Directing Solution-Phase Nucleation to Form Organic Semiconductor Vertical Crystal Arrays

Control over the out-of-plane molecular orientation of solution-processed organic semiconductors is a long-standing challenge in the organic electronics community. Here, a generalizable strategy using nanoconfinement to direct the nucleation of small-molecule organic semiconductors during solution-phase deposition is presented. Using a facile dip-coating process, triisopropylsilylethynyl-derivatized acene molecules were deposited onto nanoporous anodized aluminum oxide (AAO) scaffolds with average pore diameters ranging from 60 to 200 nm. Preferentially oriented nuclei were found to form within the cylindrical AAO nanopores such that the fast growth direction (i.e., the ?-stack direction) aligned with the long axes of the pores. Crystal growth then propagated above the scaffold, resulting in the formation of vertical crystal arrays with the high surface energy ?-planes exposed at the crystal tips. The diameters and heights of these crystals were tunable over ranges of 100-600 nm and 0.8-6.7 μm, respectively, by varying the dip-coating speed and scaffold pore diameters. Photoluminescence (PL) experiments further revealed an 8-fold enhancement of the PL signal from vertical crystal arrays compared to horizontal crystals deposited on flat SiO₂ substrates due to waveguiding along the crystal length. Critically, this strategy is compatible with continuous deposition techniques that will enable the high-throughput, large-area manufacturing of flexible and inexpensive optoelectronic devices.