Bridging the gap between organic and silicon electronics
Combining all-printed organic and silicon electronics in smart hybrid labels is a difficult challenge due to a lack of technology that (de)multiplexes signals between the Si chip and the printed electronic devices. A solution to this problem comes from all-printed integrated circuits based on OECTs.
Our connected society urges for widely distributed electronics that can be added onto any desired item. In this respect, printed electronics hold promise to address this challenge by providing low-cost and intelligent technology platforms that can be implemented and integrated into large-area sensors, detectors, medical diagnostics systems and surveillance technology. The printed devices and systems, built up from organic and inorganic electronic inks, can be added onto stickers, labels, posters, packages, smart cards, construction elements, solar cells, etc. The only limit is our imagination! However, the relatively low mobility of organic semiconductors makes it difficult to provide high speed processing and communication which is a key advantage of Silicon (Si).
Si-based labels (e.g. RFID) are still awaiting their breakthrough. One reason for their slow market evolution is the relatively high cost to manufacture a single label; a price that is too high for low margin products, for example packaging. The added value for an RFID function is not high enough to motivate a 10-50 cents cost addition per item. However, if the label also includes sensors and communication interfaces (e.g., displays), then such additional cost becomes acceptable. The cost of a Si chip is to a large extent dictated by the Si chip area. In the Si-label technology, most of the area is consumed by the large pads, which are necessary to enable easy interconnects and bonding. Therefore, the cost of a chip carrying a large number of contact pads (necessary for the multifunctional Si-label described above) will be prohibitive for most applications. A solution to this problem may come from a hybrid integration of smart electronic labels (Fig. 1).
Fig. 1 Concept of hybrid electronics systems combining printed electronics and Si chips. Application-specific integrated circuits (ASIC) require a large number of pads to implement multiple functionalities. Printed digital circuits can significantly reduce the number of pads. The combination of printed electronics and Si-based electronics into hybrid electronics systems enable ultra-low-cost applications.
Our research groups at Linköping University, together with Research Institutes of Sweden (RISE), have designed and manufactured, by using screen printing as the sole deposition technique, all-printed large-scale integrated circuits that (de)multiplex signals between Si chips and printed electronic devices. We developed all-printed 4-to-7 decoders and 7-bit shift registers by using organic electrochemical transistors (OECTs) as the circuit building blocks. The OECTs represent a class of printable devices that operate at approximately 1V, with exceptional threshold voltage stability and high transconductance characteristics, even outperforming more traditional field-effect transistors (FETs). This is due to the fact that charges in OECTs are accumulated and transported throughout the entire bulk of the transistor channel, resulting in current densities that are typically orders of magnitude higher than those attained in FETs. Most importantly, the OECT electrical characteristics are not sensitive to the morphology of the channel materials, an attribute that makes them easy to manufacture in non-cleanroom environments by using large-area printing techniques. For example, we fully screen printed – on a flexible substrate – 7-bit shift registers, made of over 110 OECTs, 170 carbon resistors, and 400 via interconnects. We further demonstrated monolithic integration of the 4-to-7 decoder with a seven-segment electrochromic display, resulting in an all-printed electronic label having a total area of ~40 cm2, smaller than a credit card. All individual components (OECTs, resistors, via interconnects and display segments) need to be functional in order to obtain a fully operational printed electronic system, which also gives an indication of the robustness of this technology.
The possibility to manufacture large-scale integrated circuits entirely from solution and without the need of cleanroom facilities demonstrates the potential of all-printed OECT circuits for countless applications. In this hybrid electronic system, the large-scale printed circuits share the job of the Si chip and drastically reduce the number of required pins, thus becoming cost effective. For example, an 8-pin microcontroller – costing < 1 US$ – is able to drive all-printed 7-segment displays without being deprived of the ability to acquire and process the signals coming from multiple sensors. These all-printed large-scale circuits operating at low voltages also enable a myriad of Internet of Things applications on various substrates, such as skin-conformable elastomers, paper, and fabrics. If this technology is then combined with an array of wearable sensors monitoring our health status, it offers an unreached solution when targeting diagnostic applications.
These results were recently published in Nature Communications:
"All-Printed Large-Scale Integrated Circuits Based on Organic Electrochemical Transistors", Nat. Commun. 10, 5053 (2019)