Solution printing holds the promise to manufacture future electronic materials in an energy-efficient, low-cost, high-throughput fashion. However, major challenges remain, in controlling the morphology and molecular packing of the printed thin films, which critically impact the printed device performance. We introduce a new methodology to control thin film morphology during solution printing – flow enhanced crystal engineering, or FLUENCE, which leverage the unique characteristics of meniscus-guided solution coating methods, such as solution shearing and roll-to-roll printing. Using this approach, we have recently demonstrated for the first time large area coating of aligned single-crystalline thin films for organic field effect transistor applications. The high crystalline quality of FLUENCE printed thin films has enabled us to resolve the structures of newly discovered polymorphs of small molecule organic semiconductors, thereby uncovering that the charge carrier mobility can be altered by orders of magnitude among structurally similar crystal polymorphs.
We further extend the FLUENCE concept for controlling micro-phase separation in printed all-polymer solar cells. The design concept is verified via finite element based fluid simulations. The resulting morphology of the FLUENCE-printed thin films were characterized using resonant soft X-ray scattering to extract the domain size, and by grazing incidence X-ray diffraction to analyze the relative degree of crystallinity of the polymer donor. We found that FLUENCE significantly improved the crystallinity of the thin film, and reduced the characteristic length scale of microphase separation, both of which contributed to improved power conversion efficiency of the solar cell devices. We expect our strategy of flow-enhanced crystal engineering to go beyond printed transistors and photovoltaics to impact the greater community of printed functional materials.