In addition to surface engineering, composition adjustment of PNCs is also a feasible strategy. Kimet al. introduced guanidinium cations (GA⁺) to occupy A sites in FAPbBr₃ PNCs (Fig. 2(a)), which can well passivate surface-exposed Pb²⁺ due to extra amino groups and the well distributed positive charges on GA^+[9]. Because of high PLQY (93.3%) and structural stability of GA⁺-doped FAPbBr₃ PNCs, the LEDs offered an EQE of 23.4% (Fig. 2(b)) and a current efficiency of 108 cd/A. By contrast, doping cations on B-site modulates the energy band of PNCs (e.g. Mn²⁺, Cu²⁺, Sn²⁺ and Sr²⁺)^[36-39]. Shenet al. converted CsPbI₃ PNC from n-type semiconductor to nearly ambipolar semiconductorvia doping Zn²⁺ (Fig. 2(c)), leading to more balanced carrier transport within LEDs^[40]. The resulting red LEDs achieved brighter EL emission with a luminance of 2202 cd/m² and an EQE of 15.1% (Fig. 2(d)). Besides, doping Mn²⁺ into perovskite lattice induced a second emission peak at ~600 nm, which resulted from the energy transfer from perovskite to Mn²⁺ (Fig. 2(e)). Doping Sr²⁺ in CsPbI₃ PNCs caused new trap states near the conduction band edge but simultaneously generating an excitonic state at a lower energy level, which prevented the trap-assisted non-radiative recombination, yielding a PLQY of ~95%^[39]. Such PNCs showed enhanced stability, due to the increased formation energy of cubic CsPbI₃ after being doped with Sr²⁺. Based on Sr²⁺-doped CsPbI₃ PNCs, Chenet al. realized efficient and stable red LEDs with an EQE of 17.1%^[41].

Several issues impede the application of PNC-LEDs. (1) Poor stability. More conductive capping matrix can facilitate charge injection into PNC emitters, e.g. 3D perovskite and metal of frame (MOF)^[42,43]. Besides, balanced charge transport favors to combat the efficiency roll-off and enhance device stability^[44]. (2) Lead toxicity. Lead-free PNCs always present inferior luminescent properties, e.g. relatively low PLQY and wide emission spectra^[45]. More studies on bandgap structure, surface properties and carrier dynamics of lead-free PNCs are needed. In addition, effective encapsulation of lead-halide PNCs may also be a solution.

Lewis bases containing carbonyl (C=O), carboxylate (–COO^–), phosphate (P=O) or sulfonate (–SO₃^–) groups can also passivate uncoordinated surface Pb²⁺ and suppress nonradiative recombination in PNCs^[30-33]. Zenget al. employed phosphine oxide molecules to passivate both top and bottom surfaces of CsPbBr₃ PNC film to suppress trap-assisted nonradiative recombination (Fig. 1(b)). The LEDs gave an EQE of 18.7% and a prolonged half-life of 15.8 h^[31]. Zhaoet al. used 2-naphthalenesulfonic acid (NSA) to passivate the uncoordinated surface Pb²⁺ in FAPbBr₃ PNCs, and the green LEDs offered a luminance of 67115 cd/m² and an EQE of 19.2%^[32]. The removal of redundant Pb²⁺ from PNC surface is an effective strategy to keep the high efficiency. Hassanet al. reported that ethylenediaminetetraacetic acid (EDTA) and the reduced L-glutathione could eliminate excess surface Pb²⁺ in MAPb(Br/I)₃ PNCs due to their strong interaction with Pb²⁺ (Fig. 1(c))^[34]. The PNCs with flattened surfaces presented boosted PLQY and suppressed phase separation, yielding an EQE of >20% for the LEDs with a stable EL peak at 620 nm. Biet al. utilized hydrogen bromide (HBr) to facilitate the desorption of OA ligands and induce the removal of imperfect [PbBr₆]⁴⁻ octahedra from CsPbBr₃ PNC surface. By using didodecylamine (DDDAM) and phenethylamine (PEA) passivating ligands, the damaged PNC surface was recovered with reduced trap density, yielding a pure-blue LED with enhanced stability and a luminance of 3850 cd/m^2[35].

Compared to traditional emitters, the ionic bonding and relatively low formation energy of perovskite lattice enabled the facile formation of PNCs through liquid-phase synthesis^[11,12] (e.g. hot-injection (Fig. 1(a))^[1], ligand-assisted reprecipitation^[13] and ultrasonic-assisted synthesis^[14]). Thanks to the great efforts in regulating the types and ratios of precursors, modifying the reaction temperature and adjusting the solvents and anti-solvents, near-unity PLQYs have been demonstrated for as-synthesized PNCs^[15-17]. However, the luminescence properties for fresh PNCs would always be impaired during the subsequent purification, assembly process and storage in ambient conditions. The highly dynamic binding between the capping ligands and PNC surface induced the surface ligand desorption and subsequent generation of surface defects^[18,19]. In particular, the facilely formed halide ion (X^–) vacancies and uncoordinated Pb²⁺ ions on PNC surface would yield carrier trapping centers and also provide sites for the invasion of external water and oxygen^[20,21], seriously deteriorating the performance of PNCs^[22-25]. Chibaet al. utilized ammonium iodine salts, oleylammonium iodide (OAM-I) and aniline hydroiodide (An-HI), for post-treatment of CsPbBr₃ PNCs to fill in the surface Br^– vacancies and red-shift their PL emission (Fig. 1(a)), leading to pure red PNC-LEDs with an EQE of 21.3%^[26]. Similarly, didodecyldimethylammonium fluoride (DDAF) endowed CsPbBr₃ PNCs with well-passivated surface and improved resistance to thermal quenching of PL^[27]. As a result, the LEDs presented an EQE of 19.3% with a low efficiency roll-off and enhanced thermal stability. Dong et al. proposed an ordinal surface-passivating strategy involving short-chain isopropylammonium bromide (IPABr) and NaBr to coat CsPbBr₃ PNCs with a relatively stable bipolar shell, while also enhancing inter-dot charge coupling due to adequate removal of insulating OA and OAm ligands^[28]. This strategy yielded blue and green CsPbBr₃ PNC-LEDs with EQEs of 12.3% and 22%, respectively, as well as improved operational stability. Zhenget al. used n-dodecylammonium thiocyanate (DAT) to eliminate the surface defects of mix-halide PNCs without affecting their PL spectra, yielding pure blue PNC-LEDs with an EQE of 6.3% and stabilized EL spectra^[29].

With superior photoluminescence quantum yields (PLQYs), tunable bandgap, high color purity and solution processibility^[1,2], metal halide perovskite nanocrystals (PNCs) with a general formula of ABX₃ (A = CH₃NH₃⁺ (MA⁺), CH(NH₂)₂⁺ (FA⁺) and Cs⁺, B = Pb²⁺, Sn²⁺ and Mn²⁺, X = Cl^–, Br^– and I^–) emerge as promising luminescent materials in light-emitting diodes (LEDs) and solid-state lighting^[2-4]. Since electroluminescence (EL) of PNCs was first observed in CsPbBr₃ PNC-based LEDs with an external quantum efficiency (EQE) of 0.07% in 2015^[5], the efficiencies for different LEDs have been significantly boosted. The red and green LEDs demonstrated an EQE of >23% and the highest EQE for blue LEDs reached 13.8%^[6-8], comparable to conventional LEDs based on organic emitters or metal chalcogenide (II–VI) quantum dots (QDs)^[9,10].

This work was supported by the National Natural Science Foundation of China (51872014), the Recruitment Program for Global Experts, the Fundamental Research Funds for the Central Universities and the “111” project (B17002). L. Ding thanks the open research fund of Songshan Lake Materials Laboratory (2021SLABFK02), the National Key Research and Development Program of China (2017YFA0206600) and the National Natural Science Foundation of China (51922032, 21961160720).