Critical Size/Viscosity for Coffee-Ring-Free Printing of Perovskite Micro/Nanopatterns.

Inkjet printing is the most encouraging method for patterning and integrating perovskite materials into microminiature application scenarios. However, it is still challenging to achieve high-resolution, coffee-ring-free, and perfect crystallized patterns. Here, a strategy based on powerful electrohydrodynamic printing and droplet viscosity-size coordinate regulation is developed to solve the above problems. By adding a long-chain polymer poly(vinylpyrrolidone) (PVP) into perovskite precursor to tune ink viscosity and introducing electrohydrodynamic printing to print the high-viscosity ink into droplets of different sizes, we can manipulate the inside flowing resistance and outside evaporation rate of a droplet, thus revealing a critical size/viscosity under which the coffee ring effect is inhibited, showing immense potential and significance for high-quality patterning. In addition, the long-chain polymer benefits droplet spatial limitation and uniform crystallization. The as-printed luminous patterns demonstrate high resolution (structure size ∼1 μm), excellent brightness, pleasant uniformity, and fascinating compatibility with flexible substrates, which is promising for future perovskite optoelectronic device applications.