Hybrid Interfacial Layer for Enhancing Hole Injection in Quantum Dot Light Emitting Diodes

Colloidal quantum dots (QDs) have been applied in the display industry due to their outstanding optical properties, including tunable spectra, narrow full-width at half-maximum (FWHM), and compatibility with solution processing. To achieve high performance, QD light-emitting diodes (QD-LEDs), electroluminescent devices utilizing QDs, require efficient hole injection by engineering charge transport layers. This is essential for balanced charge injection, suppressing unwanted non-radiative recombination such as Auger recombination, and mitigating the hole transport layer (HTL) degradation caused by inefficient hole injection. It is typically reported that hole injection from the highest occupied molecular orbital (HOMO) of the HTL into the QD is inefficient. Therefore, techniques such as designing a stepwise injection structure using an HTL with a deep HOMO or fabricating the HTL structure with materials having high hole mobility have primarily focused on controlling injection at the HTL-QD interface. However, it is difficult to conclude that the injection characteristics at the HTL-QD interface solely determine inefficient hole injection.<br/>Herein, we suggest increasing the hole injection efficiency by inducing p-type doping of the HTL through interface engineering between the hole injection layer (HIL) and HTL. To achieve p-type doping of the HTL, we prepared a solution-processed hybrid functional thin film (PVK:PMA) by mixing the organic molecular material poly(9-vinylcarbazole) (PVK) with the inorganic material phosphomolybdic acid (PMA). Through X-ray photoelectron spectroscopy and ultraviolet-visible spectroscopy analyses, we confirmed that a charge transfer complex (CTC) was induced by mixing PVK and PMA. Furthermore, the positive charges generated in the CTC can be easily separated into free charges by the electric field, and the separated positive charges can be injected into the HTL matrix without overcoming the hole injection barrier. However, since the negative charges separated from the CTC can recombine within the HIL, it is necessary to suppress the recombination of the negative charges. Therefore, by introducing transition metal oxide between HIL and PVK:PMA layer, the recombination caused by the negative charges from the CTC, is effectively suppressed due to the deep conduction band energy levels of transition metal oxide. Moreover, using ultraviolet photoelectron spectroscopy has shown that the high ionization energy of PVK:PMA contributes to alleviating the hole injection barrier at the transition metal oxide-HTL interface. These results provide clear evidence for the increase in hole injection characteristics in QD-LEDs incorporating the PVK:PMA hybrid functional thin film. The HIL with the PVK:PMA hybrid functional thin film showed significant performance improvements compared to conventional HIL structures, with power efficiency increasing by 2 times and lifetime extending by 6 times. This approach highlights that efficient hole injection in QD-LEDs is determined by the number of holes present in the HTL, and it suggests that this approach could enable the realization of stable QD-LEDs suitable for applications requiring high efficiency and enhanced stability.