Cadmium-Free and Efficient Type-II InP/ZnO/ZnS Quantum Dots and Their Application for LEDs

Because of the spatial separation of electron-hole wavefunctions, type-II quantum dots (QDs) are considered to have a poor photoluminescence quantum yield (PLQY). In recent studies, cadmium-based type-II QDs with high PLQY have been reported¹. However, due to its toxic environment, it is required to develop non-toxic and efficient type-II QDs. Herein, we demonstrate highly efficient heavy toxic metal-free type-II InP/ZnO/ZnS core/shell/shell nanostructures².<br/>InP core structure is synthesized by the hot injection technique. Consecutive ZnO and ZnS shells are grown by thermal decomposition and successive ionic layer adsorption (SILAR), respectively. Optimization studies of ZnO and ZnS shell thickness result in a highly efficient nanostructure that has a PLQY of 91%. The effect of ZnO and ZnS shell growth on the non-linear absorption and ultrafast decay properties is investigated via femtosecond pump-probe spectroscopy.<br/>Small-angle X-ray scattering (SAXS) and transmission electron microscopy (TEM) measurements were used to determine the size and shape of the core/shell/shell QDs. The SAXS analysis shows that multiple ZnS shell growth induces the transition of nanocrystals from spherical to ellipsoidal shape which is in accordance with TEM results. Finally, we integrate the core/shell/shell QDs into light-emitting diode (LED) die in liquid state to reduce the host-material effect. The resulting liquid-LED device exhibited 9.4% of external quantum efficiency (EQE) and 6.8% of power conversion efficiency (PCE), respectively, making it the most efficient type-II-based LED to date. This research found that cadmium-free type-II QDs can achieve significant levels of efficiency, which could lead to new devices and nanomaterials for bioimaging, display, and lighting.<br/><br/><b>Keywords:</b> cadmium-free, type-II band alignment, liquid LED<br/><br/><b>Reference:</b><br/>(1) Ca, N.; Hien, N.; Luyen, N.; Lien, V.; Thanh, L.; Do, P.; Bau, N.; Pham, T. Photoluminescence properties of CdTe/CdTeSe/CdSe core/alloyed/shell type-II quantum dots. <i>Journal of Alloys and Compounds</i> <b>2019</b>, <i>787</i>, 823-830.<br/>(2) Eren, G. O.; Sadeghi, S.; Bahmani Jalali, H.; Ritter, M.; Han, M.; Baylam, I.; Melikov, R.; Onal, A.; Oz, F.; Sahin, M. Cadmium-free and efficient type-II InP/ZnO/ZnS quantum dots and their application for LEDs. <i>ACS Applied Materials & Interfaces</i> <b>2021</b>, <i>13</i>, 32022-32030.<br/><b>Acknowledgment:</b> S.N. acknowledges support from the European Research Council (ERC) under the European Union’s Horizon 2020 Research and Innovation Programme (grant agreement no. 639846). S.N. also acknowledges the support of the Turkish Academy of Sciences (TUBA-GEBIP; The Young Scientist Award Program); the Science Academy of Turkey (BAGEP; The Young Scientist Award Program); and Bilim Kahramanlari Dernegi (The Young Scientist Award Program).