(Nanowerk Spotlight) Perovskite materials have attracted great attention in the fields of optoelectronics due to their significant optoelectronic properties. So far, the applications of perovskite thin-films have been limited to solar cells because the required high-definition patterning for optoelectronic devices hadn't been achieved yet.
Now, though, researchers in Korea have realized a high-resolution spin-on-patterning (SoP) process for the fabrication of optoelectronic devices arrays such as image sensors.
"Unlike previous works, which can only pattern perovskite materials on metal, patterning perovskite materials on the electron and hole transport layer is possible by using our novel SoP method," Dae-Hyeong Kim, a professor at Seoul National University, tells Nanowerk. "We expect that our work could help develop perovskite optoelectronics such as perovskite cameras and displays."
Spin-on-patterning (SoP) of a inorganic–organic hybrid perovskite thin-film. Snapshot images at various states of the SoP process, captured from a slow-motion video, see below. (Image: Professor Dae-Hyeong Kim, Seoul National University) (click on image to enlarge)
Although there were similar research efforts on patterning organic/perovskite materials by manipulating the surface energy of metal oxide, patterning materials during spin-coating is difficult to realize.
In order to solve this problem, Kim's team observed and analyzed the intermediate steps of patterning perovskite with fast-speed cameras and subsequently optimized their SoP process.
Briefly, a densely packed hydrophobic self-assembled monolayer (SAM) is deposited on the hydrophilic oxide substrate. The photoresist is spin-coated and patterned. The photoresist-patterned oxide substrate is then treated with oxygen plasma using a reactive ion etcher to remove the SAM selectively, expose the oxide, and make that region hydrophilic. The removal of the photoresist leads to a patterned substrate with different surface energies (hydrophilic oxide region and hydrophobic SAM-modified region). Then, iii) the solution of perovskite precursors is spin-coated, which forms a thin solution film. Finally, the
patterned solid-state perovskite film is obtained by thermal annealing.
The following video shows the detailed process:
The team notes that their SoP process is compatible with almost every spin-coated perovskite material and deposition technique (e.g., mixed-precursor, two-step, hot-casting, and solvent-engineering) as well as microfabrication processes used in the industry for mass production of multiplexed image sensor arrays. As such it can be applied to high-performance, ultrathin, and deformable optoelectronic device arrays.
As a demonstration, they integrated the patterned perovskite photodiodes (photodetecting unit) with silicon nanomembrane blocking diodes (active electronics) to build a multiplexed, ultrathin, lightweight, and deformable perovskite-on-silicon image sensor array.
However, large-scale fabrication with uniform pattern is crucial to commercializing perovskite optoelectronics. Although the researchers showed that wafer-scale fabrication is possible by using the SoP process, uniform patterns are not achieved (because the uniformity of the spin-coating process decreases with the increase of the number of pixels in the array). This problem will be addressed by future work.