Speaker: Mr. Rohit Kumar Rohj
Research Supervisor: Prof. D. D. Sarma
Topic: Photophysics of 2D organic- inorganic hybrid lead halide perovskites: Layer edge states
Date & Time: Thur, 22nd September 2022 at 4:00 PM
Venue: SSCU Auditorium, Old SSCU Building
Two-dimensional (2D) Ruddlesden-Popper (RP) organic-inorganic lead halide perovskites have emerged as exciting materials for a wide range of applications in optoelectronics, owing to their tunable band gap, quantum confinement effect and long-term stability. These 2D systems are generally described as A2Bn-1PbnX3n+1 where A is a long organic alkyl/aromatic spacer cation, B is a small organic cation, and X is halide ion. Then integer ‘n’ is the number of inorganic perovskite layers between the organic spacers (A) which can be tuned by controlling the stoichiometry of A, B and Pb cations in the solution.
Quantum and dielectric confinement effects in these layered 2D structures lead to large exciton binding energies ranging from 400-100 meV for n = 1-4 and a tunable exciton emission/absorption can be obtained by changing the ‘n’ value . These properties make 2D structures an ideal candidate for light emitting and other optoelectronic applications. Large exciton binding energy in these 2D systems leads to quick exciton recombination and therefore faster photoluminescence (PL) decay. Recently, Blancon et al. found that excitons in layered 2D perovskite single crystals showed efficient exciton dissociation through a lower-energy state located at the layered crystal edges . These “edge states” are believed to be responsible for the improved performance of 2D perovskites in applications such as solar cells. Feng et al. showed that these edge states facilitate the collection and transportation of photogenerated carriers in layered 2D perovskite single crystalline nanowires for the demonstration of ultrasensitive 2D nanowire photodetector . Zhao et al. found that the edge state emission of (BA)2(MA)n-1PbnI3n+1 can be controlled by MA+ /BA+ cation exchange process . Whereas Shi et al. pointed out that the moisture exposure is the origin of the formation of these edge states . The behavior of excitons and/or free carriers is intimately correlated with the optoelectronic properties and thus their applications. Therefore, understanding the photo physics of 2D organic-inorganic hybrid lead halide perovskites is essential to consider exciton dissociation and free carrier properties of 2D RP perovskites when designing optoelectronic devices.
To date, the understanding of the edge states in 2D RP perovskites is insufficient and many questions remain unsolved. There are many experimental and theoretical results in the literature, but a consensus is not yet reached. In my talk, I shall discuss about some of the investigations done in probing the origin and the physical properties of the edge states.
1) Blancon, J. C.; et al. Scaling Law for Excitons in 2D Perovskite Quantum Wells. Nat. Commun. 2018, 9, 2254.
2) Blancon, J. C.; et al. Extremely Efficient Internal Exciton Dissociation Through Edge States in Layered 2D Perovskites. Science 2017, 355, 1288− 1292.
3) Feng, J.; et al. Single-Crystalline Layered Metal-Halide Perovskite Nanowires for Ultrasensitive Photodetectors. Nat. Electronics 2018, 1, 404−410. 4) Zhao, C.; et al. Controlling the Property of Edges in Layered 2D Perovskite Single Crystals. J. Phys. Chem. Lett. 2019, 10, 3950–3954.
5) Shi, E.; et al. Extrinsic and Dynamic Edge States of Two-Dimensional Lead Halide Perovskites. ACS Nano 2019, 13, 1635−1644.