Engineering of Ruddlesden-Popper planar faults in CsPbBr3 perovskite nanocrystals for improved performance in light emitting devices

We developed a procedure that allowed triggering post-synthetic growth of CsPbBr3 perovskite nanocrystals (PNCs) via fusion. Pursuing the study, we discovered that the growth of CsPbBr3 PNCs was accompanied by the formation of Ruddlesden-Popper (RP) planar faults -- well known for perovskite oxides, yet not observed in perovskite allinorganic halides until then. In a mutual effort with the researchers at the University of Washington, it was discovered that the RP planar faults were predicted to produce significant band-off sets in the conduction (+134 meV) and valence bands (-193 meV), thus repelling both electrons and holes at room temperature. In other words, RP faults were projected to serve as built-in potential wells, confining excitons within the perovskite domains. Following this lead, we proceeded exploration of CsPbBr3 PNCs with RP planar faults with an intent to define the impact of such RPs on the optoelectronic properties of CsPbBr3 PNCs. The work consists of three parts. In part one, we analyze and provide further support to the suggested earlier mechanism behind the formation of the RP faults. We demonstrate that the injection of diethylzinc into the PNCs solution results in a removal of stabilizing ligands from the surface of the PNCs, and their subsequent fusion accompanied with the formation of RPs. By modifying the procedure, we find that the concentration of the occurring RPs is higher when the injection is performed in the excess of CsBr. We reveal that the in the absence of high moisture content RP-CsPbBr3 PNCs display an extraordinary resilience to photodegradation when compared to PNCs without RPs. In addition, we demonstrate that the RP-CsPbBr3 PNCs can undergo anion-exchange reaction while successfully preserving the RP faults. This discovery opens up the avenue for future exploration of the observation for the first time mixed CsPb(I/Br)3 PNCs with RP planar faults. In part two, we study optoelectronic properties of RP-CsPbBr3 PNCs with an attempt to define the structure-property relationship for PNCs with the RPs. Our findings reveal that RP-CsPbBr3 PNCs exhibit higher binding energies and longer exciton lifetimes. The former is attributed to the quantum confinement induced by the RPs, resulting in tightly bound excitons within the perovskite domains cleaved by the RPs. The latter is ascribed to a plausible electron-hole spatial separation across the RP fault analogous to the one reported for quantum dots with the type II energy alignment. We also study the upconversion photoluminescence in both types of PNCs and reveal a significant difference in the response. These findings demonstrate explicitly that the RP faults largely transform optoelectronic properties of CsPbBr3 PNCs. Higher binding energies and longer lifetimes, combined with the exceptional resilience to photodegradation establish the RP-CsPbBr3 PNCs as excellent candidates for light emitting devices (LEDs). In part three, we postulate and experimentally verify the enhanced performance of RPCsPbBr3 PNCs in thin-film perovskite LEDs. Both type of PNCs are put to a test in the devices with identical architectures. We demonstrate that RP-LEDs outperform LEDs without RP faults. Specifically, they display stronger electroluminescence response, significantly higher air stability and better reproducibility. This is the first demonstration of thin-film LEDs based on all-inorganic CsPbBr3 PNCs with RP planar faults. Lastly, we summarize our findings and conclude the work with the suggestions for future work.