Deciphering exciton-generation processes in quantum-dot electroluminescence

• Published in: Nature communications 2020-05, Vol.11 (1), p.2309-2309, Article 2309
• Main Authors: Deng, Yunzhou, Lin, Xing, Fang, Wei, Di, Dawei, Wang, Linjun, Friend, Richard H., Peng, Xiaogang, Jin, Yizheng
• Format: Article
• Language: English
• Subjects: 639/301/357; 639/624/1020; Colloiding; Confinement; Crystals; Electroluminescence; Electrons; Excitons; Holes (electron deficiencies); Humanities and Social Sciences; Injection; Light emitting diodes; multidisciplinary; Nanocrystals; Quantum dots; Radiative recombination; Recombination; Room temperature; Science; Science (multidisciplinary); Spectroscopy; Spectrum analysis
• ISSN: 2041-1723; 2041-1723
• DOI: 10.1038/s41467-020-15944-z

Electroluminescence of colloidal nanocrystals promises a new generation of high-performance and solution-processable light-emitting diodes. The operation of nanocrystal-based light-emitting diodes relies on the radiative recombination of electrically generated excitons. However, a fundamental question—how excitons are electrically generated in individual nanocrystals—remains unanswered. Here, we reveal a nanoscopic mechanism of sequential electron-hole injection for exciton generation in nanocrystal-based electroluminescent devices. To decipher the corresponding elementary processes, we develop electrically-pumped single-nanocrystal spectroscopy. While hole injection into neutral quantum dots is generally considered to be inefficient, we find that the intermediate negatively charged state of quantum dots triggers confinement-enhanced Coulomb interactions, which simultaneously accelerate hole injection and hinder excessive electron injection. In-situ/operando spectroscopy on state-of-the-art quantum-dot light-emitting diodes demonstrates that exciton generation at the ensemble level is consistent with the charge-confinement-enhanced sequential electron-hole injection mechanism probed at the single-nanocrystal level. Our findings provide a universal mechanism for enhancing charge balance in nanocrystal-based electroluminescent devices. Today it remains unclear how excitons are electrically generated in individual nanocrystals. Here, the authors propose the identification of the longlived intermediate QD– state for exciton generation of CdSe-based QD-LEDs by a room temperature electrically-pumped single-molecule spectroscopy.