## 2 Ecofriendly Strategies to Synthesize High-Quality PNCs
### Capping Ligands from Natural Sources  
In line with employing green chemistry in the preparation and stabilization of colloidal PNCs, it is also remarkable that some of the capping agents can be obtained from natural sources such as seeds, fruits, and vegetables, among others. This is the case of the introduction of soy lecithin during the synthesis of CsPbBr\({}_{3}\) PNCs, studied first by [PERSON] and coworkers [63]. This interesting ligand is a zwitterionic phospholipid presenting a mixture of saturated and unsaturated long hydrocarbon chains with some functional ramifications, such as palmityl, stearyl, oleyl, linolyl, and linoleyl chains. Unlike colloidal solutions based on OA/OLA, where high PNCs concentrations cause material aggregation, lecithin-covered PNCs preserve a fully dispersed state even at ultrahigh or ultralow nanocrystal concentrations. This feature keeps the PL emission peak positions and PLQY \(\sim\)80% (Figure 3a,b) unaltered, indicating a good PNCs size homogeneity without any sign of aggregation, with a low density of surface defects to improve the radiative recombination pathway. Additionally, diluted colloidal PNCs can resist several purification repetitions with non-polar and polar solvents as in the case of acetone, acetonitrile, and isopropanol, providing a stable PLQY close to 100%. The benefits of this natural capping agent have been taken advantage of by [PERSON] and coworkers [64], synthesizing red-emitting CsPbI\({}_{3}\) PNCs with long-term stability for around 6 months under open-air and room conditions and maintaining a PLQY of up to 100% for the solution, and 60% in films (Figure 3c). Lecithin is anchored to the PNCs surface through oxygen incorporation, generating Cs-O and Pb-O bonds [14]. Here, it does not exist any hydrogen bonding interaction, which is exhibited by the use of an OA/OLA combination to promote surface lattice strain [65; 66]. In the absence of hydrogen bondings, the possibility of reaching a crystalline phase transformation, for instance, \(\alpha\)-to-\(\delta\) phase transition is avoided, which is pivotal to obtaining a stabilized photoactive phase for optoelectronic applications. At this stage, the authors have fabricated highly efficient bright red-LEDs with an electroluminescence peak position of \(\sim\)634 nm at 3.5 V initial voltage (Figure 3d), a luminance of 1391 cdm\({}^{-2}\), EQE -7.1%, and an operational lifetime of 33 min. This performance is a breakthrough in Rec. 2020 coordinates for bright-red emissions. Here, the linked lecithin molecules fill iodide vacancies from the nanocrystal surface, suppressing non-radiative carrier traps, which promotes better carrier injection and mobility into the device. With the aim to hinder the \(\alpha\)-to-\(\delta\) phase transformation into CsPbI\({}_{3}\) PNCs and generate high-quality perovskite inks for 3D printing applications, a previous contribution has investigated the use of \(\alpha\)-tocopherol (\(\alpha\)-TCP) for the first time [67]. This molecule is the active component of the vitamin E. By performing a post-synthetic treatment of the nanocrystals by adding diverse concentrations of \(\alpha\)-TCP, the intrinsic properties of perovskite can be stabilized for more than 2 months under room conditions and ambient air, reaching PLQY values up to 100% (Figure 3e). The addition of \(\alpha\)-TCP also restores the PL properties of aged PNC colloidal solutions, also keeping their structural integrity for 60 days. Through the phenolate moiety contained in the \(\alpha\)-TCP structure, halide vacancies are filled, inducing an effective ligand coverage against O\({}_{2}\) and humidity. Hence, a suitable \(\alpha\)-TCP content produces an enhancement of the radiative recombination dynamics into the nanocrystal, increasing the PLQY. In contrast, an excess of vitamins can affect the  
Figure 3: (**a**) Photographs of the lecithin–modified CsPbBr\({}_{3}\) PNCs by varying the concentration of the nanocrystal product under visible (up) and UV illumination (down).