Photophysics of fused ring electron acceptors for photovoltaic applications

<p>Organic photovoltaic cells (OPVs) have received significant interest over the last decade as they offer the potential of cheap renewable energy via direct photon to electron conversion of abundantly available Sun light. Compared to the inorganic equivalents, they offer lightweight, low-cost, and flexibility advantages. Conventional OPVs are typically based on blends of electron-donor materials and fullerene-based electron-acceptor materials that form bulk-heterojunctions (BHJs). But the attention has recently shifted to organic non-fullerene acceptors especially fused ring electron acceptors (FREAs) owing to their attractive properties including flexible energy levels, tunable band gap, crystallinity, and planarity. Thus the power conversion efficiency (PCE) of OPVs has recently attained a record of 16% by synthesizing FREAs with modified chemical structures. Owing to the high crystallinity and packing orientation of acceptors, FREA based OPV systems are characterized by large and pure phases sized 20-50 nm. This is consistent with the observation that charge generation dynamics in these systems lacked the ultrafast component that characterizes most fullerene blends. To test the hypothesis that the optimal phase size can be large due to facile exciton diffusion in FREAs, a planar indacenodithiophene (IDT) based FREA, IDIC is selected as a model system to study the exciton dynamics. Chapter 3 includes the exciton diffusion measurements in IDIC films using transient absorption spectroscopy which resolves a substantially high, quasi-activationless diffusion coefficient that exceeds that of typical organic semiconductors. The study also includes a deep insight of the key factors behind the enhanced exciton diffusion in IDIC and is shown to arise from different molecular and packing factors which enhance the long-range resonant energy transfer. Rapid exciton diffusion in IDIC films introduced the possibility of solution-processed bilayer devices. Chapter 4 comprises both device and photophysics of planar bilayer devices with a PCE of 11.1% which is readily accounted for the material and device design. By pairing a mid-band gap polymer donor with a range of FREAs, it is shown that the combination of high molecular packing densities and absorption coefficients, long exciton diffusion lengths, and efficient, resonant, long-range energy transfer between donor and acceptor layers enable efficient bilayer devices. By designing new materials with these characteristics, along with orthogonal solubility for layer-by-layer deposition of clean bilayers, it is suggested that the clear connection between material design and function in the bilayer structure will accelerate the development of more efficient organic photovoltaic devices. The molecular packing of active layer components has a crucial role in the device performance of OPV devices. Especially for FREA based OPV systems, the longrange structural order induced by end group 𝜋 - 𝜋 stacking is considered as the critical factor for achieving high PCEs. Chapter 5 includes a deep spectroscopic insight into the exciton and charge transport processes in a series of FREA based OPV systems having different molecular packing and ordering. Here the molecular stacking manipulation in FREAs is achieved by changing the length of alkyl side-chains so that the FREA backbone is changed from a 𝜋 -𝜋 stacking mode to a non-stacking mode. Transient absorption spectroscopic analysis of neat FREAs and blends reveals that exciton diffusion and intermolecular charge transfer processes do not necessarily rely on the molecular 𝜋 -𝜋 stacking, while close atom contact can also enable these processes. This work provides new insights into the design of advanced materials for next generation organic photovoltaics considering diverse transport channels formed by close atom interactions. Chapter 6 discusses the advantages of ternary OPV strategy in BHJ world with a 13% efficient ternary device based on a highly efficient FREA, FOIC. The introduction of a mid-band gap small molecule donor TR into the binary blend PTB7- Th:FOIC improves the open circuit voltage (Voc), short circuit current (Jsc), fill factor (FF), and thereby the overall device performance. Transient absorption spectroscopy reveals ultrafast resonant energy transfer from TR to PTB7-Th domains which is consistent with their intermixed morphology. Additionally, the study also includes the rapid long-range energy transfer from PTB7-Th to FOIC phases that</p>

Photophysics of fused ring electron acceptors for photovoltaic applications

<p>Organic photovoltaic cells (OPVs) have received significant interest over the last decade as they offer the potential of cheap renewable energy via direct photon to electron conversion of abundantly available Sun light. Compared to the inorganic equivalents, they offer lightweight, low-cost, and flexibility advantages. Conventional OPVs are typically based on blends of electron-donor materials and fullerene-based electron-acceptor materials that form bulk-heterojunctions (BHJs). But the attention has recently shifted to organic non-fullerene acceptors especially fused ring electron acceptors (FREAs) owing to their attractive properties including flexible energy levels, tunable band gap, crystallinity, and planarity. Thus the power conversion efficiency (PCE) of OPVs has recently attained a record of 16% by synthesizing FREAs with modified chemical structures. Owing to the high crystallinity and packing orientation of acceptors, FREA based OPV systems are characterized by large and pure phases sized 20-50 nm. This is consistent with the observation that charge generation dynamics in these systems lacked the ultrafast component that characterizes most fullerene blends. To test the hypothesis that the optimal phase size can be large due to facile exciton diffusion in FREAs, a planar indacenodithiophene (IDT) based FREA, IDIC is selected as a model system to study the exciton dynamics. Chapter 3 includes the exciton diffusion measurements in IDIC films using transient absorption spectroscopy which resolves a substantially high, quasi-activationless diffusion coefficient that exceeds that of typical organic semiconductors. The study also includes a deep insight of the key factors behind the enhanced exciton diffusion in IDIC and is shown to arise from different molecular and packing factors which enhance the long-range resonant energy transfer. Rapid exciton diffusion in IDIC films introduced the possibility of solution-processed bilayer devices. Chapter 4 comprises both device and photophysics of planar bilayer devices with a PCE of 11.1% which is readily accounted for the material and device design. By pairing a mid-band gap polymer donor with a range of FREAs, it is shown that the combination of high molecular packing densities and absorption coefficients, long exciton diffusion lengths, and efficient, resonant, long-range energy transfer between donor and acceptor layers enable efficient bilayer devices. By designing new materials with these characteristics, along with orthogonal solubility for layer-by-layer deposition of clean bilayers, it is suggested that the clear connection between material design and function in the bilayer structure will accelerate the development of more efficient organic photovoltaic devices. The molecular packing of active layer components has a crucial role in the device performance of OPV devices. Especially for FREA based OPV systems, the longrange structural order induced by end group 𝜋 - 𝜋 stacking is considered as the critical factor for achieving high PCEs. Chapter 5 includes a deep spectroscopic insight into the exciton and charge transport processes in a series of FREA based OPV systems having different molecular packing and ordering. Here the molecular stacking manipulation in FREAs is achieved by changing the length of alkyl side-chains so that the FREA backbone is changed from a 𝜋 -𝜋 stacking mode to a non-stacking mode. Transient absorption spectroscopic analysis of neat FREAs and blends reveals that exciton diffusion and intermolecular charge transfer processes do not necessarily rely on the molecular 𝜋 -𝜋 stacking, while close atom contact can also enable these processes. This work provides new insights into the design of advanced materials for next generation organic photovoltaics considering diverse transport channels formed by close atom interactions. Chapter 6 discusses the advantages of ternary OPV strategy in BHJ world with a 13% efficient ternary device based on a highly efficient FREA, FOIC. The introduction of a mid-band gap small molecule donor TR into the binary blend PTB7- Th:FOIC improves the open circuit voltage (Voc), short circuit current (Jsc), fill factor (FF), and thereby the overall device performance. Transient absorption spectroscopy reveals ultrafast resonant energy transfer from TR to PTB7-Th domains which is consistent with their intermixed morphology. Additionally, the study also includes the rapid long-range energy transfer from PTB7-Th to FOIC phases that</p>

Pharmacokinetic Analysis of Epithelial/Endothelial Cell Barriers in Microfluidic Bilayer Devices with an Air–Liquid Interface

As the range of applications of organs-on-chips is broadening, the evaluation of aerosol-based therapies using a lung-on-a-chip model has become an attractive approach. Inhalation therapies are not only minimally invasive but also provide optimal pharmacokinetic conditions for drug absorption. As drug development evolves, it is likely that better screening through use of organs-on-chips can significantly save time and cost. In this work, bio-aerosols of various compounds including insulin were generated using a jet nebulizer. The aerosol flows were driven through microfluidic bilayer devices establishing an air–liquid interface to mimic the blood–air barrier in human small airways. The aerosol flow in the microfluidic devices has been characterized and adjusted to closely match physiological values. The permeability of several compounds, including paracellular and transcellular biomarkers, across epithelial/endothelial cell barriers was measured. Concentration–time plots were established in microfluidic devices with and without cells; the curves were then utilized to extract standard pharmacokinetic parameters such as the area under the curve, maximum concentration, and time to maximum concentration. The cell barrier significantly affected the measured pharmacokinetic parameters, as compound absorption through the barrier decreases with its increasing molecular size. Aerosolizing insulin can lead to the formation of fibrils, prior to its entry to the microfluidic device, with a substantially larger apparent molecular size effectively blocking its paracellular transport. The results demonstrate the advantage of using lung-on-a-chip for drug discovery with applications such as development of novel inhaled therapies.

Permeability of Epithelial/Endothelial Barriers in Transwells and Microfluidic Bilayer Devices

Lung-on-a-chip (LoC) models hold the potential to rapidly change the landscape for pulmonary drug screening and therapy, giving patients more advanced and less invasive treatment options. Understanding the drug absorption in these microphysiological systems, modeling the lung-blood barrier is essential for increasing the role of the organ-on-a-chip technology in drug development. In this work, epithelial/endothelial barrier tissue interfaces were established in microfluidic bilayer devices and transwells, with porous membranes, for permeability characterization. The effect of shear stress on the molecular transport was assessed using known paracellular and transcellular biomarkers. The permeability of porous membranes without cells, in both models, is inversely proportional to the molecular size due to its diffusivity. Paracellular transport, between epithelial/endothelial cell junctions, of large molecules such as transferrin, as well as transcellular transport, through cell lacking required active transporters, of molecules such as dextrans, is negligible. When subjected to shear stress, paracellular transport of intermediate-size molecules such as dextran was enhanced in microfluidic devices when compared to transwells. Similarly, shear stress enhances paracellular transport of small molecules such as Lucifer yellow, but its effect on transcellular transport is not clear. The results highlight the important role that LoC can play in drug absorption studies to accelerate pulmonary drug development.

Synthesis and characterisation of a carbazole-based bipolar exciplex-forming compound for efficient and color-tunable OLEDs

The single-layer and bilayer devices showed blue monomer, electromer or interface exciplex emission.