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Organic transistors presentation for Graduate Course.
- 1. FLEXIBLE & ORGANIC TRANSISTORS EE6183 By Joshua H Rouleau Russ College of Engineering and Technology, Ohio University
- 2. OUTLINE • Key History • Introduction • Background • Theory • Research Examples • Challenges • Simulation Tools • Conclusion Courtesy of Someya Group, Organic Transistor Lab of the Department of Electric and Electronic Engineering school of Engineering, University of Tokyo
- 3. KEY HISTORY OF THE TRANSISTOR • 1945-Team assembled at Bells Lab • 1947-Bardeen and Brattain discovered the point-contact transistor. • 1948- Shockley invents Junction transistor. • 1965- Moore’s Law is Born. • 2017 – Current Era of Transistors Moore's Law as expressed in Density versus Time.
- 4. INTRODUCTION OF THE FLEXIBLE/ORGANIC TRANSISTOR • Lightweight • Flexibility • Low cost (manufacturable) • Low temperature • Biodegradable Above- Figure A and B both show Flexible organic transistors arrays on plastic substrates. .
- 5. THEORY/CHARACTERISTICS • HOMO/LUMO • Crystal structure • Purity • Orientation • Uniformity Above- Representation of a thin film transistors in three different orientations.
- 6. THEORY/CHARACTERISTICS Above- Representation of P channel versus N channel organic transistor Above- Summary of OFET performance of molecule N- channel semiconductors. Right - Summary of OFET performance of molecule P- channel semiconductors.
- 7. THEORY/CHARACTERISTIC'S Above- Organic FET drain current in initial state a saturation Below –Band Diagram of Organic TFT. Above- Drain Current as a function of Voltage
- 9. TRANSISTOR ARRAYS BASED ON SOLUTION PROCESSED CRYSTALS – Sensory and scanning feature devices – Biofeedback devices – Tracking of physiological functions – Low cost – Wearable sensors – Implantable electronics
- 10. ULTRA FLEXIBLE ORGANIC AMPLIFIER WITH BIO- GEL ELECTRODES • In vivo electronic monitoring systems – Using biocompatible gel – Highly Conductive – Multi walled carbon nanotube disperse sheet.
- 11. APPLICATIONS • Displays (OLED) – Consumer products (phones, computers, T.V.’s, etc.) – Industrial products • Solar cells (OPV) • Smart Textiles – Wearable sensors – Integrated transistor clothing • Lab-on-a-Chip – Medical detection
- 12. CHALLENGES • Instability in organic material • Shorter Life spans (dependent on stable environment) • Manufacturing reliable electrical contacts • Low carrier mobility (smaller Bandwidths) • Conductive Polymers have High Resistance
- 13. SIMULATION TOOLS AND EXAMPLES • Bulk Heterojunction Morphology Generator • Exciton Annihilation Simulator • Exciton Dynamics Simulator • Polymer Modeler • NuSIMM – Molecular Simulator • Circuit on Cellulose: Presentation
- 14. SUMMARY AND CONCLUSION • Lightweight • Flexibility • Low cost (manufacturable) • Low temperature • Biodegradable
- 15. REFERENCES • Sekitani, Tsuyoshi, et al. “Ultraflexible organic amplifier with biocompatible gel electrodes.” Nature Communications, vol. 7, 2016, p. 11425., doi:10.1038/ncomms11425. • Bredas, J. L., et al. “Organic semiconductors: A theoretical characterization of the basic parameters governing charge transport.” Proceedings of the National Academy of Sciences, vol. 99, no. 9, 2002, pp. 5804–5809., doi:10.1073/pnas.092143399. • Deiptii Das Follow. “Organic transistors.” LinkedIn SlideShare, 18 Aug. 2012, www.slideshare.net/deeptishankardas/organic-transistors. • “JST ERATO Someya Bio-Harmonized Electronics Project.” Members | Someya Group Organic Transistor Lab, www.ntech.t.u- tokyo.ac.jp/en/introduction/index.html. • Mazhari, Baquer. “Introduction to organic electronics (EE611 lecture 1 2015).” YouTube, YouTube, 4 Jan. 2015, www.youtube.com/watch?v=RI_NkUHMpaY. • Sekitani, Tsuyoshi, et al. “Flexible organic transistors and circuits with extreme bending stability.” Nature Materials, 7 Nov. 2017. • Sirringhaus, Henning. “25th Anniversary Article: Organic Field-Effect Transistors: The Path Beyond Amorphous Silicon.” Advanced Materials, vol. 26, no. 9, 2014, pp. 1319–1335., doi:10.1002/adma.201304346. • Steckl, Andrew. “Circuits on Cellulose: From Transistors to LEDs, from Displays to Microfluidics on Paper.” NanoHUB.org, 2017, nanohub.org/resources/25874. • “Turning clothing into information displays.” Phys.org - News and Articles on Science and Technology, phys.org/news/2015-09-turning- clothing-into-information-displays.html. • Zhao, Xiaoli, et al. “Conformal transistor arrays based on solution-Processed organic crystals.” Scientific Reports, vol. 7, no. 1, 2017, doi:10.1038/s41598-017-15518-y.
Editor's Notes
- Basic Outline for this slide show. Key history of the simple Transistor leading into the current Era. An introduction to familiarize ourselves with Flexible/Organic Transistors. Background of the Organic Transistor The theory of the transistor itself, including some characteristics. Some research examples for which have been or are being explored. The challenges of Flexible/Organic transistors. Some common simulation tools for which can be found. Lastly a conclusion of Flexible/Organic transistors. Photo- University of Tokyo showing a macro level picture of a Flexible/Organic cluster of transistors on a flexible substrate.
- In 1945, as a response to the invention of the ATT’s triode vacuum tube, Bells lab director assembled a team to explore semiconductors. The team consisted of Bill Shockley, Walter Brattain, and John Bardeen among other support scientists. In this time, Shockley invented a preliminary design for a semi conductor device using field effect properties. Design did not work as expected and Bardeen and Brattain were tasked with the investigation into the reasons why. In 1947, Bardeen and Brattain started experimenting with Shockley's design and discovered the point-contact transistor. The two found that the original idea for how electrons behaved in a crystal was incorrect. In 1948, Shockley invented the junctions transistor, which was more rugged, practical and more manufacturable than the point-contact transistor. In 1965, Gordon Moore, Fairchild Semiconductor and Intel, observed that the transistor density in a IC doubled every year. After revision he found that the law was in fact doubling every 24months. This trend would continue into the next century, 21st century. Fast forward to the current year, microprocessors with 18 to 19 billion transistors with feature sizes as low as 10nm. Photo shows the growth of transistor density as a function of time. Starts with the electromechanical era, to solid-state, Vacuum tube, to the transistor, integrated circuit and beyond,
- Why not just use non-organic based transistors? Flexible and organic transistors are the future for flexible, next generation, electronics. These transistors hold the properties essential for flexible or rolling applications. This is due to the transistors simple and large area processing, low manufacturing cost, mechanical flexibility and mass, and optical absorption/emission. These characteristics coupled with a plastic substrate will make for some interesting technologies. Low cost including no vacuum processing, no lithography, and low cost substrates (plastic, paper, cloth, glass). Most importantly there are two characteristics for which necessitate the use of Organic Transistors. First is the substrate. Typical inorganic transistors need to be built or deposited on a crystallin substrate, organic materials can be deposited on many different substrates due to being amorphous in nature. Secondly, temperature. The temperature used to manufacture non-organic transistors is quite high. But inorganic transistors require far less temperature. Thus allowing for a more diverse substrate selection.
- To achieve an acceptably operating Flexible/Organic transistor, there are some initial requirements for which need to be met. In order to ensure current flow, a designer must ensure that the highest occupied and the lowest unoccupied orbital energies of a particularly used molecule needs to be in the correct zone. This will ensure that holes can be induced at a applicable electric field. The crystal structure or the material needs to provide overlap. Without this overlap, there will be a inefficient charge migration. The material should be pure. This will prevent charge carrier traps and allow the unrestricted flow of electrons and holes. The orientation or the molecule can affect the performance. The molecule should be orientated in such a way where the molecule runs parallel to the FET substrate. The domain for the semiconductor should cover the same area between the source and drain. Moreover, the domain should contact the source and drain uniformly at the convergence plane. Photo- This photo shows some simple representations of a thin film transistor and its different orientations.
- Photo – Shows a organic transistor either taking the path of a N or P semiconductor. It is worth pointing out that organic and inorganic semiconductor don’t share the same characteristics. For instance, an in-organic semiconductor basis its N or P type based upon majority carrier type due to doping. The organic semiconductor is based upon the gate voltage sign. Semi-Conductor tables – Shows N and P type organic semiconductor materials and their mobility with on/off current ratio. Pentacene, or P5 on the chart, is the most studied semiconductor for OFET’s. As can be seen in the chart, Pentacene has a mobility of 1.5 and an on/off current ratio of 10^8th. Pentacene also exhibits near zero threshold voltage and a subthreshold slope of 1.6V per decade. Some Pentacene devices have been produced with carrier mobility's of 3 and sub threshold slopes of 1.2V per decade. 6T on the chart is another highly studied organic material. The 6T family shares a very high mobility. Which proved that polycrystalline organic films could obtain these characteristics.
- A few interesting characteristics to be know: Current Drain Equation – W is the channel width, L channel length, u is the field effect carrier mobility, Ci is the capacitance per unit area, Vt is the threshold voltage, Vsd the source drain voltage, and Vsg is the source drain voltage. This first initial equations holds true when Vsd is less than Vsg. This second saturation equation holds true when the drain voltage exceeds the gate voltage. The band diagram shows some interesting characteristics in organic thin-film transistors. When there is zero bias, as shown in the examples, there is a state of band bending causing a non conductive channel. But when a positive bias is applied, the conduction band bends towards the fermi level creating a conductive channel. The plots on the left show the typical drain current versus source voltage. Moreover, the drain current versus the gate voltage.
- One last characteristic, I will touch on is the bending characteristics with organic transistors. This is an important field of study for when a transistor is bent, different characteristics propagate. As shown in the chart above, there is a nice comparison for several TFT compositions. They are being compared to each other through field-effect mobility, operating voltages, and finally there minimum bending radius. As shown for comparison, organic TFT”s have much smaller bend radii versus there silicon counterparts. When bending there is some significant changes in a transistor. One of which is the gate current. As the bend becomes “tight” or as the radius becomes smaller the gate current starts to increase. This is important to understanding that there are limits to bending organic transistors.
- One study, for which has been accomplished recently, was the progress of transistor arrays based on solution processed crystals. This development in organic transistors bring a promising route to fabricate conformal FET’s based upon Pentacene single crystal Nano wire. The capabilities for this study was to allow for sensory and scanning feature devices, biofeedback devices and physiological tracking devices. The study was performed to drop cast an organic solution on an anti-solvent photolithography compatible electrode with bottom contact configuration. This allowed the transistor array to be conformable to uneven objects. Moreover, this allowed for 100% field effect mobility, low threshold voltage, and good device uniformity. This study opened up the door to next generation wearable and implantable technology.
- In Vivo electronic monitoring system has been investigated there has been promising results. The monitoring system uses a biocompatible gel, for which is highly conductive. Comprising of multi-walled carbon nanotube sheets 100um in length and 5nm in diameter. The gel displays an admittance of 100mS cm-2 even during low frequency ranges which is uncommon. While implanted the human body, the sensor, showed little foreign-body reaction compared to metal electrodes normally implanted. Moreover there is a 1.2um thick polyethylene film to help amplify, by a factor of 200, weak bio signals. The added composite allowed the use of the sensor for electrocardiography for which was easily spread over the uneven heart tissue.
- Some of the possible applications are shown here. Displays – OLED displays are simply a layer of organic light emitting diodes for which produce a visual display. The applications can be used in the consumer realm and the industrial market. Solar Applications- Solar application have risen for organic transistors due to its low cost and environmentally friendly characteristics. Current max efficiency with OPV is 11% and rising. Smart Textiles – Some applications for smart textiles is a wearable sensor for which can detect several gaseous materials. Moreover, another application is having a woven temperature array into clothing to monitors body heat and external temperatures. Lab-on-a-Chip- This technology is used to scale multiple lap processes down to a single chip. Given the manufacturing efficiency and low cost of organic transistors. Photo 1 – OLED screen produced by Samsung for the next generation iPhone. Photo 2 – First stretchable ad conformable thin-film transistor driven LED display. Created by Holst Center, imec, and CMST at Ghent University.
- Instability in organic material seems to be a large problem. When exposed to air, water of UV light the transistor can degrade rapidly while loosing all original electrical properties. Moreover, with non-stable environments comes shorter lifespans. Unlike traditional semiconductors. While its easy to manufacture organic transistor materials, it is challenging to manufacture a consistent and uniform contact between the electrical contacts and the organic material. Also, it Organic materials have a lower carrier mobility than its silicon counterpart. Thus, having a smaller bandwidth limiting the organic transistor from high speed RF applications. Lastly, conductive polymers have high resistance, therefore are poor conductors of electricity.
- Nano Hub has several simulation tools mainly centering on OPV’s or polymers in general. First of which is: Bulk Heterojunction Morphology Generator -- This tool, as per Nano Hub, allows the user to generate the morphology for a bulk heterojunction for the use in organic photovoltaics. Exciton Annihilation Simulation – This tool, also found on Nano Hub, simulates light absorption and the subsequent exciton annihilation behavior of organic semiconductors measured with pump-probe spectroscopy EXCITON Dynamics Simulator -- This tool like, like the previous simulation, simulates the dynamics in the exciton event in organic photovoltaic devices. Polymer Modeler – This tool allows you to build thermoplastic polymer chains to study the mechanical properties of the designed chain. nuSIMM: Nano Hub User simulation interface for molecular Modeling – The, if interested, can simulate polymerization, equilibration and characterization of molecular models. Circuit on Cellulose: From transistors to LED’s, from Displays to Microfluidics on paper. Photo – This Photo shows the simulation of the Exciton Annihilation Simulator. As can be seen, the simple simulator takes the Material optical properties and some simulation parameters and produces a exciton concentration as a function of Pump Pule Power.
- In summary, organic transistors are extraordinary semiconductors. They are light in weight, extremely flexible, low is manufacturing temperature and cost. Lastly, they are biodegradable. There is so much to learn from this field for a more in depth look into organic transistors, please consult my references. There are still many hurdles to overcome with OFETS. But I believe that these can be overcome with time. OFET’s could be used to potentially save lives or could be to create that new heads up display for the military. I hope to help this technology along its way.