Abstract
AbstractMicrofluidic technologies are highly adept at generating controllable compositional gradients in fluids, a feature that has accelerated the understanding of the importance of chemical gradients in biological processes. That said, the development of versatile methods to generate controllable compositional gradients in the solid‐state has been far more elusive. The ability to produce such gradients would provide access to extensive compositional libraries, thus enabling the high‐throughput exploration of the parametric landscape of functional solids and devices in a resource‐, time‐, and cost‐efficient manner. Herein, the synergic integration of microfluidic technologies is reported with blade coating to enable the controlled formation of compositional lateral gradients in solution. Subsequently, the transformation of liquid‐based compositional gradients into solid‐state thin films using this method is demonstrated. To demonstrate efficacy of the approach, microfluidic‐assisted blade coating is used to optimize blending ratios in organic solar cells. Importantly, this novel technology can be easily extended to other solution processable systems that require the formation of solid‐state compositional lateral gradients.
Highlights
Control of compositional gradients has been exploited in multiple materials to engineer new physical and chemical properties as well as novel functionalities
To generate a binary compositional gradient, we utilized a microfluidic mixer incorporating two inlets and a symmetric branched manifold that allows the distribution of the two precursor solutions into three different streams
We focused our initial experiments on polymer:fullerene blends, namely poly[N-9′-heptadecanyl-2,7-carbazole-alt-5,5-(4′,7′-di-2-thienyl2′,1′,3′-benzothiadiazole)]: [6,6]-phenyl-C71-butyric acid methyl ester (PCDTBT:PC70BM), which constitutes a high performing material combination in organic photovoltaics (OPV), and one which has been recently subjected to combinatorial screening.[12]
Summary
Control of compositional gradients has been exploited in multiple materials to engineer new physical and chemical properties as well as novel functionalities. Gradual changes in absorption can be used to create tunable density filters,[1] and subtle variations in surface energy can drive water drops uphill.[2] compositionally graded materials allow the investigation of the role of compositional effects on a desired property of interest Examples in this respect include the screening of biochemical signals on cell behavior,[3] the maximization of piezoelectric coefficients,[4] the rapid exploration of material blend phase diagrams,[5] as well as the determination of the optimum composition in ternary photovoltaic devices.[6]. Films are formed by blade coating of solutions delivered from the outlet channels of a microfluidic device We show that such approach enables the efficient and large area transfer of compositional gradients generated in solution to solid state thin films onto virtually any substrate. We optimize the chemical composition as well as the performance of polymer:polymer blend-based organic solar cells Such systems represent a formidable challenge, due to their intrinsic high viscosities and difficult mixing dynamics
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