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Halide perovskite nanocrystals hold promise for printable optoelectronic and photonic applications. Doping enhances their functionalities and is being investigated for substituting lead with environmentally friendlier elements. The most investigated dopant is Mn 2+ that acts as a color center sensitized by the host excitons. The sensitization mechanism is far from understood and no comprehensive picture of the energy-transfer process has been proposed. Similarly, the role of shallow states, particularly abundant in defect tolerant materials, is still unknown. Here, we address this problem via spectroscopic studies at controlled excitation density and temperature on Mn:CsPbCl 3 nanocrystals. Our results indicate a two-step process involving exciton localization in a shallow metastable state that mediates the thermally assisted sensitization of the Mn 2+ emission, which is completely quenched for T < 200 K. At T ≤ 60 K, however, such emission surprisingly reappears, suggesting direct energy transfer from band-edge states. Electron spin resonance supports this picture, revealing the signatures of conformational rearrangements below 70 K, possibly removing the potential barrier for sensitization. Our results demystify anomalous behaviors of the exciton-to-Mn 2+ energy-transfer mechanism and highlight the role of shallow defects in the photophysics of doped perovskite nanostructures.

Colloidal nanoplatelets (NPLs), owing to their efficient and narrow-band luminescence, are considered as promising candidates for solution-processable light-emitting diodes (LEDs) with ultrahigh color purity. To date, however, the record efficiencies of NPL-LEDs are significantly lower than those of more-investigated devices based on spherical nanocrystals. This is particularly true for red-emitting NPL-LEDs, the best-reported external quantum efficiency (EQE) of which is limited to 0.63% (EQE = 5% for green NPL-LEDs). Here, we address this issue by introducing a charge-regulating layer of a polar and polyelectrolytic polymer specifically engineered with complementary trimethylammonium and phosphonate functionalities that provide high solubility in orthogonal polar media with respect to the NPL active layer, compatibility with the metal cathode, and the ability to control electron injection through the formation of a polarized interface under bias. Through this synergic approach, we achieve EQE = 5.73% at 658 nm (color saturation 98%) in completely solution processed LEDs. Remarkably, exposure to air increases the EQE to 8.39%, exceeding the best reports of red NPL-LEDs by over 1 order of magnitude and setting a new global record for quantum-dot LEDs of any color embedding solution-deposited organic interlayers. Considering the emission quantum yield of the NPLs (40 ± 5%), this value corresponds to a near-unity internal quantum efficiency. Notably, our devices show exceptional operational stability for over 5 h of continuous drive in air with no encapsulation, thus confirming the potential of NPLs for efficient, high-stability, saturated LEDs.

Colloidal quantum wells combine the advantages of size-tunable electronic properties with vast reactive surfaces that could allow one to realize highly emissive luminescent-sensing varnishes capable of detecting chemical agents through their reversible emission response, with great potential impact on life sciences, environmental monitoring, defence and aerospace engineering. Here we combine spectroelectrochemical measurements and spectroscopic studies in a controlled atmosphere to demonstrate the ‘reversed oxygen-sensing’ capability of CdSe colloidal quantum wells, that is, the exposure to oxygen reversibly increases their luminescence efficiency. Spectroelectrochemical experiments allow us to directly relate the sensing response to the occupancy of surface states. Magneto-optical measurements demonstrate that, under vacuum, heterostructured CdSe/CdS colloidal quantum wells stabilize in their negative trion state. The high starting emission efficiency provides a possible means to enhance the oxygen sensitivity by partially de-passivating the particle surfaces, thereby enhancing the density of unsaturated sites with a minimal cost in term of luminescence losses.

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.