Low‐Voltage, High‐Frequency Organic Transistors and Unipolar and Complementary Ring Oscillators on Paper

DOI: 10.1002/aelm.201800453 anti-counterfeiting or tracking features to the existing passive security features on banknotes.[6] One possibility for implementing active electronic circuits on paper is the use of organic thinfilm transistors (TFTs), since these can usually be fabricated at relatively low temperatures, which simplifies their fabrication on temperature-sensitive substrates, such as paper. Organic TFTs have been successfully fabricated on various types of paper, including commercial packaging paper,[2] printing paper,[7–9] photo paper,[10–14] homemade paper,[15,16] starch paper,[17] and banknotes.[6,18] In some cases, one or several coatings were applied to the surface of the paper prior to TFT fabrication in order to smoothen or seal the surface.[2,10–13,19,20] For example, Li et al. fabricated bottom-gate, top-contact TFTs based on solution-processed C8-BTBT on parylene-coated photopaper and reported a carrier mobility of 1.3 cm2 (Vs)−1 and an on/off current ratio of 108.[13] Minari et al. used parylene-coated photopaper as the substrate and fabricated topgate TFTs using drop-cast C8-BTBT as the semiconductor and Au nanoparticles for the source and drain contacts and measured a carrier mobility of 2.5 cm2 (Vs)−1.[20] This is still the highest carrier mobility reported to date for organic transistors on paper. Grau et al. demonstrated roll-to-roll printing of polymer TFTs on kaolin-coated packaging paper.[2] The majority of the organic TFTs fabricated on paper in the past have required operating voltages of at least 10 V,[13,16,20] which is likely too high for most of the applications envisioned for paper electronics, as these are primarily targeting mobile or wearable devices powered by small batteries or energy-harvesting concepts. One possibility to reduce the operating voltage of organic TFTs and circuits is the use of electrolyte-gated or electrochemical TFTs,[7,10,21] while electrolyte-gated and electrochemical TFTs can provide very large transconductances at very low operating voltages,[22] they usually suffer from large signal delays, as they involve the inherently slow movement of ions. Another option for fabricating low-voltage organic TFTs and circuits is to utilize very thin gate dielectrics,[23] e.g., consisting of a thin aluminum oxide layer and a self-assembled monolayer of alkylphosphonic acids, and following this approach, organic transistors and inverters have previously been demonstrated on paper.[6,24] Here, we report on the current–voltage characteristics and the contact resistance of low-voltage pand n-channel organic Paper substrates for flexible, mobile, and disposable electronic applications draw academic and commercial attention due to their many advantages, such as cost efficiency, low weight, mechanical flexibility, and environmental compatibility. For portable electronic applications, it is important that the electronic devices and circuits can be operated with voltages of a few volts, due to the limitations of mobile power supplies, such as solar cells or energy harvesting devices. Here, low-voltage organic p-channel and n-channel transistors as well as unipolar and complementary ring oscillators are fabricated directly on the surface of a banknote. It is demonstrated that low-power organic integrated circuits on paper substrates can be operated with supply voltages of less than 3 V and with frequencies of several hundred kilohertz. These results emphasize the prospects of organic transistors for smart paper applications.