Printed organic field-effect transitors (OFETs) have been considered for many novel applications towards large area and flexible electronics, since they can enable pervasive integration of electronic functionalities in all sorts of appliances, their portability and wearability. Applications are countless: from personal devices (e.g. wearable health monitoring devices) to large-area sensors (e.g. electronic skin, bio-medical devices), and smart tagging of products with radio-frequency identification tags. However, printed OFETs fabricated with scalable tools fail to achieve the minimum speed required for example to drive high-resolution displays or to read the signal from a real-time imager, where a transition frequency (fT), i.e. the highest device operative frequency, above 10 MHz is required. In this contribution we present effective strategies to increase fT in polymer devices by combining only printing and digital, laser-based direct-writing techniques.
By controlling the self-assembling properties of conjugated copolymers, in combination with simple, roll-to-roll compatible coatings, it is possible to achieve well-ordered and efficient charge-transport nanostructures over large-areas. In particular, by exploiting the one-dimensional self-assembly of model conjugated polymers, such as naphthalene diimide based co-polymers, highly controlled printed anisotropic thin films with excellent transport properties are demonstrated. By controlling the ink flow directionality with a bar-coating deposition technique, shear-aligned thin films with a highly oriented functional surface are realized at a coating speed of few meters per minute, without the need for additional post-processing steps. This approach produces a marked FET mobility anisotropy and greater performance uniformity with respect to the spin-coating deposition, and excellent electron mobility along the printing direction. The simple adoption of this fast coating approach allows to strongly enhance the highest operational frequency of FET devices, achieving a transition frequency in the MHz range.
By combining printing and laser-based direct-writing techniques, additional strategies to boost the transition frequency of polymer based devices can be pursued. First, by combining inkjet printing and femtosecond laser ablation to obtain small channel lengths, all-polymer FETs operating in the MHz regime can be fabricated on plastic without the use of any mask. In particular, an engineered layout of the contacts allows to achieve a transition frequency of 4.9 MHz. Alternatively, narrow, micron-scale metallic electrodes can be sintered on plastic through femtosecond laser sintering. The combination of such electrodes with fast-coated polymers allow to achieve the higher transition frequency for a mask-less fabricated polymer transistor to date, reaching 20 MHz.