Luminescent Inorganic–Organic Hybrid Nondoped Double Perovskite Nanocrystals for Optoelectronics

One of the key issues of lead-free halide double perovskite nanocrystals (DPNCs) is their low photoluminescence quantum yields (PLQYs), and the PLQYs of nondoped pristine DPNCs are currently below 2%, though they can be enhanced via strategies such as ion doping or surface passivation. Herein, we report an organic–inorganic hybrid approach to significantly increase the PLQYs of nondoped pristine DPNCs. With Cs2NaBiCl6 NCs as the model, we selected methylammonium (MA+) and formamidine (FA+) as organic A-site ions and for the first time successfully synthesized the organic–inorganic hybrid in-direct bandgap DPNCs, e.g., MA2NaBiCl6 and FA2NaBiCl6. The PLQYs of the prepared Cs2NaBiCl6, MA2NaBiCl6, and FA2NaBiCl6 are 1.9%, 6.2%, and 15.4%, respectively. The enhancement of PLQYs can be due to the lattice distortion on using the unsymmetric rod-like MA+ and V-like FA+ to replace the symmetric Cs+, which yields faster charge transfer, enhanced excitation, and reduced nonradiative recombination, which are combined to make the self-trapped excited state brighter with significantly enhanced PLQYs. Overall, our results clearly demonstrate that the organic–inorganic hybrid strategy is an effective way to tune the lattice distortion and significantly enhance the emission of nondoped DPNCs, which will open doors for designing functional optoelectronic materials.