The recent developments in organic photovoltaic (OPV) field make this technology highly promising with record power conversion efficiencies (PCE) of 19% in single junction solar cells achieved.¹ This printed and low temperature processes technology is very attractive due to several advantages such as a low energy payback time, light weight, flexibility and transparency. However, one of the drawbacks of OPV is the high toxicity of the solvents used to process the active layer which are mainly aromatic and/or chlorinated.

In order to render this PV technology more environmentally friendly, we have been working on the development of water-based organic semiconductor colloidal dispersions.² In this communication, I propose to present our recent developments in this field, on the control of the size and the morphology during the nanoparticle synthesis to fabricated highly efficiency OPV devices. Two strategies have been investigated. On the one hand, attempts to fabricated pure donor or acceptor nanoparticles with diameter below 30 nm have been done. To do so, we have been working on millifluidic systems to control precisely the size of the nanoparticles synthesized by nanoprecipitation. By increasing the flow rate of water anti-solvent, turbulent regimes were achieved in the mixing chambers, leading to smaller nanoparticles size. Using such kind of continuous flow devices, the effect of different additives such as surfactant on the nanoparticle size have been investigated. On the other hand, composite donor/acceptor nanoparticles were fabricated and control of the nanoparticle internal morphology was targeted. To this aim, we have been studying the influence of the surface energies of the donor and acceptor materials on the morphology of donor/acceptor composite NPs. We showed that matching the surface energies of the donor and the acceptor plays a major role to control the internal morphology of the NPs: a large interfacial energy between the donor and the acceptor leads to core-shell structure while a small one tends to give intermixed morphology. Organic photovoltaic devices were fabricated from water-based inks with varying donor/acceptor combination. Optimal performances were with PTQ10:Y6 NP, a donor/acceptor system presenting low interfacial energy. As a result, a NP-based active layer with optimal intermixed morphology was achieved and high efficiency devices with up to 9.98% PCE were fabricated.³ This work highlights the importance of selecting donor/acceptor combination with matching surface energy to ensure an optimal nanoparticle morphology in miniemulsion processes and, ultimately, reach highly efficient water-processed organic photovoltaic devices.