Bandgap-Engineered High-Efficiency Blue- and Green-Emitting CdZnSeS/ZnS Quaternary Alloyed Core/Shell Colloidal...

Developing solution-processed blue emitters with high stability and photoluminescence quantum yield (PL-QY) is strongly desired for advanced optoelectronic devices. However, achieving high-efficiency blue emitters has been challenging, as the growth of shell layers required for passivation of nonradiative recombination pathways induces a considerable red shift toward longer wavelengths in colloidal nanocrystals. To address this limitation, in this work, we propose and demonstrate a meticulous synthetic approach to develop highly efficient CdZnSeS/ZnS quaternary alloyed core/shell nanoplatelets (NPLs) with controllable shell thickness and core composition, exhibiting blue or green emission, depending on the core composition. Starting with the CdSe_0.7S_0.3 alloyed core NPLs, a thin ZnS shell was first grown through the hot injection (HI) technique, followed by a Cd-to-Zn cation-exchange (CE) reaction, which blue-shifts the absorption/emission peaks. Then, a wide-gap ZnS shell was grown a second time to passivate the surface and obtain high-efficiency thick NPLs with a PL-QY of >70% over a broad spectrum (ca. 460-560 nm). Despite the increased thickness, the thick-shell quaternary NPLs exhibit a minimal PL red shift. The blue light-emitting diode (LED) device fabricated using these bandgap-engineered NPLs demonstrates an exceptionally high external quantum efficiency (EQE) of 11.3% at 482 nm with a low turn-on voltage (V_T) of less than 2.5 V, and a maximum luminance (L_max) of 12,451 cd/m². These advanced heterostructures of NPLs with highly efficient tunable emission in blue and green provide a great platform for developing high-performance light-emitting devices, especially for LEDs and lasers.