope. Suppressing these undesired properties is essential to make PFETs industrial viable. It should be noted that the double-slope has been a long debate without a solid explanation in the OFET research community.

In sum, this thesis shows a comprehensive understanding of the structure-processing-property relationship of PFETs fabricated with well-aligned polymers. New findings in the thesis provide important guidelines for molecular design and device engineering of high-mobility and practical PFETs. These guidelines have been successfully demonstrated by us and our collaborators at UCSB.