Department of Chemical and Biomolecular Engineering , University of Illinois Urbana-Champaign , 600 S. Mathews Avenue , Urbana , Illinois 61801 , United States.
Department of Materials Science and Engineering , University of Illinois at Urbana-Champaign , 1304 W. Green Street , Urbana , Illinois 61801 , United States.
ACS Appl Mater Interfaces. 2018 Nov 28;10(47):40692-40701. doi: 10.1021/acsami.8b13790. Epub 2018 Nov 13.
Meniscus instability during meniscus-guided solution coating and printing of conjugated polymers has a significant impact on the deposit morphology and the charge-transport characteristics. The lack of quantitative investigation on meniscus-instability-induced morphology transition for conjugated polymers hindered the ability to precisely control conjugated polymer deposition for desired applications. Herein, we report a film-to-stripe morphology transition caused by stick-and-slip meniscus instability during solution coating seen in multiple donor-acceptor polymer systems. We observe the coexistence of film and stripe morphologies at the critical coating speed. Surprisingly, higher charge-carrier mobility is measured in transistors fabricated from stripes despite their same deposition condition as the films at the critical speed. To understand the origin of the morphology transition, we further construct a generalizable surface free energy model to validate the hypothesis that the morphology transition occurs to minimize the system surface free energy. As the system surface free energy varies during a stick-and-slip cycle, we focus on evaluating the maximum surface free energy at a given condition, which corresponds to the sticking state right before slipping. Indeed, we observe the increase of the maximum system surface free energy with the increase in coating speed prior to film-to-stripe morphology transition and an abrupt drop in the maximum system surface free energy post-transition when the coating speed is further increased, which is associated with the reduced meniscus length during stripe deposition. Such an energetic change originates from the competition between pinning and depinning forces on a partial wetting substrate which underpins the film-to-stripe transition. This work establishes a quantitative approach for understanding meniscus-instability-induced morphology transition during solution coating. The mechanistic understanding may further facilitate the use of meniscus instability for lithography-free patterning or to suppress instability for highly homogeneous thin film deposition.
在聚合物的介电质引导溶液涂层和印刷过程中，半月板不稳定会对沉积物形态和电荷输运特性产生重大影响。由于缺乏对介电质不稳定引起的聚合物形态转变的定量研究，因此无法精确控制用于所需应用的聚合物沉积。在这里，我们报道了在多个供体-受体聚合物体系中，在溶液涂层过程中由于粘性滑动介电质不稳定而导致的薄膜到条纹形态转变。我们观察到在临界涂层速度下，薄膜和条纹形态共存。令人惊讶的是，尽管在临界速度下，条纹状晶体管的沉积条件与薄膜相同，但它们的载流子迁移率却更高。为了理解形态转变的起源，我们进一步构建了一个可推广的表面自由能模型，以验证这样一种假设，即形态转变的发生是为了最小化系统的表面自由能。由于系统表面自由能在粘性滑动循环过程中发生变化，我们专注于评估给定条件下的最大表面自由能，这对应于粘性滑动前的粘着状态。事实上，我们观察到在薄膜到条纹形态转变之前，最大系统表面自由能随着涂层速度的增加而增加，并且在进一步增加涂层速度后，最大系统表面自由能突然下降，这与条纹沉积期间介电质长度的减少有关。这种能量变化源于部分润湿基底上的钉扎力和去钉扎力之间的竞争，这为薄膜到条纹的转变提供了基础。这项工作为理解溶液涂层过程中介电质不稳定引起的形态转变提供了一种定量方法。对这种机械理解的进一步认识可以促进利用介电质不稳定进行无光刻图案化或抑制高度均匀的薄膜沉积过程中的不稳定性。
