Transcap: A New Integrated Hybrid Supercapacitor and Electrolyte-Gated Transistor Device
Authors: Soavi Francesca , Sayago Jonathan , Cicoira Fabio , Santato Clara 
Published: February 2020

The recent boom in multifunction, light and flexible portable electronic equipment and the increasing need of low-energy cost and autonomy for different fields ranging from wireless sensor networks for smart environments to biomedical applications are triggering intensive research efforts towards the development of self-powered, sustainable electronic devices. Within this context, the coupling of electronic devices (sensors,transistors, etc.) with small size energy storage systems (e.g. micro-batteries or micro-supercapacitors) is actively pursued.
Micro-electrochemical supercapacitors are attracting much attention in electronics for their capability of delivering short power pulses with high stability over repeated charge/discharge cycling. One of the strategy which is pursued to increase the energy storage capability of supercapacitor is coupling electrode materials of different nature that are charged/discharged via different electrostatic and faradic modes in a hybrid configuration. The use of wide electrochemically stability electrolytes like ionic liquids, permits to achieve high cell voltage and , hence high energy. For their high specific pseudocapacitance, electronically conducting polymers are well known as positive materials for hybrid supercapacitors featuring high surface carbon negative electrodes. The processability of both polymer and carbon is of great relevance for the development of flexible miniaturised devices.
Electronically conducting polymers are even well known to feature an electronic conductivity that depends on their oxidation (p-doped state) and that it is modulated by the polymer potential. This property and the related pseudocapacitive response make polymer very attracting channel materials for electrolyte-gated (EG) transistors. In this systems electrolytes replace conventional gate dielectrics and permit low-voltage operation, which in turn makes EG transistors attractive candidates to be coupled to energy harvester/storage microsystems.
Here, we propose a novel concept of “Trans-capacitor”, an integrated device that exhibits the storage properties of a polymer/carbon hybrid supercapacitor and the low-voltage operation of an electrolyte-gated transistor. When the TransCap is ON, the polymer transistor channel is open. The p-doped polymer and the negatively polarized carbon gate electrode are storing the charge at a given gate-source voltage, hence the system is storing energy. When the TransCap is switched OFF, the channel and the gate are discharged and the delivered energy can be used for subsequent operation of the EG transistor itself or to power other components in the electronic system. This multifunctionality should positively impact on energy saving in the multicomponent electronic system where the TransCap will be used [1].
The performance of a TransCap based on a polymer channel like MEH-PPV, poly[2-methoxy-5-(20-ethylhexyloxy)-1,4-phenylene vinylene], a high surface area carbon gate, and the ionic liquid N-trimethyl-N-propylammonium bis (trifuoromethanesulfonyl) imide electrolyte are here reported and discussed.
ACKNOWLEDGEMENTS
This work was financially supported by NSERC (Discovery grants, CS and FC) and FQRNT (Nouveau Chercheur, CS). JS acknowledges fiancial support by CONACYT and CMC Microsystems. FS acknowledges financial support by Università di Bologna (Researcher Mobility Program, Italian-Canadian cooperation agreement).
[1] J. Sayago, U. Shafique, F. Soavi, F. Cicoira and C. Santato, J. Mater. Chem. C, DOI: 10.1039/c4tc02119c