Improved Ink-Jet-Printed CdSe Quantum Dot Light-Emitting Diodes with Minimized Hole Transport Layer Erosion

Ink-jet printing is a promising deposition technology, which is capable of large-area fabrication and mask-free patterning. For ink-jet-printed quantum dot (QD) light-emitting diodes (LEDs), the QDs are commonly dissolved in a mixture of solvent and thickener ink system. However, the hole transport layer could be eroded by this QD ink, leading to a rough surface morphology and resulting in the leakage of carriers and low device performance. This phenomenon was first and directly observed by using an atomic force microscope and a cross-sectional scanning electron microscope. We, therefore, redesigned the annealing process of the hole transport layer to achieve an optimized smooth surface with a reduced number of defects for ink-jet-printed QD LEDs (QLEDs). Optimized morphology brings back a maximum luminance of over 30,000 cd/m2 and an external quantum efficiency of 7.52% for the ink-jet-printed red QLEDs using CdSe QDs, which are comparable to those of the spin-coated device. Moreover, the operation lifetime of the ink-jet-printed device is also enhanced by the restored surface morphology. An enhanced T50 lifetime of the ink-jet-printed device at 1000 cd/m2 is improved from 26 to 127 h, which converted to a long T50 lifetime of 8013 h, when operated at 100 cd/m2.