| Author: | Zhao, Fangqing |
| Title: | Study of light extraction and carrier behavior of cadmium-free and lead-free quantum dot light-emitting diodes |
| Advisors: | Hao, Jianhua (AP) |
| Degree: | Ph.D. |
| Year: | 2025 |
| Department: | Department of Applied Physics |
| Pages: | xxii, 156 pages : color illustrations |
| Language: | English |
| Abstract: | Quantum dots (QDs) and their light-emitting diodes (LEDs) have shown great potential in the display industry. However, the well-studied cadmium based QDs and perovskite nanocrystals contain elements such as cadmium and lead, which pose significant threats to the environment and human health, thereby limiting their broader application. In contrast, cadmium-free and lead-free QDs and their light-emitting diodes show greater potential. In recent years, significant research efforts have been directed toward the development of cadmium-free and lead-free quantum dot light-emitting diodes (QLEDs). Despite these efforts, the performance of cadmium-free and lead-free QLEDs, particularly in the blue and green spectral regions, remains below the thresholds required for commercialization. Indium phosphide (InP) and zinc selenide (ZnSe) QD materials have emerged as promising cadmium-free and lead-free candidates for constructing high-performance QLED emissive layers (EML), especially for blue and green QLED applications. Enhancing light extraction efficiency and optimizing carrier behavior in the EML are two key factors for achieving high-performance QLEDs. To achieve good external quantum efficiency (EQE), this thesis focuses on the working mechanisms of cadmium-free and lead-free QLEDs concerning the above two factors and proposes strategies for efficiency improvement, leading to several innovative results. Firstly, we analyzed the light extraction modes within the cadmium-free and lead-free QLEDs and proposed a combined scheme to improve light extraction efficiency under limited internal quantum efficiency (IQE), thereby obtaining higher-performing QLEDs. The thesis presents an effective comprehensive strategy, involving the addition of an appropriately thick spacer layer, high-index substrates, and substrate surface roughening to sequentially extract light from the SPP mode, waveguide mode, and substrate mode to the air mode of QLEDs. This combined scheme promotes optical transmission between modes within the device through optical tunneling and microcavity design. After optimization, the light extraction efficiency (LEE) was significantly improved, resulting in an EQE increase from 6.64% to 18.50% for blue ZnSe QLEDs. Approximately threefold EQE improvement provides a more universal optical optimization strategy for cadmium-free and lead-free QLEDs. To address the issue of efficiency reduction due to unbalanced carrier injection in cadmium-free and lead-free QLEDs, an innovative design based on an electric dipole layer was proposed to enhance hole injection. An innovative electric dipole layer material, 2,2’-(perfluoronaphthalene-2,6-diylidene)dimalononitrile (F6-TCNNQ), was also applied to cadmium-free and lead-free QLEDs. The strong forward-built electric field significantly facilitates hole injection at the interface from the hole transport layer (HTL) to the emissive layer (EML). The introduction of F6-TCNNQ also regulates the interface energy level of the HTL, reducing the hole injection barrier to the emissive layer (EML). For blue ZnSe QLEDs with strong electron injection, this method bypasses the limitations of electron injection, thereby achieving a higher radiative recombination rate. As a result, the optimized EQE of blue ZnSe QLEDs was increased from 7.44% to 12.71%. To further enhance the efficiency of cadmium-free and lead-free QLEDs, the electron leakage mechanism was revealed and studied in depth using green InP QLEDs as an example to achieve less non-radiative recombination. An effective method was proposed to prevent electron leakage by applying LiF, which reduces the Fermi level difference between green InP QDs and the ITO anode. LiF modification not only suppressed electron leakage but also further enhanced hole injection through the tunneling effect. The optimized devices achieved more balanced carrier injection in the EML, resulting in the EQE increasing to a maximum of 9.14%. This thesis explores cadmium-free and lead-free QLEDs with a focus on light extraction and carrier behavior. Based on an analysis of the working mechanisms, optimization strategies are proposed from both optical and electrical perspectives to achieve improved light extraction and better carrier behavior. The optimized cadmium-free and lead-free QLEDs exhibit higher EQE, making them more suitable for display applications. The mechanisms and strategies proposed in this thesis are expected to provide valuable insights for further advancements in the cadmium-free and lead-free QLEDs field. |
| Rights: | All rights reserved |
| Access: | open access |
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