Understanding the Optical and Photophysical Properties of Organic and Hybrid Macromolecules and Polymers for Solar Cell Application

Organic solar cell materials represent a better sustainable alternative relative to inorganic materials in terms of lower cost, ease of large-scale processing, better absorptivity, ease of tunability, and unique flexibility so as to be used on different kinds of surfaces. With the knowledge of the fundamental molecular structure of an organic solar cell material, scientists and engineers can predict the electronic and optically-excited properties. Hence, understanding the structure-property-function relationships is paramount in optimizing the solar cell device performance. This thesis is structured into two sections. The first section focuses on electron acceptors in the active layer of bulk heterojunction (BHJ) device architectures, and the second section discuses organic and hybrid electron donors. In the first section, the optical and photophysical properties of several structural variations of a special class of non-fullerene acceptor compound – perylene diimide (PDI), are described in substantive details with a number of studies. In the active layer, this material functions as the electron acceptor, and acts in tandem as an excellent light-harvester. The first study on intramolecular singlet exciton fission (iSEF) in PDI trimers – the generation of two triplet charges from one photogenerated singlet, seeks to elucidate the important structural features required to obtain high yield triplet formation as a result of multiexciton generation and subsequent separation in multichromophoric PDI systems. Time-resolved spectroscopic measurements were used to show how the flexibility of the π-bridge connections in these multichromophoric PDI systems, strongly affect the triplet yield and triplet formation rate. The results obtained showed that the weak electronic coupling observed in the twisted PDI trimer is necessary to activate iSEF in multichromophoric systems. The next chapter is about the effect of ring-fusion on the optically-excited properties of N-annulated thiophene π-bridged PDI dimers. The results of this study show that ring-fusion favors ultrafast photoinduced intramolecular charge transfer (CT) and opens up the triplet excited state deactivation pathway that was absent in the unfused dimer. The triplets were formed via spin-orbit CT intersystem crossing pathway owing to the strong electronic coupling present in the fused-ring thiophene π-bridged dimer. The final chapter about PDIs involves two analogous positional isomers, exhibiting twisted vs planar geometries. The results confirm an efficient and faster intramolecular CT mechanism (symmetry-breaking) taking place in the planar PDI dimer, leading to better device performance despite strong aggregation effects. The second section involves the electron-donating portion of the active layer of BHJ devices. For donor polymers, the influence of furan vs thiophene π-bridge heterocycles and linear vs bulky sidechains on the optical properties of donor–π-bridge–acceptor polymers, was investigated. The results showed that the furan π-bridge polymer displayed better solar absorptivity and showed a more planar polymeric backbone that correlated to better CT and longer excitonic lifetimes. Also, the linear sidechain polymers showed improved solution-processability leading to better photophysical properties, like longer fluorescence lifetime, relative to the bulky sidechain polymers in BHJs. The final chapter of this dissertation covers three structural variations (Cube, Half and Corner) of hybrid silsesquioxanes, functionalized with donor chromophore(s) at the edge(s) of the cage unit. Improved excitation energy transport and ultrafast CT was observed in the Cube compound relative to the Half and Corner systems attributed to strong electronic coupling. A detailed summary of this dissertation alongside a set of molecular geometry guidelines for structure-function relationships and future direction, was provided.PHDChemical EngineeringUniversity of Michigan, Horace H. Rackham School of Graduate Studieshttp://deepblue.lib.umich.edu/bitstream/2027.42/167973/1/kizmadu_1.pd

The Burqa Ban in France and Its Potential Implications on Islamic Terrorism

Islamic terrorism has become a global problem which has resulted in human, social, political, and economic costs. Many Islamic terrorist organizations have focused their attacks on the West and its interests. They justify these acts by making reference to policies of the West, which they believe are inimical to Islam. France, a Western country, recently introduced a law which bans the Islamic face veil (the burqa) in public places in the country. This study examined the implications of this law. The research question focused on the perceived relationship between this law and increased acts of Islamic terrorism. The theoretical construct of this study was based off Huntington\u27s clash of civilizations theory. Data were collected through in-depth, open ended interviews with 23 Muslim participants who were identified through a snowball sampling technique. These participants represented a variety of nations, including France. Data were inductively coded and then organized around key themes and ideas. Findings indicate that participants did not perceive a relationship between increased acts of Islamic terrorism and the burqa ban specifically, but they did feel the burqa ban undermines Islam. These findings were consistent with the study\u27s theoretical construct as they revealed an underlying conflict between some Western countries and Islam. These findings contribute to positive social change as they highlight areas where policy makers have opportunities to build more culturally sensitive relations between the West and Islam, perhaps eventually serving to reduce future radicalization and religious extremism

Heteroatom and side chain effects on the optical and photophysical properties: ultrafast and nonlinear spectroscopy of new Naphtho[1,2-b:5,6-b ']difuran donor polymers

The photophysical and electronic properties of four novel conjugated donor polymers were investigated to understand the influence of heteroatoms (based on the first two member chalcogens) in the polymer backbone. The side chains were varied as well to evaluate the effect of polymer solubility on the photophysical properties. The donor–acceptor polymer structure is based on naptho[1,2-b:5,6-b′]difuran as the donor moiety, and either 3,6-di(furan-2-yl)-1,4-diketopyrrolo[3,4-c]pyrrole or 3,6-di(thiophen-2-yl)-1,4-diketopyrrolo[3,4-c]pyrrole as the acceptor moiety. Steady-state absorption studies showed that the polymers with the furan moiety in the backbone displayed a favorable tendency of capturing more solar photons when used in a photovoltaic device. This is observed experimentally by the higher extinction coefficient in the visible and near-infrared regions of these polymers relative to that of their thiophene counterparts. The excitonic lifetimes were monitored using ultrafast dynamics, and the results obtained show that the type of heteroatom π-linker used in the backbone affects the decay dynamics. Furthermore, the side chain also plays a role in determining the fluorescence decay time. Quantum chemical simulations were performed to describe the absorption energies and transition characters. Two-photon absorption cross sections (TPA-δ) were analyzed with the simulations, illustrating the planarity of the backbone in relation to its torsional angles. Because of the planarity in the molecular backbone, the polymer with the furan π-linker showed a higher TPA-δ relative to that of its thiophene counterpart. This suggests that the furan compound will display higher charge transfer (CT) tendencies in comparison to those of their thiophene analogues. The pump–probe transient absorption technique was employed to probe the nonemissive states (including the CT state) of the polymers, and unique activities were captured at 500 and 750 nm for all of the studied compounds. Target and global analyses were performed to understand the dynamics of each peak and deduce the number of components responsible for the transient behavior observed respectively. The results obtained suggest that the furan π-linker component of a donor and acceptor moiety in a conjugated polymer might be a more suitable candidate compared with its more popular chalcogenic counterpart, thiophene, for use as donor materials in bulk heterojunction photovoltaic devices.Support for this investigation is provided by the National Science Foundation (DMR-1709005) Polymers (TGIII) and (DMR 1410088/1640297) Polymers (MJE). (DMR-1709005 - National Science Foundation; DMR 1410088/1640297 - National Science Foundation)Accepted manuscrip

Extending the Technology Acceptance Model in the Impact of Vocational Educators' Gender, Experience and Perception

The study considered extending the technology acceptance model in learning-management-systems in TVET: The impact of vocational educators’ gender, experience and perception. A mixed research design was adopted for this study with 1314 TVET educators as the population. A sample of 386 respondents comprising of 234 and 152 vocational educators from federal and state institutions were selected based on Taro Yamene’s formula afterwards, proportionate sampling technique was used to determine the number of the vocational educators for each institution. The instrument for data collection was a structured questionnaire titled: “TVET Educators Experience and Perception of LMS Questionnaire” (TEEPLMSQ). The reliability of the instrument was determined using Cronbach’s Alpha which yielded reliability indices of (α = .761); (α = .816), (α = .832) with a high grand coefficient of 0.80 for vocational educators’ perception, LMS skills possessed and barriers militating against LMS use respectively. Data collected were analyzed using bivariate correlation, regression, and also path analysis through 5000 re-samples bias-corrected (BC) bootstrapping method, as well as confirmatory factor analysis. The result of the study revealed that vocational educators in Enugu and Anambra States of Nigeria perceived LMS as a credible ICT tool that supports the teaching of TVET courses and enhances the transfer of TVET skills to TVET students. The path analytical results revealed that lecturers’ characteristic (experience) has full mediation effects between the LMS skills possessed and level of LMS usage. Based on the findings of the study, it was recommended among others that The TVET educators should adopt LMS in teaching and learning of TVET courses in Nigerian tertiary institutions

Heteroatom and Side Chain Effects on the Optical and Photophysical Properties: Ultrafast and Nonlinear Spectroscopy of New Naphtho[1,2‑<i>b</i>:5,6‑<i>b</i>′]difuran Donor Polymers

The photophysical and electronic properties of four novel conjugated donor polymers were investigated to understand the influence of heteroatoms (based on the first two member chalcogens) in the polymer backbone. The side chains were varied as well to evaluate the effect of polymer solubility on the photophysical properties. The donor–acceptor polymer structure is based on naptho­[1,2-<i>b</i>:5,6-<i>b</i>′]­difuran as the donor moiety, and either 3,6-di­(furan-2-yl)-1,4-diketopyrrolo­[3,4-<i>c</i>]­pyrrole or 3,6-di­(thiophen-2-yl)-1,4-diketopyrrolo­[3,4-<i>c</i>]­pyrrole as the acceptor moiety. Steady-state absorption studies showed that the polymers with the furan moiety in the backbone displayed a favorable tendency of capturing more solar photons when used in a photovoltaic device. This is observed experimentally by the higher extinction coefficient in the visible and near-infrared regions of these polymers relative to that of their thiophene counterparts. The excitonic lifetimes were monitored using ultrafast dynamics, and the results obtained show that the type of heteroatom π-linker used in the backbone affects the decay dynamics. Furthermore, the side chain also plays a role in determining the fluorescence decay time. Quantum chemical simulations were performed to describe the absorption energies and transition characters. Two-photon absorption cross sections (TPA-δ) were analyzed with the simulations, illustrating the planarity of the backbone in relation to its torsional angles. Because of the planarity in the molecular backbone, the polymer with the furan π-linker showed a higher TPA-δ relative to that of its thiophene counterpart. This suggests that the furan compound will display higher charge transfer (CT) tendencies in comparison to those of their thiophene analogues. The pump–probe transient absorption technique was employed to probe the nonemissive states (including the CT state) of the polymers, and unique activities were captured at 500 and 750 nm for all of the studied compounds. Target and global analyses were performed to understand the dynamics of each peak and deduce the number of components responsible for the transient behavior observed respectively. The results obtained suggest that the furan π-linker component of a donor and acceptor moiety in a conjugated polymer might be a more suitable candidate compared with its more popular chalcogenic counterpart, thiophene, for use as donor materials in bulk heterojunction photovoltaic devices

Charge Transfer and Aggregation Effects on the Performance of Planar vs Twisted Nonfullerene Acceptor Isomers for Organic Solar Cells

Newly synthesized perylene diimide dimers were investigated as nonfullerene electron acceptors for organic solar cells. In particular, two analogous positional isomers exhibiting twisted vs planar geometries were prepared to make a direct comparison of their optical and electronic properties. These properties were investigated to provide information regarding the impact of the nonfullerene acceptor geometry on the ultimate photovoltaic performance. The two isomers exhibited strikingly different optical and photophysical properties in solution as well as in film. The ultrafast spectroscopic investigation in solution revealed the occurrence of charge transfer upon photoexcitation, which takes place more efficiently in the planar isomer. This is also supported by theoretical simulations. The planar conformation exhibits higher aggregation in the neat film as well as in the blend. However, our results suggest that the dominance of intramolecular charge transfer in the planar isomer is the crucial factor in determining the improved power conversion efficiency of organic solar cells