All-Solution Processed Green Quantum-Dot Lighting Device with PEDOT:PSS:PMA P-Type Conducting Layer

Quantum dot light-emitting diodes (QLEDs) have drawn substantial interest over the past few decades as potential large-area display devices, thanks to their remarkable quantum efficiency, pure and highly saturated RGB color expression, and broad process adaptability. Typically, the colloidal quantum dots (QDs) are spin-coated to form an emissive layer (EML) sandwiched between the hole and electron transport layers (HTLs and ETLs), essential for crafting p–n junction lighting devices. Despite their simple solution casting process, QLEDs show high efficiency and long lifetime, qualifying them for practical devices and displays. However, the overall efficiency of QLEDs is determined not just by the internal quantum efficiency of QDs, but also significantly by the carrier transport and injection capabilities of the HTLs and ETLs. Particularly, the electron mobility and concentration often surpass those of holes by orders of magnitude . This carrier imbalance leads to the accumulation of excess electrons at the interfaces around light EML, causing exciton quenching and leakage current overflow in the devices. Additionally, surplus electrons intensify the Auger recombination of quantum dots, resulting in further fluorescence quenching, which is a principal factor behind the reduced efficiency of QLED devices. Thus, the carrier imbalance poses a significant challenge in the fabrication of high efficiency devices.<br/>In this presentation, we highlight the improved p-type conductivity in solution-processed green QLEDs, achieved through the use of mixed solution of poly(3,4-ethylenedioxyth-iophene):poly(styrene-sulfonate) (PEDOT:PSS) and phosphomolybdic acid (PMA) as an HTL. The mixing ratio of PEDOT:PSS to PMA was progressively adjusted from 1:0 to 1:0.5 to determine the optimal mixing ratio for maximizing p-type conductivity and QLED lighting efficiency. Ultraviolet photoelectron spectroscopy (UPS) and x-ray photoelectron spectroscopy (XPS) were used to investigate the actual energy level shift of hole transport layers in the junction device structure. Moreover, the redox of Mo in various valence states in the mixed solution was explored concerning work function and energy level via UPS, suggesting the modulation of hole injection and electron blocking abilities. The work function of PEDOT:PSS:PMA was evaluated as well. The optimized energy level mitigates the hole transport barrier, leading to a decrease in driving voltage and an enhancement in electroluminescent efficiency. The p-type conductivity of the mixture was further validated by assessing the electrical properties of hole-only devices. This approach to enhance the hole conduction can be extended to the fabrication of many other solution-processed QLEDs, thereby advancing practical display applications.