Unveiling the Impact of Electron Accumulation on the Performance of Quantum-Dot Light-Emitting Diodes Using Electrically Pumped Transient Absorption Spectroscopy

Abstract

The quantum-dot light-emitting diode (QLED) is a new generation light emission source that holds great promise for display and laser applications. Unbalanced electron and hole injections, leading to excessive electron accumulation in the quantum dots (QDs), are believed to impair QLED performance. However, this effect of electron accumulation remains unverified due to the lack of a time-resolved technology capable of characterizing electrons in QLEDs. To tackle this challenge, we develop a unique electrically pumped transient absorption (E-TA) spectroscopy to probe the density of accumulated electrons in QD layer with a nanosecond time resolution. The E-TA result provides a comprehensive understanding of the electron accumulation dynamics in red, green and blue QLEDs, by quantifying the electron injection time (τ_r) after external voltage on, electron release time (τ_d) after external voltage off, and equilibrated electron density (N_e) accumulated at QD layer during device operation. We find that when QDs with a quantum yield of 95% are used in QLEDs, electron accumulation indeed harms device efficiency. However, when QDs with a quantum yield of less than 70% are used, QLED efficiency increases with a higher density of accumulated electrons. We expect this result to guide further QLED optimization and to promote the application of E-TA for QLED studies.