Organic electronics enables a manifold of novel and alternative applications in portable, mechanically robust and light-weight devices. Formulated into functional inks, organic semiconductors can be processed with printing technologies, thus allowing scalable device fabrication across large-area and onto flexible substrates, opening a complete new pathway for organic electronics and its integration into circuits. In particular, in the case of organic photodetectors, all-plastic short range data communication, plastic digital and conformable imagers, positions and security sensors, and interactive surfaces become possible. To date the examples of all-printed photoresponsive devices are very scarce,[1, 2] and they are limited to the case of blends comprising polymer donors and fullerene acceptors. Till now fullerene derivatives have been extensively employed as acceptor materials in donor-acceptor bulk heterojunctions due to their high electron mobility. In previous work, we have already shown that the use of small molecules, as should be also considered fullerene derivatives, into printing technology it is not ideal due to their strong tendency to crystallize. Instead an ink formulation containing the small molecules and conjugated polymer as additives was required to enable printability.[3] Neverthless, it has been recently shown that by a proper chemical design also conjugated polymers can reach high electrons mobilities. That is the case for copolymers of benzothiadiazole and fluorine and copolymers incorporating perylenediimide or naphthalenediimide acceptor units. In this work, we present the first example of an all polymer and all printed semitransparent photodetectors where a dyketopyrrolepyrrole-benzothiadiazole copolymer is employed as an electron acceptor. The entire device structure is fabricated by means of inkjet printing processes, on flexible polyethilene naphtalate (PEN) substrate and in ambient conditions. The photodetector structure included a printed layered stack of: poly(3,4-ethylene dioxythiophene):(polystyrene sulfonic acid) (PEDOT:PSS) as bottom contact, which was functionalized to reduce its workfunction by spincoating polyethyleneimine (PEI); a blend of poly-(3-hexylthiophene-2,5-diyl) (P3HT) and diketopyrrolopyrrole-benzothiadiazole-based copolymer as photoactive layer; and a top PEDOT:PSS electrode contact. Our polymer:polymer blend could be easily printed as nozzle clogging and slow precipitation of the starting solution during printing are strongly hampered with respect to inks based on small molecules. The devices showed high light vs dark current ratio ( around 10000 at short circuit) comparable to the state of art of the best optimized polymer:fullerene based organic photodetectors. Our all-organic and fully-printed photodiode, contains semitransparent contacts enabling double-side signal detection: light detection is indeed showed to occur both when light is incoming from the top and from the bottom side of the photodiode, with comparable efficiencies. Parameters of our devices satisfy specifications for the realization of a prototype pixel paving the way for their integration on an actual imaging screen.
References [1] K.-J. Baeg, M. Binda, D. Natali et al., Advanced Materials, 25, 4267-4295 (2013). [2] G. Azzellino, A. Grimoldi, M. Binda et al., Advanced Materials, 25, 6829-6833 (2013). [3] G. Pace, A. Grimoldi et al. Advanced Materials, 26, 6773-6777 (2014).