Nanoimprint-induced molecular stacking and pattern stabilization in a solution-processed subphthalocyanine film

We present a systematic study on the thermal nanoimprinting of a boron subphthalocynamine molecule, 2-allylphenoxy-(subphthalocyaninato)boron(III) (SubPc-A), which represents a class of attractive small-molecular weight organic compounds for organic-based photovoltaics (OPV). The final equilibrium imprinted feature profile strongly depends on the imprinting temperature. The highest feature aspect ratio (or contrast) occurs at a specific window of imprinting temperatures (80-90 degrees C). X-ray diffraction indicates that the nanoimprint at such a temperature window can induce high-degree molecular stacking, which can help stabilize the imprinted features. Outside this window, we observed a pronounced relaxation of imprinted features after template removal, which is attributed to the surface diffusion. Key factors affecting the final equilibrium profile of the imprinted features were simulated. From the simulation, it was found that the crystallization-induced anisotropy of surface energy stabilized imprinted features. Simulated parameters such as stable feature aspect ratio and pitch agree well with experimental data. Such work provides an important guideline for optimizing the nanopatterning of small-molecular-weight organic compounds.