Engineering of thienothiophene-based conjugated molecules for the development of efficient P-type semiconductor
Date of Issue2017
Interdisciplinary Graduate School (IGS)
Energy Research Institute @ NTU (ERI@N)
In order to reduce the cost of energy production from clean and renewable energy source, next-generation solar cells have been extensively studied. Amongst the candidates for the next generation solar cells, organic solar cells have the advantage of a higher degree of freedom in terms of materials design and selection. Decades of research in the field has resulted in impressive power conversion efficiency (PCE). However, the materials development relies heavily on trial-and-error method due to the lack of good understanding of how to translate the molecular structure to the solid-state structure in devices. In this paper, small molecules having pi conjugated backbones have been synthesized and their properties have been studied in an attempt to establish the design principles of efficient organic semiconductors for the application in organic photovoltaic (OPV) devices. Novel small molecules based on dithienocyclopentathieno[3,2-b]thiophene (DTCTT) structure which consists of thienothiophene core was synthesized and compared to the structural analogue based on Indacenodithiophene (IDT) which consists of a benzene core instead. Dense molecular packing structure due to the strong interactions between sulfur atoms in the thienothiophene unit was expected to result in an increase in the hole mobility. This is an important parameter for high-efficiency solar cells. The effect of the packing structure on optical, thermal, and charge transport properties was investigated. As expected, the DTCTT-based compound showed enhanced crystallinity and higher hole mobility than the IDT-based compound. However, when it was mixed together with the fullerene acceptor and incorporated into OPV devices, no significant increase in PCE was obtained. Although the film crystallinity of the DTCTT-based small molecule was high, uneven and rough film morphology resulted in poor device performance. The control of morphology in the solid state was the key to further improvement of device efficiency. Subsequently, derivatives of the DTCTT-based molecules with different side groups were synthesized and their self-assembled structures were analyzed in detail. This approach was attempted to understand the impact of molecular design into the solid-state structure which eventually leads to control of the morphology. Structural analysis using X-ray diffraction techniques and computational calculations revealed that the derivatives adopt a columnar structure with the slip-stacked manner of pi-pi stacking. It was also found that the derivatives with two thiophene rings in the side group showed the unique crystal-to-crystal reversible phase transition behavior in heating and cooling cycle. It was concluded that the columnar one-dimensional structure reorganized during the transitions in a similar manner to liquid crystalline materials. Due to the one-dimensional organization favorable to charge transport, a hole mobility value as high as 0.02 cm2/V·s was obtained in the field-effect transistor. The novel DTCTT-based molecules demonstrated high potential as efficient p-type organic semiconductors in solar cells and filed-effect transistors. With a rigorous structural analysis of the molecular self-assembly and the solid-state behavior, modified molecular structures were rationally proposed as a direction for the future research. The approach carried out in this thesis is believed to have provided detailed guidance for the design of efficient organic semiconductors.