Non-Conventional Building Blocks for Organic Electronics
Bulumulla, Panikki Chandima
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Organic semiconductors are potential candidates for replacing high-cost silicon electronics for low-end applications where high mobilities are not required. Owing to unique advantages such as solution processability, flexibility, lightweight, low cost with countless structural modifications, organic semiconductors can be realized for many applications using high throughput roll-to-roll fabrication techniques. Hence, a remarkable amount of scientific efforts have been dedicated to improving electronic and physical properties of these materials. Throughout the past two decades, many improvements in the field have been achieved by designing novel building blocks. Since efficiencies and mobilities in organic solar cells and transistors have stagnated, it is highly desirable to seek and develop non-conventional building blocks for organic electronics. In this dissertation, the fundamentals and recent developments of non-conventional materials are covered in Chapter 1. Operation principles, charge transport of organic field effect transistors and organic photovoltaics are introduced. Compared to conventional thiophene-based π-electron donor materials, promising non-conventional pyrrole based donor materials employed in organic electronics are discussed and summarized. Similarly, non-conventional electron acceptors could be used to fabricate organic solar cells. By using inorganic semiconducting quantum dots (QDs), organic-inorganic hybrid solar cells could be fabricated. Different systems with polymer: QD solar cells are also discussed and summarized in chapter 1. Chapter 2 describes the effect on organic field effect transistor (OFET) properties of two novel small molecules containing terminal N-dodecylthieno[3,2-b]pyrrole (TP) donors and N-dodecylfuro[3,2-b]pyrrole (FP) donors with a central thiophene flanked 5,6-difluorobenzo[c][1,2,5]thiadiazole (FBT) acceptor. The influence on frontier molecular orbital energy levels, UV-vis absorption, electrochemical properties, OFET parameters and morphological effects were investigated. In chapter 3, the effect of flanking group on banana shape small molecules is discussed by using terminal N-dodecylthieno[3,2-b]pyrrole (TP) donors, and thiophene or furan flanked benzo[c][1,2,5]thiadiazole (BT) central units. Upon changing similar flanking groups, the curvature of the small molecules was changed. Thiophene flanked small molecule showed high hole mobility of 0.08 cm2 /V s while furan flanked small molecule performs poorly due to both heteroatom effect and the degree of curvature. Chapter 4 describes the extension of thieno[3,2-b]pyrrole based small molecules to polymers. A Novel conjugated polymer is synthesized by copolymerizing N-methylthieno[3,2-b]pyrrole and 2,5-bis(2-octyldodecyl)-3,6-di(thiophen-2-yl)-2,5-dihydropyrrolo[3,4-c]pyrrole-1,4-dione (DPP) via Stille coupling polymerization. With an optimized molecular weight, the polymer exhibited high hole mobility of 0.12 cm2 /V s in OFET devices. The high hole mobility reflects the potential of the thieno[3,2-b]pyrrole building block. Inorganic QDs also can be employed as electron acceptors compared to conventional fullerene derivatives in bulk heterojunction (BHJ) solar cells. However, they do not outperform conventional fullerene counterparts. Therefore in chapter 5, a facile method is described to generate thiol functionalized block copolymers to improve the interaction between photoactive polymers and QDs. By incorporating only 17 mol% of the thiol containing block a two-fold increase in power conversion efficiency was observed. The improved interaction was supported by atomic force microscopy and photoluminescence quenching studies.