Research on Controlling Electron Injection and Leakage in Efficient Green InP-Based Quantum Dot Light-Emitting Diodes

 Research on Controlling Electron Injection and Leakage in Efficient Green InP-Based Quantum Dot Light-Emitting Diodes 

 https://doi.org/10.1038/s41586-024-08197-z

 I. Research Background and Challenges 

Quantum dot light-emitting diodes (QD-LEDs) are regarded as a core technology for next-generation displays and lighting due to their high quantum efficiency and excellent monochromaticity. However, cadmium (Cd)-based QD-LEDs, the current state-of-the-art, are limited by toxicity concerns, driving the development of non-toxic alternatives like InP and ZnSe-based QDs. While red InP-based QD-LEDs and blue ZnTeSe QD-LEDs have achieved external quantum efficiencies (EQEs) exceeding 20%, green InP-based QD-LEDs face critical bottlenecks: low efficiency (16.3%) and short operational lifetime. These issues arise from the InP–ZnSeS–ZnS core–shell–shell structure, where the ZnSeS interlayer creates a high electron injection barrier, leading to insufficient electron concentration and inadequate suppression of non-radiative recombination and trap effects. 

 II. Key Discovery: Regulation of Electron Behavior via ZnSe Interlayer 

Using electrically excited transient absorption (EETA) spectroscopy, the study directly measured electron concentrations in operating QD-LEDs and revealed that the ZnSeS interlayer reduces electron concentration in green InP-based QD-LEDs to just 0.09 (vs. 0.50 in red devices), far below the threshold for efficient radiative recombination. 

A two-step strategy was proposed: 

1. Lowering the Injection Barrier: Replacing ZnSeS with a pure ZnSe interlayer, leveraging ZnSe’s lower conduction band minimum to enhance electron injection efficiency, increasing electron concentration to over 0.15. 

2. Suppressing Electron Leakage: Thickening the ZnSe interlayer (from 2.5 nm to 4.0 nm). The Wentzel–Kramers–Brillouin (WKB) quantum tunneling model showed that a thicker ZnSe layer significantly reduces electron leakage probability while maintaining high injection probability, increasing the injection-to-leakage probability ratio by over threefold. 

 III. Breakthrough in Device Performance 

The optimized green InP-based QD-LEDs (emission wavelength 543 nm) achieved remarkable performance improvements: 

- Efficiency: peak EQE of 26.68%, a 63% increase over the previous best value (16.3%) and 1.6 times higher than comparable devices. 

- Lifetime: T95 lifetime (time for luminance to drop to 95% of the initial value) of 1,241 hours at an initial brightness of 1,000 cd/m², a 165-fold improvement over previous records, addressing long-standing stability issues. 

- Brightness and Driving Voltage: A maximum brightness exceeding 270,000 cd/m² and a turn-on voltage as low as 2.1 V, approaching the efficient injection performance of Cd-based QD-LEDs. 

 IV. Role of in Device Structure 

PF8Cz (poly((9,9-dioctylfluorenyl-2,7-diyl)-alt-(9-(2-ethylhexyl)-carbazole-3,6-diyl))), serving as the hole-transport layer (HTL), plays a critical role: 

1. Hole Transport and Accumulation Regulation: Spin-coated onto the PEDOT:PSS layer, PF8Cz forms a uniform film that facilitates efficient hole transport from the anode (ITO) to the quantum dot layer. EETA measurements showed that in devices with a thickened ZnSe interlayer, the HTL bleaching signal saturated rather than decreased with increasing voltage, indicating suppressed electron leakage and stable hole injection. 

2. Interface Energy Level Matching: PF8Cz’s energy levels align with the valence band of InP QDs, reducing hole injection barriers. Its high glass transition temperature and excellent film-forming properties enhance device structural stability, minimizing non-radiative recombination caused by interface defects. 

 V. Theoretical Model and Mechanistic Analysis 

The WKB quantum tunneling model revealed the balance between electron injection and leakage: while increasing ZnSe interlayer thickness slightly reduces injection probability, it exponentially decreases leakage probability, leading to a net increase in electron concentration. Experimental results closely matched model simulations, proving that carrier behavior can be precisely controlled by adjusting shell material and thickness. 

 VI. Conclusion and Prospects 

This study used EETA to identify the root cause of low efficiency in green InP-based QD-LEDs—the high barrier of the ZnSeS interlayer—and achieved breakthroughs in efficiency and lifetime through material replacement and structural optimization. PF8Cz aids in balancing hole transport and interface energy levels, indirectly supporting efficient electron-hole recombination. The proposed strategy provides universal guidance for cadmium-free QD-LED design and is scalable to other low-electron-concentration systems like blue QD-LEDs, driving the practical application of full-color, efficient, and environmentally friendly QD-LEDs.