Earlier reports have discussed the manifold opportunities that arise from the use of eco-friendly organic semiconductor dispersions as inks for printed electronics and, in particular, organic photovoltaics. To date, poly(3-hexylthiophene)...

Organic semiconductor nanoparticle dispersions are electrostatically stabilized with the p‐doping agent 2,3,5,6‐tetrafluoro‐7,7,8,8‐tetracyanoquinodimethane (F4TCNQ), omitting the need for surfactants. Smallest amounts of F4TCNQ stabilize poly(3‐hexylthiophene) dispersions and reduce the size of the nanoparticles significantly. The concept is then readily transferred to synthesize dispersions from a choice of light‐harvesting benzodithiophene‐based copolymers. Dispersions from the corresponding polymer:fullerene blends are used to fabricate organic solar cells (OSCs). In contrast to the widely used stabilizing surfactants, small amounts of F4TCNQ show no detrimental effect on the device performance. This concept paves the way for the eco‐friendly fabrication of OSCs from nanoparticle dispersions of high‐efficiency light‐harvesting semiconductors by eliminating environmentally hazardous solvents from the deposition process.

An entirely different route to the ecofriendly processing of organic semiconductor thin films is their deposition from aqueous or alcoholic dispersions, [4] reinforcing the unique environmental sustainability of organic solar cells. Recent reports also featured acetonitrile as a promising dispersion agent due to its high permittivity and hence its excellent screening of stabilizing charges. [5] Besides the omission of toxic solvents during coating, this deposition route i) disentangles solution processing from the need of solubility, ii) advances multi-layer deposition without the need for orthogonal solvents and hence iii) is less dependent on molecular engineering, eventually giving access to a broader choice of semiconductors for optoelectronic applications. [6] The prevailing challenge of this approach is the colloidal stabilization of the dispersion. [7,8] Only very few organic semiconductors exhibit sufficient intrinsic colloidal stability, with its most prominent example being poly(3-hexylthiophene-2,5-diyl) (P3HT). Its high intrinsic colloidal stability allows the synthesis of P3HT nanoparticle dispersions by rapid solvent exchange, where a P3HT solute is nanoprecipitated upon injection into miscible ethanol. [9,10] The high colloidal stability of P3HT also enables the formation of nanoparticle dispersions of blends of P3HT and indene-C 60 bisadduct (ICBA). Such P3HT:ICBA nano particle dispersions can be synthesized with reasonably high concentrations and can be used as inks to fabricate solar cells with maximum power conversion efficiencies (PCEs) of 4.5%. [11] Other organic semiconductors such as the latest highperformance combinations of polymers and non-fullerene acceptors do not exhibit this intrinsic colloidal stability. To disperse organic semiconductors that would otherwise coagulate rapidly, surfactants have been introduced as stabilizing agents. Best results on the stabilization of organic nanoparticle dispersions were achieved by employing poloxamers, but extensive subsequent purification steps were needed to reduce the final surfactant content which otherwise would have remained in the light-harvesting layer where it would have hampered the solar cell performance. [12,13] Still, this concept produced nanoparticulate solar cells with PCEs of 7.5%. [8] Recently, we found that the intrinsic colloidal stability of P3HT stems from its unusual but well-known tendency to electrically charge itself which in turn fosters electrostatic repulsion of the nanoparticles. [7] Accordingly, extrinsic oxidation of the light-harvesting polymers, e.g. by electrical p-doping with F 4 TCNQ or temporarily even by photodoping with visible light, can enhance the colloidal stability of nanoparticle dispersions. [5,7] Yet, the large ionization potentials of most high-performance light-harvesting polymers render High-performance organic solar cells are deposited from eco-friendly semiconductor dispersions by applying reversible electrostatic stabilization while omitting the need for stabilizing surfactants. The addition ...

State‐of‐the‐art solvents for the fabrication of organic solar cells are mostly toxic or hazardous. First attempts to deposit light‐harvesting layers from aqueous or alcoholic nanoparticle dispersions instead have been successful on laboratory scale, enabling future eco‐friendly production of organic solar cells. In this work, a scalable high‐throughput continuous‐flow microfluidic system is employed to synthesize surfactant‐free organic semiconductor dispersions by nanoprecipitation. By adjusting the differential speed of the syringe pumps, the concentration of the initial solute and the irradiation of the microfluidic chip, the synthesis can be controlled for tailored dispersion concentrations and nanoparticle sizes. The resulting dispersions are highly reproducible, and the semiconductor inks are stable for at least one year. The synthesis of the dispersions is exemplified on a polymer/fullerene combination with large‐scale availability.