Organic solar cells based on highly self-organizing semiconducting polymers

In this thesis I have studied organic solar cells (photovoltaic devices) based on a highly self-organizing polymer, regioregular poly(3-hexyIthiophene) (P3HT), because of its particular crystallization tendency leading to high charge carrier mobilities, good light-harvesting in red parts, and suitable energy band structure for an electron-donor. Prior to organic solar cell study, the pristine P3HT films have been investigated to understand their optical/electrical property and nanocrystal structure changes upon thermal annealing. As an electron-acceptor for organic solar cells, two candidates were employed: One is polymer [poly(poly(9,9-dioctylfluorene-co-benzothiadiazole) (F8BT)), another is big small molecule [[6,6]-phenyl Cgi-butyric acid methyl ester (PCBM)]. The kinds of blends used for organic solar cell fabrication were P3HT:F8BT, P3HT:PCBM, and P3HT:PCBM:F8BT. Organic solar cells were fabricated by spin-coating these blend films onto transparent conductive oxide coated substrates followed by depositing metal electrodes (sometimes inserting LiF). For better understanding of device performance changes, blend films have been examined with optical absorption, photoluminescence including time-resloved system, normal reflection mode x-ray diffraction, grazing incidence x-ray diffraction (Synchrotron), atomic force microscopy, scanning electron microscopy, high resolution transmission electron microscopy with field emission gun, transient absorption spectroscopy, and time-of-flight mobility measurement. As a result, P3HT:F8BT solar cells (maximum external quantum efficiency=~3%) showed poorer efficiency than P3HT:PCBM solar cells (maximum external quantum efficiency=~73%), though both blends have P3HT components, which is attributed to the low electron mobility of F8BT compared to PCBM. The power conversion efficiency of P3HT:PCBM solar cells has reached 4.4-5.5% at 85~8.5mW/cm^ (air mass 1.5 simulated solar illumination), which is ascribed mainly to the formation of vertical phase segregation upon thermal annealing leading to pseudo layered p-n junction. This layered structure might reduce the charge recombination between P3HT positive polaron (radical cation) and PCBM negative polaron (radical anion), a parameter that has been quantitatively analysed using a new model proposed in this work

In situ-prepared composite materials of PEDOT: PSS buffer layer-metal nanoparticles and their application to organic solar cells

We report an enhancement in the efficiency of organic solar cells via the incorporation of gold (Au) or silver (Ag) nanoparticles (NPs) in the hole-transporting buffer layer of poly(3,4- ethylenedioxythiophene):poly(styrenesulfonate) (PEDOT:PSS), which was formed on an indium tin oxide (ITO) surface by the spin-coating of PEDOT:PSS-Au or Ag NPs composite solution. The composite solution was synthesized by a simple in situ preparation method which involved the reduction of chloroauric acid (HAuCl4) or silver nitrate (AgNO3) with sodium borohydride (NaBH4) solution in the presence of aqueous PEDOT:PSS media. The NPs were well dispersed in the PEDOT:PSS media and showed a characteristic absorption peak due to the surface plasmon resonance effect. Organic solar cells with the structure of ITO/PEDOT:PSS-Au, Ag NPs/poly(3-hexylthiophene):[6,6]-phenyl-C61-butyric acid methyl ester (P3HT:PC61BM)/LiF/Al exhibited an 8% improvement in their power conversion efficiency mainly due to the enlarged surface roughness of the PEDOT:PSS, which lead to an improvement in the charge collection and ultimately improvements in the short-circuit current density and fill factor. © 2012 Woo et al.1

High efficiency, low distortion 3D diffusion tensor imaging with variable density spiral fast spin echoes (3D DW VDS RARE)

We present an acquisition and reconstruction method designed to acquire high resolution 3D fast spin echo diffusion tensor images while mitigating the major sources of artifacts in DTI-field distortions, eddy currents and motion. The resulting images, being 3D, are of high SNR, and being fast spin echoes, exhibit greatly reduced field distortions. This sequence utilizes variable density spiral acquisition gradients, which allow for the implementation of a self-navigation scheme by which both eddy current and motion artifacts are removed. The result is that high resolution 3D DTI images are produced without the need for eddy current compensating gradients or B_0 field correction. In addition, a novel method for fast and accurate reconstruction of the non-Cartesian data is employed. Results are demonstrated in the brains of normal human volunteers

Phenanthroline diimide as an organic electron-injecting material for organic light-emitting devices

We report a diimide-type organic electron-injecting material, bis-[1,10]phenanthrolin-5-yl-pyromellitic diimide (Bphen-PMDI), for organic light-emitting devices (OLEDs), which was synthesized from its monomers, pyromellitic dianhydride (PMDA) and 1,10-phenanthrolin-5-amine (PTA). The vacuum-purified Bphen-PMDI powder showed high glass transition (∼230°C) and thermal decomposition (∼400°C) temperatures, whereas neither melting point nor particular long-range crystal nanostructures were observed from its solid samples. The optical band gap energy and the ionization potential of the Bphen-PMDI film were 3.6 eV and 6.0 eV, respectively, leading to the lowest unoccupied molecular orbital (LUMO) energy of 2.4 eV. Inserting a 1 nm thick Bphen-PMDI layer between the emission layer and the cathode layer improved the device current density by 10-fold and the luminance by 6-fold, compared to the OLED without the Bphen-PMDI layer. The result suggests that an effective electron tunnel injection process occurs through the Bphen-PMDI layer. © The Royal Society of Chemistry 2012.1

Parametric cerebral blood flow and arterial transit time mapping using a 3D convolutional neural network

PurposeTo reduce the total scan time of multiple postlabeling delay (multi-PLD) pseudo-continuous arterial spin labeling (pCASL) by developing a hierarchically structured 3D convolutional neural network (H-CNN) that estimates the arterial transit time (ATT) and cerebral blow flow (CBF) maps from the reduced number of PLDs as well as averages.MethodsA total of 48 subjects (38 females and 10 males), aged 56-80 years, compromising a training group (n = 45) and a validation group (n = 3) underwent MRI including multi-PLD pCASL. We proposed an H-CNN to estimate the ATT and CBF maps using a reduced number of PLDs and a separately reduced number of averages. The proposed method was compared with a conventional nonlinear model fitting method using the mean absolute error (MAE).ResultsThe H-CNN provided the MAEs of 32.69 ms for ATT and 3.32 mL/100 g/min for CBF estimations using a full data set that contains six PLDs and six averages in the 3 test subjects. The H-CNN also showed that the smaller number of PLDs can be used to estimate both ATT and CBF without significant discrepancy from the reference (MAEs of 231.45 ms for ATT and 9.80 mL/100 g/min for CBF using three of six PLDs).ConclusionThe proposed machine learning-based ATT and CBF mapping offers substantially reduced scan time of multi-PLD pCASL

Associations among vascular risk factors, neuroimaging biomarkers, and cognition: Preliminary analyses from the Multi‐Ethnic Study of Atherosclerosis (MESA)

IntroductionLittle is known about how antecedent vascular risk factor (VRF) profiles impact late-life brain health.MethodsWe examined baseline VRFs, and cognitive testing and neuroimaging measures (β-amyloid [Aβ] PET, MRI) in a diverse longitudinal cohort (N = 159; 50% African-American, 50% White) from Wake Forest's Multi-Ethnic Study of Atherosclerosis Core.ResultsAfrican-Americans exhibited greater baseline Cardiovascular Risk Factors, Aging, and Incidence of Dementia (CAIDE), Framingham stroke risk profile (FSRP), and atherosclerotic cardiovascular disease risk estimate (ASCVD) scores than Whites. We observed no significant racial differences in Aβ positivity, cortical thickness, or white matter hyperintensity (WMH) volume. Higher baseline VRF scores were associated with lower cortical thickness and greater WMH volume, and FSRP and CAIDE were associated with Aβ. Aβ was cross-sectionally associated with cognition, and all imaging biomarkers were associated with greater 6-year cognitive decline.DiscussionResults suggest the convergence of multiple vascular and Alzheimer's processes underlying neurodegeneration and cognitive decline

Electronic Structure and Transition Energies in Polymer–Fullerene Bulk Heterojunctions

© 2014 American Chemical Society. Photocurrent spectroscopy is used to measure both the charge transfer and exciton optical absorption spectra of various bulk heterojunction organic solar cells. The energy difference between the polymer HOMO energy and the fullerene LUMO energy is obtained from the spectra, along with the disorder energy. Combining information from cells with several different polymers and fullerenes allows measurements of the energy differences between HOMO or LUMO energies for about 10 different polymers and fullerenes, with an estimated uncertainty of 50 meV. Heterojunction band offsets are obtained for the various cells, distinguishing between the excitonic and the single-carrier band offsets. The cell open-circuit voltage is shown to be closely correlated with the interface band gap. The exciton disorder energy is directly correlated to the band-tail disorder and we also consider the effects of exciton thermalization on the charge generation mechanism. The data indicate that an energy offset between the polymer exciton and the charge transfer ground state below about 0.25 eV adversely affects the cell performance, while a HOMO band offset below about 0.2-0.3 eV also degrades cell performance but by a different mechanism

Reduction of compression Artifacts (Blocking Artifacts, Ringing Artifacts) Using POSC

Our project presents an image recovery algorithm which reduced two artifacts, blocking artifact and ringing artifact, from compressed image. It is based on theory of projections onto convex sets (POCS). A new family of directional smoothness constraint sets is defined based on line processes modeling of the image edge structure. We used a divide-and –conquer (DAC) strategy for reduce the numerical difficulty. The transform-based compression has recently received considerable attention. It is widely used for both still images and video. Theoretically, the image compression is aimed to minimize the number if bits needed to represent an image without sacrificing quality. However, while we compress the information, we have to encounter undesirable artifacts. Artifacts take on several forms in images