Inkjet Printable ZnO/PEDOT:PSS Heterojunction for Thin Flexible Semi-Transparent Optoelectronic Sensors

Flexible sensors play an increasing role in printed electronics and are of interest for optoelectronic applications in flexible robotics and industrial automation. Thus, we have investigated the hybrid inorganic-organic junction between ZnO and PEDOT:PSS (poly(3,4-ethylenedioxythiophene) polystyrene sulfonate). A thin ITO (indium tin oxide) layer on PET (polyethylene terephthalate) foils was used as substrate electrode. ZnO was deposited from a nanoparticle (NP) suspension by electrophoretic deposition. For comparison, we have used three different methods for the deposition of PEDOT:PSS, namely (i) drop casting, (ii) dip-coating, (iii) inkjet printing. For the resulting ZnO-PEDOT:PSS junctions, current-voltage characteristics were recorded using miBot micromanipulators, and analysed applying adapted models of thermionic emission, according to Cheung and Cheung, as well as Norde. For dip-coated samples we obtained barrier heights of (0.78±0.01) eV and (0.73±0.02) eV, respectively, while inkjet printed and drop casted structures show smaller values (0.53 eV to 0.66 eV, respectively). Thus, dip-coating can be used for a rapid upper estimation of the ZnO-PEDOT:PSS barrier heights for developing inkjet printable optoelectronic sensors. When using low concentrated PEDOT:PSS suspensions, we observed negative photoresponse of the junction; this is consistent with the formation of a ZnO-NPs and PEDOT:PSS nanocomposite which we obtained on porous semi-transparent ZnO-NP films.