Design Strategies for Stretchable Organic Semiconductors via Systematic Synthesis and Characterization

Abstract

Developing stretchable semiconducting polymers (SCPs) for organic solar cells (OSCs) is challenging because of the conflicting requirements for high mobility and durable mechanical properties in material design rules. While the amorphous regions in SCPs with low Tg are ideal for mechanical compliance, the crystalline regions in SCP are ideal for at least charge transport. The reported approaches enable materials to be stretched under strain without cracking by lowering their tensile modulus. However, softening SCPs is not enough to achieve stretchability of materials. The softened materials behavior upon multiple stretching cycles is rarely discussed. These polymer films must maintain the same electrical / morphological characteristics after releasing strain for practical application in stretchable electronics. Although increased softness of the material may improve cracking strain, the stretchable material must resist repeated strain and fatigue after the release cycle and must exhibit robust electrical performance. Basically, in the case of elastic SCPs, elasticity is an important property because it allows the material to maintain structural integrity. This is the inspiration and subject of my Ph.D. research, and I have conducted everything from the design, synthesis, optical and electrochemical analysis of semiconducting polymers increasing deformability, elasticity, eventually stretchability. After that, the research was conducted as a process of analyzing the device characteristics in collaboration with co-workers to find out the suitability and possibility of molecular design for application to next-generation organic electronic devices. In Chapter 1, the basic properties of semiconducting polymers and the design strategy of stretchable semiconducting polymers, especially for organic solar cells, and the fabrication method for improving stretchability were briefly introduced. Chapter 2and 3 treat experimental procedures and approaches of stretchable SCPs, respectively. The design strategies are as follows: 1) I assumed that molecular miscibility among blend components can control the morphological changes under external stress condition. Therefore, I designed a new naphthalenediimide (NDI)-based random copolymer (asy-PNDI1FTVT), incorporating a fluorine atom on the dithienylethene unit (asy1FTVT). By substituting a fluorine atom, the physical properties (e.g., crystallinity, molecular miscibility) of asy-PNDI1FTVT were significantly altered, breaking symmetricity of the TVT unit, which differed from symmetric polymers (PNDITVT, PNDI2FTVT); 2) I synthesized new siloxane-based crosslinkable additive, DBr-siloxane, introduced for increasing amorphous fraction of the rigid active film. The novel DBr-siloxane can provide both of enhanced ductility by siloxane chain and elasticity by crosslinking method. The crosslinkable additive was based on radical producible bromide (Br) functional group owing to their strong crosslinking ability (i.e., reactivity of crosslinking groups: Br > azide > oxetane > vinyl). Before I developed new SCPs for stretchability, I adopted this DBr-siloxane to PTB7-Th:PC71BM system to explore a new role of crosslinkable additive that the macroscopic phase deformation is more prominent than with the non-fullerene system under harsh thermal and strain conditions. The agglomeration or dimerization of fullerenes results in severe long-term instability under operating conditions. Furthermore, in this work, I present the synthesis and characterization of a PBDB-T-based SCP containing 20 mol% of 2-ethylhexyl ester thiophene units (PBDB-COO) at the end of side chains, which can be radical cleavage under UV irradiation and reacted with DBr-PDMS. I have investigated the cross-linking between elastic cross-linker and SCPs on the morphology, and mechanical characteristics; 3) I report a series of stretchable SCPs (P-PDMSX, X = 0–20) containing a hydrophobic poly(dimethylsiloxane) (PDMS) as a backbone. The addition of PDMS-CBSs into the polymer backbone improves its pre-aggregation of a reference polymer, PBDB-T, as well as the molecular compatibility with small molecular acceptors. These results expect that P-PDMSx would form the optimal blend morphology in SCP/acceptor BHJs and exhibit excellent mechanical durability under strain; 4) I present the synthesis and characterization of new conjugation polyelectrolytes that can maintain electron mobility at thickness above 10 nm and contain high stretchability. I opted the SCPs than small molecular ETMs, which can clearly show high mechanical durability under strain. Additionally, I expect that near amorphous property of PFN-based SCPs (i.e., weak donor – weak acceptor type) certainly increase the molecular deformability than conventional NDI-based CPEs (i.e., weak donor – strong acceptor type). I synthesized new PFN-based CPE containing trifluoromethanesulfonyl imide as a counter ion, and developed new ETM composites by blending with a highly stretchable ionic conductor, N50, in various weight ratios. I changed the counter ion of existing CPEs (PFN-Br) to TFSI ion, which can act as a plasticizer due to combination of size and high degree of charge delocalization. In this chapter, I have figured out the effects of the side chains covering the ion contained in SCPs on the polymer solubility and film morphology. Moreover, I have investigated the ionic cross-linking between SCPs and stretchable ionic conductor and SCPs on the mechanical properties, morphology, and optoelectrical characteristics of OSCs. In conclusion, in this study, based on an in-depth study on the development of stretchable semiconducting polymers for organic solar cells through molecular structure design, the optimal molecular structure and synthesis route were found. I believe that this result suggests a promising new strategy in the field of stretchable organic semiconductors research for the commercialization of not only organic solar cells but also various organic electronic devices.