Role of Nonradiative Defects and Environmental Oxygen on Exciton Recombination Processes in CsPbBr₃ Perovskite Nanocrystals

Abstract

Lead halide perovskite nanocrystals (NCs) are emerging as optically active materials for solution-processed optoelectronic devices. Despite the technological relevance of tracing rational guidelines for optimizing their performances and stability beyond their intrinsic resilience to structural imperfections, no in-depth study of the role of selective carrier trapping and environmental conditions on their exciton dynamics has been reported to date. Here we conduct spectro-electrochemical (SEC) experiments, side-by-side to oxygen sensing measurements on CsPbBr NCs for the first time. We show that the application of EC potentials controls the emission intensity by altering the occupancy of defect states without degrading the NCs. Reductive potentials lead to strong (60%) emission quenching by trapping of photogenerated holes, whereas the concomitant suppression of electron trapping is nearly inconsequential to the emission efficiency. Consistently, oxidizing conditions result in minor (5%) brightening due to suppressed hole trapping, confirming that electron traps play a minor role in nonradiative decay. This behavior is rationalized through a model that links the occupancy of trap sites with the position of the NC Fermi level controlled by the EC potential. Photoluminescence measurements in controlled atmosphere reveal strong quenching by collisional interactions with O, which is in contrast to the photobrightening effect observed in films and single crystals. This indicates that O acts as a scavenger of photoexcited electrons without mediation by structural defects and, together with the asymmetrical SEC response, suggests that electron-rich defects are likely less abundant in nanostructured perovskites than in the bulk, leading to an emission response dominated by direct interaction with the environment.

Investigation of the Role of the Environment on the Photoluminescence and the Exciton Relaxation of CsPbBr3 Nanocrystals Thin Films

“…Despite this, the effects of the environment on the optical properties of lead halide perovskites NCs have been surprisingly poorly investigated to date, with only few experiments on CsPbBr 3 nanocubes grown by hot-injection method, curiously finding contradictory results. In particular, the PL intensity in air has been reported to be lower than in vacuum [3,28], or not affected by the environment [27]. These results are different from the ones on single crystals, bulk polycrystalline thin films, and even nanosheets and nanowires [3], all showing a PL intensity enhancement in air.…”

Surface Halogen Compensation for Robust Performance Enhancements of CsPbX3 Perovskite Quantum Dots

“…. Moreover, the luminescence features can even be affected by the ambient atmosphere and this phenomenon was also assigned to surface defects . When preparing CsPbBr 3 QDs or films, equal moles of CsBr and PbBr 2 do not result in the highest QY and the QY values often vary in a wide range, depending on the mole ratio .…”

Optimizing the Atomic Layer Deposition of Alumina on Perovskite Nanocrystal Films by Using O2 As a Molecular Probe

“…O 2 was then used as a molecular probe to compare and contrast the ability of the proposed ALD modes to grow gas diffusion barriers. Recent reports have demonstrated reduction in the PL of CsPbBr 3 NCs in the presence of molecular O 2 . The authors have suggested that this behavior is due to the interaction of O 2 with the NC surface.…”

“…Herein, we optimize the above‐mentioned ALD protection scheme for CsPbBr 3 NC films by using O 2 as a molecular diffusion probe to assess the uniformity of the encapsulating AlO x layer. The O 2 quenches the PL by extracting photogenerated charges . The better the gas barrier properties of ALD layer are, the less quenching is expected when the CsPbBr 3 /AlO x nanocomposites are exposed to an O 2 ‐rich atmosphere .…”