The interesting optical and electrical properties of semiconductors can be modified by reducing their size to a few nanometers scale. I have experience in the preparation of various perovskite nanocrystals such as CsPbX₃, MAPbX_3, and FAPbX₃. I was among those first people who reported a simple one-step synthetic route to prepare well-defined FAPbX­₃ nanocrystals at room temperature (Chem. Mater. 2017, 29, 13, 5713-5719). The synthesized FAPbX₃ nanocrystals show high thermal stability, very broad color tunability (408 nm ≤ λpeak ≤ 784 nm), high PLQY (75%), and narrow FWHM (18‒48 nm).

Blending polymer into halide perovskite is a simple and effective method to stabilize the fluorescent emission of perovskite materials. I have demonstrated a unique technique called “size exclusion lithography” to self-assemble perovskite nanoparticles in polymer composite film (Adv. Mater. 2018, 1802555). This technique enables us to fabricate FAPbX₃-polymer composite films with the capability of high-resolution patterning (≥ 2 µm) and excellent resistance to various aqueous and organic solvents. Both positive- and negative-tone patterns of FAPbX₃ nanoparticles are created by controlling the size-exclusive flow of nanoparticles in polymer networks. The position of nanoparticles is spatially controlled in both lateral and vertical directions.

Formamidinium bromide (FABr) is well-known to be an A-site cation in conventional three-dimensional APbBr₃ perovskite structure, which is extremely difficult to construct low-dimensional perovskites. In contrast, I first time demonstrated that FABr can also act as a spacer cation to prepare a new class of (110)-oriented low-dimensional perovskites with a formula of FA_m+2Pb_mBr_3m+2 (Adv. Funct. Mater. 2021, 31, 2011093). The prepared FA_m+2Pb_mBr_3m+2 films showed a phase transition to 3D FAPbBr₃ when exposed to humid conditions and UV light. PVDF can stabilize FA_m+2Pb_mBr_3m+2 phase. The phase-transition behavior of FA_m+2Pb_mBr_3m+2-PVDF films in response to humidity, UV light, and solvents has been explored for the phase-change memory films.

I found that DMF, which typically used as solvent for perovskite fabrication, has undergone photoreaction to produce perovskite structure. The reaction occurred under high intense UV irradiation with the presence of PbX₂ acting as both precursor and catalyst. The photoreaction is compatible with various polymer matrices, which are utilized to fabricate free-standing and highly stable fluorescent pattern perovskite-polymer composite films. Notably, DMF can be completely removed by temperature, resulting in a long-lasting fluorescent pattern. These fluorescent composite films are highly potential to apply for flexible, stretchable LED color filters and physical unclonable functions.

In the current project, I focus on the fabrication of intrinsically stretchable perovskite light-emitting electrochemical cells (is-PeLECs). The is-PeLECs are expected to overcome numerous limitations in stretchability, stability, material variations, and efficiency of previously reported traditional LEDs such as polymer LECs (PLECs), organic light-emitting devices (OLEDs), and perovskite LEDs (PeLEDs).

In the current project, I focus on the fabrication of intrinsically stretchable perovskite light-emitting electrochemical cells (is-PeLECs). The is-PeLECs are expected to overcome numerous limitations in stretchability, stability, material variations, and efficiency of previously reported traditional LEDs such as polymer LECs (PLECs), organic light-emitting devices (OLEDs), and perovskite LEDs (PeLEDs).

Semiconducting nanocrystals even under constant illumination almost exhibit intermittency in their emission often calls “blinking”. Blinking arises from the escape of either one or both of photoexcited carriers to the nanocrystal surface that limited various optoelectronic device applications of semiconducting nanocrystals. In the collaboration work, we have shown that FAPbBr₃ nanocrystals can act as good single-photon sources with very low multiphoton emission probability achieved by extremely fast nonradiative Auger recombination (ACS Photonics 2018, 5, 12, 4937-4943). By analyzing the high (ON) - low (OFF) intensity time distribution and fluorescence lifetime intensity distribution (FLID), we have demonstrated that type-A and type-B blinking simultaneously contributed to FAPbBr₃ nanocrystal blinking (Sci. Rep. 2020, 10, 2172). From these understandings, we demonstrated that TiO₂ film coated below perovskite nanocrystals can suppress the blinking phenomena. The electrons from TiO₂ filling the trap states of FAPbBr₃ nanocrystals during Fermi-level equilibrium, which can reduce the possibility of capturing the hot electrons from perovskite nanocrystal into the trap states (APL Mater. 2020, 8, 031102).