Organic field-effect transistors (OFETs) have gained significant attention due to their potential in flexible electronics, low-cost fabrication, and compatibility with large-area processing. A critical aspect of OFET performance is the electrode-contact interface, which directly impacts charge injection efficiency and overall device characteristics. The choice of electrode materials and contact engineering strategies plays a pivotal role in minimizing contact resistance and optimizing device performance.

Electrode materials such as gold (Au), silver (Ag), and graphene are commonly used in OFETs due to their distinct electrical and structural properties. Gold is widely favored for its high conductivity, chemical stability, and suitable work function (~5.1 eV), which aligns well with many organic semiconductors. However, its high cost limits scalability. Silver offers higher conductivity and lower cost but suffers from oxidation under ambient conditions, which can degrade contact performance over time. Graphene, a two-dimensional material, presents unique advantages such as mechanical flexibility, transparency, and tunable work function (4.5–5.0 eV), making it attractive for flexible and transparent OFET applications.

Work function matching between the electrode and the organic semiconductor is crucial to minimize the energy barrier for charge injection. A large mismatch leads to high contact resistance, reducing device performance. For p-type OFETs, gold is often preferred due to its work function proximity to the highest occupied molecular orbital (HOMO) of many organic semiconductors. However, for n-type OFETs, lower work function materials like calcium or aluminum are typically required, though they suffer from poor environmental stability. Interfacial doping can mitigate injection barriers by introducing charge-transfer layers that facilitate carrier injection. For example, doping the semiconductor-electrode interface with strong electron acceptors (e.g., F4-TCNQ) can enhance hole injection in p-type OFETs.

Contact engineering strategies are essential to further reduce contact resistance and improve charge injection. Self-assembled monolayers (SAMs) are widely used to modify electrode surfaces, tuning work functions and improving interfacial compatibility. For instance, thiol-based SAMs on gold electrodes can adjust the work function by up to 1 eV, depending on the terminal group (-CH3, -CF3, -NH2). Buffer layers, such as transition metal oxides (MoO3, WO3) or conjugated polyelectrolytes, can also enhance injection by forming ohmic contacts. These layers act as charge-transport bridges, reducing the energy barrier between the electrode and semiconductor.

The deposition method of electrodes significantly impacts device performance. Evaporated electrodes, formed through thermal or electron-beam evaporation, provide high-purity, well-defined contacts with precise thickness control. This method is ideal for research-scale devices but is less scalable for industrial production. Printed electrodes, including inkjet-printed or screen-printed conductive inks, offer cost-effective, large-area fabrication but often suffer from higher roughness and lower conductivity compared to evaporated films. Recent advances in nanoparticle-based inks and post-processing techniques (e.g., thermal annealing, laser sintering) have improved printed electrode performance, narrowing the gap with evaporated contacts.

Comparative studies between evaporated and printed electrodes reveal trade-offs in OFET performance. Evaporated gold electrodes typically yield lower contact resistance (10^2–10^3 Ω·cm) due to their smooth morphology and intimate contact with the semiconductor. In contrast, printed silver electrodes may exhibit higher resistance (10^3–10^4 Ω·cm) due to grain boundaries and interfacial defects. However, printed graphene electrodes, while still under development, show promise with contact resistance values approaching those of evaporated metals when optimized with doping or SAM treatments.

Interfacial morphology also plays a critical role in charge injection. Rough electrode surfaces can lead to poor semiconductor coverage and increased trap states, degrading device performance. Atomic force microscopy (AFM) studies confirm that evaporated electrodes generally exhibit smoother surfaces (RMS roughness <1 nm) compared to printed electrodes (RMS roughness >5 nm). Techniques such as surface planarization with polymer interlayers or post-deposition annealing can mitigate these issues in printed electrodes.

Long-term stability of OFET contacts remains a challenge, particularly for printed electrodes exposed to environmental stressors. Silver electrodes are prone to sulfidation and oxidation, increasing contact resistance over time. Encapsulation strategies, such as thin-film barrier coatings or inert SAMs, can improve environmental stability. Graphene electrodes, with their inherent chemical inertness, offer superior stability but require further development to achieve consistent performance in large-area devices.

In conclusion, electrode materials and contact engineering are critical for optimizing OFET performance. Gold remains a benchmark for p-type OFETs due to its work function and stability, while graphene emerges as a versatile alternative for flexible and transparent applications. Contact engineering through SAMs, buffer layers, and interfacial doping effectively reduces injection barriers. Evaporated electrodes provide superior performance in research settings, whereas printed electrodes offer scalable manufacturing potential with ongoing improvements in materials and processing. Future advancements in electrode design and deposition techniques will further enhance OFET performance, enabling broader adoption in next-generation electronic applications.