Comparative Study of Red/Green/Blue Quantum-Dot Light-Emitting Diodes by Time-Resolved Transient Electroluminescence

Abstract

To understand the electronic processes in quantum-dot light-emitting diodes (QLEDs), a comparative study was performed by time-resolved transient electroluminescence (TREL). We fabricated red, green, and blue (R-, G-, and B-) QLEDs with poly­(9,9-dioctylfluorene-co-N-(4-sec-butylphenyl)­diphenylamine) as the hole-transporting layer with conventional structures. The external quantum efficiency (EQE) and current efficiency were 19.2% and 22.7 cd A–1 for R-QLEDs, 21.1% and 93.3 cd A–1 for G-QLEDs, and 10.6% and 10.4 cd A–1 for B-QLEDs, respectively. The TREL results for B-QLEDs were remarkably different from those for R- and G-QLEDs because of the insufficient electron injection crossing the type II heterojunction between the emission layer and the electron-transporting layer. We further applied poly­(N-vinylcarbazole) as the hole-transporting layer and obtained much better performance for B-QLEDs, with EQE and current efficiency of 15.9% and 15.4 cd A–1, respectively. Concomitant with the increase in EQE are an increase in the turn-on voltage from 2.3 to 3.7 V and a transient electroluminescence spike after voltage turn-off.

Understanding and minimizing non-radiative recombination losses in perovskite light-emitting diodes

“…Additionally, it could provide an energy level profile of different traps. Time-resolved transient electroluminescence (TREL) 88 has been widely used in QLEDs to investigate the pertinent kinetic processes in operational devices. This method can also be utilized in understanding the dynamics in operational PeLEDs to optimize the performance.…”

Alleviating Electron Over-Injection for Efficient Cadmium-Free Quantum Dot Light-Emitting Diodes toward Deep-Blue Emission

“…In deep-blue QLEDs, although poly­(9-vinylcarbazole) (PVK) is commonly used as the hole transport layer (HTL), its hole mobility (2.5 × 10 –6 cm 2 v –1 s –1 ) lags behind ZnO by three orders of magnitude. Simultaneously, the large hole injection barrier (1 ± 0.3 eV) between PVK and QDs hinders effective hole injection, resulting in electron over-injection during device operation, thereby inducing severe charge injection imbalance. ,,, Excessive injected electrons accumulate at the QDs’ interface to aggravate nonradiative Auger recombination and even tunnel directly into the HTL to decompose it, ,, which degrade device efficiency and operational stability. The abovementioned phenomenon is particularly significant in the wide bandgap deep-blue QLEDs.…”

Intense Electrical Pulsing of Perovskite Light Emitting Diodes under Cryogenic Conditions

“…The intensity of such overshoot reaches more than six times of the stabilized EL value at the end of the pulse at a relatively low pulsing voltage of 3.8 V. With an increase of pulsing voltage, the overshoot effect diminishes. The EL overshoot has been previously observed in thin‐film optoelectronic devices based on organics, [ 27 ] quantum dots, [ 28 ] and perovskites. Gegevičius et al have modeled and verified this effect in a typical perovskite solar cell at RT, showing that the mixing of charge carriers that were accumulated near the transport layer/perovskite interfaces is responsible for this phenomenon.…”