Streamlined Inkjet-Printing of Stiff Platforms and Organic Photodiodes on Elastomeric Substrates for Stretchable Light Sensors

The swift progress in IoT, robotics, and wearable electronics has introduced new challenges and heightened the demand for advanced sensing technologies. These sensors must not only deliver customized performance but also possess properties such as stretchability and flexibility while remaining compatible with industrial manufacturing processes.<br/><br/>In this study, we present a streamlined approach to fabricating stretchable organic photodiodes (OPDs) arrays using exclusively inkjet printing. This is achieved by printing stiff islands and interconnects directly onto elastomeric substrates. The structures were fabricated from the UV-curable photoresist SU-8 to offer a solvent-resistant platform for creating the OPDs. Simultaneously these structures provide a mechanically stable foundation that protects the devices from mechanical stress. We thoroughly investigated and optimized the ink formulation and printing process of these structures, as well as their impact on the mechanical properties of the silicone-based elastomeric substrate. After printing the structures, we achieved elongations of up to 60% before observing any delamination or structural failure of the subtrate. Challenges encountered during the printing of Ag electrodes on the curved island surfaces were resolved through a two-step printing process and optimized annealing steps. Cyclic conductivity measurements of printed Ag layers on top of them did not observe any degradation for up to 500 cycles.<br/><br/>Finally, we fabricated fully-printed OPDs based on polymer: non-fullerene acceptor bulk heterojunctions (BHJ). The inkjet printing process of the multilayer architecture Ag/SnO/BHJ/PEDOT:PSS needed to be adapted to the new substrate properties and morphology. The devices exhibited state-of-the-art peak responsivities of 340 mA/W (l = 730 nm). Furthermore, they showed specific detectivity (D*) of 2.8 × 10^11 Jones (at 100 Hz and -2V). The light sensors remained operational for over 600 mechanical stretching cycles at strains up to 7%. This fully inkjet-printing approach can potentially simplify the fabrication steps needed to fabricate stretchable devices and enhance compatibility with industrial printed electronics processes. Furthermore, its digital nature will enable a broader application in other types of (opto) electronic devices and emerging stretchable applications.