Chemical Structure Engineering of Molecular Building Blocks Towards Ambient Stable OFETs

Giri, Indrajit (2024) Chemical Structure Engineering of Molecular Building Blocks Towards Ambient Stable OFETs. PhD thesis, Indian Institute of Science Education and Research Kolkata.

Text (PhD thesis of Indrajit Giri (18RS074)) 18RS074.pdf - Submitted Version Restricted to Repository staff only Download (7MB)

Abstract

Recent outstanding achievements in the field of organic semiconductors (OSCs) have been constructed throughout the decades in point of view of being structurally designable, lightweight, low cost and mechanically flexible. It has always been projected towards the next-generation electronic devices and has immense potential for industrial applications in organic field effect transistors (OFETs), solar cells, light-emitting diodes (LED), sensors etc. Specifically, the OFETs are diversely utilized in displays, logic circuits, amplifiers etc. OSCs with high carrier mobility and good ambient stability have been robustly demanded to meet the benchmark performance. As reliable electron-transporting n-type OSC materials are lagging behind p-type OSCs, and for the industrial advancement of OFETs, p-n heterojunction and integrated circuits assembled by both types of materials, the improvement of n-type OSCs including good environment stability and high μe is in research hotspot. This thesis is derived from our efforts towards developing such materials and devices. Chapter 1 outlines the background, architecture, working principles, and defining parameters of OFET devices. Detailed discussion on the challenges faced by n-channel OFET devices in real-life applications and a brief overview of the approach towards the resolution is provided. Findings summarized in Chapter 2 emphasize the significance of balancing flexibility and rigidity in side-chain analogues to manage film state disorders and enhance solution-processed fabrication for effective charge transport in OFETs. An in-depth analysis of n-hexyl (flexible) and tertiary-butyl-cyclohexyl (rigid) NDI derivatives revealed that the flexible analogue introduces dynamic disorder, resulting in significant bulk molecular disorder. On the other hand, the rigid analogue demonstrates minimal disorder and improved μe. This study highlights the pivotal role of side-chain engineering and processing conditions in enhancing the electronic performance of OSC films. Chapter 3 focuses on enhancing ambient stability in solution-processed n-type OFETs. This project demonstrates that reducing the LUMO is not the only method to improve ambient stability; substituting appropriate end-groups in the active layer materials can also significantly enhance stability. Specifically, it compares two core-substituted naphthalenediimide (NDI) derivatives, NDIFCN2 and EHNDICN₂, which possess alkyl and perfluorinated end groups. The superior stability of devices made from NDIFCN₂, which remained stable for over 25 days under ambient conditions, was attributed to fluorinated end groups. These groups act as hydrophobic barriers, preventing moisture infiltration into the active layer and thus achieving ambient stability (with relative humidity exceeding 65%). In Chapter 4, the foremost importance is given to synthetic easiness to ensure the scalability and economic utility of active layer n-type materials. Need of intermolecular interactions to minimize the longitudinal slip between π-stacked molecules and to reduce the lattice vibrations, special care is devoted to the molecular design strategy. Additionally, the substitution should favor effective solid-state aggregation which is crucial in achieving high carrier mobility and device stability. Substitution of planar aromatics with electronegative atoms or electron-withdrawing groups to achieve n-type materials is a known concept. Such additional groups should not only endorse deep LUMO to ensure efficient electron injection at low operational bias, but also potentially lead to substantial electrostatic interactions. Here, we have achieved excellent dopant-free μe as high as 1.0 cm² V⁻¹ s⁻¹, and a very high on-off current ratio (Ion/Ioff ~10⁹) in bottom gate-top contact OFET devices fabricated by solution process deposition (spin coating). Chapter 5 concentrates on enhancing the performance of OFETs by tackling the issues of high contact resistance (RC) and threshold voltage (Vth) using EHNDIBr₂ as the OSC. High RC, caused by the energetic mismatch between metal electrode work function (WF) and the LUMO of EHNDIBr2, impedes efficient charge injection. To address this, two strategies have been implemented: self-generated interlayers and the application of a pre-applied gate voltage (Vg). The results show substantial performance improvements, offering a promising method for optimizing organic semiconductor devices.

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