Solution-Processable A2XY4 (A = PEA, BA; X = Pb, Sn, Cu, Mn; Y = Cl, Br, I) Crystals for High Light Yield and Ultrafast Scintillators

Two-dimensional (2-D) Ruddlesden-Popper (RP) hybrid organic-inorganic perovskite (HOIP) crystals, A2XY4 [A = Phenethylammonium (PEA), Butylammonium (BA); X = Pb, Sn, Cu, Mn; Y = Cl, Br, I] have been a subject of interest for solution-processable scintillators for the past two decades, due to the possibility to grow high-quality and large crystals with low-cost techniques. We start the review from PEA2PbBr4 and BA2PbBr4 crystals, which have light yields >10 photons/keV and scintillation decay times < 15 ns. Then, we extend our review to iodide compounds from the perspective that the smaller bandgaps and the heavier anions can allow higher light yields and shorter absorption lengths, respectively. In our previous experiments, we observed that the iodide crystals are bright while they have 1 ns optical decay times. Another approach is the investigations of the ion-doped PEA2PbBr4 and BA2PbBr4, in which Li-doped PEA2PbBr4 has 23 photons/keV light yields. An additional feature is the thermal neutron detection and the discrimination with gamma-ray. Finally, we investigate lead-free perovskite variants (Sn, Cu, and Mn) as they are more friendly to environments, and the emission is shifted from blue to green or red for better sensitivity with current X-ray imaging detectors. Unfortunately, the light yields are much lower than the Pb counterparts, while the decay times are considerably slower due to different exciton mechanisms. This comprehensive investigation helps us to direct our review to the identification of the ultimate 2-D RP HOIP scintillators with high light yield, ultrafast response, and environmental friendliness.