All inorganic perovskite nanocrystals of CsPbX₃ (X=Cl, Br, I) have superior properties such as high fluorescence quantum yield, narrow half-peak width and easy adjustment of emission wavelength, etc., which have wide application prospects in light-emitting diodes, solar cells and photodetectors. However, CsPbX₃ nanocrystals have a poor stability under light, moisture and heating conditions due to their ionic properties, restricting their practical application. Metal ion doping can affect the electronic band structure of CsPbX₃ nanocrystals, thus effectively improving their photoluminescence properties. The doped metal ions can be divided into main group metal ion, rare-earth metal ion and transition metal ion. Transition metal ion has a lower price, and it can efficiently improve the stability of perovskite nanocrystals. This review represented recent work on the effect of transition metal ions (i.e., Mn²⁺, Ni²⁺, Cu²⁺, Zn²⁺, etc.) doping on the properties of perovskite nanocrystals. In addition, the challenges in the development of doped all-inorganic perovskite nanocrystals were analyzed, and the future development direction was also prospected.

Transition metal ion doping CsPbX₃ nanocrystals (NCs) are explored extensively. Mn²⁺ doping can improve the formation energy of CsPbX₃ NCs with a high stability. Mn²⁺ doping exists in wide-band-gap perovskite hosts where the excitation energy is transferred to Mn d-state, resulting in short-range tunable yellow-orange d-d emissions. Low concentration Mn²⁺ doping is beneficial to enhancing the exciton emission, and the Mn²⁺-doping sample has a long PL lifetime. Ni²⁺ doping can eliminate the structural defects of CsPbX₃ NCs, resulting in the improvement of the lattice order. Compared with the undoped sample, doping Ni²⁺ also induces the formation energy of CsPbX₃ NCs and triggers the phase transition of CsPbBr₃ from orthorhombic to cubic, which is attributed to the lattice strain due to the size mismatch of Ni²⁺ and Pb²⁺. Ni²⁺ doping reduces the non-radiative recombination efficiency and increases the fluorescence intensity of the CsPbX₃ NCs. Cu²⁺ doping sample maintains a cubic crystal structure of the initial phase. When Cu²⁺ with the smaller radius (i.e., 0.72 Å) replaces Pb²⁺ ion with the larger radius (i.e., 1.19 Å), the shrinkage of lattice contraction regulates the tolerance factor of the [PbBr₆]^4- octahedron, and increases the lattice formation energy, resulting in high stability and fluorescence intensity of NCs. Compared with Pb²⁺ (i.e., 1.19 Å), Zn²⁺ has smaller ion radius (i.e., 0.74 Å) and a lower toxicity. The introduction of Zn²⁺ can effectively passivate the surface and internal defects of CsPbX₃ NCs. Doping Co²⁺ can increase the formation energy, improve the short-range order of the lattice, and enhance the different monochromatic band edge emission without introducing new composite channels. Doping Cr³⁺ can regulate the structure and the optical properties of NCs. Doping Fe²⁺/Fe³⁺ has some advantages like low cost, high electrical conductivity and magnetic properties. Moreover, an appropriate amount of Fe²⁺ doping improves the size uniformity of CsPbX₃ NCs, as well as increases the PLQY and the average PL lifetime due to the decrease of the defect state and nonradiation recombination of NCs. Furthermore, the application of transition metal ion doped CsPbX₃ NCs in white light-emitting diode (WLED), solar cells and laser devices was outlined. The employment of doping transition metal ions into perovskites can improve the optical properties and stability, which energetically facilitate their applications. Finally, the main future research directions of transition metal ion doped CsPbX₃ NCs were concluded.