Overcoming Processing Constraints in Organic Electronics: Crosslinking Strategies and Ultra-Thin Film Integration

Abstract

The evolution of modern electronics has been dominated by Si-based technologies for many years, as they have enabled reliable, powerful, and compact devices. While the silicon-based technologies are powerful, they face severe limitations in flexible, biodegradable, and large-area electronics. In this context, organic field-effect transistor offers a compelling alternative to the traditional inorganic transistor as it provides solutions-based processing, low temperature fabrication, and compatibility with various flexible and biodegradable substrates. To fully unlock the performance potential of Organic Field-Effect Transistors (OFETs), precise patterning of the polymer semiconductor layer is essential. Such patterning plays a critical role in reducing crosstalk between adjacent transistors, lowering parasitic capacitance, and ultimately improving device speed and scalability. However, achieving this level of precision is particularly challenging with organic materials. Unlike their inorganic counterparts, polymer semiconductors are often incompatible with conventional lithographic processes due to their sensitivity to solvents and processing conditions, which can lead to performance degradation in polymer-based devices. This challenge becomes even more pronounced in advanced OFET architectures, where we need to define a pattern on top of the polymer layer. Deposition and patterning of functional layers such as metal electrodes, dielectrics, or encapsulants on top of the polymer impose additional challenges as the underlying polymer material must remain chemically stable and electrically active throughout the fabrication process. To address all these challenges, a direct patterning method based on electron-beaminduced crosslinking (EBIC) is developed. This technique allows the effortless patterning of P3HT without the need for conventional preprocessing elements, such as resists. A comprehensive suite of characterization techniques, such as Raman spectroscopy, photoluminescence (PL) spectroscopy, atomic force microscopy (AFM), ultraviolet photoelectron spectroscopy (UPS), and Kelvin probe force microscopy (KPFM), is used to assess the impact of EBIC patterning on P3HT. Each technique provides insights into different aspects of the P3HT material properties post-patterning, from chemical structure to electronic properties. Further, the electrical properties of the EBIC-patterned P3HT OFETs are meticulously evaluated across different e-beam dose values to determine the optimal conditions that preserve the intrinsic properties of P3HT while enhancing device performance. Dose values of moderate beam energy are optimized to circumvent any potential performance degradation during the patterning process. These advancements have significantly enhanced the performance of organic field-effect transistors (OFETs) by increasing the ION/IOFF ratio by about six orders of magnitude, reducing the subthreshold swing, and decreasing the interface trap density by an order of magnitude. This work presents a direct and efficacious approach to pattern P3HT-based OFETs, demonstrating their substantial potential to overcome challenges in polymer patterning, reduce device dimensions, and improve overall device performance. Further, the lithography process on polymer surfaces is successfully demonstrated using an orthogonal photoresist for polymers exhibiting no solubility in acetone through the fabrication of top-contact OFETs. DPP-DTT, PBTTT-C12, and P3HT top-contact OFETs are fabricated using this technique. To evaluate potential degradation in device performance caused by the lithography process, both bottom- and top-contact OFETs are fabricated, and their device parameters, operational stability, and bias stress reliability are analyzed. The top-contact devices fabricated using orthogonal photoresist exhibited no significant signs of deterioration and demonstrated superior operational stability compared to their bottom-contact counterparts. This work paves the way for the fabrication of more advanced multilayer OFET architectures and their potential for large-scale integration. In parallel with patterning strategies, an ultra-thin P3HT film with precise thickness control is developed. Ultra-thin polymer films have remarkable physical, chemical, and mechanical properties that the bulk films cannot achieve. They are more promising for flexible and transparent organic electronics due to their enhanced flexibility, transparency, and lightweight. This work enables reproducible tuning of P3HT film thickness from 0.9 nm (monolayer) to 10.7 nm (four layers) by systematic variation of e-beam energy and dose, along with lateral patterning. The transparency of these ultra-thin P3HT films increases with decreasing P3HT film thickness. PL spectra show that all these ultra-thin P3HT films have dominating intrachain coupling. UPS confirms that increasing the e-beam energy and dose values reduces the work function, consistent with progressive trap passivation and n-type doping. Small channel OFETs fabricated with these ultra-thin P3HT films demonstrate high ION/IOFF (exceeding 108), low subthreshold swing (0.98-1.88 V/dec ), and low contact resistance (44.6 kΩ ). This work provides a scalable and versatile platform for fabricating patterned ultra-thin polymer films with state-of-the-art electrical performance, thereby enabling their potential integration into high-sensitivity flexible sensors, photodetectors, memory devices, and wearable electronic systems. In the end, the application of polymer patterning is demonstrated by fabricating a depletion load inverter circuit. In order to fabricate a depletion load inverter circuit, the enhancement-type driver OFET is fabricated using the EBIC technique, and the depletion-type load OFET is fabricated using the optical lithography technique. The electron beam used in EBIC patterning induces n-type doping in P3HT films, enabling threshold voltage tuning. Top-gate, bottom-contact OFET structures employing Al2O3 as the gate dielectric are fabricated for the realization of a depletion-load inverter circuit. Voltage transfer characteristics of the depletion load inverter circuit show good switching characteristics and gain higher than that of the resistive load optically and EBIC-patterned P3HT inverter circuit.