Perovskite quantum dots (PQDs) combined with blue/UV Micro-LEDs have the potential to realize full-color Micro-LED, which hopefully solves the low efficiency of red LEDs in small sizes and mass transfer of three-color chips, meeting display requirements in AR/VR. The premise for the application of PQDs in the color conversion Micro-LEDs is to pattern PQDs into pixel arrays. Photolithography has shown great potential in patterning solution-processed nanomaterials for integration into advanced optoelectronic devices. However, photolithography of PQDs has so far been hindered by the incompatibility of perovskite with traditional optical lithography processes where lots of solvents and high-energy ultraviolet (UV) light exposure are required.
Recently, researchers led by Prof. Haizheng Zhong and Dr. Gaoling Yang at Beijing Institute of Technology demonstrate a direct in situ photolithography technique to pattern PQDs based on the photopolymerization catalyzed by lead bromide complexes. By combining direct photolithography with in situ fabrication of PQDs, this method allows to directly photolithograph perovskite precursors, avoiding the complicated lift-off processes and the destruction of PQDs by solvents or high-energy UV light, as PQDs are produced after lithography exposure.
The direct in situ photolithography is shown in Fig. 1 and described as follows:
(1) Functionalize substrates with ethenyl/thiol groups exposing by using silane coupling agents, creating covalent bonding sites for the resultant polymer.
(2) Cast the photosensitive perovskite precursor solution (PPR) directly onto the functionalized substrate. PPRs are prepared by mixing perovskite precursor solution with multifunctional thiol and ethenyl monomers, which can be cured under UV exposure without any external initiators and catalysts.
(3) Expose the PPR with UV light through a photomask, a photochemical reaction between thiol and ethenyl monomers, leading to solidification.
(4) Remove the unexposed PPR by spin-washing with chloroform as a developer. Cured products on exposed areas can adhere to the substrate.
(5) Anneal the prepared perovskite precursor patterns to evaporate residual solvents for in situ fabrication of luminescent perovskite patterns.
Perovskite precursor solutions are colloid, where halides and solvent molecules coordinate with lead ions to obtain a variety of lead halide complexes. For the first time, researchers discovered the photocatalysis capacity of lead bromide complexes, which can catalyze thiol-ene free-radical photopolymerization to enable fast photolithography without any external initiators and catalysts. As shown in Fig. 2, the photochemical mechanism was demonstrated that the lead bromide complexes catalyze thiol groups to generate sulfur radicals by hole transfer.
Using direct in situ photolithography, PQD patterns with excellent fluorescence uniformity, thickness up to 10 μm, high resolution up to 2450 pixels per inch (PPI), and good stability are successfully demonstrated (Fig. 3). This work opens an avenue for non-destructive direct photolithography of high-efficiency light-emitting PQDs, and potentially expands their application in various integrated optoelectronic devices.
For more details, please refer to “Direct in situ photolithography of perovskite quantum dots based on photocatalysis of lead bromide complexes, Pingping Zhang, Gaoling Yang, Fei Li, Jianbing Shi, & Haizheng Zhong, Nature Communications, 2022, 13, 6713, https://doi.org/10.1038/s41467-022-34453-9.