Investigating the photodegradation mechanism and effects of excess iodide on the photostability of all-inorganic lead halide perovskite

• Main Author: Soo, Shawn Zuo Xiang
• Other Authors: Nripan Mathews
• Format: Final Year Project
• Language: English
• Published: Nanyang Technological University 2024
• Subjects: Chemistry; Engineering; Perovskite; Solar; Energy
• Online Access: https://hdl.handle.net/10356/176135
• Institution: Nanyang Technological University

Perovskite solar cells (PSCs) have risen in popularity following its rapid advancement in performance over the years since its invention in 2009. Hybrid PSCs such as MA/FAPbI3 have exhibited excellent power conversion efficiency (PCE) and good intrinsic stability, but do however suffer greatly from its poor extrinsic stability. Although lacking behind in performance, scientists have gravitated towards the use of all-inorganic PSCs due to its excellent extrinsic stability over its hybrid counterparts, particularly CsPbI3 mainly due to its suitable bandgap (~1.7eV). However, all-inorganic PSCs still suffer from poor phase stability and much research have been carried out to improving intrinsic stability, whereas little attention have been placed on its extrinsic stability, especially its photostability. Herein, we aim to investigate the full photodegradation reaction and mechanism of CsPbI3, as well as to identify the products formed from the photodegradation of the perovskite. When exposed to light for a prolonged duration, we have shown that CsPbI3 perovskite film will undergo photodegradation to form cesium iodide (CsI) crystals and an intermediate product of lead iodide (PbI2), before it undergoes further photodegradation to form lead (Pb0) metal and iodine (I2). With regards to the photodegradation mechanism, we propose that photodegradation is driven by the ion migration of iodide anions (I-), which drives the formation of I2 and more anionic vacancies, ultimately resulting in the formation of Pb0 metal. When excess I- and thiocyanate anions (SCN-) were added to reduce the ion migration of the existing I- present in the lattice, it is shown that the photostability of CsPbI3 generally improves, however only with the addition of an adequate amount of excess anions, whereas any insufficient or excess anions present in the lattice is shown to be detrimental towards the photostability of CsPbI3. This is likely attributed by the lack of anions added to passivate all the anionic vacancies or due to the surplus of anions that are incorporated the lattice, resulting in the formation of interstitial defects. Hence, calculation of the intrinsic defect density or judicious optimisations with respect to the quality of the perovskite films may prove to be a useful method to accurately determine the appropriate amount of additives to add, in order to effectively passivate all vacancy defects and ultimately improving the photostability of the perovskite film.