Hybrid Quantum Dot Light-Emitting Diodes: Design, Fabrication, and Characterization

Abstract

The authors have designed and fabricated the colloidal quantum dot (QD) light-emitting diodes based on a thin CdSe/ZnS QD layer sandwiched by organic hole and electron transport layers. The device configuration can be depicted as ITO/PEDOT:PSS/poly-TPD/QD/BCP/Alq <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">3</sub> /LiF/Al. The optimized light turn-on voltage is about 7 V and the maximum brightness is 141 cd/m <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sup> at 11.5 V. It has been observed the second organic layer, 2,9-dimethyl-4,7-diphenyl-1,10-phenanthroline (BCP), not only served for the electron transporting but also for hole blocking. It can greatly improve the device performance by enhancing the probabilities of electron and hole injection into the QDs. In device characterization, room temperature photoluminescence (PL) and electroluminescence (EL) peaks are at 625 and 628.6 nm, respectively. The red-shift may be from the local Joule heating induced by the injection current. Emission from Alq <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">3</sub> at 516 nm is also observed in all EL spectra. Specifically, the QD-dominant EL spectra indicate the QD emission is mainly from the direct injection and radiative recombination of carriers rather than the radiative and nonradiative (Forster) energy transfers from excitons in organic materials shown in single-layered hybrid devices. Since the emission spectra are dominated by the inorganic QDs, the improvement of reliability of hybrid devices can thus be expected. However, the luminance efficiency (~0.014 cd/A) is still not as high as that of pure organic or polymer LEDs. To increase the brightness and efficiency, the device structure has to be modified and equivalent injection of electrons and holes is necessary.