Microfluidics: Continuous‐Flow Synthesis of Nanoparticle Dispersions for the Fabrication of Organic Solar Cells

Abstract

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.

The Importance of Semiconductor Miscibility for the Formation of Light‐Harvesting Polymer:Fullerene Nanoparticle Dispersions

“…The performance of P3HT:IC 60 BA solar cells is comparable to previous studies. [ 15,28 ] Due to the simultaneously enhanced absorbance of the C 70 core versus C 60 in the visible spectral range and the enhanced V OC , nanoparticulate P3HT:IC 70 BA solar cells achieved the best PCE in this study of 4.5%, which marks the highest reported performance of nanoparticulate P3HT:fullerene solar cells to date—despite the still moderate light‐harvesting layer thicknesses.…”

“…Bearing in mind that the nanoparticle formation by nanoprecipitation occurs within milliseconds, [ 28 ] we picture the formation of nanoparticle dispersions as follows: within this very short time frame, the individual polymer chains with their negligible solubility in the chloroform:ethanol mixture are immobilized instantly and solidify. Fullerenes within the polymer matrix are trapped and incorporated into the nanoparticles.…”

“…The nanoprecipitation was performed at room temperature (20 °C) under irradiation from a chip‐on‐board light‐emitting diode with a power of 1 A (30 W) as described earlier. [ 28 ] After nanoprecipitation, the beakers containing the dispersions were placed inside a water bath (47 °C) to remove the chloroform, to reduce the non‐solvent and to concentrate the dispersion to yield the original semiconductor concentration. To remove larger nanoparticle aggregates, which would later affect the quality of the thin film, the dispersions were centrifuged (Eppendorf, MiniSpin plus, 14,500 rpm, 2 min) and the supernatant was used for further investigation.…”

Iodine‐Stabilized Organic Nanoparticle Dispersions for the Fabrication of 10% Efficient Non‐Fullerene Solar Cells

“…[45,46] Only by using a chemically modified Y6 for less demixing, an efficiency of 11.6% was achieved. [47,48] Here, the "freezing" of the J71:Y6 bulk-heterojunction in an non-equilibrium state during the very fast nanoparticle formation within 100 ms [49] may well account for the improved performance of the solar cells.…”

Water-based solar cells over 10% efficiency: designing soft nanoparticles for improved processability

“…The nanoprecipitation technique not only remains extremely simple for large-scale transfer with continuous flow production, 49 but the absence of additives and the use of lower temperatures are also attractive for industrial manufacturing and/or the use of flexible substrates. To conclude, green and efficient water-based organic solar cells were prepared, with an average efficiency reaching more than 90% of the average performance from organic solvent references (10.8%).…”