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Report

Teaser, summary, work performed and final results

Periodic Reporting for period 1 - NEXTLEDs (Chemical Engineering of Atomically-Flat Colloidal Quantum Wells for Next-Generation Light-Emitting Devices)

Teaser

Summary

The introduction of light-emitting diodes (LEDs) offering energy-efficient solutions has revolutionized the solid-state lighting and display technologies. With the widespread use of LEDs, the electricity consumption for lighting in Europe has significantly decreased from 19% in 2006 to below 15% in 2015, enabling saving 85 billion â‚¬ annually together with the dramatically reduced carbon footprint. In addition to the energy efficiency, the paradigm shifts towards to the achievement of high colour quality for the next-generation display technologies. To meet these future demands, the NEXTLEDs project was dedicated to the development of solution-processable and high performance LEDs showing improved colour purity and brightness by using colloidal quantum wells (CQWs) as a novel light-emitting layer.
As a new class of colloidal semiconductor nanocrystals (NCs), CQWs exhibit distinct electronic structure and optical properties compared to the commonly used organic and inorganic light-emitting materials in LEDs. Owing to their atomically-flat surfaces and well-defined vertical thicknesses, they possess the narrowest emission linewidth, giant oscillator strength and suppressed Auger recombination. In addition, the orientation of dipole moments in these atomically-flat CQWs enabled the observation of directional emission, which may help to overcome the theoretically limited light-outcoupling efficiency issue observed in the LEDs. With these appealing features, the effective utilization of CQWs in the LED architectures has hold great potential for the development of the high-performance LEDs with improved colour quality, increased brightness and enhanced outcoupling efficiency. To successfully achieve our goal in the NEXTLEDs project, we target the following objectives:
- to develop advanced heterostructures of CQWs showing narrow emission linewidth together with the improved photoluminescence quantum yield (PLQY) and stability,
- to control the assembly of the synthesized CQWs with desired configuration,
- to design and fabricate CQW-LEDs exhibiting high efficiency and high colour quality.

Work performed

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 âˆ¼46000 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: UniversitaÌ€ 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.

Final results

To achieve the overarching goal of the NEXTLEDs project, it was crucial to synthesize CQWs exhibiting very high PLQY and stability. However, the state of the art core/shell CQWs were generally synthesized at room temperature by using colloidal atomic layer deposition approach and they suffered from the poor crystallinity, limited stability and low PLQY. Thus, the realization of outstanding performance of CQWs in light-emitting devices has been hampered.
With our new synthetic approach, we showed that it is possible to obtain highly crystalline and uniform core/shell CQWs showing very high PLQY and we managed to push the efficiency levels of anisotropically shaped CQWs comparable to the state of the art isotropically shaped NCs. We believe that this synthesis approach will also open new directions for the synthesis of carefully designed and engineered heterostructures of CQWs. In addition, the quasi two-dimensional nature of CQWs providing a better suppression of Auger recombination enabled us to the achievement of superior performance in LEDs including a peak external quantum efficiency of 9.92%, ultra-high brightness levels up to 46000 cd/m2 and low efficiency roll-off characteristics. Our results has already shown the great potential of CQWs in LEDs and make them highly appealing for the development of next-generation optoelectronic devices, which may even challenge their widely used epitaxially-grown counterparts.