Modulating Bulk Charge Carrier Density in Organic Semiconducting Layers and its Implications

Amjed, Khader K T (2025) Modulating Bulk Charge Carrier Density in Organic Semiconducting Layers and its Implications. Masters thesis, Indian Institute of Science Education and Research Kolkata.

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Abstract

Precise control over charge carrier density in organic semiconductors is essential for realizing their full potential in next-generation optoelectronic and spintronic devices. This work presents a comprehensive investigation into two complementary doping strategies— photosensitized transient doping and steady-state chemical doping—to modulate bulk carrier density in naphthalene diimide (EH-NDI) based semiconducting layers. In the first approach, we develop a light-responsive system employing a photosensitizer (Mes-Acr) and sacrificial donor (DIPEA) to achieve dynamic, reversible n-type doping under visible-light illumination. This method enables modulation of conductivity and synaptic-like plasticity behaviors, including short-term and long-term plasticity, laying the foundation for organic neuromorphic devices. In parallel, we explore a robust chemical doping technique using hydrazine to achieve permanent reduction of pristine and brominated EH-NDI derivatives. Spectroscopic analyses (UV-Vis, FT-IR, and EPR) confirm the formation of radical monoanions and dianions, with brominated species showing enhanced doping efficiency and radical stability. Magnetic characterization reveals that at low temperatures, spin-rich doped materials exhibit emergent ferromagnetic behavior, particularly in EH-NDI-Br4, demonstrating potential for organic magnetic materials. Together, these strategies illustrate how the temporal and chemical nature of doping can be leveraged to engineer distinct functionalities— from synaptic emulation to spin alignment—within the same organic semiconducting framework.

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