Overcoming fundamental challenges in n-type organic field effect transistors

Chhetri, Shant (2026) Overcoming fundamental challenges in n-type organic field effect transistors. PhD thesis, Indian Institute of Science Education and Research Kolkata.

Text (PhD thesis of Shant Chhetri (20RS043)) 20RS043.pdf - Submitted Version Restricted to Repository staff only Download (8MB)

Abstract

The development of molecular semiconductors that can overcome the intrinsic constraints of extrinsic structural heterogeneity and intrinsic dynamic disorder is essential to the progress of organic electronics. The interaction of charge transport physics, supramolecular assembly, and molecular engineering in naphthalene diimide (NDI)-based n-type semiconductors is methodically investigated in this thesis. This work establishes important design guidelines for high-mobility organic field-effect transistors (OFETs) by tackling important issues such as lattice stiffness, polymorphism, and halogen-mediated packing. The theoretical framework is established in Chapter 1, which also describes the device architecture, operating principles, and defining parameters of OFETs. It offers a thorough analysis of the unique difficulties that n-channel organic devices face, especially with regard to ambient stability and charge trapping, and it establishes the context for the particular design techniques used in this study. The molecule EhB2 is identified as an example of the "ideal" organic semiconductor in Chapter 2, which concentrates on intrinsic transport processes. EhB2 demonstrates a unique combination of a highly rigid lattice that minimizes relative disorder (σ/J = 0.23) and enormous effective electronic coupling (Jeff ≈ 167 meV) through careful side-chain engineering. This material acts as a structural blueprint, showing that the key factor in obtaining quasi-band-like transport is immunity to dynamic disorder via robust 2D brickwork packing. Chapter 3 examines the effects of polymorphism in the semiconductor C12B2, moving from intrinsic design to supramolecular control. The work shows a clear dimorphism in which the production of either high-mobility 2D sheet-like structures (Agg2) or low-mobility needle-like aggregates (Agg1) is controlled by solvent polarity. Expanded lamellar packing in the Agg2 phase results in a 50-fold increase in electron mobility (μsat = 0.95 cm²V⁻¹s⁻¹) with high on/off ratios (108), underscoring the need to manage phase thermodynamics for device repeatability. The particular function of halogen bonding in a number of brominated NDIs is explained in Chapter 4. The study shows that dispersed bromine orbitals cause a thermally stable, longitudinal reorganization into a brickwork layout and greatly improve electronic coupling. In contrast to non-brominated equivalents, this work offers the first comprehensive proof that diffuse orbitals control molecular alignment and film smoothness, reducing trap-rich grain borders and producing better charge transmission. Post-deposition processing for pyridine-functionalized NDIs (o, m, and p-PyNDI) is covered in Chapter 5. It has been demonstrated that heat annealing causes a crucial change in molecular-scale assembly, especially for the m-PyNDI isomer, which greatly improves device performance. Together, these results offer a thorough road map for creating high-mobility, defect-tolerant organic semiconductors through orbital engineering, processing protocols, and intermolecular interaction manipulation.

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