Professor Dr. Stefan Mannsfeld

Technische Universität Dresden
Center for Advancing Electronics Dresden                                                   

Project Summary

Even though organic field effect transistors (OFETs) have now been researched for more than 20 years there are only very few reports of air-stable OFETs switching at frequencies exceeding 1MHz. On the other hand, there is a great demand for inexpensive analog, and flexible circuit applications such as high-frequency amplifiers or low-voltage low-noise receiver circuits working at tens of MHz or even close to GHz frequencies. In order to enable applications in which transistors work, for example, in amplifier circuits operating at common wireless frequencies within the Industrial, Scientific and Medical (ISM) radio bands (13.56MHz or 27.12MHz), the transit frequency of the individual OFET needs to be significantly improved and pushed into the range of 50-200MHz. The transit frequency is mainly limited by the transistor geometry/dimensions and the semiconductor material’s intrinsic charge carrier mobility. So far, most of the recent reported improvements have focused on the transistor geometry and several promising concepts, especially layouts with vertical channels have been demonstrated. However, the potential to combine these existing device concepts with ultra-high mobility organic semiconductor materials has not been fully exploited yet. The main objective of this work is to develop a robust fabrication scheme that enables OFET on flexible substrates to work at switching speeds above 50MHz. We have recently been able to form ultra-high performance OFETs from solution-deposition methods that are compatible with flexible substrates. By careful control of the processing conditions we have obtained charge carrier mobilities as high as 11cm^2/Vs from printed highly crystalline films of the commercially available semiconductor TIPS-pentacene, and mobilities as high as 43cm^2/Vs for spin coated blended inks of the also commercially available C8-BTBT with the polymer polystyrene. The proposal objective will be achieved by combining these technological breakthroughs in obtaining very high electrical performance thin films with existing approaches to fabricate OFETs with short channel lengths. One great advantage of our approach is that it is compatible with one of the key ideas behind the vision of flexible, plastic wireless technologies: low cost! The result of this project will be a solution-deposition (printing!) method by which OFETs with transit frequencies above 50 MHz can be inexpensively fabricated on plastic substrates and from air-stable and commercially available semiconductor materials.