Defect Passivation of Mn²⁺-Doped CsPbCl₃ Perovskite Nanocrystals as Probed by Scanning Tunneling Spectroscopy: Toward Boosting Emission Efficiencies

In recent years, Mn-doped CsPbCl3 perovskite nanocrystals (NCs) have received widespread attention for solid-state lighting applications. However, a low PL quantum efficiency/yield coupled with a short lifetime of the PL emission forbade the material from further applied advancements. Defect states have been assumed to act as nonradiative recombination sites yielding a truncated PL quantum yield (PLQY). In this direction, simultaneous codoping with transition metals in the host lattice emerged as an effective strategy toward the improvement of PL intensity. However, despite intense research, a complete understanding of this approach remained elusive. Hence, further in-depth studies on the underlying mechanism are essential for their successful use as an optoelectronic material in different devices. In this work, we revisited energy levels of Cu2+–Mn2+ codoped CsPbCl3 perovskite NCs as a case study. Through scanning tunneling spectroscopy (STS), we observed direct evidence of defect states in the NCs and the microscopic origin of defect passivation upon codoping. The STS studies provided the density of states of the NCs and thereby band- and defect-state energies of the pristine semiconductor; the studies also infer defect passivation of the NCs through doping and codoping processes. The codoped perovskites yielded a high PLQY of 80%, which is one of the highest in the inorganic perovskite to date. This work on perovskite NCs shows the unique advantage of STS measurements over traditional PL studies or decay dynamics toward achieving a complete understanding of defect passivation in the light of their energy levels.