In the beginning of the project, different heterostructures of CQWs were identified for the synthesis including CdSe core-only, CdSeS alloyed core-only, CdSe/CdS core/crown, CdSe/CdSeS core/alloyed crown, CdSe/CdS core/shell, CdSe/CdZnS core/shell and CdSe/CdS/CdZnS core/crown/shell CQWs. By using the optimal approaches for the identified CQWs, their syntheses were optimized and different heterostructures of CQWs were successfully obtained. We also developed a new two-step synthetic approach for the synthesis of core/shell CQWs at high reaction temperatures. This new synthesis approach enabled us to precisely tune the composition of shell layer. We found that core/shell CQWs having CdS buffer and CdZnS gradient shell layers exhibit the highest PLQY up to 89% together with the narrower emission linewidth down to 21 nm.
In the next step of the project implementation, we investigated the film formation characteristics of the synthesized CQWs. We identified three different approaches for the uniform film formation including drop-casting, spin-coating and self-assembly. We observed that by using spin-coating approach, we produced highly uniform films with desired film thickness in a reproducible way.
In the last part, we focused on the fabrication of LEDs by using our successfully synthesized CQWs. We firstly identified several organic and inorganic charge transport layers and studied the charge and/or energy transfer kinetics between the charge transport layers and CQWs. Secondly, we optimized the thicknesses of the each layer in device structure. By using our optimized device architecture, we also investigated the effect of shell compositions on the device performances. We found that CdSe/CdS/CdZnS core/graded shell CQWs exhibit the best performance in LEDs. The additional growth of CdZnS gradient shell had a profound effect on the resulting characteristics of CQW-LEDs and exhibit significantly improved external quantum efficiency value of 9.92%, which is one of the highest reported efficiency value in the literature. Finally, the low efficiency roll-off characteristics of CQW-LEDs enabled us to observe ultra-high brightness up to 46 000 cd/m2 with an electroluminescence peak centered at 650 nm.
The most significant results achieved during the project implementation can be summarized as followed;
- Development of a two-step synthetic approach for the synthesis of core/shell CQWs at high reaction temperatures enabling the synthesis of core/shell CQWs with various shell composition and the systematic investigation of relationship between the shell composition and resulting optical properties of CQWs.
- Synthesis of CdSe/CdS/CdZnS core/graded shell CQWs showing significantly improved optical properties including enhanced PLQY (up to 89%) and narrow emission linewidth (down to 21 nm).
- Fabrication of LEDs by using CdSe/CdS/CdZnS core/graded shell CQWs showing remarkable properties: one of the highest external quantum efficiency value of 9.92% as compared to the other CQW-LEDs, and the achievement of the ultra high brightness of ∼46000 cd/m2 with an electroluminescence peak centered at 650 nm among the CQW-LEDs.
Two peer-review articles have been published on the basis of the results obtained during the project implementation in ACS Nano and Small. The results were presented in the NaNaX-9 conference held in Hamburg, 2019. During the project implementation, collaborations with the universities and research centres have been performed: Università del Salento (Italy), Bilkent University (Turkey), and other research groups at ETH Zurich (Switzerland). Dr. Kelestemur also supervised a visiting student during the period of fellowship. The results of the project were discussed biweekly with Prof. Kovalenko and presented in the group meetings regularly.
