Organic Field-Effect Transistors (OFET) and Organic Electrochemical Transistors (OECT)

Organic thin-film transistors (OTFT) like OFETs are used for various applications, including flexible displays, smart sensors, organic photovoltaics and printed electronics.

Silicon-based OFET substrates provide a high-precision patterned foundation for the fabrication of organic field-effect transistors (OFETs) and allow the deposition of organic semiconductor layers that determine the electronic properties of the OFET. The resulting OFETs can be flexibly designed due to the adaptability of the materials as well as source and drain contacts.

Due to their reliability and reproducible preparation, these substrates are used worldwide by all major developers of organic semiconductor materials as part of standardized material monitoring.

The Si bulk acts as gate electrode and controls the current flow between the source and drain gold electrodes. A suitably doped Si-SiO2 interface in CMOS quality guarantees a reproducible gate contact. The gold electrodes with patented adhesive layer suppress the formation of injection barriers between the gold electrodes and the organic in the transistor channel, even for p-type semiconductors, so that reliable ohmic source / drain contacts are formed in the back-gated OFET.

In the standard layout, 60 chips of size 15 × 15 mm² each with a total of 960 individual transistor structures are realized on 200 mm wafers. Each chip contains four groups of four identical transistors with channel lengths of 2.5, 5, 10 and 20 µm. Identical layouts with graduated channel widths as well as the flexible choice of oxide thickness allow adaptation to a wide voltage and conductivity range of the materials under investigation. Customized layouts with modified electrode geometry are possible at any time.

Organic Electrochemical Transistors (OECTs) and Organic Field-Effect Transistors (OFETs) are both integral components of Organic Thin-Film Transistors (OTFTs). OECTs operate by modulating the conductivity of an organic channel through the electrochemical doping process, which offers high sensitivity and low operating voltages, making them suitable for bioelectronics and sensor applications. In contrast, OFETs control the current flow via an electric field applied through a gate electrode, providing high switching speeds and stability, which are advantageous for display technologies and flexible electronics. While OECTs excel in environments requiring aqueous compatibility and biocompatibility, OFETs are more suited for applications demanding high-speed operation and device integration. They are particularly useful for bioassays and neuromorphic circuits.

In bioassays, OECTs can detect biological signals with high sensitivity, making them ideal for applications such as monitoring glucose levels, detecting pathogens, or measuring neurotransmitter concentrations. In neuromorphic computing, OECTs can mimic the behavior of biological neurons and synapses, offering potential for developing advanced computing systems that replicate brain-like processing.

Arrays of OECTs can be designed to mimic neural networks, facilitating the development of neuromorphic computing systems. These arrays can replicate the behavior of neurons and synapses, enabling advanced computational tasks such as pattern recognition and learning.

Both types of transistors leverage the unique properties of organic materials, offering complementary strengths within the broader category of OTFTs.

Overall, the precise design and selection of electrode materials and geometries are essential for the optimal performance and reliability of OECTs and OFETs within OTFT technology.

Electrode design plays a crucial role in both OECTs and OFETs, significantly influencing the efficiency and performance of the transistors. In OECTs, the electrodes must not only provide good electrical conductivity but also be biocompatible, especially for applications in bioelectronics. Materials such as gold, platinum, or conductive polymers are commonly used. The surface of the electrodes can be functionalized to enhance interaction with biological molecules.

When using silicon as the substrate for OECTs, it provides a stable and highly reproducible platform that supports precise electrode design. Silicon substrates allow for the integration of microfabrication techniques, enabling the creation of highly reproducible and finely patterned electrodes. This precision in electrode design enhances the performance and sensitivity of the OECTs, saving time and increasing the relevance of the experiments.

The most common materials for OFETs are organic polymers or small molecules. For use in applications, these materials are applied to substrates such as glass, plastic or paper.

Testing Materials with Substrates

1. Material Selection: the organic material that has the desired electrical properties must be selected.

2. Substrate Preparation: The substrate must be clean and free of contaminants. It may be necessary to treat the substrate to improve the adhesion of the organic material.

3. Applying the Material: The organic material can be applied to the substrate using various methods

• Chemical Vapor Deposition (CVD)
• Physical Vapor Deposition (PVD)
• Atomic Layer Deposition (ALD)
• Spray-coating
• Spin-coating
• Dip-coating
• Dispensing
• Printing
• Sputtering

4. Characterization: After manufacturing the OFET, tests are conducted to measure the electrical properties such as charge carrier mobility, threshold voltage, and current on/off ratio.

Storage

Please store the wafers at a cool and dark place and protect them against sun. Storage temperature: between 15°C and 25 °C.

Recommendation for resist removal

• Step 1: Keep the OFET under purified acetone for 15 minutes covered with a lid. Avoid scratching of OFET substrates with one another thus preventing damage to the gold structures.
• Step 2: Remove the chips after the 15 minutes from acetone and rinse them immediately with IPA, to keep them wet after Acetone treatment. Then blow drying them with Nitrogen 5.0 to prevent the stains from drying.
• Step 3: Put the substrates into a new Acetone bath for further 5 minutes.
• Step 4 : The OFET substrates are then individually taken out of the acetone bath and rinsed immediately (keep them wet !) with IPA before blow drying with Nitrogen 5.0.
• Step 5: Keep the substrates for 10 min under purified IPA.
• Step 6 : Transfer the OFET samples into a clean beaker with purified IPA and subjected to ultrasonic bath for 5 minutes (at 60 °C). The ultrasonic bath makes sure the resist particulates which were left behind from previous steps naturally fall off during the process leaving behind a clean OFET substrate. Care should be taken when handling the substrates after cleaning to prevent any damage or contamination of the transistor structures. Follow the safety instructions by using ultrasonic bath in combination with IPA !
• Step 7 : After the ultrasonic treatment, the substrate with OFET samples should be cleaned with deionized water followed by blow drying with Nitrogen 5.0

• Towards implementation of logic circuits based on intrinsically reconfigurable organic transistors

• Modelling and physical implementation of ambipolar components based on organic materials
• Improving the selectivity to polar vapors of OFET-based sensors by using the transfer characteristics hysteresis response
  
• Dicyanovinyl substituted oligothiophenes: Thermal stability, mobility measurements, and performance in photovoltaic devices
• Characterization of liquid crystalline phthalocyanines for OFET
• Electrostatic discharge sensitivity investigation on organic field-effect thin film transistors
• Deposition of composite materials using a wire-bar coater for achieving processability and air-stability in Organic Field-Effect Transistors (OFETs)
• Increase the field-effect mobility of regioregular poly (3-hexylthiophene) by introducing fixed acceptor molecules
• Improved photon harvesting by employing C [sub] 70 [/sub] in bulk heterojunction solar cells
• Long-term stabilization of sprayed zinc oxide thin film transistors by hexafluoropropylene oxide self assembled monolayers

Recent publications by Fraunhofer IPMS

• T. Stoppe, A. Graf, T. Ludewig, O. Hild: Characterization of Organic Semiconductors and Conductors by Means of Conductivity and Field Effect Using the Example of Graphene. 
• T. Stoppe, A. Graf, O. R. Hild: Electrical characterization of silicon-based OFET substrates spray coated with water-based graphene solution