Area-selective growth of functional molecular architectures.

Over the last two decades, organic semiconductors have attracted increasing attention because of the applications of their inorganic counterparts in a growing number of devices. At the same time, the further success of these materials will require device processing techniques for organic semiconductors that produce high performance and high integration over large areas. Conventional top-down patterning techniques based on photolithography have served powerful methods for the surface patterning of inorganic materials. However, researchers cannot simply transfer these techniques to organic semiconductors because organic semiconductors can include small, fragile organic molecules. Alternatively, researchers have developed several nonconventional techniques, including shadow mask, printing, and vapor jet writing. However, no leading technique has emerged, and researchers are still trying to realize batch-to-batch, and even device-to-device, reproducibility. This Account summarizes recent research in our group aimed at developing methods for patterning small organic molecules that are compatible with standard device processing procedures for inorganic semiconductors. Our concept is based on classic growth dynamics by gas-phase deposition but leads to different selective growth mechanisms: "pre-patterning and patterned growth" instead of the traditional "film growth and patterning." As a result, both "foreign body" and "step edge", two possible nucleation positions for atoms and molecules during thin film growth process, can be enlarged to the mesotropic scale to define molecules within pre-determined areas. The techniques can do more than patterning. We demonstrate that these techniques can produce heteropatterning of organic structures that cannot be obtained by conventional photolithography and printing techniques. Through a combination of different growth modes, we can separate molecules at given locations on the mesotropic scale, which could lead to applications in the production of organic solar cells. Taking advantage of the differences in emission of molecules in different aggregation states, we can achieve tunable single, double- and triple-color patterns using two types of molecules. We also show that these materials can lead to devices with improved performance in features such as carrier mobility. In addition, we believe that this new photographic compatible procedure in small molecular organic semiconductors can address some issues in device performance, such as carrier transport in organic field effect transistors, by controlling domain size and numbers, and allow researchers to explore new nanoscale properties of these materials. The techniques are still in their infancy, and further research is needed to make them applicable, such as transferring the technology to cheap substrates, for example, glass and flexible plastic. For organic electronics, high-level integration, addressable, and cross-talk free device arrays are critical for producing high-performance devices at a low fabrication cost.